I see lots of very sophisticated projects here with people doing smart stuff with Arduino and Pi and all sorts. So I feel suitably chastened in posting this but may be of some interest to someone.
Having finished my first Chipamp I was left with a Cambridge Audio A1 case and some of the internals and decided to repurpose them into a preamp to feed the Chipamp. All very simple stuff. I sourced a modest 30VA 15-0-15 toroid. I stripped out the old regulator system (just Zeners) from both the main preamp board and the input board and installed a proper linear regulator based on L78/7915. I did some rerouting on the input board to cut out the the tape loop and give myself just another line level input. Then obviously I had to rewire everything together and in doing so tried to adhere to some better routing than the original design allowed (given that it had a comparatively massive power amp board in there).
So the design (as per original) is simple. An inverting buffer on input, feeding a shunt feedback Baxandall tone circuit (so again inverting), feeding a pot and then straight to output. So this last stage is a 'passive' preamp. I use short cables and the pot is 20k (ln) so no issues there, but I will be adding a final buffer stage just for versatility to allow longer cables if wanted (may enable both buffered and non-buffered outputs).
On to opamps. So much written I know but I will keep it brief. It came with TL072 in both stages. I am a great fan of TL072 from long back and in these low or unity gain circuits I didn't see a compelling reason to change, but I had socketed the boards and so experimented a little. Tried 4558, NE5532 and AD712 (as used in more upmarket Cambridge Audio A3i). I measured all of them for noise (pretty negligible in all cases but measurable differences) and did some listening tests.
OK my ears are not good and I am not therefore a great guide on sound quality - whatever that means. In all honesty I really couldn't reliably tell them apart. Its quite useful that the tone circuit is switchable so its possible to switch one op amp out of circuit and this did reveal a very slight degradation due to the 4558 - very slight, but female vocals just didn't sound as convincing (at least not to me). For the others I couldn't have reliably told whether the tone circuit was on or off with a blind test. So on this criteria alone the 4558 drops out of contention.
On to noise. Interesting. The differences are tiny and all versions are getting close to the noise floor of my ancient Levell muV meter but here it is. The noisiest (in this circuit) was the TL072, the quietest was the AD712. The total measured difference (full volume, tone on) was only around 10muV from best to worst but given the low levels I was measuring (20-30muV) I guess this is quite a large percentage. However .... it was impossible to tell them apart in practice unless my ear was right up against the speaker. In that case I would say the TL072 noise was a bit more objectionable / noticeable as in being more popcorn like.
Then I am going to add a criteria. DC offset. No surprise here the NE5532 was worst by a country mile. So what? Well the NE5532 implementation would be unusable without some switch on delay relay / switch off mute. I say that because if ever by accident you were to switch off that preamp first (or switch it on last) with NE5532 on board there would be very loud, very unpleasant and possibly damaging loudspeaker cone excursions. I tried it without tone enabled and volume turned down and it was bad enough. The TL072 by comparison was relatively benign (but you would still want to avoid a mistake of powering down preamp with power amp on). The AD712 was just a dreamboat. I couldn't measure any DC offset and it starts up and shuts down almost silently.
Perhaps no surprises - a circuit designed with JFET opamps works better with JFET opamps. But then the performance of the AD712 in comparison with TL072 did take me aback. Received wisdom is this is an ancient 80s opamp and offers no real benefit over the TL072. And I hadn't realised the benefit of DC precision is potentially simplifying a design. It was therefore measurable better and much more practical. In fact for the audiophile approach, DC offset is so low i could easily remove at least two DC blocking capacitors per channel if I wanted to. Picture of new internals just for the hell of it.
Having finished my first Chipamp I was left with a Cambridge Audio A1 case and some of the internals and decided to repurpose them into a preamp to feed the Chipamp. All very simple stuff. I sourced a modest 30VA 15-0-15 toroid. I stripped out the old regulator system (just Zeners) from both the main preamp board and the input board and installed a proper linear regulator based on L78/7915. I did some rerouting on the input board to cut out the the tape loop and give myself just another line level input. Then obviously I had to rewire everything together and in doing so tried to adhere to some better routing than the original design allowed (given that it had a comparatively massive power amp board in there).
So the design (as per original) is simple. An inverting buffer on input, feeding a shunt feedback Baxandall tone circuit (so again inverting), feeding a pot and then straight to output. So this last stage is a 'passive' preamp. I use short cables and the pot is 20k (ln) so no issues there, but I will be adding a final buffer stage just for versatility to allow longer cables if wanted (may enable both buffered and non-buffered outputs).
On to opamps. So much written I know but I will keep it brief. It came with TL072 in both stages. I am a great fan of TL072 from long back and in these low or unity gain circuits I didn't see a compelling reason to change, but I had socketed the boards and so experimented a little. Tried 4558, NE5532 and AD712 (as used in more upmarket Cambridge Audio A3i). I measured all of them for noise (pretty negligible in all cases but measurable differences) and did some listening tests.
OK my ears are not good and I am not therefore a great guide on sound quality - whatever that means. In all honesty I really couldn't reliably tell them apart. Its quite useful that the tone circuit is switchable so its possible to switch one op amp out of circuit and this did reveal a very slight degradation due to the 4558 - very slight, but female vocals just didn't sound as convincing (at least not to me). For the others I couldn't have reliably told whether the tone circuit was on or off with a blind test. So on this criteria alone the 4558 drops out of contention.
On to noise. Interesting. The differences are tiny and all versions are getting close to the noise floor of my ancient Levell muV meter but here it is. The noisiest (in this circuit) was the TL072, the quietest was the AD712. The total measured difference (full volume, tone on) was only around 10muV from best to worst but given the low levels I was measuring (20-30muV) I guess this is quite a large percentage. However .... it was impossible to tell them apart in practice unless my ear was right up against the speaker. In that case I would say the TL072 noise was a bit more objectionable / noticeable as in being more popcorn like.
Then I am going to add a criteria. DC offset. No surprise here the NE5532 was worst by a country mile. So what? Well the NE5532 implementation would be unusable without some switch on delay relay / switch off mute. I say that because if ever by accident you were to switch off that preamp first (or switch it on last) with NE5532 on board there would be very loud, very unpleasant and possibly damaging loudspeaker cone excursions. I tried it without tone enabled and volume turned down and it was bad enough. The TL072 by comparison was relatively benign (but you would still want to avoid a mistake of powering down preamp with power amp on). The AD712 was just a dreamboat. I couldn't measure any DC offset and it starts up and shuts down almost silently.
Perhaps no surprises - a circuit designed with JFET opamps works better with JFET opamps. But then the performance of the AD712 in comparison with TL072 did take me aback. Received wisdom is this is an ancient 80s opamp and offers no real benefit over the TL072. And I hadn't realised the benefit of DC precision is potentially simplifying a design. It was therefore measurable better and much more practical. In fact for the audiophile approach, DC offset is so low i could easily remove at least two DC blocking capacitors per channel if I wanted to. Picture of new internals just for the hell of it.