2. amps-preamps
  3. 1291501029

Help needed to diagnose preamp channel unbalance.

Hello all. The preamp is a new dual mono linestage with both balanced and single-ended connections. The volume control is an Alps motorized pot. There is no balance control, just a single stereo volume control.

The problem is that at low volumes, there is a channel mismatch resulting in the right channel being noticeably louder than the left. The problem was confirmed to be with the preamp through cable/channel swapping and listening with matched channel test CD's. Correct phase was also confirmed. When the volume is increased to moderate levels, the left channel seems to come fully on-line so that proper balance is restored. This is obviously an unacceptable situation for low level listening.

Has anyone had a similar problem? Is this likely a tracking problem with the volume pot? Is it possible this will improve as the electrical components and volume pot break in, since the preamp is new with only about 15 hours on it? - Thanks
mitch2

Showing 2 responses by johnsonwu

johnsonwu
410 posts
 
If the volume control is discrete resistor based, then its one of the following possible scenarios:

Cathode and plate resistor mismatch between the channels.
(no, tube mismatch wont affect gain unless the cathode is grounded and you are using a biased grid)

negative feedback resistor mismatch or broken

B+ completely off (I mean it has to be off by some 20%) on one channel vs the other.

But if its just a cheapie "alps" or more likely Chinese counterfeit alps, then its the volume pot.

johnsonwu
410 posts
 
Alps volume is like "Vishay resistors"
They have different grades, starting at a few bucks each.
Chinese amps have "alps motorized volume pots",
probably made by the same people who make fake gucci handbags.

6550Cs problem is likely the preamp has a grounded cathode and grid-biased using a constant voltage reference. In which case your gain is at the mercy of the gm of your particular tube, so matching is essential.

Counterpoint SA-3.1 and SA-5 are examples of preamps with grid biased with negative power.
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