MagneQuest/Peerless Forum: CCS Preamp by John Swenson

OK, here is the schematic for the CCS based preamp. Again this is based on gary Pimm's self bias CCS, but this time there is no pentode, just MOSFETS. The "grid bias" is provided by a 9V battery in this case. The impedance of the CCS generated bias voltage was much lower than I wanted so it was easier to use a bettery than modify the CCS bias system (which I could do, and might still do someday)

The input attenuator takes some explaining. Its a stereo stepped attenuator with balance control. The topology is a series/shunt attenuator, with variable series resistor to implement a balance control. I do not like the mono controls like in the FP, but if you use a stereo switch you can't do balance adjustment. Sooo this uses a stereo switch, but adds high value resistors in parallel to the main series resistor to decrease its value, thus decreasing the output voltage for a given setting of the shunt resistor. For each chanel one "side" is open (thus no paralleled resistor) and the other side has resistors that cause a .5db change in output per step. In actuality these are 24 position switches with 2 db steps on the attenuator and .5 db steps (+-6db total) on the balance control. This gives me exactly what I want: stereo volume and fine balance control.

The gain of the stage is set by the ratio of R20/R1, 2,2 in this case. With the 9V bias there is 4V threshol on Q1, so there is 5V across R1, thus in this case with R1 = 1K, there is 5 mA flowing through Q1 and Q2. Changing R1 changes the current and R20/R1 sets the gain. Now this is both AC and DC gain, so high gain means high voltage drop across R20. To get around this you can either bypass R1 with a cap and low value resistor (which needs a LARGE cap) or use a plate choke or transformer. Theoretically you can go with a lower bias voltage, but the stage does not work nearly as well with low voltages across R1, 5V is already pretty low, Gary likes 12V better.

The gain of 2.2 and the gain of the CCS Amps was just perfect, primary listening position is in the 12:00 to 3:00 range, I still have room to turn it up for REALLY loud and have plenty of range for soft levels.

The THD on this preamp is .04%, not too bad, with a perfect harmonic series, 2nd highest and decreasing as you go with 4th barely visible and the rest down in the noise. The frequency response was down .1db at 20Hz and up very slightly, .05db, at 20KHz, probably a cap resonance. The noise level was right at the sound card's noise floor (about -100db) so the noise of the preamp itself is at that level or even better. This is one QUIET preamp. There was some 60Hz about 60db down, it was definately 60 not 120, so it seems to be hum getting picked up by the wiring rather than PS ripple causing this.

This preamp out tubes the tube preamps, it has great bass, deep, full rich, VERY smooth, yet highly detailed, loads of dynamic downrange. Music is powerfull, subtle, lush, smooth and detailed all at the same time! Very "real" sounding, just effortless music. Huge amounts of details, texture etc are audible, but its not "etched" or "in-your-face". For me at least it beats out a tweaked FPII, I don't have an FPIII to compare it with though. Its completely SS, yet sounds like the best tube designs, I was very surprised at how good this sounds. I haven't spent any time tweaking this at all, just threw it together to see if any sound would some out at all and was blown away.

With this design I can easily hear differences in coupling caps, I guess it just kind of gets out of the way, so the differences are audible.

This gain stage also works extremely well as a cathode follower, so a CF stage could be added easily, but in this case I don't think it is needed since the output impedance was low enough to work well with the CCS amps as is. The two stages could be DC coupled, it would take a little more voltage to do that though (60V or so).

All in all a surprizingly good sounding preamp, especially for SS!!!

John S.