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February 08, 2023, 09:29:33 PM

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Power amp transistor choices??

Started by NPreston, May 08, 2021, 09:22:13 AM

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I've just successfully designed and assembled my first highish power amp.  Rails at +/- 45 volts into 16 ohm cab and using up all the headroom.  Currently using tip33/34c for output transistors and tip29/30c in the VAS and drivers.  For the sake of a guitar amp do I have much to gain in using faster transistors I the VAS/driver/ output stages??


Faster?    Unless you are amplifying radio signals, I am not sure what faster would get you.


Look for something with a bigger Safe Operating Area (SOA).


So I believe the devices I am currently using have a ft of 3 MHz.  From reading in the diy audio forums I'm seeing a lot of vas transistors people talking about over 100 MHz and output devices where ft is about 20 or 30 MHz.  Being that these devices signals become negative feedback in the power section was just wondering what audible difference if any this might make in the case of an overdriven guitar signal?



An old saying from an old bugga who has been around and seen/read enough to know this simple rule in regard to Guitar amplification;
"If you want to build a great guitar amp sound (Especially Overdriven) then build a really crappy amp"

If you do some simple maths you will realise that 90% percent of every fundamental freq /note you can play on a guitar is UNDER 1kHz.  add the essential harmonics and you are still well under 5kHz.
Basic rule of thumb, Anything past ~4kHz will just mess up the magic.

If all you ever do is strum a few chords with a clean signal then wide bandwidth won't matter much BUT the moment you introduce distortion and crank it up that excess bandwidth will just sound crap.
Wide bandwidth clutters the spectrum with way too much hash and you actually loose note definition

I know the average Joe will assume MORE is always better but wide bandwidth will just frustrate you and you will spend a fortune adding mare and more gizmos and never get anywhere close to the classic sound of years past.
Go listen to Carlos Santana's guitar sound,, You are hearing a very narrow and limited bandwidth.
If you took Carlos's guitar and plugged it into the latest over hyped fancy Amp sold these days (often with 20/20 bandwidth digi crap added) it would sound crap.
                          ***It's what you DO Not Hear that makes the magic *** 8|
Rant over and hope it helps. ;)


Quote from: Loudthud on May 08, 2021, 11:40:04 PM
Look for something with a bigger Safe Operating Area (SOA).

I agree. :tu: The power devices in my last amp build were way over speced on current and voltage.


Thanks for the advice!!   I should probably spend more time finding a better heat sink and fine tune the vbe multiplier instead of shopping for more transistors.. only blew one pair of outputs so far...  got my pnp/npn mixed up for instant death.. lol.   Since then I've been playing full blast and going a little more deaf every day!! 


Never lose sight of the fact this is just a guitar amp, not lab equipment.   And a lot of those audio discussions are from hifi people whose goal is an amp flat from almost DC up to radio.  Totally not needed even for them and certainly not for guitar amps.