Buzzing power amp (Sherwood AM-8500) [UPDATE: Now measuring]

[Kaktus_Kontrafaktus]

yes, just glue.

You need to check the DC power supply voltages with an oscilloscope.
Also the derived voltages (need schematic) need checking.

Likely candidates: big capacitors and regulator circuits. Maybe the bridge rectifiers ?

Not happening, I'm going to have to find a professional for that.
Which brings me to another question: How much does it make sense to spend on repairs?

 

[solderdude]

How much does it make sense to spend on repairs?

That's hard to say. When it would be completely silent after repairs I would not go above $ 200.- (revised)
A decent amp with similar output power will likely cost more.

I figure a repairguy would be busy for quite a few hours. Labour is expensive unless you can find someone like like @restorer-john interested in doing a revision. Say someone named Restaurator-Günther oder so etwas.

 

[Kaktus_Kontrafaktus]

Would you expect whatever's causing the buzzing to do more damage over time?

The buzzing annoys me as a matter of principle but it has little impact on my actual listening experience.
I'd take my time finding Günther...

 

[PaulD]

The only other thing I can think of is that the amp's power transformer is humming because of DC in the AC line (aka dirty AC from your local grid). In that case a DC blocker will do the trick.

DC on the mains is the easiest to problem to eliminate and quickest to test for. It's caused me problems more than once in some locations.
https://sound-au.com/articles/xfmr-dc.htm

 

[Kaktus_Kontrafaktus]

DC on the mains is the easiest to problem to eliminate and quickest to test for. It's caused me problems more than once in some locations.
https://sound-au.com/articles/xfmr-dc.htm

I guess I could pick up this thing and sell it on if it doesn't make a difference:
https://www.ebay.com/itm/312850024724

 

[solderdude]

I guess I could pick up this thing and sell it on if it doesn't make a difference:

That device could only help if the hum is coming from the transformer inside the amplifier AND you are plagued with DC on the mains.
In that case more than one device in the house may hum audibly from the device itself.

It will do absolutely nothing when hum is from the speakers.

When the amp is connected to speakers only and nothing else and the hum is constant regardless of the volume control position it is most likely the power supply smoothing caps. When it is silent (or very low hum with ears to the speakers) then it is nothing to worry about.
When the hum varies (again only speakers attached) with the volume control it may be open inputs that pick something up or a regulator inside that may not function right.
Devices of this age are likely to need some capacitors replaced (dried out).
When there is only hum when other devices are connected (say a DAC via a PC) then chances are it is a ground-loop.

 

[Kaktus_Kontrafaktus]

That device could only help if the hum is coming from the transformer inside the amplifier AND you are plagued with DC on the mains.
In that case more than one device in the house may hum audibly from the device itself.

It will do absolutely nothing when hum is from the speakers.

It's coming from both.
Right above the amp: https://voca.ro/379C4hw5pav
Tweeter: https://voca.ro/8z0gKiUfhDU
Midrange driver: https://voca.ro/aGbvjNhFFCO
(all with no input connected)

The only other audibly humming device is the RCD/GFCI in the bathroom.

When the amp is connected to speakers only and nothing else and the hum is constant regardless of the volume control position it is most likely the power supply smoothing caps.

When the hum varies (again only speakers attached) with the volume control it may be open inputs that pick something up or a regulator inside that may not function right.

It's a power amp, there is no volume control.

When it is silent (or very low hum with ears to the speakers) then it is nothing to worry about.

I can hear the noise from the tweeters from 2-3m away (when the room is quiet enough).

When there is only hum when other devices are connected (say a DAC via a PC) then chances are it is a ground-loop.

The hum is there without anything connected, disconnecting all other devices in the flat or disconnecting the amp from ground doesn't change anything either.

 

[restorer-john]

Sorry for not weighing in earlier, I must have missed the thread.

When the amp is connected to speakers only and nothing else and the hum is constant regardless of the volume control position it is most likely the power supply smoothing caps.

This is highly unlikely in this amplifier and should be discounted in my opinion if we are talking main HV rail capacitors.

It is a dual channel, dual transformer unit with entirely separate supplies for each channel's power stage. The likelihood of both sets of power supply filter caps causing the same issue at the same time is extremely unlikely. People love to blame capacitors and replace them en-masse, only to suffer the same issues after replacement. Golden rule in repair is to fix the problem first, before throwing components at it, shotgun style.

Dual transformer amplifiers (on the same chassis) often have hum issues. Many can be traced to circulating eddy currents in the chassis, poor mounting and electromagnetic interference between the transformers. Poor earthing practices are the biggest issue, along with cable routing and inadequate regulation/decoupling in the VAS stages.

The front end buffering for both channels is derived from a single regulated +/- supply using 78/9xx regulators. The filtering up front is only 470uF per rail, which is at the lower end of what I would expect for a ripple free supply. That said, it may be sufficient.

I would be shutting one channel down completely as a first step. Basically, pull the primary fuse for the right channel and fire it up. (Should be a rear panel pair marked left and right @7A or so) Report back with your findings. Is the hum still there? You cannot pull the fuse for the left channel as it also runs the front end PSU for both channels.

 

[Kaktus_Kontrafaktus]

I would be shutting one channel down completely as a first step. Basically, pull the primary fuse for the right channel and fire it up. (Should be a rear panel pair marked left and right @7A or so) Report back with your findings. Is the hum still there? You cannot pull the fuse for the left channel as it also runs the front end PSU for both channels.

Pulling the "R" fuse doesn't change anything. CORRECTION: The hum on the right channel disappears after a while.

When I pull the "L" fuse, the amp keeps humming (though the LEDs don't light up) but the speakers are silent.

 

[Kaktus_Kontrafaktus]

I found this in another forum:

Check the solder points at the RCA terminal preamp input boards, I have a hunch you will find them loose or disjointed. I have repaired many SW and New Castle receivers/ amps with this problem. The latest was an AM 9080 with three preamp input boards with broken/loose solder points at the RCA input very bad buzz hum. I re soldered the boards did a resistance check and now the client has a new amp.

https://www.avsforum.com/forum/90-receivers-amps-processors/1152555-sherwood-amp-buzzing.html

Would that be something worth checking? The input board is fairly easy to access.

 

[solderdude]

Mechanical hum from the amp itself may be hard to lower.

With the RCA input of the amp shorted does the hum become less ?
When the shield of the RCA loose I would expect a very loud hum when another device would be connected.

Any reports of other owners (there won't be that many ?) reporting hum ?
It seems to me that if this amp did this also when new it would have gotten poor reviews and people would complain about it.

 

[Kaktus_Kontrafaktus]

With the RCA input of the amp shorted does the hum become less ?
When the shield of the RCA loose I would expect a very loud hum when another device would be connected.

Connecting an input device doesn't change the hum in any way, I'll try shorting the inputs and report back.

Any reports of other owners (there won't be that many ?) reporting hum ?
It seems to me that if this amp did this also when new it would have gotten poor reviews and people would complain about it.

I couldn't find any similar complaints or reviews for that matter. Best I could find is the ranking of the german magazine Stereoplay:
https://www.docdroid.net/Upd8Taf/1349855217.pdf#page=8 (the power amp ranking is page 7-9)

They put the AM-8500 in the same class as e.g. these:
Yamaha MX-70 & MX-1000, Pioneer M-73, Onkyo M-5890, Denon POA-5000, Kenwood L-1000M

 

[Kaktus_Kontrafaktus]

No change with shorted inputs.

 

[solderdude]

As the power supply of the left channel seems to be doing something for both channels (don't have the schematics, John does it seems) maybe there is a common factor there that could induce a high-harmonics hum in both channels.
When it is something like wire routing or another structural thing then most likely it has always been there from day one.
May have been masked by the previous owner using inefficient speakers for instance.

 

[Kaktus_Kontrafaktus]

(don't have the schematics, John does it seems)

 

[solderdude]

I would suspect R501 and R502
These are fusible resistors that have a tendency to increase resistance.
This could be checked by measuring the voltages on CNT11.
When one of these voltages drops below 20V this could directly cause hum via the DC servo loop or the input opamps (via the power rail).

This is the obvious common path in case it did not hum when new.

 

[Kaktus_Kontrafaktus]

I would suspect R501 and R502
These are fusible resistors that have a tendency to increase resistance.
This could be checked by measuring the voltages on CNT11.
When one of these voltages drops below 20V this could directly cause hum via the DC servo loop or the input opamps (via the power rail).

This is the obvious common path in case it did not hum when new.

Finally, a task I'm not totally underqualified for, though it seems I'm going to have to pick up a new multimeter...

New set of pictures, just in case they're useful for anything:

 

[Kaktus_Kontrafaktus]

Here's the view from the bottom:

 

[solderdude]

Below the 2 resistors in question (green body orange, orange, gold, gold) you see a brown connector with a red and a blue wire.... don't cut the red one nor the blue one.... even though in movies you have to.

When you have a multimeter you must measure the DC voltage between the red wire and black wire.
Should be around +27V.
Between the blue and black wire there should be around -27V.

 

[Kaktus_Kontrafaktus]

Below the 2 resistors in question you see the red and the blue wire.... don't cut the red one nor the blue one....

When you have a multimeter you must measure the DC voltage between the red wire and black wire.
Should be around +27V.
Between the blue and black wire there should be around -27V.

Don't worry, I'm not quite that daft.
This DMM should do for this sort of thing, right?
https://www.amazon.com//dp/B00KXX2OYY/