ImPasse Preamplifier - Page 53

luvdunhill · 6th February 2012, 02:38 PM

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

luvdunhill · 7th February 2012, 12:08 AM

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.

luvdunhill · 7th February 2012, 12:54 AM

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?

SY · 7th February 2012, 01:40 AM

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.

luvdunhill · 16th February 2012, 12:50 AM

... 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.

BDP · 25th February 2012, 02:53 AM

Quote:

Originally Posted by luvdunhill

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

BDP · 6th March 2012, 04:02 AM

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

luvdunhill · 6th April 2012, 07:16 PM

yes!

In my case it was an overloaded isolation transformer

Once rearranged my equipment it's dead quiet. Sorry for the slow response!

jackinnj · 7th April 2012, 12:44 PM

You might want to try a ground loop breaker -- I use the one a la ESP --

Tony · 16th April 2012, 10:40 PM

there is another thread here linking SY's article: ImPasse Preamp now online

7th February 2012, 01:40 AM	#524
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

16th April 2012, 10:40 PM	#530
Tony diyAudio Member Join Date: May 2003 Location: Palatiw, Pasig City	there is another thread here linking SY's article: ImPasse Preamp now online __________________ http://www.electronicslab.ph/forum/i...?topic=32688.0

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6th February 2012, 02:38 PM	#521
luvdunhill diyAudio Member Join Date: Jul 2006	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

7th February 2012, 12:54 AM	#523
luvdunhill diyAudio Member Join Date: Jul 2006	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?

16th February 2012, 12:50 AM	#525
luvdunhill diyAudio Member Join Date: Jul 2006	... 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.

6th March 2012, 04:02 AM	#527
BDP diyAudio Member Join Date: Apr 2002 Location: Washington State	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

6th April 2012, 07:16 PM	#528
luvdunhill diyAudio Member Join Date: Jul 2006	yes! In my case it was an overloaded isolation transformer Once rearranged my equipment it's dead quiet. Sorry for the slow response!

7th April 2012, 12:44 PM	#529
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	You might want to try a ground loop breaker -- I use the one a la ESP --

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