Adcom 555II - Nelson Pass ?

Ok, time for silly questions/comments, it's Friday.

1. Did Nelson Pass design the Adcom 555II amplifier for Adcom,
if yes, then more questions.

a. What other Adcom models did he design?
b. What type of topology is used ?
c. Since there are so many of these amplifiers on Ebay,
can the design be modified to run lower impedances,
and/or can per channel power be increased, ie paralleling outputs, bigger power supply. Of course this will require major hacking - hehe

I have one of these in the garage for my speaker
testing, it works quite well, I like the mono bridged
mode for 600w.

I'm debating on whether to;
a. build my own amp using "off the shelve" schematics found
on the internet, this will take time - heh
b. or just settle on getting more 555II on Ebay.

I need 6 channels of power, 200-400w per channel would
be nice, and need 2 more channels with 400w-800w,
but after listening to the 555II bridged, I would love to
have 6 channels of 600w - WooT !!

I don't want to get into exotic amplifiers (Krell, etc.)
that cost a bazillion dollars - ha!

The only thing that caught my eye so far was the holton 1kw
amp design I found on the internet.
The one and only
Joined 2001
Paid Member
I designed the 555. I believe the Mk II added a couple
of what were considered improvements, and it seemed
to me that it included triple darlington outputs instead
of 2 follower. I might have the schematic around, but
all told it was a simple bipolar design with a current
sourced input diff pair followed by a common-emitter
gain device that was also current sourced. This drove
the followers.

It was typical of all the 5XXX amps that I did the first one
and then they improved it until it had too many parts,
and then I would design the next in the series :)

Americo Borza sent me an old 555 in the hopes that I
would update it, and it still sits here, a perfect candidate
since so many of them were built.

Personally, I would score a few of these and run them till
they die and then update them. If you wait long enough,
an update will be issued.
The original GFA555 as built by Adcom had a couple of small problems. The 2SA1011/2SC2344 driver transistors blew when the fuses went, driving low impedance speakers (Infinity). The Adcom fix was the 2SB633/2SD613 pair, selected for high voltage. These were a real problem as the un-selected part was only 100V. and every now and then the 'selected' part would puke. I used the Motorola MJE15030/31, a 150V part, without any problems. The power switch had a short life expectancy. I added a CL-30 Keystone inrush current limiter and a Hafler DH500 relay board with a three pole relay, the third pole shorted out the CL-30. The bass sounds much tighter if you add a 22µF 100V to each of the four 15,000µF main filter caps. The MKII version was strictly a cosmetic upgrade. The GFA585 had the triple output stage with cross coupled dual differential inputs and power supply bypass caps, and a DC servo. My modified 555s had much more 'slam' than the typical PA power amps of the era, and cost cost me considerably less. I used modified DH500s for mids and highs.
555's and 565's

Are the 565's pretty much the same as the 555's but set for mono only? also...seems like the drivers run VERY hot on these. I would really like to see a schmatic of the 555's and 565's as they look like they would be fun to work on but I don't have the skill or knowledge to do anything to them without a map.

Very interesting discussion..Thank you,

Varada niktoo,
D525 and B595? Wow, what was Adcom thinking? Those are only 100V parts. It is easy to see what the bean counters at Adcom were thinking, "we can sub the C2344/A1011 for those expensive Motorola parts Mr. Pass used and save a ton of money." and when those 1.5A parts didn't work "maybe there was a reason Mr. Pass used an 8A part, let us try this 5A 100V part, I mean the rails are only +/- 80V, right? A 100V part should be enough?"