Building a pimeta with a BUF634 pseudo-ground?
May 13, 2012 at 10:34 PM Post #16 of 110
By "basic" I meant just the necessities, not cheap.  I am not familiar working with diodes and regulators at the point.  So far I've only built amps that run off a 24V wall wart I have.

Yes, I'm going for LMH6321 buffered OPA627 for L/R.
 
May 14, 2012 at 2:26 PM Post #18 of 110
Thanks for the schematic.
So something like this should work for converting AC->DC.
Negative Regulator
Positive Regulator
Since there's a voltage drop across the regulators, should I get a 28VAC transformer instead of a 24VAC?
Why do I need power caps on both sides of the regulator?  Should I use 1µF film or tantalum caps?
 
May 14, 2012 at 2:39 PM Post #19 of 110
Those regulators will do nicely.
The bridge rectifier is also good.
 
The caps on the input side of the regulators smooth out ripple from
the rectifier. The cap on the output side of the regulator improve
the transient response. Use film for the 1uF...it helps reduce any
high frequency noise. The 1uF is optional and all of the cap values
are just examples. For the electrolytics, I would recommend Panasonic
FM series. 35 volts should be good for the input side and 16 volts
on the output side.
 
May 14, 2012 at 2:51 PM Post #20 of 110
cost and time savings can be had by just buying a "open frame" line input dual supply module
 
a linear, agency safety rated PS can be had for less than Glass Jar Audio's sigma-22 kit price - you can even get Medical safety/leakage current speced modules for that price
 
line cord, sw and fuse are all that would be needed
 
May 14, 2012 at 2:56 PM Post #21 of 110
Quote:
cost and time savings can be had by just buying a "open frame" line input dual supply module, a linear, agency safety rated PS can be had for less than Glass Jar Audio's sigma-22 kit price
 
line cord, sw and fuse are all that would be needed

Like this?
 
May 14, 2012 at 3:09 PM Post #22 of 110
yes, I have bought Condor branded MAA15-0.8-A which is medical rated for ~$60 in singles not too long ago - a bit bigger than optimum, will require good sized desktop enclosure
 
May 14, 2012 at 3:24 PM Post #23 of 110
Yes, adding the transformer has already doubled my intended size and increased the cost more than I'd like, but I am grateful for the learning opportunity.
 
Another question: I'm measuring 3mV-8mV DC offset from my valab DAC.  Is it safe for me to omit input caps?
 
May 14, 2012 at 3:51 PM Post #24 of 110
That depends on how much gain you are going to use...
 
If have a low gain, say 3 or less, and never turn the
volume up past, say, 1 o'clock, then you should be safe.
 
If you use a gain of five, and turn it up all the way, then
that offset can be 15 to 40 mV.
 
If you add a servo to your design, then input offset is
pretty much irrelevant.
 
May 14, 2012 at 5:44 PM Post #25 of 110
I'm also adding variable bass boost.  I think i read something about it adding some offset as well.  Is that right?
Nvm, i found it.  Better to go with input caps
 
May 14, 2012 at 8:13 PM Post #26 of 110
Now, for biasing the opamps to class A, do I need to bias the ground channel as well?  What's a good amount of current draw for the OPA627?

For this, would these work well?
I am lost with how to bias class A...  I'm reading tangent's writeup of biasing opamps into class A, but I'm still not getting it.
 
May 15, 2012 at 7:14 AM Post #27 of 110
I thought you weren't going to use the ground channel...
There is no point in having a dual rail power supply if
you are using the three channel design.
 
According to the data sheets, J113 would be a better match...
I'm running 2mA of class A bias in my PPA, (with OPA627), but
I have not played with it much so I can't say if that is an "ideal"
bias point for the OPA627 or not.
 
May 15, 2012 at 7:15 AM Post #28 of 110
About 2 mA would be fine,although it's not really necessary for modern high-quality opamps.Why are you building a ground channel?
 
May 15, 2012 at 12:33 PM Post #29 of 110
I might be confused... Using a dual power supply, I don't need to have an opamp+buffer in the ground channel?
 
Thanks for the J113 suggestion.  I've been haunted by this question for a while now.
 

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