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.
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.
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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.
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.
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?
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?
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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?
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.
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.
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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.
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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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.
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.
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Nigel:
I think you misunderstood that I was talking about a motor resistor at the output, not the Zobel network. I already have the Zobel components all soldered in.
I think you misunderstood that I was talking about a motor resistor at the output, not the Zobel network. I already have the Zobel components all soldered in.
The ratio Rf to R3 determines the gain. Gain is Rf/R3+1. If you change any so that the ratio changes, you change the gain. Do you want to change the gain? Why do you want to change R3?
Jan
Thanks, but I think I'm okay with the gain resistors. I'm changing R3 because I'm changing the gain resistors. Otherwise, I'd likely have DC imbalance problems.
My real question now is what do for a resistor to prevent motorboating. As I said, Linkwitz recommends a 2 ohm resistor at the ouptut, but doesn't specify power rating or tolerance. Can anyone comment on this?
My real question now is what do for a resistor to prevent motorboating. As I said, Linkwitz recommends a 2 ohm resistor at the ouptut, but doesn't specify power rating or tolerance. Can anyone comment on this?
Tolerance is unimportant, even 10% is fine.
If all is fine there is very little power dissipated, only when it starts oscillating it gets hot, but that is normally not the case. A 1W should be fine here, that's what I always use.
BTW it is not against motorboating, it is against high frequency instability. It terminates the amp for high frequencies where the output cabling would inductively separate the load from the amp. The R (with the C in series) makes sure the amp is always terminated.
BTW R3 IS a gain resistor. As I said, changing R3 changes the gain, which depends on the ratio of R3 and Rf. So changing R3 changes the gain, unless you also change Rf in proportion. I understand your concern for equal input impedances for bias current balance, but since it varies with the input potmeter setting there's no single resistor value that balances it, unless you use a coupling cap on the signal input.
If you use a coupling cap after the pot wiper, both inputs see 22k for the bias currents and it will be balanced.
Jan
If all is fine there is very little power dissipated, only when it starts oscillating it gets hot, but that is normally not the case. A 1W should be fine here, that's what I always use.
BTW it is not against motorboating, it is against high frequency instability. It terminates the amp for high frequencies where the output cabling would inductively separate the load from the amp. The R (with the C in series) makes sure the amp is always terminated.
BTW R3 IS a gain resistor. As I said, changing R3 changes the gain, which depends on the ratio of R3 and Rf. So changing R3 changes the gain, unless you also change Rf in proportion. I understand your concern for equal input impedances for bias current balance, but since it varies with the input potmeter setting there's no single resistor value that balances it, unless you use a coupling cap on the signal input.
If you use a coupling cap after the pot wiper, both inputs see 22k for the bias currents and it will be balanced.
Jan
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BTW on Rm: TI recommends (see data sheet) to draw a minimum of 0.5mA from the mute pin. The mute pin will sit at -2.6V.
So the max Rm value would be: (-supply - 2.6) / 0.5 .
Example: -supply = 35V, then Rm nominal = 32.4/0.5 = 64.8k. So anything smaller than this would be fine. 10k is fine but so is 22k or 38.3k etc. Except that 38.3k is an uncommon value and needlessly expensive.
Jan
So the max Rm value would be: (-supply - 2.6) / 0.5 .
Example: -supply = 35V, then Rm nominal = 32.4/0.5 = 64.8k. So anything smaller than this would be fine. 10k is fine but so is 22k or 38.3k etc. Except that 38.3k is an uncommon value and needlessly expensive.
Jan
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1) just follow the datasheet recommendations until you have a better reason not to do so.
Which so far you don´t.
2) that amplifier has a minimum gain rating under which it must not go, just check the datasheet suggestions.
22k/680 ohms is fine and guaranteed stable, 20k/1k is not.
Meaning some amps would work with no trouble, others maybe not.
Why would you attract gremlins to your project for no good reason?
Which so far you don´t.
2) that amplifier has a minimum gain rating under which it must not go, just check the datasheet suggestions.
22k/680 ohms is fine and guaranteed stable, 20k/1k is not.
Meaning some amps would work with no trouble, others maybe not.
Why would you attract gremlins to your project for no good reason?
jan.didden:
Linkwitz wrote in his notes for the LM3886 circuit:
quote:
"R0 - to isolate signal from power ground in case of hum or motorboating due to poor wiring layout etc."
As for an input "potmeter". I don't know that means, but I'm assuming potentiometer? I won't be including one on these amps, so IIUC, that won't apply here.
JMFahey:
Why would I attract gremlins? Like I said earlier in the post, I'm going with Linkwitz's recommendations where possible. With all respect, I think Linkwitz knows more than most of the people in this forum combined.
I forgot the exact number, cause my notes are downstairs, but the gain will still be about 26, which is well within spec.
I was also advised to follow Linkwitz recommendations by someone who knows a fair bit about building amps, but can no longer help due to illness. Maybe you didn't read the earlier posts?
Okay, will go with 2R, 1W 5 or 10% resistors, then.
Thanks.
One more question to post and I should now exactly what parts to order to (hopefully) finish up these things.
Linkwitz wrote in his notes for the LM3886 circuit:
quote:
"R0 - to isolate signal from power ground in case of hum or motorboating due to poor wiring layout etc."
As for an input "potmeter". I don't know that means, but I'm assuming potentiometer? I won't be including one on these amps, so IIUC, that won't apply here.
JMFahey:
Why would I attract gremlins? Like I said earlier in the post, I'm going with Linkwitz's recommendations where possible. With all respect, I think Linkwitz knows more than most of the people in this forum combined.
I forgot the exact number, cause my notes are downstairs, but the gain will still be about 26, which is well within spec.
I was also advised to follow Linkwitz recommendations by someone who knows a fair bit about building amps, but can no longer help due to illness. Maybe you didn't read the earlier posts?
Okay, will go with 2R, 1W 5 or 10% resistors, then.
Thanks.
One more question to post and I should now exactly what parts to order to (hopefully) finish up these things.
Am I overstaying my welcome if I ask one more question in this thread?
It's been recommended to me to use a capacitor of 1.0 to 2.2 uF as a high pass filter and to block DC at the input. What type of cap what would I use (voltage, radial or not etc.
It've also been advised to use a low pass filter in the form of a cap of value 1500pF to 3300 pF . What do you folks recommend? Will difference values in these ranges make much of an audible difference?
Thanks.
It's been recommended to me to use a capacitor of 1.0 to 2.2 uF as a high pass filter and to block DC at the input. What type of cap what would I use (voltage, radial or not etc.
It've also been advised to use a low pass filter in the form of a cap of value 1500pF to 3300 pF . What do you folks recommend? Will difference values in these ranges make much of an audible difference?
Thanks.
I use none polarised capacitors on the input as I don't know what preceding device will output. Polyester is good.
220pf is a good for low pass. Depends on input resistor.
But with 22k input resistor put the 220pf in parallel with it.
Well you said 20k and 1k, that is NOT 26 gain.
I know personally Linkwitz and he is a very experienced designer, but you need similar knowledge to understand what he says. Is that the case?
For instance, the ground isolation resistor (which you call motorboating) is important in Linkwitz' system as he has 8 or even 16 amps all on the same power supplies and grounding. Are you using more than 2 for stereo?
Jan
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