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Old 19th May 2013, 07:17 PM   #31
iggy111 is offline iggy111  Croatia
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Boy, that escalated quickly Thanks guys!

Now I did some calculating based on all this, but first a few questions:
1) Should I use the stock R1 (1k) if I'm using an input cap? I presume I should, so I should calculate Input RC with 1k+22k = 23k. Right?
2) If R1 (Rs?) is 1k, then RF decoupling cap is 680pF - solder it at the input RCA or at the LM3886 pins?

Now for Input RC to be 1,7Hz, C should be around 4uF when R is a given 23k (22+1).
Using 3,3uF which I have at hand, the frequency goes up to 2,09Hz, and using that stock Ci of 47uF together with 680R the NFB RC would be 4,98Hz - but it should be 1,45Hz max!

Long story short - how does this sound:
Input RC: C=3,3uF, R1=1k, R2=20k (lower) >>> 2,29Hz (69ms)
NFB RC: C=100uF (higher), R3=1k (higher) >>> 1,59Hz (100ms)
69ms*sqrt(2)=97,6ms

The next problem are the PSU caps, which are 10mF per channel. I do have another pair of 10mF, but they are Jamicons general usage, so I'm not sure if I should use them or buy another pair of Panasonic TS...

PS Andrew - took me a while to see that the HF and MF decoupling caps are already in my amp, what you suggested is to solder them directly to the pins, not add new ones... Will try to do that, though the MF caps legs are probably too short now.
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Old 19th May 2013, 07:24 PM   #32
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You could use a DC detection/speaker protection circuit on the output instead of making the amp A/C coupled...
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Old 20th May 2013, 08:11 AM   #33
AndrewT is offline AndrewT  Scotland
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the HF decoupling must be attached with a very short round trip route = short leads and short traces.
The MF decoupling can have a longer route. The Pi filter effect benefits from the extra resistance and extra inductance of the longer route. But the round trip route still needs to be reasonable. 50mm to 100mm of total round trip route would be acceptable. Whereas the HF total would be 5mm to 15mm.
Quote:
Input RC: C=3,3uF, R1=1k, R2=20k (lower) >>> 2,29Hz (69ms)
NFB RC: C=100uF (higher), R3=1k (higher) >>> 1,59Hz (100ms)
seems good. It needs smoothing RC>=140ms.
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Last edited by AndrewT; 20th May 2013 at 08:24 AM.
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Old 20th May 2013, 08:20 AM   #34
AndrewT is offline AndrewT  Scotland
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Default RF and interference attenuation

RF and interference attenuation:
I put the main input filter on the input to the amp PCB. There it can attenuate RF and interference coming from inside the Chassis and any external fields (entering via the cable Signal/Hot) that leak through the Chassis.
I also fit a VHF attenuator at the input socket to catch the very strong external fields as soon as they arrive at the equipment. This is a differential mode filter.

Recently I posted an enquiry about attempting to catch both the differential mode and common mode interference at the XLR input socket. But I have had no response to my suggestion and question.
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Old 20th May 2013, 09:26 AM   #35
minifly is offline minifly  South Africa
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Quote:
Originally Posted by godfrey View Post
Quote:
Originally Posted by minifly
I remember seeing some scope tests that showed that even a small capacitor between the + and - signal inputs caused the amplifier to take a while to stabilise on HF input signals - perhaps try removing that to see if it goes away?
Yes, that's generally a bad idea but he's not doing that (according to the schematics he showed earlier).
He posted this schematic in post 3, which shows a 220pF between +IN and -IN. That cap should go from +IN to A_GND, not -IN if you want a RF filter.
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Old 20th May 2013, 11:17 AM   #36
iggy111 is offline iggy111  Croatia
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Quote:
Originally Posted by AndrewT View Post
the HF decoupling must be attached with a very short round trip route = short leads and short traces.
The MF decoupling can have a longer route. The Pi filter effect benefits from the extra resistance and extra inductance of the longer route. But the round trip route still needs to be reasonable. 50mm to 100mm of total round trip route would be acceptable. Whereas the HF total would be 5mm to 15mm.seems good. It needs smoothing RC>=140ms.
So this is the plan - desolder the 100nf bypass caps from the board, solder them together and directly at the V+ and V- pins as you stated before.

The MF decoupling caps are about 5cm apart on the board as is, so do you think it would be ok to leave them there? I should measure to be sure, but total route seems to be around 10cm. Another option is to desolder the cap leads connected to the ground and connect them to the HF ground with a trace as you stated before.
I'm just thinking how to solve this without buying new caps because of the short leads on the existing ones...

The smoothing caps are a problem, I will leave them at 10mf per channel for now until everything else is in place, listen to the amp, and then try with the extra 10mf Jamicons.
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Old 20th May 2013, 11:47 AM   #37
iggy111 is offline iggy111  Croatia
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Quote:
Originally Posted by minifly View Post
He posted this schematic in post 3, which shows a 220pF between +IN and -IN. That cap should go from +IN to A_GND, not -IN if you want a RF filter.
Hmm, I'm pretty sure I saw a schematic somewhere that had that cap in the same place. Is this a mistake then, is my 220pf cap doing any good or bad there?
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Old 20th May 2013, 11:55 AM   #38
AndrewT is offline AndrewT  Scotland
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There are a few articles/papers stating that great care must be exercised when attaching any capacitance to the -IN pin of an opamp. This applies equally well to chipamps. They are opamps with a heavy duty output stage.
The warning is there because some added capacitance on the -IN pin makes for worse stability.
There are some papers that show how to use added -IN capacitance on the -IN pin to improve stability, but the values are usually very low and usually not much bigger than the parasitic capacitance of a PCB.
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Old 20th May 2013, 11:55 AM   #39
AndrewT is offline AndrewT  Scotland
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50mm for MF decoupling is by my understanding not a killer.
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Old 20th May 2013, 11:59 AM   #40
iggy111 is offline iggy111  Croatia
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And Andrew, what do you make of this reverse math, using the existing 10mf per channel and some standard values:

Smoothing caps: 10mf, Rspk=8R >>> 1,98Hz (79,5ms)
NFB RC: Ci=100uf, R3=560R >>> 2,84Hz (55,6ms)
Input RC: Cin=3.3uf, R1=1k, R2=10k (original is 22k) >>> 4,38Hz (36,1ms)

Is -3dB point of 4,38Hz too high?
And does changing R2 from 22k to 10k, and R3 from 680R to 560R affect the chip much? Although the changes are not big, so I guess it would work fine
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