Measurement Microphony

[RayDunzl]

I have an audio interface hooked attached to the output of my power amp, to see what's there.

When the amp is on things look normal. Lots of power harmonics, though, which I'll investigate eventually.

When the amp is in standby, something is microphonic.


Idle standby:

Handclap 10 feet away:

When the amp is fully powered and ready to play, the microphony goes away.

Same reading, with or without handclap.

???


Test cable attached to interface (XLR with alligator jumpers on the amplifier end) but removed from amp and no termination, no microphony:

How can the amp, in hot standby, apparently act as a microphone?

 

[Blumlein 88]

Does the amp in standby turn everything on except the output stage or what is it doing?

Worked on an old ML monster amp once which had standby and slow turn on. There was a capacitor that was on in standby or until it came on 20 seconds later (the cap was drained thru a large resistor to act as a timer). It was somehow microphonic until turned on via the capacitor. Would never have known this except at one point they were hooked up to Khorns and those were so sensitive you could hear something.

 

[amirm]

When the amp is in standby, something is microphonic.

Your speaker is acting as a microphone. Indeed in a pinch, I have used speakers as microphones in the past!

 

[RayDunzl]

Your speaker is acting as a microphone. Indeed in a pinch, I have used speakers as microphones in the past!

I know that!

I didn't think about that.

When the amp is on, that extraneous voltage must be opposed by the amplifier's feedback loop, and no longer seen.

Thank you.

Does the amp in standby turn everything on except the output stage or what is it doing?

That's a good question.

They draw about 30W on standby, and 100 when ready/idle.

"Stand-by Mode: A low power consumption status that keeps the audio and regulator circuits at idle. Krell recommends leaving the component in the stand-by mode when it is not playing music."

There are three LED on the front:

http://krellonline.com/assets/support/0162_020_MAN.pdf page 7

Standby, Regulation, Bias - Standby stays on, Regulation, and then Bias light as it turns itself up.

 

[GrimSurfer]

So this raises the question whether reflected energy causes the speakers to be microphonic during music playback... particularly at high sound levels?

 

[LTig]

I have an audio interface hooked attached to the output of my power amp, to see what's there.

When the amp is on things look normal. Lots of power harmonics, though, which I'll investigate eventually.

When the amp is in standby, something is microphonic.
[..]
How can the amp, in hot standby, apparently act as a microphone?

Alexa?

 

[GrimSurfer]

LOL. Good one, @LTig

 

[SIY]

So this raises the question whether reflected energy causes the speakers to be microphonic during music playback... particularly at high sound levels?

With the amp on, the speakers see essentially a short. So highly unlikely to any significant extent..

 

[RayDunzl]

So this raises the question whether reflected energy causes the speakers to be microphonic during music playback... particularly at high sound levels?

In my case, for the dynamic woofer, yes.

For the electrostatic panels, very slight.

The crossover is at 180Hz.

Pink noise played through the right speaker as seen at the left standby amp.

Red - Peak during test
Black - Idle

White noise played through the right speaker as seen at the left standby amp. A small rise in the high frequencies.

 

[SIY]

Now try it with the silent amp powered, i.e., not on standby. Maybe disconnect the input.

 

[GrimSurfer]

Interesting. I wonder if things might be different if the damping factor was higher?

 

[RayDunzl]

Now try it with the silent amp powered, i.e., not on standby. Maybe disconnect the input.

A tiny bit in the low frequencies (maybe).

Retesting, I think maybe not. And a more definitely maybe not.

 

[RayDunzl]

Interesting. I wonder if things might be different if the damping factor was higher?

Damping factor doesn't apply when the amplifier output stage is turned off.

 

[GrimSurfer]

Damping factor doesn't apply when the amplifier output stage is turned off.

Understood. Thanks for the reply, Ray.

What you reported is significant because it could add colour to an amp's output, the occurrence of which could be space- and loudspeaker-specific.

 

[tohoho4]

How can the amp, in hot standby, apparently act as a microphone?

As amirm said, I think speaker is acting as a microphone. When amp is stand-by, the impedance which looks from speaker to amp is higher than audio interface's input impedance. Therefore, currents caused by speaker as a microphone is flowing to audio interface (not to amp) which lead to spectrum change. When amp is fully powered and ready to play, the impedance which looks from speaker to amp is turned to lower than audio interface's input impedance, so currents caused by speaker as a microphone is flowing to amp (not to audio interface) and no change in spectrum.

 

[solderdude]

Would be easy to test by making the clap close to the speaker instead of the amp.

 

[Blumlein 88]

For anyone who needs a speaker for a microphone, headphones works better. You just have to speak into the earpiece real good.

So are you satisfied Ray, that the back EMF is feeding into the interface? Seems likely.

 

[RayDunzl]

So are you satisfied Ray, that the back EMF is feeding into the interface? Seems likely.

Of course it is.

What else could it be?

Dead amp? Cable laying on the floor? Magic?

It just slipped my mind - didn't think about the speaker itself at all...

It started with thinking other inputs to the interface, or the UMIK-1 attached to the PC, or "did I leave the guitar plugged in?", some kind of bleed.

After disconnecting/disabling everything else I could think of, and it was still there, duh!

 

[Krunok]

I have an audio interface hooked attached to the output of my power amp, to see what's there.

When the amp is on things look normal. Lots of power harmonics, though, which I'll investigate eventually.

When the amp is in standby, something is microphonic.


Idle standby:

View attachment 27456

Handclap 10 feet away:

View attachment 27457

When the amp is fully powered and ready to play, the microphony goes away.

Same reading, with or without handclap.


View attachment 27458

???


Test cable attached to interface (XLR with alligator jumpers on the amplifier end) but removed from amp and no termination, no microphony:

View attachment 27459

How can the amp, in hot standby, apparently act as a microphone?

It looks like that amp is affraid of you when clapping hands. Have you been beating it with your spoons lately?

 

[RayDunzl]