Distortion on p2p LM3875 - probably my fault :D

quickshift · 5th February 2006, 06:16 PM

Hello

As a little project and to try to make a case I have knocked up a p2p (mostly) LM3875 but... It sounds really distorted.

I have used a chipamp rectifier board and added some additional capacitors and stuff - the power supply but checks out OK and all that.

I have changed 2 of the resistor values - the one which goes from pin 3 to 8 is 10K instead of 22K and the one that goes from pin 8 to ground is 330 - IIRC, this should give me a bit less gain as the ratio is 30 instead of 32 but should still work properly. The input resistor is 1K. I have used different diodes >> http://uk.farnell.com/jsp/endecaSear...U=250697&N=401

Anyway... The sound is really distorted - it's like the input levels are way too high.

Is this problem likely caused by the substitution of the resistors?

The resistors are all in the correct places etc and nothing is shorting out. Both channels are exactly the same.

Any ideas appreciated

Nordic · 5th February 2006, 07:45 PM

why not take your circuit and draw what you see in a circuit diagram for us.

quickshift · 5th February 2006, 08:21 PM

OK, good plan!

Here is the diagram.

Thank you.

Vikash · 5th February 2006, 11:24 PM

Try adding a resistor to ground on the input and/or reducing the input resistor R1: http://www.purplestorm.co.uk/vikash/audio/gainclone02/

Eva · 6th February 2006, 12:19 AM

The input RF filter is missing.

Tube_Dude · 6th February 2006, 01:22 AM

Quote:

Originally posted by quickshift

Anyway... The sound is really distorted - it's like the input levels are way too high.

Have you used a pot at the input?...if not , you need to put a resistor from pin 7 to ground to bias the non inverting input.

Nordic · 6th February 2006, 05:32 AM

it is also advisable to add a small (2.2. - 4.7uf) poly cap in the input path to protect against high DC offsets on the output, that can fry your speakers.

Here is Nuuks diagrame

Some notes, he used 2 resistors in paralel on the feedback to get to a bit over 200k, also that input cap is 4.7uf not 47...

This is an inverted design, so its not going to like a pot too much

that 56k resistor can be soldered directly between live and ground on the input RCA, keep wires from this ground to the ground star thinner than the rest of your earth wires...

quickshift · 6th February 2006, 06:30 AM

Thank you everyone

I'll add the resistor to ground and change the input resistor for a lower value.

I did use pots but I think I wired them in wrong (I couldn't find any diagrams of how to connect them) so removed them - they didn't do anything when I turned them

I thought I was copying the chipamp design with a 1K input resistor and nothing going to ground but I guess I got it wrong..

DC offset is 0.3mV and 0.4mV with an 8 ohm load but adding blocking caps is a good idea just in case as I prefer my speakers unfried.

Thanks again

I really should read a book about all this stuff...

Nordic · 6th February 2006, 06:41 AM

Look, I think your circuit should work, and its more than likely a dry solder joint... would especilay look at connections to the feedback resistor, also just double check your resistors, I once accidentaly took a 1k resistor instead of a 10k on the input and ended with a gain of about 200, which sounded just horrible.

Igla · 6th February 2006, 11:24 AM

I would say that the input is overloaded.
What do you have in front of your GC? CD player, computer soundcard, a preamp ?

5th February 2006, 06:16 PM	#1
quickshift diyAudio Member Join Date: Jun 2005 Location: West London	Distortion on p2p LM3875 - probably my fault :D Hello As a little project and to try to make a case I have knocked up a p2p (mostly) LM3875 but... It sounds really distorted. I have used a chipamp rectifier board and added some additional capacitors and stuff - the power supply but checks out OK and all that. I have changed 2 of the resistor values - the one which goes from pin 3 to 8 is 10K instead of 22K and the one that goes from pin 8 to ground is 330 - IIRC, this should give me a bit less gain as the ratio is 30 instead of 32 but should still work properly. The input resistor is 1K. I have used different diodes >> http://uk.farnell.com/jsp/endecaSear...U=250697&N=401 Anyway... The sound is really distorted - it's like the input levels are way too high. Is this problem likely caused by the substitution of the resistors? The resistors are all in the correct places etc and nothing is shorting out. Both channels are exactly the same. Any ideas appreciated

5th February 2006, 11:24 PM	#4
Vikash diyAudio Moderator Emeritus Join Date: May 2003 Location: UK	Try adding a resistor to ground on the input and/or reducing the input resistor R1: http://www.purplestorm.co.uk/vikash/audio/gainclone02/ __________________ "The human mind is so constituted that it colours with its own previous conceptions any new notion that presents itself for acceptance." - J. Wilhelm. (But I still think mine sounds better than yours.)

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5th February 2006, 07:45 PM	#2
Nordic diyAudio Member Join Date: Sep 2005	why not take your circuit and draw what you see in a circuit diagram for us.

6th February 2006, 12:19 AM	#5
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	The input RF filter is missing.

6th February 2006, 05:32 AM	#7
Nordic diyAudio Member Join Date: Sep 2005	it is also advisable to add a small (2.2. - 4.7uf) poly cap in the input path to protect against high DC offsets on the output, that can fry your speakers. Here is Nuuks diagrame Some notes, he used 2 resistors in paralel on the feedback to get to a bit over 200k, also that input cap is 4.7uf not 47... This is an inverted design, so its not going to like a pot too much that 56k resistor can be soldered directly between live and ground on the input RCA, keep wires from this ground to the ground star thinner than the rest of your earth wires...

6th February 2006, 06:30 AM	#8
quickshift diyAudio Member Join Date: Jun 2005 Location: West London	Thank you everyone I'll add the resistor to ground and change the input resistor for a lower value. I did use pots but I think I wired them in wrong (I couldn't find any diagrams of how to connect them) so removed them - they didn't do anything when I turned them I thought I was copying the chipamp design with a 1K input resistor and nothing going to ground but I guess I got it wrong.. DC offset is 0.3mV and 0.4mV with an 8 ohm load but adding blocking caps is a good idea just in case as I prefer my speakers unfried. Thanks again I really should read a book about all this stuff...

6th February 2006, 06:41 AM	#9
Nordic diyAudio Member Join Date: Sep 2005	Look, I think your circuit should work, and its more than likely a dry solder joint... would especilay look at connections to the feedback resistor, also just double check your resistors, I once accidentaly took a 1k resistor instead of a 10k on the input and ended with a gain of about 200, which sounded just horrible.

6th February 2006, 11:24 AM	#10
Igla diyAudio Member Join Date: Jun 2003 Location: Slovenia	I would say that the input is overloaded. What do you have in front of your GC? CD player, computer soundcard, a preamp ?

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