ADCOM GFA-555 II Audio Amplifier Repair And Test

[@williamcampbell3868]

I have a Adcom 555 mk2 that I bought in 1990. It barely gets warm and runs flawlessly with tremendous bass extension and definition. Its sound staging and depth are also amazing. I am biamping with the Adcom on lower midrange and bass and a Conrad Johnson mv55 on upper midrange and treble. I alternate with a Hafler 120 during the spring and summer and the Connie-J. at fall and winter. The Adcom is indestructible and the only maintenance I perform is checking all connections and dusting out the interior every 18 months. Its going on 34 years and I'm confident it will reach 40!! My Hafler dh120 is 37 and both have seen different speakers over the years, Sansuis, H.H.Scotts, Ohm Walsh 3's, Vandersteen 2ci's, Energy veritas, and the Maggie 1.6's along with the Vandersteen 2w's which are a end game for me!

[@williamcampbell3868]

I forgot to mention Polk Audio monitors also!☺️

[@fzesgru]

What a great way to start your channel. I have three of these and love them, and your video is most appreciated.

[@EriksElectronicsWorkbench]

Glad the video was enjoyed, thanks for the nice comment!

[@longshot9078]

I have this exact same amp!!! it's a dream 200 watts at 8 ohms. i use it too power my front left and right channels i use a outlaw mono block for the center! works so well as the front stage! best part only paid $200.00 for it !!

[@hoobeydoobey1267]

You have great knowledge and skill. Explanation was on point. Great job!

[@craighoffman6876]

That amp is likely good to go for another 35 years. Well done sir.

[@blazemonkey]

Excellent video! I don't have this amp or need to repair it, but I came across the video while looking at Adcom's newer amp which shares the same chassis, design and base model name. As a former electronics engineering student, this is an excellent breakdown of troubleshooting. I enjoyed!

[@EriksElectronicsWorkbench]

Thank you for the compliment and I'm glad to know you enjoyed the video.

[@fastlayne9076]

I have a GFA555 that one channel stopped working. The rail fuses are all testing good. I am a weekend electronics hobbyist from 30 years ago and not sure the high-end audio world passed me by while I raised my family and am now empty nest and looking to a) repair/refurb my GFA - or - b) replace it (with what that would sound as good as the GFA without spending $4,000). I built several Dynaco kits back in the day and even a 100W/channel into 8 ohms RCA (schematic) power amp from scratch built into an old AM carrier current gutted chassis that had some minor hum because I didn't do a good job of grounding - but that amp still sits on a shelf somewhere in my basement and likely still works albeit, it's just there to make me feel accomplished as I am a self-taught electronics hobbyist and really only have a VOM on my bench and nothing else to work with other than the minor knowledge I have to do what I've done - and the desire to want to see my GFA555 working again. Do I rebuild and if so, where do I get the parts? Or do I replace and, if so, with what and at what silly stupid expensive price? I LOVE my GFA555 and my ears tell me nothing on today's market will sound any better...............so why replace if I can repair/refurb it? PLEASE - any help, guidance/direction you can give me Erik, would be greatly appreciated! As well, I don't know how to reach you (email?) so I have to post here to ask. A musician, high-end hi-fi enthusiast, and weekend auto and electronics hobbyist who has enough skills to be dangerous - and capable! Thank you in advance for whatever you can do to help me with my GFA555 dilemma. Mitch

[@EriksElectronicsWorkbench]

Mitch, they way I would troubleshoot the amp is to check the power supply of the dead channel. If it is ok then apply a test signal to the input (like a 1 KHz tone) and use an oscilloscope to measure the test signal and see where it disappears. Work from the input to the output stage. You can trace the 1 KHz signal with a VOM on AC volts and appropriate range but an o'scope is normally used. The GFA555 is definetly repairable/rebuildable but the cost and involvement is hard to guess at without knowing what went wrong. Study the schematic and work methodically through the circuit and there is a good chance of finding the problem yourself. The amp is a traditional 3 stage design, lots of info online about how they work. Parts sources I typically use are Mouser Electronics and Digi-Key. Have fun!

[@mrturtle80]

awesome video and sweet setup

[@jameswarren1831]

Excellent video! I worked on one a couple of years ago that had a failed power supply and a bad zener diode.

[@justicelut]

Enjoyed your presentation, thanks!

[@bpalpha]

Gawd! I love Adcom. What day and age is it that such a boss audio company goes kaput?! See a lot of the pre amps around on the used market, not so much the amps. Thanks for the vid.

[@vegaslimoguy2376]

They are still currently in business they are being made over in Taiwan. They have stuck to the same design and the Nelson pass design as well. You can find many of their top amps on eBay right now but they are pricey! Also, there is tons of Adcom gear on the internet you can find many used amps and preamps on reverb or Craigslist or audiogon even the older items are bringing huge prices right now these things were built like tanks back in the '80s and '90s I have several models I have a huge collection of Adcom gear, most of them have been trouble free for over 30 years.

[@robertfournier7050]

Up here in Canada I had an Adcom. I really found the sound cold and uninteresting. Well built. Perhaps even very well built. Just I look at sound from being expressive and yet delicate. I mean I could push the amp to it's max and it had no distortion. One other amplifier circuit was not as simple but was effectively more robust while able to produce very high quality sound. Rebuilding a damaged amplifier takes knowledge of more than reading a schematic diagram. Understanding

[@PeriodWoodworker]

Glad I found your channel. Nice video and thanks for sharing, Steve

[@EriksElectronicsWorkbench]

Thanks for the compliment! Glad you enjoyed the video.

[@g.fortin3228]

Very very detailed, and helpful. Appreciated !

[@EriksElectronicsWorkbench]

Thanks for watching and happy to know it was helpful!

[@RanTausi]

@@EriksElectronicsWorkbench Hello Erik , I have a strange phenomena, when I power on the amp I see on both of my speakers the woofer cone is pull in then go to normal position , when I play music it seems all is good, but this phenomena wasn't occur before. Any idea ? Thanks

[@EriksElectronicsWorkbench]

@@RanTausi The woofer cone moving as you describe is a sign of DC offset voltage. You should measure across the speaker outputs to see what the DC voltage is, nothing connected to amp's input or speaker jacks. It should only be a few millivolts DC, lower is better. If the amp only momentarily has DC offset and then settles to near zero it is not a problem. These amps are known for turn on and off thumps and noises and it won't hurt the speakers. Any continuous DC offset more than a few millivolts is a problem in the amp and can cause speaker damage.

[@RanTausi]

​@@EriksElectronicsWorkbench Many Thanks Erik , I will check the Bias and I will see If I have voltage on the speakers terminals. I know thump on power on is OK, but the cone movement of the woofer is new phenomena that doesn't happened before so I am obviously a little concerned.

[@EriksElectronicsWorkbench]

@@RanTausi It is good to check the bias voltage but that may not be causing the DC offset. I agree if this is something new the amp should be tested. The DC offset could be a failing transistor in the differential input stage, the OP amp as I showed in the video, or a failing output stage transistor. Those are key areas to look at but not the only components to cause such a problem.

[@garynichols1502]

Good video, I love the Adcom 555

[@MoparTech]

Thank You for this Cool Insight on these Very Powerful and under appreciated Adcom amps. I am now subscribed to your channel. I have 3 of these Bad Boys, 2 of them have been fully restored and matched to each other all of them Driving Reconed Cerwin-Vega 12s and 15s from one common/daisy chained preamp (Yamaha RX797) output. Front Row Concert Sound in my Basement. -When My wife is not home lol.

[@EriksElectronicsWorkbench]

Glad you enjoyed the video and thanks for subscribing. Your system sounds impressive, I know how loud this one unit plays, I can just imagine three of them!

[@johnlake6321]

Thanks for this video. I've had an Adcom 555 Mk11 in my shop for months while I get my nerve to tackle it. (Dead on one channel).

[@EriksElectronicsWorkbench]

Glad you enjoyed the video. Hopefully it will help in successful troubleshooting on your amplifier.

[@RixtronixLAB]

Creative video, thanks :)

[@EriksElectronicsWorkbench]

Glad you enjoyed the video!

[@rafaelbalcazar4983]

Hello Erik great video! I have an old GFA 555 AMP that needs repair. I think it is maybe 40 years old. Can it be repaired? good idea? thank you!!!

[@EriksElectronicsWorkbench]

Hi, glad you liked the video. The GFA 555 is certainly repairable and worth doing as it is a nice amplifier. You might look at this link for helpful information hoppesbrain.com/adcom-faq/

[@zulumax1]

I have two modified Adcom GFA-545 mk II amplifiers, and a GFA-535 mk II amplifier. I must have a later revision board of the 545 amplifiers. There are extra components that are not in the schematic. Is there information on board variations somewhere?

[@EriksElectronicsWorkbench]

I'm only familiar with the schematics as shown in the service manuals which on my amp matched exactly. Sorry I don't have info on board revisions. You might take a look at the info here hoppesbrain.com/adcom-faq/

[@zulumax1]

@@EriksElectronicsWorkbench I have Hoppesbrain saved in my book marks. A great site and tribute to Nelson Pass.

[@Muzzman1]

Great Video! @EriksElectronicsWorkbench, do you have a service center? I have a GFA-585, and 2 DBX-BX1's Id love do send in for maintenance. Maybe you are are in the LA area, so I dont have to ship those beasts?

[@EriksElectronicsWorkbench]

Thanks for the compliment Ammar. Sorry I am not currently taking in equipment to service.

[@johnbravo7542]

Beautiful condition subbed,I like your channel

[@EriksElectronicsWorkbench]

Thank you!

[@georgeho8997]

Hi Erik, I tried your biasing method on my GFA-555 II but couldn't hold down a stable reading like you did, could you tell me what are the usual cause for this to happen? thanks and hope you can help. love you channel.

[@EriksElectronicsWorkbench]

If the unstable readings are happening on both channels I suspect the issue is with the meter or probes you are using, perhaps picking up noise as the meter is trying to measure in the millivolts range. Keep sources of noise like switch mode power supplies away from the test area. The bias potentiometers may be dirty and making poor connection. The AC line voltage could be varying excessively. If just one channel bias is unstable it could be its bias pot, or a defective transistor. Make sure the transistors are securely attached to the heatsinks and with thermal paste. Look for broken solder connections on the power transistors and emitter resistors. Heating and cooling over years can stress the solder joints. I hope these ideas help. And thanks for watching!

[@georgeho8997]

@@EriksElectronicsWorkbench I tried it with the clip type this time but still no luck, I got all zeros with every range setting. Perhaps I have a different/earlier version of the GFA 555 II with different bias points and 1800VA at the AC input whereas the later version has 1440VA, don't know what that means but that's the different I've spotted. That may sound unlikely but I just can't think of why am I not getting any meaningful reading. Anyway, thank you so much Erik.

[@EriksElectronicsWorkbench]

@@georgeho8997 The bias is taken across an emitter resistor. The upper resistors are easier to reach. You don't have to use the test points, technically, to read the bias as any of the resistors will give a reading. You may not have bias due to a fault thus no voltage reading. The bias pots are very sensitive to adjustment. No bias will result in cross over distortion. Excessive bias will make the amp run hot or even blow a fuse. Make sure your meter is on DC millivolts, not AC volts. The VA rating is not related to the bias, that's just the AC power consumption under full load.

[@georgeho8997]

@@EriksElectronicsWorkbench What might be the reason for the problem if there is a fault like this?

[@EriksElectronicsWorkbench]

@@georgeho8997 if there is no bias voltage it could be a bad bias potentiometer, bad bias transistor on heatsink, a bad transistor or component in the preceding stages. Just some general ideas. If you have no bias reading on both channels look at your meter setup and the test points you are using as it would be unusual to lose bias on both channels.

[@-loki-7727]

Do you want to freshen up a Nakamichi Receiver 1? Can't find anyone local.

[@EriksElectronicsWorkbench]

Hi, thanks for the offer but too many current projects going to take on any more so I will have to pass on it for now.

[@carlespy7470]

Erik - could you point me toward the speaker protection relay and monitor circuit for each speaker output? You said that "these are commercially available and could be installed inside the amplifier," but I need help finding the items. Thanks, Carl Espy

[@EriksElectronicsWorkbench]

Carl, one example is listed here diyaudiostore.com/products/soft-start-speaker-turn-on-delay-and-dc-protector-boards you need to make sure the protection board can handle the power rating (watts per channel) of the amplifier. I don't know if the item at the link I listed meets that power handling requirement, but I found it quickly and just used as a reference. You might also try a Google search for "speaker protection relay kit" or similar keywords. -Erik

[@carlespy7470]

@@EriksElectronicsWorkbench Eric - the one I'm looking at requires a 12 VAC source inside the amp. Where would that be in this amp?

[@EriksElectronicsWorkbench]

@@carlespy7470 The two options I can think of.. There are two small blue wires on the secondary of the transformer. They were for an optional cooling fan. Measure the AC voltage on those wires and see if it is in the range you need. If it is not correct you would have to install a small transformer (at whatever current rating the protection board requires to operate) to provide the 12 volts. The transformer primary would be connected so it is switched with the main power switch.

[@carlespy7470]

Eric - sorry to keep pestering you but before I do all this, couldn't I put a low ESR blocking capacitor in the speaker line to block any high current DC from the amp yet not affect the signal? If so, what value would be appropriate?

[@EriksElectronicsWorkbench]

@@carlespy7470 The blocking capacitor won't work for this application. It would have negative affects on the frequency response (attenuation), no capacitor would handle the power levels the amp delivers (even if the cap had the proper voltage rating), and the amp would effectively be driving a purely capacitive load which it was not designed to do.

[@carlespy7470]

Hi, Erik - my GFA 555II is silent when no inputs are connected, but it hums very noticeably when connected to my preamp. Thinking this was ground loop hum, I took everything apart and couldn't locate the source of the hum. Finally, I installed a different power amp and there was no hum. What could be the issue with the 555? I would greatly value your guidance.

[@EriksElectronicsWorkbench]

It does seem like a ground loop problem. Inside the GFA555II the input connector ground side first connects to a 100 ohm resistor and then to ground. Check that this resistor is not open. Disconnect input cables and measure with an ohm meter from the RCA outside shell to the amp's chassis. Should be 100 ohms for each input. If that test is ok a solution could be to use this device to break the ground loop www.parts-express.com/Rolls-HE18-Buzz-Off-Hum-Eliminator-Balanced-Unbalanced-Conve-245-1032?quantity=1&srsltid=AfmBOorTiJzXfjFAfwW-TlMsBcDu6nJ0eM6GrwB-6vSzVbY42MLK6McO

[@carlespy7470]

Erik - Thanks. Each side measures 100 ohms. I already tried the Rolls "Buzz Off" device and it didn't help. The amp hums with any source. I even bypassed the power distribution component and went straight out of my dedicated 20A outlet without improvement. I tried lifting the ground to no effect. It seems that the 555 introduces hum when amplifying any signal but it is quiet when the inputs are unplugged. Can you think of any other potential cause in the amp? The substitute power amp plays; no hum.

[@EriksElectronicsWorkbench]

@@carlespy7470 unusual problem as hum if coming from within the amp is usually present all the time. Perhaps it is more of a distortion issue. Really the only way to test is to input a known good sinewave signal from a signal generator and then use an oscilloscope and probe starting at the input jack and move through the amplification stages until the noise is found. It could be a defective transistor but equally any other component failure could cause distortion so it is not really possible to say the cause without testing. Also check the power supply rails and make sure they are equal in DC value and very low ripple voltage. Also check the output bias setting.

[@carlespy7470]

Eric - thanks for the advice. I'll give all your suggestions a try. I do have the equipment but have limited expertise.

[@jimomertz]

I’m shocked that this amp doesn’t have output protection circuitry to prevent that kind of offset to get to the speakers! 🤯 I have the Adcom 545 and check it every so often. But it can always fail when in use. 😱

[@EriksElectronicsWorkbench]

My understanding is ADCOM didn't want relay switching in the speaker outputs in an attempt for sonic purity. The trade off is the risk of DC offset voltage and speaker damage. There are modifications that can be done to add protection if it is desired. Since I made this video I have put a lot of hours on the amp and so far all is ok! I also have the 545II, great amp, never given me any trouble.

[@jimomertz]

@@EriksElectronicsWorkbench I understand the rationale to avoid relays. It also helps the damping factor number. But is very risky to use these amps with expensive speakers. I have had my 545II for almost 30 years with no trouble, but it is used in my secondary system.

[@carlespy7470]

I take it that adding a fuse in line with the speakers won't help resolve this?@@EriksElectronicsWorkbench

[@EriksElectronicsWorkbench]

@@carlespy7470 A fuse is not the best solution.. fuses have DC resistance across them, although small, there will be a voltage drop. More importantly they will not protect unless there is drastic output stage failure, and perhaps for some duration of time, since the fuse would have to be rated for the peak currents of the amplifier's output to avoid opening too soon. In the case of this amp's failure the output DC offset was not going to the full rail voltage meaning the output stage was not turning full on. It would never have blown a fuse rated for the amp's normal output power. The speaker would still have been subject to DC voltage. The best solution is a sensing circuit that monitors for even the slightest DC offset and opens a speaker line relay or otherwise disables the rail power supply.

[@zulumax1]

@@EriksElectronicsWorkbench There are aftermarket kits you can add. xraytonyb just did a video on a couple. I was thinking of adding it to my two 545 mkII amps. Just thinking about it mind you, damping factor being the drawback as you mentioned.

[@deepakkantgir]

Hi Eric my GFA 555 II hadn't been in use for a while and recently when I turned it on I noticed the right channel heat sinks ran too hot and the channel tended to shut down. I can't remember which one but I did notice some fluid leakage on one of the boards. I gave it to a local shop for service and they claim it isn't troubling anymore. Although I noticed the right heat sinks running hotter. I have no knowledge of electronics but don't want to harm the amp by being ignorant. Specifically could you advise what to check and what to replace as a way to.ensuring the amp doesn't have the over heating problem. Thanks.

[@EriksElectronicsWorkbench]

Hi Deepak, all the electrolytic capacitors should be, at minimum inspected and tested, and probably all should be replaced. The large filter caps may be ok, they don't often fail but should be tested. If the capacitors don't solve the problem then continue with testing.. The overheating problem could be: a problem in the right channel bias circuit, a failed op amp as my unit had, a fault causing a DC offset (differential amp transistor or resistor etc), or a defective power output transistor(s). Only testing will show the actual fault, sorry I can't pinpoint more specific. After the capacitors have had attention test the bias as I show in the video and test for DC offset voltage at the speaker terminals (no speaker connected when testing). Those tests should point to the area that needs further repair. Also note that if an electrolytic capacitor leaked the fluid is conductive and will cause problems so the surrounding area must be cleaned very well.

[@thompjo76]

I’m getting some static in my right channel that almost sounds like it’s getting a radio signal from a router or phone. Curious if this is an issue with my amp? Also where are you located if I can have you look at my amp?

[@EriksElectronicsWorkbench]

Have you tried unplugging the inputs on the amp and see if the problem goes away? If so the amp is not the source and the interference is being picked up in the cables or preamp or some other audio component. Any strong radio transmitters nearby? They can induce interference.

[@carlespy7470]

I am having hum from my 555 II that might be ground loop hum, maybe not. To make the grounds common, I plugged in the amp and the surround sound processor into the same wall outlet. (Both have polarized, two-prong plugs.) No other devices were powered on. Still humming. But there's no hum when inputs to the amp are taken out. The amp by itself is quiet. Then I plugged in my old Dynaco PAS 3X preamp to the amp and hum is there again, so I guess the problem is not in my surround sound processor. And the hum increased by the volume control. Is there anything internally in the amp that could cause this. Do you have any other thoughts? By the way, in normal operation, none of the other surround amps hum, only the Adcom. Thanks, Carl

[@EriksElectronicsWorkbench]

It sounds like a ground loop problem. You might try this product, I have had good results with it... www.parts-express.com/Rolls-HE18-Buzz-Off-Hum-Eliminator-Balanced-Unbalanced-Conve-245-1032?quantity=1

[@carlespy7470]

Thank you, I'll give it a try!

[@DavidDyche]

I have had this amp for 30 years and I have been trying to get it going and it is having the exact problem as this one, same right side too. How much would it cost me to have you fix it?

[@EriksElectronicsWorkbench]

Sorry I'm not taking on additional repair jobs right now. However if it is having the same symptoms I would recommend replacing the op-amp as shown in the video. If that does not solve the DC offset it might be the differential amp transistors. If you are unsure about desoldering and soldering components I cover that in my other videos. There is also Adcom tech info and parts at this link hoppesbrain.com

[@DavidDyche]

@@EriksElectronicsWorkbench Wow thank you for your response I may develop the courage to try it. I blew out my subwoofer on my Chorus II speaker, it rattles why not doing much in the way of bass. I am thinking there is no way to fix it and I just bought a replacement but if you know something different about that please let me know I can cancel my order. AGAIN THANK YOU SO MUCH!

[@EriksElectronicsWorkbench]

@@DavidDyche a complete replacement is probably the best solution but if you want to try and fix a speaker check out www.parts-express.com

[@DavidDyche]

@@EriksElectronicsWorkbench Again thank you for helping me! wow. So I am going to check out your videos on the basics and see if I can do this but you truly inspire me. Your level of expertise is beyond my understanding but I am a fast learner and with your help (videos) I bet I can do it. I will keep you posted. TY

[@EriksElectronicsWorkbench]

@@DavidDyche I hope you find the other videos helpful. Have fun on your repair project!

[@DaveMcAnulty]

What is the disconnected blue wire pair with white JST connector on it on top of the transformer?

[@EriksElectronicsWorkbench]

It is for an optional cooling fan. The fan is only needed in special applications, most of the amps don't have it installed as convection cooling is enough. There are also temp sensors preinstalled on both heatsinks for the optional fan control circuit.

[@danny51577]

I just purchased an adcom 7607 which is a 7 channel amplifier. I purchased it as parts or repair. It won't power on. I thought I could find someone familiar with these but having zero luck. Without taking up a bunch of your time, would you have any idea what I may be able to check quickly to possibly resolve without having to ship the amp cross country for 3 months?

[@EriksElectronicsWorkbench]

I don't have a schematic for this model but based on the design layout I see there are two transformers. The large toroidal to power the individual amplifier boards and a small power transformer to power miscellaneous support circuitry. Each power amp board has its own bridge rectifier and filter capacitors. Since the whole unit does not power up you should check that both transformers have AC being applied to their primary windings. Then look at secondary winding voltages. Check all the internal fuses. Then check the power supply related to the small transformer in the front left corner. If nothing turns up unplug each power amp board and one by one plug each board in (cycle the power off when making these connections) to see if one of the power amps is the trouble. I don't know if this amp can detect DC offset, thermal overload, etc but a protection circuit could be shutting down the amp. Be safe, you do this work at your own risk, best of luck on the repairs.

[@danny51577]

@@EriksElectronicsWorkbench hey thanks man. I figure I'll get a look, see if it's something I can figure out. If not, prob keep looking for someone to send it to

[@davidnguyen5028]

Hi, I’m a subscriber. My class A amp, (adcom gfa5802) just recently developed a slight buzzing hum on the left channel. Do you think the filter capacitors for the left channel are bad? I checked the DC voltage at the speaker terminals and they’re at 0.025V, so I think the coupling capacitors are fine. Or do you think it’s something else? Thanks in advance.

[@EriksElectronicsWorkbench]

Hi David, DC offset is fine. Unplug inputs to the amp and make sure the noise is not from an external source or ground loop. If the noise remains use a DMM set to *AC volts* and measure the DC supply rails of both channels. This will show you the ripple voltage. It should be very low down in the millivolts range. If the noisy channel reads a high AC ripple compared to the good channel then the filter caps are bad. The GFA-5802 is known for cooking capacitors because it runs hot. A complete recap with 105C rated caps might be the best course of action. But if that does not solve it then you would need to use an oscilloscope and start at the speaker terminals and work back (or from the input connector and work forward) and find where the noise begins. Schematic is listed here www.hifiengine.com/manual_library/adcom/gfa-5802.shtml

[@davidnguyen5028]

@@EriksElectronicsWorkbench Thanks Erik.

[@andymouse]

Cheers.

[@julian65886]

Hi Erik: I have the exact same amplifier purchased new sometime in the 1990s. The amp has been used very little in the last 20 years. Once a channel went out way back when and I had someone look at it. It was so long ago I don't remember. I sometimes power up the amp and about three years ago a fuse was blown. I replaced the fuse and it has worked OK. However, I only power this amp up once every three months or so. Now I want to use the amp again on a regular basis , however I am told if it fails it can destroy the speakers. I have B&W 802 speakers. Should I go ahead and use the amp or should I have it check? I am using the B&W 802s on my home theater, but the AVR amp is probably not as nice as the Adcom.

[@EriksElectronicsWorkbench]

The amp has no built in speaker protection. If a DC offset voltage occurs because of a failed component (i.e. output power transistor or something like my video covers) you can have DC current flow through the speaker(s) which can overheat the voice coils (usually the woofers) and destroy them. You can try this test: Disconnect the input signal and speaker wires, power up the amp, and connect a digital voltmeter (DVM) to the speaker terminals (one channel at a time) and measure the voltage. On the meters most sensitive range you should have only a few millivolts of DC voltage on the speaker terminals. If the speaker terminals have more than a few millivolts the amp has a problem. A typical failure will put a few volts to 50 or more volts on the terminals. My video covers measuring the DC offset voltage and typical readings on a working amplifier. Also, if the DC bias is incorrectly adjusted too high the amp can be damaged.. it will run very hot and destroy the power transistors. My video shows that adjustment too.

[@julian65886]

@@EriksElectronicsWorkbench I have a voltmeter, so I will give it a go. Should I have it service even if the test comes out OK? One would think the caps are old. BTW, there is a guitar amp You Tuber named Uncle Doug. Have you seen his channel? IMO, you may be better than Uncle Doug.

[@EriksElectronicsWorkbench]

@@julian65886 The main filter caps are probably ok, they rarely give trouble. They could be tested if you are concerned. The other caps have aged some no doubt, for peace of mind I'd say replace them. You can see in my video none of the caps were actually failed (shorted or leaking) but some showed age with higher ESR. Thanks for the compliment on my channel, I am not familiar the the channel you mentioned.

[@julian65886]

@@EriksElectronicsWorkbench Thanks so much for replying. I am waiting for a new voltmeter to do the test you suggested. If it fails the test I will try to find a shop in Maryland. Uncle Doug is very gifted and works on vintage tube guitar amplifiers, he has 117k subscribers. He also builds and designs amplifiers. I believe the two of you have a similar style. Thanks so much!

[@julian65886]

@@EriksElectronicsWorkbench Erik: I am happy to report I measured 0.8 mV and 0.9 mV at the amp speaker terminals using the most sensitive range in the voltmeter. My ordered a new one as my old one was a Radio Shack model that was very flimsy. My GFA55IIA amp in the back is identical to yours but does not say made in Japan. I read that at one point Adcom moved production overseas. Perhaps the USA made amp are of better quality. Not knocking the Japanese as they are obviously great manufacturers. Next I will open the amp to see if the caps are blown. I will not touch anything. Thanks again!!!!

[@f430ferrari5]

Not sure if the 555ii simply had more units sold than the Adcom 5500. I do see way more vids and repair vids on the 555.

[@EriksElectronicsWorkbench]

I'm not sure how the sales volume of the amps compare. The GFA-5500 is a MOSFET amp (not BJT like the 555), it uses a traditional amplifier design, but the way the MOSFET's handle heat, high currents, etc may play a part in the types of repairs and failures that occur.

[@f430ferrari5]

@@EriksElectronicsWorkbench I have the GFA-5500 since 1995. Never an issue. Yes it has the Mofset but not sure if it makes it better or not.

[@thompjo76]

Can you repair mine? I need to see if anything is wrong. Seems the input is very weak even with a good pre amp.

[@EriksElectronicsWorkbench]

Sorry I'm not taking in additional projects at this time as I have a backlog of projects waiting.

[@charlesclifton5896]

I have the Adcom GFA5500. The amp seems to function fine except when I power it off - I get a low pop on the right speaker and the actual cone of the speaker moves out when this occurs. Anyone have any idea if this is dangerous or not?

[@EriksElectronicsWorkbench]

It's normal for the GFA5500 to pop when powering off. There is no speaker protection relay so the speakers are connected to the output stage at all times. As the power supply rails collapse the amp becomes unstable and a short pulse of voltage is applied to the speakers. It is not damaging, and the voltage is less than what the speaker is subjected to when playing loud music.

[@charlesclifton5896]

@@EriksElectronicsWorkbench Thank you. Maybe I should clarify. The pop I referred to is more of a "whumph" (not loud but audible). The cone movement is quite noticeable and, as I mentioned, the event is only on one speaker. Your assessment is probably correct and there is nothing to worry about but I am going to sell this amp and do not want to pass on a faulty product. I'll see if there is a local expert on this amp. For now I am using a Schiit amp and I am very pleased with it. Much thanks!!!!!!

[@EriksElectronicsWorkbench]

@@charlesclifton5896 I would check the amp for meeting rated output power, no distortion issues, etc and if all looks good then the turn off thump is not a concern and for whatever reason the one channel is just more prone to it. Agree good idea to have it tested to be on the safe side.

[@carlespy7470]

Did you test the main power supply 15k uf capacitors in circuit? I don't have the Peak meter you have, but my Fluke meter shows each one (in circuit) is very close to 15k. Couldn't measure ESR, though.

[@EriksElectronicsWorkbench]

I tested them both ways, initially in circuit and later when wiring was disconnected, just to compare, I retested out of circuit. Both ways measured the same, very close to 15K uf. The Peak meter does a very good job measuring in circuit. The ESR was well within spec. Large filter caps on 60/120Hz ripple voltage are often reliable, they are not stressed as much as a filter caps in a switch mode supply with very high frequency ripple voltage.

[@RanTausi]

@@EriksElectronicsWorkbench A year ago I was lucky to buy GFA 555 MK2 rare ! New never used in closed original box , mine s/n start the same number 4.... I had just to set it to 220 V from 120V , and I have checked Bias and DC offset was within the specs. Thanks for your great video. ADCOM the best ❤

[@EriksElectronicsWorkbench]

@@RanTausi Nice find being new in the box! Glad you liked the video.

[@RanTausi]

@@EriksElectronicsWorkbench I also own GFA 545 MK2 which is a fantastic amp by himself some say it is sounds sweeter then the 555, I have GFP 565 pre amp, but lately I put it a side and I am using Fosi audio tube P1 preamplifier and it sounds awesome 👍.

[@carlespy7470]

@@EriksElectronicsWorkbench Erik - thanks for your reply. I am hoping to cure a low level but audible hum that has developed in both channels. So I suspected the power supply's large capacitors. Also, it's going to be hard to get them out of the chassis.

[@najzain]

I heard these amps do no have speaker protection. What is the best way to protect the speakers, playing them at low volume?

[@EriksElectronicsWorkbench]

It is correct that there is no protection circuit. The danger is having an amplifier fault that creates a DC voltage at the speaker terminals (DC offset). That is what my amplifier was doing. This will cause a high current to flow through the speaker and burn out the voice coil. You would need to add a speaker protection relay and monitor circuit to each speaker output. These are commercially available and could be installed inside the amplifier.

[@najzain]

@@EriksElectronicsWorkbench thank you for the quick response! Also, are these amplifier Class A or Class A/B?

[@EriksElectronicsWorkbench]

@@najzain You're welcome and it is class AB.

[@scotthullinger4684]

@@EriksElectronicsWorkbench - What is the real likelihood of having a problem if I don't have one of those protection circuits? Is it mostly to prevent problems from any stupid things I might do? Or is is a common problem which otherwise actually needs to be addressed?

[@EriksElectronicsWorkbench]

@@scotthullinger4684 that is a difficult question to answer because components can fail even if the amp is not being overworked. There is always that small possibility of failure. But that chance is slight in my opinion. The amp in my video has seen regular use since the video was made and had no further issues. If you push the amp and it runs hot then the chance of failure is higher although the amp is well designed. So my opinion is if you don't abuse the amp you are not likely to have a failure and don't need the protection modifications. Consider the cost of your speakers too, the cost and time to add a protection circuit would be like buying insurance for the speakers. -Erik

[@reubenedwards5286]

Hi l do have 555 ii amp works good, how can I have it check out.

[@EriksElectronicsWorkbench]

Hi, some basic checks would be the bias setting on both channels and the full power output test both which I show in the video. Also testing the various capacitors which can be done in circuit with the tester I show in the video. That would give you an overall health check on the amps condition and performance.

[@RanTausi]

Can you please list the capacitor values you have changed ? thanks

[@EriksElectronicsWorkbench]

2x 4.7uf 50V changed to 4.7uf 100V, 2x 470uf 25V changed to 470uf 35V, 4x 47uf 160V changed to 82uf 200V, 4x 15000uf caps not changed. Replacements are high quality 105C rated by name brands like Nichicon, etc.

[@RanTausi]

@@EriksElectronicsWorkbench Dear Arik, In the short future I would like to have the C104/154 and C109/159 be replaced , for de soldering the parts from the input board I guess the board have to be removed for having the back better access. How do you removed the input board a side ?

[@EriksElectronicsWorkbench]

@@RanTausi You will need to desolder the wires on the RCA input connectors. Unplug two plugs that have three wires on them. Remove ground screw on bus strap and cut zip ties. Turn board sideways as far as possible and unsolder two wires on bottom side. Note positions, they must be reconnected exactly. Then board can be turn over completely for service.

[@Dave-lr2wo]

Where are you located? Are you a business? Thanks.

[@EriksElectronicsWorkbench]

Not a business, sorry not taking on projects for repair at this time.

[@schummiehugo]

nice amp. bat erik, i had a problem whit mi amp.. i dont no how match bias volt i need. the amp is a realisitc 32-9003. from the 80ste. ant i ripaer it, bat dont now the volt of it. can you help mi ( pls ) it runs now good, so now the setting bias. schematic is on internet.. if i send you te link??? is it possible ant look for a minit ? gr huug sorry for mi shitty englis 😐😐

[@EriksElectronicsWorkbench]

I'm not familiar with that amplifier's bias setting spec. However if you input a sine wave signal (1 KHz) and monitor the speaker output on an oscilloscope, turn the bias to zero, then increase until the crossover distortion is gone. (crossover distortion is where the sine wave crosses zero volts and has an offset above and below zero reference, you want a smooth transition from positive to negative) This will get you very close to the proper bias setting without setting too high of bias.

[@schummiehugo]

@@EriksElectronicsWorkbench thanks for react. yee hahahaha i dont haf the stuf for messer it. so that way a am ask somwone. somwere i reed, 20 mv ant som say 30 or 40 mv so i realy dont now i put it on 20mv. ant ist not hot. afther 20 a 30 min. ant if i turn the volume. ant ist running nice warm. ( not 2 hot bat little hot.) so now i look you vid, ant c 10mv.. so a reali dont now.. ( i am not a electr man. , i ripear a lot batt haf not the eqipments for it. ) bat thanks for reacting. gr