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Old 27th December 2005, 03:58 PM   #1
sachi is offline sachi  United States
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Location: Silicon Forest
Default Lm4780 running. but...

i finally got around to wiring up Peter's LM4780 in paralell mode but am getting DC offset of 226mv.
Am testing it with my 35 year old murphy's.
No hiss or hum, but the DC worries me.
Only modification i have done on the original ckt is using a 22k for mute and fusable .1ohm(2.5watt) output resistors.
transformer secondaries are 20V.

One thing i would like to point out is that i have moved into my new house and am yet to get a permanent power connection.
The present connection is only sanctioned for 1KW(for all lights and applicances put together) and I must say the voltage fluctuates pretty badly.
Primary is way below the normal 230Volts.
usually around 222V and often drops to 212 for more than 30secs.
Primary winding on the transforerms are for 230V.
Do you guys think that this affects the DC offset.

Measured the input DC at the signal in at around 6mV.
Factoring the gain of 33 the 230mV seems about right.

So what should i do?

I am using this parallel LM4780 for driving the midrange(San Speaks) in my 3 way active.
I am using a 2.2uF cap in the input of the crossover to stop any DC.
but as of ow, am running it off a old CDP(Akai ****) and my NAD320BEE as preamp.

Any views to resolve the problem would be greatly appreciated.


P.S i donot plan to add a DC protection for the moment at least. i will do so at a later stage.
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Old 27th December 2005, 05:33 PM   #2
lineup is offline lineup  Sweden
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Location: the north
I dont know if you are using this setup.
This is bridge mode.

What you can try
is to change RB2 at the other OP-amp input.
See attached circuit.

Bias current in this resistor, creates an voltage offset.
Changing resistor will change voltage offset.

By default this is set to 3.3k.
I do not know what is value in your amp.

We can hope it helps. But I am not sure.
As I can see it, it will not effect amp operation in a major way.
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File Type: png bridge_lm4780.png (11.8 KB, 202 views)
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Old 27th December 2005, 05:47 PM   #3
sachi is offline sachi  United States
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Join Date: Dec 2003
Location: Silicon Forest
HEy lineup,
i am also using a bridged setup for my lows but have had no problems with it.
The DC offset is less than .5mV in the bridged mode.
and RB2 is the default value itself.

The circuit am using is the one attached below. i have not employed a zobel in my circuit.
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File Type: pdf lm4780_parallel.pdf (93.2 KB, 49 views)
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Old 28th December 2005, 05:33 AM   #4
sangram is offline sangram  India
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Join Date: Sep 2002
Location: India
You need an input cap for the amp chip, if you want the amp to not amplify the input-offset. P-A is kindly helping on another similar thread.

I too am suffering high offset but I think one channel of my chip just has a high dC-offset, with very little I can do otherwise...
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