Modifying a HiFi amplifier to work well for guitar

[joecool85]

If you want the hands on build challenge, fine with me, just get the Velleman board, either as a kit or on its own, use your own heatsink and transformer.

I agree, if you want to do this circuit or similar, the Velleman kit is the best premade option.  Alternatively, you could create your own board in Kicad or similar, and get it printed through one of the various online PCB retailers.  I use OSHPark.com typically.

[megatrav]

I think that PCB is not well designed, you will need a good heatsink on the power transistors but the transistors are right in the middle of the PCB, right behind the transistors where you should mount the heatsink bracket are 2 small footprints for 7 watt resistors.
Have I been asleep for centuries and 7 watt resistors have now shrunk right down in size ?
I would throw that PCB in the bin, I would never try to build that amp since it will undoubtedly lead to heartache.
You can buy a 100 watt class D amp for less then the cost of the components you need to complete it, and the transformer will be expensive as well. There are so many better and cheaper options out there.

Fully agree.

Checked KM Tech site out of curiosity, thinking it might be a Chinese site, it reeks of it.

To my surprise, it IS UK based, but otherwise the description fits .

There is a Business Model based on ordering stuff in China , wholesale, and reselling it.
PCBs can be ordered from fabricators such as PCB Way and others for peanuts, think 10 boards for $5 or so.
https://www.pcbway.com/
https://www.pcbway.com/orderonline.aspx?x=100&y=100&num=10&Layersquote=2&Thicknessquote=1.6

KM seems to be a hardworking PCB design office, churning out tons of PCBs.
Their catalog shows over 200 different designs, they have sold over 20000 empty boards, so the Business model is working.

Most are for the Retro Gaming market, think Amiga, Playstation, Atari, Amstrad, etc. but they also offer some Audio boards.

Most look reasonable, but when being so broad banded sometimes a lemon sneaks in.

Their LM3886 power amp looks fine, the Chipamp is "heat sink mountable" along an edge, they show a populated picture, proof it is "buildable":





Now the "150W Darlington Amp" looks like it was hastily thrown together.
It may meet the "electrical" connections but layout is ludicrous, the thing is unbuildable; so much so that they show a *simulation* where to boot Power Transistors are NOT inserted 
Because they can´t.



Bonus points:
* as noted above, real 7W resistors will NOT fit there, no way.
* speaker ground return track is impossibly thin, it will explode on the first drum roll or something.

Funny notice: LM 3886 board has the "Use heat sink" warning printed on the silkscreen side ; "150W" one does not even mention them. 

That said, IF you already bought it and have no option, it *may* be built, sort of (assuming design is fine, of which I am not sure) by mounting TIPs on a heatsink proper, and running wires to PCB holes.
Same with 7W resistors.
But why bother to correct a botched board design which to boot is also poor electrically?

If you want the hands on build challenge, fine with me, just get the Velleman board, either as a kit or on its own, use your own heatsink and transformer.

If you find another kit that looks suitable, feel free to post/link it here so we can have a look.

That's a lot of good information and I am going to take your advice and shelve this one, at least for now.
I haven't bought any parts for it, so I'm only out the money for the board.
I may come back to it once I have a little more experience.

The LM3886 board is really interesting to me.

Knowing that they aren't as careful or worried about making tested and functioning amplifier PCBs I think I would rather buy from someone who is.

If you (or anyone else) can recommend a good LM3886 and/or a good Darlington or MOSFET PCB that would work well as a guitar power amp, please send links!

I would like it to be in the 60 - 100 watt range.
Ideally it will be used for playing live.

Thanks everyone for the info and advice. I really appreciate it and hope you have a great New Year!

[J M Fahey]

I love LM3886 but they are out of production for years now.

Only non original ones freely available.

Most are straight junk; *maybe* a honest chip factory (in Asia of course) cloned one or makes something which reasonably works, who knows?, but that is still betting on unknown horses, how would you know who is who?

For "just one" not worth the risk.

The real ones are made by TI; they are not "officially" discontinued but typically on heavy backorder because they have WAY more profitable products to make, think car, solar energy, electric vehicles, etc. stuff.

A hobby type (commercial factories have switched to Class D) market is not even a drop in a bucket.

There is a specific tread on LM3886 unavailability on DIY Audio.
Currently 29 pages long, maybe that means something

https://www.diyaudio.com/community/threads/whats-going-on-with-lm3886-availability.386577/

Now and then a few appear at Mouser or Digikey or Farnell, they are swept out within hours.

*Maybe* you get lucky, I commercially make amplifiers and can not work with that uncertainty.
 

[megatrav]

I love LM3886 but they are out of production for years now.

Only non original ones freely available.

Most are straight junk; *maybe* a honest chip factory (in Asia of course) cloned one or makes something which reasonably works, who knows?, but that is still betting on unknown horses, how would you know who is who?

For "just one" not worth the risk.

The real ones are made by TI; they are not "officially" discontinued but typically on heavy backorder because they have WAY more profitable products to make, think car, solar energy, electric vehicles, etc. stuff.

A hobby type (commercial factories have switched to Class D) market is not even a drop in a bucket.

There is a specific tread on LM3886 unavailability on DIY Audio.
Currently 29 pages long, maybe that means something

https://www.diyaudio.com/community/threads/whats-going-on-with-lm3886-availability.386577/

Now and then a few appear at Mouser or Digikey or Farnell, they are swept out within hours.

*Maybe* you get lucky, I commercially make amplifiers and can not work with that uncertainty.
 

I had no idea they were so rare now.

I guess I should look for TDA7293/4 or transistor based boards.

I wish that I could find one that was designed with guitar in mind, is in the 50-100 watt range, and works well to "replicate" tube amp behavior (current drive/feedback or some other manner of raising output impedance and lowers damping).

Maybe I should try to have PCBs made and use schematics from an existing guitar power amp (Yamaha G100/50, Randall RG, Peavey Transtube, etc)

[megatrav]

Actually, I just looked and DigiKey has LM3886T and LM3886TF. Are these not the same?

LM3886T

LM3886TF

[g1]

The T has the metal tab, the TF is fully plastic encapsulated, so no insulator required.
Not sure if the metal style transfers heat better to the heatsink.

[Loudthud]

I love LM3886 but they are out of production for years now.

Only non original ones freely available.

I don't think this is true. There was some availability issues around the time of Covid, and there was a switchover from National to TI when TI bought National, but I think TI is still cranking them out. I will check with a guy I know who works at TI, may take a day or two.

That's not to say there aren't fakes out there. Like always, buy from a known good source like Mouser or Digi-Key.

[J M Fahey]

HAPPY to see Mouser now has them in stock, in large amounts.

It was not so for *years* , where they did not straight say "forget it" but gave ridiculous lead times for restocking.

Oh well, all is well that ends well.

In this case, it becomes again an option for a Guitar amp.

Personally much prefer the metal tab version

A slice of mica or a laminated silpad or even Mylar insulator will *always* be thinner (so better heat conducting) that some Epoxy or any other plastic that has to be *injected* into a die, and flow to fill it.

The OP can use that KT something board or any other tried and proven one.

There is a guy at DIY Audio called "Brian GT" who has been supplying high quality LM3886 boards for years now, hundreds have been built by Forum Members with success.

[joecool85]

HAPPY to see Mouser now has them in stock, in large amounts.

It was not so for *years* , where they did not straight say "forget it" but gave ridiculous lead times for restocking.

Oh well, all is well that ends well.

In this case, it becomes again an option for a Guitar amp.

Personally much prefer the metal tab version

A slice of mica or a laminated silpad or even Mylar insulator will *always* be thinner (so better heat conducting) that some Epoxy or any other plastic that has to be *injected* into a die, and flow to fill it.

The OP can use that KT something board or any other tried and proven one.

There is a guy at DIY Audio called "Brian GT" who has been supplying high quality LM3886 boards for years now, hundreds have been built by Forum Members with success.

I thought Brian got out of the chipamp biz?  He used to run chipamp.com and had great LM3886 and LM1875 kits.  I actually helped him test the LM1875 kits back when I was in college.  Last I knew, he had sold chipamp.com maybe 10ish years ago.

**edit**
Yup: https://www.diyaudio.com/community/threads/my-first-try-at-a-lm3886-layout-any-comments-suggestions.49813/page-15#post-5147555

According to this post from Brian in 2017, he had sold chipamp.com to a friend and it went downhill.  In this thread he provides the gerber files if people want to print their own LM3886 and LM1875 boards though - which is awesome!

[joecool85]

I have compiled all the LM1875 and LM3886 files here: https://www.ssguitar.com/index.php?topic=5506.0

[Loudthud]

Sorry for the delay but here's what I got from my guy at TI. He's not an actual employee as far as I know, more of a paid consultant. The TI engineers are talking about those designs where you use two chips in parallel or two chips in a bridge drive configuration. I think Marshall has a design that uses four chips for amp. Hard to keep working blowing up chips all the time.

Yea LM3886 is back in stock. I've printed a few PCB batches
for a guitar and Bass power amps to build with them but after
talking with TI engineers they steered me away from them to
use Class D. I've yet to build any amps with the LM3886.  They
pretty much implied it was designed for single module application
or dual in stereo and not recommended for multi modules designs
to double the power. They said rail voltages needed to achieve the
result I expected would exceed the maximum continuous voltage
and cause them to fail. So I get they exaggerated the true power
output and adding additional chips don't increase total power
output in expected quantities. They also exaggerate about the
Class D chips. In reality the advertised output is at 2 ohms and
maximum rail voltages before it burns!! So if you want 200 watts
in the real world you must get 700watts to come close.

[megatrav]

Sorry for the delay but here's what I got from my guy at TI. He's not an actual employee as far as I know, more of a paid consultant. The TI engineers are talking about those designs where you use two chips in parallel or two chips in a bridge drive configuration. I think Marshall has a design that uses four chips for amp. Hard to keep working blowing up chips all the tome.

Yea LM3886 is back in stock. I've printed a few PCB batches
for a guitar and Bass power amps to build with them but after
talking with TI engineers they steered me away from them to
use Class D. I've yet to build any amps with the LM3886.  They
pretty much implied it was designed for single module application
or dual in stereo and not recommended for multi modules designs
to double the power. They said rail voltages needed to achieve the
result I expected would exceed the maximum continuous voltage
and cause them to fail. So I get they exaggerated the true power
output and adding additional chips don't increase total power
output in expected quantities. They also exaggerate about the
Class D chips. In reality the advertised output is at 2 ohms and
maximum rail voltages before it burns!! So if you want 200 watts
in the real world you must get 700watts to come close.

I think a single LM3886 chip amp at 50-ish watts would be plenty for a guitar amp.
Especially if it's going through a 2x12 or 4x12

I think it would be better to use class d for bass amps.

If the TI chips can't really achieve their listed wattage, I bet the ICEPower modules can.

There are other options too.