Parallel resistors in LM3886 feedback loop? - diyAudio
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Old 4th August 2014, 08:55 AM   #1
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Default Parallel resistors in LM3886 feedback loop?

Hi, In my active system ptp gainclones have 33x gain on bass while le Monstre drives mid treble at 10x gain. Thus far I have simply atteneuated bass signal by a 3rd. Sounds good but attenuation not ideal. LM3886 stable down to 10x gain? anyone have experiance of this? Will soldering A 10k paralleled with existing 22k feedback resistor (for 11.1x gain) ,between pins 3-9, cause instability? Any thoughts before I dismantle my amps and give it a go. If not I will crack on and report back. Thanks in advance.
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Old 4th August 2014, 09:02 AM   #2
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Smile Typo, attenuatated bass signal by 2/3rds To 1/3rd.

Oops.
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Old 4th August 2014, 01:46 PM   #3
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The TI data sheet says, "The LM3886 is designed to be stable when operated at a closed-loop gain of 10 or greater..." so you'd be OK with that.

But hold everything. 22k in parallel with 10k gives a value of about 7k, not what you want.

I'm looking at the factory schematic. Using standard value resistors, about the best you can do is parallel your 22k (Rf1) with another 22k, giving a value of 11k. The gain formula for this noninverting amp is:

Gain = Rf1/Ri + 1

So your gain would now be 12. This might be a bit more than you want, but parts have tolerances so it's wise to leave some slack, just in case.

Ummm...your amp doesn't have a volume control? The factory schematic does. http://www.ti.com/lit/gpn/lm3886
.

Last edited by bentsnake; 4th August 2014 at 01:54 PM.
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Old 4th August 2014, 01:47 PM   #4
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Quote:
Originally Posted by duckanddexter View Post
Hi, In my active system ptp gainclones have 33x gain on bass while le Monstre drives mid treble at 10x gain. Thus far I have simply atteneuated bass signal by a 3rd. Sounds good but attenuation not ideal. LM3886 stable down to 10x gain? anyone have experiance of this? Will soldering A 10k paralleled with existing 22k feedback resistor (for 11.1x gain) ,between pins 3-9, cause instability? Any thoughts before I dismantle my amps and give it a go. If not I will crack on and report back. Thanks in advance.
Manual:
"The LM3886 is designed to be stable when operated at a closed-loop gain of 10 or greater, ...".
So it should be stable. (ahh too slow )
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Old 4th August 2014, 09:23 PM   #5
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Thanks for reply. Ri is 680ohm instead of the 1k on schematic giving a gain of just over 11x with 7k. Shall give it a whirl. When stuff works I am reluctant to experiment. Thanks again.
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Old 5th August 2014, 01:18 AM   #6
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" Gain = Rf1/Ri + 1"

This is only if your circuit is set up for a non-inverting configuration.

If it is a inverting configuration then your gain would be, "Gain = Rf1/Ri ".

FWIW

jer
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Old 5th August 2014, 09:27 AM   #7
AndrewT is offline AndrewT  Scotland
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the gain and phase margin are linked.

Look at the gain/phase plot.

When gain is +0dB the phase margin has gone very low. Probably oscillation.
When gain is +20dB the phase margin is roughly 61. The output will exaggerate fast transients.
When the gain is +26dB the phase margin has improved to more than 70. The overshoot on fast transients is now getting quite low.
When gain is +30dB the phase margin has improved even further to more than 80 and overshoot (exaggeration of fast transients) is gone.

You will see many 3886 reports stating that gains around the +27dB to +30dB sound nice. This is because the transient exaggeration has gone.

+20dB (10times) is stable, but is likely to not sound nice.
capacitive loading (cables and speaker/s) decrease the phase margin even more.

10times gain has the potential for sounding worse than "not nice".

I have just built a +20dB 3886 with noise gain compensation and added feedback cap. Not had sufficient listening experience to give useful guidance.
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regards Andrew T.

Last edited by AndrewT; 5th August 2014 at 09:31 AM.
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Old 6th August 2014, 09:04 PM   #8
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Fascinating. Thanks Andrew. Been reading up on phase margins since your post. I naively assumed feedback amps either oscillated or were stable. So much to learn. A Dac with the usual op amp outputs (around 2v out) only need about 10x Vgain even for the mediocre efficiency, 87db, speakers I have tried. Pre/power amp Gain structures, developed in the age of analogue sources, seem excessive in our digital age? Any suggestions for lower gain ,class b. diy amps? Don't want to waste class a heat dissipation on the bass. My brain cant cope with feedback compensation circuits. I get a "pole splitting" head ache just thinking about it! Thanks again.
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