Hi.
I've drafted up a class A MOSFET amp that I'm going to experiment on, with the aim of getting a wall-wart powered, tube-sounding amp capable of about 2W to 8W power.
My questions are: 1. Is there a demand for such an amp?
2. Am I making any mistakes in the design?
thanks for any comments or help.
Here's the design so far;
http://members.optusnet.com.au/~jethro.dog/mypic156.jpg
1. ABSOLUTELY! if you can get MOSFETs to sound like tubes, you'll make friends with all kinds of amp builders!
2. I am by no means an expert... but it looks like it should work. i don't know anything about MOSFETs (used them, but never really learned about them) or speaker loading (nothing in that section yet) so you should probably wait for actual advice. :wink:
you should make a different output optional! a push-pull setup, in case we want it louder! :)
Quotein case we want it louder!
The main problem with higher power is that wall-warts aren't feasible as power supplies.
In fact, I have wondered whether the MOSFET should have a Vcc of 15V, so that (a) it draws less current, and (b) it clips harder. For some reason, a heavily clipped 5W sounds louder to me than a clean 10W. RG has commented on this, too.
isn't that what the Tubeworks amps were trying to accomplish?
How many watts do you think it will produce?
Also, note true class A design!
looks workable.
i would regulate the MOSFET separately.
that way, when drawing large currents, won't affect the
input fet biasing. The OA probably would B more tolerant of
supply fluctuations, but, wouldn't hurt. So, i say run 2 regs.
U have potential for overdrive in input stage *and* output "stage".
i've been thinkin bout just a single MOSFET, TO3, powertranny
for a fuzz box. I gotta bunch of 2N6766's that i've been dying 2 try.
stayMOSed
tone
I looked at the Tubeworks amps. They are high-power designs, and people seem to like them. I'm aiming for something slightly more powerful than a Ruby (0.5W) or the excellent (but expensive to build) tube amp the Firefly (~1W).
As for power output, the MOSFET is only a voltage follower for the op-amp. The op-amp output can swing nearly rail-to-rail (+6V, -12V). If the MOSFET manages to get within 3V of the rails, it might swing (+3V, -9V) into 8 ohms, for about 8V RMS x 1 amp, or 8W RMS. Maybe??
I know very little about MOSFETs and power devices, so I'm making some uneducated guesses here.
Oh, yeah, credit is due to others that inspired the "components" of the amp. The input section is based on a Stratoblaster (General Guitar Gadgets). The tone section is from a big muff, with values substituted that provide scooped mids. The buffer for the tone section is kinda like a tubescreamer "drive" section, but with less drive and biased so that it turns the MOSFET on. The MOSFET voltage-follower design is mine (well, something had to be!).
Thanks toneman. Yeah, 2 supplies sounds good. I'll run the JFET and op-amp at 18V, the MOSFET at 15V. That'll lower the power demand, too.
See:
http://www.passdiy.com/pdf/zenamp.pdf
http://www.passdiy.com/legacy.htm
http://www.passdiy.com/amps.htm
and
http://sound.westhost.com/project36.htm
Thanks RG.
Also found this at the ESP site, concerning using a MOSFET as a voltage follower for audio output. It seems promising.
http://sound.westhost.com/project83.htm
As he says"How does it sound? Wonderful, regardless low or high volume. Entire spectrum from bass to high is perfect. It only needs a good preamplifier."
The question that I ask myself now is whether the zener-Darlington combination froviding resistance on the source of the output MOSFET is better than using a fixed resistor. It seems great in theory, but would it be better in practice? RG? Anyone?
If you can get hold of a back issue, Elektor had a 'FET Amp with Valve Sound' feature in the 11/2003 issue.
MOSFET output in a Mu-type configuration. Never built it, but interesting schematic (liked the power arrangement) 8)
There's a schematic of a mark I type circuit which used a choke to help shunt DC past a speaker directly connected to the drain of a single power MOSFET. Personally, I wouldn't chance it, but clever idea (about as simple as it gets)...
Isn't there a problem in clipping the OA rather than the MOSFET?
(I'm not familiar with discrete poweramps...)
Or is it the MOSFET that really gives the harmonics?
QuoteMOSFET output in a Mu-type configuration. Never built it, but interesting schematic
Yeah, I've seen those schematics in a couple of places. One is the Zen amp that RG mentions above. Some people like it, others dislike it, but most are referring to it in audio/hi-fi terms. I might build one and see how it behaves when driven hard.
QuoteIsn't there a problem in clipping the OA rather than the MOSFET?
(I'm not familiar with discrete poweramps...)
Or is it the MOSFET that really gives the harmonics?
That was one reason why I was going for a voltage follower. The MOSFET would start clipping (gracefully, one would hope) well before the op-amp. Hopefully, any nasty op-amp clipping would not be reproduced in the ouput (ie the MOSFET can't swing its output nearly as wide as the op-amp).
I'm going to incorporate some changes and re-post a circuit later today.
cheers
Run at low voltage, those single transistor class a amps are not likely to ever run higher than about two watts. You'll need a pretty hefty power supply capable of providing lots of current and good heat sinking.
Here are a few more similar amps:
http://www.rason.org/Projects/transaud/transaud.htm
ISTR a Zen amp of some type that used a light bulb as a current source. Kind of a neat, bizarre idea. Would make a nice pseudo-tube conversation piece.
QuoteRun at low voltage, those single transistor class a amps are not likely to ever run higher than about two watts
That's what I was aiming for. Low power, maximum tone (given that it's solid state).
A 24V 1A wall wart regulated to 18V will give me 18W of heat to dissipate in total. I expect up to 4W might appear in the speaker coils, and about 5W in the MOSFET, 3W in the Darlington pair and 2W in the load resistor (new schematic using a constant current source appearing soon). So yes, the heatsinking will be considerable!
thanks for the comments, Tim
Here's version 2 for those intersted.
Unfortunately it is more complex (now 3 output semiconductors), but I *think* it is significantly better.
I'll start working on pcb tomorrow.
http://members.optusnet.com.au/~jethro.dog/mypic157.jpg
Quote from: brettQuoteMOSFET output in a Mu-type configuration. Never built it, but interesting schematic
Yeah, I've seen those schematics in a couple of places. One is the Zen amp that RG mentions above. Some people like it, others dislike it, but most are referring to it in audio/hi-fi terms. I might build one and see how it behaves when driven hard.
QuoteIsn't there a problem in clipping the OA rather than the MOSFET?
(I'm not familiar with discrete poweramps...)
Or is it the MOSFET that really gives the harmonics?
That was one reason why I was going for a voltage follower. The MOSFET would start clipping (gracefully, one would hope) well before the op-amp. Hopefully, any nasty op-amp clipping would not be reproduced in the ouput (ie the MOSFET can't swing its output nearly as wide as the op-amp).
I'm going to incorporate some changes and re-post a circuit later today.
cheers
Still, after the first clipping of the FET (let's say at 80% of the OA clipping) the OA will clip itself, resulting in a "double clipping" (like a fuzz before a Tube screamer). I would instinctively put diodes around the OA that have offset voltage a bit higher than the FET clipping point. This will result IMO in a sweeter high gain setting.
And so, the complexity grows and grows, until we have yet another solid state amp to ruin our tube-world. Lol, just kidding. I really like the idea though.
Any sample sound clips anytime soon?
------------
Jimbo
Something I'd like is a Small Amp, that sounds better than my Little 386-er that I use...it' ok...
Brett, I appreciate the interest and everyone who's helping out this thread !!! ...And will probably begin construction of whatever it is ya'll settle on here soon.
Mosfet outputs...some mfr. was making "Mosfet' amps for a time...wasn't that Marshall ? They were all the rage ... I never tried one though...
Quote from: petemooreBrett, I appreciate the interest and everyone who's helping out this thread !!! ...And will probably begin construction of whatever it is ya'll settle on here soon.
Same thing here :D I hope i can get the output stage semis (or a suitable replacement) around here :twisted:
Quote from: JimRaydenAnd so, the complexity grows and grows, until we have yet another solid state amp to ruin our tube-world. Lol, just kidding. I really like the idea though.
Any sample sound clips anytime soon?
------------
Jimbo
Well, even when just kidding, I kinda DO LIKE searching for non-tube electronics. Because let's be realistic: We don't need to search any more for tube electronics: we know everything.
Maybe one day one can say: wow! what a nice holy grail
sound! realizing that there's no tube sound at all.
The Fuzz face is the most known distortion device in the world: just try making a fuzz face sound with only tubes! Good luck I would say.
No univibes, no fuzz faces, no treble boosters, ...
That would be quite a step back.
:?: :shock: :!: ...
When asked 'Tubes Or Transistors" I reply "I Gotta Have BOTH !"
Quote from: StebenQuote from: JimRaydenAnd so, the complexity grows and grows, until we have yet another solid state amp to ruin our tube-world. Lol, just kidding. I really like the idea though.
Any sample sound clips anytime soon?
------------
Jimbo
Well, even when just kidding, I kinda DO LIKE searching for non-tube electronics. Because let's be realistic: We don't need to search any more for tube electronics: we know everything.
Maybe one day one can say: wow! what a nice holy grail
sound! realizing that there's no tube sound at all.
The Fuzz face is the most known distortion device in the world: just try making a fuzz face sound with only tubes! Good luck I would say.
No univibes, no fuzz faces, no treble boosters, ...
That would be quite a step back.
Yea, transistors totally rock the stompbox world. And actually the best digital devices out there have the exact copies of tube sounds. It's just... it's not the sound. To me, I like to know that my guitar signal goes through those big hot glass bottles and sound really really good. I don't have anything against transistor amps, maybe I'm just a lil' old-fashioned (although I was born well after the invention of transistor :P). While I'm not too fond of transistor amps, there is one thing I can't stand - digital modelling amps. (You might think this is totally senseless but this is me.)
Well, you can give me a digital amp and tell me it's a tube amp and as long as I can't hear any difference, I'll be happy with it.
It's a psychological stuff. I love tubes. :) Tubes love me. :)
But yea, I think I'm going to build your amp once it's ready, and make an A/B test with my tube amp. Maybe I'll change my way of thinking.
Oh, and if it's possible, and if you haven't thought of that yet, maybe you could add an OT to it. Or at least an OT simulator.
----------
Jimbo
Hi.
QuoteI love tubes. Tubes love me.
Me too. That's why I'm aiming for a GOOD, tube-like, cheap solid state amp. FETs and MOSFETs are the closest things we have in solid state to valves. In fact, their operation is quite analogous to valves, so I don't see why we can't get some reasonably valve-sounding amps if we use them. (Though someone hear might explain that I'm misguided).
The other advantages are that it is low-cost and low-voltage. I costed the parts for my MOSFET design last night, and I think this would cost no more than $80 (AUD, =$US50) as a head, including the cost of a plugpack ($20) and cheap case ($15). Anyway, it's Friday, and a long weekend (Labour day), so I'll do my best to put this thing together, see how it sounds and get back to you all.
thanks for the comments.
Quote from: JimRaydenQuote from: StebenQuote from: JimRaydenAnd so, the complexity grows and grows, until we have yet another solid state amp to ruin our tube-world. Lol, just kidding. I really like the idea though.
Any sample sound clips anytime soon?
------------
Jimbo
Well, even when just kidding, I kinda DO LIKE searching for non-tube electronics. Because let's be realistic: We don't need to search any more for tube electronics: we know everything.
Maybe one day one can say: wow! what a nice holy grail
sound! realizing that there's no tube sound at all.
The Fuzz face is the most known distortion device in the world: just try making a fuzz face sound with only tubes! Good luck I would say.
No univibes, no fuzz faces, no treble boosters, ...
That would be quite a step back.
Yea, transistors totally rock the stompbox world. And actually the best digital devices out there have the exact copies of tube sounds. It's just... it's not the sound. To me, I like to know that my guitar signal goes through those big hot glass bottles and sound really really good. I don't have anything against transistor amps, maybe I'm just a lil' old-fashioned (although I was born well after the invention of transistor :P). While I'm not too fond of transistor amps, there is one thing I can't stand - digital modelling amps. (You might think this is totally senseless but this is me.)
Well, you can give me a digital amp and tell me it's a tube amp and as long as I can't hear any difference, I'll be happy with it.
It's a psychological stuff. I love tubes. :) Tubes love me. :)
But yea, I think I'm going to build your amp once it's ready, and make an A/B test with my tube amp. Maybe I'll change my way of thinking.
Oh, and if it's possible, and if you haven't thought of that yet, maybe you could add an OT to it. Or at least an OT simulator.
----------
Jimbo
Jim, I'm not questioning Tube amps (at certainly not power amps)! :wink: I was simply argueing that solid state design is the one to be thinked through and redesigned in order to achieve even newer tones, not just tube emulating.
Tube amps are what they are since half a rock century: the best in simple tone. By the way: this IS stompbox world and although it's funny to see some experiments: Tubes do not belong in small metal cases.
I`m allowed to break rules, and love to bring stompboxes back to their sound roots :P
I never said anything wrong with solid-stateness either. As soon as you get the amp ready and tested, I'll build it. If it sounds good, I'm gonna stick it into a cab and buy a celestion for it. And start using it for home practice. And then apologize to the gods of solid-stateness, for I have had the wrong impression of them.
Oh, and Steben, I think exactly the same. Transistors are good for making new tones, etc. But most of it can be done with tubes also. For example take a look at puretube's work, wich I admire. :)
-----------
Jimbo
thanx for the flattery, Jimbo :)
upon popular demand I`ve re-opened my sites...
I just found this ol' thread again. I've been thinking about this project. Are there any threads of this one completed? I'd sure like to hear that.
---------
Jimbo
I've been intrigued by this schem for a couple years....wishing someone might try a build & give a review. It sure looks cool to me...might be a bit beyond my skills...but not TOO far.
http://www.redcircuits.com/Page52.htm
Hi.
The schematic above is for a fairly conventional small amp. Circuits such as this have been largely replaced by single-chip devices. Although they're no without their problems (such as thermal shutdown mid-riff), they are probably the way to go for smallish SS amps (5 to 10W) where tone isn't a big factor.
I'm still looking at using a MOSFET as a voltage follower in a small class A amp. Progress is slow because there's too many other things on...
cheers
I guess a non-FET design won't do the job. If you get distortion from the preamplifier (in this case the two JFETs) why not use a IC-pcwer amp indeed.
I really think the design of a MOSFET-including power amp is still usefull.
http://www.elektor-electronics.co.uk/Default.aspx?tabid=28&art=50483
Did anybody try this one???
The schem looks like high power Jack Orman minibooster
The Elektor design is well up my 'to-do' list, but will take a little design time since I discovered that it would cost me £25 to get the PCB from Elektor (including postage). NO WAY! :evil:
Hey Squidsquad,
I was looking into that design as well, and emailed the designer about it. He put me on to an Australian company who are marketing it. For what it's worth, go to their site :
[url]http://www.ledeaudio.com/audio_kits.html
and scroll down until you see "10w guitar amp". They don't seem to have any sound samples though, or any more info than the designer's site.
Just thought I'd give this a bump and ask brett if there was any progress?
Cant see the schematic "404 Not Found"... Anyway...
If I had to build a mosfet/fet amp I will follow http://runoffgroove.com ideas ( eighteen, professor tweed ).
I'll bet to a 2 or 3 mosfet/fet preamp and then the power amp in A or AB config, just like any tube amp.
See http://www.18watt.com . I'll just adapt this simply design to be used with mosfets/fets.
No ICs or silicons transistors.
But I'll use high voltage, 100V at least... And I'll buy a good death insurance... just in case ;)
Why? Imagine a photo of the signal into a tube at 110V. Then resize it down to 9 - 30V, as in any IC or transistor. You'll loose the fine details.
mac
http://www.geocities.com/guitarfxs
I think you're on the right lines there.
What I want to see is a super simple mosfet power amp mated to one of those runoffgroove 'emulators' or better still a Plexi front end.
Or even have a multi channel input stage, with a doctor tweed and a Plexi. That would be brilliant if you ask me. 1/2 watt would be just fine through a 1x12. Also, i'd like it to run off 18-24v so as not to kill me for being careless.
Got that? Ok, go make me one ;)
Quote from: mac on October 29, 2005, 05:43:34 AM
But I'll use high voltage, 100V at least... And I'll buy a good death insurance... just in case ;)
Why? Imagine a photo of the signal into a tube at 110V. Then resize it down to 9 - 30V, as in any IC or transistor. You'll loose the fine details.
mac
http://www.geocities.com/guitarfxs
That doesn't make any electronical sense to me. You have no idea how tiny the fine details can go if they do exist in your signal. If you have an analogue signal, you'll have to take it down to electron sizes -in both voltage and frequency- if you want actual blur in your sound. Well that's my vew at least. It's the actual clipping process that we're after, the preservation of it is simple. :)
Though correct me if I'm wrong.
---------------
Jimbo
just my two cents. the marshall mosfets were actually quite good. not100% by any means but they still had the feel of it cause you had to crank it to get it to respond right.
jimbo.
Mosfets are quantum devices. It was Mr. Werner Heisenberg :icon_eek: and his uncertainty principle dE = hdF what gave me that idea at first sight. Calculation involves deviations from the mean value, that's why I'll increase a priori the working voltage.
Anyway, the blurring effect could be imperceptible to our ears... I'll have to do the math.
If you don't see any reply soon is because I was terribly wrong :icon_redface: lol
mac
http://www.geocities.com/guitarfxs
More on why high voltage supply... now in english :icon_biggrin: (should be in spanish for me...)
I've been GOOGLEing and reading a lot of technical reasons why it is better to use high voltage supply on very specialized devices, among them a better freq response, although at values far away from the audio range. Others are a better hum-to-voltage ratio, more headroom, better slew rate, etc.
But I read a paper that may be used as a guide: :icon_idea:
http://www.vtl.com/pages/whytubes.html
As I have to see it to believe it, I built a simple 2-stage preamp using BF245. First at 6V and then redisigned for 25V. Yeap, a very subtle improvement. Dont ask me why! :icon_confused:
Hope an elect. eng. may throw some light here.
mac
http://www.geocities.com/guitarfxs
the mosfet preamp i was working on was a high voltage tube style one. based off an article that pops up around here from time to time. i liked that. but i got stuck in a rut i ended up with three channels[keep in mind this is over 6mos ago and i havent' touched it since] the first channel was completely clean and it had ample headroom run in the 50+volt range. i ran the medium gain channel [what i called the acdc channel] on 24v and i ran the ultra smack high gain channel on 6v or so. i liked the sounds it produced but there was no dialing back the gain on the dist channel it was on full tilt.
Clean Channel was Irf820 or 520 whatever came from fairchild as a sample pack. and i used bs170's for the medium gain channel and high gain was originally the 170's but i opted for a different one. which i will have to dig up a no. on it. i haven'ttouched it since i blew the cheap tranny that was in there. i plan onbreaking it out and redoing it wheni move
Quote from: Penguin on October 30, 2005, 04:32:58 PM
the mosfet preamp i was working on was a high voltage tube style one. based off an article that pops up around here from time to time. i liked that. but i got stuck in a rut i ended up with three channels[keep in mind this is over 6mos ago and i havent' touched it since] the first channel was completely clean and it had ample headroom run in the 50+volt range.  i ran the medium gain channel [what i called the acdc channel] on 24v and i ran the ultra smack high gain channel on 6v or so. i liked the sounds it produced but there was no dialing back the gain on the dist channel it was on full tilt.Â
Clean Channel was Irf820 or 520 whatever came from fairchild as a sample pack.  and i used bs170's for the medium gain channel and high gain was originally the 170's but i opted for a different one. which i will have to dig up a no. on it. i haven'ttouched it since i blew the cheap tranny that was in there. i plan onbreaking it out and redoing it wheni move
Want to share the schematic so far ? 8)
So? Has there been any recent developments in single-ended mosfet technology? :)
Some hints
Look at a PRS solid state amp. I have one
Look the the fender patents from the 70's there was a cool one IIRC using transistors.
Read R.G.'s Vox page
Now read all you can on pentodes and then how crazy the math gets with a pentode PP or SE and an output transformer and clipping.
look at the curves for triodes, BJTs and mosfets
Also read the "Art of Electronics" there is a circuit fragment in there.
I have a cool se mosfet circuit I built no big deal.
There is an older book "blackburg" series? about mosfets and there is a 4 watt SE Mosfet amp circuit Late 70's book IIRC. I think I have it somewhere in the house..