9v Guitar Amp w/out LM386

[Don Perry]

Hello all,
Last week my employer commissioned me to build a small guitar amp that would fit inside a tiny cigar box. The caveat is that I'm not allowed to use an LM386. I'm suddenly finding myself way over my head. I've been crawling these forums like crazy and I've found a lot of good information, but I'm not sure that I've seen anyone actually pull it off.

My target is to run a small speaker, I'm gathering that 64Ω is the norm but I've seen those Smokey amps drive 4/8Ω speakers. How is this possible?

Should I be looking at push-pull circuits? Class A or Class AB? I'm just not even sure where to start. I've some, but very limited, experience with analog circuits.

Any Ideas?

 

[davenn]

Last week my employer commissioned me to build a small guitar amp that would fit inside a tiny cigar box. The caveat is that I'm not allowed to use an LM386......

so, what are you saying ? .... that it must be discrete transistors ?
Why you your "employer " ask for such a crazy limitation ?

 

[Don Perry]

Aye, It must be transistor driven.

 

[davenn]

Aye, It must be transistor driven.

OK, weird , but whatever

have you googled for circuits for low power transistor audio amplifier circuits ?

there's a zillion out there

Dave

 

[Don Perry]

Yeah I've been all over the internet. I've got the preamp figured out but I'm not sure what comes next. I'm using a common emitter for the pre part but... Should I use push/pull for the second stage? Is it possible to create a circuit that can drive an 8Ω speaker similar to a Smokey w/out having to use a 386 chip?

EDIT: I think part of the problem is that there really are a million different designs to choose from, which is best for this kind of application? My head is just swimming from all the possibilities.

Thanks again

 

[davenn]

EDIT: I think part of the problem is that there really are a million different designs to choose from, which is best for this kind of application?

it's just a low power amp a couple of watts or less ... design isn't all that critical at that power level

did you google the text I bolded ? probably not, there were plenty of ideas there

http://www.bing.com/images/search?q=low+power+transistor+audio+amplifier+circuits&FORM=HDRSC2

just one of the examples shown ......

Dave

 

[Don Perry]

I've seen this circuit a hundred times. It's a push pull, yes?

 

[davenn]

It's a push pull, yes?

yes

 

[Don Perry]

OK thanks! Just looking for a push in the right direction I guess. I'll report back with the results when I finish building!

 

[Audioguru]

Most speakers are 8 ohms.
The most important spec for a power amplifier is, um, maybe its output power?? The amount of output power at clipping tells you the power supply voltage that is needed.

You mentioned an LM386 that produces 0.45W at clipping into an 8 ohm speaker when the battery is brand new 9.0V. Then the peak current is 5.4V/8 ohms= 675mA. Little transistors have a maximum allowed current too low so a little power transistor like TIP31 and TIP32 must be used at the output. Their minimum current gain at 675mA is about 35 then their peak base current is 675mA/35= 19.3mA for the driver transistor to produce.

Here is a very simple 0.45W power amplifier. Use Ohm's Law to calculate its resistor values. Look up "bootstrapping" in Google.

 

[davenn]

Here is a very simple 0.45W power amplifier. Use Ohm's Law to calculate its resistor values. Look up "bootstrapping" in Google.

that's the same circuit I posted up the page, just a variation in transistor types

 

[Don Perry]

YEAH!

 

[Audioguru]

A simple circuit is the same when it is copied. Its sound and performance are awful. But some people like a guitar amp to sound awful and have terrible performance.

I was wrong about the peak voltage. The peak current is half of 675mA which is 338mA and the peak driver transistor current is only 7.5mA.

 

[Don Perry]

Funny thing about that. We actually do kind of want a really raw, basic sound.

 

[Don Perry]

In my feeble mind, those two diodes will only cause more resistance, and reduce amplification, I would lose them. Then I would take that tripple threat deal back to the egg..I would loop it back through the system 6 times before reaching cascade amperage, then dump the new amplified current straight out the output without a cap to stumble over.

This is way over my head

 

[AnalogKid]

In my feeble mind, those two diodes will only cause more resistance, and reduce amplification,

0 for 2. First, they do *not* "cause more resistance". Second, if they did, that resistance would be in the voltage amplifier collector and *increase* the "amplification". Do you know why the diodes are in the circuit?

I would take that tripple threat deal back to the egg..I would loop it back through the system 6 times before reaching cascade amperage, then dump the new amplified current straight out the output without a cap to stumble over.

What dressing goes best with word salad?

ak

 

[AnalogKid]

This is way over my head

No, it isn't. *Your* head is fine.

ak

 

[AnalogKid]

Are all integrated circuits banned, or only the LM386? There is a variation of an opamp output current booster that is more efficient than the 386 or the circuits posted.

ak

 

[73's de Edd]

Confucius . . . . . he say . . . . .

" Pre-biasing with Diodes "
http://diodetech.blogspot.com/2013/07/crossover-distortion.html

 

[Don Perry]

Are all integrated circuits banned, or only the LM386? There is a variation of an opamp output current booster that is more efficient than the 386 or the circuits posted.

ak

aye, no ICs unfortunately.