How to setup an oscilloscope to trigger when an AV receiver is clipping? - Page 1
Forum section: electronics / beginners

author: BlackICE
author_rank: Regular Contributor
timestamp: April 03, 2022, 07:11:02 am
content: Most moderately priced AV receivers have marketing BS for specs. The only spec that is somewhat valid is the power and distortion number for 2 channels driven over a specified freq range. In real use 5 or more speaker channels driven, so when is the receiver's amp out of juice? As a rough guide, using a generous 80% efficiency number, the input watts to the receiver to estimate the best case output watts. Another big unknow is what is demanded by the program material.I thought that an scope may help find the answer. First I tried finding the line level to speaker output multiplier. Setting the scope to divide output/input I should get a constant (except for zero crossing, infinity). The number will be smaller if clipping occurs. Another idea I had was faking the probe multiplier for the input channel so that it would match the output channel. Then subtract the two channels. Should be near 0 most of the time unless clipping occurs.However my Siglent 1204 scope doesn't have trigger on math function output. So neither of these methods would work. Am of forced to trigger on high levels of speaker output and inspect input vs output manually, or is that a way to auto trigger on any clipping events?

author: james_s
author_rank: Super Contributor
timestamp: April 03, 2022, 07:30:37 am
content: All you need to do is feed the receiver a sine wave and increase the volume until you observe clipping. Back in the days before everything went class D you could just measure the supply rails under load and use that to calculate the maximum power.

author: BlackICE
author_rank: Regular Contributor
timestamp: April 03, 2022, 08:30:02 am
content: I'm trying to find out under typical use if the amp is clipping. Testing with sine waves isn't real program material so wouldn't represent real life use. Given the passive crossover in most speakers they aren't resistive loads, combine that with varying loads on 7 speakers depending on material, sin wave testing is futile. If I willing to remove the case the measuring a drop on the supply rails would be the easiest. Triggering on a significant voltage drop on the rails. But I would have to remove all the cables and reinstall to do the test.

author: tggzzz
author_rank: Super Contributor
timestamp: April 03, 2022, 09:34:43 am
content: You can't trigger on the absence of a peak, as you realise.Consider taking a representative source waveform, set the scope to trigger on its peak.Apply the source to the UUT, and observe the output waveform. If you don't have a two channel scope, apply the source waveform to the scope's external trigger.

author: CaptDon
author_rank: Super Contributor
timestamp: April 03, 2022, 05:04:26 pm
content: One thing for sure, when it flat tops it will sound like crap, so when it sounds like crap it's clipping! 7 speakers??? How many output channels does this amplifier have? I guess you could be well into clipping on one output channel but other channels with a lesser input signal may be clean? Just hook the scope to the output and see what Pk to Pk voltage it starts to clip at and do the math. If the amplifier is one of those 'bridged output' jobs your scope MUST be floating or you will short out one side of the output when you connect the probes ground lead

author: james_s
author_rank: Super Contributor
timestamp: April 03, 2022, 06:05:33 pm
content: Even if it's a bridged amp it will have the isolation of the power transformer so a scope ground shouldn't cause a short.

author: bdunham7
author_rank: Super Contributor
timestamp: April 03, 2022, 06:23:04 pm
content: Quote from: BlackICE on April 03, 2022, 07:11:02 amYou're on to the right idea here. I don't see how you could distinguish between distortion caused by the amplifier and distortion intentionally put into the program material. Detecting distortion on a given music signal is something your ears and brain can do, but probably not a scope. A ratio-calibrated null, which is what you are proposing, is the way to go. Unfortunately I'm not aware of scopes that trigger on a math signal, so you'd have to make a small nulling amplifier, which is one way that measurements of audio amplifiers are done on a budget.

author: CaptDon
author_rank: Super Contributor
timestamp: April 03, 2022, 07:35:59 pm
content: Isolation not required......Not So! Usually the power supplies 'one being positive and one being negative' have their returns at chassis reference and the chassis may be or probably is grounded via the mains cord. One side of the bridged output is swinging positive (referenced to chassis and ground) while the other side is swinging negative (referenced to chassis and ground) and if the scope is also grounded there is a potential problem. Rule of thumb, NEVER ground either side of a bridged output because the centerpoint of the power supplies could well be chassis and ground referenced.

author: tooki
author_rank: Super Contributor
timestamp: April 03, 2022, 09:19:21 pm
content: Quote from: CaptDon on April 03, 2022, 05:04:26 pm5.1 and 7.1 are the most common surround formats these days (and have been for years). More channels are common on the high end. A common amp configuration is front L/center/R, rear L/R, subwoofer, and then another uncommitted L/R pair that can be assigned as either “back surround” for 7.1 surround, as a second front pair (or for bi-amping the front pair), or as a separate “zone 2” that can play a different source (e.g. for a different closed room).In the newest stuff with Dolby Atmos, height is also a thing, so you can have amps assigned for that, too.

author: TimFox
author_rank: Super Contributor
timestamp: April 03, 2022, 09:45:18 pm
content: Quote from: bdunham7 on April 03, 2022, 06:23:04 pmA circuit to compare the input and scaled output to detect clipping will need to be phase-compensated. This can be adjusted with sinusoidal voltages below the clipping point, but will be a function of frequency due to the amplifier response. Assuming the amplifier itself has heavy negative feedback, one could (carefully) look at the error voltage at the feedback node to see when it increases dramatically.

author: bdunham7
author_rank: Super Contributor
timestamp: April 03, 2022, 10:54:17 pm
content: Quote from: TimFox on April 03, 2022, 09:45:18 pmMost people that do this don't bother and their null measurement ends up being distortion + noise + phase + anything else and that is probably good enough for a good solid-state DC power amplifier because there should be minimal phase shift anyway, even compared in real time to the input. Just to find the clipping point is fairly straightforward because you can look at the null signal and ignore any noise, residual signal due to phase shift or null errors and so on. I've attached two screenshots of me doing this quickly using an AWG, a fairly powerful audio amplifier and a scope with math. It's not great and I don't have gain or anything dialed in because I managed to smoke my load resistors and that was the end of the show! However, you can see that it works--the distortion is very evident on the sine wave, but on music program material it wouldn't be--but the large MATH signal would be.The error voltage indicator you mention is exactly how the peak indicators on one of my very large stereo receivers work.

author: David Hess
author_rank: Super Contributor
timestamp: April 04, 2022, 01:14:58 am
content: Quote from: BlackICE on April 03, 2022, 07:11:02 amAnalog oscilloscopes and some early DSOs trigger on the ADD waveform using the VERTICAL trigger source, which originally was literally the signal from the vertical deflection plates.

author: oPossum
author_rank: Super Contributor
timestamp: April 04, 2022, 02:45:44 am
content: Soundcraftsmen used a clever clipping detect circuit in many of their amplifiers. The LTP of the VAS is used as both part of an op-amp (when not clipping) and a comparator (to detect clipping). When there is a gross mismatch between input and output T4 will turn off, T5 will turn on via R9, and the clip LED will glow. This could be easily retrofitted to many discrete transistor amplifiers. It is quite common to use a LTP VAS.For class D amps it is often just a matter of detecting when the pulse width falls below some small value. Some 'scopes can trigger on that condition.

author: bdunham7
author_rank: Super Contributor
timestamp: April 04, 2022, 02:49:58 am
content: Quote from: David Hess on April 04, 2022, 01:14:58 amYes, that works in this case--I just tried it with my Tek 2221A and I can get it to stay blank until a certain level of distortion arises and then is displays a trace. It's also a lot easier to 'null' out the channels with a test signal by twiddling the CAL knobs instead of entering a formula in the function editor.It occurs to me that it wouldn't be that hard (maybe) to add a real-time differential function like this to any DSO, especially one with 2 ADCs.

author: David Hess
author_rank: Super Contributor
timestamp: April 04, 2022, 03:34:13 am
content: Quote from: oPossum on April 04, 2022, 02:45:44 amI had some difficulty following your description. Integrator T6 is the VAS. The differential pair made from T1 and T2 are more commonly known as the IPS or input stage and only provide a small part of the open loop gain.