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

[JohnRoberts]

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.

I killed a lot of brain cells over the years thinking about (and doing) mic preamps.

Using modern off the shelf technology they are pretty much transparent. However when trying to merchandise a new SKU as better, it helps if it at least sounds different. One obvious way to do that is with unconventional input termination (other than the typical 1.5-2k).

Another mic preamp variant I saw one popular company use in recent years is to add a soft limiter to the mic preamp. This allows the operator to literally overdrive it with only minimal audible artifacts. Since louder is always perceived as better, this is perceived as better my some.

If serious I suggest null testing to compare two otherwise similar audio paths, a deep null of the two outputs when subtracted from each others suggests that they are the same. FWIW microphones are not accurate enough to null with each other. Too many variables.

JR

 

[Voyager10]

Since louder is always perceived as better,

This is the elephant in the room.

I've often wondered how to reconcile the countless reports of "I swapped X for Y, and it sounds so much better", with an absolute scarcity of concrete results confirmed by repeatable, properly controlled experiments.

I am fairly certain that for small level differences (1dB or less), I hear changes in "sound quality" - brightness, clarity, whatever - without recognising them as actually louder or quieter. In other words, many "X vs Y" differences which disappear when examined under lab conditions may simply be a matter of inaccurate level matching when listening.

Also: alcohol and tiredness are (in my experience) massive confounding variables in what you think you're hearing.

 

[JohnRoberts]

This is the elephant in the room.

I've often wondered how to reconcile the countless reports of "I swapped X for Y, and it sounds so much better", with an absolute scarcity of concrete results confirmed by repeatable, properly controlled experiments.

I am fairly certain that for small level differences (1dB or less), I hear changes in "sound quality" - brightness, clarity, whatever - without recognising them as actually louder or quieter. In other words, many "X vs Y" differences which disappear when examined under lab conditions may simply be a matter of inaccurate level matching when listening.

Also: alcohol and tiredness are (in my experience) massive confounding variables in what you think you're hearing.

I have shared this anecdote before, but back in the late 70s early 80s I was spending some time inside a friend's recording studio control room, tweaking a delay line/flanger design (Loft 440/450). Efx designs are pretty subjective but I was spending time in a real recording studio to get access to good source material, with accurate studio monitors for playback.

After a few hours the group was getting burned out and needed a cigarette break. Since I don't smoke I grabbed a truly mediocre cardboard cup of coffee from the machine in the break area.

Returning to the circuit tweak I was working on after a 15-20 minute break, what I was hearing was dramatically different from what I thought I heard before. I am not sure what to blame other than human fallibility. While that studio delay line flanger product was successful, it pretty much got spanked in the market by lower cost digital delay lines.

Since then I prefer to work with audio paths that can be objectively characterized with a competent test bench. I learned decades ago that my ears were not absolutely reliable. I mainly use my ears to confirm that I didn't neglect to measure something important.

JR

 

[ruffrecords]

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.

And also quite a bit of work has been done on this topic over the decades. It would be worth googling.

Cheers

Ian

 

[MidnightArrakis]

I grabbed a truly mediocre cardboard cup of coffee from the machine in the break area.

Returning to the circuit tweak I was working on after a 15-20 minute break, what I was hearing was dramatically different from what I thought I heard before. I am not sure what to blame other than human fallibility

What about the effects of caffeine on the human auditory sensory system? Perhaps the dilation of blood vessels due to the caffeine may have an effect on one's hearing response? If ingesting some LSD can allow you to "hear in color", then maybe just a bit of some coffee caffeine might allow you to simply hear somewhat "better". Just a thought.....

/

 

[JohnRoberts]

What about the effects of caffeine on the human auditory sensory system? Perhaps the dilation of blood vessels due to the caffeine may have an effect on one's hearing response? If ingesting some LSD can allow you to "hear in color", then maybe just a bit of some coffee caffeine might allow you to simply hear somewhat "better". Just a thought.....

/

You forgot about the blood sugar change.

I never heard sounds in color while on LSD, but that experience is likely different for different folks. No LSD was dropped that day in the studio.

The lesson I drew was that human hearing can be a little too variable to trust blindly.

JR

 

[user 133392]

Line level signal.

You eliminated the microphone and an acoustic source.
With a line-level source you performed a more consistent test. Very good.

If you used a line level signal there must have been a resistive pad, likely external, and common to all the DUT preamps.
Tell me more...

What type of pad did you use? Was it a U-pad? If not what type?
How much attenuation in the pad/gain in the preamps?
What type of line level output drove the pad? Was it a floating transformer output or an active output?
If it was an active output, was it balanced with both legs driven symmetrically or single-ended with one pad input leg grounded and one driven?

My questions, depending on the answers may actually lead somewhere...

 

[freshtapescent]

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

I haven’t read the whole thread but I disagree with your beginning premiseit doesn’t and won’t ever come down to any one measure-able electrical specification -in isolation-. The mic’s electrical properties and mic amp comprise a resonant system. All of the resulting properties you’ve mentioned AND more are what gives an amp its character and would differentiate it from a different design.

 

[user 133392]

The mic’s electrical properties and mic amp comprise a resonant system.

Agreed, but the OP did later clarify he was using a line-level source, presumably padded, so the microphone variable is eliminated.

A null test, as JR has pointed out, would provide some measure of what the preamps are actually doing to the signal. The residual un-nulled components of the different preamps he tested could be instructive.

 

[Cqwet Dbdfte]

I haven’t read the whole thread but I disagree with your beginning premiseit doesn’t and won’t ever come down to any one measure-able electrical specification -in isolation-. The mic’s electrical properties and mic amp comprise a resonant system. All of the resulting properties you’ve mentioned AND more are what gives an amp its character and would differentiate it from a different design.

Likely, if a mike, any resonances would be acoustic, not electric.
Unless an anechoic chamber is used, with a consistent sound source, i.e. not a human.

This topic is as old as audio itself. Measurement methods from the 1930's are still used, and while
you can measure equipment differences, assigning a value scale is harder and has not been done.
People with money to burn spend a lot on things that LOOK better, which has a different purpose other than sound itself.
In measurement systems simple signals are always used, never music. There has never been a value scale assigned to levels of odd harmonics, and their perceived negative effect.
The assumption that a lower number of non linearities would easily be translated into its reciprocal quality factor. As to harmonics, no common system is distinguishing between odd and even order is used.
%THD can be all odd or all even, with the same reciprocal goodness factor.
It is pretty well established that odd order dominance subjectively sound worse.
Soft clipping sound better than hard clipping, which points to odf vs. even differences.
If there was a quantitative scoring system invented we would not be having this discourse.
IMD is of course a major subject, with the same scoring problems as single tone measurements.
While Mr. Black's invention of negative feedback has improved static measurements it is not a guarantee that more negative feedback is the solution.
Music production has more creative freedom than REproduction the same underlying issues prevail.
I would propose a study of artificially generated insertion of specific harmonics and develop a scale on their effect on perceived sound.
I'm not sure if white noise testing with a frequency section nulled out and then measured has been done.
While not quantitative it could show differences between equipment.
So, more questions than answers.

 

[JohnRoberts]

Likely, if a mike, any resonances would be acoustic, not electric.
Unless an anechoic chamber is used, with a consistent sound source, i.e. not a human.

This topic is as old as audio itself. Measurement methods from the 1930's are still used, and while
you can measure equipment differences, assigning a value scale is harder and has not been done.
People with money to burn spend a lot on things that LOOK better, which has a different purpose other than sound itself.
In measurement systems simple signals are always used, never music. There has never been a value scale assigned to levels of odd harmonics, and their perceived negative effect.
The assumption that a lower number of non linearities would easily be translated into its reciprocal quality factor. As to harmonics, no common system is distinguishing between odd and even order is used.
%THD can be all odd or all even, with the same reciprocal goodness factor.
It is pretty well established that odd order dominance subjectively sound worse.
Soft clipping sound better than hard clipping, which points to odf vs. even differences.
If there was a quantitative scoring system invented we would not be having this discourse.
IMD is of course a major subject, with the same scoring problems as single tone measurements.
While Mr. Black's invention of negative feedback has improved static measurements it is not a guarantee that more negative feedback is the solution.
Music production has more creative freedom than REproduction the same underlying issues prevail.
I would propose a study of artificially generated insertion of specific harmonics and develop a scale on their effect on perceived sound.
I'm not sure if white noise testing with a frequency section nulled out and then measured has been done.
While not quantitative it could show differences between equipment.
So, more questions than answers.

My judgement is that there are more answers than questions. Audio technology is quite mature now.
==
I've shared this story so many times that even I am getting bored, but back in the 70s I discovered the inadequacy of trying to characterize RIAA phono preamp stages with simple THD+N measurements. The RIAA eq, effectively a low pass filter, also significantly attenuated the HF harmonic distortion. I had to DIY my own two-tone IMD analyzer using 19kHz and 20 kHz 1:1 (I modified an old Heathkit SMPTE IMD analyzer). This revealed a lot of distortion that was not being captured by conventional bench measurements. Now modern analyzers do this and more.

JR

 

[Cqwet Dbdfte]

I remember your two tone 19+20 IMD deal.
Can you point to where I can find a quantitative/qualitative harmonic analysis?

 

[ccaudle]

Can you point to where I can find a quantitative harmonic analysis?

Not for preference, but for audibility the place to look is codec research papers.

 

[Estomagado]

I have shared this anecdote before, but back in the late 70s early 80s I was spending some time inside a friend's recording studio control room, tweaking a delay line/flanger design (Loft 440/450).

This one?

https://modularsynthesis.com/loft/450/Loft 450 Owners Manual.pdf

 

[JohnRoberts]

Can you point to where I can find a quantitative/qualitative harmonic analysis?

I don't know what that is. If interested in perception look into the field of psychoacoustics, perhaps more specifically perceptual distortions. Over the years I have used a few perceptual tricks in the course of designing dynamic processors. Human hearing tends to overlook some short term phenomenon, this can be useful when designing side chains for compressor/expanders.

This one?

https://modularsynthesis.com/loft/450/Loft 450 Owners Manual.pdf

yup, that's the old soldier.

JR

 

[Mr. Moscode]

From what I've read here everyone's opinions are valid. So many variables, so few ways to measure.

There are more factors than just the op amp. The power supplies and regulators, wiring, cap quality, resistor quality, circuit topography, pole points, gain, layout, op amp quality, discrete component designs, tubes, hi NFB, low NFB, constant NFB w gain pads, active eq, passive eq, just to name a few things that alter the qualities of sound we hear but may not show up on the bench.

Ears - AM or PM?, over what period of time are you evaluating? I usually listen for a week or 2 before I feel I have consistent repeatable results. And I agree what you put in your body affects hearing. I found that Ibuprofen seems to aggravate tinnitus, also fatigue and constant exposure to loud sounds alters hearing. Loud sound triggers a reflex in the ears that tightens the eardrum to help attenuate loud sound. Like a mic capsule skin, the tension changes the response. A good night's sleep works well for me.

I made a preamp for our RCA 77 ribbon mics with 3 AD797s wired as an instrumentation amp. No transformer. Low noise, high gain, plenty of punch and presence. Sounded great. Much better than standard single ended approach.

That's my story and I'm sticking to it. YMMV

 

[AdrienPerinot]

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

Well not that much, it seems. I've done a lot of research online, and apart from "beliefs" and "feelings", I was not able to find science-backed answers.

What about the effects of caffeine on the human auditory sensory system?

Love that. It can be both a joke and reality

won’t ever come down to any one measure-able electrical specification

There are more factors than just the op amp. The power supplies and regulators, wiring, cap quality, resistor quality, circuit topography, pole points, gain, layout, op amp quality, discrete component designs, tubes, hi NFB, low NFB, constant NFB w gain pads, active eq, passive eq, just to name a few things that alter the qualities of sound we hear but may not show up on the bench.

I saw multiple claims that it's "a set of factors" that influence the sound, but do we have actual measurements of that ? Especially when it comes to transient response, nothing is clear. Maybe I did not find the good studies, and of course it makes sense that many parameters can influence the result - but I'd like to see some clear comparative study of, say, a tube preamp, a clean solid state one and a colored solid state one with transformers, that would highlight what is actually different. It doesn't have to be exhaustive, but I wish it could show more than simply "a different frequency response".

A null test, as JR has pointed out, would provide some measure of what the preamps are actually doing to the signal. The residual un-nulled components of the different preamps he tested could be instructive.

I did a null test for instance between two opamps transformers on my Warm Audio TB12. There is a difference in the mids/high-mids in particular, but it doesn't sound particularly different to me. So I'm not sure what to conclude : maybe phase is affected, but it doesn't change the sound ? It's a rabbit hole lol.

 

[gyraf]

..recovering from overload in an graceful way is probably an underestimated parameter when it comes to preamps. Somewhat in line with the soft-clipping suggested by JR earlier

(great thread, btw..!)

 

[MidnightArrakis]

https://modularsynthesis.com/loft/450/Loft 450 Owners Manual.pdf

>> WOW!!! FLASHBACK!!! -- XLR Pin-3 = HOT!!!

/

 

[user 133392]

@AdrienPerinot Please refer to my post, #27, and then tell me more about the pad and driving source.

https://groupdiy.com/threads/preamp...armonics-what-is-it.87436/page-2#post-1148627