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Old 6th February 2012, 02:38 PM   #521
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If I remember, 10kHz square wave wasn't pretty.

I believe I have the FFT software installed, at least I can hit Math and select FFT for one of the channels and it shows up as a red line on the graph.

So, what exact test setup would you suggest? Something similar to page 17 here:

http://www.cs.washington.edu/lab/fac...s/71034901.pdf
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Old 7th February 2012, 12:08 AM   #522
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I think I figured it out. I'm using 12.5k + 10nF as the zobel here. Using just 12.5k with no capacitor makes little difference in this graph.
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Old 7th February 2012, 12:54 AM   #523
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Also, one more question for SY when he gets back...

Why not just use a 10K attenuator and the 10K:10K transformer and not add anything in-between?
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Old 7th February 2012, 01:40 AM   #524
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You can, and that will get you 95% of the best possible performance. But if you want it closer to perfect, you also have to consider the Miller capacitance of the tube and strays. I think you want to tune with a 50k pot and a 365pF variable cap. Or do what I did and tack solder a bunch of different parts in to zero in on the optimum values.
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Old 16th February 2012, 12:50 AM   #525
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... to be clear, you mean 50K || (12.5K + 365pF). I think for now I'll just stick the pot in and the 12.5K resistor and go from there and see how it sounds.
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Old 25th February 2012, 02:53 AM   #526
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Quote:
Originally Posted by luvdunhill View Post
Interesting. I am still trying to isolate the problem, it's definitely the preamp and is clearly 60 Hz sounding... But it is tough to reliably reproduce the issue.
Did you figure out the 60 Hz noise problem?

BDP
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Old 6th March 2012, 04:02 AM   #527
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There was a slight 60 Hz hum when running amps with gain like the Aleph and F3 with my 95 dB speakers. I thought about using a regulator but the HT transformer voltage is just not high enough which means another transformer. Just not enough space in the case, so I ran the heater wiring through a bridge rectifier and CRC. With a 1 Ohm resistor the heater voltage came out to 6.25 volts. The 60Hz hum is now inaudible.

From an earlier post about noisy tubes, I tried a TUNG SOl 6SN7 and its working great. None of the EH6922 tubes have caused me any problems.

BDP
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Old 6th April 2012, 07:16 PM   #528
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yes!

In my case it was an overloaded isolation transformer Once rearranged my equipment it's dead quiet. Sorry for the slow response!
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Old 7th April 2012, 12:44 PM   #529
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You might want to try a ground loop breaker -- I use the one a la ESP --
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Old 16th April 2012, 10:40 PM   #530
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there is another thread here linking SY's article: ImPasse Preamp now online
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