advertisement


Preamp channel imbalance

kasperhauser

kasperhauser

pfm Member
Perhaps not exactly "classic", but as it involves a classic-era preamp, I'll ask here.

I posted awhile back about my new/old preamp, made by the Quintessence group in the early 70s. It has developed a channel imbalance; left is down ~10 dB in very non-scientific testing. Happens on all inputs, and it's always there, not intermittent.

My question: is there a "usual" culprit when this happens? Should I look at the volume and/or balance pot, or is it more likely to be components in the circuit? I should note that the volume pot that's in there appears to be a replacement. It isn't as old or nearly as robust looking as the balance. Also, there's never been any hint of scratchiness that might indicate crud in any of the controls.

If it's likely to be pot-related, I can try cleaning or even replacing, but I'll seek professional help for anything beyond that, which will be difficult as there are no repair shops left in my town, so a 100-mile drive would be required.

Any advice is welcome.
 
I should try spraying inside the pots (volume and balance) first, assuming they're not a sealed design, with Deoxit, or perhaps better still, use isopropyl alcohol. That's often the usual suspect. If that doesn't work the next step might be to check the channel bias but you need to know where the test points are. The next step after that gets a little more involved and that would be changing out the capacitors as they're likely pretty much past it if they haven't been changed before now.
 
By "classic era" do you mean valves? If so switch each of the valves from one channel to the other if that's possible and see what happens. If output is good and normal from one "good" valve in either channel, it may point to replacing the "bad" valve. But that's a crude test - you really need to check all the voltages against the schematic, but that requires skill and knowledge. It could be anything, but it sounds like a fault and it should be corrected rather than keep on using it.
 
I’ve had this. I solved it by turning each pot from end to end about 20 times (whether overkill or not!) & working all switches a few times. A squirt of electrical lube in each first doesn’t hurt. Do all this with the preamp turned off of course.

I do this kind of maintenance either a couple of times a year or when things start to sound a bit “off”. I then leave it overnight for the volatiles in the lube to evaporate.
 
Had similar problem on DH110 - the culprit was a switch.
What switches are common to all inputs but tend to not get used?
 
You must log in or register to reply here.


advertisement


Back
Top