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Old 26th October 2012, 05:04 AM   #1
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Default SSE as Single Stage Power Amp

So I built my SSE as a power amp with EL34s triode strapped and a 12BH7 driver to reduce gain for an active preamp. Turns out, even with the much lower gain of the 12BH7 and with the tube's cathode bypass cap eliminated, it has a sensitivity of just over 1V to hit speaker breakup and less than 2V to reach the start of clipping on a 1kHz sine wave.

I just built an Aikido 6SN7 preamp with 20 dB gain, which means it can output 20V from a 2V source, and it can do so with a completely flat freq response and .05% thd. This is a killer voltage amp.

So I just did an experiment and removed the SSE's driver section by pulling one leg of R14/24 and C11/21. Then I simply moved the input signal wires coming from the RCA jacks to the EL34's grid stoppers.

The result is pretty awesome. The Aikido can drive the crap out of the EL34s, in fact it sounds like it has an iron grip on them, dynamics are improved, as well as clarity and detail.

Gain is perfect, but there is none to spare. I don't have the measuring equip on it now, but the Aikido is driving the EL34s within a couple dB of clipping and right on the ragged edge of my speaker's power handling abilities. You need a 20 dB+ preamp to make this work out. An Aikido w/6SL7s would give more than enough gain.

Basically this amounts to replacing the SSE's input stage with an active preamp... but since each amplification stage now has it's own power supply, you're now getting the benefits that a pre/power amp are supposed to provide... voltage gain in one box and power stage in a separate box, each with their own power supplies.
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Old 3rd November 2012, 09:36 PM   #2
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I was going to try something similar, However are you sure 20v is enough? is that 20vPk-Pk or 20Vrms?
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Old 3rd November 2012, 10:04 PM   #3
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That's 20V RMS, and it is an estimate based on measuring the preamp and amp BEFORE I made this change. With the 12BH7 driver the total input sensitivity was .2V RMS to reach a slightly clipped 6W output, the Aikido has 20 dB gain, which is 10x, so the amp by itsself has a 2V sensitivity, or just under...

I am guessing the 12BH7 with CCS load and no cathode bypass cap has a stage gain of around 20 dB, it's mu is about 16... I am not sure how to figure gain with the CCS load, but I think it'll be a bit more than mu/2.

In any case, my DAC (Schiit Bifrost) has a 2V RMS output, which is capable of driving my speakers into breakup right at full volume, so it is working fine for my setup.

If you need more gain you could always use an input tube with higher mu than the 6SN7, or add a cathode bypass cap to the bottom input triode on the Aikido if it's close but you just need a little more gain. 6922s with a mu of ~33 might be a good option or 6SL7 in Octal, but a mu of 70 might be a bit too much...

I really need to get some test equipment.
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Old 20th November 2012, 12:27 AM   #4
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I really want to do this, I'd take it to the next level and make a full scale preamp out of it. I just finished an Aikido Headphone amplifier. Thanks for the advice.
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Old 6th December 2012, 08:36 PM   #5
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With a single-stage tube amp driven directly from a pre-amp, you can parallel the EL34s for more output. Try 4 of them with a lower impedance transformer.
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