Op Amp Mismatch

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Lav750
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#1 Op Amp Mismatch

Post by Lav750 »

Hi to all. First post is a question. I have a small collection of TDA1541 equipped CD players including Cambridge Audio CD3, CD2, and yes a two box CD1. I have changed Op Amps, caps, clocks and PSU parts on the CD2 and CD3 to great effect after reading online about it's suitability. The CD1 presently has OPA27 Op Amps (which I doubt are original) I am thinking of changing these to either LME49710's or AD797's. Not because the sound is poor, but just that I'd like to see what difference it makes. I may try them first in an old series 1 type Philips machine just for confirmation. Does anyone have any thoughts on whether these later two are compatible with the OPA27's as I'd not like to toast the Philips player, and certainly not the CD1!, (this Philips CD304 machine has NE5534's).
Many thanks.
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Nick
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#2 Re: Op Amp Mismatch

Post by Nick »

Hi,

In theory the LME49710 will be ok, the pinout is the same. I would check the supply voltage the LME is a 17v part, whereas the OPA is 22v. The AD is 15v. My worry with the AD is if the original design used pin 8. In the OPA its part of the offset null, and in the LME its n/c so even if the original circuit used it it should be ok. But in the AD its used as the distortion null input. Also, the AD is very dependent on decoupling and can be unstable if its supply rails are not clamped down with ceramic caps close to the pins. The original circuit probably expects a slower, of its time part so will be less concerned with this. Unless you have a scope that could check for instability, what you hear as a sonic difference may just be the op amp singing in the RF area. Sockets can also be a problem for the AD for the same reason.
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Lav750
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#3 Re: Op Amp Mismatch

Post by Lav750 »

Hi Nick.
Thanks for the information. I will check on the Philips CD-304 first, then go on from there. Hopefully even though the parts are of different max voltage specs the actual PCB's will be outputting less. The NE5534 which I assume it had as standard is +- 15V I think.
All said and done, the CD1 is a great thing, it's space and musicality is wonderful. Even compared to the Tube Technology Fusion which is no slouch.
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jack
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#4 Re: Op Amp Mismatch

Post by jack »

One point about the AD797 (lovely device that it is) is that the GBP is huge and that unless compensated, it can introduce RF oscillation.
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Lav750
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#5 Re: Op Amp Mismatch

Post by Lav750 »

Hi Jack.
Thanks for that.
As an update I tried them (797's and 49710's) in the CD304 and they were OK, then tried them in the CD1 and got no sound what so ever! Was wetting myself for a moment just in case I had damaged it! Luckily not as all was well when I put the OP27's back. As the DAC's and other parts are potted i'm loath to risk anything, though it would be nice to hear what it would be like with the newer OP Amps.
:(
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