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Old 12th October 2012, 08:10 PM   #11
Boscoe is offline Boscoe  United Kingdom
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Originally Posted by akis View Post
This morning I experimented with 10R base stoppers in my PCB test platform (ie no breadboard weirdness), and they did absolutely nothing to stem the oscillations of the compound pair under load and driven close to the rails from the op-amp and without any global feedback. However placing a 1/10 resistor in series at the output of the op-amp (R9/R10) helped reduce the oscillations visibly, albeit with a small voltage loss at the load. In the end it is only the caps C6/C7 that stop the oscillations completely. There are no oscillations under no load and the test load is purely resistive. To drive an inductive load I have used a 1R/18nF Zobel at the output (not shown on schematic).

I attach a very simplified schematic.
Weird way of doing things and I can image distortion being high?
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Old 12th October 2012, 08:11 PM   #12
Boscoe is offline Boscoe  United Kingdom
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It's odd, I have being looking at the exact same thing the past few days. The same op amps but I have a better way which I will post (once refined) in a week or so!
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Old 12th October 2012, 08:12 PM   #13
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You two have made it all too complicated!
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Old 12th October 2012, 08:32 PM   #14
Boscoe is offline Boscoe  United Kingdom
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I can't want I haven't built one with the OPA445 yet only with a single NE5534 on 20V rails. I have mimicked STD03N/Ps with discrete components, one HFE category is 8000 and over so don't need anything in between them and the Op amp and I have proved this on the bench. I added a second OPA445 for added loop gain to get that distortion down but it isn't really needed as they are expensive. Bias is 96mA.

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Click the image to open in full size.
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Old 12th October 2012, 08:58 PM   #15
Boscoe is offline Boscoe  United Kingdom
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Simulated max power into 8R is ~57W 0.001% THD 4R ~80W 0.002% THD. Not too shabby.
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Old 12th October 2012, 10:30 PM   #16
akis is offline akis  United Kingdom
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Weird way of doing things and I can image distortion being high?
Why weird?

I do not have the rig to measure distortion, I just look at the curve on the scope and am happy when it looks clean and when the output shape on load matches the input shape. I cannot simulate it either - none of the oscillations were evident on the simulator. many designs look perfect in simulation when in fact they suffer from temperature instability, oscillations, variation of components values etc.
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Old 12th October 2012, 11:44 PM   #17
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@akis

I also don't understand your circuit. What is the purpose of the first op-amp?
What is C3/C4 needed for? The bias spreader already puts the required VBE between the transistor bases, so there is no need to AC couple a signal to both of them. Also the feedback loop is not around the output stage which increases distortion (unless the output stage is very linear at first).
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Old 13th October 2012, 12:38 AM   #18
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The source of my oscillations was simply a wiring error on the breadboard.
Ah, that makes more sense. Easy fix too. Interesting sims; thanks for sharing.

Yes, so far as I know SOT23 is the only package for FMMTx91. I looked at several of the Diodes BJTs for my amp but ended up designing for the Sanken STD03N and STD03P for bias stability and small size (the layout I have sitting around fits two channels into a 2x3 inch two layer PCB including 15,000+uF per rail; not bad). Unfortunately Sanken doesn't publish models for them and characterizing the 03Ps I picked up for cheap hasn't happened--no fT spec but Sanken's other Darlingtons are in the 45 to 80MHz range so hopefully the STD03s are fairly fast.

2pF is probably comparable to the stray capacitiance on the board. Disengages the outer loop at what, 100MHz? The AD797, LME49713, LME49990 are the only audio op amps I know of where that might matter. Something I've been contemplating though is lowpassing in feedback loops. That's as simple as stacking a cap on top the feedback resistor and a first order roll off at, say, 100kHz provides a significant reduction in the bandwidth the op amp will try to control the output stage over. Good for noise rejection and avoiding slew rate consumption by high frequency wibbles so that the excess loop gain stays fully engaged over the audio bandwidth. However, the LME49710 channel I'm operating without any feedback filtering is already better than any of the measurement gear I have access to. So if filtering were to help it out I've no objective way to verify that.

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Your GBP formula is a nice rule of thumb.
Thanks.
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Old 13th October 2012, 09:13 AM   #19
akis is offline akis  United Kingdom
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@akis

I also don't understand your circuit. What is the purpose of the first op-amp?
What is C3/C4 needed for? The bias spreader already puts the required VBE between the transistor bases, so there is no need to AC couple a signal to both of them. Also the feedback loop is not around the output stage which increases distortion (unless the output stage is very linear at first).
My circuit is similar to yours in that there is a push pull with compound pairs and unwanted oscillations. That is where the similarity ends I think.
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Old 13th October 2012, 10:40 AM   #20
Boscoe is offline Boscoe  United Kingdom
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Why weird?

I do not have the rig to measure distortion, I just look at the curve on the scope and am happy when it looks clean and when the output shape on load matches the input shape. I cannot simulate it either - none of the oscillations were evident on the simulator. many designs look perfect in simulation when in fact they suffer from temperature instability, oscillations, variation of components values etc.
You can't see millivolts on top of volts on a scope so you can't see the distortion, it's very hard. There's no point in C3, C4 and R10 there should be global closed feedback to minimize distortion perhaps use one BD139 instead of three diodes? It makes for easier mounting and overall simplifies the circuit. You can at least simulate distortion in multisim as well as use frequency gens so you don't need to make them. Yes simulation is way off real life but it can give use proof of concept.
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