After moving house, thanks to the lockdown, i had time to clear off some old projects. And since I have these beautiful rubber wood chassis, I just have to utilize them. Hence what you see below…
On the left is the passive preamp, sitting on top of Mr Ew’s excellent Rapsberry Pi – AD1865 DAC. On the right is Anna! An All Russian Push Pull amplifier.
The passive preamp is a restacked Intact Audio autoformer. By itself, Dave Slagle’s autoformer is already excellent, but restacked for more inductance to fit your system, that’s what a proper DIYer should do!
Note below, wired with Elma switches. I bought these years ago, back then could only afford the 12 steps… These switches are fantastic! The touch, the feel… it’s like Leica camera, you’ll understand if you are of particular vintage… 😉
Here’s internals of Anna. It’s a heavy 2 chassis affair, done mainly because I want to hide the ugly power transformer and chokes inside. And also I don’t want anyone to accuse me of building a “lightweight” amp. Count the number of irons please…
Here’s a close-up of signal wiring. The iron arrangement… went through so many iterations before arriving at this arrangement. Mainly to minimize induced hum… Once it hums, it’s no fun… Resolving hum is just as fun as a root canal…
What is Anna? At first glance, looks like the Simple EL84 concept but instead of using differential long tailed pair as phase splitter, using iron here.
Driver stage is a plate choke loaded 6E5P. My previous experience with 6E5P wasn’t that good. Must have done something right this time… Circuitry wise, nothing special here, other than the 5 Schottkies at the cathode.
Next, it drives a center tapped choke. Yes, it’s an audio choke with its mid point tapped out. As this center tap is grounded, driving one end of the choke will induce an inverted signal at the other end. In other words… a phase splitter! It works very well here and I have no idea why we rarely see such design. So simple and elegant!
After that is our favorite SV83 in triode mode in Push pull fashion. All for a grand 4000 miliwats… Again, nothing special here…
So the magic is in the phase splitter. Push pull can be done right if you get the phase splitter right. None of that “push pull” veil here.
I would add that if you look at the music signal, every stage it sees lots of inductance.
From DAC output, it goes to an autoformer (an inductor with very high inductance ~200H)… Then to the 6E5P (high inductance on its plate 100H)… then driving another high inductance phase splitter choke (easily >100H)… then SV83… Output transformer (PP output transformers easily have >100H)…
Hmm… every stage it sees >100 Henries… Any idea what does it do?
Sorry, I’m not smart enough to tel you what it does. So I’ll just tell you what I hear here.
This is the most dynamic amplifier I have ever built. It’s only 4W but the dynamics, transients is striking. This raises a new bar to the word – realism.
Enuff said.
If you were to remove one of the output tube from this amp, would it then become a Single Ended amp ?
I wonder how it would sound then….
hi dean
thanks for your comment.
the push pull output transformers wouldn’t like the dc current, so inductance will be reduced.
i doubt it’ll sound good.
yeo
I’m still thinking of your desigh ? So could the center tap choke be something like a 115+115 toroidal power transformer ?
I’ve been reading that power supply toroidals can be used as output transformers to good success but the 115+115 is best suited to tubes that like a 2k to 3k primary impedance. And you multiple the expected amps power output by 3 or 4 to get the Volt Amp rating of the toroidal to use. Shoog on diyaudio was using them.
hi dean
hmm…. use the center tap of a power transformer? well, it depends on the bandwidth of the power transformer right? power transformer’s main job is to pass through 50/60 hz of sine wave. if this particular power transformer has the adequate bandwidth to cover the audio band (20-20khz), then yes, it’s doable.
second question will be how balanced is both sides of the secondary. i think this is probably less important than bandwidth issue above.
yeo