ruffrecords
Well-known member
Following the passing of Rupert Neve I was pondering the longevity of his designs and others of the period versus the myriad integrated op amps now available that wax and wane every few years.. Is it more than just nostalgia that causes people to think those old designs sound better or is there something tangible that actually creates a well liked sonic signature?
If there were a difference, it would have to be more than just the supposed mojo of vintage components; there would have to be some common thread in the designs that sets them apart from your typical integrated circuit op amp. In the past I have postulated that it may be that the most favoured vintage discrete designs are class A. Others have suggested that the transformers are a major contributor (except Helios mixers had very few transformers yet were considered very musical).
It then occurred to me that there may be another factor that makes a significant difference to the sonic signature. Let me explain.
In the dim and distant past, tubes were the only active devices available. Practical problems due to the large dc voltage difference between inputs and outputs meant that the NFB loop could not easily be closed at DC leading to low frequency stability problems. Miller effect at the high frequency end led to similar NFB stability issues at this end of the audio spectrum. These two factors alone meant it was not really possible to make something like a mic pre with a gain that could be set to a wide range of values (the REDD 47 is a good example of this) because the stability criterion at each gain setting is different.
However, the ancients did have a solutions to these problems. They realised the problem was in the fixed open loop gain. To vary the gain you only had to alter the loop parameters which meant the stability parameters changed with gain. So they cleverly arranged things so that both the open loop gain AND the loop parameters were varied together such that the stability criteria were near identical at all gains. The first instance of this design I can find is in the German V41 which appeared in 1949. The almost identical circuit was repeated in the well known V76 which appeared in 1959.
i do not know if it was deliberate or not, but this technique of varying open loop gain at the same time as closed loop gain was used in all Rupert Neve's three transistor class A designs of the late 1960s and in the Helios and Cadac three transistor and Calrec four transistor designs of the same era.
So, what is it about this technique that affects its sonic signature. In all designs, the open loop response falls off at some high frequency. Which frequency this occurs at depends an many factors but for stability, the frequency at which this begins should be such that by the time the open loop gain as reached unity, the phase shift is less than 180 degrees - this is one of the basic stability criteria that stops your mic pre becoming an oscillator. The bottom line is that with this type of design, this frequency is usually well above the audio band (Rupert once suggested it should be around 75KHz). What this means for sonic signature is the amount of NFB is the same at all frequencies in the audio band and so the distortion at any frequency is the same. Hence any distortion that is introduced does not alter the relative levels of harmonics in the original sound.
Moving on now to integrated op amps we find they are built differently. First they have enormous open loop gains (100dB is not unusual). They do not have the ability to alter the open loop gain as the closed loop gain is altered so they have a serious high frequency stability issue. To fix this and make it easy for almost anyone to deign with them, they include a capacitor inside the chip that starts the HF roll of well before the top end of the audio spectrum - for instance the TL072 starts it at about 12Hz. This allows you to set the closed loop gain easily with a couple of resistors without having to worry about it oscillating. But what is the price? It is that the open loop gain drops at 20dB per decade over nearly all the audio spectrum. This means that the amount of NFB falls at the same rate which in turn means the distortion rises at 20dB per decade. Hence the distortion introduced does affect he relative levels of harmonics in the original sound. In particular it emphasises the higher harmonics and as we all know, the higher the harmonic the less the ear likes it.
So it is my suggestion that this fundamental difference in design may well be what separates vintage discrete designs from modern integrated circuit ones. As a caveat, I will add that not all op amps are as bad as the TL072 in this respect. For instance, the internal capacitors of the NE5532 start the HF drop at 1KHz.nearly 100 times higher than the TL072. Perhaps this goes some way towards explaining why the NE5532 is considered a relatively nice sounding op amp.
All the above is simply an observation based on what I know of the various designs themselves. I am not saying it is certainly an explanation for the sonic difference between the two types of amplifier, only that it is a possible explanation.
Cheers
Ian
If there were a difference, it would have to be more than just the supposed mojo of vintage components; there would have to be some common thread in the designs that sets them apart from your typical integrated circuit op amp. In the past I have postulated that it may be that the most favoured vintage discrete designs are class A. Others have suggested that the transformers are a major contributor (except Helios mixers had very few transformers yet were considered very musical).
It then occurred to me that there may be another factor that makes a significant difference to the sonic signature. Let me explain.
In the dim and distant past, tubes were the only active devices available. Practical problems due to the large dc voltage difference between inputs and outputs meant that the NFB loop could not easily be closed at DC leading to low frequency stability problems. Miller effect at the high frequency end led to similar NFB stability issues at this end of the audio spectrum. These two factors alone meant it was not really possible to make something like a mic pre with a gain that could be set to a wide range of values (the REDD 47 is a good example of this) because the stability criterion at each gain setting is different.
However, the ancients did have a solutions to these problems. They realised the problem was in the fixed open loop gain. To vary the gain you only had to alter the loop parameters which meant the stability parameters changed with gain. So they cleverly arranged things so that both the open loop gain AND the loop parameters were varied together such that the stability criteria were near identical at all gains. The first instance of this design I can find is in the German V41 which appeared in 1949. The almost identical circuit was repeated in the well known V76 which appeared in 1959.
i do not know if it was deliberate or not, but this technique of varying open loop gain at the same time as closed loop gain was used in all Rupert Neve's three transistor class A designs of the late 1960s and in the Helios and Cadac three transistor and Calrec four transistor designs of the same era.
So, what is it about this technique that affects its sonic signature. In all designs, the open loop response falls off at some high frequency. Which frequency this occurs at depends an many factors but for stability, the frequency at which this begins should be such that by the time the open loop gain as reached unity, the phase shift is less than 180 degrees - this is one of the basic stability criteria that stops your mic pre becoming an oscillator. The bottom line is that with this type of design, this frequency is usually well above the audio band (Rupert once suggested it should be around 75KHz). What this means for sonic signature is the amount of NFB is the same at all frequencies in the audio band and so the distortion at any frequency is the same. Hence any distortion that is introduced does not alter the relative levels of harmonics in the original sound.
Moving on now to integrated op amps we find they are built differently. First they have enormous open loop gains (100dB is not unusual). They do not have the ability to alter the open loop gain as the closed loop gain is altered so they have a serious high frequency stability issue. To fix this and make it easy for almost anyone to deign with them, they include a capacitor inside the chip that starts the HF roll of well before the top end of the audio spectrum - for instance the TL072 starts it at about 12Hz. This allows you to set the closed loop gain easily with a couple of resistors without having to worry about it oscillating. But what is the price? It is that the open loop gain drops at 20dB per decade over nearly all the audio spectrum. This means that the amount of NFB falls at the same rate which in turn means the distortion rises at 20dB per decade. Hence the distortion introduced does affect he relative levels of harmonics in the original sound. In particular it emphasises the higher harmonics and as we all know, the higher the harmonic the less the ear likes it.
So it is my suggestion that this fundamental difference in design may well be what separates vintage discrete designs from modern integrated circuit ones. As a caveat, I will add that not all op amps are as bad as the TL072 in this respect. For instance, the internal capacitors of the NE5532 start the HF drop at 1KHz.nearly 100 times higher than the TL072. Perhaps this goes some way towards explaining why the NE5532 is considered a relatively nice sounding op amp.
All the above is simply an observation based on what I know of the various designs themselves. I am not saying it is certainly an explanation for the sonic difference between the two types of amplifier, only that it is a possible explanation.
Cheers
Ian
