Push-Pull Class A 2A3 Stereo Amp

Last Update - 11/27/95


Here is the schematic for one channel of my amp:


By popular demand ... click here to download a postscript version of the schematic. (90k)

Design Details

Input stage

The input stage is comprised of a both halves of a 6SL7 octal dual hi-mu triode in a differential amp configuration with a 1ma constant current cathode load. I'm using field-effect (constant-current) diodes for simplicity. the diff amp approach was chosen for good power supply rejection, ease of balancing, good gain, and ease of application of *feedback*, if desired (hey, i like to keep an open mind...). It also takes care of phase-splitting right up front.
Note: I've returned the constant current diode cathode to -9V instead of ground in order to avoid the non-linearities about the pinch-off region (Vpo ~= 1.5V).

Driver stage

The driver stage is comprised of a both halves of a 6SN7 octal dual medium-mu triode, also in a differential amp configuration, with a 4.7ma constant current cathode load. Because the normal bias gives me about 8.5V on the common cathodes I can return this constant-current diode directly to ground (Vpo ~= 3V).

Output stage

Push-Pull 2A3's using cathode bias. I'm currently using a quad of 750 ohm, 5W resistors bypassed by a 100uf/100V *electrolytic*. Output Transformers are stolen from a Grundig console EL84 amp. Impedance ratio is 6K:4ohm, but I'm using 8ohm speaker for 12K plate to plate load. This is about 3-4X the recommended load per RCA. Power is down considerably from what is could be. (For example, 8.5W with 4k load vs. 5.6W with 12k load.) I decided to go with this transformer for now figuring that the distortion would be minimized. Don't Know if I've passed the point of diminishing returns but there is only one set of secondary leads so I can't experiment too easily.

An other *interesting* feature of this transformer ratio is that if I simply pull out one tube from each channel I am left with a Single-ended amp with a 3k load. This is right out of the book...3k load for Eb=250V. Cool....

Of course there is the matter of 60mA unbalanced DC. We'll just have to see how it works out. So far I've only briefly tried this *Single-ended* mode. Not sure if it's the naturally higher distortion, the OPT's saturating, or if I'm just turning up too much (due to my low efficiency speakers) but the PP mode seems to sound much better. Even at lower levels. I'll save this feature for when I build some Horns. This way I can make a fair comparison.

Power Supply (not shown)

Power supply is basically robbed from an old Fisher console. It uses a Power Transformer with a 120VAC secondary connected to a doubler using solid state rectifiers and *BIG* 100uf/250Vcaps. The main 6.3V filament windings feed 6SL7's and 6SN7's for both channels. So far I haven't found the need to go to DC filaments - I think the diff. amp topology of the gain stages helps to cancel hum. Another 6.3V winding is bridge rectified to give me my -9V diff. amp sink supply, mentioned above.

Parts Selection

I'm currently using Sovtek 6SL7's and 6SN7's and Chinese 2A3's. Two Triad 2.5VCT/10A filament transformers - one for each channel. Chinese ceramic 2A3 sockets and ordinary moulded Octals for the input/driver tubes. No real hi-grade components for now: Metalized film tubular coulping caps, run of the mill carbon comp resistors (hand matched - tho), mostly teflon insulated hook-up wire - all the things that I read that I must avoid. Eventually, maybe I'll upgrade in some areas...


Construction Details

I built the entire amp on a 17" x 13" aluminum plate. This plate drops into a nice burly maple box that my brother, Rich, made for me. I cut most of the tube socket holes using a hole-saw on a low speed power drill. I use WD-40 as a "cutting-oil". The 2A3 sockets were a pain to mount. I had to cut (4) 1/2" holes in a diamond pattern then cut the inside out with a low-speed sabre-saw. I mounted all components between the socket terminals and strategically placed terminal strips (from Radio-Shack).


Performance

Calculated power output should be about 5.6W for 12k load (8ohm spkr). Measured output is 4.5W at 1kHz before clipping (under investigation)
Overall gain is about 29dB.
Input sensitivty is about 220mV.
I'll present better frequency response data as soon as I build a new signal generator. (The wien bridge that I'm presently using is a real pain to work with.) Gain is relatively flat from 36Hz to 20KHz with about a 1dB peak at 200Hz. At the high end measured -3db at about 40Khz. Estimated low freq -3dB to be at about 10Hz.

Tweaks

i got *purist* one day and replaced the icky solid state constant current diodes with simple resistors (kept same bias conditions). i found that the former constant current diodes gave a better attack to percussion tracks (especially noticeable on snare drum "rim-shots", etc). so ... back went the diodes.

i got *scientific* another day and added some overall feedback. first about 10db, then 20, then 26. can't say that the smaller amounts did much for the distortion or bandwidth. larger amounts cut gain significantly (which allowed me to finally use the full range of the input pot) but made distortion come on a bit more gradually. the problem was that the sound closed up. i lost the separation (maybe even the "sound-stage" i hear everyone talking about), especially at lower volumes. so ... out came the feedback.


Listening Impressions

So far this amp sounds better than any other that I've built or owned. The dynamics are outstanding at all listening levels. The *apparent* power is surprizing - even with my relatively low efficiency speakers. This amp sounds like it puts out at least 25W. Even a 1000W-blow-your-windows-out-car-stereo-havin', muscle-head friend of mine swore that this was a 50W amp (ya' kno 12V watts are smaller than 300V watts ...).

One last anecdote: While I was working in my shop (read: laundry room with a work bench) I had the amp playing in the adjoining room (read: garage). I was minding my own business letting the solder fumes take me away when I noticed something. I said to myself "Who the hell is having a party in my garage!!!" As it turned out it was Mick, Keith, and Ron in the background of the Stones' "Hot-Stuff". First of all, I'd never noticed all of that carying-on in the background of this album before I heard it through this amp, but to sound so life-like from another room ...hoo boy, I like it!




Email your comments/suggestions/critiques/questions to Bob. D. at rdaniela@astro.ge.com