preamp to poweramp question

[bamera]

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

[J M Fahey]

Hi Daniel.
You needn't burn your brain with very exact calculations, for the very good reason that the "standard" guitar player, with a "standard" guitar, "standard " pickups ..... strings ... picks .... song .... etc. ... hasn't been defined yet. (Undependable b*st*rds these crazy guitar guys, eh?)
So there's an empirical "unwritten rule", which comes at least from the early Fender amplifiers and "states": design your preamp "as if" the guitar puts 50 to 100 mV signal, and have enough gain so the volume on 5 , 6, or 7 starts to clip the power amp.
In your case that means designing the preamp with a 10x to 20x gain and use a Logarithmic/A/Audio taper Volume pot.
That's why the clean (or only) channels of 95% of the World's guitar amps, start crunching between 5 and 7, with 6 being a most popular choice.
And why is that?: so that that mythical standard guitarist can reach full power on 6, with some gain to spare, 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.
On a regular Log pot, "10" means 10x or 20dB higher level than "5", an impressive reserve of gain on tap, with a very simple circuit.
The famous exception are Marshall Plexis , which saturate on 2.

[bamera]

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

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.

[Minion]

I"m running about 4v right into my guitar amp(as measured with my dmm which strumming My onboard preamp has 20x gain and pickups put out over 200mv) I then have a Tube pre in my amp that puts out over 20v with the gain maxed into the eq section and then into the TDA7293 Power amp and I haven"t had any problem with clipping or anything and no tripping the speaker protection relay (has a threshold of 2v dc at the speaker terminals) ....

I dont have a clue why that is , I haven"t had the amp up to 10 (plenty loud enough to jam on 5) so it might clip at a higher volume ...... I have run my hefty guitar signal into the TDA7293 but it was barely audible so I don"t know what is up with Chipamp input voltage sencitivity specifications but for me I have allways had to put a much higher voltage into them , maybe it has to do with RMS voltage or something ....

[J M Fahey]

Hi Minion

I"m running about 4v right into my guitar amp(as measured with my dmm which strumming

Good. You are clipping some of that on strong chords, because a 9V powered preamp can't put out more than 3 V RMS (clean).

My onboard preamp has 20x gain and pickups put out over 200mv)

Did you measure that or it's a calculated value?

I then have a Tube pre in my amp that puts out over 20v with the gain maxed into the eq section

So the tube gain is 5x. (20V/4V)

I haven"t had the amp up to 10 (plenty loud enough to jam on 5) so it might clip at a higher volume

Definitely. Just play a couple chords on 10 and post what happens.

I have run my hefty guitar signal into the TDA7293 but it was barely audible so I don"t know what is up with Chipamp input voltage sencitivity specifications

A regular guitar , amplified 20x , and, obviously, with volumes on 10, *will* be enough to push a regular chipamp into the beginning of crunch.It will be very loud too. If it does not, either the pickups are not exactly hot, or the preamp has less gain than expected, or volume controls are eating signal. An audio taper pot on "5" provides 1/10th the input voltage, which causes the power amp to supply 1/100th of its power (-20dB).