Hi there,
I have measured and built a good number of speakers over the past 15 years doing measurements using old amps that I had or found at thrift shops. I am thinking of building a small portable amp to measure and design speakers. I have a couple of questions:
I know that Class AB is typically the recommended type of amp. I believe that Class D (or T) is not recommended due to a floating ground but I may be mistaken and love an explanation regarding this.
Also, if anyone has built a very basic amp just for this purpose, I would love the opportunity to copy your work. I have looked at LM3886 builds and although I think I will try one some day, for now, it seems like overkill for my immediate purpose.
Thanks,
Jay
I have measured and built a good number of speakers over the past 15 years doing measurements using old amps that I had or found at thrift shops. I am thinking of building a small portable amp to measure and design speakers. I have a couple of questions:
I know that Class AB is typically the recommended type of amp. I believe that Class D (or T) is not recommended due to a floating ground but I may be mistaken and love an explanation regarding this.
Also, if anyone has built a very basic amp just for this purpose, I would love the opportunity to copy your work. I have looked at LM3886 builds and although I think I will try one some day, for now, it seems like overkill for my immediate purpose.
Thanks,
Jay
Most cheap digital amps are bridged, with half the output on each terminal, and present problems with grounding during measurements. Noisy, too.
-can't be "bridged".
..with "dual-channel" it doesn't really matter (assuming it's decent, isn't bridged, etc.) about the quality of the amp.
Pretty much the "go-to" amp for this:
the t.amp PM40C – Thomann United States
..and yes, it's not a DIY project ..but for that kind of money you aren't going to do better.
IF you absolutely need to DIY and aren't shy about spending money:
Modulus-86 R2.4: Composite amp. 70W (4Ω) @ 0.0001% THD
..with "dual-channel" it doesn't really matter (assuming it's decent, isn't bridged, etc.) about the quality of the amp.
Pretty much the "go-to" amp for this:
the t.amp PM40C – Thomann United States
..and yes, it's not a DIY project ..but for that kind of money you aren't going to do better.
IF you absolutely need to DIY and aren't shy about spending money:
Modulus-86 R2.4: Composite amp. 70W (4Ω) @ 0.0001% THD
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Excuse my ignorance: what is the problem with a bridged amp for measurements?
I‘ve been using a tiny PAM8403 recently and it seemed ok, but would rather change that if I can improve things.
I‘ve been using a tiny PAM8403 recently and it seemed ok, but would rather change that if I can improve things.
Class D amps have LC filters at the output and due to that are non linear depending on load impedance. It might not be an issue but is worth consideration. I sometimes deal with things like 30 Ohm tweeters and am happy using an LM1875 opamp for measuring.
On a bridged amp, the speaker negative is often live (at Vcc) and connecting to the test equipment will cause a short and the amp auto shuts off or you may blow blow a fuse, or worse, damage your gear.
I am using my old Rotel RB980 for loudspeaker testing. I added a stepped volume control that allows me setting the voltage gain of the amp from -20 / -10 / 0 / +10 / +20 dB. However, the amp keeps blowing fuses when things go bad on the workbench (and yes, I try to be careful). Then I have to open the amp and find a suitable fuse, which isn't exactly convenient. An amplifier with some sort of self protection that does not require opening the amp after a mishap would be a lot more convenient.
Regarding class-D, my experience was that they are noisy and don't have a flat frequency response (I have an old one that is down by more than 1 dB at 10 kHz).
Hi Scott,
I was under the impression that T amps are bridged and have an RC filter. Is this not the case?
Jay
Apart from the yet mentioned drawbacks: If you want to determine a driver's TSP's, you need a common ground for the amplifier and your test equipment, 'cause you'll have to insert an accurately defined resistor into the speaker's ground lead and measure both the voltage drop over it and the total voltage that is applied to the speaker.
Best regards!
Are there any amps that will do down to DC? Or like 0.1 Hz? Wouldn't have to be particularly clean, nor super powerful. The purpose is to push a very low frequency into a woofer for a talk about how speakers work, so people can see the cone move back and forth. (A true DC amp is also useful for MLSSA to directly measure DC resistance, though I don't have a MLSSA any more). Cheap is good. And so is pre-built, I have no time for projects. And if tax deductible and guaranteed to keep me cancer-free that's good too as long as I am wishing. ;-D
Maybe I am a bargain hunter, but any time I've examined my testing tools noted below, they are closer to perfect than I need anyway. That includes anything in my older Mac laptops, cheap old DACs, cheap new us$23 Behringer DACs, laptop mics north of the bass (no kidding... see my posted curves), $60 condenser mics without bothering with the calibration fixes, ancient Radio Shack SPL meter....
Even my smartphone SPL meter is not too far off the mark, at least for my testing purposes.
And speaking of grounds and bargains, for cheap on eBay you can get interface transformers (like Pyramid that cost me us$12) with astounding FR going near-flat well below 20 Hz.
B.
Even my smartphone SPL meter is not too far off the mark, at least for my testing purposes.
And speaking of grounds and bargains, for cheap on eBay you can get interface transformers (like Pyramid that cost me us$12) with astounding FR going near-flat well below 20 Hz.
B.
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That would work perfectly fine. I had the IC in the parts bin and used other bin parts to wire it up point to point on a (parts bin...) heat sink. I already had a matching bipolar supply but I feel for speaker measurements two 9V batteries should work without danger of clipping the rails.
Any OpAmp should work fine.
There probably is a lot about this online already, but I was wondering how to calibrate the SPL in REW to get an idea (or something better than just an idea) of speaker efficiency. I the "Analyzer" app on my iPhone to be trusted? +-1dB accuracy would be nice.
It's actually not so difficult to use one of these TI chips like a TPA3244 or similar.
The only thing is that you have to use two differential opamps to "average" (compensate error) the positive and negative and get rid of the DC.
Otherwise my pick would be also the t.amp PM40C and convert it to a composite amp (that's not difficult to do)
The only thing is that you have to use two differential opamps to "average" (compensate error) the positive and negative and get rid of the DC.
Otherwise my pick would be also the t.amp PM40C and convert it to a composite amp (that's not difficult to do)
In one of the amps you have laying around, you could try shorting the unity gain capacitor in the nfb loop to see how high the DC offset gets. It may still be low enough for your demo. Or you could increase the nfb capacitor value by a factor of ten or so, but this may still give tilt on the lower frequency square waves.
Good question.
Aside from doing the T/S math, the issue tends to be how hard are you driving the amp. Much nonsense posted. You need to see amp voltage output on a scope to see how small it is.
I find there is practically no situation where I need to know with any but the roughest guess about loudness esp using REW. For testing, "Real Loud" works for me if checking for max's and most measurements are comparative or iterative.
Some devices have web-published calibrations (such as old RS meters). Even a single frequency comparison to a trusted SPL meter owned by a friend would suffice.
BTW, there's a truly wonderful hearing test using warble tones. Again, precise calibration just doesn't matter here either. (Hint - you can use a speaker with an NR35 earplug in the other ear.)
Online Hearing Test & Audiogram Printout
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