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Old 13th August 2012, 02:57 PM   #21
ddd is offline ddd  Bulgaria
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Will you try the "true" circuit now?
The pot position mustn't make any difference.
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Old 13th August 2012, 03:16 PM   #22
AndrewT is offline AndrewT  Scotland
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12mVdc of output offset for a complete sweep of the vol pot is considerably better than the 100mV to 200mV of offset changes I have seen in other Direct Coupled implementations.

Cvp,
do not become complacent.
You must ensure that faults elsewhere do not damage your speakers.
Detect DC and correct excessive DC and isolate if outside the correction range is a minimum requirement for a DC coupled implementation.
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regards Andrew T.
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Old 13th August 2012, 04:31 PM   #23
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First I'll do the same measurements but with a 8ohm load and input signal.

On the second board I'll try the fixed polarisation resistor (aprox 20k as the feedback).

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Quote:
Originally Posted by ddd View Post
Will you try the "true" circuit now?
The pot position mustn't make any difference.
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Old 13th August 2012, 05:00 PM   #24
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Bob Cordell's book "Designing Audio Power Amplifiers" has a very nice LM3886 design -- he takes advantage of the lower distortion when the inverting input is used. Use a dual FET opamp first as buffer, then as inverter (OPA2604). He also uses another OPA2604 as servo. If you take a look at the documentation for the LM3886 -- specifically AN-1192 you'll get the idea.

Bob demo'd his design at one of the NJAS meetings a couple years back and it sounded very good indeed. He also built in a bit of loop compensation --
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Old 13th August 2012, 06:29 PM   #25
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Here's one I drew up for the LM47890 -- which is a pair of LM3886 -- mostly follows Cordell:

Click the image to open in full size.
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Old 13th August 2012, 07:37 PM   #26
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Thank you, the design you suggest is fine but the requirements from the start were: simple and small amp.
Best regards

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Originally Posted by jackinnj View Post
Here's one I drew up for the LM47890 -- which is a pair of LM3886 -- mostly follows Cordell:
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Old 13th August 2012, 08:03 PM   #27
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Quote:
Originally Posted by AndrewT View Post
12mVdc of output offset for a complete sweep of the vol pot is considerably better than the 100mV to 200mV of offset changes I have seen in other Direct Coupled implementations.
Hi Andrew,

To be honest we don't have here a real "Direct Coupled".... there is still the Cin before the pot.

PS: In front of the amp will be a Technics SH-GE70
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Old 14th August 2012, 01:25 AM   #28
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Quote:
Originally Posted by cvpho1 View Post
Thank you, the design you suggest is fine but the requirements from the start were: simple and small amp.
Best regards
I've probably sold 800+ PCB's for various iterations of the LM3886. This example I think to be the best.

A servo will prevent heartbreak.
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Old 14th August 2012, 08:42 AM   #29
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Measurements in normal mode, after 180min listening jazz at normal level for a 20m2 room.

WARM after 180 min
L R
3mV 2mV min 0%
3mV 2mV med 25%
1mV 0mV med 50%
-2mV -3mV med 75%
-8mV -10mV max 100%

Note 1: temp went from 29C to 38C

Note 2: as before, above is mentioned (in %) the position of the pot, not the value of its resistance
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Old 15th August 2012, 07:32 PM   #30
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I've redone the measurements with signal at the input after a few hours of listening. Basically the values are the same:

L R
3mV 2mV min 0%
3mV 2mV med 25%
1mV 0mV med 50%
-2mV -3mV med 75%
-8mV -10mV max 100%

I noticed that at max volume the DC offset is fluctuating.

Any idea what are acceptable values?
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