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Old 23rd June 2012, 07:37 PM   #1
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Join Date: May 2012
Default some help with dac opamp capacitors

I am installing 2 AD797BRZ on brown dogs

dac output has 2 10uf 50v caps on output along with 10k resistor inline of signal

anything I can do about that?

also 47u 6.3v bipolar on op-amp output with 1k resistor in signal path

then 220p across rails before rca

I will remove muting transistors but my question is what can I do to improve these circuits ?

anything?
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Old 24th June 2012, 03:23 AM   #2
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You really don't give enough info, but odds are that you need to keep the caps on dac output(indicating a voltage out dac), as most v-out dacs run from single supply and thus have B+/2 dc voltage on the output pins. So, I would install 10 to 33uf Nichicon KZ or FW lytics & parallel with at least 0.47uf of as nice film cap/caps as will fit. Assuming the opamp is fed by a dual supply, you should not need dc blocking caps after the opamp, as you should see very little or zero dc voltage on the outputs. The 220p accross the output is a pretty harmlessly tiny value, but may sound better or worse if removed. Depends on dac, OS rate, etc.
Personally, I find the AD797 sounds super clean, but entirely lacking tonal richness. Have yet to find a chip that beats the OPA134/2134, as it's just as clean, but has natural richness of tone that is not artificial(unlike the OPA627, which I think is a terribly colored chip).
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Old 24th June 2012, 04:16 AM   #3
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thanks for the reply

this is a Marantz cc4000ose 5 disk, so space is at a premium

I did get silmic 2 and bi polar nichicon es for both outputs but was wondering about options

if I check op-amp output to ground (is that right?) and see no dc while playing can that change with frequency and or time?

heard dc can build up in op-amp?

no room for film

why is there an open air inductor across op-amp

I`m obviously a novice

how to post skemo?

Last edited by GLENZWORLD; 24th June 2012 at 04:22 AM. Reason: add skematic
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Old 25th June 2012, 02:36 AM   #4
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Quote:
Originally Posted by GLENZWORLD View Post
thanks for the reply

this is a Marantz cc4000ose 5 disk, so space is at a premium

I did get silmic 2 and bi polar nichicon es for both outputs but was wondering about options

if I check op-amp output to ground (is that right?) and see no dc while playing can that change with frequency and or time?

heard dc can build up in op-amp?

no room for film

why is there an open air inductor across op-amp

I`m obviously a novice

how to post skemo?
I've just looked up the service manual. You absolutely CAN eliminate the 100uf caps on the output of the opamp. There should never be any significant DC there under any circumstances, except a blown opamp. You do, though, need the 47uf blocking caps between the dac chip and the opamp, and I would certainly use the Nichicons for that. But, even if you only have room for a skinny little 0.33 or 1uf polyester cap accross those caps and accross any power supply caps for the dac/audio stages, it's much, much better than nothing. The coil in the feedback loop of each opamp channel is simply part of the EQ for the stage. And I would positively use an OPA2134 in this unit.
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Old 25th June 2012, 07:26 AM   #5
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thanks for looking that up and helping

too late for opa2134, just received ad797brz x2 on brown dogs (was told the 797b is tube sounding highs)

tube sound from an op amp we shall see

caps that are after dac are 10uf 50v

also noticed that on the skemo cc4000 gets 47uf 25v, so smaller caps on ose or f model

how do you figure out cap size? trail and error?

I will use nichicon es on both dac out and op-amp out unless I remove those

so if I run a volt meter on opamp output caps on each cap lead (while playing)
and if no dc....I can then remove or jumper cap?

is that right?

if thats so what if I run a vishay resistor 200ohm strait from opamp out cap in (or positive side) on board right to rca jack?

bypass output transistor and 1000 ohm resistor and the 220p across rails?

I will post pics of this soon on photobucket and send link

all the mechanical stuff is done, just finished today damping case and made a transformer shield (external, no shorted turns), in fact its isolated from ground now

may ground one side

grounds are another time lol

thanks
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