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Old 9th March 2004, 06:49 AM   #41
tcpip is offline tcpip  India
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I also have another schematic for a finished non-inv amp for the LM3886, from Elektor. In that schematic, they've handled the mute pin too, so one can simply pick up the resistor values based on what they've put in it. (They've put in the option for a mute switch, which I don't intend to use.... I'll just keep the circuit permanently "un-muted" when I design the PCB.)

I have this article in PDF format, in case you want me to mail it to you. It's a one-page thing with schematic and PCB layout.
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Old 10th March 2004, 04:14 AM   #42
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the circuit as shown has a gain of about 3. it needs to be boosted by a bit.

why 2 42k resistors in parallel. also the input capacitor may now be needed.
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Old 10th March 2004, 06:18 AM   #43
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Quote:
Originally posted by theChris
the circuit as shown has a gain of about 3. it needs to be boosted by a bit.
You're right! I'll have to make R2 into 1K, and probably make C2 to 47uF to compensate and keep the same roll-off frequency.

Quote:
why 2 42k resistors in parallel. also the input capacitor may now be needed.
The two resistors in parallel are because some people on the chip amps forum say that a higher power rating for the feedback resistor seems to improve the sound a bit. I thought this would be a good way to reach the 0.5W rating using two 0.25W MFR.

And can you please explain the comment about the input cap? You think it may now be needed? I already have one.
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Old 10th March 2004, 01:40 PM   #44
tcpip is offline tcpip  India
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Default Fixed the gain

Is this okay?
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File Type: png gainclone-noninv.png (9.0 KB, 1529 views)
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Old 10th March 2004, 03:54 PM   #45
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Hi Tarun

I don't get how increasing the power rating of the f/b resistors can improve the sound. Anyway, electrically it should be the same. Some may say that heating effects in smaller resistors can raise the temperature enough to change the resistor value, leading to distortion. Could be, but IMO, the effect won't be audible except to the golden eared. Esp. if you're using metal film, which has a good tempco.

Please verify (using SPICE and an ordinary opamp model in place of the 3875) the effect of the 50pF cap in the feedback loop. Or else some simple math will tell you the 3dB point of the ckt (no, I'm recovering from flu, and you can't tempt me even with an Uncle Harry knob to do the math for you! ). If there's a rolloff of more than 1dB in the audio band, it's gonna be a lot more apparent than the effect of resistor heating.

On splitting the grounds apart, you could do the following: drill many holes on the PCB along the edge where the ground planes are split. You can solder short bits of wire across these, which you can later clip off if the hum gets annoying. You can also use a 50 - 100 ohm resistor here, because in case the input connection to the amp is removed (remember Murphy's law: sh*t happens!), you can say goodbye to whatever load is connected. The resistor will provide an "emergency" ground connection. In normal use (when there is an input), the ground return path will have a lower impedance than the resistor, so it shouldn't matter.

Madhu, welcome to DIYAudio. Using a supply of 35 volts with this chip is a bit risky. I once blew out a pair of 3886s when I used a 30-0-30 trafo. Of course, no-load DC is 42V here, but under load it should be lower than the chip's rating. Well, I guess turn-on transients did the 3886s in. Using a 25-0-25 (to get a nominal of 32V) is probably a safer idea. Cin of 10uF means using an electrolytic (err... BAD!) or a Z5U ceramic (also bad). You probably wont need LF response to go down to 0.72 Hz anyway. Use a 1uF film capacitor (get them at Cee Pee electronics. Ask for "box type" capacitors, he'll know). Perhaps you can use two of them in parallel.
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Old 10th March 2004, 04:04 PM   #46
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Looks like Tarun already did the math... Maybe I should read a bit more before I post?

One more thing: you mentioned high tech diodes. These are probably Schottky or fast recovery types. I can't get why they're used in an audio amp supply. They're meant for switchers. In a linear supply, their fast turnoff will inject harmonics of 50 Hz into the power lines, which if not adequately filtered, can cause problems. Look at the PSU to any low-noise measurement equipment from a quality firm, for instance an HP spectrum analyzer. The psu diodes will have capacitors across them (these must be able to withstand the AC voltage being applied) to slow down the diode switching. Thus, turn-off transients (which can sometimes go up to several megahertz!) are avoided at the source. The downside to all this is power loss in the diodes... small price to pay IMO.

Aside: ultra-fast switching diodes (backward diodes, some schottkys, etc) are sometimes used as "harmonic generators" to multiply the frequency of an RF signal. Introduce switching harmonics, then filter out the one you need.
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Old 10th March 2004, 04:29 PM   #47
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Here is my gainclone amp schematic. Note J1, a wire link between grounds. There will be an inductor wound around R7, which is not showin in the schematic or board.
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Old 10th March 2004, 04:31 PM   #48
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And here is my LM3886 PCB. Of special interest (maybe?) is the board size. Note that this PNG was exported at 150 dpi from Eagle.
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Old 10th March 2004, 05:20 PM   #49
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I am a little confused.

Output connection X2-1 and X2-2. If the load (speaker) goes between these 2 points, you will not get any potential. Your speaker will see the equivelant of a 12.7 ohm resistor in series with a 100nF cap accorss it's terminals. The amp output just flapping in the wind, with no path to ground.

Maybe there is supposed to be a ground/common connection between the C8 and terminal point X2-1?

A quick look at your PCB seems to refelct what I think is an error in the schematic.

Aud_Mot
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Old 11th March 2004, 02:57 AM   #50
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roadkill, you left off the inductor on the output of the amp. as it is you have just the 10ohm resistor
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