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Old 3rd October 2004, 04:20 PM   #1
pro is offline pro  Italy
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Default LM3886 current feedback problem.

Hi averybody
I am playng around a LM3886 arranged in current feedback, my goal is to try how does the bass section of my sealed box sounds drived in current mode. After many tests, the schematic attached
seems work sonically very well.
I only have one problem that I can't solve.
If I connect the amp to the loadspeaker with the bass section of an Audioquest Slate cable (the mid-high section is connected to a Zen V4) I have about -200mV of offset, while if I use a saperate cable (cat5, 4 lead for+ and 4 for-) the offset go down at about
-25mV.
Any ideas of what can I do to eliminate the problem?
Ciao.
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Old 3rd October 2004, 05:22 PM   #2
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Are you sure it is DC offset? There may be low level oscillations due to the cable, which may show up as offset on a DC meter. Did you check with a scope?

Jan Didden
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Old 3rd October 2004, 06:12 PM   #3
Konrad is offline Konrad  Norway
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If the input is unconn or left open, the input is floating. BAD design practice. Insert a gnd reference resistor for your positive input FIRST.
Abt the opa remember its stable for gains higher than x so dont make it lower than x in bouth regions ie also as current drive.
Further passlabs should have som info abt voltage/current feedback. As i understand the there is a compromise, and the constant power is more the goal rather than current feedback alone. Audioexpress just has a articlle abt current driven fullrange.
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Old 3rd October 2004, 09:36 PM   #4
johnnyx is offline johnnyx  United Kingdom
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I would make the resistances on both +ve input and -ve input the same, so that bias current gives equal voltage drops.

You don't need a cap C1, it may give you unwanted phase shifts leading to lf instability. If you remove the 5k6 resistor connection from the output of the opamp and connect it to the junction of the speaker and R2, the dc feedback loop is still there via the speaker.

R2 is high, if you use R2 = 0.47 ohms, then gain of Rspeaker/R2 will be more than 10, and will probably be stable. The voltage gain will be higher too, so less input volts will be needed.

This is in addition to the previous suggestions.
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Old 3rd October 2004, 11:03 PM   #5
pro is offline pro  Italy
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Thanks for your help.
Jan: there arn't low level oscillations, but same high frequency yes. I measured a signal of +- 100mv 400KHz without load that rises to +-500mv 5MHz when load is connected.
Is this a value to high of oscillation or could be normal?
The temperature of the LM3886 doesn't rise over then 30 C.
Konrad and Johnnyx:
I know that gain is too low (shuld be at least 10, while now it's 4.6) but I have it low because of the low gain (13.5dB) of the Zen on the mid-high section. I am planning to modify the gain of the Zen V4 to about 25dB, so I can use a 0.47 resistor as R2.
C2 is used to avoid to the DC to rise at VDC when no load is attached. This can cause oscillations to.
I found one problem in the fact that I have left disconnected one of the two C2. Now the offset is about 40mv with Audioquest, and about 4mv with the cat5, but I think it's still too high.
I will try to equalize R4 and R1, but I don't know if this will solve the offset problem.
Anyway, the sound of this amp in current feedback is much more inpressive that the voltage feedback one. Basses are much more fast and clear (Impedence of my 12" woofer in sealed box vary from 6 to 10 hom so it's a quasi resistive load. Crossover freq is at 700 hz 12dB slope).
It is not on the schematic, but a 47k resistor grounds the input.
Avery other suggestion is wellcome.
Thanks again.
Ciao.
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