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Looks like I have a bad volume pot. Channel balance changes with volume level. Just after the 1/2 point, balance moves far right.
When I change the pot, I will bypass the loudness switch. (Cut all connections to the loudness switch, connect 4.7k resistors between taps 1-4 and 2-8 correct ?)
I anticipate using the RS 100k Alps pot.
What will happen to gain if I make these 2 changes ? Will I have to turn the volume control up or down vs stock to get the same level ?
Best,
Ross
Follow Ups:
That replacement will do fine, but you may want to try disconnecting everything from the loudness tap, first. If those parts go off value, they can cause a large balance shift just as the volume control wiper passes the spot where the loudness tap is.
I assume you have already tried cleaning the volume pot...
If you do replace the pot, all you have to do is leave the loudness circuit parts disconnected from the pot entirely. You do not need to do anything else. They will be out of circuit.
GP49
I have not attempted to clean the pot...will do so.
RE the loudness circuit, do I cut all connections between the volume pot and the loudness switch or do I have to use the 4.7k resisters to connect between 1-4 and 2-8 ? I have seen reference to both ways.
Thanks,
Ross
If you disconnect the loudness contour circuit from the volume control at the loudness tap, you remove it from the circuit entirely. Those who say otherwise don't know how to read a schematic and should not be regarded as experts.Sorry to seem so blunt, but at times in this hobby there are those who don't know anything, but pretend to.
GP49
Edits: 02/12/12
This statement is absolutely wrong.When the loudness switch is in the "OFF" position there is a 4.7K ohm resistor tied from the loudness tap to ground. When the loudness is "ON" there is a 0.1uF cap in series with the 4.7K resistor. Therefore if you are going to bypass the loudness switch it is necessary to tie a 4.7K ohm resistor between the loudness tap and the ground lug for each channel.
The stock Dynaco volume pot typically suffers from poor channel to channel tracking and replacing with the RS 100K pot is a good solution. Note that they do track reasonably well however we hand select ours to deliver 0.5db channel to channel tracking.
Edits: 02/12/12
But, Joe!
When the loudness switch in its OFF position bypasses the 0.1µF capacitor, there is NO frequency contouring in effect. The 4.7KΩ resistor by itself doesn't cause any. None. The only thing the resistor does is to alter the effective resistance of the lower portion of the volume control by paralleling it with 4.7KΩ. Its effect on frequency response is none...the same as if the resistor were not there at all. In both cases the loudness compensation is out of circuit.
A selected RS dual 100KΩ is indeed a good replacement (if its shaft type matches that of the knob on the particular Dynaco...and they DO differ) and I have installed many of them, without connecting anything to the loudness tap.
GP49
The 4.7K resistor changes the attenuation taper (and I never claimed it has any frequency contribution) - and it should be included. I get a lot or e-mails (and see postings on the forum) from folks that find that the replacement volume control changes volume too quickly on the low side. If the resistor is included - the taper follows the stock pot. So - bottom line - 1) the 4.7K resistor is in the original circuit when Loudness id OFF and 2) it should be retained if the user wants to maintain the original taper.
I suppose I took issue with your strong comment ("If you disconnect the loudness contour circuit from the volume control at the loudness tap, you remove it from the circuit entirely. Those who say otherwise don't know how to read a schematic and should not be regarded as experts") - because it is incorrect.
What is accomplished by only removing the 0.1 cap and leaving the 4.7K resistor in place? That is no different than simply leaving the loudness switch in the off position.Yes, it does maintain the original contour of the volume control, but I can’t understand how it can be considered removing the loudness circuit, or bypassing the switch, as the OP asked. If David Hafler and Ed Laurent had originally designed the circuit without a loudness switch, there wouldn’t be a fourth tap on the pot to begin with, would there?
I also don’t know why someone would want to keep the original contour of the pot when there are so many complaints on the web about the jump in volume after the 12:00 position (where the loudness tap is located). There may be an equal number of complaints about volume changing too quickly on the low side, but I haven’t seen that nearly as often, and haven’t experienced it myself. Maybe that’s from all the folks with 104 db efficient speakers! Otherwise, it’s usually a matter of having the volume control at 9:00-10:00 versus 1:00-2:00 for normal listening volumes; not an issue as far as I can see.
Mike
Edits: 02/26/12
I see your point, Joe. I was considering only the frequency-shaping function of the loudness circuit.
My preference is not to have another resistor in the circuit where it might affect the performance of the preamp.
GP49
After reading the responses concerning the volume pot and the loudness switch, I am thinking a balance problem I am having with my PAS-2 may be faulty volume pot. I have to adjust the balance control almost to the 9 o'clock position on the left channel for a balanced output. It seems to start out ok but progressively gets worse. I have switched tubes and the problem still persists. Would the problem be a faulty volume pot?
It could, and probably is; but parts in the loudness contour circuit that are out-of-tolerance between the two channels could do that, too.
GP49
Edits: 05/11/12
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