Amplifier measurements may require improvement

[mhardy6647]

The "right" way to do it, of course, is to measure the amp's electrical performance into a reactive load that simulates a loudspeaker. It is, of course, not a perfect test (like Camelot*, it's only a model ) but it is controlled and reproducible.
Stereophile has done this for millennia... well, for a long time, at least.

Real-Life Measurements

As mentioned by two readers in this month's "Letters," amplifiers are used to drive loudspeakers but are almost exclusively measured into resistive loads. The reasons for this are twofold: 1) real loudspeakers both produce neighbor-annoying sound levels and tend to break when driven with typical...

www.stereophile.com

(after Ken Kantor, per @John Atkinson)

A pretty smart dummy load, if you ask me.

________________
* Camelot.

 

[Djano]

No, you just don't understand. The speaker is the same, but the amps are not. How an amp handles very low impedance can be very different, and that is the whole point. We already know it clips earlier with low frequencies, but we also know these chips aren't very good at driving ultra-low impedances. So this may even make the effect worse, and that was the whole point.

You're right; I didn't understand that.
I'll summarize what I now understand, and you can tell me if it's correct:

• Low-frequency clipping is a classic issue, known for a long time.
• Low impedance often leads to clippin
• -> So, if the speaker's impedance drops at low frequencies, we encounter both phenomena = guaranteed clipping.

Even if this is true, it still remains that your graph on the speaker's impedance doesn't perfectly match what is observed. The impedance clearly rises around 60Hz, which is the heart of the range where the Fosi collapses. That said, I'm not sure if the graph you sent corresponds to S.A.'s speaker. I would need to check.

In any case, it's clear that I didn't have the knowledge to interpret and understand your argument about impedance correctly. Now, it makes much more sense to me. Sorry for categorizing it as wrong objections.

 

[voodooless]

Then OP tries to understand why there doesn't seem to be clipping issues below 45 Hz. Which may well be uncovered only if we know the exact phase/impedance trace from the used speaker. -Which isn't the one published in post #17.

Because that's where the impedance peak is, and below that there isn't much output.

The no-clipping-below-45Hz would also invalidate @amirm's point BTW because the behavior was seen at 20 Hz

 

[Djano]

S.A. did his tests with a Tannoy DC8 Ti. I can't find the impedance curve

 

[Ron Texas]

I lack the technical expertise to give a meaningful opinion. However, my inclination is to trust the test done by @amirm and therefore conclude someone else is trying to stir up a controversy when there is none.

 

[Holdt]

Because that's where the impedance peak is, and below that there isn't much output.

The no-clipping-below-45Hz would also invalidate @amirm's point BTW because the behavior was seen at 20 Hz

The impedance peak is around 65 Hz on the posted trace.
The used speaker from the video may well be turned lower and has the peak at a lower frequency. Otherwise I agree.

 

[Holdt]

I lack the technical expertise to give a meaningful opinion. However, my inclination is to trust the test done by @amirm and therefore conclude someone else is trying to stir up a controversy when there is none.

I don't assume people do that just to stir up a controversy. I assume that some data are missing and/or misunderstood.

 

[voodooless]

The impedance peak is around 65 Hz on the posted trace.
The used speaker from the video may well be turned lower and has the peak at a lower frequency. Otherwise I agree.

Yeah, I said as much in the post

 

[Frank2]

The "right" way to do it, of course, is to measure the amp's electrical performance into a reactive load that simulates a loudspeaker. It is, of course, not a perfect test (like Camelot*, it's only a model ) but it is controlled and reproducible.
Stereophile has done this for millennia... well, for a long time, at least.

Real-Life Measurements

As mentioned by two readers in this month's "Letters," amplifiers are used to drive loudspeakers but are almost exclusively measured into resistive loads. The reasons for this are twofold: 1) real loudspeakers both produce neighbor-annoying sound levels and tend to break when driven with typical...

www.stereophile.com

(after Ken Kantor, per @John Atkinson)

A pretty smart dummy load, if you ask me.

It does not drop below 6 ohms, so seems very amplifier-friendly to me.

 

[Frank2]

This is impedance graph of the Tannoy DC8T: https://www.hi-fiworld.co.uk/images/stories/Loudspeakers/Reviews/tannoy-definition-dc-8t-z.gif

According to the test, "The bass unit has a very low DCR of 2.7 Ohms"

Tannoy DC8T (March 12 issue)

Tannoy DC8 T review in Hi-Fi World

www.hi-fiworld.co.uk

In the "problem area", the impedance may drop enough to make the Fosi amp current limited.
The graph explains the problems in the 80-125 Hz area, but not the one around 50-60 Hz.

 

[mhardy6647]

It does not drop below 6 ohms, so seems very amplifier-friendly to me.

It could, though. Just Ls, Cs, and Rs (and a calculator).
That said...

There's a reasonableness clause to be applied to loudspeaker models, don't you think?
In 2023, would it be reasonable to test amplifier performance into, e.g., a simulated (or real) Infinity IRS Beta or a Quad "ESL-57"?

Infinity IRS Beta loudspeaker Measurements

Sidebar 3: Measurements (from June 1989, Vol.12 No.6)

www.stereophile.com

Fig.1 Infinity IRS Beta, impedance magnitude of woofer tower (top above 500Hz) and midrange/treble panel (2 ohms/vertical div.).

The "ESL-57" is, in essence, a large and peculiarly designed capacitor -- albeit a fine-sounding one.

SDS Audio Labs - Quad Loudspeaker Refurbishing Main

Both of the above were/are renowned amplifier killers in their time.
Just one anecdote that happened to be easy to find.

QUAD ESL-57's destroyed a Carver TFM-55x amplifier

I sold a Carver TFM-55x to a guy on another forum who tried to use it to power a pair of unrestored QUAD ESL-57 speakers. Amp quit rather quickly. Preliminary testing shows blown outputs, resistors and diodes across both amp sections as well as a blown 12A slo-blo fuse. Any idea what happened...

audiokarma.org

 

[fpitas]

Just one anecdote that happened to be easy to find.

It's easy to appreciate why old SS amps got their bad rep.

 

[Djano]

This is impedance graph of the Tannoy DC8T: https://www.hi-fiworld.co.uk/images/stories/Loudspeakers/Reviews/tannoy-definition-dc-8t-z.gif View attachment 312163
According to the test, "The bass unit has a very low DCR of 2.7 Ohms"

Tannoy DC8T (March 12 issue)

Tannoy DC8 T review in Hi-Fi World

www.hi-fiworld.co.uk

In the "problem area", the impedance may drop enough to make the Fosi amp current limited.

Thank you ! Can we assume DC8T = DC8Ti ?

If so, I have a problem with the fact that impedance is climbing at 50Hz-60Hz, which is precisely the moment at which the V3 starts to struggle (@voodooless said that the problem was low impedance, and not the opposite)

 

[Ron Texas]

I don't assume people do that just to stir up a controversy. I assume that some data are missing and/or misunderstood.

It's dangerous to go that far with assumptions. You may note the source of this "missing data" appears in an advertising laden Youtube video which is a sure warning sign that its author is trying to stir up controversy.

 

[BR52]

Following post 20 will give a better pic and a lot of speculation are out.

 

[Frank2]

Thank you ! Can we assume DC8T = DC8Ti ?

If so, I have a problem with the fact that impedance is climbing at 50Hz-60Hz, which is precisely the moment at which the V3 starts to struggle (@voodooless said that the problem was low impedance, and not the opposite)

Don't know if the Ti differs.
What is very odd to me, though, is that the white noise sounds very different through the Yamaha compared to the Fosi, even at low volume levels. Listen from 14:40 in the video:

This is strange since the response graphs look identical for low volume levels.

He routes the center channel signal to the amplifier. Maybe the high pass settings were different, I wonder. If he also has subs installed, the area below 60-80 Hz could have been produced by his subs. I don't know the guy, so can't say he would be making such mistakes.

 

[Djano]

It's dangerous to go that far with assumptions. You may note the source of this "missing data" appears in an advertising laden Youtube video which is a sure warning sign that its author is trying to stir up controversy.

Please, stop with these unkind accusations.
A quick review of the thread would show you that I am genuinely trying to understand what happened.
When a satisfactory explanation was offered, I acknowledged that it could suffice to explain the result that surprised me. Since then, additional explanations have been proposed, and I'm trying to grasp their combination.

If your contributions consist solely of saying that I'm trying to create controversy, and that you lack the technical knowledge to say anything other than ad hominem things, then I don't see their purpose.

 

[Ron Texas]

Please, stop with these unkind accusations.
A quick review of the thread would show you that I am genuinely trying to understand what happened.
When a satisfactory explanation was offered, I acknowledged that it could suffice to explain the result that surprised me. Since then, additional explanations have been proposed, and I'm trying to grasp their combination.

If your contributions consist solely of saying that I'm trying to create controversy, and that you lack the technical knowledge to say anything other than ad hominem things, then I don't see their purpose.

There's no accusation against you. It's just a matter of whether I trust our host who does not accept advertising or someone else who is running a for profit Youtube channel. If anything, you are accusing me of bad behavior. I would not have linked that video to ASR. You don't have to be an engineer to smell fish.

 

[Djano]

There's no accusation against you. It's just a matter of whether I trust our host who does not accept advertising or someone else who is running a for profit Youtube channel. If anything, you are accusing me of bad behavior. I would not have linked that video to ASR. You don't have to be an engineer to smell fish.

I used the graph provided by S.A. The least I could do was mention the source. It would not be acceptable to publish someone's work without referencing the exact source.

I didn't insert a large YouTube video embed. Just a hyperlink of a few characters (something like "S.A. video").

As for S.A., what matters is not the purpose for which he produces videos. Even assuming he generates substantial profits (with 3k subscribers, I doubt it...), he provides us with objective measurements. That's all that interests us.

I propose we stop this superficial controversy here, far from the real topic of discussion. Otherwise, the thread will lengthen around a subject that no one cares about and lose readability. This happends too often.

 

[Holdt]

You both have reasonable points. But I do find it a bit strange that the author of the video hasn't chimed in. He should be @SimpleTheater here on ASR, I've been told.