I have just finished building a very minimalistic LM3886 based chipamp, which has just three resistors in the signal path, nothing else. From the design point of view it is nothing spectacularly new, but I have spent some thoughts building it very compact and avoiding closed loops. I have posted a detailed and illustrated building report on my website, which may be very instructive especially for beginners.
Here is the schematic and some pictures giving you a first impression. (As usual, I constructed the circuit on a jig and transfer it to a case after extensive testing).
The amp works fine, DC offset is 30 mV for each channel and it sounds very good.
Note: Take care if you build this amp, it has no input capacitors and no zoblel
Enjoy,
Mick
Here is the schematic and some pictures giving you a first impression. (As usual, I constructed the circuit on a jig and transfer it to a case after extensive testing).
The amp works fine, DC offset is 30 mV for each channel and it sounds very good.
Note: Take care if you build this amp, it has no input capacitors and no zoblel Enjoy,
Mick
I've made two pairs of the exact circuit. Very easy to P2P. I agree they sound very good, even with cheap components.
Note: Your schematic does not show the jumper from pins 1 to 5.
Note: Your schematic does not show the jumper from pins 1 to 5.
David, I dont have a jumper from pin 1 to 5. Pin 5 is disconnected. I dont think it is necessary, do you?
I have a jumper from pin 4 (V-) to pin 11 (nc) in order to get a more symmetrical pinout, but this is indeed not in the schematic.
Mick
PS. I forgot to give the component values in my previous post.
Rg 22k
Rf 22k
Ri 680R
Rm 10k
Cs 2200uF || 100nF
I have a jumper from pin 4 (V-) to pin 11 (nc) in order to get a more symmetrical pinout, but this is indeed not in the schematic.
Mick
PS. I forgot to give the component values in my previous post.
Rg 22k
Rf 22k
Ri 680R
Rm 10k
Cs 2200uF || 100nF
I noticed that you have no Cap to ground in your NFB. What does this mean for your low pole roll-off, since 1/(2*pi*Ri*Ci) equals the -3db pole. If Ci is only the capacitance inherent in the wires (and that would very low) then wouldn't the pole frequency be ridiculously high?
alitaido said:
That capacitor is optional and I left it out as I wanted no capacity at all in the signal path. Note that the cap, if it is there, forms a low-pass filter with the feedback resistor. Without the cap, Ci=0, which means that the cutoff frequency goes to infinity. 😉
Mick
curious if anyone can chime in as to any problems encountered leaving the Ci out... i etched boards once with an oops where i had to leave it out (didn't want to dremel up the board). seems i've had better results with no Ci or setting it to 33 or 47 uf rather than the datasheet recommended 22 in concert with 1k (instead of 680). datasheet mentions to prevent gain at dc and just wondering when that comes up or what side effects/glitches would be present
Yes, that is what I thought. I'm curious, how does this effect DC stability and the overall sonic quality? Wouldn't the DC offset at the output increase (if there were any at the input, that is) ?
Well, you should only use this design if you are sure that you dont have any DC at the input. The previous stage should take care of that.
As for the sound quality, I am sure that less capacity in the signal path increases sound quality. Moreover, if you use that cap, the gain is frequency dependent in the low range, which I dont find very appealing.
Afaik this cap has no effect on stability. If stability is an issue, consider a Zobel or caps between V+ and V- and/or in+ and in-, respectively (see datasheet).
Mick
As for the sound quality, I am sure that less capacity in the signal path increases sound quality. Moreover, if you use that cap, the gain is frequency dependent in the low range, which I dont find very appealing.
Afaik this cap has no effect on stability. If stability is an issue, consider a Zobel or caps between V+ and V- and/or in+ and in-, respectively (see datasheet).
Mick
I've built 4 stereo chipamps. Two on PCB and two P2P. No capacitors (in signal path), no problems.
PS Mick, I had a look at your site, Nice! Interesting idea for low value resistors.
PS Mick, I had a look at your site, Nice! Interesting idea for low value resistors.
The Ci capacitor (as National calls it on the schematics) has nothing to do with stability only to set DC gain to unity. Once there tho it also creates a high pass filter so the correct value should be chosen for the correct bass response. The input capacitor on many schematics is only there to block DC from the previous stage.
Connect a wire to Pin 5. Trust me on this. I know for a fact you should connect both Vcc pins as they show. The N/C pins are floating with nothing connected to them so you can do what ever you want with those pins.
-SL
Connect a wire to Pin 5. Trust me on this. I know for a fact you should connect both Vcc pins as they show. The N/C pins are floating with nothing connected to them so you can do what ever you want with those pins.
-SL
SL, I will follow your advice and try, but just for curiosity, why does pin 5 need to be connected. I use two amps without the connection and they work well. Is there a sonic difference?
Mick
Mick
If one were to remove some of the pastic so that the die was visible and look at it to see what it looks like you would find that on the left of the die there are 3 pairs of pads. The top pair has one as -Vee going to frame (what is called a downbond to the DAP, die attach pedal) and the other one goes to pin 1 (+Vcc). The middle pair will have one wire each both going to pin 3 (output). The bottom pair will have the one to pin 5 (+Vcc) and the other pin as a downbond again. Then on pin 4 you would see two wires as downbonds. So the design is that -Vee goes through the frame to the pads as well as the back of the die being held at the lowest potential. The other wires you would see are Mute, GND, and the inputs. The N/C pins have nothing connected to them at all. By not connecting pin 5, you're basically only using one path for supply when it really needs two for the current handling. So long story short, connect pin 5 to Vcc or jumper it to pin 1.
-SL
-SL
And I will try to see this for myself. I have a faulty LM3886 here, which I am going to dissect.
shouldn't the title be four-resistor LM3886?
Just curious, btw that looks very simple, the LM3875 would be a three-resistor chip amp though 😀
Just curious, btw that looks very simple, the LM3875 would be a three-resistor chip amp though 😀
ifrythings said:
I was waiting for that comment, and of course you are basically right. However, the mute resistor is not at all in the signal path, so it is not counted.