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In Reply to: RE: The Bottom Line posted by Triode_Kingdom on October 19, 2014 at 08:58:40
So as not to simply abandon this discussion to the theoretical, here's what I suggest to remedy the shortcomings of the driver circuit of the Baby Ongaku. Everything is derived from SPICE simulations, so tweaking will be necessary to optimize the circuit with live components. Also, these changes assume that one desires to keep the topology in its current form (pseudo-SRPP), and that a low distortion amplifier is the goal. If the warm sound of abundant even-order harmonics is preferred, don't do this.
According to simulations, the circuit clips at a somewhat low 2.8V RMS input. In addition, while I don't rely on SPICE for distortion analysis, deformation of the output sine wave is visible well under clipping, beginning somewhere around 2V. These results imply that the onset of audible distortion probably occurs at 1V RMS or less.
The solution is to A) remove the 220uF bypass cap at the lower triode, and B) increase both cathode resistors to approximately 2K ohm. When this is done, distortion should remain inaudible until just below the threshold of clipping, and clipping will occur at approximately 5V RMS. The tradeoff to these changes is a loss of 3dB in gain.
There are alternatives to this, but nothing easier, more reversible, or less expensive to try.
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Buy Chinese. Bury freedom.
Follow Ups:
Here are my suggestions. Note, this is only a half way point, simple modifications that would provide VERY tangible benefits despite the compromise of type of tube used and lower than ideal HT voltage. Two large improvements are a result of swapping out the bypassed cathode R in the driver for Schottky diodes and DC coupling.
A little power is given up by increasing the R value on the cathode of the OP tube but the ability to transition well into A2 will more than make it up.
The driver is far more linear with a choke instead of an R and I find that a choke in this position is not detrimental to sound in the way a typical plate choke is.
I still prefer the sound with drive taken from the bottom tube's plate but you lose any worthwhile A2 transition (call it headroom).
Naz
Edits: 10/19/14
Did you analyze this circuit before you posted it? It's not Class A, and it clips much earlier than the totem pole we were discussing. Currents within each of the sections are also extremely non-linear. Let's let this go now. I've said what needed to be said about SRPP. If you want to discuss something else, it might be better to start a new thread. Just so you understand that I did take the time to analyze what you posted, the result is below. It's shown with a drive level of 1V RMS. Again, this isn't a circuit I'm interested in discussing at this time.
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Buy Chinese. Bury freedom.
You've made some erroneous statements so before you take your bat and ball they need to be corrected.
1) Yes I've analyzed the circuit and yours too.
2) I have no idea where you got your currents from but if it's not class A what is it?
3) Mine handles less input voltage swing because it has more than twice the gain of yours for equivalent output swing due to a far better load line. Hence it's more linear, not less.
4) Say what you like about the currents it's the result at the speaker terminals that counts and overall the amp is much more linear than the mods you have put forward. At comparable output (0.5V in on my circuit), 2ndH -60db for yours compared to -90db for mine. 3rdH, -80db for yours Vs -90db for mine. Higher order H, -85db for yours, Vs negligible for mine. I can post the ffts if you like but you don't trust them.
5) Finally, my circuit is not SRPP. As I said I am putting forward simple changes to the original circuit that I would make (in line with the original poster's request before the thread was hijacked). With the proposed changes this circuit would be better served with higher HT as I've already stated but even as is I guarantee it will sound better than original or with the mods you proposed. Anyone out there willing to try compare and report back, I'd be most grateful.
Now I'm done too!
Naz
It's not just the (simulated) output that matters; it's how you get there. If that wasn't true, all our amps would have 30dB of NFB.
"I can post the ffts if you like but you don't trust them."
And for good reason. None of these topologies will produce harmonic energy at -90 dBc (~.003%) in real life.
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Buy Chinese. Bury freedom.
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