Will an Iron Shield Work?

J

Jamier

Member
I have a Cary SA 200.2. I have had it for many years but haven't used it much as it has a vicious 120 Hz buzz in both channels. Disconnecting the inputs does not help and it does not change with the volume control on a preamp. I thought about pairing it with a BP2, but it is not great candidate for that as it's input impedance is 22K. Despite the buzz, it is an otherwise nice sounding amp, runs very cool and is built like a brick Sh......., well, you know. I took it apart today to investigate the problem and found: The input jacks, both Single Ended and Balanced, are mounted on a PC Board which is located right above the IEC power inlet. There is not a switch to choose between input type, so both sets are simultaneously live. It is a simple board and the parallel traces from the input jacks pass right over the 120V input, about 2 inches above that power input. Is this the most likely source of my buzz?

Would a 3-sided Iron shield covering this board (above,below and on the side) help to shield this noise from the inputs. If so, what gauge steel should I use? I figure that I can screw it to the chassis to secure and ground it, but  it will take some effort to make. I included some photos.

Jamie









 

Attachments

  • IMG_7139.jpg
    IMG_7139.jpg
    1.8 MB · Views: 20
  • IMG_7140.jpg
    IMG_7140.jpg
    2.2 MB · Views: 16
  • IMG_7141.jpg
    IMG_7141.jpg
    1.7 MB · Views: 10
  • IMG_7142.jpg
    IMG_7142.jpg
    1.8 MB · Views: 11
22K is no big deal for a BeePre 2, especially fully upgraded.

If you had 60Hz hum, that might indicate a need for shielding.  The IEC power entry module is not a source of 120Hz noise.  120Hz buzz can come from things like loose earthing or worn out power supply filter caps. 

Have you contacted the manufacturer?  It looks like they still make this amp.  What speakers are you pairing this with?  If you are using huge horns, you might just be listening to the noise floor of the amp.
 
PB, thanks for the quick reply. The amp is less than 10 years old with low hours and has always had this noise. The speakers have been many but all are under 92db efficient. I checked the grounds and they all seem well attached, I checked continuity to the chassis where I could and they are connected. The buzz may have some hum in it as well (which might be 60Hz?). If the filter supply caps are the problem then they were bad from the start, but I kinda doubt it is that. The amp has a torroid the size of small birthday cake so maybe that board is picking up something from that?

Jamie
 
A toroid will also throw a 60Hz field.  It may be helpful to listen to a 120Hz and 60Hz sine wave tone to be 100% sure of what you're hearing.  Also, if you plug one of those 3 bulb outlet testers into your outlet, does it check out? 
 
Quick test, unscrew that input board and lift it out of the chassis to see if the noise changes.

If you need a shield i have had great results using steel plate ~1mm thick lined with MuMetal.  Search for Ultraperm 80.
 
PB, I tested the outlets a while back with one of those diagnostic plugs and the grounds were connected, but I will check again.The thing about those plugs is they don't tell you anything about the quality of your ground connection. I, know this house was originally not grounded (built in 1961) and a half-ass attempt to make "local" grounds to plumbing pipes was made. The plumbing is copper, but the service panel does not have a large, main grounding rod as one would prefer. So, essentially, the outlets are individually grounded, locally. I don't have noise problems with any other amps. My SIIs have some noise, but I'm pretty sure it is the filament noise that would be reduced by the DC filament upgrade.

Mark, I considered doing exactly what you suggest but that board has a ground connection to the chassis which will limit how far I can pull it out. Can I,safely, extend it with a clip lead for the purpose of this test? Another thing I noticed about the amp is that the signal inputs run from this input board to the R/L boards with shielded cable, but the shields do not appear to be connected at the R/L boards. I remember that when you sent me pictures of your F4 channel boards, your shields were connected at the board with the ground leads, so that was something that I made sure to do when I built mine. I get no noise from that amp. Maybe this is contributing to the problem?

Jamie
 
There are R and L signal cables leaving that input board.  With the amp running, take your needle nose pliers and carefully touch both contacts in one of those plugs to see if the noise changes behavior.  That will short the input to the amplifier and help determine whether the noises you're hearing are coming through the inputs. 

A loose outlet earth would create the kinds of problems you're experiencing.
 
PB, can’t I just disconnect one of the input wires, it’s terminated in an easily removable  plug?
Jamie
 
PB, I shorted the input at the input board, as you suggested, and the shorted side( the left side) still had the buzz. Then I completely disconnected it by pulling the plug out at the left channel board and the buzz was still present, as vicious as ever. I guess it’s not a problem with the unshielded input board.

The outputs to the binding posts are not twisted and now I wonder, even more about the unconnected shields on the input wires that run from the input board to the R/L channel boards.

It appears that the amp has separate power supplies that are built into the channel boards. Could this be part of the problem?

Jamie

Edit: If completely removing the input did not help, then it is not the unconnected shields. Also, I twisted the outputs with no improvement.
 
Solid state amps have gobs of feedback and generally don't have this kind of issue.  I would be looking more at your outlet earthing above all else.
 
PB, I tested it today with a test plug and it indicates that I have a ground connection. I removed the receptacle and inspected the ground wire and, to the extent that I can, have determined the earth connection to be good. Would covering the torrid completely with an Iron shield be of any value?

Jamie
 
No, covering it with an iron shield is only likely to destroy the resale value.  Like I have repeated over and over again, magnetically coupled hum is 60Hz and so far you have no indicated the presence of any 60Hz hum.

I would suggest taking the amp to a different location and testing it out on a completely different electrical system to see if it behaves differently. 
 
When making a shorting plug, do you absolutely need a resistor between the center pin and the ground? For a quick experiment can you just connect them?

Jamie
 
Yes, a half wave rectified supply will have predominately 60Hz noise.  I'm not sure I've ever seen a solid state amplifier with half wave rectification for anything. 
 
PB, I'm sure  this amp is full wave rectified. I spent quite a while listening to tone generator samples and I really can't say what I hear anymore. The amp noise sounds like a 60Hz tone, but it does have a slight buzzing character to it. It definitely doesn't sound like 120Hz. 120 has a higher pitch than what I hear from the amp. My ground is as good as it can be. I crawled under the house and traced the wire to a 1/2 inch copper pipe. Where that pipe enters the ground I can't say but I do know that there is a bonding strap on the exterior hose bib at the back of my house. I suppose all that could be meaningless if my water main is schedule 80 PVC, which it might be. But if that were the case why does my diagnostic plug indicate positive ground. I have a raised foundation so those pipes probably only ground at the main. Any way, I removed the toroid mounting bolt and tilted the toroid on it's side which allowed me to pull it back from the channel boards a bit. I didn't think this would solve it, but I figure if it is the transformer the noise would change, get better or maybe worse, some change to indicate that the toroid was the cause, but the noise was unchanged. So, now I wonder if it is power supply ripple. I know you said the ton of feedback in SS circuits generally prevents this, and I know I said it sounds like 60Hz, but it could be 120 Hz noise, I'm just not sure. I think I did mention that the Power Supply Caps are on the board with the audio circuit. I don't know if this is bad or not, but in the First Watt Clone builds the PS is completely separated from the channel boards.Those are my only SS experiences. Oh well, I'll have to put this aside for a while, it's Tax time.

Jamie
 
How about taking the amp elsewhere to see if it still makes the noise. The manufacturer is still in business, how about contacting them?
 
You must log in or register to reply here.

Similar threads

T
Buzz on left channel
2
Replies
20
Views
2K
Tabaia
T
O
Sublime amp buzz/whine/noise
Replies
3
Views
12K
Doc B
Doc B
D
PreBee2 “Tape” output
Replies
4
Views
96
Dzrout
D
pvannest
Resistance and power issues.
2
Replies
22
Views
240
Paul Birkeland
Paul Birkeland
M
"Ticking" interference [resolved]
Replies
4
Views
2K
mcrushing
M
Back
Top