Building a guitar amplifier

Thread Starter

drk

Joined Mar 8, 2008
41
Hi, I'm planning on building a guitar amplifier, but have some doubts about what to use.

I'm not really interested in some big high power amplifier, but something simple and small/light.


The basic blocks I'm thinking are :

- initial signal conditioning (I think this is how its called) :

I've been said to use a differencial pair, but I think an op-amp would work fine too right? Or is there advantages to using with discrete transistors?

What kind of circuit would be preferable?

- effects :

this I'm more at ease, still not sure what I'll do, but at least a distortion effect.

- power-amp :

low power, so that the amplifier isn't big and heavy.

someone mention me about using class D amplifiers. Would this be a good choise? I read a bit about it, but it seems to be more difficult to build (you have to deal with high frequencies, for example), but on the other hand, it'll require a smaller heatsink/transformer..

Would it be a good idea to use ICs for the power-amp, or build it with discrete transistors gives better results?


- power supply :

depends on what I choose above
 

#12

Joined Nov 30, 2010
18,222
Most guitar players prefer tube amps but there are a thousand ways to do this. Input signal conditioning is about increasing the voltage and lowering the impedance. An op-amp is perfect for this, doing both in one stage. (TL071)

I've done this with everything from a single j-fet to a 12AX7 tube. The results can be anything from the size of a cigarette pack to something you could use to defend yourself in a bar room brawl. You need to get a bit more clear about your goals in order to procede from here.
 

russpatterson

Joined Feb 1, 2010
353
So much of a guitar amp is what type of tone you're after. You mentioned you wanted it to be small. Is power consumption a concern? Will it be battery powered?

Take a look at some of the amp boards they sell at Parts Express. You can build one of those kits, and if you like the tone, then use it a reference design for your own.
 

KJ6EAD

Joined Apr 30, 2011
1,578
I think I've seen schematics available online for some of the commercial 40 - 50 W practice amplifiers. They might be worth looking at to get some ideas. I've been told that there are also kits available.
 

Thread Starter

drk

Joined Mar 8, 2008
41
I'm planning on a transistor amplifier (no tubes), not battery powered.
In term of power, something around 10-20 Watt works, but it isn't very important to me. What matters is that its lightweight (easy to transport).

So, its not necessary to have a buffer in the beginning of the circuit? (as in, does it makes any difference?)

Thanks for the links, I'll look through it.
 

MrChips

Joined Oct 2, 2009
23,577
If this is something you want to build yourself but wish to keep it as simple as possible,
choose a TL071 preamp followed by tone controls, another TL071 and an LM1875. This will make a nice 20W amp.

The secret to good sound is the size of the speaker you choose and good cabinet construction.
 

Audioguru

Joined Dec 20, 2007
11,249
Electric guitar speakers have no bass (or have no deep bass and have a boomy upper bass), no highs and have a peak at about 5kHz.

Acoustic guitar speakers are wideband with a flat frequency response.
 

Thread Starter

drk

Joined Mar 8, 2008
41
...audioguru, what? wrong thread maybe :)

Anyway, I've got new doubts :)

I've simulated the power-supply, I think its something like what I added as an attachment, but I don't know how to choose the capacitor's value.

Also, what value would the fuses be? (and type?)

I was thinking of using the LM1875, but I got a LM4700TF, which seems to be a good choice too.

For the transformer, I'm thinking of buying this one, would this be ok for the LM4700?

And finally, I have no idea how to choose the heatsink. I got some here, and can get one off an old computer power-supply (see pics).
Would any of these (or combination of) work?
 

Attachments

Audioguru

Joined Dec 20, 2007
11,249
The output power of the amplifier when it is powered from that transformer (its voltage is a little low) will be about 23W into 8 ohms.
The total power from the power supply is about 36W so the 50W transformer is a little more powerful than is needed but it will work fine.
The maximum current from each half of the power supply is about 0.75A so the main filter capacitors should be 1500uF each, not 100uF each.

You should not guess about a heatsink. You should calculate what is needed then buy one that has those spec's.
 

Thread Starter

drk

Joined Mar 8, 2008
41
[Heat-sink]

So, from the datasheet:

sink_to_air = ((temp_max - temp_ambient) - PDmax * (junction_to_case + case_to_sink)) / PDmax

sink_to_air = 6.133 ºC / W

with:

temperature max = 150 ºC
temperature ambient = 25 ºC
junction_to_case = 2 ºC / W
case_to_sink = 0.2 ºC / W (depends on what thermal compound you use, I guess)

PDmax = Vcc^2 / ( w * pi^2 * R_load ) (in the datasheet there's no parentheses.. but its probably a mistake)

Vcc = 48 V (total supply voltage)

R_load = 8

PDmax = 14.6 W

Or can be seen from the "Power Dissipation vs Output Power" graph, that is around 15 W.

This means, that I can use any heatsink that is below 6 ºC/W right?

but it has to be in the right format, I'm thinking about this one.


[fuses]

For the mains fuses, since this is a 50VA transformer --> 50VA / 220V = 0.23, so a 0.3 A time delay fuse?

For the secondary, with a maximum current of 0.75A (how did you get this value? is it from the 1.39A corrent limit? 1.39 / 2 = around 0.7A), maybe something around 0.7A?
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,249
The datasheet for the heatsink is in a foreign language so I cannot read it.
You need a heatsink witrh a thermal resistance of 8.1 degrees C per Watt or better.

The total power (output plus heating) is about 38W. Then the primary current is 38W/220V= 173mA. Use a 200mA slow blow fuse.
If you have a primary fuse then you do not need a secondary fuse.
 

Thread Starter

drk

Joined Mar 8, 2008
41
here's in english

The model is the SK08. To be honest, I don't understand what that mm in the graph is referring to, but the thermal resistance is well below 8.

I think I'm all set for now, I'll buy the materials and see how it goes :)

thanks for the help!
 

THE_RB

Joined Feb 11, 2008
5,438
I think the heatsinks in your photos are woefully undersized...

If you want a reliable amp for guitar use I would get a heatsink many times larger, and make sure it is mounted on the rear of the amp in free air, with the fins vertically.
 

Thread Starter

drk

Joined Mar 8, 2008
41
I got other heatsink, not going to use those.

Could you give me an example?

What do you mean with in free air?
 

Audioguru

Joined Dec 20, 2007
11,249
Free air means the heatsink is not enclosed in a sealed cabinet.
The fins should be vertical so convection can let the hot air to rise then cool air will come to the heatsink from the bottom.
 

THE_RB

Joined Feb 11, 2008
5,438
Thanks guys, you are both correct of course about "heatsink in free air" I meant in the air of the room (not inside the enclosure) with the air free to rise past the heatsink and carry the heat up and away from the amp.

I've repaired a lot of practice amps with TDA chips, which cook and blow one side of the chip short circuited (speaker shorted to a power rail). Repair is always to simply replace the chip.

Many of the new chips are much superior and they have inbuilt overcurrent protection and dynamic overheat protection "zones", I think the LM1875 is one of the new types. They should last much better in a guitar amp application.
 
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