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Old 2nd May 2006, 06:30 AM   #1
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Default Problems with dual opamp to 2 singles

Hi all,

I'm having a problem with a recently purchased Entech 205.2 DAC. One problem is that I don't have the schematic, and Monster said they don't have any either...hmmmmm...

Anyway, the original output opamp was an opa2134. I took that out and placed a socket so I can "play" Swapping other duals cause no problem; however, there seems to be a problem when I try two singles (opa627) mounted via Browndog.

There is a "buzzing" of that may be some kind of grounding issue or rf of some kind. I have tried 2 .1uf film caps across pins 4 and 7 of each 627, and that helped a little (I only had film caps available right now though I know ceramics and tantalums are better at rf). I tend to think its rf as I first tried putting the 627s in sockets on the Browndogs at it was a lot worse.

I'm sure it's not an overvoltage situation as the power supply itself is only rated at 16v.

Any thoughts? If its grounding, is there a way to "ground" the Browndogs?

Any thoughts are much appreciated (except the ones that say "throw it out and get a better one.... ).
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Old 2nd May 2006, 10:58 AM   #2
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Duals are usually internaly compensated, to reduce oscillations, a lot of singles have external compensation that has to be put on to make them stable, maybe yours? time to look at the spec/data sheets.

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Old 2nd May 2006, 03:05 PM   #3
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Hello George,

I'm pretty sure it's not oscillation. It appears to be a some kind of grounding or rf issue. If I place my hand on the case, it reduces the buzzing.

Maybe I should try finding a nearby ground and tapping into that with bypass caps from the v+ and v- pins of each 627 (star ground style of course).?
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Old 2nd May 2006, 05:09 PM   #4
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Quote:
Originally posted by wtyamamoto
Hello George,

I'm pretty sure it's not oscillation. It appears to be a some kind of grounding or rf issue. If I place my hand on the case, it reduces the buzzing.

Maybe I should try finding a nearby ground and tapping into that with bypass caps from the v+ and v- pins of each 627 (star ground style of course).?

Hey.

How many op-amps are there??
Howmany have you changed to OPA627s?

Andy
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Old 4th May 2006, 07:50 AM   #5
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Default Re: Problems with dual opamp to 2 singles

Quote:
Originally posted by wtyamamoto
Hi all,

I'm having a problem with a recently purchased Entech 205.2 DAC. One problem is that I don't have the schematic, and Monster said they don't have any either...hmmmmm...

Anyway, the original output opamp was an opa2134. I took that out and placed a socket so I can "play" Swapping other duals cause no problem; however, there seems to be a problem when I try two singles (opa627) mounted via Browndog.

There is a "buzzing" of that may be some kind of grounding issue or rf of some kind. I have tried 2 .1uf film caps across pins 4 and 7 of each 627, and that helped a little (I only had film caps available right now though I know ceramics and tantalums are better at rf). I tend to think its rf as I first tried putting the 627s in sockets on the Browndogs at it was a lot worse.

I'm sure it's not an overvoltage situation as the power supply itself is only rated at 16v.

Any thoughts? If its grounding, is there a way to "ground" the Browndogs?

Any thoughts are much appreciated (except the ones that say "throw it out and get a better one.... ).

Hi.

Anyone there ?

Andy
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Old 4th May 2006, 02:04 PM   #6
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Sorry,

Work kept me away from the net for a few days...

As stated, the original chip is an opa2134 (dual). 2132 works fine, but it is essentially the same chip I got a Browndog since I had a couple of 627s available. With the Browndog and 2 627s, the noise is there. Maybe oscillation, maybe rf, maybe ground problem.

I'm starting to wonder if it's a layout/bandwith problem as I recently tried an 8066 (dual) soldered to a DIP adapter as well as two AD847s and the same noise problem occurs. The 8066, 847, and 627 have much higher bandwidths, and may be picking up some kind of circuit noise or rf that the 2132/2134 just can't see.
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Old 4th May 2006, 04:05 PM   #7
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Quote:
Originally posted by wtyamamoto
Sorry,

Work kept me away from the net for a few days...

As stated, the original chip is an opa2134 (dual). 2132 works fine, but it is essentially the same chip I got a Browndog since I had a couple of 627s available. With the Browndog and 2 627s, the noise is there. Maybe oscillation, maybe rf, maybe ground problem.

I'm starting to wonder if it's a layout/bandwith problem as I recently tried an 8066 (dual) soldered to a DIP adapter as well as two AD847s and the same noise problem occurs. The 8066, 847, and 627 have much higher bandwidths, and may be picking up some kind of circuit noise or rf that the 2132/2134 just can't see.
Hi.

I asked how many dual opamps you changed. As far as I can tell from various sources, this DAC uses a GI circuit per channel as well as a normal filter. This may be the source of the problem rather than the I/V , filter or buffer sections.

I would expect to see 2 opamps per channel.

Andy
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Old 4th May 2006, 04:22 PM   #8
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Hi Andy,

The circuit uses one 2134 (which I swapped for a 2132 that works fine). I swapped the one 2132 for two 627s on a Browndog.

That's when I noticed it. It's more noticeable with no music on, and the volume of my preamp turned up to about 2 o'clock - so it's relatively soft, but with the 2132 or 2134, it's not there at all.

It is also present with an ad8066 and also there with two ad847s on Browndog.

Wayne
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Old 4th May 2006, 04:29 PM   #9
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I forgot to add....this dac uses the CS8412 and CS4329...

I may have to just leave it as is...instead of playing with...

It's kind of irritating that Monster does not have the schematic for it when they're the manufacturer. What does that say about product support. I know it's a few years old now, but how hard is it to keep a record of the schematic in this age!
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Old 4th May 2006, 04:35 PM   #10
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Quote:
Originally posted by wtyamamoto
Hi Andy,

The circuit uses one 2134 (which I swapped for a 2132 that works fine). I swapped the one 2132 for two 627s on a Browndog.

That's when I noticed it. It's more noticeable with no music on, and the volume of my preamp turned up to about 2 o'clock - so it's relatively soft, but with the 2132 or 2134, it's not there at all.

It is also present with an ad8066 and also there with two ad847s on Browndog.

Wayne
Hi.

you really need a circuit diagram [or work one out].
I must admit I'm a little baffled by only 3 opamps.

As I said, I trawled the net. The 205.2 seems to have a GIC filter which would be 1 chip per channel unless it shares a dual ic.

Andy
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