Please help me in getting rid of the buzz. - diyAudio
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Old 13th May 2008, 11:18 PM   #1
rdk845 is offline rdk845  United States
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Default Please help me in getting rid of the buzz.

I have a buzzing Dared VP-845 amp and did bunch of tests as described below. These did not change the buzz at all.

Any help in nailing down the cause of the buzz would be greatly appreciated. I'm particularly interested in diagnosing the power and interstage tranny.

To be clear, the buzz of concern sounds like transformer buzz but it comes through speaker, and large enough to bother listening music. My power transformer gives mechanical buzz, but I donít hear that when music is playing. It is NOT a hum that I can hear when I rotate the hum pot out of optimal position.

Here is what I did so far, all of which had no influence to the buzz.

1. Tried different 12AX, 6l6g and 845s

2. 845 bias currents adjusted.

3. Buzz is unchanged whether source is connected or not. Shorting the input signal to the ground didnít change the buzz.

4. Tried to shield the input wiring.

5. Measured the DC and AC on the various stage of amp as recommended here to make sure they are low enough not to cause problems.

6. Padded one of the interstage tranny with silicon gasket since it vibrates a bit.

7. Shorted the grid of 6l6g and saw whether buzz is coming from tube stage before. No change.

8. Bypassed all of the caps in power section. No change in the buzz

9. Replaced the bridge rectifiers for 845 heater power supply.

For what itís worth, the buzz does seem to change a bit over time as I turn on and off.
Thanks!

-John

Below is the main section schematic.
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File Type: jpg vp-845 main.jpg (76.3 KB, 512 views)
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Old 13th May 2008, 11:19 PM   #2
rdk845 is offline rdk845  United States
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Default power section schematic

Here is the power section schematic. This is shared PSU, not mono block.
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Old 14th May 2008, 12:11 AM   #3
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What happens if you disconnect the pentode end of that .22uF?
Does the buzz still occur, or does it come from the earlier stage?
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Old 14th May 2008, 12:47 AM   #4
rdk845 is offline rdk845  United States
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I didn't try to disconnect the coupling capacitor but I did ground the grid of 6l6g (no 7 of the first post), and buzz didn't change. Does this accomplish same thing as disconnecting coupling cap test wise?
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Old 14th May 2008, 03:22 AM   #5
korey is offline korey  United States
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Did you check your power filter capacitors??
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Old 14th May 2008, 05:23 AM   #6
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When you say you bypassed the cap,I assume you bridged them with know good ones.Im talking about the ones in the power supply.this means all caps.the hum could be from grid bias hum,high voltage hum or heater hum.Im assuming its also coming from both channels.If you missed bridging any now is the time to try this.
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Old 14th May 2008, 11:35 AM   #7
rdk845 is offline rdk845  United States
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By bypassing, I meant that soldering + and - lead of known good caps to the circuit board traces connecting to + and - lead of power filter caps, respectively. And yes I tried this for all caps listed in the power section of schematic.

Yes, the buzz is coming from both channels, but buzz from each channels sounds a littl bit different in their pitch from each other. Some time left side is more prominent, some times not.

By the way, at one point in time, I used a 845 for which the plate was glowing a little. The tube might have been defective or its power dissipatoin rating might have been too low. But this was only for a short time period.

thanks for your help.
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Old 14th May 2008, 01:36 PM   #8
SY is offline SY  United States
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Something that isn't evident to me- was the amp ever quiet? I.e., has it always buzzed or is this something new?
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Old 14th May 2008, 04:31 PM   #9
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Quote:
Originally posted by rdk845
By bypassing, I meant that soldering + and - lead of known good caps to the circuit board traces connecting to + and - lead of power filter caps, respectively. And yes I tried this for all caps listed in the power section of schematic.

Yes, the buzz is coming from both channels, but buzz from each channels sounds a littl bit different in their pitch from each other. Some time left side is more prominent, some times not.

By the way, at one point in time, I used a 845 for which the plate was glowing a little. The tube might have been defective or its power dissipatoin rating might have been too low. But this was only for a short time period.

thanks for your help.
Well thats the only ideas that I had.There is something majorly wrong if the plate is glowing!The plate current must be to high.Is it possible that you are gettting some type of low freq feedback.You menson that the frequency of the buzz seems to change frequency some time so Im thinking the amp is squealing at a low freq and this may be overdriving the 845 causing it to glow red.Like someone else asked,has it ever worked right? If not it could even be a wiring problem.
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Old 14th May 2008, 06:50 PM   #10
rdk845 is offline rdk845  United States
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Yes I bought it new and it used to have no buzz at all from speaker. The power transformer have been giving low mechanical buzz from new, but I never heard them from speaker until recently.
I don't recall whether the buzz started when the plate glowing incident happend, but it is possible. Although with new tubes there is no glow. Bias currents are set fairly low (~ 65ma) and they are stable. If a bad 845 demaged some part, what would they like be?

Is there any way I can diagnose whether low fequency feed back is there and if so, how would I fix it?

Thanks!
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