Questions and rumbles about solid state amplifiers

[Gus]

Hi,
so the JLH version did sound ok... but not completely happy with it. It had some annoying fuzz/fizz/buzz when overdriven... that could be fixed, but if it is there why bother taking it out!  So I've turn to the most simple amp that I could think of, a mosfet and a light bulb at the drain (as PRR suggested). It sounded also ok, as I recall from my previous experiments. I've already seen some of the amps made by Nelson Pass (delite amp is the mosfet+bulb version) and specially the Zen amp that uses a active current source. Here is my hack on it (biased at 1A):



So, how does it sound? It is the most amazing amp I've build. It stable as a rock, no fuzz/fizz/buzz, it has an incredible warm sound, so amazing that I've plug it in to a hi-fi set of speakers (what a sacrilege!) and still is amazing! As R.G. point out earlier, there is no such thing as a specific guitar amplifier. It does get hot, much hotter than the JLH.

I found this topology in  "Design of VMOS Ciruits" book from the 80's (the same book that Jack mentions on his BS170 booster webpage) but Pass's patent for a very similar topology is dated from 1998. Go figure?!?

Cheers.

There is something you need to add to the sim.  The source impedance "feeding" it.  Power mosfet have gate to drain and gate to source capacitance. So some of the "warm" sound can be the low pass frequency.  You might want to add a small series input resistor to the gate close to the mosfet.

Here is something to think about .  Musicman amps, some were IC and transistor even to the output stage(class b? push pull cascode with a transistor in the bottom part of the cascode)  I think a lot of the tube sound is the output transformer output tube interaction.

Also jimi had a post with a cool Kay transistor amp schematic .  SE transformer driving a push pull transformer output.  R.G. has written about the VOX solid state amps.

[tca]

> There is something you need to add to the sim.  The source impedance "feeding" it.  Power mosfet have gate to drain and gate to source capacitance.
> So some of the "warm" sound can be the low pass frequency.
I've done that also, but did not show it... I've also substitute the upper CCS with a car 6.8Ohm light bulb and it works nicely. It also needs a decent preamp, which I also have. I'll post the complete schematic in a few days.

> You might want to add a small series input resistor to the gate close to the mosfet.
Done that also.

> I think a lot of the tube sound is the output transformer output tube interaction.
That is what almost everybody says. I have to build a amp with an output transformer to see what that *really* means, my biggest problem is where to find one with the right specs.

> Also jimi had a post with a cool Kay transistor amp schematic.  SE transformer driving a push pull transformer output.  
I have a lot of schematics, from late 60's and 70's, with that kind of topologies with Ge and Si devices, as always, the problem is where to find the transformer.

> R.G. has written about the VOX solid state amps.
Already read that, thanks.

Cheers.

[tca]

Here goes my light bulb amp



I've been playing with it for quite some time now. It does get hot (I'm using a 10cmx10cmx3.5cm heat sink, it does not take the skin off my fingers!) and has roughly 1.5A idle current. No quite certain about the preamp I've made... But the power amp part is done.


Cheers.

P.S.
It does distort nicely when overdriven.