MagneQuest/Peerless Forum: 9 Watt Transconductance Amp by John Swenson

Finally here is the schematic for the 9 watt version of the CCS based transconductance amp. Its very similar to the EL84 version With the following differences:

Different tube (of course!) I originally designed this for the EL509 but the 6550 or KT88 will also work, you might have to change one resistor value to handle different tubes, there will be a separate section on this below.

Input impedance has been raised to 100K instead of 30K, closer to a more normal tube input impedance, easier to drive from passive pres, BUT more sensitive to picking up noise, layout is a bit more critical.

Q3 and Q4 are now connected to their own voltage divider, they are only rated for 500V which was just a little too close for comfort to the 450V B+. R25 and R26 should be 1 watt metal film types.

All caps are motor run or other high voltage film types except for C5, its a 2.2uf electrolytic 50-100V. C1 is probably the most sensitive to "quality", I've had very good results with motor run caps for this. The exact value is not that important, I've run it from 5uf to 50uf, the amp works anywhere in that range, you will get different tradeoffs in "bass impact" "airines" with different values, feel free to experiment. Different tubes will also need different values. Start with 25uf and go from there.

I chose the Robin Hood PF OPT for this design, its a perfect match and will sound exceptional without costing a fortune. You can also use an EXO-50 if you have them on hand, Yes it will probably sound better than the RH, but not by a huge amount, only you can determine if its worth the price. A TFA2004 will also work but you won't get the full power output. I designed this for an EXO-03 for its low price, you could go with a BAC-80 and get a little more power output, but the price goes up quite a bit, the EXO-03 and the RH-PF are a real "sweet spot" for this design.

If you build it as in the schematic the sensitivity for full output power is about 3V RMS. If you are using an active preamp with some gain this should be fine. (It ought to work great with an FPIII) If you are using a "passive pre" or just want a little more gain, make R1 47 ohm and R10 68K, this gives an input sensitivity of 2V RMS for full power at a VERY slight decrease in sonic purity. With 2V you should be able to get full power from a CD player or DAC output with a passive pre. I don't recommend going to any higher gain.

Tube choice, as mentioned above you can use EL509, 6550 or KT88 tubes. I don't recommend El34 or 6L6. The NOS EL509 takes a special large 9 pin socket and plate cap, but I really like these tubes. There is supposedly an octal based version out now but few have seen one of these. The more traditional 6550 and KT88 will also work. The gain of the amp is NOT tube dependant, it is determined soley on the value of R1. The CCS automatically adjusts the bias for the proper current so tube rolling is really easy and you don't have to worry about "runaway" or any such weird effects. You might have to tweak R3 to get the screen voltage in the range for proper operation. All of these tubes will work just fine in this circuit with a fairly broad range of screen voltages so its not very critical. Even if its set wrong it will NOT harm anything, you just won't get full power output.

There is one adjustment you will have to make, adjusting the pot R11. This sets the idle current, the manufacturing tolerances on the MOSFETS are fairly loose so you need to adjust this. You want to adjust R11 so there is 65mA or so flowing through R1. For the 75ohm R1 thats about 4.8 volts across R1. For the 47ohm R1 thats 3 volts.

You will need to experiment to find the best output tap to use, its going to depend on the speaker and cabinet used. The transonductance amp is specifically designed to handle single driver speakers that have a rising impedance in the bass. I usually set mine for 16 ohms so the bass region which rises to 16 ohms gets full power. The midrange has lower impedance so it doesn't get full power but it doesn't need it. If your cabinet does not exhibit very much bass impedance rise you might be better off going with the 8 ohm tap. Again experiment and find out what sound best to you.

As a little side note I've been testing out the El84 version with a spectrum analyzer, its got some pretty good specs, the noise floor is 120db below 1 watt and I can't see ANY harmonics at all so I don't know what the distortion number is, but its pretty darn low if they are all less than 120db down!! There IS some 120Hz PS bleed through, but its pretty far down, and no visible IM distortion from the PS on audio signals (I expected to see some 120Hz side bands, but they are completely absent).

A word on the power supply, the venerable MQ PGP8.1 will work to drive one channel of this amp which is why I put it on the schematic (This IS the MQ forum after all), but given all the posts flying around on low DCR PS design it may not be the optimal solution. There are so many different transformers and chokes available that will work for this design that its hard to come up with the one "best PS", so all I'm going to say right now is you need 450V at 70mA B+ and up to 2A at 6.3V for heaters, how you get there is up to you! I'll certainly be glad to help design a PS for this amp given some diretion from the user as to how they want to go.

Oh yeah, what is this thing good for? This is specifically designed for single driver speaker systems, Lowthers, Fostex, AER etc. With this amp the current through the speaker is proportional to the input voltage, this works great for speakers without crossovers, but does not work very well for most multiway speakers with crossovers. The inspiration for this was Nelson Pass's work over at firstwatt.com.

A note on construction, you can either build the CCS part by hand on perfboard or use a PC board. Gary Pimm sells little boards that can be used very well with this design. (Just make sure you use these values, not the values he has in his instructions) Try and keep the input and CCS as far away from the PS as possible. If you can't physically separate them a metal shield (aluminum plate, piece of unetched PC board etc) between CCS board and PS helps a lot. The big MOSFETS might get pretty hot (it depends on the tube and screen voltage) and so need heat sinking. I would just do it to begin with and not worry about it. You can either mount a small heat sink to the transistor or mount the transistor on the underside of the board and screw it directly to the top plate (or other mounting plate). A small standoff raises the board the right distance from the plate. Gary has some nice pictures of this on his site. You can get the the IRF820s with either insullated or non-insullated tabs, the inullated is more expensive but a lot easier to mount to a heatsink, you don't need any special electrically insullating hardware. (the insullateded version is IRFI820 (note the extra 'I' in the part number))

I guess thats it for now, have fun!

John S.

Gary Pimm CCS page (Open in New Window)