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Old 8th January 2017, 08:39 AM   #1
GJB is offline GJB  Australia
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Default Solid State version of a VOX AC-30 Guitar Amp

Hi people... I would like to get a few comments please...
Some time ago I started building custom Guitar Amplifiers... not because I am a Muso... but rather because my (then teenage) son was.

Fast forward 15 years...

I have most recently completed a few different versions of a Solid State Guitar Amplifier modeled on the (classic) VOX AC-30. Its key attributes are:-
1. Does not use ANY Integrated Circuits (in the signal path)
2. Does not use ANY bi-polar transistors
3. Uses FET semiconductors throughout ... JFET’s (small signal) and specialist MOSFET’s (power)
4. Amp architecture / signal path / tone controls based on the (classic) VOX AC-30 amplifier
5. Uses an Output Transformer... like ALL Valve Amps... which provides significantly tonal differences to direct-coupled circuits
6. Output Damping factor of transformer is much “looser” than solid state amps
7. Lower damping factor substantially changes the dynamic characteristics of the speakers
8. Is switchable between Class A(?) (like the VOX AC-30) and Class B for more “conventional” operation. This provides some tonal differences liked by musicians (Clean / Crunch)

While I could write lots more I will perhaps wait for any specific questions and/or general interest first.

Thanks
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Old 9th January 2017, 08:55 AM   #2
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I would love to hear more. I've daydreamed about doing a lot of the same things you did, particularly the JFET/ MOSFET/ output transformer thing. Also keeping away the large amounts of negative feedback found in most integrated opamps and power amps, for more progressive clipping behaviour.

You might also consider posting to the "Hundred Buck Amp Challenge" thread in this forum. A lot of oddball and unusual guitar amp ideas were discussed on that thread.

Elsewhere on the Instruments & Amps forum, things tend to be much more conservative, and you tend to find mostly questions about repairing mass-produced guitar amps and other electronics. Love for novel solid-state guitar amp innovations might be a bit thin on the ground.

One reason I have an interest in a decent-sounding solid-state guitar amp, is that I have a friend with a physical impairment that makes it hard for him to handle anything heavy. For some time I've been wanting to build him a decent-sounding guitar amp.

It needs to be inexpensive (because of my current financial situation, and his) and solid-state (to keep weight down). I've been thinking "JFET / MOSFET" for some time, but haven't really made a start on anything other than the housing.

-Gnobuddy
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Old 9th January 2017, 10:21 AM   #3
GJB is offline GJB  Australia
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Hi Gnobuddy... sounds like we are on parallel paths here... albeit that I might be more advanced! :-)
My design outlook was, as you have suggested, to get away from the "op-amp" standard design and I have, as far as possible, used the same "architecture" as the AC-30 - same number of stages; same type of stages; no negative feedback; scaled component values to match operating voltages / currents; same tone control network; etc; etc.
I have had a few Musos try it and they seem impressed. It definately has its own "sound"... rather than connecting your guitar to your hi-fi system!
As for your requirements of light weight and low cost... that might be a problem!
1) The use of an output transformer adds weight. Similarly I have used an iron-core mains transformer rather than a toroid... or even a switch-mode PSU, and,
2) The transformers add to the cost... as do the FETs - particulalry the output MOSFETs which need to be lateral (audio) devices - rather than the el-cheapo switching types.
When I get a chance I will have a look at the "Amp challenge".
Happy to continue the conversation in the interim! :-)
Cheers
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Old 9th January 2017, 09:29 PM   #4
JohnDH is offline JohnDH  Australia
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That sounds like a great project. Im sure youve seen what they have done at runoffgroove.com with modelling tube amps with fets? Though I dont think they went right through with a power amp with OT. Ive built some stompboxes that use jfets for crunch and they can sound quite convincing if setup right..
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Old 10th January 2017, 02:35 AM   #5
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Quote:
Originally Posted by JohnDH View Post
I'm sure youve seen what they have done at runoffgroove.com with modelling tube amps with fets?
John, that website, and a few clips of surprisingly good-sounding Runoffgroove pedals I found on You Tube, was one reason I wanted to experiment with JFETs in a guitar amp.

I had been kicking around some theoretical reasons, too, before I ever stumbled across Runoff Groove. In a nutshell, I realized that most of the problems with typical solid-stage guitar amps come from the use of BJTs. With BJTs comes a huge excess of ugly sounding distortion, a huge excess of voltage gain, and a huge excess of negative feedback to tame the ugly distortion.

That recipe works wonderfully well for Hi-Fi, but when you apply it to a guitar amp, the cleans are sterile and "too clean". And if you try to overdrive it, it instantly goes from "too clean to be any good" to "so harsh that it's really nasty".

So the obvious starting point for a better analogue solid-state guitar amp seemed to be to go with discrete JFETs in the preamp and MOSFETS in the power amp. I was pretty sure that this would make things better, but would it make a good amp, or just a "less awful" amp?

Reading between the lines, I think GJB had/has come to some of the same conclusions independently, and I'm hoping he's about to share his answer to that question with us.

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Originally Posted by JohnDH View Post
I've built some stompboxes that use jfets for crunch and they can sound quite convincing if setup right..
Some years ago I was quite impressed by some clips of a pedal called the "FET Dream" by C&E pedals. It was an expensive pedal, so I never seriously considered buying one, but it did serve as a demonstration that FETs at least could sound good.

-Gnobuddy
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Old 10th January 2017, 03:08 AM   #6
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Originally Posted by GJB View Post
Hi Gnobuddy... sounds like we are on parallel paths here... albeit that I might be more advanced! :-)
I haven't even got off my backside and actually built anything, so yes, absolutely, without a doubt, you are far more advanced!

Quote:
Originally Posted by GJB View Post
It definitely has its own "sound"... rather than connecting your guitar to your hi-fi system!
Exactly!

I have a solid-state amp intended for electro-acoustic guitar and vocal microphone, or as a small monitor or P.A. for keyboards, etc. It's designed to be very clean, and as "Hi-Fi" as the manufacturer could make it, given limitations on price and intended use.

I also have a (valve) Princeton Reverb reissue. An attempted (factory) copy of a vintage valve guitar amp that gained a solid reputation for good-sounding, "valvey" clean tone.

It is interesting to plug the same guitar into each amp in turn, and listen for similarities and differences. Once I dial in a similar EQ curve on both amps, guitars with very low-output pickups actually sound very similar through both amps.

But turn up the signal from the pickup, and you can hear the subtle "valvey" harmonics start to add shimmer and richness to the sound.

My friend plays rhythm guitar 90% of the time, and attempts to play kinda-sorta bass lines with his (non-bass) guitar the rest of the time. For his style of playing, he doesn't need an accurate AC-30 emulation. But the little Roland Microcube he's using now has that too-clean solid-state sound, combined with a speaker the size of my palm stuck in an open-back cabinet with a deep, deep bass response reaching all the way down to...800 Hz or so.

What I'm hoping to create for him is a better bass response, and richer clean tone (i.e., several percent of second harmonic distortion, and maybe a smattering of higher order stuff too.) The Microcube sets the bar for clean tone and bass response very, very low - so it shouldn't be too hard to do a bit better.

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Originally Posted by GJB View Post
As for your requirements of light weight and low cost... that might be a problem!
I would love to see as much of your design as you are willing to share, but I won't attempt to duplicate all of it for the project I have in mind. Corners will have to be cut, otherwise it won't happen at all.

I'll just have to be as smart as I can in terms of which corners I can cut without sounding as bad as the Microcube!

-Gnobuddy
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Old 10th January 2017, 06:28 AM   #7
GJB is offline GJB  Australia
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Default More on FETs

Hi guys... I haven't had a chance to read your info and links at present... but will do asap.
In the interim a couple of thoughts on FETs. Firstly I noted that John's VOX box has variable resistors in the FET drains. Not a bad idea since one of the issues with FETs is that their parameters... even of the "same" type... can vary widely. Some FETs types are suffixed with Y, GR and others to try to narrow this down. Even those still vary.
However I have taken a different approach.
I tested the INDIVIDUAL Vgs and Igs of every FET I used and then used the formulas described in an article I read about how to make a FET sound like a (triode or pentode) valve. I will post a link to this when I can (re) find it.
I have not used diode limiters on the FET gates... but I am assuming this was done primarily get the over-drive characteristics?
If anyone is interested I could probably post my schematic... and talk about the output MOSFETs... and its associated transformer that I have used???
Cheers
PS... I have re-thought the $100 challenge... assuming this is in US$ it could probably be done. I work in AUS$ so that would be $140 currently! :-)
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Old 10th January 2017, 09:07 AM   #8
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Originally Posted by GJB View Post
...one of the issues with FETs is that their parameters... even of the "same" type... can vary widely.
I've found that using dual (plus and minus) supply rails helps a lot with this. The gate is tied to ground with a large value resistor, the source to a negative rail via a source resistor, the drain to the positive voltage via a drain resistor in the usual way.

The idea is that Vgs might vary, say, from -1V to -3V from device to device (a huge and unacceptable 300% variation). But if the source resistor is fed from, say, a (-12V) rail, the voltage across the source resistor varies between 13 and 15 volts; that's 14 volts plus or minus about 7%, an entirely acceptable variation.

This approach works quite well to set the bias current roughly where intended. But I don't know how well it does in terms of sounding like a triode.

I've read at least one article on biasing a JFET to have a similar distortion characteristic as a triode. ("Fetzer valve"). I've been a bit skeptical, because, as far as I know, an actual triode doesn't have a three-halves law once you add an anode load resistor to it! The 3/2 law only applies when the anode is held at a constant positive potential.

But if you're getting JFETs to sound enough like an AC-30 to impress guitarists, that's no minor feat!

Quote:
Originally Posted by GJB View Post
If anyone is interested I could probably post my schematic... and talk about the output MOSFETs... and its associated transformer that I have used???
I'm interested. I'd love for you to do those things.

Quote:
Originally Posted by GJB View Post
PS... I have re-thought the $100 challenge... assuming this is in US$ it could probably be done. I work in AUS$ so that would be $140 currently! :-)
The original challenge began some years ago (2011, I think), and eventually petered out to an unplanned end of sorts. But the thread was so full of good ideas, that some of the regulars kept it going, long after the original challenge was over and done with.

The new intent of the thread was, more or less, to explore interesting guitar amps built on a fairly small budget. No more $100 constraint if you want to go over that. No more "no solid state in the signal path" constraint once imposed by the "valve purists". Just good clean guitar-amp fun.

-Gnobuddy
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Old 10th January 2017, 10:16 AM   #9
JohnDH is offline JohnDH  Australia
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A dual supply is a neat idea. In my builds, I was just trying to go for the best pedal sound in a single 9V format. I tried a few different jfet models, particularly J201 and MPF102, but settled on 2N5457 as the sweet spot. But as you say, they are all wildly inconsistent, though out of my 100 pack, most have a Vgs in the range around 1.5V to 1.9V, which seems to work well with guitar signals and a simple common source circuit.

One thing I did find, is that the softness of clipping as the jfet is pushed is related to how much current is running through it, and the more that runs through at a given bias voltage, the softer the clipping. I bias them to about 5V at the drain, using typically about a 15k drain resistor to get a tone that I like. The source resistor is whatever needs to go with it for each individual jfet to bias it to a 5V drain.

I tried a 'Fetzer valve' build, but didn't find it very interesting.I tried a few of my own designs, some with tone control and some without.

This is the simplest and the last one I did, with two class A stages and a buffer. In the final version, there is a volume and a gain pot, and as gain is increased, the extra dB's are added progressively to the low mids and highs. There is a sound clip there too:

BlueJuice - an Overdrive using JFETs | GuitarNutz 2

I'd also be very interested in your AC30 build.
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Old 10th January 2017, 07:55 PM   #10
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Originally Posted by JohnDH View Post
...the best pedal sound in a single 9V format.
That makes perfect sense from the point of view of practicality. Just about every electric guitar player has other 9V pedals, and very likely, a 9V power supply for them.

I do feel that the 9V legacy is sometimes a technical limitation, though. Too bad early guitar pedals didn't settle on +/- 15 V, like early opamp audio circuits did!

Quote:
Originally Posted by JohnDH View Post
I tried a few different jfet models, particularly J201 and MPF102, but settled on 2N5457 as the sweet spot.
I'm out of luck there, I have a double handful of one of the "J" series (J211 or J212, I think), but no 2N5457s.

Discrete JFETs are almost an extinct species now. Supplies are falling and prices are soaring. I think we can expect them to virtually disappear from the market within the next few years.

Quote:
Originally Posted by JohnDH View Post
One thing I did find, is that the softness of clipping as the jfet is pushed is related to how much current is running through it, and the more that runs through at a given bias voltage, the softer the clipping.
So a bigger Idss produces softer clipping? That sounds like a very useful thing to know!

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Originally Posted by JohnDH View Post
Some very nice sounds there! Thanks for sharing!

-Gnobuddy
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