Summer / Consolation Amplifier: 1995 Adcom GFA-5200 - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 27th September 2011, 06:59 AM   #1
diyAudio Member
 
Andersonix's Avatar
 
Join Date: Jan 2004
Location: Silicon Valley
Default Summer / Consolation Amplifier: 1995 Adcom GFA-5200

My two DIY amp projects, F5 and BA w BJT outputs have stalled for now, but look what fate (and craigslist) brought to me: a cute little Adcom GFA-5200 (an all-mosfet amp which NP designed, so that's why this post is here). I especially wanted the 5200, as it has a single output transistor per rail (whereas the 5300 has duals) and a single pcb with lots of empty space to work around. This will make a nice little summer amp to drive the top of my ESL hybrids, especially once I've completed my to-do list.
After a major vacuum, minor check-up, and revealing photo-shoot, I've punished it for over a week (making sure to test and re-test the thermal protect) playing the SL3's full-range. So far, no real guts in the bass and not so silky highs in stock form, but it shows promise.

Any chance of someone showing me a schematic?

Here's my immediate wish-to-do list:
Replace all the electrolytics
More PS caps (in the space thoughtfully reserved for them)
Thermistor to protect switch
New rectifier bridge made from 15A soft-recovery On-Semi MSR diodes and moved off-board
Re-route input leads (as they now take the scenic route close to the transformer!)

Long-term:
More heat sinking (and maybe redesign the cover to yield more space)
Bottom-mounted fan in silent housing
Transistor clamps + Kerafol insulators
More bias
Transformer-shielding and chassis-bracing steel plate
Attached Images
File Type: jpg Adcom GFA-5200 ADS1.JPG (375.3 KB, 656 views)
  Reply With Quote
Old 9th November 2011, 03:57 AM   #2
diyAudio Member
 
Andersonix's Avatar
 
Join Date: Jan 2004
Location: Silicon Valley
Stage One was completed about a week ago (see pic): I added two old 31mF caps to the PS, swapped all the electrolytics for a mix of Elna RFS and Panasonic FC, and squeezed in a 2u2 WIMA as input cap. Bass is noticeably tighter, but the highs are still not so smooth.

And Stage Two was completed a couple of days ago: I took the rectifier bridge off the pcb and made another bridge from 8A OnSemi MSR diodes. Also, the bias and DC offset pots were changed for Murata multi-turns. Now it sounds better in the highs, and I expect it will get even better when I add some heat sinking and raise the bias. Over the next few days I'll raise the bias a little with the stock heat sink and hear what happens....

Does anyone know the factory bias setting? It's currently set at 0.11A.

I'm considering swapping the 3W 0R1 emitter resistors for 5W Ohmite 10 Series axial wire element current sensing resistors (as they're available at DK and in my price range).

Then when Apex Jr gets in some 50V Panasonic caps I'll replace the four 6800uF in the PS and add a 0R25 in a CRC. And I have some old heat sinks to add to the stock hs and make another cover (and forgo the fan).
Attached Images
File Type: jpg Adcom GFA5200 stage1.JPG (360.4 KB, 578 views)
  Reply With Quote
Old 9th November 2011, 09:22 AM   #3
diyAudio Member
 
Join Date: Jun 2011
Location: Costal Virginia
I'm interested in your project. I am impressed that you squeezed those large electrolytics in that small package. I bought a beater 5200 off eBay and I am trying to get it operational. If I can get it working, I might try some of your upgrades. I don't suppose you have a schematic for this thing? I've searched in vane and found nothing other than an incomplete service manual from hifiengine. I'm forced to use the schematic for a 5300, which is similar. Good luck and I'll be following your progress.
  Reply With Quote
Old 10th November 2011, 01:47 PM   #4
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
I would bias it until the sinks run 50 deg C after an hour or two. If you don't
have a thermometer, this would be the temperature that you can leave your
hand on the sink for 10 seconds.

  Reply With Quote
Old 10th November 2011, 01:55 PM   #5
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
>ZM ; JB mode On<

Gotch ya !

<ZM ; JB mode Off>
Attached Images
File Type: jpg TEMP.jpg (39.5 KB, 523 views)
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM

Last edited by Zen Mod; 10th November 2011 at 01:57 PM.
  Reply With Quote
Old 13th November 2011, 06:42 PM   #6
diyAudio Member
 
Andersonix's Avatar
 
Join Date: Jan 2004
Location: Silicon Valley
Quote:
Originally Posted by jsfinusa View Post
I'm interested in your project. I am impressed that you squeezed those large electrolytics in that small package. I bought a beater 5200 off eBay and I am trying to get it operational. If I can get it working, I might try some of your upgrades. I don't suppose you have a schematic for this thing? I've searched in vane and found nothing other than an incomplete service manual from hifiengine. I'm forced to use the schematic for a 5300, which is similar. Good luck and I'll be following your progress.
I'm glad you're interested, jsfinusa!
No, I have not seen a schematic for any of the Adcom mosfet amps in this series.

A future option is standing up the PS caps, so they will balance the look once there is more HS added (in the manner shown, but with an additional HS opposite it).
Attached Images
File Type: jpg 5200 w HS and caps on-end.jpg (505.8 KB, 494 views)
  Reply With Quote
Old 13th November 2011, 06:48 PM   #7
diyAudio Member
 
Andersonix's Avatar
 
Join Date: Jan 2004
Location: Silicon Valley
Quote:
Originally Posted by Nelson Pass View Post
I would bias it until the sinks run 50 deg C after an hour or two. If you don't
have a thermometer, this would be the temperature that you can leave your
hand on the sink for 10 seconds.

Thank you, Nelson! I think I'm pretty adventurous (and patient!) with dialing up the bias, so I'll report back my findings. (I will probably put in some Kerafol insulators before getting too wild...)
  Reply With Quote
Old 22nd December 2011, 11:53 PM   #8
diyAudio Member
 
Andersonix's Avatar
 
Join Date: Jan 2004
Location: Silicon Valley
Bias is at 0.25A (stock was 0.11A), and it sounds much better in the highs, but still not there somehow. So I'm going to R&R the 3W sand cast 0R1 emitter resistors. Would it be OK to go even lower in value here, say, 0R15//0R15 = 0R075? (That opens up options of using 2W resistors.) What is the emitter resistors' purpose?

Since this is an AB amp, I can't quite yet turn up the bias as far as NP suggests, as it gets quite hot while playing loudly. So I need to add more hs (and/or install a fan, which I am reconsidering).

While bass control has improved with the extra 31mF caps, since I had them on-hand, I put in a couple of 68mF caps instead (as shown), so it now has six times the original PS capacitance. This further improved bass control, but I feel there's still room for improvement. And it makes me wonder why Adcom chose to use so little originally....
Attached Images
File Type: jpg Adcom w 2x 68mF.JPG (382.8 KB, 448 views)

Last edited by Andersonix; 22nd December 2011 at 11:59 PM.
  Reply With Quote
Old 18th August 2012, 03:46 PM   #9
diyAudio Member
 
Join Date: Jun 2011
Location: Costal Virginia
Andersonix

I know this thread is a little old but I wanted to follow up and say I finally got my beater 5200 working. It took replacing all output devices and several small electrolytics to restore operation.

In the discussion about bias, I set mine to what is called for in the service manual: 20 mV across 0R33 (R035/R135), which is about 60mA. That is about 1/4 of where yours is set. It makes me wonder if I should set mine a little higher? My amp sounds very good and seems to work fine though it does get quite warm after 1/2 hour or so.

BTW, I used the schematic from the 5300 and it is close enough for troubleshooting the 5200. At this point, I will live with mine the way it is though I may eventually follow your lead with some of your tweaks.

Thanks for posting your mods and pics.
  Reply With Quote
Old 6th October 2012, 09:38 PM   #10
diyAudio Member
 
Andersonix's Avatar
 
Join Date: Jan 2004
Location: Silicon Valley
Well done on getting yours working, jsfinusa!

If you are referring to the Adcom service manual for the 5200, then beware: Adcom was sloppy in editing the service manual for the 5300 to create the one for the 5200, and there are errors. Note that the source resistors (R035, R036, R135, R136) in the 5200 are 0R1 (on mine at least) and not 0R33 as in the 5300. Also, while the part numbers are correct, the biasing directions (20mV across R35) are taken from the 5300 and if followed will lead to more bias than intended. The bias on my 5200 when I got it was 0.1A. In your case, jsfinusa, 20mV/0R1 is 0.2A of bias...

With the stock heat sinking, I biased my 5200 to 0.25A, and it ran warm with the cover off. Now, with more heat sinking added, I have it adjusted to 0.4A (28mV over my new 0R07 emitter resistors), and it runs warm to hot.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Adcom GFA 4304 amplifier HVACGUY Car Audio 3 28th February 2008 06:25 AM
Adcom Gfa 565 Amplifier Service Manual bainesni Solid State 8 23rd December 2007 01:06 AM
FS: Adcom GFA 545 amplifier, Ontario Canada boogs Swap Meet 0 28th August 2006 04:35 PM
FS: Adcom GFA 555II amplifier, Ontario Canada boogs Swap Meet 0 28th August 2006 04:19 PM
car amplifier ADCOM AMP GFA-4402 zuki Solid State 5 9th September 2005 06:24 AM


New To Site? Need Help?

All times are GMT. The time now is 05:46 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2