Mosfet Amp [Simple

[petemoore]

http://www.vt52.com/diy/diypage/aren/fet/aren.html
  This looks simple enough.
  I'd want a different 'left side' [preamp].
  The amp itself looks like it could be hacked from the schematic...
  Using the 8.2 ohm resistor from source to ground, then biasing the gate...somehow...maybe just a resistor to ground or a more workable bias arrangement, like a variable divider across the rails, a large resistor from the wiper of the divider pot to gate...since there's no current to the gate I could use smaller rated resistors [1/4w ?]
  Then use a Mosfet Boost or whatever to drive the gate...
  Looks too easy, other than the heatsink an hour or less of wiring...we'll see I guess. 

[Steben]

http://www.vt52.com/diy/diypage/aren/fet/aren.html
  This looks simple enough.
  I'd want a different 'left side' [preamp].
  The amp itself looks like it could be hacked from the schematic...
  Using the 8.2 ohm resistor from source to ground, then biasing the gate...somehow...maybe just a resistor to ground or a more workable bias arrangement, like a variable divider across the rails, a large resistor from the wiper of the divider pot to gate...since there's no current to the gate I could use smaller rated resistors [1/4w ?]
  Then use a Mosfet Boost or whatever to drive the gate...
  Looks too easy, other than the heatsink an hour or less of wiring...we'll see I guess. 

Damn! 4069 inverters as power amp drivers!  Stunning!

[petemoore]

   Yupp and the 'making a present for my son' line too..lol.
  The CMOS were a 'default' choice, an 'it's either these or a trip to the store' type decisions.
  And I'd go CMOS except by default...I don't have 'em here, and I thinka Mosfet'll drive it enough to tell if it's goin', besides I'll probably opt for CCS before it's all over.
  I could probably find an amp to buy, but then what fun would that be?
  For that matter I probably have a buncha amps around here in tuner/amps/preamps I've picked up here and there...I guess I just like to read, then try to comprehend, then try to apply...
  It all pretty much makes sense when I'm reading now, but the CCS equations..adjusting a resistor value at a transistor input to control the current on the output...I'll go read through some more, see if I can find a CCS equation, then try to work with it.

[Steben]

   Yupp and the 'making a present for my son' line too..lol.
  The CMOS were a 'default' choice, an 'it's either these or a trip to the store' type decisions.
  And I'd go CMOS except by default...I don't have 'em here, and I thinka Mosfet'll drive it enough to tell if it's goin', besides I'll probably opt for CCS before it's all over.
  I could probably find an amp to buy, but then what fun would that be?
  For that matter I probably have a buncha amps around here in tuner/amps/preamps I've picked up here and there...I guess I just like to read, then try to comprehend, then try to apply...
  It all pretty much makes sense when I'm reading now, but the CCS equations..adjusting a resistor value at a transistor input to control the current on the output...I'll go read through some more, see if I can find a CCS equation, then try to work with it.

Remember the disadvantage of CCS: bad clipping behaviour. CCS pumps the "useful Watts" by increasing the linear area of the output, very good for Hifi, but for guitar I dare to argue that those linear Watts are not needed. If you need them, just build a single IC power amplifier or a big push-pull tube amp (you achieve them Watts rather by adding tubes than equations ;-) ).

[petemoore]

  Board populated [I used an old keyboard PC about 1.5'' x 5'', had a TC and Vol on it, transistor fit the hole pattern nicely, mounting the PC through transistor to the heat sink / chassis].
  I made gate bias variable w/20k pot, and a 680K [1w resistor] found some 10ohmers [huge] and seriesed them for total 20ohms source to ground...I know this raises the source voltage and reduces headroom, but it's how I'm controlling current [I think]..
  @@ Rate, thanks for the big help, everyone [Steben, Brett Alex, etc.] for showing interest !
  I connected 15v / 200ma supply, speaker clicked, I added a HB pickup as source and could hear the strings through the speaker! Success !
  I have a TS type handy and turned that all the way up to preamp the output section, pretty good, not real loud, not hot either though, [I haven't run the underrated for what I want, I'll get the bigger one out now] WW very long, but the Q and the two 10ohm resistors warmed slightly.
  I think now I'll drop to 10ohm maybe a bit higher if I can find another big one of like 3k3 or so, and add 30V [probably measures a bit higher] @500ma. and see what happens there.
  All is good !! First effort achieved speaker being driven, and sounding 'acceptable', meeting the primary goals.
  I believe I will try a differential pair for the next round, after tweeking this unit some more.

  Edit, My guess it blown transistor after 30v / 500ma went through the 10ohm/source/drain channel, that or the PS died, because it seems to buzz now, I'll try the other PS again, lol.//still buzzing means probably the PS is either connected to the speaker in some fashion or the transistor is still working...well see soon.

[petemoore]

 Back In Service !!!
  This little jobby's driving two 8'' speakers @ 4ohms quite nicely.
  Cheating' on the power supply [from printer does 30v @ 500ma] made this build an afternoon's effort, including the 15v/200ma startup tests, actually the performance was pretty good at 15v, better volume, bass, and less noise for some reason with the bigger supply, probably better filtered, noise is nilch.
  Sounds just like a nice little guitar Mosfet Amp is what it sounds like, Bass is a touch rough, especially when pushed, mid bass on up has a nice sound, very Mosfet Amp sounding.
  Oh, I'm using a 7B EQ pedal, with 7V supply as 'preamp'.
  I have a total of 2.39 I think is what the Q cost. I re-used a small PC Board from a toy keyboard, and put 20k pot as divider [reminds me I should up the 680k gate bias resistor], which allows too much control, but it's easy to find bias, starting from gate pulled toward ground and turning up.
  The transistor and source resistors are getting kinda hot..lol @4ohms and..
   The sound appears to be to have 'low listener fatigue' as Aren Van Waarde stated it would.
  Thanks to Aren Van Waarde for sharing experiences, know how and the design schematic of a very simple, great sounding amp circuit.

[Steben]

Back In Service !!!
  This little jobby's driving two 8'' speakers @ 4ohms quite nicely.
  Cheating' on the power supply [from printer does 30v @ 500ma] made this build an afternoon's effort, including the 15v/200ma startup tests, actually the performance was pretty good at 15v, better volume, bass, and less noise for some reason with the bigger supply, probably better filtered, noise is nilch.
  Sounds just like a nice little guitar Mosfet Amp is what it sounds like, Bass is a touch rough, especially when pushed, mid bass on up has a nice sound, very Mosfet Amp sounding.
  Oh, I'm using a 7B EQ pedal, with 7V supply as 'preamp'.
  I have a total of 2.39 I think is what the Q cost. I re-used a small PC Board from a toy keyboard, and put 20k pot as divider [reminds me I should up the 680k gate bias resistor], which allows too much control, but it's easy to find bias, starting from gate pulled toward ground and turning up.
  The transistor and source resistors are getting kinda hot..lol @4ohms and..
   The sound appears to be to have 'low listener fatigue' as Aren Van Waarde stated it would.
  Thanks to Aren Van Waarde for sharing experiences, know how and the design schematic of a very simple, great sounding amp circuit.

Clean <-> distorted?

[petemoore]

Clean <-> distorted?
  Yes..
  To be continued.
  Right now the cleans are having a 'fiss' sound attached, but I think that may be in the 'pre-amp', an EQ pedal is showing the greatest ability to drive the amp from the selections I have available.
  The distorteds are great !, but heavy bass /w bass definition is not a strong feature of this design.
  Next order of design is to find or design a suitable pre-amp with wide enough voltage swings, and clean or slightly distorted nature.
  Noise w/ WW adapters supplying amp and preamp..the power supply will probably have a tendancy to want to get into the preamp, that is where most of the amplitude multiplication happens.
  I might have to opt for a better supply solution than using a simple WW for the pre.
  All suggestions welcome in the preamp dept. !

[petemoore]

  I should say makes an 8'' er sound real nice, but doesn't push it hard.

[Steben]

Clean <-> distorted?
  Yes..
  To be continued.
  Right now the cleans are having a 'fiss' sound attached, but I think that may be in the 'pre-amp', an EQ pedal is showing the greatest ability to drive the amp from the selections I have available.
  The distorteds are great !, but heavy bass /w bass definition is not a strong feature of this design.
  Next order of design is to find or design a suitable pre-amp with wide enough voltage swings, and clean or slightly distorted nature.
  Noise w/ WW adapters supplying amp and preamp..the power supply will probably have a tendancy to want to get into the preamp, that is where most of the amplitude multiplication happens.
  I might have to opt for a better supply solution than using a simple WW for the pre.
  All suggestions welcome in the preamp dept. !

I would go for my Blue amp suggestions: two triode-biased Fetzer stages with J201 and MPF102, followed by a marshall stack. Harder to bias than tweak it to 1/2V but it's worth it on all "clean" stuff. The marshall stack really can save the bass, but can cut it too for less flabby distorted power amp.

[Stompin Tom]

I imagine something like ROG's Supreaux box would sound pretty good as a pre... I like to put it in front of a SS chipamp I have. But it really only does that one sound...

[petemoore]

  See here's the deal [I think
  It's a source follower, ie, it doesn't really add gain, just makes alotta current to drive the speaker, the preamp is what brings the voltage swings 'up' and where the gain increase of the signal swings occurs.
  I'm figureing @ 9v for a 'preamp', I'm not swinging the gate, and hence the source to swing quite half way...cause...I guess less than 20v [the gate source glass is 20v thick after all] can be applied to the gate, and the source will follow [or try]..probably something like 15v signal swings into the gate would be conservative enough?
  So, I'm thinking that starting with the power supply voltage of about 20v...and a circuit that will use up some of that...with a volume control of course..should put me in the ballpark of what'll swing the gate near it's 'stops'.
  And...that's liable to appear as a Max 1044 @ 18V or split supply with 9v +/-...or some other, more elegant solution.
  Any pages on what 'dude' used to whack together a power supply with scrap junk...I think it's about time I dig through my ol iron collection and take some ohm readings on some coils...build a 'real' PS for once.

[petemoore]

  I put a mosfet gain stage in to drive the source follower/output amp.
  That helped, running the PS @ 15vdc opened up the headroom, alot more clean-ish volume available now.
  This makes sense [that I was hitting the rail ceiling no matter what I tried @9v supply], because the PS of the output stage should swing near rails which are 15v...
  Real nice clean spank with SC pickup as source, next I build a preamp for the left side and add boost/distort to input !

[mac]

This is exactly what I was looking for. A simple SE power amp.
I'd like to see the final schem.
Thanks pete.

mac

[Steben]

This is exactly what I was looking for. A simple SE power amp.
I'd like to see the final schem.
Thanks pete.

mac

I'm curious how curvy the response is of a source follower. Is it also square law - second order as a common source?

[alextheian-alex]

This is exactly what I was looking for. A simple SE power amp.
I'd like to see the final schem.
Thanks pete.

mac

I'm curious how curvy the response is of a source follower. Is it also square law - second order as a common source?

WOW... a CMOS driver... whoddathunkit?!  My CMOS experiments have been limited to oscillators, clippers and switching logic... makes me want to build this up and put a scope on it to see how ti actually responds.

Off-the-top-of-my-head speculation:
I would imagine that the response would be pretty darned linear in the limited range that we need it for guitar amp use.  With a CCS it would be pretty dead linear within reason, but with the resistor source, you are going to have the gm dancing around as the conductance changes (not sure how much), so you'll probably get a more Child's law-esque slope... but i can't even predict it without picking some operating points and running a simulation, but the resistor will be more likely than a CCS to add more harmonics, so it is porbably the more 'tubey' way of doing it... if you don't mind the drop in efficiency.

In comparison to the common-source ZEN style, it LOOKS like you should have lower distortion (maybe not good in this case), but higher input impedance and lower ouptu impedance.  you'll need more voltage swing at the input, and have comperable efficiency.  Those are all just educated guesses based on the topology.

Bear in mind that even though the output stage gain is so low, the input capacitance will be very high, so the preamo output impedance should be well under 8k-10k even to keep the high end bandwidth sufficient for guitar amp use.  But it makes for a convenient place to roll off the highs to simulate a tube amp's output transformer , and you can tweak the output capacitor to simulate the low end rolloff.  And again... if you use a current source, you can up the efficiency signifigantly... by that i mean, approaching 20%.  The premp will have to swing a bunch of volts too.

[petemoore]

This is exactly what I was looking for. A simple SE power amp.
I'd like to see the final schem.
  Pretty simple, I didn't draw anything.
  The S/D channel is used to drive the speaker, the output coming off the source, so set the source resistor...I set it 'high' to start with, simply because I wanted to make sure the transistor...how hot it gets after about 5 min playing..the resistor gets hot also, having two of them dissipates the heat better. So the amp part goes ground>10ohm>10ohm>SourceDrain>V+
  See Geo Updates to mosfet boosters...and I biased the gate using a pot, just about any pot over say 10k, outside lugs across the power rails [Gnd and V+] the wiper getting a big [I used 1meg] that biases the gate, start with the pot turned toward ground, then slowly up until the mosfet turns on..wasn't really too twitchy...see the bias options RG offers.
  Input cap value...choose, then put Huge cap on the output from source...that's about it, power it up and you should be able to hear just enough to find bias, very little gain [if any...surprizing a HB drives the amp to twiddle the speaker, but it barely does...add boosted input].
  Then the booster....can be about any compressor or booster...higher than 9v supply is good, my WW for the amp puts out 20.1v, I decided to limit the booster to 18v [IIRC] but adding a zener diode to the V+ which dropped it a few volts, this way the booster can't quite exceed the Gate/Source max voltage of 20v.
  I put a Mosfet transistor for gain in front of the source follower [speaker amp], @ 18v supply the amp will crank the 8''er, overall improvement in output, better bass [but not for thick stuff, does nice pick harmonic bass string notes..]. For the preamp, a PS filter reduced noise hugely, I used the 100ohm/BFCapacitor filter.
  If you have a decent little speaker to drive, and a WW of 'enough' voltage, this little project is a good one.
  AFas the preamp tone goes, well I have plenty of 9V analog tone circuits, I just needed something with a higher supply voltage to boost the input of the amp adequately for marked performance improvement...18v was just easy for me to get from what I had...20+V and a zener diode...but I would think 12v or 15v might do a good job, 9v 'does'...but only up to a certain volume.
  I know Mosfets have been used for guitar amps [Marshall..etc.], and might like to take a look at trying something more elaborate...I'm thinking with the right number of Mosfet transistors doing the job, a very fine sounding amp could be made...but then again BJT's handle high current better [or what I read, I guess the Supertex would be worth a view...] and make a good amp.
  The amp itself is like an 'addendum' when compared to the power supply or speaker, I have like 5 buxx per amp in these...and some jacks 'n WW's and huge resistors I found around here.
  Two transistors for each 8'' speaker, stereo echoed and effecto'd, works pretty dern well, and is a fairly simple affair to wire up, Mosfet effect sound.
 

[petemoore]

  on this page
  http://www.passdiy.com/projects/zenlite4.htm
  Pass shows a balanced 'differential amp'.
  I get the idea the signal is split into inverted/noninverted versions, each side is amplified then 'compared' by the speaker...one amp moves toward + when the other amp moves toward -, the speaker connected between the amp outputs instead of one amps output and ground.
  Thing is don't I need a phase splitter [word?] to provide symmetrical but mirror signal swings..I've been looking and reading all over, typing in
  Differential Amp
  Differential inverter
  Phase Inverter
  ...other stuff into google...' 
  Thinking maybe a center tapped transformer would do it [but will I need 'accessories' like buffer to drive or tailoring freq. response.
  If a mosfet or other transistor could be used as a phase splitter [equal value reisistors on Source/Drain] that'd be cool, I just haven't done anything like this for an amp application and haven't been able to find texts about others who have..perhaps I'm not googling the right phrases..maybe 'mirror amp' I'll try that...
  any suggestions...?

[Steben]

  on this page
  http://www.passdiy.com/projects/zenlite4.htm
  Pass shows a balanced 'differential amp'.
  I get the idea the signal is split into inverted/noninverted versions, each side is amplified then 'compared' by the speaker...one amp moves toward + when the other amp moves toward -, the speaker connected between the amp outputs instead of one amps output and ground.
  Thing is don't I need a phase splitter [word?] to provide symmetrical but mirror signal swings..I've been looking and reading all over, typing in
  Differential Amp
  Differential inverter
  Phase Inverter
  ...other stuff into google...' 
  Thinking maybe a center tapped transformer would do it [but will I need 'accessories' like buffer to drive or tailoring freq. response.
  If a mosfet or other transistor could be used as a phase splitter [equal value reisistors on Source/Drain] that'd be cool, I just haven't done anything like this for an amp application and haven't been able to find texts about others who have..perhaps I'm not googling the right phrases..maybe 'mirror amp' I'll try that...
  any suggestions...?

bah, reduction of 2nd harmonic... 

[petemoore]

  Yea, second Harmonics I dont have a problem with.
  Not messing around with the amp is the problem I have...lol.
  Sounds pretty dern good to my ears as it sits..but you know it needs more power, it can't blow the speakers yet [not close really]..
  So I got to thinking if one 'amp' like I have could be 1/2 the amp [differential amp or whatever] the power and bass [which is respectable but I'd like more control or definition or loudness or whatever], would...double [electronically speaking] because now two amps [and mosfets] are being used to drive the speaker where one was.