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Old 9th September 2013, 12:43 PM   #561
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If i take a look at the circuit R27 might adjust the voltage or Bias at Q5 and Q6.
R24 woult adjust the current gain only.
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Old 9th September 2013, 12:53 PM   #562
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Okay, i just clamped 220K over the 68K R27 and the voltage drop at R17 is at 440 mV.
I didn`t check if the offset till now.

So thanks for your help and of cause 6L6 for the right hint.

Last edited by oliverniekamp; 9th September 2013 at 12:58 PM.
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Old 9th September 2013, 01:40 PM   #563
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I honestly don't know what is going on with the circuit as a whole, I do know (from adjusting it on my amp) that a change in R27 will change the bias of the output devices, which you can measure by voltage drop across the output source resistors.

Hopefully somebody smarter than I can explain the why that works...
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Old 9th September 2013, 02:22 PM   #564
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Altering R27 is changing Bias, but so little.
No i have R27 at 33K instead of 68K and the voltage drop changed from 550mV to 525mV
This is irritating, so i will end at 10K or what?

so 68K, 47K, 33K next will be 22K, but does that change the current gain and the sound?
I hope i don`t destroy it.

Last edited by oliverniekamp; 9th September 2013 at 02:30 PM.
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Old 9th September 2013, 02:29 PM   #565
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Remember you should adjust offset to zero after each change in R27 - that should help.

You had almost the proper bias with 220K parallel with the 68K... do that again, re-adjust offset to zero and measure bias.
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Old 9th September 2013, 02:43 PM   #566
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of cause i do that, anyway offset isn´t changing very much, just like the bias.
precisly the offset voltage is changing much less than the bias voltage drop.
At 22K i`m still above 500mV voltage drop, it´s warming up now.

Last edited by oliverniekamp; 9th September 2013 at 02:46 PM.
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Old 9th September 2013, 04:56 PM   #567
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As the total of R27 decreases bias will go lower, and this will not affect the current gain. For larger changes in bias swap out the Source resistors. Going 1 ohm would cut the current in half compared to 0R47.

The static current set requires that the voltage across R16//17 + R15 equal the Vbe of Q4, ~0.6V. Lower values of R27 increase the current through R15 and reduce the component of R16//17.

Increasing R18 and R19 will allow you to increase the value of R7, which will increase the open loop gain. As is, you are operating with less feedback than the original, which may or may not be a good thing.

Offset adjustment seems ineffective because you are in the range of 250 ohms to get the output stage the Vge it needs.
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Old 9th September 2013, 05:21 PM   #568
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"As the total of R27 decreases bias will go lower, and this will not affect the current gain."

Thats what i needed to know, so i will put in 10K for R27, that should be low enough to adjust the voltage drop to 450 mV.
With 22K and 490mV it´s still to hot for my taste.

Thank you
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Old 9th September 2013, 05:56 PM   #569
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Oliver, You're looking at roughly 2 amps total bias. That's about where we would expect the amp to be. If too hot, see if you can increase airflow to the amp or increase the size of the sinks.

One thing you are doing that affects the sound somewhat is setting R7 to ~250 ohms. Open loop gain of a differential pair is determined by the load impedance. (That's why you see high impedance current mirrors in so many designs) With vertical mosfets R7 would be in the range of 800R. The net result is less feedback. Distortion numbers will be slightly higher, but whether it impacts the sound is your call.

If you increase R18 and R19 to 1R you will be able to increase to ~440R and restore most of the open loop gain. It might be a worthwhile experiment to see if you can hear the difference once you get your bias settled.
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Old 9th September 2013, 08:25 PM   #570
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Yes i was considering some really huge heatsinks, or some pc fan for airflow.
But the sinks i have got are way too big.

First i wanted to adjust the bias and this appears to be a bigger problem than expected.

Now i have R27 at 10K and the bias is at 406 mV, but i can´t adjust the bias with R8 anymore, it just doesn´t change.
When the R8 resistance gets to low the amp gets unstable and the offset starts moving up and down, or i have like 14 V offset.

With R8 at 2K i can adjust offset with R7 and everything is stable.
So it appears that i could leave R8 fixed at 2K and adjust the bias with R27.

I build Aleph 3, Aleph 30 and Aleph X before, but this never happened.
Of cause i never used lateral Fets before, only in F5 without any problem.
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