Building a discrete multi-stage guitar amp

tracecom

Joined Apr 16, 2010
3,944
Please also specify which circuit you've made. Thanks in advance. :)
It's a kludge of ideas mostly found on the web. It uses an MPF102 front end and an LM386. It has a drive control, a tone control, a gain control (for the LM386), and a switched "overdrive" adder. I am going to proceed with ordering 10 PCB's, with which I can do some component substitutions and comparison testing before finalizing the schematic.
 

Audioguru

Joined Dec 20, 2007
11,248
Silly question, but I'm curious. Why is a JFET chosen over a MOSFET or BJT?
A Jfet with a source resistor biases itself.
A Mosfet (a little 2N7000?) needs complicated biasing.
A BJ Transistor also needs complicated biasing and is difficult or impossible to get a very high input impedance.
 

Thread Starter

blah2222

Joined May 3, 2010
582
A Jfet with a source resistor biases itself.
A Mosfet (a little 2N7000?) needs complicated biasing.
A BJ Transistor also needs complicated biasing and is difficult or impossible to get a very high input impedance.
Thanks, I'm assuming the self-biasing is due to the depletion-mode operation (have yet to study JFETs)...
 

THE_RB

Joined Feb 11, 2008
5,438
AudioGuru said:
... "Depending on your personal tastes, there may be a potential down side to the 386 designs. The input impedance is listed at 50k, according to the datasheet. This is quite low and causes some loading of the guitar signal. There is some degree of high end signal loss."
Well, 1; modern magnetic guitar pickups are too high in treble and generally you turn the treble down, and 2; it's a fact that pro-designed amps where done with a single LM386 and no JFET preamp. I spent years repairing amps.

With a good speaker choice and LM386 driven form a 9v battery can be "turn that damn thing down!" loud. Louder than an acoustic guitar.

As for your freq response into X impedance chart, what type of pickup was that? It looks very pessimistic.

Not everybody thinks a JFET is needed. Also keep in mind that almost all electric guitar players will use effect boxes before the amp, and they have quite a low output impedance and pretty good output voltage, again making the JFET superfluous.



Tracecom said:
...Do you happen to know what the output is from a typical piezo pickup?
Crappy! Any decent piezo pickups for acoustic guitar have a little preamp built in, usually in a box on the lead. Who plays those silly acoustic things these days anyway? ;)

As for building a class D portable amp, ebay is full of finished d-class amp modules for pennies like this one;


http://www.ebay.com.au/itm/Mini-Dig..._Audio_Amplifiers_Preamps&hash=item3f1bed981f

10W+10W from 12v for about $11.

You would spend more on a 12v rechargable battery and some type of charger! ;)
 

Audioguru

Joined Dec 20, 2007
11,248
The datasheet for the PAM8610 class-D stereo amplifier shows that its output power is 5W per channel at low distortion, 8W at medium distortion and 10W at high distortion when its supply is 12V.
 

tracecom

Joined Apr 16, 2010
3,944
Stunning example of buy-versus-make weighted very heavily to BUY.
Sad, but true. I was in an MBA program in 1992 and the buzz was about a coming global economy. I remember thinking at the time (and actually said it in class) that I didn't believe it would benefit the US. Now, I get the benefit of super cheap electronics and the US unemployment rate is over 15%. Hoodathunkit?
 
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wayneh

Joined Sep 9, 2010
17,496
Got my MBA in '89. If I only new then what I know now. Like, why didn't I buy 1000 shares of Apple instead of 100?
 

tracecom

Joined Apr 16, 2010
3,944
Got my MBA in '89. If I only new then what I know now. Like, why didn't I buy 1000 shares of Apple instead of 100?
I just read the latest biography of Jobs. He was very reluctant for a long time to trust outside manufacturers...even some right across town in Cupertino. Now, I suppose all Apple products are from China. I haven't bought an Apple product since the original Mac; between the initial purchase and "upgrades," I sunk over $4,000 in mine and it still didn't have a hard drive. I sold it in 1988 for $1500 and bought a PC. The Mac still was amazing for its time (especially without a hard drive.)
 

THE_RB

Joined Feb 11, 2008
5,438
The datasheet for the PAM8610 class-D stereo amplifier shows that its output power is 5W per channel at low distortion, 8W at medium distortion and 10W at high distortion when its supply is 12V.
Yeah I expected as much. But still a good 5w with low distortion is a huge step up in sound power to the previously discussed LM386 running from a 9v battery.

A real 5W into the right speaker will be seriously loud as a practice amp. It might also be possible to make a few mods on the PCB and bridge the two stereo amps to give 10-20W? I'm not sure if that particular D-class amp chip is bridgeable.
 

Audioguru

Joined Dec 20, 2007
11,248
It might also be possible to make a few mods on the PCB and bridge the two stereo amps to give 10-20W? I'm not sure if that particular D-class amp chip is bridgeable.
A bridged amplifier that has no saturation voltage loss with a 12V supply has an output of 9W at clipping into 8 ohms. But all amplifiers have some loss so the output is actually only 5W.

But a 12V lead-acid battery is usually 13.2V when charged, then the power of a bridged car radio amplifier IC is 14W at clipping into 4 ohms or about 8W into 8 ohms.

The class-D amplifier is already bridged and you cannot bridge it again.
 

Audioguru

Joined Dec 20, 2007
11,248
I'm going to try out the TDA2040. I'll let you know how that turns out at 12V.
Why bother trying it. Instead look at its datasheet.

It is designed to use a 32V supply, not just 12V.
With a 32V supply its output power into 4 ohms at clipping is 22W.
With a 32V supply its output power into 8 ohms at clipping is 12W.

But with a 12V supply its output power into 8 ohms at clipping is only 1.8W, a little more than a cheap clock radio.

EDIT: Look at the TDA2005. It is a bridged amplifier that with a 12V supply produces about 7.2W into 8 ohms at clipping.
 

tracecom

Joined Apr 16, 2010
3,944
EDIT: Look at the TDA2005. It is a bridged amplifier that with a 12V supply produces about 7.2W into 8 ohms at clipping.
I see from the datasheet that the minimum Vs is 8 V. What would be the power out into 8 Ω with an 8 V supply, and, if you don't mind, point out in the datasheet where you find that information. Thanks.
 

Audioguru

Joined Dec 20, 2007
11,248
The TDA2005 is made by an Italian company. Their English datasheet is missing a lot of information. Maybe the details are on the Italian datasheet but I have never seen it and I do not know Italian.

The English datasheet shows many graphs of output power at different supply voltages and with different load resistances but only when severely clipping at 10% distortion for each channel of a stereo amplifier. Almost no information is shown when it is bridged nor with an 8 ohm load.

When bridged and with an 8V supply (why 8V when a little 9V battery quickly drops to 6V?) its output power at clipping into 8 ohms will be almost nothing.
Also it is overdriven too much (to increase the output power) but then the clipping distortion is horrible at 10%.
 
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