What Measurements Show These Things?

[Wes]

Instead of asking What Cannot Be Measured (or perhaps, What Should Not be Measured) I thought this might be a better approach.

I will start with a comment I saw on another audio forum:

What measurement shows how fast an amp can ramp up current delivery?
(answers calling for weighing the power transformer will be disqualified: post an electronic measurement)

obviously, this gets at things often described subjectively as "dynamics" or transient speed...

 

[q3cpma]

Pretty sure it can be seen in the frequency response as long as we're talking about band limited signals.

 

[amirm]

How did someone ascertain fast or slow to begin with? What did they measure?

 

[Sam1879]

"slew rate" is sometimes given in amplifier ratings and refers to changes in voltage output of the amp (Voltage-Change / Time) as far as i know, but so has a direct connection to current i think. (P = U * I)
i'm not sure if it is the correct start for an answer to your question, but maybe there is some informarion about slew rate around?

 

[mansr]

What measurement shows how fast an amp can ramp up current delivery?

Square wave.

 

[sergeauckland]

Slew rate is measured in Volts per microsecond, and is a measure of bow fast an amplifier can change output voltage. Clearly, a high power amplifier outputs more volts, so needs a higher slew rate than a low power amplifier.
An amplifier has sufficient slew rate if it can output its full power into a load with low distortion at the highest frequency of interest The maximum rate of change of voltage is at the zero crossing of the sine wave, so simple geometry can determine what the slew rate needs to be for any output.

Suffice to say that there are no modern SS amps (i.e. made in the last 40 years) that have insufficient slew rate.

S

 

[DonH56]

Instead of asking What Cannot Be Measured (or perhaps, What Should Not be Measured) I thought this might be a better approach.

I will start with a comment I saw on another audio forum:

What measurement shows how fast an amp can ramp up current delivery?
(answers calling for weighing the power transformer will be disqualified: post an electronic measurement)

obviously, this gets at things often described subjectively as "dynamics" or transient speed...

Current slew rate, the change in current versus change in time (di/dt), usually measured with a step input (square wave or similar). Amplifier bandwidth (frequency response) will show it as well (slew rate = 2*pi*f*A where f is the frequency and A the amplitude, solve for f). If you use a known load you can derive the current slew rate from the voltage slew rate (step or square wave response).

Typical amplifiers have more than high enough slew rate (and bandwidth) for the audio band. It is required to meet frequency response specs, distortion specs, and often for stability.

 

[amirm]

Square wave.

If such a measurement exists, by definition then OP's question is moot. Surely whoever asked that question had not already measured the problem. They are just imagining such a problem and hence my question. For example they may think it takes time for electrons to move from power supply to the amplifier and if it can be made faster, it would improve the sound.

If there is insufficient current, that is not a "speed" issue. Ripples would show up in power supply output which then shows up in FFT in my dashboard. Peak vs max power measurements I show will also manifest this.

 

[Wes]

what effects would measuring into a complex impedance introduce vs. measuring into a resistive load?

and... how many power amps out there might have such problems, esp. if a low power amp is connected to a difficult to drive speaker??

 

[Vini darko]

This issue was solved 40 years ago. In the late seventies TIM (transient intermodulation distortion) was a selling point. The sansui aux1 was the peak of this USP.
Here's a quality video from xraytonyb about it.

Modern amps should have this sorted , class D in particular are really good at "speed" due to carzy bandwidth (assuming the power supply is strong of course).
Edit: video blocked on third party sites sorry

 

[Wes]

How did someone ascertain fast or slow to begin with? What did they measure?

It was a speculation by one poster, when another posted complaining about "lack of dynamics" in his setup. That OP was using an expensive 40 W amp.

I can pm you the thread if you want, but didn't post it here as I agree with your admonition to not run down other forums.

 

[Vini darko]

It was a speculation by one poster, when another posted complaining about "lack of dynamics" in his setup. That OP was using an expensive 40 W amp.

I can pm you the thread if you want, but didn't post it here as I agree with your admonition to not run down other forums.

Dynamics in that case I suspect is down to a lack of power from the amplifier. As we know its roughly 10x power for a doubling of volume. 40W doesn't go far. The op may well be suffering from needs a bigger amp or more efficient speakers.

 

[amirm]

It was a speculation by one poster, when another posted complaining about "lack of dynamics" in his setup. That OP was using an expensive 40 W amp.

Lack of dynamics ascertained subjectively has to be first demonstrated to be a current delivery speed. We can't show measurements for things that are not objectively there.

 

[Wes]

Dynamics in that case I suspect is down to a lack of power from the amplifier. As we know its roughly 10x power for a doubling of volume. 40W doesn't go far. The op may well be suffering from needs a bigger amp or more efficient speakers.

my thought also

 

[escksu]

Square wave.

Yes, a square wave will be a good one.

 

[escksu]

Instead of asking What Cannot Be Measured (or perhaps, What Should Not be Measured) I thought this might be a better approach.

I will start with a comment I saw on another audio forum:

What measurement shows how fast an amp can ramp up current delivery?
(answers calling for weighing the power transformer will be disqualified: post an electronic measurement)

obviously, this gets at things often described subjectively as "dynamics" or transient speed...

Yes, as many have pointed out, injecting square ware will be able to show how fast the amp could ramp up. The time it takes for the voltage rise and fall show how fast the amp can ramp up/down. The straight line at peak show how well it can maintain the voltage. However, amps mostly do not have problems with a single frequency at low power. So some measurement take it further. So some tests generate alot more waves (eg. 16 or 32) and different voltage levels.

Of course, the higher the load on the amp (need to deliver more power), the harder it is for the amp to maintain its ramp up/down time. ITs also harder to maintain a constant voltage at peak.

 

[SIY]

Yes, as many have pointed out, injecting square ware will be able to show how fast the amp could ramp up. The time it takes for the voltage rise and fall show how fast the amp can ramp up/down. The straight line at peak show how well it can maintain the voltage. However, amps mostly do not have problems with a single frequency at low power. So some measurement take it further. So some tests generate alot more waves (eg. 16 or 32) and different voltage levels.

Of course, the higher the load on the amp (need to deliver more power), the harder it is for the amp to maintain its ramp up/down time. ITs also harder to maintain a constant voltage at peak.

Don’t confuse rise time with slewing.

 

[KSTR]

What measurement shows how fast an amp can ramp up current delivery?

Short amplifier input so that output voltage = 0. Inject a reasonably small AC current (sweep) into the output and look for high frequency limit.
This basically tests output impedance, of course, but that is the ultimate limit.
Increase current level for the sweep until you see additional effects, especially if there is a current slew limit. Remember, the output voltage is still nominally zero and when the the amp cannot source or sing current fast enough (because it goes into transient or steady starte SOA limiting, etc) the output will move, the feedback loop will go open and the whole amp is out of its usable range.
One needs a reference class current source amplifier for this which is better than the DUT.
Alternatively, you can measure into a quasi-short but it harder to control the exact amount of current but at least it can be monitored and things extrapolated from that.

 

[ZööZ]

For example they may think it takes time for electrons to move from power supply to the amplifier and if it can be made faster, it would improve the sound.

And of course it does take electrons quite some time to travel such distances, and in an ac circuit they barely move at all , luckily the electric field, which is the magic that makes the stuff work, propagates at relativistic speeds.

 

[David Harper]

A long time ago (40 years) I had an SAE 2200 power amp and among it's specs were "rise time" and "slew rate".
I didn't know what those meant but they sounded pretty impressive. That amp seemed very cool. Don't remember what happened to it.
It had handles at each end on the front which really impressed my friends. And me. It was very heavy. which was also pretty impressive.