Thanks for the useful info J M, I am now closer to understanding how this all works.

Quote from: J M Fahey on November 18, 2009, 10:07:45 PM
a heavy handed guy will reach that around 5, and a weakling, without technique, with poor guitar and pickups still has some extra gain to reach full power.

Your comment quoted above has opened my eyes more than you can imagine. I wasn´t even taking these wild differences into consideration... I don´t plan on using a limiter circuit so maybe respectful distance from poweramp clipping is the key and not worry about max power.

Hello to all

I have the GGG Lm3886 poweramp up and running on a +/- 24V 3A transformer. 

It works great but I´d appreciate some guidance to take things one step farther. I would like to hook up my Professor Tweed or Dr Boogey or any similar project as a permanent preamp but I´m not sure how efficient the power transfer is.

R.G gave an excellent clue on the diystompboxes forum ...." 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. " 

Well I guess my question is: How do I calculate the gain out of a Professor Tweed from a typical guitar signal? At what voltage would the Tweed need to be powered to get as close to max power out of the 3886 without clipping?

Hope someone can help, Daniel

Hey Joecool85,

I got it working, the transfomer ground wasn´t hooked up to the circuit... something that hadn´t brought me any problems with my TDA2050 (R.G gave an explaination as to why).

The thread at DYIstompboxes is interesting and might help other beginners if you decide to point them that way

Thanks for taking an interest,

Daniel C

It was suggested I post here in my DYIstompboxes thread... I feel a little sneaky copying and pasting here though

My problem is simple I hope. 

I´m using a +/- 18V 3A transformer with a full wave rectifier bridge (1N5408 diodes) and 10000uf smoothing capacitors as my power supply. I tested it with the TDA2050 PDF split supply application circuit through an 8ohm 200W speaker and it sounded great, no problems at all.


I decided to try the Lm1875 split supply circiut from the PDF. I put it together exactly like the schematic and tested it, it worked perfectly (excellent quality sound) for about 10 seconds and shut down.
I tried again and the same thing happened.

The problem is that my heatsink is quite large and gets barely warm so I can´t see this as temperature issue. I used arctic silver thermal paste for further testing and still the same problem. I had the same exact problem with the LM3886, after two circuit builds of that one, I gave up.

Any tips on what the problem might be besides the heatsinking.....   

Thanks, Daniel C

P.S I wonder if this has anything to do with the giant capacitors I used for the PSU. 2 x 10000uf. The amp takes a while to discharge the large caps and will only start again once they are discharged  (I use the on/off LED on the secondaries to montior this).  have you ever heard of this phenomenon?