My complete guitar amp schematic - Comments?

[Wynand]

Hi.

I've been playing with this circuit for a week or two and want to build it.

At the moment I've got the circuit running without the buffer after the tone controls.

It's an inverting stage wih 10 x gain followed by a Marshall tone control circuit. I got the tone control schematic of a tone stack calculator program.

The LM3876 will probably get changed for a LM3886 since the speaker I'm getting is 50W and I want the amp just a little stronger.

I'm using a LM833 for the op-amp, but will change it for a TL072(?) if recommended.

Does it look ok or am I missing something?

Any ideas or improvements is welcomed(wanted).
I'd like to finish the schematic before playing with the PCB design.

I'm thinking of a bypass switch for the tone controls and diodes for clipping, also with a bypass switch.

[Wynand]

Sorry for the zip file but my image could not be posted

[teemuk]

Here´s few suggestion:
At the input, connect diodes from the signal path to both Vee and Vcc so that any peak exceeding either Vee or Vcc plus the diode´s forward voltage will clip. It´s a cheap protection and could/should be used at the output stage too - there it would protect your power chip from the speaker´s inductive behaviour. The gain of 10 at the 1st stage of preamp also seems rather high and you might experience clipping when you strum the guitar hard. A simple solution: reduce the gain and convert the buffer to a gain stage to make up the gain. The 1uF cap at the input seems overkill: You could get away with few hundred nF and avoid the use of an electrolytic. Wouldn´t the gain stages need some high frequency NFB for stabilization? (At least be prepared for it and leave a place for NFB caps to the PCB). Maybe a capacitor-resistor Zobel to the output as well? I would AC couple the output. I know it´s extra cost, reduces bandwidth and adds some distortion - and on top of that is unneccessary in dual supply amps - but since it´s a guitar amp you don´t need the bandwidth to extend to the very lowest frequencies and that extra protection against DC at the output is worth considering. (There will be DC from i.e. square wave distortion signal from some guitar pedals or a clipping opamp stage). That cap would smooth out harsher clipping as well.

[runebivrin]

Well, big thing is the input impedance; it's only 1 kOhm, which is way too low for a regular guitar pickup, and rather low for most other sources.

Change U1:A to non-inverting config.

There's no indication of how the transformer is connected to GND, but I suppose there's a centre-tap somewhere.

Changing the chip to 3886 isn't going to affect your output power one iota unless you raise the supply voltage or lower the speaker impedance.

As an aside: The schematic looks good, but it's better posted as a GIF/PNG. Smaller, and with a defined resolution.

Rune

[teemuk]

Originally posted by runebivrin
Changing the chip to 3886 isn't going to affect your output power one iota unless you raise the supply voltage or lower the speaker impedance.

Theoretically LM3886 has more output power than LM3876. Even though gain and load impedance remains the same the amp should be able to push more power with a higher input signal. Assuming the rail voltage is sufficient, of course. He doesn´t state the amount of higher rail voltages in the schematic though.

A side note 2: Reducing the amount of colours from 16 million to 2, would recude the image size as well. Works fine with B&W schematics.

Edit: I just went through datasheets and noticed that LM3886 has less power to 8 ohms than LM3876.

[Wynand]

I'm just changing the schema, thats why I'm taking a while. Be back now.....

[Wynand]

Here's what I've done sofar.

Diodes are at the input. Are they correct. I dont know where to put the other diodes.

Inverting to non-inverting op-amp.

Gain spil between the two op-amps. 5 x 2 gain.

C18 and R19 (C19 and R20) are zobel networks if I'm correct.
Should I put it in at both op-amp outputs? and/or main amps output?

Wouldn´t the gain stages need some high frequency NFB for stabilization?

How do I do that?

The problem with the schematics is that I have to convert from TIFF first, but this one is in GIF format. Does it look better?

I used the Overture Design guide and saw an increase in power with the same Load and Supply Voltages... I'll double check quickly.

I hope I haven't made many mistakes because I'm rushing.

Thanks

[Wynand]

Checked again

LM3876, 8R load, 30V supplies = 42.15
LM3886, 8R load, 30V supplies = 44.06

Not much higher, but when I put a bigger transformer in with higher voltages, It can do more.

Besides, I can get the 3886 for cheaper

[Wynand]

PS The tone controls have been moved to a seperate pcb, for the faceplate, and the three pads in their place would run between them (IN, OUT, GND)

[teemuk]

The output diodes are hooked up similarly but to the output node (of course) and to V+ and V- rails that supply the power amp. A zobel is needed only at the output node. I'm not sure about the values though, if you want to do it right use:

Capacitor = Le/Rc^2
and Resistor (Rc) = 1.25 * Re,

where Re = Voice coild DC resistance and Le = voice coil inductance in Henries.

In most guitar amps the designer has just sticked in a 10R and 100nF zobel and it seems to be OK. If you want high frequency NFB it´s easier just to parallel a small value cap across the feedback resistor.

Edit: BTW, your preamp total gain ignoring the tone stack losses is now 18 (6*3) instead of 10. Is this intentional?