Hello,
few days ago I've been given a Dual CV 1250 amplifier (and s.me more components) for free. The elder lady told me that the amp wasn't working. Today I tested it, and it works as it should.
Here's a link to the service manual, including the schematics on pages #3 and #4. It seeme a bit strange to me that the volume control sits in front of most active stages. Thus it protects against overdriven preamp stages, but in terms of S/N ratio this is not the best solution.
Another item that puzzles me are those RC4559 dual opamps in all small signal stages. The only time I've seen this IC before was in a TrekII retrofit reverberation amplifier for Hammond organs. I know the 4558, and it seems that this one doesn't have the best reputation here. I must admit that I don't see too much differences between both. Does anyone see clearer than me?
Would it be a good advice to swap this opamp for another quieter one, especially in the phono section? Which one would be preferable? Would even a NE5332 be an improvement? Some time ago I've bought a handful of Motorola MC33078P's. Would they fit and be advantageous?
Best regards!
few days ago I've been given a Dual CV 1250 amplifier (and s.me more components) for free. The elder lady told me that the amp wasn't working. Today I tested it, and it works as it should.
Here's a link to the service manual, including the schematics on pages #3 and #4. It seeme a bit strange to me that the volume control sits in front of most active stages. Thus it protects against overdriven preamp stages, but in terms of S/N ratio this is not the best solution.
Another item that puzzles me are those RC4559 dual opamps in all small signal stages. The only time I've seen this IC before was in a TrekII retrofit reverberation amplifier for Hammond organs. I know the 4558, and it seems that this one doesn't have the best reputation here. I must admit that I don't see too much differences between both. Does anyone see clearer than me?
Would it be a good advice to swap this opamp for another quieter one, especially in the phono section? Which one would be preferable? Would even a NE5332 be an improvement? Some time ago I've bought a handful of Motorola MC33078P's. Would they fit and be advantageous?
Best regards!
The 4559 slightly improves on the 4558 (such as slew rate) but it is still considered poor compared to other devices.
If you are just experimenting then by all means try the 5532, it excels in applications like these, and yes, its ideal for the phono section. One practical point, if you change the opamps you need to measure the DC voltage across all coupling caps (and also feedback returns caps) and then refit them as necessary to suit the measured offset.
For the line stages (not the phono) you could try the TL072. Don't discount it until you have tried it and made your own mind up about it.
If you are just experimenting then by all means try the 5532, it excels in applications like these, and yes, its ideal for the phono section. One practical point, if you change the opamps you need to measure the DC voltage across all coupling caps (and also feedback returns caps) and then refit them as necessary to suit the measured offset.
For the line stages (not the phono) you could try the TL072. Don't discount it until you have tried it and made your own mind up about it.
It seems to me that in comparison with NE5532 the MC33078P is a bit better in terms of noise, GBW product and both input offset current and voltage. It is somewhat inferior, though, in terms of maximum output voltage swing, but this wouldn't matter in the Dual CV 1250 amplifier with it's volume control in front of the line stages.
And yes, the RC4559 appears to be the most inferior in this row.
Best regards!
And yes, the RC4559 appears to be the most inferior in this row.
Best regards!
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