Bottlehead Forum
General Category => Technical topics => Topic started by: Jamier on February 02, 2021, 05:17:02 PM
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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
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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.
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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
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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?
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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.
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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
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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.
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PB, can’t I just disconnect one of the input wires, it’s terminated in an easily removable plug?
Jamie
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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.
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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.
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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
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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.
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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
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Jamie, i connect them, never had an issue.
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Aaron, thank you for that. Next question: how do you short a balanced input, or can you, safely?
Jamie
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To short an XLR input, connect all the pins together.
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PB, would PS ripple from a half-wave rectified PS be 60Hz? And from a full wave, 120Hz?
Jamie
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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.
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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
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How about taking the amp elsewhere to see if it still makes the noise. The manufacturer is still in business, how about contacting them?
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PB, I did contact Cary about this. They are making a new version of this amp. They offered to install a new version of the input board and make some other, unspecified, upgrades. They wouldn’t confirm a definite cost but the estimate started at $450 plus $125/hr labor. When I calculate shipping for a 75 pound amp both ways, it starts to look pretty pricey. I’m thinking it gets close to the price of the BP2 I want, so, honestly, I think the BP2 is a better place to spend the money. I do have other amps, too many other amps. I will take the amp to another location at some point, but it’s just not convenient at this time. Also, I really don’t think that the input board is the problem, but when I questioned the Cary Tech about that, he inferred that the other, unspecified upgrades, would take care of that. I don’t feel good about the prospect of spending a grand to get the amp back with the same noise it left with.
Jamie
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Yeah, that is a tough choice to make. Shipping monster amps is very pricey. Heck, I get a lot of inquiries about fixing Foreplay I/II preamps, and I always warn that the shipping costs will be more than the cost of the preamp.
If Cary said they have a fix for the noise you're getting, then I wouldn't worry too much about doing anything else on your end.
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PB, My gut feeling is that the noise is the result of a fundamental flaw in the design of the amp. It had this noise 5 years ago when I lived in another local where I had a confirmed positive earth ground. I think they have probably redesigned the amp to address this problem and now they are willing to upgrade my older version but they won't tell me exactly how they will accomplish this. If I knew, with certainty, that they could correct the problem, if they gave more detail on the unspecified upgrades, I might consider sending it, but without something to base that certainty on, I just can't pull the trigger on that.I guess, for me anyway, when someone tells you they will replace something that doesn't seem to be a problem, and then they are going to do some other stuff, but they won't tell you what that stuff is, it doesn't inspire confidence.
Jamie
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Test it somewhere with known good earthing. Someone elses modern house, anywhere. Yes you want it to work in your house but tick off this box first. Did You buy it new? You say it has been like it since new. If so it should have been sorted under warranty. I note that you have talked to them. They will add a $50 card for $450 plus.............
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I bought this amp about 8 or 9 years ago. At that time, I thought the noise was due to my sources and that I would sort that out in time. Shortly after the purchase, I began preparations to move, so I boxed it for the move and all these years later, I pulled it out to re-evaluate it as a possible mate for a BP2. I thought the input impedance was a little low, but PB assures me that it will be OK, especially with the upgrades. So, yes, if I had, had more sense at the time, I would have pursued this then. I should have opened it up and pulled the inputs out of the channel boards at that time to diagnose it, but I just didn’t take the time to do it.This is the only big class AB amp I own and for some speakers you just need that power, so I would like to get that noise out. The cost of the upgrades isn’t so much the issue, but, as I mentioned before, the shipping will actually be much more, and the total cost is just not worth it to me at this point.I will try a few other things before I give up on it.
Jamie