Here we are on holidays and surprisingly I struggle to find any free time in which I can sit down in front of a tablet and write this blog entry. This is actually quite good as it’s a sign of me being disconnected and looking after my daughters. Life is good.

As said earlier on my previous post, I have a pile of experiments and stuff to share which I’ll hopefully do in due course. The first one which sprang to mind was the folded cascode experiments which I conducted last year. This came around due to two main quests I was working on. First one, my phono stage which actually uses one 50dB gain stage made up of a folded cascode E810F triode-strapped. Marvelous results and hats off to Rod Coleman for building a small PCB kit for his circuit. Second quest, was to optimise my pre-amp/driver stages in my DHT system. Well, it seems like a never ending story. I kind of like it that way as it pushes me constantly to find new avenues and circuits. This was what ignited me on looking at combining the higher gain and benefits of the folded cascode topology, with the unique sound of the DHTs.

I’m not going to cover the folded cascode topology nor its circuit details. I’ve done that to death in previous posts. You can search around and will get a lot of information about it. So please, don’t send me an email asking question about how the circuit works, etc. You can do a bit of research yourself.

For obvious reasons due to my personal preferences, I picked up on the VT-25/10Y as my first DHT victim on this circuit. The VT-25 has an average transconductance for a DHT: 1.5mS (a 300B had 5mS for comparison purposes). In perspective, the gm is low for this circuit and we will see soon why.

I wanted to combine the sound of the VT25 with a higher gain which I wanted to get from end to end on my system. I targeted 30dB which obviously is beyond what you can do with a normal hybrid mu-follower stage which is mu the maximum gain possible. With a DHT normally this is around 8 and you could push it to 15-20 with some specific triodes.

When I built the first prototype test, I resourced the available PCBs I have to build this circuit like a Lego game. I used a CCS PCB, a pair of Source Followers (SFs) for output buffer and for voltage references and then a simple shunt PNP transistor as the pass element in a PCB prototype. You can avoid all this complexity and get a simple and compact PCB from Rod. I did this so I could play around and adjust the circuit and learn through experimentation.

First test was made with a VT25 DHT. I used the following configuration:

CCS load:

1. IXTP08N100D / DN2540 cascode

2. Trimpot 100R in parallel with 270R

3. Padding resistor = 10R

4. Folded Cascode device: ZTX951

5. Rc = 33K wirewound 7W

6. Bias supply for shunt cascoded: Gyrator PCB hacked for bias supply. This was later replaced with a SF board with a reference voltage generated by a CCS.

Here is the initial test using fixed bias, you can see the reduction in gain to about x40 and HF response limited to 23kHz when loaded with 100K:

Given the higher output impedance, the frequency response and overall gain was compromised with a 100K load. So I added a source follower: DC-coupled with the Shunt Cascode and an output capacitor of 220nF to the load.

VT25 Shunt Cascode. Experiment 2 (addition of SF output buffer)

As you can see in the above plot, the frequency response is now much better as the SF isolates the load from the high-impedance output of the shunt cascode stage.

Well, all this looks nice and rosy as we are having a nice DHT stage with lots of gain. So, how this stage actually performs?

Although distortion is acceptable at 4Vrms (0.05%) it increases up to 0.52% for 20V output. What I then tried was to implement some cathode degeneration to help with the linearisation of this stage. With a resistor of 300R as Rk distortion results where much better and the frequency response wasn’t impacted as I was using a low impedance voltage regulator for the filament circuit, not CCS based. Otherwise the circuit using a CCS-type filament regulators (e.g. Rod Coleman’s) will lead to a lot of hum picked up by the DHT unfortunately. The gain is reduced to 35 with same Rc and this is due to the effective gm is reduced due to the cathode resistor. This is the level of gain I’d like to have in my system, so it was great. Frequency response was good up to 86kHz with the source follower as previous test.

The current of the Shunt cascode was adjusted (CCS) to minimise distortion at maximum level. 144V at the Rc is high and limits the output swing (ideally 100V should have provided more headroom), however the distortion is minimised for a voltage across Rc about 130-145V:

This is the final stage I ended up playing with. It sounds very nice but not as good as the 01a-VT25 dual stage I have. Likely being the high harmonics introduced at louder volume levels. It doesn’t have the same headroom as the other 2 valve setup. So I wouldn’t use it unless is for small signal amplification, not to drive +100Vpp. Here is the circuit I implemented. Again, if you want to build this, ask Rod for his kit. It’s way more simpler than the below and is all included in a tiny PCB:

If you were to implement this circuit with Rod’s board, it would look like this:

01a DHT Folded Cascode

Of course after the relatively success of the VT-25 folded cascode stage, I couldn’t resist the temptation of building a similar one with the 01a. Knowing that this topology relies on good gm on the lower device, I knew I was trending waters which likely were going to conduce me to a nice outcome. Nevertheless it was worth trying it. Implementing the same circuit with 01a, I struggled with the following:

1. With cathode degeneration (Rk=1k6). The filament acts as an antenna and you need to be careful to pick up noise. With a low voltage filament regulator works well. However, the gm is reduced to a level that overall gain is just 10 with the Rc of 39k. I will probably need a resistor of 78k or 80k to deliver a gain of 20.

1. With filament bias, Rfil = 27R (which is biased a bit too high) the distortion is high:

I could only minimise the distortion by reducing the collector voltage to about 30V:

The 01a stage is a no go. Not enough transconductance is available to provide a linear stage. Here is the oposite example with a 300B:

Clearly the higher the Gm (degenerated) the more linear the stage is. 28dB gain with 22K Rc impacted slightly by 100K output load. There is no SF buffer and the frequency response is very good.

300B folded cascode frequency response

If you want to implement a DHT in a stage like this, the 300B is a winner given its gm. I’d likely try replacing the cathode resistor with an array of SiC diodes. However, would need to check distortion and optimal point. I ran out of testing time when was playing with this breadboard and should probably get back to it.

When I was exchanging emails with Rod about my findings here’s what Rod commented on the gm impact:

Yes, higher gm, lower distortion is exactly what I found. In almost every case, taking a Shunt Cascode circuit and replacing the valve with one of higher gm lowers distortion, and this action has a good record of improving the sound. High gm lowers distortion, and allows lower values of load resistor – without affecting the anode current in the valve.

For fun, I went back to my old lab book from May 2005 to check what I did when I developed the circuit: The first round of tests of the Shunt Cascode designs were used to compare EL84, EF80, PC86, EF95, Western Electric 717A [all IDHTs] and the DHT LP2, Marconi Osram manufacture.

They were measured for gain, and then tested for sound with the help of a Classical musician. The DHT came last in those tests, the record shows.

It’s not too hard to see why. DHTs owe their superior qualities to the lack of curve-compression in the curves’ high voltage, low-current segment. This gives measurably better results with wide swings, especially with reactive loads (i.e. speakers or long cables), which draw the load line out into an ellipse.

As you know, Shunt cascode applies a vertical load-line to the triode. In most cases, the DHT advantage is not helpful, because the worst-distorting zone of indirectly-heated triode curves is not covered in the signal swing.

DHTs in shunt cascode may not work best with the usual operating points: in many cases, lower anode voltage and higher current will be better. But still, a high-gm frame-grid 1950s/60s designed valve will usually be better, in the same position. Even the (maybe still plentiful?) EF80 is very fine-sounding.

For a preamp, the 6Э5П as shunt cascode will give the same outstanding performance that it does as a driver. It can be optimised by running a lower anode voltage and higher current, if desired. A lower value of load resistor will also help, and you can get usefully low drive impedance this way.

The 6Э5П is a superb Shunt Cascode valve, partly for its property of gm-saturation, at currents over 30mA. So running lower voltage, and higher current (35-38mA or even more) with this one (and also, but to a lesser extent – the 6Э6П) may give even better results!

More generally, low anode voltage (80-150V) in Shunt Cascode stages permits the use of many different kinds of low-cost frame-grid pentodes & triodes for a preamp – including many that are not well-thought of, for conventional amps.

Rod Coleman (2018)

And indeed, a 6Э5П / 6Э6П makes a much better driver in shunt cascode. I posted before about this and would recommend this to anyone who wants to build a higher gain driver for a 300B or a transmitting valve output stage.

Folded Cascode in Phono Stages

I intend to cover this in detail at some point. However, due to its subject complexity, it will take me a while to summarise my work on this.

Nevertheless, I wanted to show an example of how this folded cascode circuit can perform brilliantly when married with a high-gm valve. I’m using the E810F in triode mode as part of my phono stage. This valve is configured as second stage of amplification with a whooping 50dB gain and superb linearity. It’s situated in between a split RIAA network which leverages the advantage of the output Rc impedance of the folded cascode as part of the RIAA network.

I built and tested the FC PCBs from Rod. Stuffed it with an IXT08N100D top MOSFET and used 27K wirewound RC resistor. The E810F valves are biased at about 160V and have a 47R cathode resistor.

Really nice performance with 60dB gain, 45kHz bandwidth and distortion is very low. Nice harmonic profile. Good job on this circuit from Rod!

If you look at the above, you will see that the stage has an amazing low distortion for 60Vpp / 20Vrms and 50dB of gain! Difficult to achieve whilst providing low input capacitance thanks to the cascode configuration. Also the harmonic profile is very nice. And it sounds great too!

Here is a view of my initial phono prototype with Rod’s board:

Hope you have enjoyed this post and got the bug of the folded cascode!

Here’s a sample of the stick sound. Early days, learning and a bit of loops

Enjoy

loop 1

For quite some time now I wanted to share one of the variations of the famous VT-25/10Y pre-amplifier. As usual due to work, business travel and family duties, I’ve struggled to find the time to update the blog.

Anyhow, here is the variation of this great preamp with a slight modification of the filament bias arrangement. I discovered time ago that introducing the SiC diodes instead of the wirewound resistors provided an improvement on the overall sound and detail level. You can read around this point on my ETF.18 lecture or over here.

The circuit looks like this:

The only change I’ve made is replacing the filament resistor for an array of 6 SiC diodes. Happily used my SiC PCB which holds also six diodes. You can bolt them to the chassis or can leave them without heatsink. They will withstand individually the power dissipation as each will get about 1.2W. You will need good ventilation though.

Fabio’s build

Fabio Valente from Italy built this preamp recently and sent me his feedback and pictures. This encouraged me to dig the circuit out from my archive and post his impressions and comments which may encourage others to build this great preamp:

Ciao Ale,

I want to send you a couple of pictures of my preamp. Last week I optimized a few details and installed SIC diodes for the bias (in place of a 3R 12W resistor). With four of them I get 5.1V (1.27V on each diode), I can’t install five as I have 16VAC only transformers. I’d only have 0.5VDC headroom.

The B+, and also the 4VAC feeding the AZ1 mesh rectifier’s filament, is regenerated. There’s a 50Hz DDS based generator, preamplified and suitably delaied, feeding two powerful class AB amp followed by step-up transformers (EI with separated windings everywhere). Filtering is a LCLCLC with the last one in the preamp’s box. Only russian KBG paper and aluminium foil are used. 18+18+12uF only.

The volume is a relay based resistive logarithmic attenuator connected to the DHT’s grid. On the signal only USA made Jupiter PIO and copper foil capacitors are used.

I’m very happy with the sound of this preamp. I wouldn’t know how to try to improve it. But I know that it’d be possible somehow

Fabio

Fabio, Valente. Italy.

Why not? Less is more and if well achieved the task, you will end up with an amazing result. To do this you will need a good quality OPT and the valve which can do this task. It will need high mu where possible and linear. There are just a handful of DHT that could do the job (like EML20/30) or you can look for the high gm pentodes (e.g. E55L, E810F, etc.) which are very expensive generally or Russian valves like my beloved 6e5p which is cheap as chips (still).

Depending on the step down ratio of your OPT you will need likely a step up input transformer to drive the 6e5p to full tilt. Here is what I experimented with many years ago. Unfortunately I didn’t take any distortion measurements but it sounded really nice.

If you look at the performance of one bottle, we can squeeze out a pair of watts easily out of this valve:

Ignore the THD projection, it’s much lower than that in reality. I will hook this up again and measure to contrast. The 6e5p in triode mode is extremely linear valve so you should expect somewhere below 5% at full swing. You can see that we can get out 2W when valve is biased at 40mA/250V. Input drive it’s about 11Vpp or close to 3.8Vrms. So at least a 1:2 SUT will be required.

As you can see on the above diagram, a pair of 6e5p can do 4W. I omitted the SUT as I was driving this stage with my preamp. At the time I used a Lundahl transformer, but any 3K-ish transformer would work well here. The filaments are DC regulated but you can do as you please. The cheap DC-DC SMPS converters can do great job here. Also the HT can be derived with a HV DC-dC converter plus a cap multiplier stage. All it will be powered from a small 12V DC wall-wart supply.

The SiC diode stage performs great in terms of sound and low dynamic impedance. You can use these PCBs to hold them easily.

If you have good gain but not enough (current) drive into the grid of the 6e5p to handle the miller capacitance, you could add the follower stage like this one:

Of course you end up adding an extra stage (follower) and a pair of capacitors, but is worth trying to listen to HF improvement due to slew rate distortion removal.

You could also power the follower stage from a 24V bipolar supply and tweak the R1/R4 resistor divider to provide DC coupling into V1/V2 by removing C4. You will need to ensure the MOSFET is biased to about 0V. This would help with A2 drive if needed. But all this is extra complexity not needed here. I think it goes against the SPUD ethos but wanted to put it out there as many of you like experimenting a bit around the circuits (like I do).

So the above implementation is what I’d do again. The addition of T2 (1:2 or 1:4) provides the voltage gain needed. You may need (or not) to add the Zobel network (Rz1 and Cz1) to keep HF response smooth depending on the SUT of your choice. I could recommend Sowter, Lundahl or Dorin’s Bodea (DVB) transformers depending on where you live.

An amazing stage and no excuses to build this one. Only big expense is the OPT pair. Other than that, you can build this easily and enjoy the 4W out of it. Enjoy!

I made the following drawings one afternoon last week in which I took a few days off before getting back to the daily grind in full first week of September.

You can see what a nice couple of red wine glasses open you up to draw

Pete Millett asked me for these drawings as he likes them much. I’m flattered for him to do so, honestly. These were all made in a tablet, so originals are actually in electronic format

I will bring some nice whisky which Pete likes to ETF, likely one bottle of the Scotch Malt Whisky Society, which I became a member a year ago.

This year round at ETF there will be an interesting shootout competition, however due to lack of time I couldn’t commit to build anything in advance. I hope things will get better when my youngest daughter grows up a bit more.

A few weeks back we managed to meet at Jon’s with a few of the London Audio Circle members. I haven’t seen the bunch for a long time due to family and work travel commitments, however it was a great opportunity I didn’t want to miss this time round.

One of the intended purpose of the meeting , which for obvious reasons these meetings never goes as planned due to the random nature of DIY, was to shootout two headphone (HP) amplifiers over modded HD800 headphones.

Jon’s HP amplifier was a simple stage formed by a C3g (triode-strapped) valve loaded with a nice amorphous-core Valab choke. Output cap-coupled to a TVC to his main system when he uses it as preamp. Otherwise cap couple to HD800s headphones.

On the other hand, we had Geoff’s HP amplifier. Geoff undertook the crazy quest of building my 2P29L HP amp design in a single enclosure! After a lot of tinkering with the grounding layout and physical layout and orientation of transformers and other components, he managed to get it dead quiet. An amazing achievement for a DHT in a single enclosure! After discussing it with him, obviously regretted having done so as it was a big headache which left his metal case with a several holes from moving stuff around.

Geoff fitted a pair of Kemet 100μF/500V  large Polypropilene capacitors. He also had the option to hook in a custom-made (by himself) of 4:1 output transformers. 

His design is very close to my original circuit here:

The only differences are obviously the output cap and OPT and Geoff only has 6 SiC diodes. He runs the stage at about 17mA which provides just enough the necessary current for a 300Ω HP.

Listening to the HP amps

We had a long series of unplanned technical bits during the day which deviated us from the intended purpose of the day, however it was fun.

Listening to both amps was more than a pleasing experience. Source was the Starlight CD player with the discrete DAC. In this case we tried a low-impedance DAC board which Tom managed to develop recently. A great achievement as it can drive the TVCs directly and sounds superb.

Everyone agreed that the 2P29L sounded slightly better than the other one. Perhaps is the subtle sound transparency and detail that the 2P29L brings and also the benefits of the hybrid mu-follower, which has proven many times is a real winner.

I have to admit personally I preferred Geoff’s amplifier with the OPT, rather than the direct drive (which is very convenient). This was my personal preference

I have a pair of huge Mundorf EVO Oil 100μF/350V to try on my new HP breadboard. So stay tuned!

Modding the HD800 headphones

Here’s Geoff’s take on his HP modification: 

The mods on my hd 800 as first the “anaxillus” modification and second the “Dupont”. They are both designed to reduce a sharp peak in the phones response at 6khz. Its highly probable that this peak gives the phones its imaging and detail retreival strengths but at the expense of excessive brightness. So those who were trying to reduce the peak were careful not to reduce it too much and rob the phones of their strengths. I think both mods have succeded with this objective. The “anaxillus ” mod tries to deal with this peak by adding a ring of dampening foam in the main ear cup of the phones. The type and thickness of the foam was arrived at after considerable measurement and experimentation. It does indeed succeed in reducing THD 6kHz peak but also has some effect on other resonances within the ear-cup.  The overall result was widely acknowledged as a success. There is a thread on the web describing how to make and fit this mod. The “dupont” modification tries to deal with the peak in a different way. It uses a Helmholtz absorber to damp the 6Khz peak. It is made from rings of specific thickness and density foam to produce a small chamber with a small port which fits into a small cylinder in the ear-cup. The damping effect comes from frictional losses in the port. Again this was arrived at by lots of measurement and experimentation. Sennheiser themselves use something similar in the hd800s but which appears to be a little more effective  probably because they they can use bespoke materials. Again there is a thread describing how to make and fit this mod plus a ready made one can be bought from Dupont.  When I first bought mine  with the anaxillus I compared them to Jons which only had the Dupont  and thought Jon’s had less of the 6Khz peak problem. So I orderedand fitted a Dupont one but kept the anaxillus fitted. Mine now does have a reduced 6khz peak but also now sounds slightly softer. Its difficult to know which is best.I think it depends on what the headphone amp sounds like. I quite like the double mod with the direct o/p on the 2P29L. 

Geoff Mead (2019)

More than 8 years ago, I played with this valve after Thomas Mayer wrote about it here. It seemed like a good option but despite a few bench tests, I never got around building an amplifier with it.

I’m preparing for ETF.19 and will be bringing my eTracer to the event so I built a few socket adaptors. It was the time for the 12-pin Compactron to come to life.

Through my few Compactron valves, I bumped into the 6HS5 again, so I decided to put it to the mercy of the tracer. I wanted to see the positive grid current behaviour, as haven’t seen curves around with it:

What can we say of the above curves? Well this triode could do 7-8W easily as Thomas says. It has an anode power dissipation of 30-35W. The anode slope is between 7 to 10K so local feedback will be needed. You will need to aim for 600-700V bias at least, probably higher with slightly negative bias. However, you will need a hefty source follower to drive the grid. Look at the grid current. It can be 50mA at 5V and creeps up the higher the grid voltage it goes.

Interesting valve with a 6.3V/1.5A heater, and the high-mu (80-100) can work out as a SPUD amplifier with local feedback. Driver should be able to handle the grid current and low impedance. Not an easy task though.

Experimenting on headphone amplifiers, not always turn out the way you expected. Well, that’s the nature of experimentation and some smoke may come along the way.

Here is my new take on the headphone amp. Modified the 2P29L to fit the 6e6p-dr valve. I love this IHT one, so wanted to experiment more.

Instead of the classic HT, I used some Chinese 300-450V SMPS modules. I used the 70W version with success before, not these tiny ones.

Individually tested all parts and worked fine. Fired up the SMPS with the hot filaments and the 6e6p-dr running at 20mA each and the SMPS oscillated. They went to 450V instead of the 250V dialled and one of my cap multiplier PCB boards got damaged. Damn!

Back to the drawing board. I will probably stick to the simple HT supply

My adventure with the HT SMPS modules continued. Did further testing with higher power modules which worked ok.

The headphone stage (HP) has a 1MHz bandwidth, impressive. Here is the circuit breadboarded after the modification of the 2P29L HP amp:

The cap multiplier isn’t mandatory as the gyrator PCB has a high PSR so a good well-filtered HT supply will work. I added the cap-multiplier for 2 reasons. I have the PCBs made and also I was looking to improve the supply noise of the SMPS modules. My intension was to power this up with an LT supply module I have based out of 2 variable SMPSs.

The Chinese 40W HT SMPS modules that can do 450V work fine. They need a good HF filter as well as a proper LC stage to decouple and filter the ripple to make it absolutely dead quiet.

Here is the variable HT supply I build some time ago which I use for multiple circuits:

With this stage I have the choice of swapping the output caps. Either a massive (aka expensive) Mundorf EVO Oil 100uF/350V for direct coupling of the 300R headphones, or via the Sowter HP output transformer with a smaller cap.

I personally prefer the OT coupling in this circuit, but both sound extremely well.

More work and measurements to follow.

I’ve been running with this HP amplifier for a few weeks now and I have to say I’m delighted with it. I matched a pair of 6e6p-dr on my eTracer and after fiddling with the operating point a bit, I settled for Ia=25mA and Va=180V. Anode voltage will depend on the valve as said before, the Russian valve parameters tend to be all over the place. Anyhow, expect anode voltage to be around 170 to 185V. The beauty of the gyrator PCB is that you can adjust the anode voltage with the trimpot.

I’m using the fantastic panel meters from situbes. They are brilliant to monitor the anode current. As described before, I’m reusing a pair of PCBs I designed for fixed bias cathode protection with a fuse and also a measuring point for the anode or cathode current.

The cathode bias is provided with the SiC diode array. I installed a SiC PCB which has capacity for 6 diodes and multiple tapping points. I’ve done this to allow me testing different valves. I have a set of D3a, E282F, 12HL7 and a few other pentodes (triode-strapped) which have same pinnout and am looking to test how they sound in comparison.

I played for a week the cap-coupled output configuration with the huge 100uF Mundorf EVO Oil caps you can see on the back from the picture above. Sound is very detailed, clarity and bass is strong. I used my Sennheiser HD650 as well as my Beyerdynamic DT770 pro headphones.

Last night switched back to the Sowter 8665 output transformers and replaced the 100uF for the 4.7uF Mundorf EVO oil caps. These are much smaller as you can imagine. Smaller cap, happier listener to me.

I have to say and ratify, that I prefer the clarity and overall sound of the output cap. Indeed.

Don’t want to put away any builder who wants to build this without the expense of the Sowter Output Transformers. Believe me, the difference in sound to me is minor and if you want to build the HP amp within budget, you may spend less on the output capacitor by avoiding the output transformer.

I still need to try a standard 100uF Film cap, which are significantly cheeper than the Mundorf EVO Oil. Likely you will get a very good sound, as I’ve listened to Geoff’s 2P29L version recently.

An interesting finding was how sensitive this amp can be due to its gain and wide bandwidth. I put the RCA input sockets as well as the stepped attenuator mounted in a small piece of clad copper PCB. I forgot to ground it. What happened? Playing the HP amp I could hear some very soft hum and also eventual/random HF oscillation could kick in after a period of playing. When I touched the plate with my fingers, everything will stop. Aha! nice antenna. I grounded and ended story. Lesson learned, never forget to ground those metal plates (even in a breadboard)

I have to say that this 6e6p-dr HP amp has nothing to envy to his brother DHT incarnations. Without the DHT power supply complexity, this is an amazing HP amp worth to be added to your collection.

I’ve got a set of teflon sockets from Jakeband. These are fantastically made to order. Luciano from Jakeband sent me in addition some samples which I will use in the eTracer which I will bring along to ETF.19.

Honestly, these are fine pieces of craftwork. For example, you can measure and let Luciano know the diameter of your 845 (or any other transmitting valves) when ordering these valves so they fit perfectly.

I’ve used these sockets on my projects for years and am very pleased have to say. Of course you pay a premium price as these are hand-made with fine materials. In my opinion, these are worth every cent.

Also I had a pair of RCA sockets, look at them:

They are made on a single piece of copper. Sterling job!

You can contact Jakeband directly to order your sockets. Just use the form I posted years ago here.

I’ve been working on a few PCBs lately with great results. I started with an HT power supply PCB which allows the use of full silicon rectifier bridge, hybrid rectifier (e.g. for damper valves) and any combination of either CRCRC or CLCLC stages with external chokes. Last capacitor is bulky WIMA DC-Link film instead of electrolytic. That worked really well and I’m using it now in a +50/-300V supply for the fixed bias and source follower drivers in the amps.

Then I moved to modify the design for a low-tension (LT) supply. This is much needed for any DHT stage and could also be used for any sort of filament supply.

With the same concept in mind, I wanted to have the flexibility to use (or not) external chokes. You need to get enough space for the “snap” type of electrolytic capacitors (like the great quality KEMET 22mF / 25V). Here is the generic PSU circuit topology:

You can see a classic snubber formed by C1 and R2. I use low-drop Schottky diodes (in TO-220) albeit there is an option to use classic diodes as well. First cap (C2) is the tuning cap (choke-input vs. cap-input) and then you can either use L2 and L3 or resistors R1 and R4. R1 and R3 provide the input resistance to main bulky cap C3. Then you have C4 before the common mode choke L1 (I will rename it as L3 to make it consistent). C5 is the second bulky electrolytic cap. Then you have room for a 100nF and a 220pF ceramic cap for HF. There is a bleeder resistor (R5), the on-board fuse (F1) and the indicator circuit, an LED plus its resistor.

Here is the board before assembling:

And here is the board finished:

Ok, here is the circuit I tried. I wanted to avoid the external chokes on this first test, so here it goes:

I have a nice set of 100VA JMS transformers with split bobbins and shielded with copper screen. It does have taps at: 0-0.5-1-2.5-5-7.5-10-20V and all these options allow me to configure the transformer to the desire output voltage.

I made a few tests with 0.9A and 1.8A loads and output ripple was between 0.7mV and 1mV. Really good!

I may run these PCB in a production batch. However, I don’t have time but more importantly I don’t have the time to answer emails of people asking me for help in designing their power supplies. I don’t know, perhaps I’ll do it.

As I promised, I took the HP amp back to the workshop for a bit of abuse on the bench. Here are some interesting measurements which correlate with some of my listening impressions so far.

I added a set of 300Ω dummy loads at the output to simulate the HPs. Firstly I tested the optimal circuit which has the Sowter 8665 output transformer in Parafeed mode (4:1) to drive the load. You can get a very clean and flat response across the audio band from 3Hz up to +90kHz. There is a minor hump to be tuned due to the RLC circuit formed by the parafeed circuit. I used a 4.7μF Mundorf EVO oil cap. It’s pretty flat to me, so will leave it there.

The performance of the circuit is amazing. Distortion is very low, up to 0.07% at 100mW (which is VL=5.47Vrms):

Harmonics from H3 onwards are very low level and distortion is primarily driven by H2. This measures as good as it sounds. Great stage.

Well, let’s look at the stage with the 100μF Mundorf EVO oil cap instead. This is a completely different beast. We are asking the hybrid mu-follower to drive a 300Ω which is pretty tough. The stage has low output impedance and current drive. The maximum current drive for 100mW will be around 18mA. Anyhow, the load will be steep and a tough one for this stage without the output transformer. Although the sound is very nice at low level, I could easily prefer the Sowter circuit during the listening tests. Lets see why.

As you can expect from the hybrid mu-follower topology, the frequency response is very good:

However, distortion is significantly higher: THD is 1.7% for 100mW in 300Ω (5.47Vrms). You should expect 0.5% or less at normal listening levels though.

The main system is made up of 833a choke loaded and parafeed output. The driver is a mu-follower made up of a 6N6P loaded with a D3a. Second stage is a 300B with an IT (step-down) into the 833a valve.

Shearer horn w/ JBL 2220 drivers, mid horn w/ RCA Mi-9486 dual compression drivers and Dittmar’s tweeter horn

Whether it was a good idea or not to bring the eTracer to ETF was yet to be seen. Fortunately enough, I escaped smartly to become a prisoner of my own suggestion.

Did quite a few tests with the tracer and contributed to avoid auctioning a worn out 45. Instead, the auctioned pair became a single 45. It felt like a nice contribution.

On the flip side, tracing Pete’s 50 mesh stash was a joy. Handled with care his pension investment when tracing them. However, we found a gassy and a open filament one. Ouch. At the same time, saved the curves of a 50 measuring 100% which was really nice.

The bass horn is Jeffrey’s designed conical horn with an Altec driver. The woodwork was kindly made by Tim over in Bellême. On top sits an WE 24a mid-range horn with a 555 driver.

What a beauty, I keep saying to Jeffrey that there’s no point in building a stereo system with this configuration. It’s just perfect and even your brain gets tricked with the 24a geometry to create a pseudo-stereo imaging when listening to certain tracks. It’s just amazing.

Finally, there is a tweeter EMIA (Experienced Music Impact Audio 60Hz field coil with Le Cleac’h horn.

Looking at the amplifier, which has a unique aesthetic design which I’m very fond of, you can see Jeoffrey’s design using Dave Slagle iron at length. Rod Coleman regulators for all DHTs. Mercury Rectifier 394a. The clever stuff is on the output transformers which are in series for substractive crossover between the 555 and Altec drivers. There is a Mundorf cap to cross over the EMIA field coil driver.

From a valve point of view, the driver this year is a rare Electromekano DHT triode. The output valve is was either a 205d or 50 (mesh).

From a source point of view, the turntable is a Garrard 301 refurbished by Audio Grail. Schick tone arm and Slagle’s cartridge based on modified DL103. RIAA stage is EMIA LCR stage into Slagle’s Autoformer control.

Listening the system

So we put the 50 mesh valve on Jeffrey’s system. That was a mind-blowing fit. It just married perfectly with it. Sound was so detailed, clear and just perfect coming out of the horns.

We listened to Art Pepper, Art Tatum and Charles Bradley before the gassy 50 mesh showed a deadly blue which forced us to stop the music. Damn, it sounded so rounded and good.

Changing the 50 to the 205d wasn’t a straightforward match. We sat down and played Mingus “Changes” – which is a record I know by heart – and realised a bumpy bass response was evident and not right. The higher output impedance of the 205d didn’t marry the horn system without adjustment which Jeffrey put away for a later modification and returned to the 50 valve, not a mesh one of course.

After diner, we spent a nice time listening to Fred McDowell’s Alan Lomax recordings with JC Morrison, Herbert and other nice ETF folks.

Vegard brought a delicious Aquavit which I enjoyed whilst sitting and feeling myself in the Mississippi. What a joy ETF is.

First and foremost, Merry Christmas! I hope you’re having a great time with your loved ones and good music.

Without a mood for public introspection this time, I have to say that I will celebrate this holiday the best I can. I will be hosting today with a lot of meat, wine and single malt. What else can I do? When I escape from the little ones, will get back to spin some records and hopefully work on the projects.

Headphone Amp Quest Continues

Obviously, I needed to pursue this project further. I’ve been lately listening a lot to HPs as simply is the best choice for me when the young family is in bed. In addition to this, I find the HP setup to force me to connect a bit more to what I’m listening to as I don’t get easily distracted.

Anyhow, the testing of the previous HP amp evolved a bit. From the great 6e6p valves, I moved to the D3a (triode-strapped) which I feared of delivering too much gain into my system and hence noise would be a problem potentially.

The results where more than interesting. As expected, the D3a delivers a superbly clean and detailed sound. I love it but yes, gain is too much. Noise wasn’t a problem though as the system I built is dead quiet. Good job on this one.

The operating point I used for the D3a was Va=170V/Ia=24mA and biased with 2 SiC diodes. This was very easy as just needed to move the wire jumper on the SiC bias PCB to set it for 2V and job done.

I can try several pentodes with minimum readjustment providing bias is 6V or less and have same pinout than the 6e6p/D3a amongst others.

The breadboard morphed into a vertical setup as per below picture. You will see a Cap Multiplier for the HT (Which also helps setting the HT level to desired one) . A pair of SI-Tubes current meters (thanks Pete Millet for these ones), the SiC bias PCBs and a pair of anode/cathode current protection + sensor PCBs which I use to take the anode current for the SI-Tubes. I have also multiple connectors for DHT filament supplies. A pair of LV regulators for the IHT filaments which I also use instead of Rod Coleman regulators for simplicity in this bread board. The gyrator PCBs are mounted vertically and the LEDs are set for always-on operation. As you have probably seen on the previous posts, the parafeed caps are mounted with 2mm connectors to allow me swap between them and use / bypass the Sowter step-down transformers:

The next valve I tried was the E282F. A fantastic pentode I used primarily as a driver. I have a few on my collection, mainly Siemens and Valvo.

After playing a bit with the operating point, I settled for 180-183V / 5 SiC diodes (about 4.8V bias voltage) which delivers a quiescent anode current of about 20mA:

Another fantastic triode-strapped pentode for the HP design. Microphonic noise is minimal and you don’t get any warm up “dings”. Sound again, very similar to D3a, strong bass, detailed and as best as you can get with an IHT.

This is a very simple circuit to build. Tom Brown sent me an HT SMPS module he design and built for the 2P29L HP amp. This is powered with a good-quality 48Vdc SMPS supply. You can use a simple stepdown isolated converter from 48V to 6V to feed the filaments directly. If you want to use a DHT you can add a BJT cap multiplier and with a pair of isolated converters you can build a stereo system powered from a single 48V converter.

Merry Christmas!

Here is a nice sharp cut-off RF pentode with low capacitance, high gm and nice curves: 6AG5.

Also the triode curves are interesting with a mu of about 50:

I spent a bit of time this morning developing a pentode model:

There is a bit of saturation above 16mA hence the gap between curves. However, this isn’t important as the valve won’t be used near this area.

Spice models

Pentode:

Triode:

This valve was recommended to me by my friend Nick W. He has used it with great results as a driver to a local feedback output DHT stage. Perfect candidate which I will put in use at some point.

Hope these curves and the models are useful to you.

After few tests and revisions (Rev 04) I updated the capacitor multiplier board I developed time ago

Despite you can place WIMA DC-Link film caps across the entire board, the use of high-quality electrolytic capacitors for the input and multiplier places is more efficient and convenient. I leave the expensive WIMA for the last cap after the SiC Mosfet:

The board has the flexibility to feed C2 with either the CCS or a simple resistor divider from the input HT. With the CCS you get above 120dB rejection of noise across the band average which is fantastic. However, there is no free lunch as you are obliged to drop 30 or 50V between the CCS for proper operation and good performance (HF in particular). The toll is carried on by the pass element (M5 or T5) which will have to dissipate the voltage drop across its shoulders.

The pass element is C3M0280090 which has a gm of 3.6S @3.5A or about 430mS @ 100mA. It also has very low Crss and output capacitance (Coss) which is ideal for good frequency response and filtering of HF coming from mains or raw supply.

I also added the feature of HV indication via an INS-1 neon bulb which is soldered straight on top of the PCB. Very handy to indicate normal operation.

With the 500k trimpot, I can regulate well a very stable voltage between 0 and 450V – won’t get close to that as it’s the limit of the electro caps and current protection ZTX458 transistor.

I’m using this board in my headphone amplifier. It’s very nice to have the slow current turn-on feature of this capacitor multiplier. With a 47uF capacitor multiplier you get above 120dB rejection. No point to increase the value of it.

I measured 300uV of noise at 250V for 50mA load. That’s -118dB which is pretty close to the Spice simulation. Extremely good!

My headphone amp is amazingly dead quiet with this HT multiplier. A great addition to it.

I made a couple of boards extra which I’m prepared to let go. Drop me an email if you’re interested.

Here’s an interest pentode to consider for phono stages. It was used extensively in instrumentation up until 1980s. There are a few examples around and you can check the Brimar data sheet to see what I’m taking about.

I checked a quartet of the military spec CV5086 which they measured bang on 100%. Here are some curves and Spice model which I’m sure you will find useful. In triode, it does give a mu of about 20.

And here is the Spice model: