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Old 17th October 2017, 10:32 PM   #1
Hogwild is offline Hogwild  Canada
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Default Substituting stock Rm with another value in LM3886

Hi everyone:

Me again. Zero electronics knowledge, except soldering.

I'm trying to finish up a project with the Chipamp.com dual mono LM3886 boards. Schematic below.

I've decided, based on advice from a knowledgeable friend who was helping me, to subsitute resistors R2, and Rf with 20k resistors.
I'm also changing R3 to 1k. My friend and I are following Linkwitz' writings.

Unfortunately, my friend is no longer able to advise me, due to illness. That's why I keep coming back here.

My question is: In his notes, my friend suggests substituting the current Rm (10k) with a 38.3k resistor. This is also what Linkwitz uses.

If I make the above changes, is it necesary to change Rm, or should I just leave it as is?

Please, keep it simple. Remember, almost no electronics knowledge here.

Thanks.
Attached Images
File Type: png Chipamp.com stock schematic.png (98.5 KB, 89 views)

Last edited by Hogwild; 17th October 2017 at 11:47 PM.
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Old 17th October 2017, 10:59 PM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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Someone in another thread changed Rm and had trouble with crossover distortion.
I would stick to datasheet component values.
22k to 20k shouldn't make much difference though.

In your project make sure path for Rf is as close to LM3886 as possible or you could get oscillation. Same goes for decoupling.
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Old 17th October 2017, 11:51 PM   #3
Hogwild is offline Hogwild  Canada
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Hi Nigel:

Thanks for that. For Rf, I am just substituting the 20k where the stock resistor was already on the PCB. Since I haven't seen many complaints about sound quality from people who built these kits, I'll assume it should be okay.

And the change in R3 from 680R to 1k? That shouldn't create too much of a problem either?

This thread
LM3886 Mute Disable

makes it sound like Rm is only used to disable mute functionality. What is Rm used for?

Last edited by Hogwild; 18th October 2017 at 12:03 AM.
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Old 18th October 2017, 12:11 AM   #4
infinia is offline infinia  United States
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Quote:
Originally Posted by Hogwild View Post
If I make the above changes, is it necesary to change Rm, or should I just leave it as is?
Hi Hogwild
Chipamp.com designs have many satisfied builders.
I remember you from a couple of years ago. How far did you get or why are you changing the design? I think I can E-mail the original manual by chipamp.com for your project. Got pictures?
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Old 18th October 2017, 12:25 AM   #5
Hogwild is offline Hogwild  Canada
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Infinia:

I have four amp boards and 5 PSU boards soldered. That was finished about a year ago. However, my friend didn't tell me about the mods he suggested until after everything was finished.

So now, I'm replacing 2 or 3 resistors on each amp board to change gain, and adding in a few other things (a motorboating resistor, and some input RC filters, about which I'll ask advice in another thread.)

I won't change my mind about this, just in case you were planning on trying.

By pictures, do you mean photos or schematic? I posted a schematic in the OP up top. Can you not see it?
Oh and thanks for your offer, but I already have two versions of the Chipamp.com manual, plus one marked up by my friend with all kinds of other useful
info, mods, and additional info.

Quote:
Originally Posted by infinia View Post
Hi Hogwild
Chipamp.com designs have many satisfied builders.
I remember you from a couple of years ago. How far did you get or why are you changing the design? I think I can E-mail the original manual by chipamp.com for your project. Got pictures?

Last edited by Hogwild; 18th October 2017 at 12:54 AM.
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Old 18th October 2017, 12:29 AM   #6
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by Hogwild View Post
Hi Nigel:


And the change in R3 from 680R to 1k? That shouldn't create too much of a problem either?
?
That's should be fine.
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Old 18th October 2017, 01:01 AM   #7
Hogwild is offline Hogwild  Canada
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Thanks Nigel. Can anyone suggest what wattage and tolerance a motorboating resistor at the outputs should be? Linkwitz specifies a 2R value, but no wattage rating or tolerance.
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Old 18th October 2017, 01:02 AM   #8
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by Hogwild View Post
Thanks Nigel. Can anyone suggest what wattage and tolerance a motorboating resistor at the outputs should be? Linkwitz specifies a 2R value, but no wattage rating or tolerance.
The zobel circuit on the output should be 2r7 3 watt and 100nf capacitor.
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Old 18th October 2017, 01:16 AM   #9
infinia is offline infinia  United States
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Quote:
Originally Posted by Hogwild View Post
Infinia:

I have four amp boards and 5 PSU boards soldered. That was finished about a year ago.

I won't change my mind about this, just in case you were planning on trying.

This? what do you mean change your mind it's your friends idea to change/modify chimpamp.com schematics based on who knows what And then throw in linkwitz labs, & various DIY Audio members ideas.
IMO seems yer just waffling. Youre the guy building the thing, make a decision and move out.

Pictures of your project of course! what chassis/box do you have , power supplies, connectors or volume control. Have you put power to anything? seems a fair request after all this time and efforts by the good folks here assisting you. I'm surprised by the lack of progress Vs the number of threads you have started over the years. It's a kit for Christsakes, just build it per the manual and enjoy the music.


Pictures from you will help us help you! spotting any mistakes etc...
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Last edited by infinia; 18th October 2017 at 01:32 AM.
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Old 18th October 2017, 03:26 AM   #10
Hogwild is offline Hogwild  Canada
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Infinia:

Pictures were posted over a year ago in other threads. I am very grateful for the help I've received here, but let's just say my life has some very very serious problems right now which prevent me from moving forward quickly.

I can't say for sure, but I will trying to post something back soon which may help other newbies. That's about all I can give back in my current situation.

I'm not waffling. How do you get that I'm waffling when I just told you I wasn't interested in being persuaded otherwise.

No case. No reason for a case until I complete and test the basics. And until I lay everything out for how it might fit into a case. But you can see I'm a distance from there. Remember...no electronics knowledge.



Quote:
Originally Posted by infinia View Post
This? what do you mean change your mind it's your friends idea to change/modify chimpamp.com schematics based on who knows what And then throw in linkwitz labs, & various DIY Audio members ideas.
IMO seems yer just waffling. Youre the guy building the thing, make a decision and move out.

Pictures of your project of course! what chassis/box do you have , power supplies, connectors or volume control. Have you put power to anything? seems a fair request after all this time and efforts by the good folks here assisting you. I'm surprised by the lack of progress Vs the number of threads you have started over the years. It's a kit for Christsakes, just build it per the manual and enjoy the music.


Pictures from you will help us help you! spotting any mistakes etc...

Last edited by Hogwild; 18th October 2017 at 03:30 AM.
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