Adcom GFA-555ii and Martin Logans

[willick]

I had been driving my Martin Logan Sequel IIs with my Marantz SR-9300 receiver but always felt that the sounds was lacking a little something in the midrange (I know these speakers don't shine here). The Marantz did however show amazing clarity... Live recordings allowed you to hear the room better than I thought was possible with recorded music. The receiver says its rated for 6ohms only and the Logans although they're rated for 4 ohms (and the Marantz tech I spoke to said they should be OK) they dip to 1.7ohms at 20khz. It sounded great - just a little anemic.

I did a whole bunch of reading and was almost sure that I needed a Carver or Sunfire to drive the Logans. The reviews of the Carvers said that with the transfer function modification (tube amp matched) they were strong in the low end, low impedance stable but softer in the high end. Lots of people seem to like these combinations. The Adcom GFA-555ii is also good bang for the buck and rated for 1 ohm but I saw more than one review of the Adcom that said "grainy" high end with Logans. Since the clarity was what I was looking for, I steered away from the Carver stuff and toward the Adcom.

I purchased mine about 2 months ago and am running it off of the pre-amp of the Marantz receiver. The bass is strong and effortless, the midrange sounds great, the highend sounds fine, but that "magic" that I heard on the weaker amp seems to be gone... slower transients? highend rolloff? I'm not sure what it is. I just can't describe it any other way than that the magic is gone.
Being older amps, as everyone knows the Adcoms could potentially stand for a recap but I had presumed that the need for a recap would mean power on thump or would affect the bass - not the high end. So, my long winded question is this: What am I hearing and can I sort it out or do I need to find another amp?

I'm comfortable with repairs and testing the thing's operation, I just wanted some advice from someone who might have seen this before...
Am I hearing unmatched or old transistors? The Adcom graininess I should have expected?

Anyone?

[willick]

I thought about this a little more and suppose that by "magic" I'm talking about the tiniest details. A singer breathing, guitarist's fingers on the fretboard, or the pedals being pushed on the piano... On the Adcom, the Logans sound a little too much like normal speakers.

[anatech]

Hi Willick,
Yup. First, leave the main filter caps alone. The input coupling cap is a nice film, let it there as well.

Graininess is usually associated with poorly matched diff pair transistors and / or iffy capacitors. Nuke the ceramic caps if you find any - I can't remember what's in there exactly. Matching the complimentary devices as best you can may also help with this. Some careful supply bypassing near the voltage amp stage will also help. Output transistors should be matched as well. I seem to remember that On Semi once made a TO-3 MJ1302A and MJ3281A - briefly maybe. Failing that, the new MJ21195 and MJ21196 would be a good direction to move in. Beef up the drivers as well, an MJW type number may fit in there.

Many of these amps have defective capacitors on that PCB. The electrolyte is nasty stuff. You will need to strip the PCB and clean the heck out of it. This stuff is clear, but not odorless when you hit it with a soldering iron tip. You must get rid of absolutely all of this stuff. Clean or replace the components as well, replace all those PCB mounted capacitors. I use an ultrasonic cleaner.

Another thought for you. It is entirely possible that you need a buffer to drive the input stage of the Adcom. Many BJT input amps have this characteristic, whereas J-Fet input amps generally do not. There's got to be something to that. Read Doug Self's new book. It's well worth having as a reference.

Take your time. Take lot's of pictures and written notes. Do not do any work after you've had a beer! Basically, work carefully and take your time. Oh, I figured out how to make most Carver amps sound good and not harsh.

-Chris