Discrete audio amplifier project PT14.1 oscillation fixed and first power test 

Thanks James. I learned something new.

Their oscillations were modulating.

It's oscillation not the noise from power supply.

It's not noise, it's the emitter follower output stage oscillating. This is a well known phenomenon with emitter followers, all it takes is a bit of inductive reactance in the collector circuit and you form a resonant circuit with the intrinsic base collector capacitance. A resistor in the base connection adds damping to this resonant circuit. As John says, a good pcb layout with a ground plane and local decoupling at the collectors will fix this.

Plus, you can see the brand and model of the power supply. Mind spending 30 seconds searching up on Google?

I believe that it would have helped because when I added the 100pf caps across the collector to base of the drivers it had no effect. It had to be the output transistors oscillating. Supply bypassing was the biggest problem with this amp.

Phase Linear did it w the original slow Output Transistors But seem not to work for the New Faster Transistor Good Luck

The feedback will carry the signal back to the input so that it can appear at places in the amplifier where it wasn't created. It shows up on the rails to some extent as well.

I would use an isolating washer and screw so that you can ground the heat sink.

@@JohnAudioTech I'm not sure, but I think the tab is connected to ground. In that case, isolation is necesary anyway? My concern is the heat. I have seen that, screwed directly to the heatsink, improves heat dissipation.

@@Kenshindegozaru The tab is connected to pin 3 which is the negative supply voltage when the IC is used in its normal dual supply configuration. The mica insulator and insulating shouldered washer for the screw is good enough provided the heat sink is of proper size.

@@JohnAudioTech thank you very much!

I would say you are okay to use them. Newer caps tend to be smaller in size. The caps must be rated to handle the ripple current. For audio amplifier use, I doubt there would be an issue.

thanks m8, looking forward to yr next vid

well that didn't work out, noticed the smaller caps were only 85C instead of 105 C, looks like its an ebay job, suppose 80v10000uf 85C for audio would be out of the question (got a doz of those lying around)

Without getting into deep explanation, the R stopper value should be kept as low as possible to take care of the oscillation as its presence in the circuit can have an adverse effect on performance. It is important to take care of other issues first such as supply bypass and layout before resorting to stopper resistors.

@@JohnAudioTech thank you for the reply!. it is my first PCB layout and indeed, i didn't place supply bypass near the output stages but instead, directly on the main supply caps (0.47uF film caps. i'm silly enough to put them on main caps instead of PCB)

LM1875 is a single channel amplifier. using two of them for stereo is just like making a dual mono module, nothing else required. if you want to parallel them, using 2 per channel, you'll need a small value resistor to each output legs before the output got paralleled for current balancing. if you want to bridge them, you'll need to invert the signal on one of the chip.

Just want to use one per channel in a DIY stereo amp. Thanks

@@MrDunk66 Just hook them straight up, then. you'll be fine. good luck!

S. Kojina adequately answered your other question. As for the bias pot, bias is internally set within the IC. There nothing to adjust so no reason to worry about it.

Thanks, I am fixated on the LM1875 at the moment (building an amp) and every time I see a a similar package I think it is a chip amp. Yours is made of discrete components - - I need to wake up 🤪

Vas cap is the miller cap from collector to base on the predrivers. I have a vintage amp and it is pretty much factory, I cant solve the oscillation. I got better grounding all over and somehow improved but I have a 4Mhz osc, that goes up when I turn the volume up (no signal). And once the amp want to do its job with signal, the oscillation is all over the test tone, sounding extremely harsh ..... It is a tipical TIP41-42 with RCA 01C16/17 predrivers in TO-39. It is an split power supply design, no oput cap. Rail are pretty contmited with oscillation ... but is more a a consequence of the amp itself. Where I should start looking. I got really lost scoping. Noise is everywhere ... due to feedback. Any trick I can use ?. Thanks

Wtf is wrong with you? Your son died?

nah theres a schematic for that online

@@aljivenalejo3904 be nice to have a PCB though

seriusly?? man if i want to tell you i gave you some pcb will you trade me some cash??

Kind of the upper limit for the transistor, frequency where gain has fallen down to unity.

@@HillsWorkbench thanks for that

the circuit might not oscillate with slower output stage but the performance itself might be degraded.

No. Using slower transistor may cause other issues and/or won't perform as good.

As I find more problems with this design, you will have to cancel your canceled cancellation!

That would reduce the current gain of the output stage as it shunts more current from the base of the output transistor.

@@JohnAudioTech , got it! Thank you!

The supply I use is a linear type. But you are right, bypassing the supply near the outputs cleared up the problem.

JohnAudioTech Be careful of setting the parts as close as you may be able to. A star-point ground works better for improved ground impedance. The close loop part of the amplifier must always be isolated from others, learned that the hard way. If your final design would employ a double-layer board, make sure the audio traces stay near to ground, and not any other power potential. Just saying. 02ルイ

@@ruikazane5123 Yes, Each ground lead has its own connection to the ground around the bypass capacitors and each block of the amp has its own supply leads off the bypass bank. It turns out I just had too long of supply leads running out to the output stage.

JohnAudioTech Sounds about right. It can also help seperating the preamlifier stage's power supply connections and the power stage connections as well. And you may then try connecting the two together. Inductance itself is created with frequency, causing oscillations which are typical in Class B/AB amplifiers, and hazardous on pure class A topology amplifiers...I killed my amp because of that!

package size

@@johnyang799 Which is?

@@kennmossman8701 Use your own eyes. Apparently you think you know everything so you should be able to work it out.

@@ferrumignis Hahahaha Great answer - it shows how ignorant you are!

That you don;t know is obvious. Just a bunch or imbeciles sitting around in a circle jerking each other off saying how clever you are.

I was thinking the same, that would slow down the whole amp. High frequency gain rolloff

That won't do any good.

A small capacitor, C to B, or stopper resistors as John described.

@@HillsWorkbench stopper resistors? wtf did you get all this freaking bent terminology

it's actually a simplified terminology. it's easier to understand for non-engineer peoples i guess?.

@Hill's Workbench stopper resistor is actually base resistor. what i know, this resistor acts as damper for the previous stage that might be overloaded and causing instability. i don't know the technical explaination, though.

@@n.shiina8798 so a brain is a 'thinker', fingers are wriggly things, a zener diode is a voltage capper? bloody ridiculous

I can use square waves for the fast rising edge if that's what you need to see. Do you mean that you want me to set the current limit on my supply to 100ma? What are are you checking for?

@@JohnAudioTech , I want to check for the effect on the amplifier due to hard clipping. Or, I must clear things up a little bit and say that, I want to see the effect on the amplifier due to highly unequal supply rails.

I will be performing square wave tests (step response) soon. I can adjust the input signal to drive the amplifier into clipping to see if there is any stability issues there. If the supply rails have unequal voltage. The output will clip on the rail with the lower voltage first. This should not cause output offset voltage problems.

@@JohnAudioTech , excellent test! And you can further reduce one of the rail voltages, like you can say, for example, to supply the amplifier with +ve rail having +36V or more and -ve rail having -9V or less, at 100mA current limit. And see there, if you can simulate any stability problems or not.

It is exactly that. A high frequency waveform modulated by a lower frequency.

Watts RMS, JAT's favorite term! :) (He did a rant on that one...)

78 Watts with sinusoidal signal. This is the proper term.

@@johnyang799 There is peak, average, RMS, etc. To say only Watts without qualification is BULL-SH^T.

@@kennmossman8701 Lol. Vrms is shown clearly on the scope and the load is 4ohm So what? Can't you calculate for yourself?

@@kennmossman8701 There is no RMS for power. Average it is if you wish. But square wave and dc can also shown as average power so it's simply useless. Another way to effectively imply the condition is to use 1% thd.