Bought a XY LM3886 Kit. - Page 38

AndrewT · 10:28 AM

Quote:

Originally Posted by Aguilabrava

........... I highly recommend upgrading at least the C4, but the ideal would be to replace both, R4 and C4, using 680 Ohm for resistance, and at least 100uF for capacitance.

These cap value are not absolute.
They in conjunction with the resistor value determine the RC time constant.
That RC must be related to the input filter RC to ensure near zero AC voltage across the NFB capacitor.

Go back and re-read the formula. Post54. The RCs are linked to each other.

Aguilabrava · 22nd May 2012, 02:31 PM

Quote:

Originally Posted by AndrewT

C4 should be chosen so that in normal operation it has virtually zero DC voltage across it and virtually zero AC voltage across it.

Choose C4 value by using the equation
C4 >= C5 * Sqrt(2) * [R5+R6] / R4

The bigger you choose for C5, the bigger that C4 becomes.

Aguilabrava · 22nd May 2012, 02:53 PM

Andrew, I read each and every one of your posts... Don't tempt me here to use the "P" word, please!

Anyhow, I never said the values were absolute, my point is precisely that, they are not, and they influence very much the quality of sound that comes off the speakers.

Now, the formula above includes C5, but I have not found any reference in National's datasheet about this cap affecting the sound, they only refer to "AC coupling at input and output for single supply operation". They only mention Ri and Ci as a low frequency pole (highpass roll-off) and Cf, Rf and Ri which are the "Zobel RC Network" as a high frequency pole (lowpass roll-off).

Based on this, I will be using a 2.2uF 63V MKS type capacitor for AC coupling, this is the only size I could finf that will fit the space on the Pemo PCB's. On the other hand, instead of 47uF or 100uF, I will be using 220uF in Ci and 680 Ohm in Ri as you suggested.

Is this allright, or the 2.2uF 63V MKS type cap will cause troubles?. I thought it was only for "AC coupling"...

AndrewT · 22nd May 2012, 03:59 PM

The DC blocking cap on the input is a high pass filter.
I reckon that this filter should passively roll off the bass response for good sound. This follows on from others that have posted here before me.

That is where the formula came from.

avbuilder · 08:13 PM

The 2.2uF capacitor is the DC blocking capacitor as Andrew mentioned. This means that it prevents DC from entering the amp and destroying your speakers.

You can read about it on the My RefC thread where people have even bypassed it if the DC at output is very low or if your preamplifier already has a DC blocking output cap.

The higher the value of the input capacitor the lower the bass frequencies will cut off (high pass filter as told by Andrew). To explain in a simpler way with an arbitary example, say 1 mfd capacitor will pass frequency of over 40 Hz (so you lose the 20-40Hz band) while a 2.2 mfd one goes down to 20 Hz while a 10 mfd may gown down to 5Hz. Since healthy humans can hear from 20-20KHz, a 10 MFD will be an overkill (and enormous too). One can use from 1 microfarad and above however 2.2 to 4.7 will be the best.

After searching a lot of sites for a good yet small input cap I only found that the panasonic polypropylenes are the ones that fit.
The value of R3 feedback resistor determines the gain of the amplifier.

Polypropylenes are better than polyester ones. Try to find a 2.2 polypropylenes that fit. Wima MKP / FKPs are bigger however you may solder them from below. Here is my schematic and BOM:

Pemo · 23rd May 2012, 09:21 PM

Hi again, this is PEMO.
Continuing my research on LM3886 boards, I also found this highly respected kit from BrianGT.
Here you can see part of one of his boards, where you can see that power ground and signal ground are.....tada.... connected.
briangt3886board.jpg
NOTE: (Brian has the PDF document where I took this picture, protected. So if I am doing something wrong please let me know ASAP, to delete this message.)

3 out of 3. Why?

Best regards,

PEMO

AndrewT · 24th May 2012, 08:39 AM

Your pic does show the connection.
Is the connection on the bottom side?

Pemo · 24th May 2012, 01:07 PM

Quote:

Originally Posted by AndrewT

Your pic does show the connection.
Is the connection on the bottom side?

Hi Andrew, this is the only picture I found, I think this is the upper side of the board.

Why these three designers have grounds connected?
Why they sound so good?
Is this grounding issue just for safety or also to improve sound?

I really think that we can get something good out of this thread, at the end we will have a new board design, one that includes all the changes that have been discussed here.

Best regards,

PEMO

bcmbob · 01:48 PM

Here are some pictures of the bottom of the BryanGT amps. Hope they help

Edit: Long leads in pic #1 are just the excess from Uriah's Resistor Re-placer.

Aguilabrava · 24th May 2012, 02:32 PM

Avbuilder:

Thank you for your explanation "in simple terms" on the AC coupling capacitor. I tried to get the Panasonic ones you suggested, but since they only had them in their warehouse in Europe, just for shipping charges they wanted $32.00, which is way too much. I had to look for something they stock locally, and I found the Wima MKS-4, 2.2uF, 63V. Here is the link:
MKS4C042204C00KSSD - WIMA - CAPACITOR PEN FILM 2.2UF, 63V | Newark
I understand that the Panasonic are better, or any Poplypropylene cap for that matter, these Wima above are "Polyethylene Naphthalate", but I'll have to do with them since the price on the Panasonic ones became too high when shipping charges were applied.

Now, we have two issues that need to be worked, the connection on the signal and power ground in the PCB's, and the capacitor, resistor values....

22nd May 2012, 07:44 PM	#375
avbuilder diyAudio Member Join Date: Nov 2009	The 2.2uF capacitor is the DC blocking capacitor as Andrew mentioned. This means that it prevents DC from entering the amp and destroying your speakers. You can read about it on the My RefC thread where people have even bypassed it if the DC at output is very low or if your preamplifier already has a DC blocking output cap. The higher the value of the input capacitor the lower the bass frequencies will cut off (high pass filter as told by Andrew). To explain in a simpler way with an arbitary example, say 1 mfd capacitor will pass frequency of over 40 Hz (so you lose the 20-40Hz band) while a 2.2 mfd one goes down to 20 Hz while a 10 mfd may gown down to 5Hz. Since healthy humans can hear from 20-20KHz, a 10 MFD will be an overkill (and enormous too). One can use from 1 microfarad and above however 2.2 to 4.7 will be the best. After searching a lot of sites for a good yet small input cap I only found that the panasonic polypropylenes are the ones that fit. The value of R3 feedback resistor determines the gain of the amplifier. Polypropylenes are better than polyester ones. Try to find a 2.2 polypropylenes that fit. Wima MKP / FKPs are bigger however you may solder them from below. Here is my schematic and BOM: Last edited by avbuilder; 22nd May 2012 at 08:13 PM.

23rd May 2012, 09:21 PM	#376
Pemo diyAudio Member Join Date: Dec 2009 Location: Echegaray, Naucalpan, Estado de México	Continuing my research on lm3886 boards... Hi again, this is PEMO. Continuing my research on LM3886 boards, I also found this highly respected kit from BrianGT. Here you can see part of one of his boards, where you can see that power ground and signal ground are.....tada.... connected. briangt3886board.jpg NOTE: (Brian has the PDF document where I took this picture, protected. So if I am doing something wrong please let me know ASAP, to delete this message.) 3 out of 3. Why? Best regards, PEMO

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22nd May 2012, 02:53 PM	#373
Aguilabrava diyAudio Member Join Date: Jan 2012	Andrew, I read each and every one of your posts... Don't tempt me here to use the "P" word, please! Anyhow, I never said the values were absolute, my point is precisely that, they are not, and they influence very much the quality of sound that comes off the speakers. Now, the formula above includes C5, but I have not found any reference in National's datasheet about this cap affecting the sound, they only refer to "AC coupling at input and output for single supply operation". They only mention Ri and Ci as a low frequency pole (highpass roll-off) and Cf, Rf and Ri which are the "Zobel RC Network" as a high frequency pole (lowpass roll-off). Based on this, I will be using a 2.2uF 63V MKS type capacitor for AC coupling, this is the only size I could finf that will fit the space on the Pemo PCB's. On the other hand, instead of 47uF or 100uF, I will be using 220uF in Ci and 680 Ohm in Ri as you suggested. Is this allright, or the 2.2uF 63V MKS type cap will cause troubles?. I thought it was only for "AC coupling"...

22nd May 2012, 03:59 PM	#374
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	The DC blocking cap on the input is a high pass filter. I reckon that this filter should passively roll off the bass response for good sound. This follows on from others that have posted here before me. That is where the formula came from.

24th May 2012, 08:39 AM	#377
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Your pic does show the connection. Is the connection on the bottom side?

24th May 2012, 02:32 PM	#380
Aguilabrava diyAudio Member Join Date: Jan 2012	Avbuilder: Thank you for your explanation "in simple terms" on the AC coupling capacitor. I tried to get the Panasonic ones you suggested, but since they only had them in their warehouse in Europe, just for shipping charges they wanted $32.00, which is way too much. I had to look for something they stock locally, and I found the Wima MKS-4, 2.2uF, 63V. Here is the link: MKS4C042204C00KSSD - WIMA - CAPACITOR PEN FILM 2.2UF, 63V \| Newark I understand that the Panasonic are better, or any Poplypropylene cap for that matter, these Wima above are "Polyethylene Naphthalate", but I'll have to do with them since the price on the Panasonic ones became too high when shipping charges were applied. Now, we have two issues that need to be worked, the connection on the signal and power ground in the PCB's, and the capacitor, resistor values....