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Old 8th February 2010, 06:44 AM   #11
Mooly is offline Mooly  United Kingdom
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External PSU is fine if that's what you want.
If I were making this I would use a small transformer for what they cost, something like,

BLOCK|AVB3.2/2/12|TRANSFORMER, 3.2VA, 2 X 12V | CPC

and build it all as one.
Get the grounding correct and there should be zero hum from the PSU electrically. That means taking the ground connection (star) from a small spur from between the two main smoothing caps. That's then the only connection you use to the amp etc.
As to induced hum from the magnetic field of the tranny... at these signal levels it shouldn't be a problem. A bit of distance tranny to circuit works wonders and rotating the tranny to cancel the field helps to in some cases. At the impedances of the CMOY and the levels involved I would try all in one box tbh... and make it work

If you have "hum/buzz" and it's "harsh" sounding because of harmonics, then that points to a wiring problem. Pure low frequency hum is induced or caused by stray pickup.
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Old 8th February 2010, 04:02 PM   #12
TB is offline TB  United States
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Thanks Mooly.
Those trafos are cool. I have not seen them in US though.
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Old 8th February 2010, 04:54 PM   #13
Mooly is offline Mooly  United Kingdom
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The OPA134 draws around 4 milliamp.
Little transformers should be available... isn't it Mouser you all use over on your side of the pond.
020-5378-0 Pulse Transformers

They call it a pulse transformer... it's not. Click the data sheet from the above link.
Use 79L12 and 78L12's and it would be perfect I guess.
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Old 10th February 2010, 06:43 PM   #14
TB is offline TB  United States
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Hi,
I have one more question about regulated PSU. I still would like to build external one and I am not sure where to install regulators. In majority designs it is recommended to install regulators as close to chip as possible. But, in my case would be easier to do it inside of the external PSU case. Does it matter for this particular project? What might be important info, I do not want to use long umbilical cord between PSU/Amp. For this purpose I want to use shielded computer power cord.
But, in the case of use of regulators in the amp case as a better solution, should I put any capacitors after bridge rectifiers (inside of PSU box)?
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Old 11th February 2010, 06:22 AM   #15
Mooly is offline Mooly  United Kingdom
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For your application it won't matter. Just remember to decouple the IC's (OpAmps) at their supply pins directly.

The supply from a regulator is "at it's best" directly on the regulator... once you feed that voltage down a wire or a length of PCB print that adds a small resistance, which means that load fluctuations "alter" that super stable voltage at the "load end" of the wire. In practice it's worse than that as the wire etc add inductance too so that the AC impedance of the supply is higher at the load end as well. So decouple well at the amp.

The good news is that for a CMOY it won't make one jot of difference
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Old 11th February 2010, 03:11 PM   #16
TB is offline TB  United States
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Mooly,
Thank you very much for the help !!!!!
I will do what you have suggested, regs close to chips. I wanted to bypass power pins at the chip anyway. In that case, should I put any smoothing caps in the PSU after rectifiers or it is not necessary?
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Old 11th February 2010, 05:31 PM   #17
Mooly is offline Mooly  United Kingdom
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You definitely use caps in the PSU lol.
The caps at the regs need not be large, they are more for stability. The problem with putting big caps "in and around" an amplifier (again yours is so low current so it's not a problem) is that any ripple, any disturbance due to the load drawing current, then that current is introduced via large caps into the signal ground causing an increase in distortion etc

So smoothing caps in the PSU, probably 1000uf at most for this low current application, then smallish caps at the regs, say 47uf, and something like a 10uf and a 0.1uf in parallel directly across the supply pins to the OpAmp.
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Old 6th December 2012, 01:51 AM   #18
TB is offline TB  United States
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Default Cmoy and Zobel

It took me two years to get back to construction of this amp. I do not want to pollute the forum with another thread about Cmoy amp, but after search I did not find an answer to the Zobel question. Maybe because this amp is usually made portable and additional parts are not desired. Anyway, after lurking through RJM site I have found his version of Cmoy. It has Zobel at the output with note it is recommended. I have not seen here anyone implementing Zobel in Cmoy. So, would using it improve the sound or improve stability with lower impedance headphones? Do you recommend it?
Thanks

This is URL to RJM site: http://phonoclone.com/diy-hclo.html
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