DR #55 - Restoring the Bass on an Adcom GFA 5500 Power Amplifier
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- เผยแพร่เมื่อ 6 ก.ค. 2024
- This Adcom GFA 5500 is suffering from a common problem: weak bass. This video documents how I restore the bass, as well as the theory behind why it was lost.
- วิทยาศาสตร์และเทคโนโลยี
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Can’t believe this randomly showed up in my feed! I acquired a GFA5500 (along with a Krell KSA 200) in a trade for a turntable I had restored. Tge deal was actually for the Krell, but he threw in the 5500 because he was clearing out some space. Neither unit was in working order. Figured I would take a look at the 5500 first, as it’s a bit less intimidating than the Krell…. Powered it up, no speakers just to do some quick measurements. No smoke - that was good :) But, I was seeing the full rail voltage on the speaker terminals. I poked about a bit, got some help on DIYAudio trying to figure out what the issue was, but no luck. So, I sent it out to a tech, and they didn’t want to repair it (Also sent them a Marantz 7T that I had done a full recap on, which was working wonderfully until I was trying to run a sub off the center channel, and right channel went pop - but that’s another story) All that to say, tech either couldn’t or didn’t want to fix that either.
I was informed that all the semis in the 5500 were bad. They also said there was a 16A automotive fuse in the 5500.
I am subscribed and excited to watch the rest of the videos in this series. The 5500 is supposed to be a very capable amp!
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If you add flux to your de-soldering wick, it pulls the solder easier, and leaves a clean pad. Great video.
It's always a treat when a new Duality video appears in my subscription feed!
Wow, brilliant repair!
Thank you Mike!!
Great video! I worked over my 5500 a few years ago. I replaced all the electrolytics on the amp boards with Elna SILMIC IIs though I think they've been discontinued. Also replaced the polyester caps with polypropylene units. Also replaced all eight of the 1000 uF electrolytics in the power supply as they were bulging. The amp sounds amazing!
Nice walkthrough and demonstration of the problem, along with solution.
Thanks!
great ad fine job
Cool! I have this amp and its 3-channel brother used perhaps 100 hours total. I bought them new and have been in the box since. I should sell.
Cool repair. Nice amp. Enough power and very high damping. 700!
Great job.. I did the same repair on my GFA 5500 a while ago. Turned out to be one my favorite power amp.. Now I got two more...
I sure hope I have time left in my life to able to make repairs like you do. Currently, I have 3 NAD amps and a NAD CD player that need TLC. I also have my late mother's 1980's Fisher system I would like to get going again. It makes a loud thump through the speakers and cuts off the output. Must be going into protection mode. Been like that since the mid 90's. Haven't turned it on since.
Very Informative video. You must be from Texas your arm shows a map of the state.
Minnesota actually. Thanks for the comment.
Excellent explanations of the problem
Thank you.
Magnepans made in Minnesota 😊
Great tutorial 👌 thx for sharing 👍
Great video. I also love the little Dynascan era BK scopes. Especially the round crt models like the 1470. Sexy! See if you can score a little 1405. So fun!
Thanks. The 1405 does look neat.
Awesome!!!! I’m glad this showed up on my feed. Liked and Subscribed!!! Great job sir!!!
Thank you very much.
Very nice video!!!
Great cinematography!
And very good explanation of the issue!
👏👏👏😃👍💥💥💥💥💥
Thank you. I appreciate the comment.
Excellent Video 👍
Great video! Very educational! What a beast of an amp! Coke can electrolytic filter caps. Geesh.
Thank you very muck Nick!!
Glad you enjoyed it Bob.
TH-cam shoved this video in my face but i didn't want to look at amp videos yet
Next minute, i end up with an amp with no bass. Had to try and remember the name lol
Wow what timing
Many thanks for this video, 5500 is a great amp. I have the GFA555 and the 545 both MK2. Please make more ADCOM stuff videos ❤ Thanks for sharing
I hope to find more Adcom gear to work on in the future.
@@dualityrepair4770I've got a GFA-5400 I can send you! When powered on the right channel sends a really bad crackling sound to the speaker, that is until it warms up. I have no clue what's going on with it.
Ahh interesting. Send me an email and we can discuss further. nick@duality-er.com
@@dualityrepair4770 E-mail sent.
Wow, seeing that the capacitor that went defective isn't subjected to a life of high ripple current or voltages near its rating, I am surprised that it failed in the way it did. I am thinking there was a very poor selection of quality when purchasing. Nice repair presentation!
that capacitor was subject to ac - the amp's output - thats the reason for the early failure. in other words, poor selection of parts by adcom
Because it is a low gain power amp stage (Av=6), the peak AC voltage of the output divided by 6 is far greater than is smart to reverse voltage a polarized electrolytic. He should have replaced it with a nonpolar cap - it will just fail again. Most amplifiers have gains around 20-30, the problem rarely happens at normal listening levels - you might get peak reverse voltages high enough to damage an electrolytic. Stupid design error trying to replace high Gm bipolar transistors with low Gm FET's. Audiofoolery at it's finest!
@@jim9930 Did you take into account that, there is a 49.9K Ohm ( lets' call it a 50K for all intents and purposes) resistor between the amps output, and the 47uF cap? With that cap being loaded to ground with a 10K resistor, that would only give it a max of 5 volts AC across it at a 250 watt - full output of the amp. That cap will also have a DC bias on it, seeing that it is connected to the gate on the negative feedback differential IRFD210, but the swing of a max AC signal would be less than that AC signal, and that AC voltage across the cap should be less than 6 volts, creating a current through that cap of less than 2mA according to a SPICE modeling circuit I made @ everycircuit. IMO, it was more of a poor quality cap being used than the circuit it was applied in.
@@poormanselectronicsbench2021 Somewhere else in the comments I explained it would have been confetti if it wasn't current limited by the 50k. Electrolytics act like a lousy diode when reverse biased - they can take some abuse. Think of it as a dartboard with a piece of paper; keep punching tiny holes in the electrolyte (hot spots) till it fails. X and Y caps (polypropylene) fail the same way from voltage spikes on the ac line.
@@poormanselectronicsbench2021 The source of the D210's will have a negative bias , gates will be at DC offset of the amplifier. (zero) High AC line, peak ac output divided by 6 could be minus 10 volts worst case - more than enough to slowly 'cook' the electrolyte - 2 or 3 volts can do it.
I would look at using a Tantalum capacitor in that application as it will last a lot longer.
When I checked the calculations, I came up with 26.5Mohms for the 0.3nF cap at 20hz. So NFB attenuation was even worse.
Yes that’s correct for the 0.3nF cap alone. I was calculating based off both caps in parallel (0.2uF + 0.3nF).
Your video is very useful and gave me the notion that the ayre mxr i owned has a similar issue. I always felt that the ayre lacks bass weight and resembles little to the reviews i have read. Would you think the ayre would have something like a "c3" lurking in the unit that is hindering the bass performance. Your insights is appreciated. Thanks
Yes it could very well have a “C3.” Unfortunately I can’t find a schematic anywhere to help point you at a specific component or area.
No worries...when I find a schematic I will come consult you again. Thanks
Random question, how difficult would it be to convert this to a dual monoblock? Is there space for a second toroidal there, or is there a smaller option available which can be placed side by side? Just curious.
Why did Adcom use an electrolytic c3in the signal chain?
Thank you, very useful information. I'm puzzled why the same capacitor failed on both channels. Design issue? Component quality, or sizing issue?
Good question. I was thinking component quality. The ratings are appropriate and the design seems solid.
Age and heat IMO. These amp boards run very hot, as you can see by how browned the PCB fiberglass is.
This cap is located directly above the input LPT MOSFETs, which run super hot.
Usually there is a copper bar glued across the tops of the input MOSFETs, that thermally bonds them so they track each other better, and it dissipates some heat. This amp doesn't seem to have the bar intact, or it was not included in this version. They do fall off! What I do is use thermal heatsink epoxy to attach a DIP-16 aluminum heatsink across the two input MOSFETs, and that dissipates heat better while also thermally bonding them. -Chris Hoppe of Hoppe's Brain.
Also the heat from the input MOSFETs tends to cause their solder joints to crack, causing a crackling sound. They just need a solder touch-up.
Heat, actually. This board runs really hot, as you can see by the browning of the PCB fiberglass. The input LTP MOSFETs in this design run especially hot, and are located directly underneath this poor capacitor. There is normally a copper heat-spreader bar glued to the tops of the transistors, which keeps them tracking thermally, and also helps with heatsinking. This amp is missing the copper bar, so it either fell off or was not included in this revision.
What I do, is attach a DIP-16 heatsink to the tops of the transistors with thermal heatsink epoxy. This dissipates heat better, as well as providing additional cooling.
Also, the heat causes cracks in the solder joints from thermal cycling, so I recommend re-touching those solder joints.
Chris Hoppe - Hoppe's Brain
Hi, I've got a similar issue on my Mission Cyrus one. Did you measure the output voltage under a resistive load?
Yes, the amp was connected to 8 ohm - 500 watt resistors.
@@dualityrepair4770 Thanks!
I am a bit confused by the feedback resistor being so high at 10K ohm wirmth the other 50K ohm that would make the overall gain only X 6 or have i missed something.
Good eye.. and you're right.. a gain of 6 is ridiculous. It turns out that the schematic isn't completely correct. The feedback resistor is indeed a 49.9K, but the resistor to ground is not 10K.. it's 1.82K, making the gain ~28. Thanks for bringing this up.
The old schematic documentatio error problem .
That is why we hone our fault finding skills to give us the tools to overcome these obstacles.
Dealing with lack of or conflicting information.
Maybe that would be a good subject for one of your future videos. @dualityrepair4770
Any tips for the GPT-500?
I’m not familiar with that model. Does it have issues or are you just planning on maintenance?
@@dualityrepair4770 it's a matching Adcom pre-amp / tuner. I just think it sounds a bit meh.. May work on it some day.
Its easy to know why caps failed like that, its caused by heat you can see discolored pcbs in almost all Nelson, Adcom and Nakamichi top end amplifiers he likes to design time bombs deliverately, that amplifier also has a small heatsink in the differential pair that almost all the times (have seen that small metal thing missing in almost all of that model amplifier) and if it didnt went missing it will short ccircuit some transistor or something after the glue fail under the heat...... the same heat that caused the video capacitor fail 😅
This isn't Nelson Pass's fault, he only provides the circuit design. Adcom did all the actual board design, including decisions on heatsinking.
O.K. but why did the elcaps went bad? Strange, but it is as it is of course.
Both channels had the same cap fail in the same way, from age or something else?
I believe they failed in the nearly the same way and at nearly the same time. The caps were probably from the same manufacturing batch, so suffered from the same issue. And since both channels would always be running together, each cap would be subject to the same amount of use.
My other thought is that one channel did fail before the other, but it wasn’t noticeable to the owner until the other channel failed too.
@@dualityrepair4770 I was an Adcom dealer back in the 90's. The GFA-565 and '5500 amps had issues due to bad caps. It wasn't Adcoms fault, the cap manufacturer had a bad run of caps. Sometimes the amps failed under warranty, and on some they ran fine for several years before they failed. Nice video!
@@dualityrepair4770 POOR design - you don't reverse voltage an electrolytic by more than a volt or so - had there been enough current, you'd be picking fuzz out of the box after the cap exploded. Peak AC voltage on that cap under worst case could be 10 volts - dumb oversight by engineering or mistake in parts procurement.
@@jim9930 Seems suspicious in most cases it might have been by design. I've seen computer CRT monitors fail back in the day, just days if not weeks after reaching the limited warranty. The failure was due to the power supply capacitors running at the cap-rated voltage. In most cases, caps should have been rated double the application voltage.
Has adcom ever been good?!
My Adcom GFA 5500 is nearly 30 years old. Works great and sounds great. 👍
From experience. I hate Adcom. I never like their sound. But then again I don't like half the amplifier's I went through other than , Spendor class a amplifier and Mackintosh 252
Then WHY are you here??? Weird
very good video. thank you for the explanation!!!☺
Perfect👍