Audioanalyse PA90 mod, help?!

[Kyuusai]

Hello!

I have some trouble with a modded Audioanalyse PA-90 integrated amp: the right side runs much hotter than the left side and is almost untouchabe.

For those interested, let me introduce Audioanalyse first. They were a French company founded in the early 1970's and continued to make amplifiers until mid eighties when they went bankrupt. Although AA made beautiful amps (the PA-90 has been on display in the NY museum of modern arts, don't know when though) they were notoriously badly constructed. Plastic snap-in binding posts, messy soldering and wiring (in my opinion) and more often than not they ran too hot: as a result of that most of the survivors now have fried pcb's.

A dutch company named Duson eventually bought Audioanalyse and continued to produce and revise them, this time with a higher production standard. Unfortunately to no avail: Duson is also long gone.

The above and some images were all google told me (apparently that is still possible!). Lacking sufficient knowledge of electronics I now need help .

Here are picture-links of the original PA-90:
- Top open: http://i43.tinypic.com/30b0fh0.jpg
- Advert: http://1.bp.blogspot.com/-2MOWeKiYY...hXK2l25npCs/s1600/audioanalyse+reklame+87.jpg

The specimen I accidentally managed to buy 2 years ago gradually broke down on me, and I have been gradually patching it up. I removed the input selector boards (chip-based...?) and the opamp/pre-stage that had all sorts of bass / treble / cutoff mumbojumbo. This cleaned up the sound considerably but left me with one dysfunctional looking amp. Then the pot started to fail and I decided to revise the chassis. Welded new back and faceplate in place, cleaned pcb's and fitted some proper connectors and an alps pot. Had some issues with an accidental ground-loop but that's sorted now.

I've attached some pictures:
- The amp as it is now.
- The new front, homemade volume knob, fuse and power-switch that I still need to install.
- The tiny pots I suspect being responsible for bias correction (If that's why it runs too hot left).

So: any advice? Please let me know if I should post more pictures, unfortunately I couldn't find a circuit schematic. Thanks!

 

[sextaafondo]

Hi, from the information I gathered from the internet that amplifier is running on class A what gives sense to the huge amount of heat generated.
Both channels should heat up evenly though, so there is some adjustment to be done.
In case you can't get hold of an squematic would be advisable that you take the amp to a knowledgeable shop for proper calibration.

 

[Ian Finch]

The spec. shown in the PA 90 advert. link you posted, is for a 2 x 120W class AB amplifier. The class A rating is for only 2x10W but it will then slip into class AB to realise full power. This will be a P-P design, so that should relate to about 600mA bias current in each output stage which should be measureable by the voltage drop across the large (output transistor) emitter resistors. There will likely be 2 complementary transistor pairs, so the current will be shared between them.

Taking care not to slip with meter probes and short anything (use IC clips or clip leads), you can measure the voltage drop across the large resistors and calculate the bias current flowing, if there is no dedicated test point(s).
Depending on whether this is really rated for 8 or 4R loads, the rail voltages will tell us what is expected. 120W 8R indicates probably 57V rails and substantial idling heat dissipation of perhaps 114V x 1.2A or near 140W for both channels.

A 4R rating may require substantially lower rails and heat dissipation, so it would help make proper suggestions for bias settings if you were to tell us what the power supply voltages are at the amplifier power rails.