Introduction
This was one of my first DHT preamps. I found a quartet of NOS CX-112a Cunningham (globe) back in Buenos Aires many years ago and built one of my first DHT preamps. I loved it. I played with it before I moved into the 26 and then started the long exploratory journey with DHTs.
The CX-112a can be easily fit in an existing 01a preamp. Take a look at what Thomas Mayer recently blogged about this valve, worth reading it.
Well, you can get more current drive than 01a (nearly double) but no thoriated tungsten filaments. Anyhow, the gain is slightly higher but is very easy to adapt to my gyrator-based circuit, that I couldn’t resist to take the quartet out of my valve stash and make them sing again after so many years.
The circuit
Again, an easy drop in without modifications. If you want to learn more about this circuit then I suggest you read the 01a blog posts. In summary, here it is:
I used my existing 01a, however I tried newer PCB boards with the IXTP08N100D top MOSFET as well as slight variation of the components as shown int the diagram. I like the fact I can drive it at 8mA, better Slew Rate and deeper bass as the BF862 transconductance is higher.
Performance
This is an outstanding preamp. I have to admit that I was even impressed with the bass, closer to the VT-25. It has good dynamics but probably the level of detail not as close as the 01a. I think it’s really good and is worth listening for some time. I will leave it running as I enjoy it very much.
The measured performance is great. Let’s have a look at the frequency response:
Flat response from 3Hz up to 700kHz. High-bandwidth, clear treble and strong bass. A real contender amongst the DHTs
Distortion can be as low as 0.005% for 1V output to 0.11% for 10V swing. The harmonics haven’t got the best cadence when at full tilt, however, at normal levels they are great:
I don’t know how easy are the CX-112a to get these days, but certainly they are worth using. The sound of the preamp stage is great. Detailed and with strong bass and clear treble. I feel at this early stage is a good contender to the 01a as well as the VT-25.
i will play it for a while. Boards and preamp overall needs some break-in time.
Cheers
Ale