update on lm3886 power amp

[donald stringer]

I have succes on this project ,I fired it up and at first thought oh know another dud. But then thought to turn up the guitar vol. in all honesty this thing is so quiet even at full vol. So far so good. I dont have my pre-amp built yet so its just the chip. If I put an trimmer pot  between pin 9 and 3 instead of the 22k would it be possible to get a tad bit more gain but its fairly loud. I guess I was expecting a bit more. By the way Its an lm3886. For anyone thats familiar with this chip.

[R.G.]

I dont have my pre-amp built yet so its just the chip. If I put an trimmer pot  between pin 9 and 3 instead of the 22k would it be possible to get a tad bit more gain but its fairly loud. I guess I was expecting a bit more.

Don't tinker with getting more gain out of the LM3886. Just build your preamp, even if it's just a gain-of-ten opamp stage. Or just a boost pedal.

You don't want an amp chip with that much power on tap to be sent into oscillation by tinkering with and losing its feedback connection. Take the time and do it right.

[dschwartz]

did u use the GGG schem?
i have a 3886 and a 24 0 24V 2A transformer lying around..maybe i can do something nice

[donald stringer]

http://sound.westhost.com/project19.htm    This is the schem. I used letter for letter. Some of of the electros I had to change just because of necessity sake. If you read the other post on the power supplies question that will give you my actual dc voltages.

Don't tinker with getting more gain out of the LM3886. Just build your preamp, even if it's just a gain-of-ten opamp stage. Or just a boost pedal.

You don't want an amp chip with that much power on tap to be sent into oscillation by tinkering with and losing its feedback connection. Take the time and do it right

I am going to take your advice on this.So far I am quite satisfied. I have had an pre-amp built. This is the one out of MKII amp build here

http://sound.westhost.com/project27.htm

I have it built and have been running it off an 9+ 9- right now I have 18 + 18- dc available and  thinking about using it providing I can transplant in a nice way. By the way I am using it for the clean sound [not the clipping diodes]  But still considering the options.I have room in the case and as of yet have not made the face plate. I just dont want to clutter it up.
_

[donald stringer]

I am considering on going  with an simple pre-  maybe something tried and true. I am visualizing an one knob input vol. for the pre-amp/ one knob output vol. for the amp. itself. Definitely and pre- amp out and input. How much more complicated it gets than that I am not sure. The faceplate will be brass. After the hole accomodations  I" m going to polish it and coat it with some ultra clear.

[sean k]

The national datasheet has 20k over 1k as the feedback and he's raised it to 22k over 1k so it looks like the man has gone down that road and found the right corner to hang out on. Lotsa, lotsa heatsink would seem to be the answer to getting power up there with those things... and a nice big (enough) input.

[R.G.]

The national datasheet has 20k over 1k as the feedback and he's raised it to 22k over 1k so it looks like the man has gone down that road and found the right corner to hang out on. Lotsa, lotsa heatsink would seem to be the answer to getting power up there with those things... and a nice big (enough) input.

The LM3886 is not stable with gains below 5. With a gain of 22, you need about 1.4V peak to drive it to full excursion with a +/-30V power supply. That implies a preamp gain of about 14 to 20 for full undistorted power from a single coil guitar.

It is a good idea to never make the LM3886 clip. It will exhibit some of the uglinesses of high feedback solid state amps by flat topping abruptly. Better to put the limiter ahead of the amp, as Thomas Organ did with the Vox Royal Guardsman and Beatle amps. They did it because the power amp they were using would literally vaporize the output stage if you overdrove it, but limiting pre-power-amp is still a valid thing to do. In fact, their limiter might not be a bad circuit to use.

[dschwartz]

+1 on the limiter..
please read teemuk book about ss amplifiers..THE BEST GUITAR ELECTRONICS BOOK I´VE READ IN YEARS!! (or ever) you can download it at ssguitar.com..
It has many limiter circuits , included the thomas organ limiter..

[donald stringer]

Theres a lot of useful info in that book. I am convinced one could design one heck of a ss guitar amp if you could digest the content. The thomas vox limiter if I understand it right this is meant to introduce some clipping in the the preamp stages. The audio signal would be at the little dot [top left] in/ arrow out and the little circuit reduces the useful amount of clean headroom available to the rest of the circuit preamp. Correct me if I am wrong or even close. The 24 volts of course would be whatever pre-amp dc you would be using and the circuit grounded to 0 volts. If that is close then then one would just have to choose where at in the pre -amp to put it.....http://www.thatraymond.com/downloads/solidstate_guitar_amplifiers_teemu_kyttala_v1.0.pdf..... I was blessed with a  fairly decent sized heatsink, vertical fins and didnt go crazy with the thermal paste. Thanks for for all the help and I do plan on posting some pictures soon.

[R.G.]

if I understand it right this is meant to introduce some clipping in the the preamp stages. The audio signal would be at the little dot [top left] in/ arrow out and the little circuit reduces the useful amount of clean headroom available to the rest of the circuit preamp. Correct me if I am wrong or even close.

The Thomas Vox limiter was the Mystery Circuit at Geofex a few years back. It doesn't have much to do with the preamp, sitting as it does between the preamp and the power amp. What it does do is to stop the input to the power amp from ever, ever, even once going into clipping, as clipping level overdrives on this power amp kills the power amp. As such, the perfect level for the limiter to be set is so that it clips at an input level that is just a hairsbreadth lower than the clipping level on the output. But your concept is close. Not much headroom is wasted if it's set right, as the power amp itself would clip a fraction of a volt higher if the limiter didn't stop it from trying to go there.

The 24 volts of course would be whatever pre-amp dc you would be using and the circuit grounded to 0 volts.

Then 24V and resistance into the limiter sets a current which in turn sets the clipping levels.

[teemuk]

It's very simple to limit the power amp as well. 
e.g. Warwick style:

Component values of the limiter naturally may need a bit of adjusting depending on the circuit.

[Thomeeque]

It's very simple to limit the power amp as well. 
e.g. Warwick style:
(img)http://img504.imageshack.us/img504/3328/warwicksoftclippingpa00.gif(/img)
Component values of the limiter naturally may need a bit of adjusting depending on the circuit.

Hello!

Did somebody stored teemuk's picture above (warwicksoftclippingpa00.gif) by any chance? I'd like to see it, but it seems to be gone from imageshack

I was just wondering if it would be possible to implement some limiter directly into LM3886's gain adjusting circuitry..?

Thanks, T.