Dead Bug: SS Guitar Amplifier

[ve3wwg]

While I was off sick, I had some time to think about and complete the tone section of this amplifier (design wise). You can read about it here:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=fet_tone_ss_amp&#tone_stack

In there are some response curves to the changes in treble, midrange and bass controls. I went with the tried and true Marshall Amp tone section for this.

Now I just got to get off my butt and order some parts to finish the preamp section of this project.

[expanoncolin]

Very cool to see the frequency response of the tone stack.

-Colin

[ve3wwg]

It took me a while to figure out how to do that in LTspice. Especially with the potentiometers.

But it essentially involved:

• parameterizing the pots, particularly the one with the wiper connection.
• inserting a .STEP directive
• Using the .AC analysis

Then the only trick required was to turn off the phase plot that comes out by default on the .AC response. You can click on the right axis to change the plotting parameters there, including hiding the phase plot. Whoo hoo!

[ve3wwg]

I've posted some cabinet pictures here:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=fet_tone_ss_amp&#cabinet_power

It is just a crude construction grade cabinet, which is good enough for prototype work like this project.  I got the power, fuse, switch and transformers installed and passed the smoke test.

Now I will be building the rectifier and power filter sections. The +/-12 Volt side will need voltage regulators for the preamp board.

[ve3wwg]

I think I have the design of the power supply finalized now:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=fet_tone_ss_amp&#power_supply_schematic

I usually just wing power supply design but this project requires more care, since I am aiming for >50 Watt amplifier.  A lot of details to plan on this one, some of which includes:

• Voltage planning (for caps and diode ratings)
• Power ratings on the bleed resistors (R1 and R4)
• Filter capacitor sizes (to meet ripple requirements)
• Voltage drop diodes to reduce power dissipation in +/-12V regulators
• Snubber capacitors for the rectifier diodes (to reduce RF interference)

That reminds me-- I need to also check into surge currents and power ratings.

The rectifier diodes can apparently provide some RF as they threshold into and out of conduction. This seems to be recommended on any quiet amplifier design. I have to say I really enjoyed reading the book "Audio Amplifier Design Handbook" by Douglas Self" this week (another installment in my used book collection).