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Old 4th May 2010, 06:37 AM   #11
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Hi Qusp,

I have the schematics of the dac, Each dac chip is configured in mono mode, 2 chips for each channel, Each dac chip outputs two + and two -, All + goes to 1 opamp and all - goes to the other, Therefore, creating summed + and summed - for a balanced signal output, Which are then combined by the differential mode output opamp. The ones nearer the rca terminals.

BTW , How would I know if the AD797 is oscillating, And what do you suggest I'd do if they oscillate? Thanks

Hello Lordearl,

I still haven't got the opamps, Due to Holidays taken by the post office. But I will report when I get to listen to them.


I can send you both the schematics if you like, only 90kb, just send me your email ad.
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Old 4th May 2010, 09:20 AM   #12
qusp is offline qusp  Australia
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yeah I looked a bit further at it and noticed all in parallel, rather novel for a wolfston. so this dac already has balanced output and you are only using half of it tapped for the RCAs? better off just tapping the trace that the + connects to directly in that case. the wolfston part is already voltage out, so no need for anything after it especially with 4 in parallel like that, in fact you may even need to attenuate it if its too hot. so that and ground connected to the RCA


you will know your 797 is oscillating when it is turned on chances are if the circuit isnt designed specifically tuned for AD797, it will oscillate. symptoms can be unnoticeable to the ear, right the way to distorted or sibilant output and the chip can be quite warm to the touch. it mostly tends to show itself in the high frequencies IME, but in all cases it is not good. the way for you to get rid of it is with an RC low pass filter (LPF), the f point of this filter should be chosen with your desired output frequency in mind, but 100khz is a pretty safe level, higher if you wish, its not so important, as long as its under 200khz I guess. look up the AD797 datasheet, its pretty large with plenty of examples, you may find an example you could use, stolen from an instrumentation amplifier circuit (a common application for the 797 due to its ultra low noise) you should use high quality resistors (metal film at the least) and film caps (PPS perhaps, or metalized polystyrene) otherwise you will be adding noise of your own. a regular electrolytic will not cut it here, even if you could find one in the PF range

in fact here is a schematic stolen from tangent's site for an instrumentation preamplifier the RC filter i'm talking about is R2/C4, the values are not specific to you because it is for different reasons in this application, but should give you an idea

Last edited by qusp; 4th May 2010 at 09:30 AM.
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Old 4th May 2010, 11:49 AM   #13
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Hi Qusp,

Thank you for the advice and help, Very much appreciated.

All the output on the chip are utilized, But I don't know much about opamp circuits or any electronics, I'm more of a speaker DIYer. So I will post the screen shot of the chips' and opamps' diagram .

I would like to ask whether the c54 ,c56, c65 and c66 acts as low pass filter to stabilize the opamp?

Thank you.
Click the image to open in full size.

Click the image to open in full size.

Last edited by marchel; 4th May 2010 at 12:03 PM.
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Old 4th May 2010, 12:01 PM   #14
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Let me add that there is also a provision on the board for balanced output connecting points, But it is no use for me as I only use single ended, But it is nice to have it there.

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Old 6th May 2010, 09:08 AM   #15
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I'd like to update this thread,

I still have'nt receive the opamps yet, But I've tweaked the Dac, I removed the spdif transformer out of the circuit .

Click the image to open in full size.

Click the image to open in full size.
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Old 6th May 2010, 11:57 AM   #16
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Looks good...but why did you do that?
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Old 6th May 2010, 02:27 PM   #17
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After a few couple of hours of listening , I have concluded that there is a very minute improvement in sound quality, The tiny bit lack of refinement that I've talked about before, Now appears to be the lack of coherence by the highest highs with the rest of the spectrum, Because Now, The highest highs sounds more harmonically connected with the music, and not like another instruments. I don't know whether I'm imagining things, But the first impression remains after further listening, just now as I write.

But it all depends on the Transport too, Just bought 2 new dvd players this week in addition to my old players, And these new sony dvd players sound best with this dac, I've tried onkyo cd player, expensive old sony dvd player, pioneer dvd player and 2 late model sony dvd player. And these late model sonys have the best sound of the bunch. I don't have highend transport or cd player to try with it, But the new sony dvd players sounds so good with this dac , That I felt I've saved alot of money, and don't feel the need to spend too much just to enjoy music. This is a very good dac indeed, if combined with a good sounding transport.

Hi Lordearl, I did it after reading a few articles like this, LOL

CD_transport_DIY
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Old 11th May 2010, 08:39 AM   #18
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HI,

I received the ad797 chips last week , I installed it and it does not seem to be oscillating.

I gave it about 25 hours of break in time , The sound of ad797 is much better than the opa134 period, If put into category , The opa134 is in the good mid-fi sound quality but the ad797 truly belongs to hi-end.

By comparison , The ad797 sound a little more forward in the upper mid, This is only because the opa134 sounds distant in that region, I would have to say that, I feel the ad797 is more accurate in this regard.

In the High frequencies, The ad797 have better details and have finer rendition of harmonics of the treble producing instruments.

In the mid bass , I feel that the opa134 is a little warmer, But this could be caused by the depression in the upper mid. I feel that the ad797 is more accurate in this regard.

On acoustic bass drums, The ad797 trumps the opa134 hands down, The Ad797 is so good that , Not only do bass drum sounds tighter, weightier and faster, The initial attack of the bass drum instrument sounds airier, Which I didn't quite noticed with the opa134.

YOu know what, I used to believe that discrete device is the only way to go if you want transparency ,But the AD797 is very transparent sounding , As if there were no op amps in the signal path, That it totally changed my mind.

I will try the opa627 in the future.
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Old 11th May 2010, 10:18 AM   #19
Luke is offline Luke  New Zealand
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Hi Marcheel,

removing the transformer undoes impeadance matching, it also removes galvanic isolation.
But congratulations for improving the sound.
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Old 11th May 2010, 12:58 PM   #20
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Thanks Luke,

Yes I understand the risk and trade off. I have looked at the sony player that I currently use this with, And it has no pulse transformer too, But it has an output buffer using a single hi-speed transistor, So I think the digital output is robust and square. As for the Isolation , I don't know. I think it's better to have some sort of isolation and pulse transformer solves this problem, But I'm not the only one who has removed the pulse transformer from their dacs , I was thinking , Why not give it a try , I can always put it back. I'm glad I did, Because I feel and think that there is improvement in sound quality , And I don't hear any ill effect of not having a transformer.

But I should state that the improvement is really small, Almost ( but I think not) imaginary or psychological. If one is not too inclined to remove it , Then I suggest to leave it as is. The biggest change in sound quality is coming from the change of op-amps. And changing the op-amps is a must , If one wants to hear the potential of this dac.

Last edited by marchel; 11th May 2010 at 01:01 PM.
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