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Old 15th January 2013, 11:20 PM   #151
AJT is offline AJT  Philippines
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a power traffo rated for 7.9 amps does not stop delivering currents at 8amps....

Va ratings are all about tolerable heating that can be allowed in a traffo....
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Old 15th January 2013, 11:30 PM   #152
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Quote:
Originally Posted by 6L6 View Post
5.7A at 70vct is 400VA An Antek AN-4235 would be exactly what you are asking for.
Tested secondaries parallel, but is secondaries series when I use it: Oops! Bit of a hard sell to say that doubling the length of the copper doesn't have any effect.

EDIT:
There was increased loss at secondaries, not at primaries. I've still got the Primaries parallel like the test conditions. So the discrepancy wasn't a factor of 2, but rather somewhat less. It is quite confusing to figure out specs that have a "somewhat" in them.
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Last edited by danielwritesbac; 15th January 2013 at 11:37 PM.
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Old 15th January 2013, 11:44 PM   #153
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@danielwritesbac, testing and normal use are two different things.... there is really no comparison.....
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Old 16th January 2013, 11:44 PM   #154
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I have a few questions....

In the build thread people are talking about a 2x45 Vac transformer 800va , if the speakers were 4 ohms what output power would that give? I wanted the amp to be flexible so I could use different speakers, maybe 8ohms at a later stage. I noted in one of your posts you said 45vac is too high for 4 ohm loads, what would happen? blow up?

2nd question (of many, yet to come). The PSU BOM states capacitors with a value of 10k uf to 22k uf, but some members are talking of just using 6k uf, what would the higher capacitance give, sound wise?

3rd question what is a snubber?

4th question - looked at some capacitors on Hi Fi collective and they we 15-30, whilst others are talking of using 5usd capacitors. Would using more expensive be better, does it make any difference in a PSU? or should high quality caps be reserved for the amp?

That will do for now, hopefully be able to order something soon.
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Old 17th January 2013, 02:39 AM   #155
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Ahh , question 1 is very timely. I am about to build some "mini badgers" in bridge mode for my sub. The trick for 4R seems to be lower Rail voltages. At 40-0-40VDC the badgers SOA doubles and at 30VDC doubles again. 2 bridged Badgers running with 35v rails would most likely drive a 2-4R sub at over 300W !!
(BTW , this would equate to a 26-0-26Vac trafo).
The SOA of even the lowly onsemi NJW pair is close to 7 amps at 30V(DC).
Of course , using the more robust MJL's will increase thermal dissipation and these have even higher SOA.


Q2 - Smaller main supply caps = more ripple.For 8R loads , I use 10Kuf x2 per channel. For lower impedance's, 15-20Kuf is suggested.
6Kuf would turn this amp into an OEM (higher ripple - less "punch")

Q3 - The "snubber" Snubber - Wikipedia, the free encyclopedia
in this amp (the 2 diodes) keeps the inductive load of the loudspeaker from destroying the output stage. The 3 amp diodes should have a short reverse recovery time. You see this technique applied to relay coils as well.

Q4 - Some audiophiles swear by esoteric capacitors. My take is that low esr
and MTBF are the main considerations.
The input cap should be high quality polystyrene ,very stable over time. The miller caps in the VAS should be silver-mica - very stable (temperature and value) no parasitic resonances.
ALL power supply caps can be any properly derated (panasonic , nichicon ,rubycon), low esr electolytic.

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Old 17th January 2013, 05:40 AM   #156
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OS,

Can you clarify a doubt...
In the BOM, R8 & R9 are specified as 100R 1%.
I'm wondering if you meant for R15 & R16 to be 1% rather than R8 & R9?
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Old 17th January 2013, 05:54 AM   #157
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Quote:
Originally Posted by solid View Post
OS,

Can you clarify a doubt...
In the BOM, R8 & R9 are specified as 100R 1%.
I'm wondering if you meant for R15 & R16 to be 1% rather than R8 & R9?
Are you referring to the XLS BOM? If yes, that one followed OS's BOM, afaik.
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Old 17th January 2013, 06:03 AM   #158
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Quote:
Originally Posted by JojoD818 View Post
Are you referring to the XLS BOM? If yes, that one followed OS's BOM, afaik.
Yes, both BOMs show the same. Just wondering if it is R15 & R16 that need to be 1%?
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Old 17th January 2013, 06:09 AM   #159
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Quote:
Originally Posted by solid View Post
Yes, both BOMs show the same. Just wondering if it is R15 & R16 that need to be 1%?
No, although making R15 and R16 1% wouldn't hurt, it is not that critical because each one is effectively paralleled to each side of trimmer R17. So using 1% or 5% doesn't matter.

R8 and R9 matters because they are in the CCS and makes changing R7 (CCS adjust) more meaningful.

HTH
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Old 17th January 2013, 06:31 AM   #160
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I fail to see why R9 needs to be 1% and because of the R7 trimmer, R8 does not seem critical either... On the other hand, closely matched R15 & R16 have the potential to make R17 unnecessary (though it's good to have for fine dc offset adjustment)

I would like to hear OS's thoughts on this, though.
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