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Adcom 555II - Nelson Pass ?
Adcom 555II - Nelson Pass ?
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Old 3rd October 2002, 07:33 PM   #31
Nelson Pass is offline Nelson Pass  United States
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Adcom 555II - Nelson Pass ?
Just for the record, I don't recognize the schematics
posted. I surmise that the circuits were developed
in-house at Adcom.
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Old 4th October 2002, 04:00 AM   #32
Jared is offline Jared
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i drive 2 dayton DVC 15's for hours with a GFA 555 II
I think the sub was wired for 4 ohm, but I suppose it could be 8.. I need to change it to 2 ohm and use my class D car amp rated at 1200x1@2 ohm, measured at 1400x1@2 ohm nto it to start the subs moving
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Old 3rd December 2003, 12:55 AM   #33
K-amps is offline K-amps  United States
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The 555Mk.II was similar to the Pass designed 555 except for the drivers. In the Mk.II Adcom used beefier drivers i.e. 2sb817/2sd1047. These were 130 watt/ 140v devices. Also they used a 33 ohm resistance across the driver emitters. The heatsinks were much more substantial than the original and construction overall much better.

The 585 and 565 were similar to each other but radically different topologies that the 535/545/555's.

They still used the Toshiba outputs but changed the drivers to 2sc3907/2sa1516 (???) pair which was a 180v device...

I always liked my 555's with lessor NFB than stock. Instead of the 22k NFB resistance, i used 30 or 33kohm and got a sweeter sounding amp..
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Old 6th December 2003, 06:29 PM   #34
Bernhard is offline Bernhard  Germany
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I am also a proud owner of this thingy.

Bought it on a fleemarket with top cover missing and fuse holders missing.

Seems for some reason that the preowner did not like the fuses

It was dead and I bought the new expensive japanese transistors.

Changed them and it worked, oh yes !

Very nice amp

Just the problem was I could not get hands on new US fuse holders, so I did what the preowner did

No risk - no fun !

After a while when the amp was on, I pulled one cinch plug - and - boom the amp blew and the speaker blew and everything blew.

Bought new transistors and put in only 1 per bank, works.

Until now I had no time to complete the work.

One thing is for sure: This amp is no toy and needs fuses
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Old 26th December 2006, 02:10 AM   #35
kennyd is offline kennyd  United States
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Default gfa5500

neilson pass ....how would the 5500 ,5800,5802 amps do at running a pair of infinity kappa 9s ?
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Old 26th December 2006, 02:19 AM   #36
anatech is online now anatech  Canada
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Adcom 555II - Nelson Pass ?
Hi kennyd,
Those models came out much later. They are mosfet and sounded okay.

The larger amps might work with Kappa 9's (silly speaker!). It really depends on how hard you drive them. It is possible to blown them up, but Adcom amps will generally put up with some abuse.

I have no idea what was going on in the Infinity design lab. Why would anyone design a speaker that stresses most amps? You want the speaker to sound good, but you've designed them to be hard to drive. Senseless.

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Old 17th February 2009, 05:26 PM   #37
docrw is offline docrw  United States
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Default DJK help with modifing the GFA-585 amp

DJK have you modifed the GFA-585 amp before? I've owned several Adcom amps and was considering something better but believe I could make my trusty 585 everything an Audiophile would want without spending the kids college money. Is this something you or another person on DIY could help me with?

Originally posted by djk
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 22F 100V to each of the four 15,000F 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.

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Old 15th October 2009, 08:15 PM   #38
a.wayne is offline a.wayne  United States
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Lifting from the dead again ... Any worthy mods for the 565 ?
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Old 11th November 2009, 05:39 PM   #39
IrishboyM4 is offline IrishboyM4  United States
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i have a adcom gfa555 and Klipsch KLF-30 speakers that are 8 ohm. do i need to worry about anything? 8 ohm is supposed to be easy... are these hard to drive?

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Old 11th November 2009, 09:11 PM   #40
djk is offline djk
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The Klipsch KLF-30 are 4 ohms, but the Adcom GFA555 will drive then just fine.

The Klipsch are not too hard to drive, despite their being 4 ohms (they are moderately efficient).
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