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New "Kit" on the Block - BrianGT /LM3886 Upgrade
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Old 18th October 2013, 07:07 PM   #51
udailey is offline udailey  United States
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Join Date: Mar 2008
Im working on a LM3886 amp using LDRs in Ri and Rb resistor positions. I think I will use Mr. Evil's new power supply to power it. Trying to make it work on single sided copper. Will share so you can all etch and build if you like. Might take me a few months. It all takes a few months, at least! Also working on an F4 and Bob is sending me some fremen editions to listen to. Hoping to be able to compare.

Hope everyone is well.
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Old 18th October 2013, 07:16 PM   #52
bcmbob is offline bcmbob  United States
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Hey U,

Sounds like great stuff. I've never etched a board so it will be a fun and instructive project. The LM3886 and LDRs sounds like a match made in heaven.

Please keep us informed.
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Old 18th October 2013, 08:23 PM   #53
carlmart is offline carlmart  Brazil
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Quote:
Originally Posted by udailey View Post
Im working on a LM3886 amp using LDRs in Ri and Rb resistor positions. I think I will use Mr. Evil's new power supply to power it. Trying to make it work on single sided copper. Will share so you can all etch and build if you like. Might take me a few months. It all takes a few months, at least! Also working on an F4 and Bob is sending me some fremen editions to listen to. Hoping to be able to compare.
I'm very glad you are back and doing things. Let's hope you get better and better.

What Mr. Evil's supply is that? Can you specify or URL?

Almost all projects can be designed on single-side pcb, even more an LDR. Designed a few for myself.
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Old 24th October 2013, 06:24 PM   #54
udailey is offline udailey  United States
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Default So far

Here is the layout so far.
Will update with a BOM later.
Attached Images
File Type: gif LDRamp.GIF (24.4 KB, 291 views)
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Old 24th October 2013, 07:06 PM   #55
danielwritesbac is offline danielwritesbac  United States
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I recently experimented with making a variable gain amp and to do that I put a little blue multi-turn cermet pot instead of the feedback-shunt resistor (not the feedback resistor).
That was great! Easily and quickly, it fine tuned up nicely.
So, I went to replace this with a fixed resistor of the same value.
FAIL!!!
There was no fixed resistor, even of the same value that sounded as nice as the little blue pot. Well, perhaps if I had found a cermet composition resistor, but only then.
Previously, I thought that resistors didn't sound much different from each other; however, I found out that they really can, if put into a high gain area, such as one leg of the feedback divider.
Result:
I ended up leaving the little blue pot in circuit since I didn't have a resistor that performed as well.
Anyone tried cermet?
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Last edited by danielwritesbac; 24th October 2013 at 07:18 PM.
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Old 24th October 2013, 07:28 PM   #56
udailey is offline udailey  United States
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So Rf is a conventional resistor. Highest quality you can afford/find. I would love to stick an LDR there of all places however they are not very stable at 20k and most importantly they will add distortion, a lot of it, when that kind of voltage passes over the resistive side. Maybe a low voltage headphone amp with an ouput transformer to step it up a little but not this amp. Should be a 1/4w resistor and it fits across pins 3 and 9. You solder it direct to the pins.
Zm, Rm and Cm are all part of a slow start or more of a delayed start. See, they force the amp to mute until the supply comes up to the zener voltage. You can use any zener you want here as long as its not close to your supply voltage. Maybe a 12V would be fine. What happens is that the major burst of power entering the supply will normally create a turn on thump. We've all heard it. This will help to lessen that thump as the supply will already be halfway full before the amp kicks in. I got this from one of Apex' amp and he got the idea from some amps Yorkville uses for guitar and PA work. I havent tried it yet. In fact I havent tried this amp at all. So far I have only replaced Rb with an LDR. Other folks have replaced Ri. I havent.
Rz and Cz are the Zobel of course and we can use a few values there. Common are .22uF and 4.7R.
See the Link? Its that white trace at the bottom. It is for powering the LDR circuits. You MUST MUST MUST never power up the amp before having calibrated/dialed in the LDRs to their correct values. So what do you do? Dont hook up anything but ground of your supply to the board. Then with no LINK inserted yet you can give positive supply voltage to the LINK side of R1. This will then power up the LDR power supply. (Z1 should be around 5V6 but could be slightly higher and .5W minimum. This zener could be as low as 3V3. The lower the output of the zener the more you must worry about its heat dissipation)

You will use a multimeter to measure the resistance of the LDRs. As you turn the trimpots P1 and P2 they will vary the current supplied to the LDRs by CS1 and CS2 which are LM334 current sources. P1 controls CS1 which controls LDR1. You know how LDR2 is controlled now. You will dial these LDRs to the correct resistance. LDR1 should be around 680R when the biploar Cf is around 47uf. You should always set LDR2 to around 1k. C3-C6 will help to further smooth supply voltage to the LDRs. I know you dont think LEDs should need this foo foo treatment but they like it and they sound better when you add caps here. The ceramic smt caps are around .1uf to .22uf and the electrolytics can be 10uf to 100uf. I have heard a difference adding caps to LDRs which did NOT have caps but I have not heard a difference between cap quality or value on the LDRs.
The white lines going out from the LDRs just indicate the wires from the resistive side of the LDRs should follow these lines to their corresponding through holes. Rb is made by LDR2 and Ri is LDR1.
TB1 is a 3 position terminal block with 5.08mm pitch. Its for power to the amp. You will use V-, Gnd and V+.
TB2 is a 2 position 5.08mm terminal block and so is TB3.
TB2 is for output to the speaker. It will allow Out+ and Out- which is actually ground.
TB3 is the signal input and uses In+ and In- which is tied to ground on the board.
You can see there is a star ground.
C1 and C2 are 100uf to 1000uf and I plan to solder .1uf Wima caps to the bottom of mine.
There are some small filled in zones on the board that do not connect to anything. These are so that your board will etch faster and lose less copper into your solution. You will want to test your board after etching to make sure that no zone touches (use your multimeter on the resistance setting) another zone. If you find tiny shorts you will want to scratch them out with something like an awl. I usually use an awl to scratch between all zones anyway.
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Last edited by udailey; 24th October 2013 at 07:34 PM.
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Old 24th October 2013, 07:34 PM   #57
udailey is offline udailey  United States
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So this is basically a copy of a BrianGT LM3886 with LDRs replacing Rb and Ri while slightly modifying the mute circuit to soft start.
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Old 24th October 2013, 08:06 PM   #58
udailey is offline udailey  United States
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Quote:
Originally Posted by danielwritesbac View Post
I recently experimented with making a variable gain amp and to do that I put a little blue multi-turn cermet pot instead of the feedback-shunt resistor
Previously, I thought that resistors didn't sound much different from each other; however, I found out that they really can, if put into a high gain area, such as one leg of the feedback divider.
Result:
I ended up leaving the little blue pot in circuit since I didn't have a resistor that performed as well.
Anyone tried cermet?
Thats a fun experiment! Glad you found a good solution and it must be great to be able to vary gain while you are listening so you can see what suits you best.
I think the best solution to your experiment and mine would be to have a 3 resistor Rf. Two resistors would be in series and one would be in parallel with one of the series resistors. Of the paralleled resistors there would be one that is variable and the other would be a regular resistor as would the other series. This way you can never adjust to much or to little gain but you can change the value inside this window of resistance.
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Old 24th October 2013, 08:14 PM   #59
udailey is offline udailey  United States
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Default PDF of board with no silk then just silk

Here is the board with out silk and then just the silk
Attached Files
File Type: pdf NoSilk.pdf (11.5 KB, 40 views)
File Type: pdf SilkOnly.pdf (8.9 KB, 30 views)
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Old 24th October 2013, 11:31 PM   #60
davym is offline davym  Scotland
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Interesting conversation this. When I suggested fitting expensive resistors to my Brian GT LM3886 boards a couple of months or so back, some members told me to save my cash because it would be a waste of time and money.

Anyway, I hope to have a couple of mono block My Ref FE's completed in the near future so that's where my limited funds are going. That and a DCB1 to go with them.

I will probably find another use for the Brian GT kit at some point though, it's good to know it can be improved further as it's pretty decent already.

I also believe it's a good idea to build in variable gain for power amp's but i was quite effectively dismissed by a certain Mr T when I suggested it
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