[markC]

I've replaced the ne5532 and the tl071 opamps in my cdp with 2604's and 627's with fantastic results, (after adding by-pass caps to the 2604's), but have left the lm741's alone, since they are described as "dc servo" op amps and I do not quite understand what their purpose is. I've read that the lower the input offset voltage is, the better the opamp is in this configuruation. Looking for any input in this area B4 I try a pair of opa604's in this application.

[Andrikos]

Mark,
you can't get much worse DC offset wise than the 741...
It's a classic but also outclassed.

Any low Dc offset opamp will be just fine.
Many BB opamps have spectacular offset and offset drift specs.
Also make sure they also have good PSRR and CMRR specs as those are offset specs too.
Good luck.

[markC]

Thanks for the response, but could you elaborate on the psrr and cmrr references? Although I am handy with a soldering iron, I don't quite comprehend electronics lingo.  (But I definatley am working on it!).

[Davey]

Mark,

The way I read your initial post it sounds like the 741's are used as servos and not directly in the audio signal path.  A servo is a closed-loop system that samples the DC level on the output of the "main" op-amp and then generates a correction voltage that is fed back to a previous portion of the circuit.  The effect is to zero or null the output DC level actively.

I think, in your case I'd be inclined to leave the 741's alone.  Yes, the 741's would have more offset than some other devices, but used in this application its job is to actually generate a DC offset.

Occam can probably elaborate further if I'm unclear.

Power Supply Rejection Ratio, and Common Mode Rejection Ratio.

Cheers,

Davey.

[markC]

psrr for 604 is 100db - 741 is 96 db. cmrr for 604 is 100 db - 741 is 90 db. Although the test parameters are different, one @ an impedence and one @ voltage, (blowing smoke)? Still, if it were anywhere near apples to apples, we're talking an average of about 7% here. I know that I could hear a 7% improvement in my system if it actually were to occur.

[eico1]

A servo is really a low pass filter with the corner frequency at about 0.3-3Hz, and errors due to the circuit are injected to the signal path along with the dc error it is correcting.

Static or dc errors aren't interesting, and any dynamic types are likely to be due to the passive components like the integrating capacitor since even lowly op-amps can handle this range of frequencies.

The main reason low offset fet op-amps like the lm412 are often used is that you can use a large value resistor with a small high quality capacitor to keep cost down thanks to the high impedence input.

steve

[markC]

Probabaly best to leave it be,( but I am in tweak mode). I may try a pair of brown dog adapters to allow two 607's to replace one of the 2604's, (formerly the ne5532's), in each channel.

[peranders]

Mark,

The way I read your initial post it sounds like the 741's are used as servos and not directly in the audio signal path.

Actually, DC servo IS in the audio path and will have influence up to 1 kHz. Therefore it's important to have low distortion to at least 100 Hz. A 741 will problably work rather good since the distortion is fairly low below 1 KHz.

Personally I have always used high quality opamps for DC servo for this reason.

Tested opamps for DC servo:
MAX420
MAX430
OP07
OPA227
AD8610