SDA SRS 2.3 with Adcom GFA 555 amp and Adcom preamp

krook

I have had these awesome speakers for 25 years. A couple drivers are bad and a couple tweeters are blown but I still listen to them regularly (at a lower volume). I have an estimate of $1144.00 to replace all the drivers except the big sub. I'm trying to decide if I want to dump that money into these speakers or sell them and start a new system? Has anyone upgraded their 2.3's to new drivers? If so how it the sound quality?

The Adcom power amp was serviced once about 10 years ago when the power switch failed. It works fine now except the left channel seems to be louder than the right. Not sure if that is due to the speakers having bad components or a problem with the amp. Does anyone have any experience with this amp/issue.

I'm sure there is a market for this stuff if I decide to go new. Anyone interested?

F1nut

Six new RD0194-1 tweeters will set you back $288.00 at the Club Polk price. Not sure how many drivers you need, but there's no reason to replace all of them. Used MW6510 and MW6511's come up on eBay often, the MW6513 donut drivers not so much. They run in the $30 to $35 range used.

The DC offset may need adjusting on your amp, but you should get the speakers fixed first.

BTW, you can't sell anything here until you get 100 posts and list it in the Flea Market section.

OldmanSRS

Unfortunately the GFA-555 has no adjustment for DC offset, only for bias. If DC offset is out of spec (40mV max, 20mV typical) a component has changed value. The input stage has a pair of transistors (Q1 and Q2) that are the usual culprits. They must stay closely matched but can drift with age (as can several other components associated with them) increasing the offset.

If your amp is quieter on one channel, swap L/R outputs to the speakers and see if the problem moves or not. If it moves to the other speaker, it's not the speakers.

F1nut

Unfortunately the GFA-555 has no adjustment for DC offset, only for bias.

Good info, I wasn't aware of that.

Faustin

I was having some minor issues with my GFA 555 and currently have it in for service. Replacing caps etc. Anxious to see how it sounds when it is finished. I think if it was me I would put a bit of money in the SDA's and Adcom.

TennMan

Spending money on the GFA-555 is definitely worthwhile. When working properly the GFA-555 is an excellent amp for SDAs. They can easily handle the extra load presented by SDAs like mine that have low DCR inductors installed.

I have learned a lot about these amps from OldmanSRS. He is very knowledgeable about these amps and knows them inside and out. I followed his progress as he disassembled each board on his GFA-555, tested each component, and replaced/upgraded the parts as needed. Then he tested it thoroughly and made adjustments to make sure all components were within factory specs. Once he put it all back together it looked like a brand new amp inside. He also added a speaker protection board to the amp that has the added benefit of eliminating any thump (or other sounds) when the amp is turned on and off.

After he completed the work on his amp we spent several hours comparing his GFA-555 to my stock GFA-555 driving my 2BTLs. The difference in the quality of the sound was immediately obvious. The bass was much tighter and controlled compared to my 555. I heard details in the sound of strings on an upright bass that I could not hear when listening to my 555. The detail of instruments in the mid-range was much improved as well.

I was able to persuade OldmanSRS to go through my GFA-555 and upgrade it with high quality resistors and caps to bring it up to the same level as his. I couldn't be happier with the results. My 2BTLs sound the best they have ever sounded. Spending the time and money to refresh and upgrade these old amps is a good investment in my opinion. I will have my GFA-555 for a long time.

embrown057

Yes, the differential pair used 2SC2240 (Toshiba). You can find originals at B&D enterprises. You'll have to match them very close but even then it's a PITA a perfect match and it will take a batch of them to go through to find two sets. You can also add the second degeneration resistor (33.3 ohm) but match them also to bring them closer. But, even then 40Mv is not a big deal in this amp. The 555 is a good amp but there are some glaring problems. Not to bash this amp, I have serviced an modified 10's of dozens of them over the years. Also there are two versions of the input board, one slightly better the other. Make sure you find the correct schematic before servicing.

krook

Thank you for the answers and advice. I appreciate you all sharing your knowledge with me. Guess I'll put some money into the equipment and keep on rockin for another 20 years.

OldmanSRS

embrown057 wrote: »

Yes, the differential pair used 2SC2240 (Toshiba). You can find originals at B&D enterprises. You'll have to match them very close but even then it's a PITA a perfect match and it will take a batch of them to go through to find two sets. You can also add the second degeneration resistor (33.3 ohm) but match them also to bring them closer. But, even then 40Mv is not a big deal in this amp. The 555 is a good amp but there are some glaring problems. Not to bash this amp, I have serviced an modified 10's of dozens of them over the years. Also there are two versions of the input board, one slightly better the other. Make sure you find the correct schematic before servicing.

I discovered there is a 555 schematic on the web that does not match the one in the service manual from HFE. My amp is the same version as the manual. My DC offset was 198mV on the left and 14mV on the right. Part of the problem was cat whiz on the input/control board. I removed the pots and the switch and gave the board a good bath in Simple Green and water. The DC offset dropped to 65mV on the left. I found that D5 was the culprit. Instead of the 0.62V diode drop it was 0.85V. This caused Q3 to conduct more than it should and got Q1 and Q2 out of balance. I replaced D5 and D6 and the 1.24V reference returned and matched the same reference reading on the Right channel. Dc offset is now 5mV Left and 14mV Right. I run the warm bias at 24mV.

What other problems or improvements can you tell me about.

Thanks,
Greg

OldmanSRS

TennMan wrote: »

I was able to persuade OldmanSRS to go through my GFA-555 and upgrade it with high quality resistors and caps to bring it up to the same level as his. I couldn't be happier with the results. My 2BTLs sound the best they have ever sounded.

TennMan,
My pleasure indeed. Nice getting to know you and hearing your 2BTL's.

Greg

bugsnt

OldmanSRS wrote: »

Unfortunately the GFA-555 has no adjustment for DC offset, only for bias. If DC offset is out of spec (40mV max, 20mV typical) a component has changed value. The input stage has a pair of transistors (Q1 and Q2) that are the usual culprits. They must stay closely matched but can drift with age (as can several other components associated with them) increasing the offset.

If your amp is quieter on one channel, swap L/R outputs to the speakers and see if the problem moves or not. If it moves to the other speaker, it's not the speakers.

Is this only true of the original GFA-555 amp? What about the MKII version?

Many Thanks for this valuable info.

OldmanSRS

The 555II version does not have user-adjustable DC offset. It has a servo feedback using an integrated circuit chip that self-adjusts DC offset when it's working as it should. Bias is user-adjustable.

embrown057

The first version of the 555 is not the same as the 555MKII. The MKII uses a op-amp and to maintain offset. The bigest problem with the MKII along with the GFA-565 are the lytic caps on the input board. In short , they leak and damage the board. It take a deep cleaning in the ultrasonic cleaner with SG and deonized water to remove the fluid. As far as improvments to the first ver 555, it's realy not worth the investment and you will only gain little in the outcome. The MKII is sonicly much better of a amp and has much better spec's. Changing the caps on the input board, replaceing the by-pass caps on the main filters with a good PP will bring her back to specs for the most part. The main filter caps are a little expensive but BCH / EPCOS has the best cap for replacements I have found. I have also seen a lot of people change the film caps on these boards, not a wise thing to do. The orginals are polycarobonate films. Now from time to time I do see a stacked film in there but that's fine also. If you enjoy the sound of the amp just freshin it up and don't stress. And by the way. 24mv of bias gives you nothing more than "more heat". Setting the bias as low as 14mv will NOT change the crossover distorshion in this amp. Lowing the bias also dose nothing to change the output of the amp. I recomend you run BIAS at 20Mv +/- 1mv.

TennMan

embrown057 wrote: »

... As far as improvments to the first ver 555, it's realy not worth the investment and you will only gain little in the outcome...

I won't get into a pissing contest but I disagree with that assessment. I have listened to two GFA-555s side by side, mine in stock form and one updated by OldmanSRS. I'm not an electronics guy so I can't explain why, but I can tell you without reservation that there was a big improvement in the sound of his updated amp. I did a lot of reading before purchasing the GFA-555. My research of comparison reviews lead me to believe the GFA-555 was preferred over the 555II. If I recall correctly I read where Nelson Pass preferred the 555 over the 555II as well. All of us have different ears and different opinions so a blanket statement that the GFA-555 is not worth improving is a little off base.

Faustin

Can someone comment as to whether or not the 555 and the 55511 are common ground, in regards to running SDA's. I thought I read that the 555 was common ground and the 55511 was not. Thanks for any input.

teekay0007

Both the 555 and the 555II are common ground and fine with SDAs.

OldmanSRS

The 555II is also common ground.

OldmanSRS

embrown057 wrote: »

I have also seen a lot of people change the film caps on these boards, not a wise thing to do. The orginals are polycarobonate films.

Please clarify. Are you saying not to change the stock film caps to good polypropolene film caps? If so, why?

I'm not looking at the schematic right now but if memory serves there are only 8 caps on the control board. 4 electolytic, 2 stacked film and 2 others (film I think) that roll off high frenquecies/increase stability in the feedback when the amp has no speakers attached.

embrown057

This is the major problems with fourms, thre always some one that want's to ague facts. You can't change a apple into a orange. I have never heard Nelson say the 555 was better the the MKII. He had no involvment into the design of the MKII. Just do some research into the spec's between the two units. Sure I have "upgraded the 555 and it sounds better, some what. I'm not some kid fooling around with guesswork. I've been doing this professionaly sence 1973 and have a full lab of test equiptment and vast experance. I'm not going to "blow smoke" here, tell it like it is.

To answer the question concering the film caps on the input board:
Polycarbonate is more stable and made a great coulping cap in that circuit, otherwise they would not have used it. You can call Electronic concepts and ask for a price on the two cap, I think you'll be suprised.

OldmanSRS

By coupling cap do you mean signal passing though it between amplifier stages?

TennMan

No argument here. My comments were about sound, not about specs. Now you say, "Sure I have "upgraded the 555 and it sounds better, some what." That doesn't seem to jive with your previous statement about upgrading them.

I guess it just depends on how you interpret what Pass wrote. I read the following as his preference being the 555:

Adcom 555II - Nelson Pass ?

Nelson Pass wrote:

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.

embrown057

I know spec's sometimes don't tell the whole story but in the case of the first 555 it never broke any audiophile records. Don't get me wrong, I belive the 555 is a solid amp big bank for the buck. That is up till the one time it smokes your speakers from shorting rail voltage to the speaker terminal. Just had a GFA5500 in a few months ago that a channel shorted and it just distroyed the crossover board. So, updating the lytic's, adding full regulation to the unit and adding speaker protection is just not worth it in my opion. One could just purchase a newer amp with the feature needed in a high current amp and get better sound and sleep at night. This is just from my experance, your experance may very . This was never to be a "Bash the 555" but just a sanity check.

OldmanSRS

embrown057 wrote: »

That is up till the one time it smokes your speakers from shorting rail voltage to the speaker terminal. Just had a GFA5500 in a few months ago that a channel shorted and it just distroyed the crossover board.

That's not happening. I added DC offset protection and on/off protection to both the 555s in this post.

sinusshaphard123

Oldmansrs,how did you add protection?how much was it?