Push-Pull Class A 2A3 Stereo Amp
Last Update - 3/12/97
Here is the schematic for one channel of my amp:
By popular demand ... click here to download a postscript version of the schematic. (90k)
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).
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).
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.
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...
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).
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.
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.
I bought a new power transformer from Antique Electronic Supply
(Hammond 272-JX, 600VCT@250mA) in the hopes of upgrading to a slightly
higher B+ and a tube rectifier! The only problem is that it won't
all fit into the existing chassis :( .
I mounted the xfmr and a 5U4G rectifier in a temporary chassis along with
some filtering elements (RC). I then bypassed the existing B+ (310VDC) with
the new supply (+350VDC). Keeping the 750 ohm cathode bias brought me up to
58V on the cathodes. For these operating conditions I estimated about 8.4W
into 12k load. I didn't have a chance to measure, but there was a definite
sense of added power over the 5W I was getting before. The bass was real
strong (perhaps too strong, or not as sharp as before). Overload also seemed
to come on more gradually. It tolerated hard rock (AC/DC) a lot better - and
at a more authoritative level. For lower level listening I noticed no real
improvement. In fact, I felt the sound became somewhat clouded (to coin a
new subjective term). Then again, i probably just blew my ears out standing
in front of the speakers blasting "Fishbone's" version of "Fred Is Dead".
Keep in mind that I only had one evening's audition of this kludged set-up.
But for now I've decided to leave the 2A3 amp's power supply as-is. The
higher power woulda been nice. I'm sure that if i played around with the
bias i might have found a better sound. But the real killer was the physical
rebuild involved. I'll probably use the tranny for a self contained separate
power supply. Then i could use it for this amp as well as some of my other on-going
Well, I used that 600VCT Power transformer in my Single Ended SV811-10 amp. I had been using that amp
a lot of the time while i worked on other things. I did experiment quite a
bit on this pp2a3 amp, though.
I tried transformer coupling/phase splitting for a while. This sounded
real nice. A lot of low level detail. The main problem was the lack of
bass. Of course, I was using the small, cheap 1:3CT ,10K:90K, suckers
from AES. If I had the $ I think I would be quite happy with this arrangement
with GOOD quality transformers...
I also experimented with the concept of a single diff amp stage using
pentodes. I figured that I would get enough gain to replace the two stage
arrangement shown in the schematic above. FURTHERMORE, I wanted to utilize
a single DIRECTLY HEATED twin pentode tube, the 1E7G. Dig it, Directly
Heated clear thru... Unfortunately, in order to get the gain I needed to
increase the plate resistors, which meant reducing the plate current. I
didn't want to reduce the plate current below about 2mA (much more would be better) because i wanted to be sure to have the current available to drive
the grids (input capacitance) of the 2A3's. couldn't get enough gain...
could have added a stage, but that would have missed the point.
thought about using the cheap 10K:90K interstage as an input transformer.
this way i'd get the full differential gain plus the step-up.
...wonder why i never tried it?! ...hmmm...
The latest thing i tried was replacing the 6SN7 _and_ the 6SL7 with
6f8-g's. these are essentially 6sn7's in a sexy ST bottle with one grid
cap. I think this sounds better than the original. There seems to be just
the right amount of gain, now.
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 (Design Acoustic
PS-10's - 90dB/1w/1m). 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 firstname.lastname@example.org
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