Bonkers MOSFET Amp

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Mike H
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#1 Bonkers MOSFET Amp

Post by Mike H »

You've seen Phillip's simple MOSFET amp, and Brian's "Class A from scrap", now put you hands together (and pray) and please give a big welcome to ~

Bonkers MOSFET Amp

aka "An Amplifier For Hotter Evenings" (© Andrew I [well almost])

It's bonkers because it seems like the wrong sorts of bits used in unexpected ways

Circuit's a bit bonkers as well


Image


The amps are two version 1 FETREG module boards, modified (somewhat extensively):

Image


Image


Now improved with "panel mounted" controls:

Image


The power supply is:

Image

If nothing else it proves a IRFP240 power MOSFET can (also) be used as an audio amplifier.
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Mike H
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#2

Post by Mike H »

PS: it's not a ZTX751, in the end, but a BC557
 
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pre65
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#3

Post by pre65 »

Is that an interstage as a 1:1 OPT ?
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G-Popz THE easy listening connoisseur. (Philip)
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Mike H
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#4

Post by Mike H »

No it's an early prototype (slightly larger) VT1087 output transformer and using the 16 Ohms taps.

PS: I modified the diagram for Q2 & C1
 
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Nick
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#5

Post by Nick »

If nothing else it proves a IRFP240 power MOSFET can (also) be used as an audio amplifier.
First Watt amps are full of them
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#6

Post by Mike H »

PS: that means the impedance ratio is 312:1 and the Volts ratio 17.68:1, the whole thing just runs on 70 mA

Max. power o/p is a whole half a Watt r.m.s. Told you it was bonkers :lol:


 
 
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Mike H
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#7

Post by Mike H »

Nick wrote:First Watt amps are full of them
That is strangely reassuring :lol:
 
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brig001
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#8

Post by brig001 »

Very nice. Out of interest, does R4 help?

I thought about degeneration on mine, but didn't try simulating it with and without. The theory being that a higher open loop gain would allow better correction from the feedback loop. I think this is something to try when I get home.

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#9

Post by Mike H »

R4 allows setting the DC biased current with any kind of accuracy, as it all depends on what's flowing through Q2, which in turn depends on supply Voltage. As R1 is effectively connected to supply rail via the transformer (DCR 230 Ohms or something). So it's not like half the supply is across the primary, cos it isn't, only about 9V or so. And that because of 35 - 40 mA of Q1 drain current.

I left it unbypassed to keep the number of caps to a minimum, seems to be happy :D
 
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#10

Post by Mike H »

NB: if it works out correctly I make it open-loop gain >1,000 at Q1 drain :?:
 
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#11

Post by Mike H »

It's worth having its own transformer now I reckon, a suitable toroid is on order as I type, as the droning fan of the bench PSU is annoying.

Bit of an ally box, IEC inlet; bridge rectifiers I got coming outa me ears, even got one on a bit of board, add on another 1,000uF, proper job.
 
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#12

Post by brig001 »

Mike H wrote:R4 allows setting the DC biased current with any kind of accuracy, as it all depends on what's flowing through Q2, which in turn depends on supply Voltage. As R1 is effectively connected to supply rail via the transformer (DCR 230 Ohms or something). So it's not like half the supply is across the primary, cos it isn't, only about 9V or so. And that because of 35 - 40 mA of Q1 drain current.

I left it unbypassed to keep the number of caps to a minimum, seems to be happy :D
Ah, I see - makes sense now.

I tried simulating it on mine, and the distortion hardly changed until it was too big to bias properly. My supply is only 15V, so finding the 4V needed for the MOSFET gate isn't really an issue.

Brian.
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Nick
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#13

Post by Nick »

Keep going in this direction and you will reinvent the F2 :-)
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#14

Post by ed »

The zen series is a brilliant tutorial on this if anybody is interested.

http://www.firstwatt.com/articles.html

Of particular interest to the bass cos the early projects use BIG lightbulbs as load resistors....

As Nick suggests, all roads of this type eventually lead to F2.

It certainly helped me understand more about what those pesky electrons get up to.

btw Mike..has it got anything to do with simplified nemesis?
http://www.linearaudio.net/letters/Volume_0_NP.pdf
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#15

Post by Mike H »

Very interesting, nope none whatsoever, it started with what could be best to drive my 4 germanium power transistors with transformer phase splitter

Nick wrote:Keep going in this direction and you will reinvent the F2 :-)
Is that a fact? lol .. :D
 
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