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Old 31st July 2012, 11:42 PM   #1
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Cool DIY 8 Watt Portable Guitar Amp

Welcome to my first real adventure in building anything useful using my soldering iron. I'll use this thread to ask for help and share my progress as I build my latest project!

The goal is to build a portable amp for my cheap Indonesian Squire Strat that is flexible in the tones that it can reproduce, capable of lasting several hours on a charge, and loud enough to compete with an acoustic guitar, cajon, vocalist etc.

I've purchased an 8 Watt mono chipamp kit, an 8" Peavey speaker, and assorted bits to both finish the cabinet and build a control panel and preamp circuit. I also have a PowerSonic PS12180 SLA battery, an NTE858M low-noise JFET dual opamp, plenty of 22ga solid core wire, 20ga stranded core wire, heavy speaker wire, and a big messy bucket of unsorted resistors, capacitors and transistors.

Amp Kit: Class-AB 8W Mono Power Amplifier Kit TDA2003
Speaker: Peavey 8" Guitar Speaker 4 Ohms
Opamp: NTE858M Dual Low Noise JFET Input Datasheet
Battery: 12V 216 Whr Power-Sonic PS-12180-NB Sealed Lead Acid Battery

I'll build the cabinet out of a scrap piece of MDF I already have, mount the components and test the design, then likely build a proper cabinet out of plywood. I'll charge the battery with a small motorcycle-type trickle charger.

My questions to the great and infinite knowledge base that is DIYaudio:

1. What sort of preamp should I be considering for this project? MXR Distortion Plus? Ibanez Tubescreamer? A simple transistor fuzz? I want to do something simple, but opamps don't scare me and I really want a flexible tone, I want to hit bluesy notes AND screaming solos.

2. I would like to include a tone stack, where do I place it in the signal path? If I build a preamp with two stages of gain, perhaps in between the two stages? What is a buffer? Do I need one? Two?

3. How loud will my little chipamp be able to push the Peavey speaker? Can I expect decent volume? The Peavey speaker is 4Ohms, 91dB 1W/1m. I also considered a Jensen MOD-8 which may be a future upgrade if this goes well.

4. Do I need to worry about the preamp sending too hot of a signal to the chipamp and damaging it? How many mV come out of my Strat? How high should it be after the Preamp? Do I get better performance out of the chipamp by sending it a lower signal (500mV) and turning up the gain? Or turning up the preamp (1500mV) and keeping the amp volume reasonable?

5. Should I be considering a voltage regulator to protect my circuits? My battery puts out 12.45-12.75VDC. Ideally I'd like to run the TDA2003 near 15VDC, but that's not likely to be attainable on the cheap, and I don't think the preamp would appreciate it. I know I won't get good output from the TDA2003 if the supply voltage goes below 12VDC (It's good for 5-15v though.) Will my battery charger damage my precious circuitry if I charge the battery while it's connected? What about while it's on?

My parts arrive today, and I'll likely assemble the amp kit tonight and run my guitar into it just to see what happens. I've attached a quick CAD of my cabinet design with the speaker, battery, and carrying handle drawn in.

Thanks all and have a great day!

DK
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Old 1st August 2012, 01:29 AM   #2
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Well, guitar amps are not built for "high fidelity"
They are built to get a certain sound.
This sound is a combination of the signal processing and the speaker / cabinet combination. Actually works better with tube amps because the speaker / cabinet combination impedance is reflected back to affect the output stage operation...more contribution to the characteristic sound of the whole package.

one thing they all have in common is a final output pot just before the final power amp.

prior to that it seems that anything goes.

preamp, tone, distortion, more tone, external FX loop, etc. mix and match.

There are a few different guitar tone stacks and somewhere on the net is a tone stack simulator. It demonstrates different stack types and you can change the component values and see the response graphically.

Search for "tone stack simulator".

IMHO: leave your design open to modification so you can truly experiment to get the sound you want.

have fun
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Old 1st August 2012, 06:32 PM   #3
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Default Day 1 - Soldered the Amp Together

Parts arrived yesterday. I soldered together the amp kit but didn't have a power supply to test it with. Tonight I'll try it with my Powersonic battery. I'm going to run the leads from the 1/4" mono jack into the input, and the leads from the output straight into the speaker. I'll take some pics and shoot a little video, just so other curious people like myself know what it will sound like.

I'm aware that guitar cabinets are not hi-fi. You basically take the all the TS variables except Fs and sensitivity (maybe Qts) and throw them out the window when building a guitar cabinet! A 3/4" birch ply box with a 1/2" hardwood baffle is where I'll likely end up if this project is successful.

The tone stack simulator is really neat. I'm using this: Tone Stack Calculator
I'm not sure how to interpret what I'm seeing, it would be cool if I could pump an audio file through the calculator and play with the knobs that way. It's very interesting to see the different tone styles.

I am heading to my parents to build the cabinet on Sunday. I would like to have the preamp built before then. What would DIYaudio recommend as an ideal high-gain home for my NTE858M opamp?

Also, I'm using shielded microphone cable for all my signal wires. I'll likely be using this to connect the preamp to the amp since they're on separate PCB's. Should I be considering a quick-disconnect solution for simple modification? Can I use male>female spade connectors or will they degrade the signal too much?
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Old 1st August 2012, 08:26 PM   #4
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Quote:
What would DIYaudio recommend as an ideal high-gain home for my NTE858M opamp?
If you are going down a road you have never travelled, look for a map! Find a similar combo amp out there in Google land and start with its schematic. About 15 years ago the kid next door blew up his little Peavey practice amp by wiring up too many speakers to it. I replaced the power amp chip and sent it home. It used a dual opamp for a preamp and had a reasonable sound for a practice amp. It was loud enough that he knew when I had fixed it (next door). Fender and a zillion others make similar designs, find one you like and start there.

I am working on a similar idea, but from a totally different perspective. I want a battery operated practice amp, but I want it to be a tube amp! Progress is very slow since I get delayed by life too often (recovering from pneumonia now). I have several LIPO batteries from model helicopters. They come in varying sizes up to about 5 AH and are very light weight. I just got the chips and transformer for the 400 volt supply. The tube amp will be the easy part. I want two different sizes. A medium size unit with 5 to 10 watts and a 5 to 8 inch speaker (I have several to choose from including one of those Peavey units) and at least a 3 hour run time. It will be built in a typical box. I also want a tiny unit that makes about a watt that fits INSIDE the guitar. I have several tiny speakers, but I may not like the sound of any of them.....only time will tell.
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Old 1st August 2012, 08:37 PM   #5
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I found the schematic to the kids Peavey. My memory wasn't perfect....it uses 2 opamps to drive a 20 watt chip amp to 15 watts.....and soes it from 12 volts worth of batteries. Maybe a good start?????

The Free Information Society - Peavey Solo Electronic Circuit Schematic
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Old 1st August 2012, 09:01 PM   #6
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Like you, I had intended to use tubes at the beginning. I found a beautifully simple power supply designed for those steampunk nixie tube wristwatches, (included a regulated 12VDC out and an adjustable 100-250VDC out) but in the end decided that I had better aim for something a bit closer to my skill level. Again, if this works, and it's fun, I'll likely try a starved tube preamp and a larger solid state power amp, maybe closer to 50W. This time with two of my Powersonic batteries for 24V!

I think I'm leaning towards the Ibanez Tubescreamer TS-9 as shown and described here: GM Arts - Guitar Amplifiers
That design will give me the opportunity to experiment with the placement of my tone stack before/between/after the opamp, as well as having a tone control in the last stage of the circuit for added flexibility.

That's also based on my assumption that the Tubescreamer will be a good fit for my opamp...will she survive? I should add, I'm placing the opamp in a socket for flexibility. I'll probably socket the transistor too, so I can replace it with others...or another JFET.
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Old 1st August 2012, 09:33 PM   #7
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Quote:
Originally Posted by tubelab.com View Post
I also want a tiny unit that makes about a watt that fits INSIDE the guitar. I have several tiny speakers, but I may not like the sound of any of them.....only time will tell.
Oh gosh. I love the sound of that. A buddy of mind has a solid body strat with all the hardware long gone that he uses as a doorstop...maybe I could chop a hole in it and mount a 3" full range speaker...hmm...hard to find one under 2" deep, and SPL is way down in the low 80dB range. @_@ CHALLENGE ACCEPTED!

Do you think it's even possible to build something decent into a solid body electric? I suppose you could put it inside an acoustic, but that's too easy...then again, the mess you make of the body's resonance by adding components to it would likely ruin the tone anyway. Flipping a hidden switch on your acoustic that magically doubles the volume and adds delay/reverb would be a pretty neat show!
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Old 2nd August 2012, 12:17 AM   #8
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Quote:
assumption that the Tubescreamer will be a good fit for my opamp...will she survive?
I don't know why it wouldn't. Fed a reasonable supply voltage to the correct pins, opamps are pretty hard to kill. Some can be killed if the output is shorted to ground or B+, so don't do that and you're OK.

Using the tube screamer to drive the chip amp will leave you without tone controls. To me that would be the deal breaker, since I like to have knobs to turn.....the more the better. Even the Peavey circuit is kinda limiting. Is a second opamp out of the question. They don't eat much power, or money.

Quote:
Do you think it's even possible to build something decent into a solid body electric?
I don't know, but I am going to find out. Back when I was a kid.....say the 1960's....there was store chain similar to what Radio Shack was 10 years ago. It was Lafayette Radio Electronics and there was a store near my house. They sold CB radios, stereos, guitars and amps. Mostly cheap stuff like you find at Radio Shack. They had an electric guitar with a solid state amp and a maybe 4 inch speaker. It might have cranked out a watt on a good day, and wasn't terribly loud, but I thought it was cool. I played it every time I went into the store. When the batteries were dying it had a cool distorted sound. Granted I was about 13, but I always wanted one...with a little more crank. I remember it was heavier and thicker than the other guitars too. So I am going to try to make one.

Fast forward 47 years, and 6 years of evening woodshop classes, and I have successfully made a few working solid body electric guitars. None have been spectacular and all used Fender necks from Ebay, but I am slowly getting there. I cut up an old waterbed frame and used the wood to make a few zero cost guitar bodies that are about 1 3/4 inch thick.

Quote:
maybe I could chop a hole in it and mount a 3" full range speaker...hmm...hard to find one under 2" deep, and SPL is way down in the low 80dB range.
I have tried a few. I got a pair of Tang Band W2-1811S 2 inch full range neodymium speakers. The SPL is low, and I was not too impressed when I connected them to a 2 watt tube amp. They are 1.6 inches deep. I also tried a Tang Bang W3-1750S 3 inch NEO "subwoofer". It is quite a bit louder, but has no high end. It is 1.7 inch deep. All 3 together still don't cut it.

These less than stellar results have led me toward a different path. Using the zero dollar waterbed frame wood, I will attempt to laminate two blanks together for a 3 1/2 inch body. Then I will have plenty of room for a tube amp, a fat battery and a 5 inch Jensen MOD. The MOD measures 2.44 inches gasket to magnet. It sounds pretty good with the 2 watt amp, not as loud as the Peavey, but the Peavey is in a cabinet and the MOD isn't. Trainwreck style feedback is at a twist of a knob......Well that may be the next obstacle to overcome.

I take the woodshop class for "therapy". It is my weekly escape from the grind of a 40 year engineering career. I have no problem spending a few weeks making something, and then tossing it into the trash. I have done it before especially with speaker cabinets, and I will do it again, maybe with a guitar or two.

I have not made it to woodshop class in about 3 weeks, maybe tomorrow.
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Old 2nd August 2012, 01:23 AM   #9
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Quote:
Originally Posted by NapoleonDK View Post
Oh gosh. I love the sound of that. A buddy of mind has a solid body strat with all the hardware long gone that he uses as a doorstop...maybe I could chop a hole in it and mount a 3" full range speaker...hmm...hard to find one under 2" deep, and SPL is way down in the low 80dB range.
Take a look at the Faital 3 and 4 inch drivers. I've been messing with a pair of the 4s for bass cab tops, but they might work pretty well for guitar too. Just measured one @ ~1 3/4" deep. If you hunt around eBay and email Faital sellers you can sometimes get 'em a lot cheaper than the usual online vendors sell them for.

Rod Elliot's (ESP) guitar preamp circuit board is a pretty nice Fender tone stack variant that you might look into as well.

Last edited by Passinwind; 2nd August 2012 at 01:27 AM.
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Old 2nd August 2012, 09:52 AM   #10
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For preamps, you may want to have a look at the runoffgroove overdrives, these are overdrives modeled after known amps. They use FETs to replace tubes, since they have similar overdrive characteristics.

For bass, the flipster is a nice overdrive, but they have several guitar options, like matchless and orange-esque preamps. I have built an Umble, modeled after a Dumble amp, very nice, pretty high gain, you may want to use something else for the low-gain channel (if you decide to go two-channel).
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