Square wave stability tests for audio amplifiers

[@ohmbug10]

That was super interesting, John. Thanks for the demonstration.

[@MsFireboy2]

Thanks for posting. Nice to see you back. I just assembled a push pull AB Amplifier circuit using a BD137 and a BD138 transistors with 10 ohm emitter resistors. Main use for this will be amplification of a Triangle Wave. This will be applied to the Collector Emitter junction of a transistor in a curve tracer circuit. Oh I will transfer the circuit to a Perf-Board to reduce ringing on the ground plane.

[@bobisyouruncle1]

Hi John - again, a very intuitive tutorial. Good stuff.

[@davidahmad6090]

Man so good to see these videos from you, thanks so much, all the best.

[@Sctronic209]

Thanks John, really enjoy the content.

[@andymouse]

Awesome tutorial John, truly fascinating stuff !

[@Mrsteve4761]

Very revealing and understandable video, very much enjoyed this one.

[@1pcfred]

Who else wanted to hear some of those sweet oscillations? Rock the noise! You and Sam Battle should to a collab video. He's a synth maniac. He knows his way around electronics though. He's built the kilodrone. It's a thousand oscillators. Just a wall of oscillators. He takes everything to the Nth degree. Does over the top projects. If one oscillator is good then a thousand of them is awesome! Phil Spector got nothing on Sam.

[@anurasenarathna1703]

I liked your videos on Audio Amps. As usual nice video.

[@twotone3070]

Very interesting. Thank you.

[@mysock351C]

Any thoughts on just testing with a pure capacitance with no load? On the face of it the idea seems extreme but with a long run of zip cord if the other end suddenly finds itself floating this is basically what you have. Probably not something we worry about in standard audio but in home theater with many channels it could conceivably happen. (Edit: In the video the resistive test load is always present, I assume? Or did I miss something?) Probably wouldn’t need to be more than 10nF (the 100nF would be quite mean.) As an aside in very old ARRL manuals there was advice on putting ceramic caps on the outputs to stop RF interference. This was removed and advised against in later additions due to reports of exploding audio equipment. Could see that happening if someone just had the ceramic caps on the output along with a a scope to make sure it’s “ok” only to find they have a medium wave CW transmitter tuned to magic smoke instead.

[@JohnAudioTech]

Yes, the cap without the resistance is part of the test. I didn't perform that in the video because it's a pain with my setup switching the wires around each time. I want to rewire my test circuit.

[@junkyoutube9060]

from Rod Elliott "A further test is to apply a low frequency square wave at about half to 3/4 power, mixed with a low-level high frequency sinewave to the amplifier. At the transitions of the squarewave, the sinewave should simply move up and down - 'riding' the squarewave. If there is any misbehaviour in the amp, the sinewave may be seen to be compressed so its shape will change, or a few cycles may even go missing entirely. Either is unacceptable, and should not occur."

[@simonkormendy849]

That coil placed in the output of the amp under test, is usually called a Zobel Network.

[@michaelspiering7585]

Is this type of testing relevant with tubes?

[@junkyoutube9060]

Hi John, is it possible for you to show us how to test for amplifier power supply sag, and the effects it has on say... a square wave ? thank you for your videos and sharing your knowledge.

[@gamiwv]

John have you ever involved with TDA7294/7293 ? I'm seeking around all of your videos but I didn't found something. A friend intrigued me about this chip amplifier (simplicity, cheap, nice low frequencies, no distortion on highs ect. It is "his poison" 😀) and I remember in one of your video that you are referring to "bootstrap capacitors". Will you point me somehow ?

[@JohnAudioTech]

I made a video a while back. Limited to under 30 volt rails with 4 ohms due to current limit, so little interest in that chip. Bootstrap video link: kzbin.info/www/bejne/f2OpgmCee9V7b8ksi=7IPPzLLpfochVjLe

[@gamiwv]

@@JohnAudioTech Thank you John . I found both videos. I wander what could be happen if you bypassed the bootstrap capacitor with a very good quality film capacitor let say 100nF at the bottom side of the pcb (or if you replace the electrolytic with a "audiofile" bipolar -MUSES Nichicon- capacitor). I have see some designs that feed the output part with about 62V and the "driver" with 78V "eliminating" the bootstrap capacitor. Lower output of course but - as they claim - much higher (Hi-End) quality

[@haraldh.9354]

This is usefull for all DIY builders - thanks -whats about JAT 801,,,,

[@Spentelectrons]

Can two LM1875 be run in parallel? Id be curious how that works out if it can.

[@JohnAudioTech]

Yes they can and it works well. Use .1 ohm series resistors on each output and 1% tolerance resistors in the feedback path for good gain matching.

[@lohikarhu734]

Why such high frequency for the square wave? After all, the step response is the only important part for high frequency response and stability, no? A lower frequency can show low-end response by any "slope" on the flat parts of the cycle, and ringing is ringing, although, on a low-cost scope there may not be sufficient sample rate at lower horizontal rates... Also, if you have an actual function generator, you can adjust the slew rate of the rising/falling edge, to emulate the effect of slew-rate limited (frequency-limited) inputs.... Anyway, square waves are pretty revealing.

[@carlfranz6805]

First?

[@JohnAudioTech]

I guess so. Congrats!