How many times I’ve been asked “how do I lower the gain of the DHT preamp?” many builders love the DHT sound and yet don’t really need that gain. Of course when you use a transformer load, step down is the obvious choice. And it works really well when you invest in a very good quality OPT.
However, when you use DHT with high rp, generally most of the low current ones, you face a tough challenge with regard to frequency response.
With the hybrid mu-follower topology (aka gyrator) we have the problem that the gain is fixed to nearly mu.
I’m in love with the DHT sound and in a preamp a 01a and the 801a/VT-25 DHT Preamp Update preamp cannot be beaten in my view. They all deliver a gain of 8 with a hybrid mu-follower topology.
This can be overcome in the following way:
We add an attenuator network at the mu output before the source follower circuit. In the above example, I configured the resistor divider R2 and R3 to provide 1/4 gain so the resulting gain is around 2 (6dB). This works like a charm and helps reducing the microphonic noise due to the attenuation of the overall signal. However, the signal headroom is limited but this shouldn’t be a problem as we would expect to run this below 5Vpp.
The rest of the circuit remains the same. The gyrator is the Rev08 and set to about 90-100V depending on the valve, the source follower is configured to operate at 15mA and the cathode SiC bias is using the C3D02060F which are in my experience the best sounding arrangement for this preamp. The filament regulators are the classic Rod Coleman regs.
It can be improved further by adding a DC level shifter to the bottom of R3 (a simple PMOS would work here like shown here) to increase the input DC level of the Source Follower. This will optimise the performance and reduce even further the distortion at higher voltage swing. Of course, this adds more complexity to the circuit.
I will likely hack my 01a preamp as don’t need the gain of 8. A simple gain of 2 would do just fine in my current 300B amplifier.