Decibel Dungeon


I had a lot of interest in the pre-amp design that I posted on Decibel Dungeon. One question often asked regarding the simple three-transistor buffer has been 'can it have some gain?' Well, I don't know how to alter that circuit to give it some gain but I can now publish another simple circuit design, this time using a single Jfet, and one that does allow for adding some gain.
The circuit comes courtesy of that well-known Gainclone builder, Carlos Filipe Machado, yes the man from Lisbon, Portugal! The UNO circuit is very simple but it does require a good PSU, either batteries, or the design that is described below. So without any more preamble, let's see how it's done!
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The design.

The Carlos FM discrete GC preamp with gain.
The main circuits - words and drawings by CFM. Please note the CSN is a standard electrolytic type, not a non-polarised type.

Well that's pretty comprehensive isn't it? Carlos even clearly shows the pin out arrangement for the 2SK170 Jfet transistor so with a bit of care, anybody should be able to construct the circuit on a piece of stripboard. Of course, if you are more ambitious, you could make up a simple PCB. Carlos has even drawn one up and you can download it and the diagrams on this page by clicking here.
Now, to provide a supply for that LM317, here is the circuit that Carlos and I have come up with. It is the one that I built to test this pre-amp.

First stage of Power supply for the CFM pre-amp.
The circuit for the PSU.

  • Note that to get the 47 volts we use both sections of an 18-0-18 volt transformer to get 36 volts prior to the rectifier bridge. We chose this because it is not easy to source a 35 VAC transformer with a low current rating!
  • After the bridge we get approximately 50 volts and then loose a few volts across the resistors.
  • The rectifier bridge can be made up from separate diodes or you can use a ready-made variety. This supply won't draw a lot of current but the bridge must handle the voltage.
  • The 20K resistor across the rails is to drain the capacitors of charge when the power is turned of. I used a 3 watt rated resistor but 1 watt is probably adequate.
  • The 2R2 resistor, 2200 uF cap, 10R resistor and 10,000 uF cap make up what is known as an RCRC filter to smooth the power supply. The resistors should be rated at 1 watt and the capacitors (in fact all capacitors for this project) rated at 63 volts of higher.
  • Don't forget the mains fuse.
  • For connecting this circuit to the one in the pre-amp, ie the LM317 circuit, use either tightly twisted wires or screened cabled to avoid picking up any noise. This is important!

The first stage of the power supply.
Simple but effective. I was lucky enough to have that transformer lying around!

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The results.

Well, I have to admit to making a couple of mistakes along the way when I tried this pre-amp. But basically, if you take your time, it is not a difficult circuit to build. Indeed, with it's single rail power supply, it should be easier for beginners to follow! Make sure that you keep the two types of ground connections separated by the 1R resistor and of course, check that you insert the Jfets, the right way.
I suggest building the power supply section first and then testing it to make sure that you have the correct voltage (around 41 volts after the LM317 circuit) before you connect up the rest of the circuit. As ever, watch out for shorts (yes I had one across a couple of tracks of the stripboard and took out the PSU!).

The finished pre-amp internals.
This simple circuit will easily fit inside one of the DD pods.

The results are well worth the effort. Like the simple three-transistor buffer, this circuit has a very pure signature. Detailed, transparent, with plenty of drive, it will be the answer to your prayers particularly if you are using items like the NOS DACs that have a lowish output. If the gain is still not enough for you, swap out the 75R resistors (RD1 and RD11) for 100R or even 120R (but don't go above that).

The finished pre-amp in a 'pod'.
Another pre-amp, another pod! Many thanks to Carlos for sharing this design with us all.

This pre amp should work equally well with class-T amps or most other power amps for that matter!

Here are some answers to questions raised about the Uno pre-amp.

  • Q. I have built the UNO and find the gain too much. How do I lower the gain?

    A. Increasing the value of the source resistor (RS1 and RS11) lowers the gain.
  • Q. What is the usable DC voltage range for the UNO ?

    A. Between 30V to 40V regulated.
  • Q. Will the transparency be improved with a different voltage ?

    A. I don't think so. But the input filter (before the volume pot) can be reduced or even removed. If you want to keep a gentle RF filter there that doesn't impact the sound, then I suggest 51R + 100pF.
  • If my amp has an input capacitor, can I remove the output caps (C1) in the pre ? The amp has an input cap of 3.3 uF.

    Yes, in that case the cap (C1 and C11) can be removed. But never forget that your pre amp has tens of DC voltage on the output. In case you connect it to a power amp without an input coupling cap. (!)
Thanks to Carlos for answering these questions.
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