Modifying a HiFi amplifier to work well for guitar

[megatrav]

Hey all!

I was recently given an interesting power amplifier PCB based around a Darlington TIP41/TIP147.

I haven't built it yet, but since its a pretty simple circuit, I wanted to see if anyone had any suggestions for how I could make it work well as a guitar power amp?
Ideally, I'd like to experiment with making a hybrid amp.

You cannot view this attachment.
You cannot view this attachment.


Would this be an opportunity to add current feedback from the speaker? 

Could any of the component values be changed to help the response?

Or is it fine the way it is and I should just focus on the preamp?


Thanks!

[tonyharker]

Definitely not HiFi, but this amp if I remember rightly was originally used as a guitar amplifier. It will not produce anything like 150w not with those transistors, but should get close to half that. i cant remember who the designer was at the moment.

[J M Fahey]

Use it as is, crude but does its job.

Later you can build a better one but it´s fine for experimenting.

It´s somewhat on the edge, use +/-40V rails or so (30+30VAC PT, 2,5A), 8 ohm load, beware it has NO short protection.

No big deal in a Combo or powered cabinet, somewhat dangerous on heads because someday you WILL meet a bad cable.

Power output is similar to a Bandit, VS8080, Crate or Laney 60, etc. , enough for playing around with a good speaker, say an Eminence or Celestion or modern Jensen

Not HiFi, Car, Home Theater class speakers.

Use good heatsinks.

[megatrav]

Thanks for the reply!  I assumed 150 watts was overstated.  60 - 80 watts is perfect.

I did actually want to build this as a stand alone head, so I think I should add in some kind of protection circuit.

[megatrav]

beware it has NO short protection.

Mr. Fahey you have helped me in the past on DiyAudio.  I appreciate your advice and knowledge.  I have read lots of posts you've made helping others, especially related to guitar amps you have built.

Do you recommend a way to include protection for the amp?  I have seen a few PCBs on ebay for speaker and DC protection. 

I haven't ever designed my own PCB so I would prefer to either buy a PCB or wire it up using another kind of board.

Thanks!

[J M Fahey]

You are lucky: exactly that same basic amp but properly made (not much more added, yet ...)is offered in kit form with an *excellent* 20 page assembly manual.

Enter the Velleman k8060.

Proper biasing, short protection, very flat assembly (board,  supply,  power transistors at the ends) make it easy to mount inside a Guitar amp chassis or a powered speaker back aluminum plate.

Circuit:  (eerily similar to the crude one):



Full  manual :  https://cdn.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k8060.pdf

from:  https://www.velleman.eu/products/view/?id=360242

They even offer a heatsink, custom drilled or not, and a suitable power transformer.

Of course you can buy only the PCB and parts kit and mount it on your own heatsink or aluminum chassis and get a cheaper and maybe closer to home ANTEK toroid.

Anything from 26+26 to 30+30 VAC, 2.5 or 3A will do fine.

Amp marketing is "optimistic" as usual, forget the meaningless "200W peak power", it has honest 70-80W RMS into 8 ohms, which is perfect for live use.

[Loudthud]

A few words about the need for short circuit protection. In addition to the bad speaker cable as mentioned by J M Fahey above, there is the very real possibility of a momentary short occurring when using Switchcraft brand jacks (or similar) on the speaker cabinet. Cliff type jacks don't have this problem.

Most Guitar amps use Phone plugs to connect speakers. When a cable with Phone plugs is inserted into a Switchcraft jack, a momentary short occurs across the cable. If an amp is Powered On and you first plug the cable into the amp, no problem yet, until you plug the cable into the speaker cabinet. The amp sees the momentary short and could blow up. Best to leave the amp off while you connect the speakers or always plug into the cabinet first, then the amp.

Those 60s solid state Thomas-Vox amps didn't have short circuit protection. How did they get away with that ? First, they used XLR type connectors for speaker connections. Those won't short the signal. Second, they used an Off - Standby - Play power switch that didn't connect the amp to the speaker until you were in the Play mode. Pretty clever

[Tassieviking]

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.

[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.

The board is 2 sided so components can be mounted on the under side.

You're right that I could pick up an ICEPower module (or an even cheaper class d module) for less than the cost to build this amp, but that wouldn't be a challenge at all

[megatrav]

You are lucky: exactly that same basic amp but properly made (not much more added, yet ...)is offered in kit form with an *excellent* 20 page assembly manual.

Enter the Velleman k8060.

Proper biasing, short protection, very flat assembly (board,  supply,  power transistors at the ends) make it easy to mount inside a Guitar amp chassis or a powered speaker back aluminum plate.

Circuit:  (eerily similar to the crude one):



Full  manual :  https://cdn.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k8060.pdf

from:  https://www.velleman.eu/products/view/?id=360242

They even offer a heatsink, custom drilled or not, and a suitable power transformer.

Of course you can buy only the PCB and parts kit and mount it on your own heatsink or aluminum chassis and get a cheaper and maybe closer to home ANTEK toroid.

Anything from 26+26 to 30+30 VAC, 2.5 or 3A will do fine.

Amp marketing is "optimistic" as usual, forget the meaningless "200W peak power", it has honest 70-80W RMS into 8 ohms, which is perfect for live use.

You're the man! Thanks for sharing.

[megatrav]

A few words about the need for short circuit protection. In addition to the bad speaker cable as mentioned by J M Fahey above, there is the very real possibility of a momentary short occurring when using Switchcraft brand jacks (or similar) on the speaker cabinet. Cliff type jacks don't have this problem.

Most Guitar amps use Phone plugs to connect speakers. When a cable with Phone plugs is inserted into a Switchcraft jack, a momentary short occurs across the cable. If an amp is Powered On and you first plug the cable into the amp, no problem yet, until you plug the cable into the speaker cabinet. The amp sees the momentary short and could blow up. Best to leave the amp off while you connect the speakers or always plug into the cabinet first, then the amp.

Those 60s solid state Thomas-Vox amps didn't have short circuit protection. How did they get away with that ? First, they used XLR type connectors for speaker connections. Those won't short the signal. Second, they used an Off - Standby - Play power switch that didn't connect the amp to the speaker until you were in the Play mode. Pretty clever

Thanks for the info!

Would I experience that issue if I used Cliff jacks on the amp and the cabinet has phone jacks?

The standby switch sounds clever and I might have to look into using that too just as an extra safety precaution

[Loudthud]

I was thinking that a Cliff type jack might briefly short the cable as the Ground contact of the jack slides over the insulation between the tip and sleeve of the plug.

[phatt]

My thoughts and experience on this issue FWIW,
You can save a whole lot of drama just by inserting a Poly switch on the output terminal of any power amp that does not have protection.
Simply by using a Poly Switch which is a resetable Solid State Fuse called a
"polymeric positive temperature coefficient device" (PPTC) Some times called polymeric PTC device or simply called Poly Switch.

These things are cheap ($2~3) and having used these on a few Amp builds I can say they work as intended.
They come on different sizes so you just need to know the full power current of your Amp circuit to match up the right one to use.
Here's a link to Altronics page I purchased mine from in Aust.
https://www.altronics.com.au/hardware/fuses/?type_1=ptc-polyswitch&srsltid=AfmBOopVtjMMNff1hDVmDTor3xGW21LYeTkEGn2Q1QzeRm4Zsm11_Hzv

I'm surprised they are not more widely used as they are perfect for the job especially as they save all the fiddly extra components needed to implement the current limiting which is often more than 6 extra resistors, a couple of diodes and 2 transistors which takes up extra PCB real estate.

I've forgotten the specs but obviously they switch lightning fast. Because I've tested my 80Watt DIY build and while at full power I Dead shorted the output over and over and that Amp was built 10 years ago and still runs fine today.

The yellow case does actually darken to orange as they get close to the limit. You can hear the Sound start to fade just before they trigger. Once triggered and Short removed they return to normal in a matter of a few seconds.

In my view ,, Perfect for DIY home build Amps.
Phil.

[phatt]

For clarity, Here is what the Poly Switch replaces in a power amp circuit.
If your amp already has the Current limit setup you can still Add the Poly Sw for extra protection. BTW The extra bonus is also protects your speakers. win win  
Phil.You cannot view this attachment.

[J M Fahey]

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.