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Old 18th October 2017, 02:38 AM   #11
Hogwild is offline Hogwild  Canada
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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.



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Originally Posted by nigelwright7557 View Post
The zobel circuit on the output should be 2r7 3 watt and 100nf capacitor.
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Old 18th October 2017, 08:28 AM   #12
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Hogwild View Post
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?
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
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Old 19th October 2017, 12:38 AM   #13
Hogwild is offline Hogwild  Canada
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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?
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Old 19th October 2017, 06:43 AM   #14
jan.didden is offline jan.didden  Europe
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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
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Last edited by jan.didden; 19th October 2017 at 06:48 AM.
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Old 19th October 2017, 07:20 AM   #15
jan.didden is offline jan.didden  Europe
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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
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Last edited by jan.didden; 19th October 2017 at 07:27 AM.
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Old 19th October 2017, 07:31 AM   #16
JMFahey is offline JMFahey  Argentina
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1) just follow the datasheet recommendations until you have a better reason not to do so.
Which so far you donīt.

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?
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Old 19th October 2017, 10:58 PM   #17
Hogwild is offline Hogwild  Canada
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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.
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Old 21st October 2017, 03:34 AM   #18
Hogwild is offline Hogwild  Canada
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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.
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Old 21st October 2017, 03:47 AM   #19
nigelwright7557 is online now nigelwright7557  United Kingdom
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Quote:
Originally Posted by Hogwild View Post
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.
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.
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Old 21st October 2017, 07:16 AM   #20
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Hogwild View Post
I forgot the exact number, cause my notes are downstairs, but the gain will still be about 26, which is well within spec.
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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Last edited by jan.didden; 21st October 2017 at 07:21 AM.
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