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Old 5th January 2005, 06:11 PM   #1
durxter is offline durxter  Netherlands
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Question DC voltage on output

Hi,

I have build a gainclone with the LM3875 with the scematic of Fedde (Chill-amp, not inverted, included zobel)
I designed my own board with 2 amps and the PSU on the same board.
When connecting it, I'm measuring a DC offset of -85 mV on the output (input shorted and output R 10 Ohms). When I switch the amp off, the DC voltage rises to 1.5 V and decreasing to 0 V. You also hear a "pop" from the speakers when turning off.

What can I do to get rid of the DC voltage? Resistor between input cap and pin 7 or or include Ci??? Oscillation...?

The amp is pretty quiet whit no input connected and it plays fine, but I think -85 mV is to much!!

Here a picture of my PCB

Click the image to open in full size.
Click the image to open in full size.

I hope someone can give me some good advise!!

D Siesling
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Old 5th January 2005, 06:14 PM   #2
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Can you post a circuit with component values?

Jan Didden
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Old 5th January 2005, 07:40 PM   #3
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Dirk(?),

The DC path to gnd from the non-inv input is 22k, from the inverting inut it is 680ohms//22k = 659.6 ohms. There is a DC bias current at the inputs, and because of the unequal resistances, this generates a DC difference voltage at the inputs which is amplified and appears at the output. Make the non-inv input 10K (your preamp can handle this unless it is a very high output impedance tube amp), replace the 680 ohms by 10 k also and change the feedback resistor to 180k. This will make the impedances almost equal, and makes the gain lower so your volume control will be less nervous.

If you really want to balance the DC paths, make the non-inverting input R to gnd up of 10k//180k.

Note that the bias current are never exactly equal so even with equal resistors you may still have some offset, so the extra accuracy probably isn't worth it.

Jan Didden

PS You can post directly to the forum, of course. No need for emails.
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Old 5th January 2005, 07:55 PM   #4
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Default Re: DC voltage on output

Quote:
Originally posted by durxter

When connecting it, I'm measuring a DC offset of -85 mV on the output (input shorted and output R 10 Ohms).
Hi Durxter

Have you used a 1 K resistor , in series ,between the input and the non inverting pin of the amp?

If not , when you shorted the input , this will produce a large DC offset..if the amp is DC coupled.

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Old 5th January 2005, 08:06 PM   #5
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Jorge,

The amp isn't DC coupled. In fact, if you would have seen the schematic (Durk send it to me) you will realise that the 1k wouldn't do anything to help either. See my above post.

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Old 5th January 2005, 08:17 PM   #6
durxter is offline durxter  Netherlands
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The circuit from Fede's site www.fedde.tk

mine is almost the same, only I use 4.7u in stead of 2 in parallel.





Click the image to open in full size.

D Siesling
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Old 5th January 2005, 08:45 PM   #7
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Quote:
Originally posted by janneman
The amp isn't DC coupled. In fact, if you would have seen the schematic (Durk send it to me) you will realise that the 1k wouldn't do anything to help either.

If the amp is not DC coupled ,why not to use the usual electrolytic capacitor in series with R2, the 680 Ohms resistor...that way the DC path will be balanced and the DC gain will be near unity.
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Old 6th January 2005, 07:50 AM   #8
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Yeah, I agree with tube_dude.
For more information look at datasheet for LM3875 at www.national.com/pf/LM/LM3875.html.
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Old 6th January 2005, 08:48 AM   #9
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Default Re: DC voltage on output

Quote:
Originally posted by durxter
I have build a gainclone with the LM3875 with the scematic of Fedde (Chill-amp, not inverted, included zobel)
I designed my own board with 2 amps and the PSU on the same board.
When connecting it, I'm measuring a DC offset of -85 mV on the output (input shorted and output R 10 Ohms). When I switch the amp off, the DC voltage rises to 1.5 V and decreasing to 0 V. You also hear a "pop" from the speakers when turning off.
I have used DC servos on both my Gainclones and one advantage is a totally silent power-on and power-off. Some of the disturbance is eliminated by the servo because it works under 12 volts which is the limit for LM3875/3886.

A simple design may have drawbacks and the one you describe is one of them if it really bugs you.

Quote:
Originally posted by durxter
The amp is pretty quiet whit no input connected and it plays fine, but I think -85 mV is to much!!
85mV isn't extremely much and will probably not have any influence on the performance of your speakers.

When your homepage is working (nothing there yet?) you are very welcome to join my webring for Gainclones

http://C.webring.com/hub?ring=gaincloneamplifi
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Old 6th January 2005, 02:38 PM   #10
durxter is offline durxter  Netherlands
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Thank you all for the help!! I will first try the cap between 680 Ohms and ground. Will the capacitor have much negative effect on the sound quality?? Do I have to use a very high quality component for it?

D Siesling
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