The audio part - proto 1

Proto 1 is up and running. I can control the equalizers by I2c, the channel switching is functional, the optional voltage drop (as in marshall amps) is functional. I had to change the last gain stage to get some distorion. However I have some noise issues. The audio equalizers only accept 1~2Vmrs max, I have to rethink the gain/noise plan of the tubes and tone control.... to be continued. Here is a picture of the setup as it is now.

My design - audio part

I started my first prototype PCB. The schematic & layout is drawn in Orcad. I based the design on the JCM2000 triple lead because of the high gain and also on the fender hotrod deville amp. In total three 12AX7 tubes. Some of the features I included:

1) Three I2C controlled tone control ICs, to have maximum flexibility.
2) First output after 1 tube stage for very clean output, could also be nice for acoustic guitars.
3) Return before the last tube stage
4) Tube output buffer with 12AX7
5) One high level output and one low level output
6) double layer PCB
7) leaded components top side, smd bottom side

This is only the audio part, the microcontroller with midi input and front panel stuff has to wait because first the audio part has to be perfect.


Yesterday I received the empty PCBs, I put on the tube and the jack sockets. The next month I have to do some building / debugging and tone seeking...

In the pipe line:

(1) Control: For the microcontroller and the front panel I have some idea's. Using the LPC2468 arm processor (olimex development board Olimex LPC-E2468) running Linux (yes I know it is overkill) in combination with a display on the front panel (Matrix Orbital GLK19264-WB). I hope that the display does not cause any noise. Probably I need some help with the i2c drivers.

(2) Power supply: The SMPS must have a +400V, +19V, -19V, +5V output. Most probably it will be a flyback with PFC stage.

First prototype

Just bought a second hand Peavey TG Raxx preamp on the internet. For this first test I connected the TG Raxx with an I2C controlled tone IC TDA7419 instead of the front controls. Optocoupler devices I don;t like because of accuracy and I don't like relays, they might fail when they are old.

So after defining the I2C map of the TDA4719 in my control software, I control the tone IC with my laptop. I had to replace the first tube with a 12AU7 type because the tone IC was clipping. But the rest of the circuit is rather straight forward. I could power the tone IC with the TG raxx itself.

Everthing under controll and no strange noise. By the way, the TG Raxx has a nice feature, DC current through the filaments. I also want that in my amp!

This setup sounds very clean, the TG raxx is from its own a clean preamp. Wit hthe 12AU7 it is even more clean. Sounds good with my Gretsch 6120SSU-GR Setzer Nashville guitar.

Background

Hi, on this site you find information about my "midi controlled guitar tube pre amp" project.

Background

I am not satisfied with the guitar amplifiers available. I own a ENGL E580, it is good amp but its clean just canot make me happy, the high gain channels are pretty good. My Fender hotrod deville sounds clean very nice, lots of body. Could this be combined into one amp? I think YES, it just has to...

Here is a list of improvements I want to make with my own tube preamp.

1) A very clean channel for rock and roll and some very high gain channels to play heavy metal.

2) Everything under midi control; no more hassle when playin in two or more bands. Or when the knobs have changes during transport

3) No more hum; I want to use a SMPS supply instead of the 50/60Hz transformer with its annoying magnetic field. The SMPS can be global mains and it can be shielded which is not possible with a transformer.

4) No noisy fan in the amp; just cooling by natural convection.