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Old 13th October 2018, 02:06 PM   #21
simonra is offline simonra  United Kingdom
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Quote:
Originally Posted by Mark Whitney View Post
Your fist mistake is wrapping the ground leads around the transformers.
Agreed, I now have the ground leads running beneath the pcbs. I was moving the wires around by trial and error, it wasn't until I drew out the block diagram that I realised I'd placed the transformers right in the middle of the loop. Interestingly moving the leads to the new position had a negligible effect on the hum.

Last edited by simonra; 13th October 2018 at 02:22 PM.
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Old 13th October 2018, 02:17 PM   #22
simonra is offline simonra  United Kingdom
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I hope this is not too much of topic, i was finishing f5 and m2 amps, and noticed slightly higher background noise with transformer in final position in the middle of the chasis. Someone suggested to rotate the toroidal transformer and observe the buz on ouput with scope. The difference was significant.
I found that different positions excited different harmonics, it was useful having the spectrum view to see when i was just shifting the problem to a different frequency. As it happened putting the band on attenuated right across the frequency range. I just need to find a supplier of grain oriented silicon steel so I can replace the bands on the transformers I stole them from.

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Old 13th October 2018, 03:45 PM   #23
Mark Whitney is online now Mark Whitney  Netherlands
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HBR. Looking at the schematic of the PAmpV4. I would have used a 1 ohm resistor between SGND and DC0. Moved output ground to where R30 connects to DC0 and used a separate transformer/PSU.
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Old 13th October 2018, 03:57 PM   #24
simonra is offline simonra  United Kingdom
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If I understand your comments correctly, looking at the schematic again it may be a bit misleading. R30 on the layout is right next to the output connectors. I usually start with R0 and then increase the value if I get noise... that would have been my next step. I've not got around to building an inverse RIAA circuit yet so when I can test that element thoroughly it may throw up a few problems.

Do you mean a dedicated transformer for the MM stage and a separate one for the buffer op amps in the later stages?
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Old 13th October 2018, 04:27 PM   #25
Mark Whitney is online now Mark Whitney  Netherlands
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Better to place resistors between the inputs and outputs.
The problem could also be a ground loop in the test setup.
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Old 15th October 2018, 01:09 PM   #26
simonra is offline simonra  United Kingdom
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Thanks for the pointer, I'll bear that in mind if I have any problems with the MM stage. The hum was down to magnetic interference in the end and not a problem any more.
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Old 15th October 2018, 01:51 PM   #27
batteryman is offline batteryman  United Kingdom
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There is no reason, especially with a preamp not to have the transformers in a separate box and connected by an umbilical and then you do not have induced hum issues. (rectifiers and smoothing in the preamp enclosure)

Some power amps eg Kraken, have the toroids remote from the amp in a small steel case, and I am building an Aleph 5 single ended 60watt Class A amp and mounting the 500vs transformer remotely, although this is partly to avoid the amp, with its massive heatsinks from requiring a fork lift to move it!
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Old 15th October 2018, 09:00 PM   #28
Mark Whitney is online now Mark Whitney  Netherlands
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Good to hear you have fixed the problem. Here is a link explainig how HBR works and can be used in almost any amp design.
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Old 16th October 2018, 09:58 AM   #29
simonra is offline simonra  United Kingdom
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Thanks for the link... I think that's the most understandable paper I've seen on the topic.
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