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I had the volume pot on a tubed preamp replaced with a vishay resistor by a tube amp modder.
It now sounds strange. Compressed, harsh. Not great.
I received the above quote in here, which if true, would be very interesting and grounds to get my money back.
Thanks for any input,
Hukk
Follow Ups:
but it won't fulfill the same function as the volume pot. You won't be able to change the volume.
Now if the function is important, if you want to be able to change volume levels, then you obviously shouldn't replace the pot with a single resistor. If changing volume level isn't important they you could replace the pot with a resistor of the appropriate value but then you've got a fixed amount of gain and, since not all discs are mastered at the same level, your listening volume is going to vary from disc to disc and you won't be able to compensate for that. It's that fact about discs being mastered at different levels which explains why you have to increase or decrease the volume for some discs in order to listen to different discs at the same actual listening volume.
So my answer is that it depends on what kind of result you want to achieve. If you're interested in having the pre-amp control your listening volume, you can't replace the pot with a single resistor because the pre-amp will then no longer be able to do what you want. If you don't want to control volume with the pre-amp, then you can replace the pot with a single resistor but you're either going to have to control the listening volume some other way, say the volume control in your CD player if it has one, or be prepared to have your listening volume vary quite a bit depending on the level each disc you play was mastered at.
Now, as to things sounding compressed and harsh, there is a possibility that the fixed level of gain the pre-amp has is equivalent to a high volume setting on the volume knob you replaced and as a result you're driving your power amp into distortion and possibly clipping.
David Aiken
As Al correctly explained below it depends on how the pot was being used in this particular case. Some circuits only use two terminals of the pot and others use three. Whether the pot bypass would require one or two resistors to pull off and retain the type of performance intended by the manufacturer depends greatly on which it is.
Can you post a schematic of the original circuit and what the modifications are? Without that we are all flying blind.
It is not a matter of a poll, there is a correct answer. A pot is simply a variable voltage divider. It can be represented by a series resistor and a shunt resistor to ground, the value of each varying according to the attenuation wanted (the position of the pot). If you don't want attenuation the pot can be replaced by a single shunt resistor of appropriate value.
Rob.
.... yet your answer (and my modder's) conflicts with the other 2 answers that have been proposed in this forum?
Or am I missing something. Seriously... I could be, because I am not good with the technical details here.
Hukk
Just perhaps you are not the only one not good with the technical details. The ability to solder does not automatically equate to understanding circut operation.
Edits: 01/30/10 01/30/10
I don't think it's completely clear what you had done. In your original post you said it was bypassed, and here you say replaced. Was the intent to remove the VC and have it set to a specific volume using a fixed resistor? If so, Al is correct, you would need a voltage divider with 2 resistors. However, it is also likely that the amp stage following the VC had a input resistor too, so could be used as the shunt leg, albeit with a resistor value that would likely be higher than optimum, and may lead to frequency response changes due to interaction with the input impedances of the following stage. Or most likely, the pot could've been left in and rewired as the shunt leg. That would've been a poor choice though, since the pot would still be part of the circuit, so little would be gained by using the fixed resistor, though it shouldn't degrade the sound too much if done correctly. It would still have a higher series resistance than you had before for the same volume, so there may be some high frequency rolloff depending on the input capacitance of the next stage.
Edits: 01/30/10
No.
Was the intent to
remove the VC and have it set to a specific volume using a fixed resistor?
Yes.
Your post may be exactly what I need in order to confirm with the modder that he did correct work. I'm sorry, I don't have enough expertise to differentiate between replacing the vol pot and bypassing it. All I know is that it's now a fixed volume. It was intended to be fixed at about half the maximum gain.
Hukk
If the attenuation is not variable and the attenuation has been set via the modification to "half gain" then your circuit has a fixed voltage divider with approximately equal value series and shnt resistors. If the shunt leg of the pot was parallel to a fixed shunt resistor then the pot could be lifted out and a single fixed series resistor soldered in. This would be scaled to give you the desired attenuation BUT will change the impedence of the voltage divider compared to the pot, (which may or may not be important depending onm the circuit).
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