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Old 8th May 2013, 10:39 AM   #311
Marra is offline Marra  United Kingdom
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@grimberg.Thanks for the excellent write up and pics.It looks like I need to order a couple of standard rectifiers just in case my Schottky's cause problems.
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Old 8th May 2013, 10:45 AM   #312
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I cannot see the rerctifiers being the issue unless they are underspec'd. The transformers are a little on the small side and could be the cause of the hum. I used mur860's in an f5t build and had no issue at all and they were being worked really hard. Grounding is a funny thing and you have to be careful in placing the blame. There is no reason that i no of that would cause a Schottky diode to induce hum when a standard didoe does not.
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Old 8th May 2013, 10:52 AM   #313
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We'll see how it goes the schottky's work perfectly ok in the F5.I guess the standard block rectifiers will always come in handy as ground loop isolators.
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Old 8th May 2013, 11:12 AM   #314
henryve is offline henryve  South Africa
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I wonder though if it is not the fact that the BrianGT PSU boards forces you to use one bridge for both secondaries of the transformer, where he now uses one bridge per secondary via the block devices.

I seem to recall a previous posting by NP indicating that sharing a bridge between 2 secondaries may induce mechanical hum in the transformers if DC is passed back into the secondaries.
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Old 8th May 2013, 11:18 AM   #315
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Very reasonable thought. You could easily bypass the onboard spot of the Brian GT boards and do the rectification off board.
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Old 8th May 2013, 09:41 PM   #316
6L6 is offline 6L6  United States
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Quote:
Originally Posted by henryve View Post
I wonder though if it is not the fact that the BrianGT PSU boards forces you to use one bridge for both secondaries of the transformer, where he now uses one bridge per secondary via the block devices.
A very fair conclusion. I have used the BrianGT PSU once before, with TO-220 high-speed devices and the amp (an F5) is amongst the quietest I've built.

That is only an observation - not trying to discount your idea, merely share my experience with it.

Quote:
I seem to recall a previous posting by NP indicating that sharing a bridge between 2 secondaries may induce mechanical hum in the transformers if DC is passed back into the secondaries.
That is my understanding of it as well.
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Old 8th May 2013, 10:18 PM   #317
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Quote:
Originally Posted by buzzforb View Post
The transformers are a little on the small side and could be the cause of the hum.
The original Aleph J uses one 300VA transformer. I am using two 250VA transformers for a clone build, how are they undersized?
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Old 8th May 2013, 10:22 PM   #318
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Guys, just pay attention to the language I used, which was carefully chosen:

"MUR and MSR diodes are not well suited for this application"

Meaning exactly what is being discussed here: a single bridge of discrete devices being overtaxed in the way they were used.

I am not discarding MUR and MSR as good diodes. I also have not tested Schottky devices. What is clear is that once I replaced the four diodes with two external bridges, the mechanical hum disappeared.
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Old 8th May 2013, 10:32 PM   #319
6L6 is offline 6L6  United States
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Quote:
Originally Posted by grimberg View Post
I am not discarding MUR and MSR as good diodes... ...What is clear is that once I replaced the four diodes with two external bridges, the mechanical hum disappeared.
I'm not surprised, I have been having similar issues with the F5T, and the pair of (slow) diode bridges is helping.

I'm sure there are plenty of builds with high-speed devices that are quiet. My (very limited) experience is showing me that the normal bridges might be a better place to begin for most builders, to keep issues to a minimum.

This leads to another question - is there any TO-220 or TO-247 discrete, slow diode of suitable rating that could be used in the BrianGT boards? Perhaps the single bridge isn't necessarily the issue -- the factory Aleph's were single bridge.
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Old 8th May 2013, 10:33 PM   #320
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My F5T was powered by single 400VA PSU and Mur 860 diodes on the ChipAMP boards with no noise issues to speak of.
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