Challenge: 1 MHz FSK shock test on amplifier output – What would your scope show?

[NTK]

Does this proposed test make any more practical sense as "any well built amplifier must pass a 2 ton crush test to show that it will survive getting runovered by a car"?

 

[MAB]

Does this proposed test make any more practical sense as "any well built amplifier must pass a 2 ton crush test to show that it will survive getting runovered by a car"?

 

[voodooless]

So, instead of endless debates, you might bring your data? Number of posters bringing something else than usual 20kHz set here is close to zero. And I can make you sure that as a designer you need to do more than a usual set. Of course in case you know what you are doing. I am saying results and work that has been done count, debates do not.

I’m not disputing your data, I’m disputing the conclusions that @tomsoon draws from it.

 

[tomsoon]

 

[SSS]

Just scroll up a bit, pma clearly explains what this is all about.

I read pma test with GHz signals. But he says there is no real outcome. My input was that there is no need to test an audio amplifier with a signal for which it was not built. Of course a GHz signal can walk through from input to output but this is not amplification. It is just bypass due to capacitancies.
Only to test RFI EMI sensibility of an audio amplifier make sense to use radio frequency signals.

 

[pma]

My point is that in some cases you need GHz Spectrum analyzer, as I have shown in This post. This is my message. There is no other way how to find the root of such issue. Not the AP, not the general purpose oscilloscope. Please those who have never found and fixed such issue, do not judge.
GHz sweep showed the ratio of suppression of the VHF signal. It was a preamp in an Al milled case, 10 mm thick. Much better than any usual audio gear. Still not very good as there was a leak - bypass the direct path. This is interesting only in special cases. But the GHz analyzer to reveal VHF oscillations is a very useful instrument. Take or leave, I am not going to be taken into endless pointless debates.

 

[voodooless]

My point is that in some cases you need GHz Spectrum analyzer

But nobody denies that There is absolutely no debate over the usefulness of these measurements.

 

[egellings]

My point is that in some cases you need GHz Spectrum analyzer, as I have shown in This post. This is my message. There is no other way how to find the root of such issue. Not the AP, not the general purpose oscilloscope. Please those who have never found and fixed such issue, do not judge.
GHz sweep showed the ratio of suppression of the VHF signal. It was a preamp in an Al milled case, 10 mm thick. Much better than any usual audio gear. Still not very good as there was a leak - bypass the direct path. This is interesting only in special cases. But the GHz analyzer to reveal VHF oscillations is a very useful instrument. Take or leave, I am not going to be taken into endless pointless debates.

GHz analyzers are not priced in a way that lets you get a couple for the kids. Most hobbyists could never afford one, myself included, and for audio, there'd be no real need for one.

 

[Philbo King]

Hi Everyone,
I am completely new here, and after looking around, I found this forum to be the most appropriate and capable place for this specific topic. It is truly an honor to be here, and it would be a privilege to get answers from the real pros on this board.
I am very curious to hear your expert opinions and see your simulations regarding a test measurement that pushes the boundaries between theory and practice.
We have a harsh test signal from a UNI-T UTG932E function generator, using a demanding FSK (Frequency Shift Keying) modulation:

• Carrier Frequency (CarrFreq): 1,000,000 kHz (that's 1 MHz!)
• Hop Frequency (HopFreq): 100,000 kHz
• Switching Rate (Rate): 10,000 kHz

Let's assume we feed this ultra-dense signal, packed with drastic frequency and phase jumps, into the input of an amplifier, while the output is loaded with a real, brutal 7.5-ohm dummy load.
My question to the experts:
How do you think a classic pure Class A, a Class AB hi-fi, or a modern high-end Class D (PWM) amplifier would react to this shock on the output?

It would be highly unusual (like Twilight Zone episode unusual) for an audio amp to pass 1 MHz.

It would be reasonable to expect that, if 1MHz output appeared at the output it would be attenuated by more than 90 dB.

 

[Philbo King]

This guys replies reek with the stench of AI.

 

[SSS]

My point is that in some cases you need GHz Spectrum analyzer, as I have shown in This post. This is my message. There is no other way how to find the root of such issue. Not the AP, not the general purpose oscilloscope. Please those who have never found and fixed such issue, do not judge.
GHz sweep showed the ratio of suppression of the VHF signal. It was a preamp in an Al milled case, 10 mm thick. Much better than any usual audio gear. Still not very good as there was a leak - bypass the direct path. This is interesting only in special cases. But the GHz analyzer to reveal VHF oscillations is a very useful instrument. Take or leave, I am not going to be taken into endless pointless debates.

Did not debate your test and findings. VHF oscillation in an audio amplifier is a very rare case but can happen. For this it can also be detected by using a rf-diode probe and a sensitive DC voltmeter but not knowing the frequency. If it was a stimulated oscillation due to the spectrum analyzer tracking generator signal at the amplifier input then this oscillation will probably not happen without stimulus in normal operating mode. And for RFI EMC sensitivity test of the amplifier it is a valid method beside others.

 

[Panelhead]

I used to think having a linear response from dc to 1Mhz could have positive improvements in audio playback. Decided bandwidth limiting has the bigger effect for playback.
Pavel, I built a line stage from a design you shared with the DIY community maybe 20 years ago. Lot of inexpensive transistors. Worked fantastic. But then had to build one with JFETS. Can never leave well enough alone.

 

[fpitas]

GHz analyzers are not priced in a way that lets you get a couple for the kids. Most hobbyists could never afford one, myself included, and for audio, there'd be no real need for one.

True, although there are some reasonable options these days that use a PC as display etc. I think pma is just saying it's a good check if you have a new design.

 

[DonH56]

It would be highly unusual (like Twilight Zone episode unusual) for an audio amp to pass 1 MHz.

It would be reasonable to expect that, if 1MHz output appeared at the output it would be attenuated by more than 90 dB.

There was a period of time when everyone was going for that sort of bandwidth, fortunately (IMO) dropped long ago by most manufacturers. Spectral has long been the champion of ultrawideband amplifiers and still produces them today. They provide (or did) special cables, probably to limit that bandwidth, and can be a nightmare for stability with some loads. Below is the specs of a current amplifier from their website. I have heard them in the primordial past but not in many years, and no need to listen again. Just an example that they are out there, whether it makes sense to me or not.

 

[fpitas]

There was a period of time when everyone was going for that sort of bandwidth, fortunately (IMO) dropped long ago by most manufacturers. Spectral has long been the champion of ultrawideband amplifiers and still produces them today. They provide (or did) special cables, probably to limit that bandwidth, and can be a nightmare for stability with some loads. Below is the specs of a current amplifier from their website. I have heard them in the primordial past but not in many years, and no need to listen again. Just an example that they are out there, whether it makes sense to me or not.

View attachment 552588

With an amp like that, you really have to worry about RF terminating your speaker cables. Not that it's a bad idea anyways.

 

[SSS]

GHz analyzers are not priced in a way that lets you get a couple for the kids. Most hobbyists could never afford one, myself included, and for audio, there'd be no real need for one.

You are right. For audio not necessary. I worked with audio devices now around for 50 years and did not need a radio frequency spectrum analyzer. But since I work also on radio and transmitters I bought used second hand hp ones for 250 and 400 EURO which work still very good. Actual new ones from asia manufacturers price is around 1000 € when not specialized. The real professional brands are much more expensive.

 

[SSS]

There was a period of time when everyone was going for that sort of bandwidth, fortunately (IMO) dropped long ago by most manufacturers. Spectral has long been the champion of ultrawideband amplifiers and still produces them today. They provide (or did) special cables, probably to limit that bandwidth, and can be a nightmare for stability with some loads. Below is the specs of a current amplifier from their website. I have heard them in the primordial past but not in many years, and no need to listen again. Just an example that they are out there, whether it makes sense to me or not.

View attachment 552588

That is crazy and not necessary except one uses such an amp for ultrasonic and other high frequency signals not audio.

 

[fpitas]

That is crazy and not necessary except one uses such an amp for ultrasonic and other high frequency signals not audio.

I agree. But there's a subjectivist contingent that fervently believes that ultrasonics are a good thing.

 

[SSS]

I agree. But there's a subjectivist contingent that fervently believes that ultrasonics are a good thing.

For bats I believe it too.

 

[fpitas]

For bats I believe it too.

More than once, it has been shown to actually introduce problems from intermod of the ultrasonics. Those wacky subjectivists.