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Old 23rd November 2010, 02:27 PM   #21
generg is offline generg  Germany
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oh I forgot,

I have mure hum. Is it possible that the two 2SK170 have less hum rejection than the Zener-IRF9610 combination to feed the sources of the J-FETs?
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Old 23rd November 2010, 02:35 PM   #22
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The voltage across R11 and R14 will tell you almost exactly what value R14 needs to be for equal current sharing through the differential pair. If you are passing 18ma through the front end then in order to be equal R14 needs to be the voltage drop across R14 divided by 9ma. If the voltage drop is 1.2 volts then R14 should be 133.3 ohms, simple math. Use a 500 ohm trimmer to fine tune it.
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Old 23rd November 2010, 02:49 PM   #23
generg is offline generg  Germany
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Bill did you mean the voltage drop over R114 or R111?

divided by 9mA?

Because there is no voltage over R114, I suppose C10 makes it impossible.

or should I take AC 50 Hz?
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Last edited by generg; 23rd November 2010 at 02:59 PM.
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Old 23rd November 2010, 03:02 PM   #24
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I'm lost in nomenclature - mixed schematics ;

boyz - first agree which one you're using for nomenclature



Generg - hum doesn't have anything with equal current through input LTP - try to find culprit somewhere else

put gate stopper resistor in LTP CCS - say that 1K is good enough
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Old 23rd November 2010, 03:12 PM   #25
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I am speaking of R11 as the resistor above the front end CS being 1Kohms. As you have measured, you have a total current of 18ma determined by the voltage drop across R11.

R14 is in one leg of the front end and should pass exactly half of the total current, which will be 9ma.

Because of the feedback supplied from the output section, Q2 will attempt to eliminate any output offset by passing more or less current through its leg.

Within reason, the amp will self adjust to keep the output offset low with no regard for maintaining the current balance between the halves of the front end, but it will tell you what voltage the R100s need on their gates to maintain the minimal output offset.

So now you know the voltage needed across R14, and you also know the proper current required to maintain current balance in the front end. So in the case of using your measurements, simply divide the voltage across R14 by the required current of 9ma and you have your answer.
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Old 23rd November 2010, 03:16 PM   #26
generg is offline generg  Germany
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Oh I mixed up in my joy the 14 and 114!

thanks!
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Old 23rd November 2010, 03:21 PM   #27
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I think R111 and R114 were typos, I'm going by his posted schematic.

Posted at same time.

Last edited by Bill Fuss; 23rd November 2010 at 03:24 PM.
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Old 23rd November 2010, 03:26 PM   #28
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I think - it's best to leave him alone for 24h , to enjoy in new amp

then - he'll sort 50% of things needed to sort , and we'll have just another 50%

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Old 23rd November 2010, 03:29 PM   #29
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Generg seems to be very proficient with a soldering iron. I think he will be back sooner.
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Old 23rd November 2010, 03:36 PM   #30
generg is offline generg  Germany
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O.K. Bill I measured 1,9 V over R14, so 211 Ohm could be right....


I thank you both for the time shared.....

the irritation was my fault I switched between the schematics of Aleph 3 and 5, and they have different numbers.
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