MagneQuest/Peerless Forum: CCS Transconductance Amp, schematic, Measurements by John Swenson

I finally got the schematic done for the transconductance amp based on gary Pimms Self Bias CCS. This is based on the idea that a slight modification to the CCS turns in into a very good transonductance AMP. I wanted a true transconductance amp to drive my Lowther speakers. This is a concept made popular recently by Nelson Pass.

This implementation is a normal parafeed SE output configuration using BCP-15 and EXO-45. The PS has a PGP-8.1 and RGC-6 choke in a simple LC configuration. There is almost 1V of ripple with this supply but due to the high PSRR of the circuit, it doesn't seem to matter, its completely quiet, even with 100db speakers.

One neat thing about this design is you can adjust the gain by changing R1, this actually chages the Gm of the stage. The 82 ohm shown in the schematic lets me get away without any input stage or input step up transformer.

I did some measurements last night, at 1 watt its 0.1% THD, with an almost ideal harmonic series, 2nd being highest and decresing smoothly from there. The distortion and harmonic serie stayed pretty much the same as I changed the output power over a wide range. There is some PS noise but its 60HZ, not 120, so its not coming from the PS ripple, but probably some coupling from the AC heater.

The frequency response is down -3db at 20HZ, (which is quite expected given the inductance of the plate choke) but its also down 1db at 20KHZ, I'm not sure whats causing this, cpcitance, leakage inductance? Any clues?

Since this is designed to be used as a transconductance amp I also measured the distotion and frquency response at different load impedances. At 20 ohms the increased properly to maintain the current flow through the higher load, this is jut what it was supposed to do. Except at the extemes, the low and high frequency droops increased as the load impedance increased. I understand why this is so at the low end, but again I'm unsure about whats happening at the high end.

These measurements brought to light that the 40H really is not enough, its fine for the 5k, but when it rises due to increasing load impedance, its not enough, the response starts drooping just when I need it to be compensating for the rising impedance of the driver. An 80H - 100H choke seems to be in order here.

Hey Mike, what do you have in the area of a 80 - 100H 60mA or so choke? I was thinking of an EXO-03 for the next version (7-8 watts with RH-PF OPT), but at 48H its not really enough given these measurements.

Sound wise its fantastic, deep, powerful, rich, smooth, extended, high resolution, lush, incredible you are thereness.

BTW I've also built a preamp using these techniques, but its all SS, no tubes, no iron (except the PS) so I can't really talk about it here (I'm not sure WHERE to talk about it) it thinks its a tube, but isn't.

John S.