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Old 6th March 2014, 01:19 PM   #1
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Default Inverting LM3886

Hi all ,

I am trying to build a stereo LM3886 based amplifier using the inverting configuration. The schematic below is for one channel.

The amplifier board will run from a +-30V regulated supply with 30000uF per rail.
The audio input will be fed to both channels by a 50K ohm dual potentiometer.

Questions:

1- What is the recommended value for C7 (bypassing the +ve terminal to ground)?
2- Are there any additional components that i am omitting from the circuit (besides the R//L output network)?
3- Where should the 220pF capacitor (RF filter) be connected, between the +ve and -ve terminals or between the -ve and ground terminals?

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Best Regards.

Last edited by sghr220; 6th March 2014 at 02:08 PM.
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Old 6th March 2014, 01:34 PM   #2
infinia is offline infinia  United States
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the gain is only -2 ( not recommended , unstable ) and the input impedance <10K , you'll probably need to add another op-amp to buffer the input. and might need to use a whole new circuit. it's been done before.
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Old 6th March 2014, 01:47 PM   #3
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Quote:
Originally Posted by infinia View Post
the gain is only -2 ( not recommended , unstable )
Sorry that's my mistake R1=1K not 10K so the gain is -20V/V.

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Originally Posted by infinia View Post
the input impedance <10K , you'll probably need to add another op-amp to buffer the input. and might need to use a whole new circuit. it's been done before.
The input impedance is determined by R1 which in this case is 1K so that should be low for the source to drive right? What about the input potentiometer before the input cap? and Why is these values recommended in the LM4780 datasheet (apart from being an honest mistake by National's engineers)?

Best regards.
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Old 6th March 2014, 09:25 PM   #4
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Quote:
Originally Posted by infinia View Post
the gain is only -2 .
I make it -200K/10k = -20

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Originally Posted by sghr220 View Post
Sorry that's my mistake R1=1K not 10K
That would make it -200
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Old 6th March 2014, 09:45 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

A very good question is why are you using the inverting configuration.
I cannot see any point to using it. Especially with a dual 50K pot.

rgds, sreten.
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Last edited by sreten; 6th March 2014 at 09:47 PM.
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Old 7th March 2014, 04:16 AM   #6
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Quote:
Originally Posted by Robert Kesh View Post
I make it -200K/10k = -20


That would make it -200
Sorry for the confusion, i updated the posted schematic to:

Rf = 200k
Ri = 10k
Rb (R7) = 10k

So the gain is -20 V/V and the input impedance = 10k approximately.

Now for my questions:

1- What is the recommended value for C7 (bypassing the +ve terminal to ground)?
2- Are there any additional components that i am omitting from the circuit (besides the R//L output network)?
3- Where should the 220pF capacitor (RF filter) be connected, between the +ve and -ve terminals or between the -ve and ground terminals?

Best regards.
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Old 7th March 2014, 04:23 AM   #7
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Originally Posted by sreten View Post
Hi,

A very good question is why are you using the inverting configuration.
I cannot see any point to using it. Especially with a dual 50K pot.

rgds, sreten.
* Better THD (no common mode distortion in the input stage).
* No electrolytic capacitor in the signal path (Ci).
* I want to try it as the title of the topic suggest.

1- What is the recommended value for C7 (bypassing the +ve terminal to ground)?
2- Are there any additional components that i am omitting from the circuit (besides the R//L output network)?
3- Where should the 220pF capacitor (RF filter) be connected, between the +ve and -ve terminals or between the -ve and ground terminals?
4- What is the recommended value for the dual pot?

Best regards.
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Old 7th March 2014, 07:41 AM   #8
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Should i dispense with the input pot completely and control the source instead?
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Old 7th March 2014, 02:19 PM   #9
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I am not sure how well the LM3886 works with relatively high feedback Rs like that. Plus, there's the problem with the 50k pot and its worst-case 12k5 output impedance (a 10k would be a better match for a 10k input impedance).

Both issues could be addressed by using an opamp buffer. (If you want to stick with the 50k pot, pick a part with low input impedance distortion and moderate current noise. Decent output driving capability much recommended.) Then you could reduce the LM3886 input resistor to, say, 2k2.

Your resistor R2 should match the DC resistance that the other input sees, i.e. (R1 + R3) || R4. BTW, the use of R3 is debatable - it is only needed to reduce the much higher input impedance of the noninverting circuit. With R3 removed, R2 = R4. (With a preceding DC-coupled opamp buffer, it's R1||R4.) You need to do this in order to keep DC offset low.

C7 should form a near-short at audio frequencies, in order to avoid introducing extra resistor noise via R2. Try 10 or so. Should be a bipolar.

The RF filter cap should go before the buffer, IMO. You can reserve a space for the cap between the inputs of the LM3886 but I'm not sure how useful it would be. If in doubt, determine cellphone sensitivity empirically.
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Old 7th March 2014, 03:01 PM   #10
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Originally Posted by sgrossklass View Post
I am not sure how well the LM3886 works with relatively high feedback Rs like that. Plus, there's the problem with the 50k pot and its worst-case 12k5 output impedance (a 10k would be a better match for a 10k input impedance).
Thorsten's original inverted gainclone used a feedback resistor Rf of 220K and input resistor of 10k so i am guessing there'll be no problem there.

I have the 10k dual pot but i am leaning towards not using an input attenuator of any kind and control the source level instead.

Quote:
Originally Posted by sgrossklass View Post
Your resistor R2 should match the DC resistance that the other input sees, i.e. (R1 + R3) || R4. BTW, the use of R3 is debatable - it is only needed to reduce the much higher input impedance of the noninverting circuit. With R3 removed, R2 = R4. (With a preceding DC-coupled opamp buffer, it's R1||R4.) You need to do this in order to keep DC offset low.
I am planning to use a 220k trimmer from Vin+ of the amplifier to the signal ground to adjust the DC offset value.

Quote:
Originally Posted by sgrossklass View Post
C7 should form a near-short at audio frequencies, in order to avoid introducing extra resistor noise via R2. Try 10 or so. Should be a bipolar.
Is there a formula to calculate the value of this capacitor?

Quote:
Originally Posted by sgrossklass View Post
The RF filter cap should go before the buffer, IMO. You can reserve a space for the cap between the inputs of the LM3886 but I'm not sure how useful it would be. If in doubt, determine cellphone sensitivity empirically.
Should the RF filter cap (300pF) connected between the amplifier terminals or between the -ve terminal and the ground?

I am not really sure i understand the "cellphone" bit completely.

Thank you for your much appreciated input.

Best regards.

Last edited by sghr220; 7th March 2014 at 03:09 PM.
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