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Old 9th January 2009, 08:18 PM   #11
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Quote:
Originally posted by chromenuts
It seemed as if the two amps had different caps being used in both the C9 and one of C1/C2 positions ( I wasn't able to confirm which of the C1/C2 caps as my abilities to decipher the schematic and compare it to the board are limited). I assumed which cap was the C9 with more certainty because it was the only cap that was supposed to have a 100uf 450v specification. That particular size cap was installed in only the amp that was working quietly in the position that I was able to narrow down to being either C8 or C9 from the schematic. The humming amp has a 47uf 450v cap installed in the same location. I'm not sure what affect this could have on that amp.

The swap that was done on the C1&C2 cap that I mentioned is in the quiet amp, although in this case the specification had been upped from the called for 22uf 450v to a 47uf 450v capacitor.
Using a 47uF cap at C9 (instead of the 100uF as shown on the schematic) will produce twice as much 120 Hz ripple on the B+ rail. This is undesirable - I'd suggest you replace C9 with the correct value cap, or maybe even a 120uF. The 450 volt rating on this cap is nice, but almost certainly not necessary. You won't ever see more than 300 volts DC out of that supply (transformer rating 210VAC times 1.414). I'd be tempted to use a 400 volt rated cap because they are smaller and less expensive. The Panasonic TS-UP series caps can be bought online from DigiKey. They come in a variety of sizes from tall/thin to short/fat. Maybe look at the P6143-ND or the P7415-ND. Check your circuit board to see what size will fit, and check the datasheet to see what sizes are available.

Using a larger cap at C1 or C2 (47uF instead of the 22uF shown on the schematic) would usually be a good thing. It should provide better filtering to the sensitive front end of the amp. On the other hand, if it is a poor quality cap(leaking DC) maybe it's making the problem worse. Or perhaps the original owner tried increasing that cap in an attempt to address the humming issue. Who knows? I might be tempted to order a replacement anyway, since you will likely be ordering a cap for C9. You can try sticking an ohmmeter across the leads for C1 and C2 while the amp is powered off and unplugged. You want to see a nice big DC resistance here - at least 100k ohms or more. I find it's easier to check this with an analog meter, since the cap will appear to be low ohms at first. Then the ohms will climb slowly as the cap charges up. Give it a minute or two before you take your final reading.
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Old 11th January 2009, 09:58 PM   #12
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Quote:
Originally posted by Ty_Bower
If it turns out not to be a ground loop, I wonder if it is residual hum coming off the power supply. I'm guessing there is about 1.15 volts of residual 120 Hz ripple on the B+. It'll mostly cancel out in the push/pull finals, and the front end looks like it has the supply reasonably well decoupled. How sensitive are your speakers?

Speakers have 92dB sensitivity and a 4 Ohm load.

From the looks of the board, you ought to be able to cut out R17 (the 100 ohm, 3w cemented resistor) without too much trouble. Replace it with a small cheap choke, like the Triad C-24X. It ought to reduce the 120 Hz ripple by nearly a factor of seven.
Interesting...I wish I already understood more about the circuitry and how this affects it. It sounds a little aggressive for me to try right now, and there seems to be more obvious things that are wrong to begin with, but I will keep this in mind...thanks.

Quote:
Originally posted by Ty_Bower


Using a 47uF cap at C9 (instead of the 100uF as shown on the schematic) will produce twice as much 120 Hz ripple on the B+ rail. This is undesirable - I'd suggest you replace C9 with the correct value cap, or maybe even a 120uF. The 450 volt rating on this cap is nice, but almost certainly not necessary. You won't ever see more than 300 volts DC out of that supply (transformer rating 210VAC times 1.414). I'd be tempted to use a 400 volt rated cap because they are smaller and less expensive. The Panasonic TS-UP series caps can be bought online from DigiKey. They come in a variety of sizes from tall/thin to short/fat. Maybe look at the P6143-ND or the P7415-ND. Check your circuit board to see what size will fit, and check the datasheet to see what sizes are available.

OK...this definitely is on the list of things to do...along with changing out any other incorrect components. Pretty simple, the specs call for it and its not there. Its good to read an explanation about how the change affects the circuit. I had looked into Parts Express as I have ordered from them before. They have the exact spec'd cap by Xicom for $5...no datasheet on their site though. Any preference for the Pansonic caps or the Xicoms?...or is it six for one, a half dozen the other? Also, do the dimensions of the leads and their spacing usually change along with the overall cap dimensions as they vary?...or are they standardized?

Using a larger cap at C1 or C2 (47uF instead of the 22uF shown on the schematic) would usually be a good thing. It should provide better filtering to the sensitive front end of the amp. On the other hand, if it is a poor quality cap(leaking DC) maybe it's making the problem worse. Or perhaps the original owner tried increasing that cap in an attempt to address the humming issue. Who knows? I might be tempted to order a replacement anyway, since you will likely be ordering a cap for C9. You can try sticking an ohmmeter across the leads for C1 and C2 while the amp is powered off and unplugged. You want to see a nice big DC resistance here - at least 100k ohms or more. I find it's easier to check this with an analog meter, since the cap will appear to be low ohms at first. Then the ohms will climb slowly as the cap charges up. Give it a minute or two before you take your final reading.
Noted...they are cheap...maybe I'll swap them all out for the 47uF caps. I also need something like a 1k 10 watt resistor to drain the caps according to a local so that its safe to work on these.

I still need to find a way to check for the proper polarity on the Auricaps that are already installed.
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Old 12th January 2009, 01:40 AM   #13
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Quote:
Originally posted by chromenuts
I still need to find a way to check for the proper polarity on the Auricaps that are already installed.
These capacitors are not polarized. The black lead indicates the outer foil. For absolute minimum noise pickup, it would typically get connected as the "input" to the cap. See paragraph five here. Also some reading here.

On your amp the black leads would be connected towards V2A. The red leads would go towards R10 and R11. For all practical purposes, I doubt it makes much difference which way they are installed. I wouldn't worry about switching them around, but feel free to do so if it makes you feel better.
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Old 12th January 2009, 03:31 PM   #14
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Quote:
Originally posted by Ty_Bower


These capacitors are not polarized. The black lead indicates the outer foil. For absolute minimum noise pickup, it would typically get connected as the "input" to the cap. See paragraph five here. Also some reading here.

On your amp the black leads would be connected towards V2A. The red leads would go towards R10 and R11. For all practical purposes, I doubt it makes much difference which way they are installed. I wouldn't worry about switching them around, but feel free to do so if it makes you feel better.
Sorry...this was a misuse of terminology...and just my way of expressing that I understood that the orientation of the leads was important to optimal installation. I had actually already read the information in those links, which is why I was preoccupied with the installation of these caps. My problem lay in the fact that I did not understand the circuit and how the caps should be installed, which you were kind enough to explain below. Thank you, yet again.
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Old 12th January 2009, 07:54 PM   #15
pchw is offline pchw  United States
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Have you rule in or rule possible ground loop? If not, I will short the input of the block that is at fault and connect only this block to the speaker. If it still hums, then you know the fault is within this block and can focus on how to trouble shoot. Eli's suggestion of DC blocker is also an easy and cheap way to eliminate another potential cause.
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Old 11th February 2009, 03:30 AM   #16
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Update:

I finally got some replacement caps in that I ordered from Digi. I replaced the C9 cap in the humming amp with a proper spec 100uF 450v Panasonic cap. I also had to re-solder the lead from the RCA jack on that amp, as the wire snapped off due to flipping the board in and out of the amp chassis so many times while I've been working on it. I also found a sloppy soldering job on one of the Auricaps.

Unfortunately, my humming is not gone. I'm pretty disappointed to say the least. I feel like I'm stabbing in the dark here now. I actually purchased enough caps to replace most of them in both amps if need be. I don't see the point if I don't have the ability to perform any diagnostics that would indicate that course of action. Any suggestions?
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Old 11th February 2009, 04:04 AM   #17
chrish is offline chrish  Australia
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From the sound of it you have a ground loop. If this is the case (and it most probably is) then changing caps and polarity of coupling caps is going to make sod all difference. You need to find the source of the loop. There have been some suggestions above regarding trying to isolate from other equipment by shorting out inputs and then reporting back...
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Old 11th February 2009, 04:43 AM   #18
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Quote:
Originally posted by chrish
From the sound of it you have a ground loop. If this is the case (and it most probably is) then changing caps and polarity of coupling caps is going to make sod all difference. You need to find the source of the loop. There have been some suggestions above regarding trying to isolate from other equipment by shorting out inputs and then reporting back...
There is no other equipment connected to the amp. It is simply plugged in and connected to the speaker. When powered up, it hums.

So far the cap that was replaced was changed because it didn't meet specification, and there was speculation as to that being a possible reason for the humming.

If someone could explain to me the proper way to "lift the ground" and/or "short the input" I would appreciate it.
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Old 11th February 2009, 06:20 AM   #19
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Did your cap replacements include C1 and C2? If not, I would replace these next. Also, check that the resistors between the ends of the heater winding and ground are ok. If these are missing or open circuit you will have floating heaters which can cause odd hum problems.
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Old 11th February 2009, 09:03 AM   #20
tmhajw is offline tmhajw  United Kingdom
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As I understand it, lifting the earth in your monoblocs will encourage the signal grounds to flow back to your preamp, as they should.
The simplest way to do this is to locate your earth wire on the immediate inside end of your power supply socket, cut it & insert a 47 ohm resistor. This should incidentally present little problem for chassis earthing should any catastrophe occur & it ever go live.
Try it and see. You can easily reverse it....
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