Building a guitar amplifier

Thread Starter

drk

Joined Mar 8, 2008
41
Hey, so I was going to start building the part with the LM4700 and got an existential doubt (lol).

What are the ratings of the resistors/capacitors in the circuit? I've looked through all the datasheet and can't find any reference to it.
Is it the 'normal' 1/4 watt resistors?
 

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Thread Starter

drk

Joined Mar 8, 2008
41
The box I had before was really small, so I end up buying a new one. I got a 275x175x65 mm.

Also got the heatsinks, I'll use two external at the back, should be ok.

I've attached an image with the latest version. I've drawn there the grounding, I'm thinking about using a star ground scheme. Does it look good?

Also, about the ratings for the LM4700 components, the tensions from the supply are about 23v, so capacitors with a voltage rating of 50V or 63v are ok.

The resistors from the input shouldn't have much current so a 1/4 watt is ok too.

My doubt is about the ones in the output (that form the filter)?...
 

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Thread Starter

drk

Joined Mar 8, 2008
41
The 12V/-12V regulators are for the pre-amplifier circuit (sorry, wasn't clear about that), the LM4700 would be supplied directly (where the LOAD resistor is).
 

THE_RB

Joined Feb 11, 2008
5,438
It's typical to have; gain, bass, mid, high, vol on the pots going from left to right. Bass is always on the left.

Re the star grounding it should be case grounding at the star point as the high gain input socket (from guitar) will be case grounded. I would connect the mains ground through a resistor to the star ground, maybe 100 ohms 1/2W.
 

Thread Starter

drk

Joined Mar 8, 2008
41

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THE_RB

Joined Feb 11, 2008
5,438
It's very unusual to see the bass knob on the right, is it some type of cheap Chinese practice amp? Anyway it's your baby so make it the way you like, only if you don't really care whcih way it is I would suggest bass mid treb from left to right.

On older amps with a metal chassis and input ground is also chassis ground, with the star ground in on the chassis itself.

If you are using an insulated ground input socket, you can couple the socket ground to preamp PCB ground, then preamp PCB ground goes to the star ground which is generally at the PSU.
 

Thread Starter

drk

Joined Mar 8, 2008
41
I'm having a problem with the L7912 regulator. I'm getting -19V at the output (with -27v at the input), but it should have -12V! I've tried with a different regulator but same thing.

I'm following the schematic as in the datasheet (split power supply, page 8). The L7812 works ok, with approximately 12v at the output.

Don't really know what is the problem :/
 

Thread Starter

drk

Joined Mar 8, 2008
41
Looking from front, from left to right:
1- ground
2- input (the -27V in my case)
3- output (the -12V)

I tried a few things.
- First raising the capacitors value, since there's a note on the datasheet that the value should be bigger if using aluminium electrolitics capacitors (L7900 series datasheet, page 8).
Didn't make much difference.

- I tried adding a resistor on the output (to work as the load), and noticed that the tension does lower with it, and with a value of around 3/4k ohm it stabilizes at the -12V. Maybe its because I was testing without the load?
Although the L7812 has 12V without any resistor...
 

Thread Starter

drk

Joined Mar 8, 2008
41
I've been working on the preamp aswell.

I've attached the circuit that I've got so far. I'm not sure about the 'line out' part, where exactly I should connect it...

Still got to test it on the breadboard though :)
 

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Audioguru

Joined Dec 20, 2007
11,249
Why is R1 only 10k ohms instead of 1M ohms?
What is U3 for? U1 can be the input and have R1 as 1M and C1 with a value much less than 1uF.
What is U13 for? The output of U2 is Line Out if it has an output coupling capacitor.
Why do you have the lowpass filter of R15 and C16? They chop all high audio frequencies that everybody normally hears except old and deaf people.
 

Thread Starter

drk

Joined Mar 8, 2008
41
Since the output resistance of the op-amp will be really small, I thought I didn't need a resistor as big as 1M.

U3 is there because the guitar signal is very weak, but I'm not sure if its needed.

Ok, I wasn't sure about U13 as well, I'll just have a capacitor there.

Yea, that's wrong :(


I've put the high-pass filters with a cut-off frequency around 15 Hz, and the low-pass filter around 19kHz.

I'm not really sure how to calculate the filters in the op-amp feedback circuit, is it the same way (f = 1 / (2 * pi * resistance * capacitance))?
 

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MrChips

Joined Oct 2, 2009
23,559
Yes, I was going to suggest adding some gain in U3 but now that you have deleted it that's ok.

Why not add some gain to U2 as well?
 

Thread Starter

drk

Joined Mar 8, 2008
41
thanks for the help!

I'll try this out in the breadboard, and add more gain to U2, and see how it sounds.
 

THE_RB

Joined Feb 11, 2008
5,438
Your distortion circuit looks like it will be a bit harsh. I would allow for a resistor (or log pot) in series with the back to back diodes to make the clipping less harsh.
 

Thread Starter

drk

Joined Mar 8, 2008
41
Your distortion circuit looks like it will be a bit harsh. I would allow for a resistor (or log pot) in series with the back to back diodes to make the clipping less harsh.
I changed a bit the distortion part... does it still apply?


I'm having a problem with switching of the distortion circuit. When I switch from clean to dist, I hear a loud 'pop'. I thought maybe its because of capacitor C16 (at the output of the dist circuit), and tried adding a 100k resistor there but didn't really work.
 

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