Adcom GFA-5500 – missing resistors?

[Tim H]

A little help, please, from anyone familiar with the guts of an Adcom GFA-5500. I have been troubleshooting a buzz; I have a used model of the version that is convertible between 120VAC and 230VAC. While looking at the main PCB, I noticed a couple of blank spaces where it looked like resistors should go. I have the schematics, and sure enough - they show that R2 and R14, both 15Kohm 2W metal oxide resistors, belong in the blank spots on my board.

Seems straightforward, right? Just put in new reistors. But I found two pictures online - both on the website of Adcom's official parts distrubutor, Affiliated Electronics - in one picture, the same two resistors, R2 and R14, are missing - but in the other picture they're there (see links below). I've written to AE about it, but so far I haven't heard anything back.

What do you think? Is it too big a coincidence to think the same resistors could be missing from my board and one of the boards at AE by accident, and that it is not a design tweak or some circuitry update that was implemented at Adcom, but not documented?

What about the bigger question - if the fact the resistors are missing is not by design but rather is unintentional - could this be the cause of the buzz I started out troubleshooting?

Thanks!

Part with R2 and R14: www dot teve21 dot com/847-805/xc/xd/adcom/pcb-assy_adcom/xa/xb/gfa-5500-power-supply-assembly dot aspx

Part missing R2 and R14: www dot teve21 dot com/847-1618/xc/xd/adcom/pcb-assy_adcom/xa/xb/gfa-5500-power-supply-fuses dot aspx

 

[slipperybidness]

Can u post the schematic so we can actually see what those resistors do?

It's likely just a different revision. Or, that same board may be used for a couple different assemblies, some use those resistors and some dont. It's pretty common to see boards with unpopulated parts. That adds flexibility. Or it may be a carryover from a prototype that is no longer needed.

About that buzz.....did the amp ever work for u without the buzz? If it did, then the problem is almost certainly not those "missing" resistors.

Typically buzz/hum on a power amp is a grounding issue

 

[TLS Guy]

Those resistors are bleeder resistors. That is to say they are there to discharge the power supply caps to make servicing safe, so the caps discharge after switching off the power.

There are two power supplies for each channel, a +60/-60 volt supply and a +73/-73 supply for each channel.

R2 is the bleeder resistor for C1, C2 and C3 of the +ve leg of one +73/-73 volt power supply and R14 is the bleeder resistor for C15, C16 and C 17 of the +ve of the other +73/-73 volt supply.

The schematic I found shows bleeder resistors in all legs of all four power supplies. Why the bleeder resistors were left out of 2 of the eight legs and present in only six on some boards I have no idea.

It has nothing to do with your hum problem.

However, since you have the resistors I would feel inclined to install them, as I think it is good practice to bleed power supply caps.

Does this unit hum if just connected to the speakers and nothing else? If not connected to anything there is no hum, then the amp is fine and you have a ground loop.

 

[Tim H]

Thanks @slipperybidness and @TLS Guy,

Regarding the buzz:

- It has buzzed since I have owned it.
- The unit does not buzz if just connected to the speakers and nothing else. More on this:

- The minimal conditions required to produce the buzz are that both inputs be connected to a pre w/ RCA cables. The buzz is there even if the pre has nothing else connected - even if its power cord is removed. Does that tell you anything about a possible grounding issue?
- I have tried changing, one at a time: interconnects, pre, speakers, and AC power source - nothing makes a difference. If it is a grounding problem I would prefer to solve it intelligently rather than stick in a $200 Jensen ISO-MAX (though I am happy to do that if nothing else works).​

Thanks very much for all the good information!

 

[TLS Guy]

Thanks @slipperybidness and @TLS Guy,

Regarding the buzz:

- It has buzzed since I have owned it.
- The unit does not buzz if just connected to the speakers and nothing else. More on this:

- The minimal conditions required to produce the buzz are that both inputs be connected to a pre w/ RCA cables. The buzz is there even if the pre has nothing else connected - even if its power cord is removed. Does that tell you anything about a possible grounding issue?
- I have tried changing, one at a time: interconnects, pre, speakers, and AC power source - nothing makes a difference. If it is a grounding problem I would prefer to solve it intelligently rather than stick in a $200 Jensen ISO-MAX (though I am happy to do that if nothing else works).​

Thanks very much for all the good information!

This is a prevalent problem with pre/power amps.

The solution is simple and best practice. DO NOT GROUND THE POWER AMP, ONLY THE PREAMP.

You do this by getting a cheater plug to lift the ground from the power amp.

This is what you need.

That will solve the issue. I never ground power amps.

 

[highfigh]

A little help, please, from anyone familiar with the guts of an Adcom GFA-5500. I have been troubleshooting a buzz; I have a used model of the version that is convertible between 120VAC and 230VAC. While looking at the main PCB, I noticed a couple of blank spaces where it looked like resistors should go. I have the schematics, and sure enough - they show that R2 and R14, both 15Kohm 2W metal oxide resistors, belong in the blank spots on my board.

Seems straightforward, right? Just put in new reistors. But I found two pictures online - both on the website of Adcom's official parts distrubutor, Affiliated Electronics - in one picture, the same two resistors, R2 and R14, are missing - but in the other picture they're there (see links below). I've written to AE about it, but so far I haven't heard anything back.

What do you think? Is it too big a coincidence to think the same resistors could be missing from my board and one of the boards at AE by accident, and that it is not a design tweak or some circuitry update that was implemented at Adcom, but not documented?

What about the bigger question - if the fact the resistors are missing is not by design but rather is unintentional - could this be the cause of the buzz I started out troubleshooting?

Thanks!

Part with R2 and R14: www dot teve21 dot com/847-805/xc/xd/adcom/pcb-assy_adcom/xa/xb/gfa-5500-power-supply-assembly dot aspx

Part missing R2 and R14: www dot teve21 dot com/847-1618/xc/xd/adcom/pcb-assy_adcom/xa/xb/gfa-5500-power-supply-fuses dot aspx

At times, does this combination not buzz? Does it buzz during the day when the house's lights aren't turned on? If so, do you have dimmers on lights and lamps? Those can introduce noise to audio/video systems.

 

[Tim H]

At times, does this combination not buzz? Does it buzz during the day when the house's lights aren't turned on? If so, do you have dimmers on lights and lamps? Those can introduce noise to audio/video systems.

I haven't run careful tests, but to my memory the buzz has always been at the same level and is present at all times. It seems pretty constant, regardless of time of day. But you remind me that I should try making sure everything else on the same circuit branch is unplugged and/or disconnected (i.e. dimmers, etc.).

 

[TLS Guy]

I haven't run careful tests, but to my memory the buzz has always been at the same level and is present at all times. It seems pretty constant, regardless of time of day. But you remind me that I should try making sure everything else on the same circuit branch is unplugged and/or disconnected (i.e. dimmers, etc.).

Just lift the ground from the power amp. This will be the only thing that solves it. Remember the power amp will still be grounded by the shield from the preamp. Peter Walker of Quad never grounded his power amps. He had special IEC cord made, in which the ground cable was missing, so it looked as if he power amp was grounded when it wasn't!

A lot of pre/power combos, if not most, will hum if you ground the power amp, as this causes the perfect ground loop.

You do not need more testing and will only have freedom from the buzz if you lift the power amp ground.

 

[slipperybidness]

Agreed, bleeder resistors are a good idea!!!

They aren't exactly necessary unless you are working on it as an electronic tech. It's a safety feature to discharge the caps when the amp is shut off.

If you don't have bleeders, then you really want to manually bleed the caps charge before servicing, or installing permanent bleeders.

 

[Tim H]

This is a prevalent problem with pre/power amps.

The solution is simple and best practice. DO NOT GROUND THE POWER AMP, ONLY THE PREAMP.

You do this by getting a cheater plug to lift the ground from the power amp.

That will solve the issue. I never ground power amps.

I am strongly inclined to go with your recommendation. But I have read on other forums that one should "never-ever" use cheater plugs because they create a potential electrocution hazard, and a legal liability in the event something should go wrong (i.e. an internal fault in the power amp that involves the chassis).

Would grounding just the pre-amp, as you suggest, avoid a potentially lethal electrical hazard (as long as the pre and power amps remained connected by interconnects)? Could severing the ground within one of the RCA cables achieve the same interruption of the ground loop, without creating the same hazard?

Like I said I am inclined to follow your advice. But if I have to choose between creating a potentially lethal hazard (with kids, pets, etc. running around) including the chronic anxiety that would come with it, versus buying a $200 Jensen product - I'd happily spend the $200. On the other hand, if using a cheater plug on just the power amp is safe, then I am happy to go with that.

Thoughts?

Thanks!

P.S. I posted this before seeing your most recent post.

 

[TLS Guy]

I am strongly inclined to go with your recommendation. But I have read on other forums that one should "never-ever" use cheater plugs because they create a potential electrocution hazard, and a legal liability in the event something should go wrong (i.e. an internal fault in the power amp that involves the chassis).

Would grounding just the pre-amp, as you suggest, avoid a potentially lethal electrical hazard (as long as the pre and power amps remained connected by interconnects)? Could severing the ground within one of the RCA cables achieve the same interruption of the ground loop, without creating the same hazard?

Like I said I am inclined to follow your advice. But if I have to choose between creating a potentially lethal hazard (with kids, pets, etc. running around) including the chronic anxiety that would come with it, versus buying a $200 Jensen product - I'd happily spend the $200. On the other hand, if using a cheater plug on just the power amp is safe, then I am happy to go with that.

Thoughts?

Thanks!

P.S. I posted this before seeing your most recent post.

This is not a hazard, and yes the power amp is grounded and safe. You are not going to use a power amp unless it is connected to something. Pro amps especially older ones, often have a switch to lift the ground for just this situation.

I bet if you look in your amp, you will see the grounding wire from the AC plug and the ground plane, have less beef to the wire than the shields in the interconnects of the cables between your pre and pro.

Contrary to a lot of bunk on consumer forums and also ignorant journalists, this is NOT a safety hazard, and is in fact the correct sensible thing to do, and your only practical solution.

I have been doing this for over 50 years and never had a problem and I have spoken to many professional engineers who have never had, or seen a problem from doing this.

Very expensive amps can get away without this problem as they have massive ground planes.

The only way you can avoid this problem is to construct a massive professional star cluster grounding system for your equipment. I have constructed one, but I still keep my seven power amps grounded by the connecting cables. To do otherwise would require re engineering the ground planes on all the power amps.

It takes very little resistance between grounds to make a hum. Power amps are a particular problem because of the currents involved, and that makes them create greater potentials between grounds. The quick translation is that they are known for hum.

This picture shows the grade of copper wire required to create a star cluster ground. It is called star cluster as everything is grounded like the spokes of a wheel, to avoid looping grounds.

Another view.

My power amps are not grounded, either because the power cable has no ground, or by ground break.

With a rig like this only best practice will bring you peace and quiet.