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PostPosted: 10 Apr 2018, 04:11 
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Joined: 19 Mar 2018, 02:57
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Hello everyone! :headphones:
I am experimenting with audio amplifiers for a while now. I learned how to do an operational amplifier with discrete components and understand all its section and sub-circuits, recently. Then I decided to just skip the hard part and use operational amplifier and output power stage for an audio amplifier to work as desired, instead of troubling with discrete components only. The goal from my earliest experimentation was to create around 100 W audio amplifier but the things went wrong as soon as applying DC to whole amplifier (no AC for now).
I have built this circuit few minutes ago on breadboard:
Image
When I checked only output power stage with VBE multiplier (without MC1458 op-amp) everything worked just fine. But when I connected op-amp to it directly (no AC coupling - switch position B), as I increased Vce of VBE multiplier just a tiny bit, then the current through both upper and lower output power quadrants suddenly increased to 2A or more and stayed there or it was swinging like there was some kind of heavy oscillation within amplifier! As soon as I added AC coupling capacitor (switch position A), the darn thing worked as expected when Vce of VBE multiplier was changed! As it was increased with potentiometer P2 slightly also the quiescent current through power stage increased slightly - as expected!

Why did such heavy oscillation occurred within my amplifier circuit when there was no AC coupling capacitor between voltage amplification stage (MC1458) and output power stage (upper and lower Sziklai pair with paralleled power transistors)?

By the way, C3, C4, C5 and C6 represent AC bypass to avoid distortion when AC is applied to such circuit. At the collector and/or emitters of power transistors Q4, Q5, Q6 and Q7 there are no low resistance resistors for improving thermal stability of those because I have no low resistance (lower than 1 Ohm) high power resistor. Also, I have replaced MC1458 with another one (same part), but the results were similar (talking about example with no AC coupling capacitor).


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PostPosted: 10 Apr 2018, 21:09 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, The first thing I would do is put a scope on it and actually see what is going on. Then you can figure out how to actually correct it. I see no reason for the coupling capacitor to fix it...unless... it is introducing a slight phase shift and that is sufficient to return the amp to a stable condition. It might be oscillating at a very low frequency...like 0.5HZ or so and not be audible or easy to detect. That might draw considerable power. I had that occur in a tube amp with a bad output transformer. Thing got very hot and didn't sound good. The scope showed a sub 1 hz oscillation at nearly full power.

Good listening
Bruce

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PostPosted: 27 Apr 2018, 00:16 
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Joined: 23 Feb 2017, 02:02
Posts: 243
Try connecting the op amp to the base of q1. (there is some voltage ~0.65v at the current connection the op amp may not like it)
Try a few pf capacitor in parallel with RF1, this will help reduce oscillations.

Try to make the feed back resistor ratio smaller, the gain is dependant on the ratio of the resistors. making the ratio smaller allows the internal capacitances of the op amp to charge up much faster resulting in higher frequency response and imprved slew rate.

https://www.electronicproducts.com/RC_F ... lator.aspx
p1+rf2 resistance and c1 should be made so that the cut off frequency is 2-5hz. I recomend having p1 and rf2 as a fixed resistor.
The gain can be adjusted via a parallel pot on RF1。 A pot cannot replace rf1 directly because if zero feedback the amp will blow. (rf1 open circuit then zero feedback)

RF2 should be made to be 820 ohms or lower. This means c1 arround 100 to 820uF. This allows internal capacitance to charge up faster.
680ohms and 100uf or so should be fine. I had my amp oscillate on me before when I made RF2 680ohm and c1 from 470uF to 1000uF.
RF2 can be directly grounded however there is dc gain and dc stablity of amp is reduced. Grounding via cap means zero dc gain

I highly recomend you to use a 0.5amp or so auto recovery fuse (PTC) on both plus and minus supply this helps prevent blow ups if something goes very wrong.
Otherwise, use a low amperage fuse like 0.1 or 0.2amp at your 110/220v pimary. You can also use a bulb indicasent like 50-100W on 110/220v to help prevent blows


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PostPosted: 27 Apr 2018, 21:49 
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Joined: 23 Feb 2017, 02:02
Posts: 243
Also try a zobel on the output, a 10to 1ohm resistor in series with 0.1uF at ouput to ground of the amp
may or may not make the oscillation better

I recomend starting with a 8r2 and 0.1uF


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PostPosted: 27 Apr 2018, 23:41 
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Joined: 09 Oct 2012, 19:43
Posts: 318
Location: Vancouver Canada
The MC1458 specs says +-18v = 36v supply as the absolute max voltage. You are giving it +23/-23v = 46v. Could be a place to look at.


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PostPosted: 28 Apr 2018, 00:24 
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Joined: 23 Feb 2017, 02:02
Posts: 243
You can use zeners or a lm317 337 to reduce the voltage.
Also try having a 82-820ohm in series with the op amp output, this will isolate the capacitve load(output stage) from the opamp reducing oscillations, I will try a 180ohm then 820 ohm to see if any diffrence.


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PostPosted: 02 May 2018, 06:52 
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Joined: 23 Feb 2017, 02:02
Posts: 243
You can find the right feedback resistance for best freq response, up to a certain point, reducing resistance would not make the amp response any higher because slew rate(frequency response) limited by gain stage capacitances and compensation capacitance.


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PostPosted: 04 May 2018, 18:01 
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Joined: 26 Dec 2016, 03:46
Posts: 181
Location: Bayarea
1) You know the amp won't work with C2. With C2, you don't have a DC feedback, the opamp will just rail. That's the reason you don't have oscillation.

2) You cannot use +/- 23V on MC1458, even MC1558 is only +/-22V.

3) Even if you can really do +/-23V, you output swing can only swing to +/-20V or so. You max output power on 4ohm is 1/2 X Vpeak X Ipeak = 1/2 X 20V X 5A = 50Wrms. You are not close to 100W output. For 8ohm speakers, you only get about 25W.

4) as for oscillation. With C2 shorted out, you get your loop closed. You have NO compensation. Of cause it will oscillate!!! It's not that easy. I suggest you read internal opamp compensation. Or better yet get the Power amplifier design book by Bob Cordell and study through it first. Normal it's easy to put a dominant pole, but since you use opamp, you don't have the internal option of miller dominant pole compensation.

5) Feedback capacitor across RF1 usually is not good enough. As the reactance of the feedback cap kick in, the Closed Loop Gain decreases, there goes your stability. You need internal compensation to do this. there is no chance this amp is unity gain stable because of the CFP output transistors stage. Only chance is a small feedback cap from output of the opamp to the -ve input of the opamp to create a miller dominant pole compensation.

I also thinking about using opamp front end like yours, it's not easy as people think. Major problem is voltage swing ( or the lack of it). You need an extra stage in the middle to get the high enough voltage swing, that defeat the good points of having an opamp as the front end.


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PostPosted: 06 May 2018, 16:41 
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Joined: 23 Feb 2017, 02:02
Posts: 243
Yungman wrote:
4) as for oscillation. With C2 shorted out, you get your loop closed. You have NO compensation. Of cause it will oscillate!!! It's not that easy. I suggest you read internal opamp compensation. Or better yet get the Power amplifier design book by Bob Cordell and study through it first. Normal it's easy to put a dominant pole, but since you use opamp, you don't have the internal option of miller dominant pole compensation.


All op amps are compensated internally, it shouldn't oscillate when connected to ampfiers outputs given the output is designed correctly and near gain of zero.
Poor op amps have poor phase margin may have poor square wave response with like 3-5 over shoot and under shoot however should not oscillate.
This poor type op amp can be compensated further at the ampflieir output or directly feeding back at the output of op amp with pF capacitors


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PostPosted: 06 May 2018, 17:53 
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Joined: 26 Dec 2016, 03:46
Posts: 181
Location: Bayarea
Not true, Output transistor create extra poles, opamp absolutely do not behave the same if you add even emitter follower in series with the output. Particular this is a CFP output stage, the frequency response is much lower and create an extra pole in the feedback loop. Or else, it'll be too easy, get an opamp, put a buffer inside the closed loop and get a high current drive opamp.

My suggestion of having a small feedback cap from the OUTPUT of the opamp back to the -ve input. This is to bypass the output stage and close the loop at high frequency to avoid the extra poles created by the output stage.

From my experience, the OPS ( output buffer stage) is the most problematic of this kind of amp. The big transistors are slower, input capacitance are much higher. When I design my amps, even the LTSpice cannot simulate stability accurately. I have to work on the closed loop feedback on the bench....even though simulation gave me like 45deg of phase margin. My suspicion is the model of the output transistors are not accurate. My design is Emitter Follower only. CFP stage like this one is inherently more unstable, which is much worst because it has local feedback and have gain within the stage.

It's quite a big deal to assure the audio power amp is stable as it has to drive complex load. On top the cable capacitance. Some high end cable can have over 4000pF. A lot of amps cannot even stay stable with those cables.

OP has to be careful about oscillation just from the CFP stage, that's the reason most people don't use this output stage in the design. Nelson Pass use it in his Threshold amps. I can tell you as an owner of the Nakamichi PA-7 ( Designed by Nelson Pass, almost identical as his Threshold S300 but better). The amp cannot stay stable even with a 1500pF across the output right on the banana output connectors.


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