Building a discrete multi-stage guitar amp

Thread Starter

blah2222

Joined May 3, 2010
582
***UPDATE***

Three great articles on discrete JFET preamp design:

- J201 preamp by Don Tillman
- MPF102 preamp by James Hawes (adaption of Tillman preamp)
- MPF102 preamp by Mike Martell


Hey, I am getting back into guitar and have wanted to put some of my EE design juices together and would like to build a guitar amp out of a couple 2n3904s.

I was thinking maybe two gain stages and then a buffer to match the speaker impedance.

The datasheet claims that they can source a max collector current of 200mA which I think would be well beyond what I need to power a small 4ohm 1 W speaker.

I'll just have to play around with filters to get rid of nasty harmonics and what-not.

I'm not expecting Marshall-worthy sounds, but just a decent project to work on. Sound realistic?
 
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wayneh

Joined Sep 9, 2010
16,390
I don't know about you, but something I've always wanted was a small practice amp - battery power capable - into which I could mix an iPod and my guitar, and output to headphones or a small speaker. That way, I could listen to and work on learning tunes without bothering anybody nearby OR take off the headphones and just have a nice little amp. Neat, self contained, rugged.

Oh, and cheap! ;)
 

Thread Starter

blah2222

Joined May 3, 2010
582
I don't know about you, but something I've always wanted was a small practice amp - battery power capable - into which I could mix an iPod and my guitar, and output to headphones or a small speaker. That way, I could listen to and work on learning tunes without bothering anybody nearby OR take off the headphones and just have a nice little amp. Neat, self contained, rugged.

Oh, and cheap! ;)
Like the AmPlug by Vox?
 

MrChips

Joined Oct 2, 2009
21,119
You will be very disappointed with a couple of 2N3904s.

I have already built a guitar amp, but not with a small 1W speaker.

I used a couple of TL071, a bridged car audio amp, driving a 450W 12" speaker, powered from a 12V SLAB.

I can use this to play guitar and bass anywhere I go, practice at home, in the garden, on the beach, around the campfire with enough audio power to play on a gig. Unplugged.

 

Audioguru

Joined Dec 20, 2007
11,249
Didn't you do the simple calculation for the peak current?
1W into 4 ohms is (the root of 1 x 4, x 1.414=) 2.83V peak. Then the peak current is (2.83V/4 ohms=) 708mA which will blow up the tiny 2N3904 transistors.

Amplifiers usually use a class-AB output stage that has a pair of complementary power transistors as emitter-followers, driven by a voltage-amplifying stage.

Your design has a class-A output transistor as a heater instead of as an amplifier.

A little LM386 amplifier with a 9V supply has an output of only 0.4W at clipping into 4 ohms and the IC gets very hot.
 

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tracecom

Joined Apr 16, 2010
3,944
I built the attached amp to play my MP3 player through a speaker, and I use it regularly into a 4Ω speaker. The audio quality is good enough for my purpose if I limit the drive to keep the output at 3.6 Vpp or less. The volume is loud enough for my somewhat diminished hearing. The IC is not even warm to the touch.

There are certainly better amplifier IC's than the LM386, but it works for some purposes. There are lots of videos on youtube of people using a guitar amp called a "ruby amp" that uses a MPF102 and an LM386.
 

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Audioguru

Joined Dec 20, 2007
11,249
An LM386 amplifier with a 9V regulated supply and a 4 ohm speaker has an output (with horrible distortion) of 0.4W and it heats with 0.9W if it has lots of sustain for almost continuous full output power.

An LM386 works best with an 8 ohm speaker. Its output with a 9V regulated supply is 0.56W when clipping a little and is 0.45W with low distortion. It heats with 0.53W if it has lots of sustain for almost continuous full output power.

A little 9V "transistor" battery will croak and die quickly when powering the amp driving a 4 ohm speaker when the peak current is 450mA.
Which gets hotter, the little amp or the little battery?

I don't have a graph of a little 9V battery being shorted with 450mA but here is 400mA:
 

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tracecom

Joined Apr 16, 2010
3,944
An LM386 amplifier with a 9V regulated supply and a 4 ohm speaker has an output (with horrible distortion) of 0.4W and it heats with 0.9W if it has lots of sustain for almost continuous full output power.

An LM386 works best with an 8 ohm speaker. Its output with a 9V regulated supply is 0.56W when clipping a little and is 0.45W with low distortion. It heats with 0.53W if it has lots of sustain for almost continuous full output power.

A little 9V "transistor" battery will croak and die quickly when powering the amp driving a 4 ohm speaker when the peak current is 450mA.
Which gets hotter, the little amp or the little battery?

I don't have a graph of a little 9V battery being shorted with 450mA but here is 400mA:
I respect your opinion and don't have the expertise (or the inclination) to argue with your data. I am simply stating my real world experience after having built the amplifier and used it on a regular basis for the last several months. Neither the amplifier nor the battery has ever gotten hot, and I have used only two batteries during the entire time period; the second one is still in use.
 

MrChips

Joined Oct 2, 2009
21,119
A JFET and an LM386 running off a 9V battery is worth trying.
I would go with an 8Ω speaker instead of 4Ω.
I'm going to breadboard this and report back.
 

tracecom

Joined Apr 16, 2010
3,944
A JFET and an LM386 running off a 9V battery is worth trying.
I would go with an 8Ω speaker instead of 4Ω.
I'm going to breadboard this and report back.
Please do. I have (solderless) breadboarded it and liked what I got so much that I have laid out a PCB, but not yet ordered. I am very interested in your design and results.
 

wayneh

Joined Sep 9, 2010
16,390
A JFET and an LM386 running off a 9V battery is worth trying.
I would go with an 8Ω speaker instead of 4Ω.
I'm going to breadboard this and report back.
I just bought a TLV272IP (dual rail-to-rail op-amp) for my parts pile. Could that be used in place of the JFET stage?

A lot of those cheap "PC" speakers use a LM386. I have a bunch laying around and I've often thought of mounting a battery holder and pre-amp inside one. Instant practice amp. I could even leave the existing transformer in place for AC operation.
 

MrChips

Joined Oct 2, 2009
21,119
I have something similar so far. I will draw up the circuit after I get a chance to test it with a guitar.
I am using a MPF102 in common-source configuration.
(560Ω on source bypassed with 10μF. 1k8Ω on drain.)
20mV input gives 2V into 10Ω load (have not tried speaker as yet).
 
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