Parasound Halo A23 problem

[jimmyjazz]

I just bought a used Parasound Halo A23, and one of the gain pots on the back is noisy. I have no intention of running it in any setting other than full open on both channels, but I have to wiggle the right pot to get it to conduct. I'm wondering if there is any reason not to have a tech open up the amp and just bypass those pots (if possible)? They seem like a silly addition to the circuit, unless I just don't understand their function. (Ostensibly, they appear to be for home theater tuning, as full open is labeled "THX". I couldn't care less -- I just want a decent amp.)

Also, any thoughts on worthwhile tweaks to the A23? My guess is that Parasound probably did a pretty good job sourcing decent components for John Curl's basic design, and that compromises might be more in layout, etc. In other words, hard to improve -- just buy an A21, correct?

[twest820]

Odds are that if you run a gain budget analysis on your system you'll find it's preferable to reduce the A23's gain towards unity. I found mine got somewhat muddy below about 10dB but never got around to tracking down what the problem was before I sold them---one starting point to consider would be bringing the inputs directly to JP1 and JP3.

My experience is the A21 is less precise than the A23. Unlike the A23 I don't have A21 schematics or measurements so I'm not sure why. If you want improvement beyond what the A23's capable of with balanced inputs and lowered gain but require comparable output powers something like this is hard to beat, particularly if fronted by an attenuator to offset the 49811's gain stability requirement, and fits in an A23 chassis. At lower powers you can just use an op amp instead of the 49811.

[john curl]

First, try to EXERCISE the pot, by turning off the amp and rotating the pot a number of times. Second, you can CLEAN the pot by getting a can of DeOxit or something equivalent, and spraying it somehow inside where the wiper is.
Third, you can turn the put full on and leave it there. That is the BEST place for the pot, as it is the quietest position.
Fourth, you can send the unit to the closest Warranty station, and get the pot replaced.

[jimmyjazz]

Thanks for the advice. Regarding gain -- my speakers are pretty inefficient (~ 81 dB/w/m), and I'm running through a passive preamp, so I need all the gain I can get.

John, I did try to exercise the pot, to no avail. I called Tony at Parasound and found out the pot is ~ $20. He also sent a schematic. Since the warranty has likely expired, I'm not worried about modifying the amp. Given this, is there any good reason NOT to bypass the pots instead of replacing them (other than the obvious -- no more variable gain)?

[jimmyjazz]

Another question: my loudspeakers (Eminent Technology LFT-VIIIb) are fairly benign loads (nominally 8 ohm) but inefficient (~ 80 db/w/m). I am using a Goldpoint passive stepped attenuator as a preamp. The combination runs out of gas at slightly less gain than I would prefer. One option I've considered is picking up a 2nd A23 and running them as bridged mono amps, which should add another ~ 5 dB headroom. However, I biwire my speakers -- is there any reason not to biwire in bridged mono mode?

[PMA]

This pot is an unhappy solution. In case you can run the A23 with maximum gain, may advice is to disconnect the pot completely and bypass it by soldering or a piece of wire. You will have no problem anymore and your sound will improve, by removing this highly compromised part.

[PMA]

This way:

[jimmyjazz]

Yeah, Tony at Parasound sent me the schematic. He said there might be 2 legs to bypass because of the amp's unbalanced and balanced option.

Anyone know if I can biwire off a bridged A23? Mr. Curl?