Functional of opamps in McCormack amp - diyAudio
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Old 28th June 2007, 04:43 AM   #1
100db is offline 100db  Thailand
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Default Functional of opamps in McCormack amp

Hi All,
There are 2 opamps per channel on the driver board in my McCormack DNA0.5 amp. The one on the left is AD712 and the one on the right is NE5532. I've tried to go through the PCB trace but it look quite complicate. Don't know if these opamps function as DC-servo or not ???

If it does, will I get any improvement changing over from NE5532 to OPA2134 or OPA627 or it just a waste of money ?

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Old 28th June 2007, 07:32 AM   #2
Rob F is offline Rob F  New Zealand
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One op-amp, the 712 from memory, operates as a DC servo, the other is the error amp for the regulator for the driver stage.

I wouldn't change these willy nilly if I were you. The servo amp will need to have good DC parameters for a start and should be fet input. BTW it is a dual op amp.

Rob.
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Old 28th June 2007, 08:45 AM   #3
VivaVee is offline VivaVee  New Zealand
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Close Rob, but not quite.

The AD712s (U1 and U2) are used as DC servos. They control the output DC offset to better than 1mV worst case and are difficult, but not impossible to improve on. The particular circuit topology used means that the opamp has less effect on the sound than is usually the case with DC servos.

Then there are two NE5532s. The LM833 opamp was also used in some versions.

The first (U3) is used to monitor the supply rails and senses if any of the supply rail fuses have blown. The second (U4) uses the first half of the dual opamp to sense any excess ouput DC offset while the second half sums the two error signals (blown fuse and/or excess output dc offset) and turns on a two relays if a fault condition occurs.

The relays mute the inputs and so ,hopefully, everything including yr speakers are protected ...

The supply rail monitor is isolated from the rail by 100k resistors so its hard to see that opamp effecting the rail. The dc offset monitor is isolated from the by resistors taht are 20x bigger than the one in the actual dc servo, so again, it is hard to see that having much sonic impact
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Old 28th June 2007, 09:13 AM   #4
AndrewT is online now AndrewT  Scotland
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Hi,
quite an achievement with one dual opamp.
Could someone post a schematic extract of the monitoring function? even a hand sketch would do.
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Old 28th June 2007, 03:12 PM   #5
100db is offline 100db  Thailand
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Thanks very much !
If I'm understand correctly, I should leave it as it is ?
And that's a wallet friendly choice
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Old 28th June 2007, 03:32 PM   #6
100db is offline 100db  Thailand
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Hi Alan,

I believe you own the McCormack amp as well, right ?
Here is existing condition of my driver board, noting morethan changing caps and mount the 10k attenuator directly between RCA & Driver board, what else I can do to make it sing better ?

Is is worth changing a rainbow cable that connect between driver & output board to a better one ?

Regards
Ball
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Old 29th June 2007, 01:50 AM   #7
100db is offline 100db  Thailand
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Old 29th June 2007, 09:03 PM   #8
VivaVee is offline VivaVee  New Zealand
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I would leave the monitoring opamps alone. Those are U3/4. You could look for an improvement over the AD712 but I think your money is better spent elsewhere for there reasons outlined above.

I assume that you do not have the Deluxe version of the DNA0.5? In this case, you could consider doing what Steve McCormack did to make yr amp the Deluxe.

Change R1/R26(1k) to equivalent value Caddock resistors.
Change R2/R27(100k) to equivalent value Caddock resistors.
Change R10/12/25/27 (249ohms) to equivalent value Vishay resistors
Change R97/98 (154ohms) to equivalent value Vishay resistors

I do not recall if this change was in the Deluxe but I would also look for C13. This is 0.1 - 1.0uF capacitor. This cap provides a shunt around the Vbe multiplier. You want to replace this with a better grade film cap. This should have a voltage rating greater than 100V and you could fit yr current fashionable favourite.

But ... do NOT fit some weirdo audiophile cap that needs flying leads to connect it to the PCB. You need a cap that neatly fits where the old one used to be. I would cosider this nearly universal advice. You neither need some flakey cap in this circuit position that breaks off nor do you need an unhealthy injection of noise at this point.

So much modification work merely changes the noise spectral density. You want to improve the circuit operation not just change it.

Finally, note that to effectively and NEATLY perform these modifications you would need to completely remove the pcb from the chassis. Please use desoldering wick rather than a desoldering gun to reduce the likelihood of track/pad damage. And of course, you are on yr own if anything goes wrong!

Still want to get in there?
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Old 1st July 2007, 03:13 AM   #9
Rob F is offline Rob F  New Zealand
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Hi Alan,

I stand corrected. Actually if I had looked at the photo I should have known the 5532 was not an error amp, since there were no
pass devices anywhere near it... Oh well my memory sucks.

Rob.
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Old 1st July 2007, 05:03 PM   #10
100db is offline 100db  Thailand
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Hi Alan,

Thanks much !
Yes, I've the DNA 0.5 standard.
From the improvment I've got after changing all the stock caps to BG, I absulutely will give a try on changing resisters as well.

My initial thought is to replace the 10 years old caps with a fresh one, and I found a positive comments about SMc's upgrade during searching for the info. about this amp. So, I end up with taking it apart and build it up again
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