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Old 18th October 2009, 09:56 PM   #41
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ok thanks for that. Can I ask if a groud loop existed and I were to connect just one input on the amp, would the hum disappear?
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Old 18th October 2009, 11:16 PM   #42
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Gaetan,

Many thanks, you have done Kenji (and other Aspen owners!) a great service.

Kenji,

If you connect just one channel to the preamp, and all is quiet, and hum appears when you connect the second channel, then yes, you have a ground loop through the preamp.

In this situation you need only disconnect the signal ground at the pcb on ONE channel.

If this works, you have identified and solved the problem.

Again, thanks to Gaetan!

Hugh
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Old 19th October 2009, 01:20 AM   #43
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Originally Posted by Professor smith View Post
ok thanks for that. Can I ask if a groud loop existed and I were to connect just one input on the amp, would the hum disappear?
Hello Kenji

Do as Hugh suggested. "If you connect just one channel to the preamp, and all is quiet, and hum appears when you connect the second channel, then yes, you have a ground loop through the preamp."

Now, if it is a ground loop, since a ground link on the 2 RCA input plugs
would be good and made your amp work good and quiet with any preamp connected to the amp, you need to do as I describe on the photo I've done.

On one PCB only, you cut the ground wire of the input wire comming from one RCA input plugs, after that you solder a wire to link the ground tabs of the 2 RCA input plugs.

After those mods, you can check again with the preamp connected to your amp to ear if all is quiet and there is no hum.

I'm french speaking, so I hope my english explanation are ok.

Bye

Gaetan
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Last edited by gaetan8888; 19th October 2009 at 01:30 AM.
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Old 19th October 2009, 01:33 AM   #44
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I am trying a passive pot as a preamp. I have read that the performance varies as a function of the volume setting. Will this be true? and in what sense is the performance affected?
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Old 19th October 2009, 02:13 AM   #45
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Originally Posted by Professor smith View Post
I am trying a passive pot as a preamp. I have read that the performance varies as a function of the volume setting. Will this be true? and in what sense is the performance affected?
Hello Kenji

Depending of the ground schema of your passive preamp you can still have a ground loop and noise, so it still better to do the modification that I've suggested you in my previous post.

And for the volume here is a text from the Aspen web site for a volume control for the Aksa amp;

"Generally, it is far too expensive to manufacture a compact logarithmic, wirewound pot. Furthermore, the wire used on a typical 50K or 100K pot is very fine, with many turns, and this creates high inductance. Sadly this makes such pots unsuited for audio, since at 20K the response is at least 2.5dB down and this rolloff is particularly noticeable, particularly on percussion.

You can law fake a linear 100 kOhm pot with a simple 15 kOhm resistor from middle terminal (wiper, or centre; output) to ground. The 47K input impedance of the AKSA combines with this resistor to give around 11K4 from wiper to ground. Signal input is taken to the top terminal, bottom terminal is earthed, and the middle (wiper) terminal is then taken to the AKSA input. This gives you an excellent and progressive volume control, with a minimum system input impedance (with the 47kOhm Zinof the AKSA) of 10K2 kOhm (at full volume), a maximum of 100 kOhm (at lowest volume), and a full range with additional sensitivity in the earlier stages for easy adjustment during quiet listening."


Bye

Gaetan

Last edited by gaetan8888; 19th October 2009 at 02:15 AM.
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Old 19th October 2009, 02:46 AM   #46
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Suppose we have a log pot. A- B -C. The input is A - C. Now why am I having such trouble understanding why it is that depending on whether we take the output between B-C or A-B we get a log curve or exponential curve? I am assuming this is correct.

thanks,
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Old 19th October 2009, 02:58 AM   #47
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By the way what is the significance of a 'log' volume pot? I have read that the ear responds in a logarithmic fashion to sound however for practical purposes any function, be it linear, log, or whatever gives us the same discrete volume settings, so why do people insist on and go to so much trouble in fabricating a logarithmic pot?
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Old 19th October 2009, 03:37 AM   #48
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People have experienced hum due to missing grounding of a pot
They all reported hum was gone once grounding of pot was established
Just a thought
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Old 19th October 2009, 05:04 AM   #49
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If you have a logarithmic pot, and the A-B connection is log, then the other connection HAS to be the inverse of this, which is exponential. This is asymmetric. I know this doesn't completely answer your question.

log pots are not good only for audio. Sometimes you need a logarithmic control knob for a special type of circuit. Then we came out with "audio taper" which was convenient for audio amps. And then, finally, we came out with "fake log", which wasn't publicly marketed but made things easier for the factories.

The pot's function is a voltage divider. If you disconnect the grounded side, its whole curve changes depending on the impedance of the amp input.

- keantoken
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Old 19th October 2009, 05:36 PM   #50
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If you have a logarithmic pot, and the A-B connection is log, then the other connection HAS to be the inverse of this, which is exponential
can you just explain what you mean more clearly? I am confused by what we are plotting, the ouput voltage as a function of distance or resistance or what?

If we take a discrete example say with 3 inputs xohms= x_1, x_1+x_2 and x_1+x_2+x_3. The output is F(x).
When we swap the wires we automatically swap the inputs which become x_3, x_2+x_3 and x_1+x_2+x_3.
I cant see how we can calculate G(x) such that G(F(x)) gives us back the original x's.

This is the diagram I refer to:

A---B---C---D
x_1 x_2 x_3

Voltage V is applied between A and D and the x's are the resistors.

Thanks.
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