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50 watt electric and bass guitar amp? Answered

good day to all, i'm new to this site hope i can gain more knowledge in electronics here, since i have been doing alot of programming lately..

i am planning to build my first higher wattage amplifier for both guitar and bass, i already have built smokey amp and its other cousins like the ruby amp and some guitar effects. i will build the two amps simultaneously and put  it in one housing..
the speakers will be separated and also the circuits and all.. does any one have schematics for this kind of amplifiers??
it should bea solid state amplifier, because its hard to find tubes here and also it is expensive..

correct me if i'm wrong
1. "instrument >>>preamp>>>poweramp>>>>speaker" ?
2. is it possible for example to use a smokey amp as the preamp section and connect it to a 50 watt power amp?
3. what type of speaker should i use for the guitar and for the bass? should it be a woofer or sub-woofer? coz i dont have a clue on speakers..

tanx in advance..



7 years ago

1) Yeah.

2) Sure, why not. If you like the sound of the smokey.

One thing (this might be over your head, as it concerns input/output impedance, which is a little slippery):

Smokey amps, like any LM386 circuit, have a large coupling cap attached to the speaker--usually in the range between 50uF and 200uF. The cap is there to block DC, and only let the audio (AC) signal pass through.

Why is the cap is so large? Because it forms an RC filter with the speaker, which is low resistance (4-16 ohms). A speaker has a very low input impedance (that same 4-16 ohms). The lower the input impedance of the next stage, the larger the coupling cap.

So power amps generally have a much higher input impedance (10K to 1M, depending on the type), so a huge cap isn't needed. The correct cap value is dependent on knowing that input impedance, however.

3) Guitar speakers are NOT full spectrum. Look at the specs for Eminence, Celestion, etc.

The high end of a guitar speaker tops out about 5K. Some of the "mojo" of electric guitar comes from the fact that speakers create their own harmonic resonance in the higher ranges. But they don't, in fact, accurately reproduce those frequencies if you pump them in directly. I.E., they are specifically designed to be low-fidelity devices, and to create harmonic distortion in a certain way.

Until the late 1960s, guitar and bass amps all used the same speakers. Now they make bass speakers specifically. If I wanted to use regular guitar speakers with a bass amp, I'd make sure the power rating for the speakers was about three times the output wattage. Large woofers would work fine for the bass...

Overall, It's always a good idea to use a speaker (or speakers) rated for more power than the amp. If you play the amp "dimed" all the time, I'd go for twice the amp's power (100 watt speaker for a 50 watt amp). Can depend on how the amp's power is rated, too (peak vs RMS, etc.)

thanks, you have answered almost 70% of my doubts in building my first amp.. i have a problem with what typical IC's i could use since i'm building an SS amp, i have some 741 & 4558, and what typical IC should i use for the PowerAmp section? i noticed in some amplifiers they have two inputs correct me if i'm wrong those 2 inputs are "hi" & "lo", what does that mean?? and i was also thinking of adding a circuit so i can plug in my mp3 player to the amp?? what typical circuit should i use?? and have you tried using a "fetzer valve" circuit?? and by the way the DIY amp you did was cool, and it inspired me to build one..=)

No biggie, thanks for the kudo about the amp...

The 741 and 4558 are op amps--preamp stuff. Great for some F/X projects.

Best bet is something similar to your LM386, like the TDA7240A. It's a 20 watt audio amp on a chip. Needs a few external components, heatsink, etc. but otherwise very simple. There's a project on Diystopmboxes call the Forum Amp, it would be a great place to start (somebody even sells PCB and kits in that thread).Talks about power supplies, etc., too. Looks inexpensive.

It uses one TL072 op amp as a preamp. but you could replace that, like you could build in a Fetzer valve, etc. or other effect as the preamp (or add that to the existing preamp). Or use a distortion FX externally for dirt.

The Fetzer valve is cool. But you wouldn't want to use that for a preamp with your Mp3 player--stick to the original design for that.

But you might need to attenuate (reduce) the signal of the MP3 player when using a guitar amp. A simple voltage divider (two resistors) can do that for you.

In fact, "hi" & "lo" inputs are usually just that--they figured out how to create an input "pad" with two "switching" jacks and a minimal amount of resistors that cuts the signal somewhat, right at the input. It's very simple. See the image below. An mp3 player might need even more attenuation than that "lo" input, however.

Twenty watts of power can be freaking LOUD with a set of decent speakers. And you could always build two...


thanks for the input.. i just need to know would this work..

i'll be using only the pre-amp stage for both my bass and guitar..

i would put this for both the guitar and bass. and can this be placed between the preamp and poweramp?

and finally this will be placed between the input and the pre.amp still for both guitar and bass, basically it's a stompbox incorporated to the amp. i have schematics of this effect. along time ago and i wanted to test this one.

would it be possible to make it switchable from 20w to 50w?? was thinking of putting 2 poweramps and putting a switch to toggle between the two..

The biggest "single chip" audio amp I know of is the TDA7256 (30 watts), but they might come bigger...

Personally I wouldn't get too caught up in wattage (although it's harder for bass guitars to be heard, so bass players usually want more). Watts and loudness are logarithmic--it takes 10 times as much wattage for an amp to be twice as loud.  I.E., the next real jump up in loudness from a 20 watt amp is a 200 watt amp.

But you certainly can use two 20 watt amp (with separate speakers--I don't now how to "bridge" those chip amplifiers). Their input impedance should be high enough that you can simply hook both up to the same preamp and it should work. You could use one or both at the same time. You could even build (or buy) a stereo speaker cabinet.

The Elliott Sound Products preamp should work fine. But compare that schematic with the "Tonemender"--the Elliott Sound Products preamp already has a three-knob tone stack (it's in the middle of the circuit), so you don't really need the Tonemender. Not that they would sound identical.

Also, you could try inserting the passive components of the Tonemender (stuff between the opamps) in place of the Elliott Sound Products tone stack. There are differences, so it might need more tweaking.  I'd try the Elliott preamp as-is for now.

I don't know if the Sansamp simulator would work well after the preamp, or if it's designed to be pre-preamp. You could search for "speaker simulator schematic", which will turn up a lot of info about altering sound post-preamp.

But you could also insert a graphic equalizer somewhere in the chain.

A few years back on ebay I bought a 200 watt (100 W stereo) power amp for my car with a built-in graphic equalizer. It only cost about $25. I'm just sayin'...

last question, how about the power supply? how many amperes do i need for the transformer? here in the philippines were using 220V? what is the relevance of the amperes when purchasing a transformer??

would this design work??


Work backwards from the power amp to figure your power supply requirements.

If you have two 20 watt power amps, add about 20% (or maybe more for a safety margin) for everything else--guestimate total power consumption at 50 watts.

Break out good ol' Ohms Law: ampere = watts/voltage.

50/12 = 4.1666 amps. So the regulation circuit needs to supply over 4 amps @ 12V.

Do the same for your transformer: 50/220 = 0.227 amps. Not too much current is needed due to the higher voltage.

Just search ebay for "12V power supply 5A"...

hahaha.. i totally forgot bout the ohm's law.. thank u for reminding that one..

thank you.. i'll start buying some of the parts that i don't have.. maybe 3 weeks from now i can make an instructable on it..

Actually, you should look at the chip amp datasheet--amplifiers aren't anywhere near 100% efficient, so if it's only 50% efficient, then you'll need to double the power supply amperage rating...


saw this schematic, it has 2 30w output, does having two, double the output. and does this circuit look feasible to build,

i have started building the pre amp, what would be a safe way to test it? because i have not build the power amp section yet.

Plug the preamp into some powered computer speakers. You should hear that fine... (you could even run it into your computer sound inputs).

what is the best material to place all the circuit all together? and what are some pros and cons of using such materials.

How to enclose the whole shebang?

It depends--on the different circuits, how many controls, etc. And what materials you have on hand. Reusing a broken case/combo is always an option. The actual enclosure doesn't matter much. Shielding is probably important, but you can use heavy foil or something similar.

Again--it partly depends on the controls. If it's advantageous to mount them all on a metal panel, then maybe an aluminum chassis or box is best. Or just a few pieces of sheet aluminum.

I'd tie the circuits together using shielded cable. All except for the speaker cable, which is high-current / low impedance doesn't need shielding--regular 2-conductor power cord is best for that.

do you have any blueprints for building a 2x10 or 2x12 cabs? and what type of materials do i need to use for a cab.

thanks for that additional info..


could this be used as a power amp?

Oh hey--you found a "one chip" 50 watt amp! H@ll yeah that would work...

But you'll need a regulated bipolar 35V power supply (+35 and -35V). And reading his notes, it should be a very good power supply or distortion would result.

i'm just wandering about the Elliott Sound Products, would the circuit behave well with guitar effects?

The preamp? I don't see why not--he describes it as being designed for guitar.

Sometimes incoming effects like a "ground reference" on the input, but you could modify the Elliot Sound Products input jacks to the Fender input pad circuit (I posted the image before).

The only addition is a 1M resistor to GND on the input signal.

delayed my work for 2 weeks i guess.. got busy on work. last week i started building the pre.amp and i like the sound of it.. i'll be working on the power amp this week. just a follow up question, how are heads and cabinets hooked up? since i haven't seen one in real life since no one sells heads and cabs.. are they hooked up by mono jacks? or what kind of connector does it use?

Speaker cables use standard 1/4 in. mono plugs and jacks. Because power amp outputs drive significant current, it's a bad idea to use a standard guitar (input) cable.

The most common type of head/cab cable is identical to standard two-conductor power cord wire that household appliances use-- like the cord for an incandescent lamp. Lots of builders make their own from that type of wire.

Because the power amp output is very low impedance (probably < 1 ohm to drive an 8 ohm speaker), shielded wire is not necessary for speaker cable. Normal two-conductor works fine.

1) That's the typical signal chain. The preamp and power amp don't _have_ to be separate circuits, but often are, for various reasons.

2) I don't know those amplifiers, so I have no opinion about whether the "smokey"'s preamp output could be accessed.

3) Ideally you want a full-range speaker, not just a woofer. Otherwise you lose some of the high frequency components of the instrument's sound, which do make a difference to the texture of the instrument. That _may_ be less true of the bass, but it depends on what you do with the instrument.

4) Is there a reason you aren't planning to have the two instruments go into separate preamps which are mixed into a signal feeding a single power amp and speaker? That's certainly how professional sound systems work -- a preamp per mixer channel, single output from the mixer into the power amp and speakers.

tanx for those inputs, i am going to add an auxiliary so i can plug my mp3 player so i can do a play along.. what typical circuit could i use?? how does an auxiliary circuit work?? does it still comprise of preamps and poweramp??

An auxilliary input may not need a preamp -- that's the difference between "line level" input and "mike level" inputs -- but will go into into a mixer stage to be mixed with your guitars before the combined signal goes to the power amp. You'll probably want independent level controls on each of the mixer inputs so you can adjust how much of each comes out the speaker.

This is one of the reasons preamp and power-amp stages are separated; the mixer and its controls get put between them. Preamps bring everything to about the same level (line level) to facilitate that sharing and to keep the mixer simple.