Preamp difference : if it's not the frequency, not the slew rate, and not the harmonics, what is it ?

[AdrienPerinot]

Hi,

I took the opportunity to measure different preamps (new SSL ones, Focusrite Scarlett, Warm Audio TB12), and investigate a bit on the topic of "audible differences between preamps".

If we run the preamps too hot, I came to the following conclusions :

• The preamps are usually rather flat in terms of frequency. At least nothing that couldn't be fixed with some basic digital EQing. Example : Measurement of an API312 here on SoundOnSound
  So it seems unlikely that frequency response is really what differs between preamps.
• The harmonics are of course different, in the case of transformer-balanced preamps. But they are usually at a rather subtle level (unless the preamp is pushed).
• The slew rate of the preamp usually seems to allow for a correct reproduction of all the audible frequencies.

So now I am wondering :

1. Why would a preamp like the API be called "punchy" ? What would "punch" be ?
2. If it's a variation of the transients (we hear about "slow" vs "fast" preamps), then where does it come from if the slew limit is > 20000 Hz ? The transformer ? If so, aren't the transformers supposed to be rather transparent, harmonics put aside for the lower frequencies, in the frequency range ?
3. Would the slew rate, if a bit too low, be able to influence the audible frequencies (even if the amplifier would, on paper, be able to reproduce those frequencies without any problem) ? It seems so : https://hifisonix.com/wp-content/uploads/2018/03/SID_and_TIM_W_Jung_77-79.pdf
4. Did I miss any phenomenon ?

I've searched all around the Internet but I can't find a clear answer.
Everyone seems to hear punch and smoothness differences, and I feel I hear them too, but where does it actually come from ? Has anyone been able to 'demonstrate' that ?

Thanks
Adrien

 

[JohnRoberts]

Hi,

I took the opportunity to measure different preamps (new SSL ones, Focusrite Scarlett, Warm Audio TB12), and investigate a bit on the topic of "audible differences between preamps".

If we run the preamps too hot, I came to the following conclusions :

• The preamps are usually rather flat in terms of frequency. At least nothing that couldn't be fixed with some basic digital EQing. Example : Measurement of an API312 here on SoundOnSound
  So it seems unlikely that frequency response is really what differs between preamps.
• The harmonics are of course different, in the case of transformer-balanced preamps. But they are usually at a rather subtle level (unless the preamp is pushed).
• The slew rate of the preamp usually seems to allow for a correct reproduction of all the audible frequencies.

So now I am wondering :

1. Why would a preamp like the API be called "punchy" ? What would "punch" be ?
2. If it's a variation of the transients (we hear about "slow" vs "fast" preamps), then where does it come from if the slew limit is > 20000 Hz ? The transformer ? If so, aren't the transformers supposed to be rather transparent, harmonics put aside for the lower frequencies, in the frequency range ?
3. Would the slew rate, if a bit too low, be able to influence the audible frequencies (even if the amplifier would, on paper, be able to reproduce those frequencies without any problem) ? It seems so : https://hifisonix.com/wp-content/uploads/2018/03/SID_and_TIM_W_Jung_77-79.pdf
4. Did I miss any phenomenon ?

I've searched all around the Internet but I can't find a clear answer.
Everyone seems to hear punch and smoothness differences, and I feel I hear them too, but where does it actually come from ? Has anyone been able to 'demonstrate' that ?

Thanks
Adrien

This is a very mature topic studied by design engineers for decades, including serious psychoacoustic studies and application of known phenomenon. Anecdotal subjective opinions are just that and everybody has one.

A serious quantification of "audibility" requires double blind listening tests carried out to realize statistical significance.

Sorry no easy answers.

JR

 

[k brown]

Slew rate hasn't really been an issue for audio circuitry for quite awhile.

Another factor (especially when using dynamic and ribbon mics) is that the synergy between a given mic and a given pre is a considerable element in the resulting sound.

 

[ruffrecords]

It's the transfer function, which unfortunately for static measurements like frequency response etc, is a dynamic property, and therefore revealed only by our ears.

Cheers

Ian

 

[JohnRoberts]

Slew rate hasn't really been an issue for audio circuitry for quite awhile.

Since the 70s inexpensive off the shelf op amps were faster than audio signals.

Another factor (especially when using dynamic and ribbon mics) is that the synergy between a given mic and a given pre is a considerable element in the resulting sound.

I wouldn't call it synergy. The more appropriate term is called "loading" and yes different termination impedances can cause audible differences.

JR

 

[ccaudle]

So it seems unlikely that frequency response is really what differs between preamps.

Don't dismiss frequency response variations so quickly, especially in transformer coupled preamps.
In the Sound on Sound measurements you linked the API clearly has a peak of around 0.25dB but across a very side frequency range, over seven octaves. That would be expected to be audible, and also complicate level matching for proper comparison tests against another preamp.

 

[k brown]

Since the 70s inexpensive off the shelf op amps were faster than audio signals.

I wouldn't call it synergy. The more appropriate term is called "loading" and yes different termination impedances can cause audible differences.

JR

I used 'synergy' in order to include complimentary non-linearilties in a particular mic and pre combo; in addition to loading factors. For example the extremely not-flat SM58 can be complimented by some 'colorful' preamps, and made worse by some others, despite offering the mic a proper load.

 

[AdrienPerinot]

Thanks everyone for the answers so far.

A serious quantification of "audibility" requires double blind listening tests carried out to realize statistical significance.

I rather agree with that, but I am mostly trying to understand the electrical phenomenon behind the supposed differences, instead of trying to assess whether those differences are audible.

Slew rate hasn't really been an issue for audio circuitry for quite awhile.

Seems so, but I saw various references of slew rates which may imply impacts below 20 kHz.

Another factor (especially when using dynamic and ribbon mics) is that the synergy between a given mic and a given pre is a considerable element in the resulting sound.

Yes, impedance matching is a thing for these situations + I guess for situations where the microphones have a transformer indeed. But I am not sure if we're only talking about frequency response, or if there are other phenomena at work in this situation ?

It's the transfer function, which unfortunately for static measurements like frequency response etc, is a dynamic property, and therefore revealed only by our ears.

I may be wrong, but the way I see it, transfer function is a way to describe the behavior. However, we can probably characterize this by doing several measurements.
I don't think frequency response would change so much (but maybe it does ?)... so which parameters do you think we should look at ?

In the Sound on Sound measurements you linked the API clearly has a peak of around 0.25dB but across a very side frequency range, over seven octaves. That would be expected to be audible, and also complicate level matching for proper comparison tests against another preamp.

You're correct. Although in this specific case, I'm not sure how it would account for the "punch" people believe this preamp has. But I agree with you, it can probably give subtle "tones" to the preamps...
In this case, my "problem" is that it's just some kind of voicing, and not particularly "specific" to the preamp. Meaning it's not really something I would call "character".

I used 'synergy' in order to include complimentary non-linearilties in a particular mic and pre combo; in addition to loading factors.

I'm sorry, I don't understand what you mean by that ? Which non linearities ?

Thanks everyone
Adrien

 

[JohnRoberts]

Don't dismiss frequency response variations so quickly, especially in transformer coupled preamps.
In the Sound on Sound measurements you linked the API clearly has a peak of around 0.25dB but across a very side frequency range, over seven octaves. That would be expected to be audible, and also complicate level matching for proper comparison tests against another preamp.

Don't dismiss frequency response variations so quickly, especially in transformer coupled preamps.
In the Sound on Sound measurements you linked the API clearly has a peak of around 0.25dB but across a very side frequency range, over seven octaves. That would be expected to be audible, and also complicate level matching for proper comparison tests against another preamp.

Indeed frequency response deviation from flat or expected can be clearly audible.

I recall seeing one hifi magazine review of a several thousand dollar phono preamp. The frequency response plot revealed a less then on dB response bump in the upper midrange. Not horrible but enough to be audible in a close audition vs a flat audio path.

Thanks everyone for the answers so far.



I rather agree with that, but I am mostly trying to understand the electrical phenomenon behind the supposed differences, instead of trying to assess whether those differences are audible.

In design we must first identify, then measure, before we can manage a parameter for a design.

Seems so, but I saw various references of slew rates which may imply impacts below 20 kHz.

Slew rate needs to be factored vs signal amplitude so lots of -10dBV nominal gear using 0.5V/uSec slew rates was not horrible. Slew rate limiting is clearly distortion so not good.

Yes, impedance matching is a thing for these situations +

Not matching termination to source, but providing the expected nominal termination. The defacto termination for microphones is 10x "bridging". So a 150-200 ohm source impedance microphone expects around a 2k termination. Some people have experimented with unusually low and unusually high impedance mic terminations.

I guess for situations where the microphones have a transformer indeed. But I am not sure if we're only talking about frequency response, or if there are other phenomena at work in this situation ?

frequency response is the most dominant audible phenomenon.

I may be wrong, but the way I see it, transfer function is a way to describe the behavior. However, we can probably characterize this by doing several measurements.
I don't think frequency response would change so much (but maybe it does ?)... so which parameters do you think we should look at ?

Whatever you want.

You're correct. Although in this specific case, I'm not sure how it would account for the "punch" people believe this preamp has. But I agree with you, it can probably give subtle "tones" to the preamps...
In this case, my "problem" is that it's just some kind of voicing, and not particularly "specific" to the preamp. Meaning it's not really something I would call "character".

Back in the 80s when I was still writing my magazine column for a recording magazine, I offered a joke glossary full of funny definitions for obscure subjective terms like "punch", another one was "balls" (make your own jokes).

I'm sorry, I don't understand what you mean by that ? Which non linearities ?

Thanks everyone
Adrien

Enjoy

JR

 

[ruffrecords]

I may be wrong, but the way I see it, transfer function is a way to describe the behaviour. However, we can probably characterize this by doing several measurements.
I don't think frequency response would change so much (but maybe it does ?)... so which parameters do you think we should look at ?


Thanks everyone
Adrien

It is but it is not a static parameter as is so often assumed. The most obvious test that actually reveals some of the dynamic behaviour of the transfer function is the intermodulation distortion test. Probably explains why this test is more revealing of perceived sound quality than a simple THD test.

Cheers

Ian

 

[JohnRoberts]

It is but it is not a static parameter as is so often assumed. The most obvious test that actually reveals some of the dynamic behaviour of the transfer function is the intermodulation distortion test. Probably explains why this test is more revealing of perceived sound quality than a simple THD test.

Cheers

Ian

I am not sure I understand the distinction you offer between static and dynamic behavior.

I am a long time fan of using IMD (intermodulation distortion) to characterize audio paths. The significant difference that I perceive between harmonic and intermodulation distortion measurements, while both measure path non-linearities, harmonic distortion generate distortion products at higher frequency overtones. Intermodulation distortion can generate much lower frequency distortion products. In the real world, reproducing high frequency audio is harder and generates more distortion. Harmonic distortion from HF signals is even higher so not very audible, Intermodulation distortion from HF signals is much lower frequency and therefore much more audible.

JR

 

[ccaudle]

I'm not sure how it would account for the "punch" people believe this preamp has

There is a lot wrapped up in that single statement you made.
First would be to check whether different people actually describe the sound of the preamp the same way. Non-sighted of course, because there is a lot of suggestibility involved with audio, meaning that if someone well known or well regarded uses a term in association with some piece of equipment, then you will find other people saying the same whether or not they would actually use that same description unprompted, or whether they could actually identify that character in a situation where they could not identify by sight what they were listening to.

my "problem" is that it's just some kind of voicing, and not particularly "specific" to the preamp. Meaning it's not really something I would call "character".

I don't think that is an internally consistent statement. Or you are making up some new definition of "character" without sharing your definition. In my usage character would refer to anything which can be used to distinguish one thing from another, and frequency response voicing is most definitely one thing which can distinguish two different pieces of equipment, so frequency response would be at the top of the list for "character."
I'm not sure what you were trying to convey by saying that the frequency response of a specific design is not "specific" to the preamp.
I found a dictionary defintion of specific as "peculiar or proper to somebody or something, as qualities, characteristics, effects, etc." A non-flat frequency response with a particular amplitude vs. frequency response that one design has would seem to fit that definition, so I would say that particular frequency response is specific to that API design, and imparts a sonic character to the design.

So to distill that into concrete steps that would help clarify your question:
1. Have multiple people listen to the pre-amp in a non-sighted fashion (i.e. the listeners cannot tell what brand or model they are listening to) and see if "punchy" is actually a common description. Perhaps a different term is more fashionable these days, the relevant point being that multiple people describe hearing a similar feature of the sound.

2. Measure as many different technical parameters as practical, and form some testable hypothesis about what may explain that.

3. Modify the units to remove an identified parameter, and modify different models to exhibit the same behavior of that parameter, and repeat step 1 to see how the perceived sound has changed.

Note that 3 could potentially done externally, e.g. in a DAW, rather than physically modifying different units.

 

[AdrienPerinot]

It is but it is not a static parameter as is so often assumed. The most obvious test that actually reveals some of the dynamic behaviour of the transfer function is the intermodulation distortion test. Probably explains why this test is more revealing of perceived sound quality than a simple THD test.

Cheers

Ian

I'll look at IMD figures, good idea

There is a lot wrapped up in that single statement you made.
First would be to check whether different people actually describe the sound of the preamp the same way. Non-sighted of course, because there is a lot of suggestibility involved with audio, meaning that if someone well known or well regarded uses a term in association with some piece of equipment, then you will find other people saying the same whether or not they would actually use that same description unprompted, or whether they could actually identify that character in a situation where they could not identify by sight what they were listening to.

Yeah I went a bit fast on that one. To give more details on the way I see it : many people claim they can hear differences between preamps. I do, to some extent. But just like audiophile topics, it's always based on "feeling". So I agree that double-blind tests and so on could be carried out, but as I'm unable to do that, I'm mostly trying to understand (for myself) if some parameters other than frequency response and harmonic distorsion can have an audible effect.

I don't think that is an internally consistent statement. Or you are making up some new definition of "character" without sharing your definition.

Yes, I was not precise enough. I was trying to say : if the difference between preamps is only an EQ curve, then we can virtually match a Neve with an API preamp with a simple EQ curve.
So I'm trying to understand if, apart from harmonics distribution, there are actual differences between preamps.

---

So basically I could rephrase my question like that : apart from static frequency response and harmonics, are there any elements that are actually different between preamps in a measurable way ?

 

[KA-Electonics.com]

I took the opportunity to measure different preamps

I'm confused.

Did you measure with equipment or did you listen comparatively?
If you listened what microphone(s) did you use?
Or did you take a line level signal, resistively pad it down, and compare results that way?

 

[ttttt]

I agree with Ian that the transfer function is important for this. I also agree that this has been an ongoing discussion for decades. Full testing can really take quite a bit of time, and would benefit from automation. I would expect that IMD measurements at a large variety of levels and frequencies may reveal things that often get missed. Transformers should generally be considered nonlinear devices.

 

[Matt Nolan]

Some ideas (and some support for things already said):

• IMD
• Behaviour on transients that go outside the nominal headroom spec
• Interaction with mics that have an impedance which is not flat with frequency in the nominal range

 

[JohnRoberts]

It is worth pointing out that between microphones and preamplifiers the microphones are the weaker link, with more variables (like pick up pattern). The only thing more variable than microphones are loudspeakers.

Audio paths generally sound the same when operating in their linear region. Overload like amplitude clipping or slew rate limiting can and will be audible. In power amps driving loudspeakers, current limiting can also be audible.

JR

 

[AdrienPerinot]

I'm confused.

Did you measure with equipment or did you listen comparatively?
If you listened what microphone(s) did you use?
Or did you take a line level signal, resistively pad it down, and compare results that way?

Line level signal. The way I understand it, it can match the behavior of transformerless condenser microphones. But maybe there is a more dynamic behavior with, say, dynamic microphones ? (no pun intended)

. I would expect that IMD measurements at a large variety of levels and frequencies may reveal things that often get missed.

Maybe yes. I need to improve my knowledge of this type of measurements (I'm familiar with the phenomenon itself, but I'm not sure what to expect when I measure it. As a first simple quick test, I've used PluginDoctor to measure that, but changing transformer or opamp on my Warm AUdio TB12 doesn't seem to change anything really. Though I haven't yet tested much the impact of level changes.
I'm surprised no one ever took the time to measure all of this and publish it.

 

[ccaudle]

I was trying to say : if the difference between preamps is only an EQ curve, then we can virtually match a Neve with an API preamp with a simple EQ curve

Yes, that would be one way to test the hypothesis, record from a Neve and an API, use external EQ to match the frequency response of both, and match levels.
Note that sensitivity testing using two copies of the same audio has consistently shown that levels must be matched within 0.1dB or audible differences can be perceived even between otherwise identical signals. That makes testing two different devices very difficult, since achieving that level of matching and stable matching over the time period of the testing can be difficult with devices using potentiometers to set gain.

 

[iomegaman]

Side bar comment here...slew rate obviously has been a non-factor in preamps for a long time, except when it isn't...Dave Hill intentionally modified his Europa preamp to add color based on slowing down the slew rate...yes its distortion, but that guys ears knew which kind was edible...

Seems like the Groove Tubes Vipre also used slew rate to "color" the sound...

By and large I think this discussion has been done almost as much as an Avengers movie franchise...

For the most part preamp designers intentionally tried to make th preamps transparent, or invisible to the process...that would make a lot of them indistinguishable from one another in a general sort of way...if you are trying to objectively sort out how they are different sounding when they were intentionally designed to not be you need more than your own set of ears to do this as everyone on this thread has suggested...if its a subjective answer you want you already have it.