Car amp to guitar amp conversion

earthtonesaudio · February 10, 2008, 04:22:29 PM

Hi,
I have an older model of this entry-level car amplifier:
http://www.garinet.com/cgi-bin/gkwebstore_shopping.cgi?ACTION=VIEW_PRODUCT&DEPARTMENT_ID=49&CATEGORY_ID=297&PRODUCT_ID=1006&SOURCE=new_release_within_products&LANGUAGE=e

And I want to use it for a guitar power amp. Why? Well, it's small and sturdy, kinda good looking, and puts out a decent amount of power pretty efficiently (220W bridged mono into 2Ω).

Just for kicks, I hooked it up with a 12V wall wart. It sounded okay at low volume (I didn't want to fry the wall wart, so I didn't crank it).

What do I need (other than a good preamp) to make this gig-worthy? I'd like to stick with using a wall wart (so I don't fry myself with line voltages), so what are my options?

Ronsonic · February 10, 2008, 07:56:50 PM

That wall wart is going to be the size of a toaster oven, but you can do it.

Or just get a charger and a car battery.

I wouldn't bother. 220 watts of car audio crap is not better than the 60 watts you can get from an LM3886 and parts you can steal from a dead stereo.

runmikeyrun · February 10, 2008, 07:59:41 PM

yeah you're going to need a big PS to keep that thing running, and it's going to get hot too. It'd be good as a slave though as it's already set to accept line inputs.

Minion · February 10, 2008, 08:02:26 PM

I can"t see why you can"t do it....You would need a good preamp but a wall mart just won"t cut it , the amp at full output (if it really is 220w) is going to suck maybe 10 amps of current maybe more so I would just get a great big 12v AC power transformer and a 20a bridge rectifier and a 10,000uF capacitor and then you have yourself a Power supply that might actually power it at full output.....

Cheers

earthtonesaudio · February 10, 2008, 10:40:42 PM

Thanks for the replies. The funny thing is, I think I can get my hands on a toaster-size transformer like you're talking about. Might have to purchase the 20A bridge rectifier and the big cap, but that shouldn't be more than about $12.

So, ignoring cost and complexity for a minute, it seems you're saying the best thing to do is just make a super high-current wall wart. Anything I should watch out for, like dangerous currents, etc?

Less like a wall wart, more like a floor goiter, right?

p.s. I totally understand the whole "why bother" factor... this is just a case of "I have these parts, and must make something with them" syndrome.

Jered · February 10, 2008, 11:17:39 PM

Something to keep in mind. Most household circuits are 15 amps. If that thing draws 10 amps, then your going to be re-setting your circuit breaker a lot. Unless you happen to have a dedicated circuit for that thing. And have you considered how heavy that thing is going to be? Toaster size transformer? Kinda heavy chunk of iron.
Jered

MarcoMike · February 11, 2008, 03:15:42 AM

on my pedalboard I used to have a switching wall wart: 14V 1.5A, not more than 100g!! the problem was certain distortions were a little noisy with it as power supply, even if regulated with a 7809. now it has benn converter to battery-charger.

the power amp "may" not be affected by this kind of noise... I don't know if 10A switching PS are available, but it can be an option! I just remembered that a bass amp manufacturer her in italy makes incredibly compact amps using switching PS. here is a link to their site, even if their name sounds similar to mine, I don't know anything more about their circuitry: http://www.markbass.it/

back on topic, usually an amp head's weight is in the range 10-20Kg.... if you build a good chassis you will probably end up with a lower weight than some commercial amps even using the toaster transformer!

dschwartz · February 11, 2008, 07:22:43 AM

Quote from: Jered on February 10, 2008, 11:17:39 PM
Something to keep in mind. Most household circuits are 15 amps. If that thing draws 10 amps, then your going to be re-setting your circuit breaker a lot. Unless you happen to have a dedicated circuit for that thing. And have you considered how heavy that thing is going to be? Toaster size transformer? Kinda heavy chunk of iron.
Jered

I really dont think that he´s going tu use 230W at full blast at home to suck 10A from the wall, if so, i would be more worried about going deaf than my circuit breaker jumping...

think about PSU´s..they have plenty of Amps at 12V, relatively clean, light and cheap!!!!

Minion · February 11, 2008, 10:55:12 AM

You can actually get pretty High amps in a pretty small transformer at 12v ,I have one at that is a regular EI transformer about 4 in square that puts out 12amps....I don"t know if a SMPS would be a good idea as the car amp allready has a SMPS inside it to convert the 12v DC to higher Voltages so I don"t know how much more switching noise you would get when haveing 2 SMPS units in series.....

Mark Hammer · February 11, 2008, 11:06:01 AM

You'd be surprised at the number of commercial guitar amplifiers (some of them even well-liked) based around power-amp chips that were originally intended for automotive use.

earthtonesaudio · February 11, 2008, 05:57:43 PM

Well the highest power wall warts I've seen from a brief search were limited to 12V, 1.5A DC. If I remember the equation correctly, P=I*V (or is it I²?), so the max power handling would be about 18 Watts.

...Which is plenty for me. Just because this thing is rated to produce 220W, doesn't mean I feel the compulsion to use all of them. I just need to hear myself over a drummer.

So here's what I'm thinking: I do a test run to determine what setting of the gain control causes the amp to consume 18W of power, with the theoretical preamp maxed out. Then I just glue the gain pot in place, and cover it up so no one can bump it and fry the power supply.

I guess this means I have to build the preamp first. Ah well.

Minion · February 11, 2008, 06:36:47 PM

Because the Adapter puts out 18w doesn"t mean the amp puts out 18w when useing that adapter.....depending on the Class of the amp it can have an effeciency of about 40% in class B or about 10% in class A or about 80% in class D, Pluss a 220W amp might suck up several watts in idle current just to run and that is without amplifying anything...You could do it much smaller and cheaper with a simple 9v adapter and a LM386 IC and make a Ruby type of amp.....Just a suggestion....

earthtonesaudio · February 11, 2008, 07:11:02 PM

Quote from: Minion on February 11, 2008, 06:36:47 PM
Because the Adapter puts out 18w doesn"t mean the amp puts out 18w when useing that adapter.....depending on the Class of the amp it can have an effeciency of about 40% in class B or about 10% in class A or about 80% in class D, Pluss a 220W amp might suck up several watts in idle current just to run and that is without amplifying anything...

Your are 100% correct. I was thinking about that when I wrote this:
I do a test run to determine what setting of the gain control causes the amp to consume 18W of power...
But I could have been clearer about it.

Quote from: Minion on February 11, 2008, 06:36:47 PMYou could do it much smaller and cheaper with a simple 9v adapter and a LM386 IC and make a Ruby type of amp.....Just a suggestion....

I built a noisy cricket already, and I love it. Actually, I think the noisy cricket is what started me on this "guit-car" amp craziness.

Jered · February 11, 2008, 08:22:19 PM

It seems like this might actually be possible. I'm curious to see how this pans out. Instead of glueing a gain knob in place. Measure the resistance of the pot at the setting you like, and use a resistor.
Jered

earthtonesaudio · February 11, 2008, 08:31:21 PM

Quote from: Jered on February 11, 2008, 08:22:19 PM
It seems like this might actually be possible. I'm curious to see how this pans out. Instead of glueing a gain knob in place. Measure the resistance of the pot at the setting you like, and use a resistor.
Jered

I just have to shop around for a suitable wall wart.
The fixed resistor would be much better. I was thinking in terms of simplicity and reversibility, but it would be almost as easy to re-do the resistors as it would be to scrape off glue. Heh, a bonus: removing the gain pot would net me a tiny pot with a tiny knob (the sucker is like 3/16" diameter).

mojo_hand · February 11, 2008, 09:04:45 PM

I've seen 12v switching wallwarts that could produce over 4A, although they're gonna cost you... http://www.action-electronics.com/acadapter.htm A "floor goitre" with a big 9V transformer might be cheaper, and could give you the assurance of a lot more clean power on tap. It would suck to not be able to use pedals because a boost might throw off your preamp's limits, causing your wallwart to burst into flames. I guess you could build in diodes to make sure that the power amp could never accept an input which would do that, but you'd be locked into one speaker impedance forever, and, I dunno, that seems like a lot of investment to make in a particular wallwart.

Overengineering always makes me feel good, and underengineering always makes me feel bad, but maybe that's just me.

earthtonesaudio · February 12, 2008, 08:42:33 AM

Quote from: mojo_hand on February 11, 2008, 09:04:45 PM
I've seen 12v switching wallwarts that could produce over 4A, although they're gonna cost you... http://www.action-electronics.com/acadapter.htm A "floor goitre" with a big 9V transformer might be cheaper, and could give you the assurance of a lot more clean power on tap. It would suck to not be able to use pedals because a boost might throw off your preamp's limits, causing your wallwart to burst into flames. I guess you could build in diodes to make sure that the power amp could never accept an input which would do that, but you'd be locked into one speaker impedance forever, and, I dunno, that seems like a lot of investment to make in a particular wallwart.

Overengineering always makes me feel good, and underengineering always makes me feel bad, but maybe that's just me.

Over-engineering good, agreed!

Re: the boost pedal problem, I would set max gain through the preamp... meaning the preamp would already be clipping at it's "max" setting. That would take care of potential boost problems.

I've heard switching power supplies are noisy for audio. Anyone have any experience with them?

dschwartz · February 12, 2008, 08:53:29 AM

did you even see my PSU recommendation?

12V, more than 15Amps for a few bucks on any computer store...

mojo_hand · February 12, 2008, 06:08:05 PM

Quote from: earthtonesaudio on February 12, 2008, 08:42:33 AM
I've heard switching power supplies are noisy for audio. Anyone have any experience with them?

My experience is that they are, but that many wall warts are, regardless of design. The only thing different about a switching power supply's noise is that it tends to be almost all high frequency (mostly 300 KHz and up), which is both a blessing and a curse. It's a blessing because you're one small inductor and capacitor away from it being squeaky clean, but it's a curse because of all the radiated noise. If you make sure that there's plenty of room and/or shielding between the PS and audio circuits, then add a very small filter where they meet, I don't think you'll have any problems.

I'm actually listening to someone play using one as I type this... it's stock, other than a 100 microhenry toroid and a 3.3 uF filmcap to kill off RF. Those set me back less than $1 from my local electronics surplus dealer, and they do the job quite adequately. You'll be needing a lot more current than me, so you couldn't get away with a teeny 2.5A inductor, but if you want to keep it cheap, you can always buy (or recycle) a core and wind it yourself, any reasonable value of inductance will work. Even 10 uH + 1 uF gives you a -12 dB slope at around 50 KHz, which works out to over 30 dB down at 300 KHz, it seems like that should do the trick.

earthtonesaudio · February 12, 2008, 07:33:34 PM

Quote from: dschwartz on February 12, 2008, 08:53:29 AM
did you even see my PSU recommendation?

12V, more than 15Amps for a few bucks on any computer store...

I saw it, but it didn't register until now. That's a pretty good all-in-one solution, but I'm still a little concerned about having that RF energy (assuming the PSU is a switch-mode) so close to a high-gain preamp. Even with proper shielding, wouldn't that still be asking for noise trouble?

Plus I found this neat little tutorial on converting one for general purpose use:
http://web2.murraystate.edu/andy.batts/ps/POWERSUPPLY.HTM

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Car amp to guitar amp conversion