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lm3886 servo help
lm3886 servo help
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Old 20th November 2017, 12:32 AM   #11
asuslover is offline asuslover  Romania
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lm3886 servo help
LM3886 Schematics + PCB
post 457
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Old 20th November 2017, 04:03 PM   #12
AndrewT is offline AndrewT  Scotland
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Here's the link to the old Thread discussing servo design.
DC Servo question...

and Gootee's site is still alive
Spice Component and Circuit Modeling and Simulation
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Last edited by AndrewT; 20th November 2017 at 04:06 PM.
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Old 20th November 2017, 04:26 PM   #13
Drummer 35 is offline Drummer 35  Spain
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Asuslover, thanks a lot for the link.

Very interesting design there. You tried it?

Tom, thanks so much for taking a look.

I thought I could use a low value cap between pins 9 and 10 without needing a RC in the feedback loop.

what about removing C2?

Yeah, I forgot the supply cons on u3, and u4. They will be connected to the power supply (+24v, -24v ,unregulated)

Regarding the zener diode... deleting it and going larger value on R7 will have the same effect? I want a "soft-on" start without having to go with regulators.

You say R4 should be 10*R3, and... R5=100*R3? right?

Thanks too for the link to your post. Very useful info there.

I think Iīll have to order some 411s to try them against the tl071.

Regards,

David
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Old 20th November 2017, 04:33 PM   #14
asuslover is offline asuslover  Romania
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lm3886 servo help
I did not , Prasi did, he said hes happy with the sound, he might have a pair of boards for you.
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Old 20th November 2017, 04:33 PM   #15
Drummer 35 is offline Drummer 35  Spain
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Andrew,

That is what I was looking for. The last post is 4 years old, so I didnīt saw it.

Thanks a lot
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Old 20th November 2017, 04:35 PM   #16
Drummer 35 is offline Drummer 35  Spain
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Yeah Asus, I think I should try, but I wanted to draw my own boards, you know...
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Old 20th November 2017, 04:42 PM   #17
asuslover is offline asuslover  Romania
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lm3886 servo help
of course,
Tom uses opa2277 for his servo. Modulus-86 Rev. 2.1: Composite amplifier achieving 0.000067 % THD.
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Old 20th November 2017, 04:57 PM   #18
Drummer 35 is offline Drummer 35  Spain
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Quote:
Originally Posted by asuslover View Post

Yes, it seems there are a lot of options to try.

Anyway, those are not cheap, and I think I would rather fry some cheap 71s while testing.
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Old 20th November 2017, 05:27 PM   #19
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by Drummer 35 View Post
I thought I could use a low value cap between pins 9 and 10 without needing a RC in the feedback loop.

what about removing C2?
The cap between the inputs of the LM3886 (C2 in your schematic) prevents quasi-oscillations as the output swings towards the negative rail. What happens is that the VAS transistor in the LM3886 enters saturation and its gain drops like a rock. This causes instability and the output will buzz as it reaches the negative supply voltage. The result is slightly lower than specified output power (and spurious oscillations). To address this, C2 is added. That messes up the transient response, so Rf2 and Cf (see Test Circuit #2 in the data sheet) are added.

Quote:
Originally Posted by Drummer 35 View Post
Regarding the zener diode... deleting it and going larger value on R7 will have the same effect? I want a "soft-on" start without having to go with regulators.
Calculating R7 so you have at least 0.5 mA at the absolute lowest value of supply voltage (low mains, bottom of the ripple valleys) will guarantee that the chip works well and starts up without any audible pops.
If you want round numbers, aim for 0.5 mA at 2/3 of the nominal supply voltage. Note that the MUTE pin sits at -3 V when the chip is un-muted, so you need to factor this in when you calculate R7. Also, round down to the nearest standard value to ensure that enough current flows.

Quote:
Originally Posted by Drummer 35 View Post
You say R4 should be 10*R3, and... R5=100*R3? right?
R5 is determined by the cutoff frequency of the DC servo. It has no relation to R3. For the tradeoffs regarding the cutoff frequency, see the DC servo thread I linked to a few posts back.

Quote:
Originally Posted by Drummer 35 View Post
I do. I'm also aiming for microvolt offsets. The LF411 is getting expensive. You can find some of the TI/Burr-Brown opamps (OPA...) that provide much better performance at the same price point.

If you haven't already, I suggest dropping the circuit into a spice simulator, such as TINA-TI (free download from TI). Simulate the DC performance with the various opamps used in the DC servo. That'll provide more insight faster than any lab experiment - at least if you're reasonably comfortable with spice simulation software.

Tom
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Old 7th December 2017, 12:39 PM   #20
Drummer 35 is offline Drummer 35  Spain
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Hi again!

Ordered some boards according to the schematic and your advices. Got it running and even sounds great,but Iīm afraid the servo is not working.

I measure from ground to lf411 pin 7 and get +15V, great!! then I measure ground to pin 4 and get -30V!!! Canīt find what is wrong.

At first I thought those -30volts came directly from PS but the power supply pumps 34,5volts so Iīm a bit lost.

I get the same reading for both boards so I think its not a faulty 79L15...

I draw the servo supply like at AN-1192 pdf, figure 16.

Whats the procedure to find out if the servo is working or not?

Need your help guys, I canīt find out what I did wrong.

Thanks again!
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