Pedalboard Mounted Power Amp

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chrismercurio

chrismercurio

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Joined 2006
#1
Hi,

I need a pedalboard mounted power amplifier. This product (almost) doesn't exist as an off the shelf style unit. Can you guys point me in a direction? I don't need a lot of power...30w mono? I was initially thinking LM3886/75 style chip amp but the supply for those is bipolar which complicates things a bit.

Best,

Chris
 
#3
That looks like a good choice.

But if you are talking 30 watts, I have to assume you are going to drive a speaker, rather than headphones. And if that is the case, why not put the amp and supply into the speaker cab instead of the pedal board? Seems to me that would simplify several things, like no mains out to the pedal board. And then it wouldn;t take up the space of a good size pedal on the board.

Run the output of the pedal board balanced, then you can run however long a cable to the speaker without tonal detriment, and it appears the ALiexpress unit is already set for a balanced input. Added bonus, you could also run the output of the board straight to a mixer or down a snake.
 
#5
Guitar speakers are generally 8 or 16 ohms. Any idea of recommended supply voltage for that application? Also, what is the necessary output cap value for reasonable F3 given that it's a guitar amp and not a hifi amp? It's been a while since I've been at this but I seem to remember that the coupling cap on the output affects frequency response depending on value.

Thanks,
 
#6
3300 uf @ 8 ohms output cap gets about 30 hz. There is no reason not to go to 4700 uf, those were cheaper than 3300 uf last time I bought some. Panasonic 4700 uf 80 v 3000 hour caps were made in USA when I bought some last spring. Lots of old single supply amps had 1500 uf or 2200 uf output caps, but 3300 uf caps cost two hours pay back in 1966. Now they are about $5 plus shipping.
Look at the LM3886 datasheet if you want to build single supply amp by yourself. It has a single supply amp as an example. I get datasheets from datasheetcatalog.com when I can.
 
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#7
You would need about 80V for 30W at 16 ohms. Mighty close to the voltage limit for the LM3886. With an unregulated supply rail you would probably have to accept less than 30W because a high line voltage would put the chip over it's maximum rating where it just shuts off, or possibly damaged.

I generally use 1000uF or 2200uF for an output cap.

Guitar amps typically use mixed mode feedback where a small resistor is installed between the minus side of the speaker and ground. Download and read more about it here: Book about solid-state guitar amplifiers
 
#8
Ever heard about this?

EH%20Electro%20Harmonix%2044%20Magnum%2044%20watt%20Power%20Amp.jpg


Electro-Harmonix 44 Magnum Power Amp Demo & Review EHX .44 mag - YouTube

Electro-Harmonix 44 Magnum 44W Guitar Power Amplifier | Musician's Friend

To be realistic, *no way* it has 44W into 8 ohms, much less into 16, only posted this as a rough idea that it can be and has been done.
 
#9
JMFahey said:
Ever heard about this?

EH%20Electro%20Harmonix%2044%20Magnum%2044%20watt%20Power%20Amp.jpg


Electro-Harmonix 44 Magnum Power Amp Demo & Review EHX .44 mag - YouTube

Electro-Harmonix 44 Magnum 44W Guitar Power Amplifier | Musician's Friend

To be realistic, *no way* it has 44W into 8 ohms, much less into 16, only posted this as a rough idea that it can be and has been done.
Click to expand...

Just going from the label on the front, like you say no way for 44W to 8 ohms - although it's not 'that far' off. Assuming it's a bridged amplifier it needs 54V p-p for 44W to 8 ohms, so a supply of 27V could do it (assuming zero losses - which won't happen of course).

But it 'could' always be using a switch-mode PSU to increase the supply rail, in which case it could manage 44W - it'd be interesting to see a circuit for it.
 
#10
Yes, it might.
I calculated , based on a very stiff SMPS (which is possible, no big deal) and 1V loss , which is possible , since they will certainly use MosFets and in that case they are resistive, there is not the minimum 1 diode drop as with bipolars: 23V pk, so 46Vpp or 33W RMS/8 ohms .
Which anyway is amazing.

The "24V - 2A" label also hints at the same: 48W DC input power available, while a 33W RMS , 90% efficient amp would need 33/0.9=37 W DC power.

Which looks generous ... until we consider that that 33W "clean" amp will have to supply twice as much if driven fully into square wave and we consider it a "perfect" SS amplifier.

Anyway I'm not dissing it at all, I consider it a remarkable advance.

FWIW I haven't yet heard any killer sound demos featuring it, sound is not bad but what is to be expected from any conventional overdriven SS amplifier connected to a *good* speaker, while its smaller brother , the .22 caliber sounds impressive.

I saw some gut pictures of the .22 , it basically featured a roughly 1" x 1" square chip, say , with a dozen pins on each face, no heatsinh visible.

Interesting indeed.

FWIW I have built 100W/4 ohms amps inside 3" x 6" x 2.5" high aluminum "bricks" (including an EI transformer inside 😱 ) as "powerboosters" mounted inside good sounding but weak little combos, which in hinsdsight *may* be mounted into a pedalboard.

And now that I think of it, might even supply lots of +9V power as an "extra job" so you can pull the original pedal power supply.

Mmmmmh, not bad, I think you gave me an idea. 🙂

I'll make one and post it here.
 
#13
JMFahey said:
Yes, it might.
I calculated , based on a very stiff SMPS (which is possible, no big deal) and 1V loss , which is possible , since they will certainly use MosFets and in that case they are resistive, there is not the minimum 1 diode drop as with bipolars: 23V pk, so 46Vpp or 33W RMS/8 ohms .
Click to expand...

Generally you lose more voltage with MOSFET's than with bipolar, MOSFET amps usually have higher voltage rails for the same output levels.

However, if the Magnum uses the Texas chip as shanx suggested (which sounds pretty spot on) then it's rated at 40W from a 24V supply, at a massive 10% distortion. So hardly 'clean power' as the adverts suggest 🙂

The datasheet suggests it's more like a 20W amp, at 0.2% distortion, which makes more sense.

Nice chip though, and looks a good little project.
 
#14
JM, your ''power brick amp'' got me thinking too. I saw that SKB makes an amplified pedal board, released in 2011, but it is pretty small, only 5 W (class AB) with a small integrated speaker.
Enough for practicing alone but not with a small group or use live as a monitor.
20W to 50W would be better(!)
 
#15
JM, your ''power brick amp'' got me thinking too. I saw that SKB makes an amplified pedal board, released in 2011, but it is pretty small, only 5 W (class AB) with a small integrated speaker.
Enough for practicing alone but not with a small group or use live as a monitor.
20W to 50W would be better(!)
Click to expand...

Well inside the comfort area for a chipamp.

Think ... think ... 😉

Your main problem is the case, the "electronics" part is easy, just get a chipamp kit.
An LM3886 or something.

You'll need an aluminum box that contains the amp and PSU and still fits in the pedalboard or, *now* that I'm thinking about it, just to get a flatter pack, the job might be split in 2:

1) a self contained PSU, with transformer + bridge + main caps supplying, say, +/-22 V or whatever the chipamp needs *plus* an auxiliary regulated +9V @ say, 1A (or 2) to power a bunch of pedals and

2) a small aluminum box or chassis with *only* the chipamp and a flat heatsink.

This way we only *need* aluminum for the chipamp dissipation, and the PSU can fit anywhere, even in a plywood homemade box or a plastic one.

3) thinking even further, the chipamp does not even need a *full* case but may fit in a section of extruded aluminum railing which may also be part of the heatsinking .......

Think ... think ...
 
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