9V guitar amp

[Frances Rhodes]

hey'all

i've been willing to build a low power guitar "bedroom" amp, battery powered.
i tried the power op-amps and was really disappointed with them, so i started looking for "simple" transistor designs, and was pretty unlucky so far.

i was thinking of trying to find circuits like the ones they must have used in old teisco guitars with built-in amps and speakers, but didn't find any information.
i tried to look for transistor push-pull designs, a little like jack orman's booster, and i was confused with them.
i can't find the information i need about the input impedance required. it seems like every low-power amp i find is designed for hi-fi purposes and i don't really know what changes to do to make them work for an electric guitar or bass.

would anyone know of a circuit ready to build that i could try? or where i could find information about how a push-pull design works so i could choose every component value to suit my needs?
i was thinking of a 3-transistor design, 1 for the "preamp" section, and a push-pull NPN-PNP pair for the output, and maybe 2 pots, 1 for gain or volume control, and 1 "master". but maybe the use of FETs would be a better idea, my knowledge is not really vast about this.

would it be possible to modify jack orman's design to hook-up up a small speaker at the output of the circuit?

any help would be precious!

regards
frances

[duck_arse]

francis - into 'a popular search engine, images', type "bd139/140 power amp". I don't know what transistors the kids use these days, but that's how we used to do battery amps way back. just add a high input impedance buffer/driver/gainer, and you should be all set. a mosfet boost or a spitfire-style or any number of runoffgroove circuits ......

[ElectricDruid]

I'd look at the TDA amplifier chips. I built a bass amp for a friend using the TDA2050. It was a mains-powered amp, so it was more powerful (TDA2050 can go up to 32W or so - my amp was maybe 25W), but there are other chips more suited for a battery amp - TDA2003, for example.
The schematics are simple, and in terms of "what do I need to do to make it work for guitar?" I'd say just put your favourite flavour of preamp and/or tone stack in front of it, and roll-off high frequencies more than you would for hi-fi.

HTH,
Tom

[Scruffie]

DOD 650.

[smallbearelec]

would anyone know of a circuit ready to build that i could try?

Velleman makes this kit:
http://smallbear-electronics.mybigcommerce.com/3w-mono-amplifier-k8066/

that would work for the output. For a preamp to get the line level drive from a guitar input, there's this other Velleman module:
http://smallbear-electronics.mybigcommerce.com/mono-preamp-k1803/

or you could build a Bare-Ass Boost:
http://diy.smallbearelec.com/HowTos/BreadboardBareAss/BreadboardBareAss.htm

[PRR]

Heavy theory-thinking---

> thinking of a 3-transistor design

I think it takes more than that.

Assume 20mV 100K input to 2V 8 Ohm output.

Voltage gain must be 100X.

Impedance change is over 10,000X.

Current gain has to be over 1,000,000X.

Check: 20mV is 0.020V. In 100K that is 0.2 micro-amps. 2V in 10 Ohms is 0.2 Amps. 0.2A/0.2uA is 1,000,000.

BTW: overall power gain 100,000,000. 80dB. Power gain of a single stage generally 20dB to 30dB. This leads to 3 or 4 stages.

A single transistor's current gain is 100 to 1,000, but only a fraction of this is available for signal amplification. With selected transistors (which may not include the 0.3A parts needed for the output) and somewhat wobbly biasing we may get current gain of 200 per stage. Two stages 200*200 or 40,000. We need a third stage with current gain >25.

JFET current gain is closer to infinity. However JFET voltage gain is generally low. It is possible you could find voltage gain of 50 or more at very low current. However in a 2-stage design the next stage probably wants quite high input current.

JFETs seem to offer much higher input impedance but very few JFETs have the ooomph to drive a speaker. MOSFETs do, but you will find that the Gate capacitance at the top of the audio band leads to a low-ish input impedance (nearer 10K than infinity).

[PRR]

> look at the TDA amplifier chips

+1. These are 4 or 5 stage amps (organized as 3 compound stages) which has long been the happy minimum for good performance. They are also favored in Car Sound because their clipping is not as abrupt and obnoxious as some other "better" audio chips.

Remember that 9VDC in a simple amp (no transformer, no bridge) is just One Watt. (9V/2.828= 3.18Vrms, 3.18V^2= 10.1Vrms, 10V in 8 Ohms is 1.26 Watts; but at 9V your "minor losses" will exceed 30%.)

> hi-fi purposes ... what changes to do to make them work for an electric guitar or bass.

Most power-amps will work best with some gain in front of the volume control. Their input sensitivity is a bit shy of what you want for guitar. Putting the volume between preamp and power amp optimizes dynamic range. For guitar tone, you often want some treble-boost. There are many-many guitar preamps to steal study.

SPEAKER is CRITICAL! Use guitar-tone speaker. Not Hi-Fi, not car-sound. Such designs "can" work but need additional tweaking to rock-out.

Small amps suggest large speakers. For better acoustic efficiency; and because you don't hear soft bass well and you need a big cone to support bass. One Watt into a Ten or a Twelve will solidly fill a bedroom. A Fifteen too boomy to use with your 18W may balance-out real nice at the 1 Watt level.

I suspect that any 1-Watt or few-Watt amp will be unsatisfying for Bass.

[deadastronaut]

1 watt  TDA7052A ""punch amp" is great on 9v psu..... (probably not too great on a batt.)

with a 12" speaker...(as paul said speaker is critical...)

does sound pretty good, and pedal friendly too..

[Elijah-Baley]

I built a "punch amp", I use it with a 9v boss psu with a 8" speaker.
It is nice with a statocaster, a bit crunchy with an Ibanez with humbuckers, but in both the cases a bit lacking of treble.

[Frances Rhodes]

thanks everyone for the answers, that gives me a very good start.

the idea was to make a smokey-amp-ish, housed in a small metal box, loaded with recycled TV speakers, so small speakers.
it's supposed to be a present for a close friend for christmas. if it doesn't sound very good, it's ok, as long as it "operates".

i still can add an external speaker plug to use a large speaker when it's possible, like the zvex nano head.

like i said, i prefer a transistor design over an op-amp circuit, so i'm gonna study the BD139/140 circuits i found and the DOD amp and try my best to make sopething good out of it!

now i just have to get to work!

[tubegeek]

I like this article a lot, it's a careful description and test of several small discrete-transistor audio amplifiers that outperform the ubiquitous LM386 amp-on-a-chip.

Homo Ludens: Small audio amplifiers

Also: I built a Smokey into a small wooden drawer from an IKEA organizer - just a little pre-made plywood box, saved me some work, it was smooth and cut accurately, easy to prime and paint.

Mine was a present for my nephew, and I used recycled TV speakers, too. Good luck! A great idea!

[Elijah-Baley]

[...]

the idea was to make a smokey-amp-ish, housed in a small metal box, loaded with recycled TV speakers, so small speakers.
i still can add an external speaker plug to use a large speaker when it's possible

[...]

It is a kind of my project. I'm working on my Smokey Amp, I think you know the Smokey.

I added an output jack (in my case there's no an internal speaker but you can do it), a gain control (search for this simple mod), a dc jack in place of battery snap and I added a 100uF power filter.
Now I'm waiting for a 25 Ohm pot to use as volume master (Little Gem style).

[Guitar Poppa]

Hey Frances

I have something for you !

My GeAmp1W has been designed one year ago. Its is germanium amplified, with level control, a special one knob tone control, 2 presets. The silicium preamp is high impedance and low noise. The overdrive [edit : is produced by] the germanium power amp, much more warm and creamy than silicium. All discreet components : not an IC pharmaceutical sound ! It works with any supply 7 to 12V, negative grounded.
The minimum output power is 0,5W with 7V and 8ohms, 1,3W RMS with 12V and 4 ohms.

Here are two pics  from my site :




You can see that product on my Website : http://guitarpoppa.com/?p=2801
The technical sheet is dowloadable on this page.


I also can give you the power amp's schematics, that are the most typical. You'll add by yourself the preamp and the tone control you want.

Waiting for your visit !
Guitar Poppa