What measures for "dynamic contrast or resolution" a.k.a. PRaT?

[Feanor]

Are there objective measures for what I call "dynamic contrast" or "dynamic resolution" -- or what I've heard the Brits call 'PRaT', Pace, rhythm, and timing? By these terms I'm referring to the subjective ability of an amplifier to handle rapid, large or small changes in amplitude. In my experience it is one of the key attributes that separate great amplifiers from merely good ones.

Possibly related is the fact that passive preamplifiers that I've tried have tended to noticable limit this dynamic quality in the downstream power amp.

 

[Daverz]

You could step into the minefield of damping factor arguments. A search should find some threads here.

For speakers, theoretically the spinorama should show any resonances that could cause a problem, if you know how to interpret them. Otherwise, look for a clean waterfall plot.

Room modes can then mess things up again, so that needs to be addressed somehow.

 

[Vini darko]

I think slew rate, bandwidth and dynamic range would be good indicators for this type of thing.
XraytonyB did an interesting series on youtube about the sansui aux1 which was engineered with speed in mind. I think it was built to minimise something called transient intermodulation distortion (TIM). The thing had so much slew and bandwidth that it was always on the edge of going into severe oscillation.

 

[RayDunzl]

Opinion:

Low noise, low resonance, quick start and stop to the sound (both the speakers and the room). You can probably add low distortion to that.

Dynamics are the difference between loud and soft - three of the above relate to the "soft" part. Distortion fuzzes up the loud.

I haven't seen evidence here for an inability to produce the "loud" part (within limits of the gear, of course).

My six foot lower distortion over-powered passives trump the smaller lower powered actives for ultimate "dynamics".

 

[wwenze]

Impulse response and step response (square wave response)

 

[Kvalsvoll]

To be able to answer this, we first need to translate those terms into technical properties of sound. Dynamic contrast can mean transient reproduction - the ability to reproduce transients. "Prat" is a constructed term made up by the hifi-press, and from subjective descriptions of sound associated, it seems to be related to ability to reproduce transients in the lower midrange-upper bass, the frequency range that contributes most to rythm and drive in music.

For amplifiers, this is quite easy, just make sure you always have enough power available - problem solved, nothing more to be gained. Once you start clipping the amplifiers, things start to get a lot more complicated.

The real differences here are caused by the loudspeakers and how they interact with room acoustics. Transients will be cleaner with a faster time decay, and are experienced to be more powerful with more late arrival energy. If the peak amplitude of the transient is reduced, it will sound less powerful and engaging. So this has to do with linear response in the time domain, and nonlinear response, mainly capacity.

But first, you should look at the frequency response. If there is a huge suck-out in the lower midrange - upper bass, the instruments that provide rythm in music will loose their energy, and the music will sound less engaging. This can not be fixed by changing the amplifier, because the fault is caused by loudspeaker - placement - room acoustics.

So these characteristics of sound are very real, we just have some problems with terminology and what words mean. Describing sound using words in a written text is difficult.

 

[pozz]

I think we're overestimating the degree to which PRaT stands for any measurable quantity at all.

It's a subjective descriptor, so for amplifiers working in their linear range it means nothing. As @Kvalsvoll said, just make sure you have enough power for your purposes. The rest are engineers' measures.

 

[JohnYang1997]

Just forget about PRaT. It's completely bullshit. More bullshit than, resolving, punchy, or any other subjective words that actually mean something.

 

[solderdude]

I didn't even include PRaT in my glossery because of the nonsense naming.

There is no way an amplifier can have different pace (change speed) rhythm (also change of speed) nor can it affect timing.
Even a DAC must have completely wrong or broken X'tals installed for pace and rhythm to be audibly off.
Well, the last bit is not entirely true when substantial phase shifts opposite the mids are present.
That would mean it is a very poor performing amplifier b.t.w. with measurable problems within the audible band.
A 10kHz squarewave at (near) full power using a realistic load can quickly confirm an amplifier is fast enough.

Then one also has to realize that 'attack', 'sharpness' is in the 5 - 8kHz range which is not a problem for any audio device.

Most likely its a combination of the aspect @Kvalsvoll mentioned and 'sighted' observations being made.
I wonder if the described differences between amps would become obvious when level multiple matched, blind tests were done to confirm this.
And even then I would really like to see simple FR, phase and distortion measurements of such amps under actual or simulated loads when the blind tests show this PRaT to actually exist.

Possibly related is the fact that passive preamplifiers that I've tried have tended to noticable limit this dynamic quality in the downstream power amp.

Noticeable but certainly not measurable. For the latter reasons there are no metrics for it. The first part (noticeable) is because the comparisons were probably performed 'sighted'. As soon as comparisons are sighted there will be no relevance to measurements.

 

[BDWoody]

I've heard the Brits call 'PRaT', Pace, rhythm, and timing.

Brits are funny...

 

[DSJR]

Thanks a bunch

 

[Feanor]

Thanks everyone for your comments (to this point).

As @Kvalsvoll mentioned, to me it seems me to be a matter of handling transients, but does Amir test for this in case of amplifiers in any way? What about 'slew rate'? What about 'square waves'?

As for square waves, comments I've heard to this point are a bit dismissive, stating that they are mainly about extended frequency bandwidth.

Like I said, passive preamps I've tried have limited what I've called "dynamic contrast", why do they do this when the source DAC or CD player has a relatively high voltage output?

"PRaT" is obvious BS which I mention only because I've heard it used in this realm of amplifier performance.

 

[Jimbob54]

Brits are funny...

"Funny" Ha ha, or "Funny" peculiar?

 

[solderdude]

Like I said, passive preamps I've tried have limited what I've called "dynamic contrast", why do they do this when the source DAC or CD player has a relatively high voltage output?

Based on what ? Sighted observations or measurements showing compression ? Do they objectively do this or are they merely perceived doing this ?

When there is a bandwidth at full output voltage of 50kHz the amp is 'fast enough'. Simply because in audio there isn't anything faster than that.
The square-wave is a test signal that does not exist in music. It is used to show engineers certain aspects of the amplifier.
The only squarewaves in audio signals are the ones that should not be there in the first place, generated by filterless DACs.

The only difference between a passive pre-amp and active one is buffering and output resistance. One should realize that the vast majority of active pre-amps uses the same volume control potmeter as the passive one. It only omits the gain stage/buffering.

What you are most likely hearing is the lack of output voltage (they can only attenuate, aside from some rare ones) or have some treble roll-off when using high capacitance interlinks at a specific volumecontrol range.
The potemeter itself cannot 'compress' dynamics in an audible way. It doesn't in a passive pre-amp nor in an active one.

 

[pozz]

Thanks everyone for your comments (to this point).

As @Kvalsvoll mentioned, to me it seems me to be a matter of handling transients, but does Amir test for this in case of amplifiers in any way? What about 'slew rate'? What about 'square waves'?

As for square waves, comments I've heard to this point are a bit dismissive, stating that they are mainly about extended frequency bandwidth.

Like I said, passive preamps I've tried have limited what I've called "dynamic contrast", why do they do this when the source DAC or CD player has a relatively high voltage output?

"PRaT" is obvious BS which I mention only because I've heard it used in this realm of amplifier performance.

Slew rate is an amp designer's measurement and is valid only when trying to figure out the source of certain distortions. It has no direct relationship to audibility.

Same goes for square wave measurements, the shape of which you see change as the bandwidth is reduced. They are, again, a tool for diagnosis.

 

[BDWoody]

"Funny" Ha ha, or "Funny" peculiar?

Indeed!

 

[krabapple]

I think we're overestimating the degree to which PRaT stands for any measurable quantity at all.

It's a subjective descriptor, so for amplifiers working in their linear range it means nothing.

And does it really come from British audiophiles? I would think they would avoid that particular acronym for anything they wanted taken seriously. I suspect PRAT actually first appeared in the usual suspect (American) journals -- TAS, Stereophile, or one of the more obscure. Anyone know for sure?

 

[SIY]

And does it really come from British audiophiles? I would think they would avoid that particular acronym for anything they wanted taken seriously. I suspect PRAT actually first appeared in the usual suspect (American) journals -- TAS, Stereophile, or one of the more obscure. Anyone know for sure?

95% sure it was British, and specifically Martin Colloms.

 

[gvl]

Possibly related is the fact that passive preamplifiers that I've tried have tended to noticable limit this dynamic quality in the downstream power amp.

This matches my experience with passive pres, I suspect the cause for it is the HF roll-off they can introduce by forming an RC filter with the power amp input or cable capacitance, so they sound a bit more dull as compared to properly buffered active preamps with low output impedance.

 

[krabapple]

95% sure it was British, and specifically Martin Colloms.

Ah. One of that happy breed whose technical training does not prevent them from asserting that e.g., speaker cables sound different. Thanks.