Complete rebuild of Parasound 806a


2010-08-04 4:20 am
I bought a supposed "lightning strike" victim on ebay, but probably a channel randomly failed, and the owner cheated the insurance company. Anyway, an 806a showed up with 5 channels working, but I want 6 channels to multi amp my speakers, and Parasound does no longer supply replacement boards for the 806.
So, I checked Ebay for kits, because I have built kits before, and now I discovered something advertised as "current feedback", and I found out that actually a Canadian had invented current feedback amplifier, Mark Alexander, and he wrote a nice article about it. Also the kit board would fit perfectly to replace the blown board. The vendor sent me some boards of exceptional quality, and a bag of parts, which did not match the schematic, so it took several hours to try to figure out what went where. Close, nearby values of resistors were supplied, completely different output transistors, but I was assured the transistors were all "hand matched". I don't even know how to properly fire up a completed kit, and there were absolutely no instructions (which tells me probably the boards were purloined, but I imagine that's basically how things work in the East - bringing home boards and selling them on ebay is probably considered part of the wage) Anyway, irrespective of whether the boards were from a production facility or not (I think yes), I finally interrogated the merchant enough to get clear instructions on how to fire it up - one trim pot gets adjusted for OVDC at the output, and the other trimpot adjusts the idling current to 100mA, which you figure out by measuring the voltage drop of the 0.22 ohm resistor at the emitter. Anyway, me and my friend who were putting it together, we were still pretty surprised today, because everything got hooked up, the trimpots behaved themselves like the should, and the amp really really sounded phenomenally good. The board is labelled M6 and I have a feeling it is a purloined Musical FidelityM6i. MF is very popular in the East and in Europe, apparently, which may account for why this particular item is showing up where it is. The Parasound power supply seems to put out +-55 VDC, which is a bit more than the kit calls for (range of 25 to 50 V is permissible, there are a couple of 50 volt electrolytic capacitors on the board). But basically I would say the sound of the amp is nothing short of glorious. Current feedback amplifiers are not limited by slew rate, and there is no power bandwidth limitation. The vendor also sells the Diamond differential amplifier kit, which I'm thinking of doing for my next project.