Maker Pro
Login Join Maker Pro
Or sign in with

Menu

Connect With Us

Network

Maker Pro

mini mosfet audio amp design

J

John Larkin

Here's an interesting schematic:

http://www.redcircuits.com/Page123.htm

I was wondering what R7 would be connected to. (It's not clear in the
picture.)

What do you guys think of the schematic, overall? A good project for
a beginner?

Michael
Click to expand...


That's some goofy European schematic convention. R4 is a pot, and R7
is connected to its wiper. R4 sets the fet idle currents.

It's OK as a project I guess. R3 and R4 wouldn't be necessary if the
bias design were more rigorous, but then you get to play with this
one, and make asymmetric clipping and crossover distortion just for
fun.

You can also fry the fets if you set the pots right/wrong. In fact,
that's pretty much the default setting.

John
 
M

[email protected]

That's some goofy European schematic convention. R4 is a pot, and R7
is connected to its wiper. R4 sets the fet idle currents.

It's OK as a project I guess. R3 and R4 wouldn't be necessary if the
bias design were more rigorous, but then you get to play with this
one, and make asymmetric clipping and crossover distortion just for
fun.

You can also fry the fets if you set the pots right/wrong. In fact,
that's pretty much the default setting.

John
Click to expand...


Fry the fets if I set the pots right, even?

default setting is Kablooie?

hmm... and here I thought my biggest problem was finding such a large
transformer to drive the thing... I considered taking a surplus ATX
power supply and working between the -12 and +12 VDC to get 24V max.

Thanks,

Michael
 
M

[email protected]

That's some goofy European schematic convention. R4 is a pot, and R7
is connected to its wiper. R4 sets the fet idle currents.

It's OK as a project I guess. R3 and R4 wouldn't be necessary if the
bias design were more rigorous, but then you get to play with this
one, and make asymmetric clipping and crossover distortion just for
fun.

You can also fry the fets if you set the pots right/wrong. In fact,
that's pretty much the default setting.

John
Click to expand...


Since it's still too early or Fourth of July fireworks shows, can you
(or anyone) recommend a link to a low-parts-count, 20W or so into 8-
ohms audio amplifier? Input will be from a laptop or MP3 player, so
no preamp necessary.

Thanks,

Michael
 
M

[email protected]

Here's an interesting schematic:

http://www.redcircuits.com/Page123.htm

I was wondering what R7 would be connected to. (It's not clear in the
picture.)

What do you guys think of the schematic, overall? A good project for
a beginner?

Michael
Click to expand...

The intent is to provide enough voltage between the two gates so that
both fets are just biased. For bipolar output stages, a VBE multiplier
would be used. For fets, a replica scheme would get you closer. I
don't see my copy of Randy Sloan's amplifier design book handy, but
that would be good reading material. I recall some design where they
used a LED for the voltage drop, but that isn't a replica bias by any
means.

Those computer switches act funny under light loads. I'm not sure I'd
suggest using one.

http://www.passlabs.com/articles.htm
has some good reading. In fact, the more I think of the schematic you
presented, the more I don't like it, if only that it looks "touchy."
Using more components is often simple if the bias scheme is more
tolerant.
 
J

Jan Panteltje

Since it's still too early or Fourth of July fireworks shows, can you
(or anyone) recommend a link to a low-parts-count, 20W or so into 8-
ohms audio amplifier? Input will be from a laptop or MP3 player, so
no preamp necessary.
Click to expand...

http://panteltje.com/panteltje/amplifier/index.html
You still need a decent transformer, forget about using PC supplies.
But this one works fine, it drives a bass speaker now.
 
B

[email protected]

That's some goofy European schematic convention. R4 is a pot, and R7
is connected to its wiper. R4 sets the fet idle currents.
Click to expand...

It's not the way I'd draw a potnetiometer, and it wouldn't pass muster
with the drafting office of any of the European firms I've worked for.
It's OK as a project I guess. R3 and R4 wouldn't be necessary if the
bias design were more rigorous, but then you get to play with this
one, and make asymmetric clipping and crossover distortion just for
fun.

You can also fry the fets if you set the pots right/wrong. In fact,
that's pretty much the default setting.
Click to expand...

You'd want to start off with no bias voltage between the MOSFET gates
(pure class B operation), and monitor the no load current drawn by the
amplifier as you used the pontentiometer R4 to increase this voltage
(to bring you into class AB).

It is the crudest possible biasing scheme. The MOSFET gate-to-source
voltage is temperature dependent

http://www.powermanagementdesignlin...ionid=KD1E2OVGD5J4QQSNDLPSKH0CJUNN2JVN?pgno=4

and tends to decrease with increasing temperature at "low" drain
currents, leading to a risk of thermal runaway.
 
P

Paul E. Schoen

Here's an interesting schematic:

http://www.redcircuits.com/Page123.htm

I was wondering what R7 would be connected to. (It's not clear in the
picture.)

What do you guys think of the schematic, overall? A good project for
a beginner?

Michael
Click to expand...

Here is a circuit I came up with using LTSpice. It can put out about 25
watts into 4 ohms, (10 VRMS) with an input of 800 mV. The power supply is
not critical, and there are no tricky bias adjustments. It draws only 400
mW (10 mA) with no input. But it works only for high input levels. This can
be fixed by using an op-amp and feedback. I'll post a circuit for that
later. But this should be a pretty good core amplifier block. Also, it uses
N-channel MOSFETs for the output power, so you don't need to worry about
complementary pair matching.

Paul

===========================================================================

Version 4
SHEET 1 1304 744
WIRE 128 -208 -192 -208
WIRE 240 -208 128 -208
WIRE 480 -208 240 -208
WIRE 128 -128 128 -208
WIRE 240 -112 240 -208
WIRE 480 -80 480 -208
WIRE 80 -64 48 -64
WIRE 48 -32 48 -64
WIRE 128 -16 128 -32
WIRE 240 0 240 -32
WIRE 432 0 240 0
WIRE 48 80 48 48
WIRE 128 80 128 64
WIRE 128 80 48 80
WIRE 128 128 128 80
WIRE 240 128 192 128
WIRE -192 144 -192 -208
WIRE 480 144 480 16
WIRE 528 144 480 144
WIRE 624 144 592 144
WIRE 656 144 624 144
WIRE 480 224 480 144
WIRE 480 224 352 224
WIRE 656 224 656 144
WIRE 352 272 352 224
WIRE 240 288 240 256
WIRE 304 288 240 288
WIRE 480 288 480 224
WIRE 128 304 128 128
WIRE -80 368 -96 368
WIRE -64 368 -80 368
WIRE 32 368 0 368
WIRE 80 368 32 368
WIRE 432 368 352 368
WIRE 240 384 240 288
WIRE 32 416 32 368
WIRE 128 416 128 400
WIRE 352 416 352 368
WIRE -96 432 -96 368
WIRE -192 560 -192 224
WIRE -96 560 -96 512
WIRE -96 560 -192 560
WIRE 32 560 32 496
WIRE 32 560 -96 560
WIRE 128 560 128 496
WIRE 128 560 32 560
WIRE 160 560 128 560
WIRE 240 560 240 464
WIRE 240 560 160 560
WIRE 352 560 352 496
WIRE 352 560 240 560
WIRE 480 560 480 384
WIRE 480 560 352 560
WIRE 656 560 656 304
WIRE 656 560 480 560
WIRE 160 608 160 560
FLAG 160 608 0
FLAG -80 368 Vin
FLAG 624 144 Vout
SYMBOL voltage -192 128 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 40
SYMBOL voltage -96 416 R0
WINDOW 3 -75 232 Left 0
WINDOW 123 24 44 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value SINE(0 1.25v 5k 1u 0 0 5000)
SYMATTR Value2 AC 1
SYMATTR InstName V1
SYMBOL nmos 432 -80 R0
SYMATTR InstName M1
SYMATTR Value STD30NF06L
SYMBOL nmos 432 288 R0
SYMATTR InstName M2
SYMATTR Value STD30NF06L
SYMBOL pmos 304 368 M180
SYMATTR InstName M3
SYMATTR Value IRF7205
SYMBOL res 336 400 R0
SYMATTR InstName R2
SYMATTR Value 1k
SYMBOL res 640 208 R0
SYMATTR InstName R8
SYMATTR Value 4
SYMBOL polcap 528 160 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName C3
SYMATTR Value 4700µ
SYMATTR Description Capacitor
SYMATTR Type cap
SYMATTR SpiceLine V=35 Irms=2.03 Rser=0.033 MTBF=2000 Lser=0 mfg="Nichicon"
pn="UPR1V472MRH" type="Al electrolytic" ppPkg=1
SYMBOL res 224 -128 R0
SYMATTR InstName R9
SYMATTR Value 10k
SYMBOL res 224 368 R0
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL diode 224 0 R0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode 224 64 R0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL diode 224 128 R0
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL diode 224 192 R0
SYMATTR InstName D4
SYMATTR Value 1N4148
SYMBOL cap 0 352 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1µ
SYMATTR SpiceLine V=25 Irms=0 Rser=0.007 MTBF=0 Lser=0 mfg="TDK"
pn="C3216X7RIE105M" type="X7R" ppPkg=1
SYMBOL njf 80 304 R0
SYMATTR InstName J1
SYMATTR Value 2N3819
SYMBOL res 112 400 R0
SYMATTR InstName R3
SYMATTR Value 200
SYMBOL res 112 -32 R0
SYMATTR InstName R4
SYMATTR Value 200
SYMBOL njf 80 -128 R0
SYMATTR InstName J2
SYMATTR Value 2N3819
SYMBOL res 32 -48 R0
SYMATTR InstName R5
SYMATTR Value 100k
SYMBOL res 16 400 R0
SYMATTR InstName R6
SYMATTR Value 100k
SYMBOL polcap 192 112 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value 100µ
SYMATTR Description Capacitor
SYMATTR Type cap
SYMATTR SpiceLine V=63 Irms=900m Rser=0.1 MTBF=20000 Lser=0 mfg="Nichicon"
pn="UPH1J101MRH" type="Al electrolytic" ppPkg=1
TEXT -176 688 Left 0 !.tran .5
TEXT 40 696 Left 0 !;ac oct 5 20 20000
 
P

Paul E. Schoen

Here's an interesting schematic:

http://www.redcircuits.com/Page123.htm

I was wondering what R7 would be connected to. (It's not clear in the
picture.)

What do you guys think of the schematic, overall? A good project for
a beginner?

Michael
Click to expand...

I finally got a reasonably decent amplifier design with LTSpice. It is
essentially a high power op-amp. It can drive 37 watts into a 4 ohm load
with 48 watts input at the start of clipping with a 40 volt supply, and it
will work from 20 Hz to 20 kHz fairly well (according to the simulator). It
only draws about 300 mW (15 mA) with no input.

There may be better circuits, but this one is simple and tolerant of power
supply variations. It works at least from 24 to 48 VDC, and to within 3
volts of the supply rails. There is some crossover distortion but it
doesn't look too bad. It's probably fine for a guitar amplifier. I might
actually build this and see how it works for real.

Feel free to modify the design and see if it can be improved. It should be
a good project for a beginner, and is especially good in that it can be
simulated with LTSpice.

Good luck,

Paul

-------------------------------------------------------------------------------

Version 4
SHEET 1 1304 744
WIRE 656 -272 -256 -272
WIRE -416 -208 -672 -208
WIRE -64 -208 -416 -208
WIRE 128 -208 -64 -208
WIRE 480 -208 128 -208
WIRE 128 -160 128 -208
WIRE -64 -144 -64 -208
WIRE 480 -112 480 -208
WIRE -416 -64 -416 -208
WIRE 128 -32 128 -80
WIRE 256 -32 128 -32
WIRE 352 -32 256 -32
WIRE 432 -32 352 -32
WIRE -256 -16 -256 -272
WIRE -256 -16 -320 -16
WIRE -256 16 -256 -16
WIRE -320 32 -320 -16
WIRE 480 64 480 -16
WIRE 656 64 656 -272
WIRE 656 64 480 64
WIRE -672 128 -672 -208
WIRE -64 128 -64 -64
WIRE -64 128 -176 128
WIRE 256 128 256 -32
WIRE 480 144 480 64
WIRE 528 144 480 144
WIRE 624 144 592 144
WIRE 656 144 624 144
WIRE -320 160 -320 96
WIRE -288 160 -320 160
WIRE -256 160 -256 96
WIRE -256 160 -288 160
WIRE -64 176 -64 128
WIRE -16 176 -64 176
WIRE 656 224 656 144
WIRE -112 240 -224 240
WIRE -16 256 -16 176
WIRE 128 288 128 -32
WIRE 256 288 256 192
WIRE 352 288 256 288
WIRE 480 320 480 144
WIRE 480 320 352 320
WIRE -176 336 -176 128
WIRE 256 336 256 288
WIRE 304 336 256 336
WIRE -528 352 -576 352
WIRE -416 352 -416 16
WIRE -416 352 -448 352
WIRE -384 352 -416 352
WIRE -224 352 -224 304
WIRE -224 352 -384 352
WIRE -208 352 -224 352
WIRE 480 352 480 320
WIRE -112 368 -112 240
WIRE -112 368 -144 368
WIRE 32 368 -112 368
WIRE 80 368 32 368
WIRE -288 384 -288 160
WIRE -208 384 -288 384
WIRE 256 400 256 336
WIRE -576 416 -576 352
WIRE -384 416 -384 352
WIRE 32 416 32 368
WIRE 128 416 128 384
WIRE -288 432 -288 384
WIRE 352 432 352 416
WIRE 432 432 352 432
WIRE -64 448 -64 176
WIRE 352 464 352 432
WIRE -672 560 -672 208
WIRE -576 560 -576 496
WIRE -576 560 -672 560
WIRE -384 560 -384 496
WIRE -384 560 -576 560
WIRE -288 560 -288 512
WIRE -288 560 -384 560
WIRE -176 560 -176 400
WIRE -176 560 -288 560
WIRE -64 560 -64 512
WIRE -64 560 -176 560
WIRE -16 560 -16 320
WIRE -16 560 -64 560
WIRE 32 560 32 496
WIRE 32 560 -16 560
WIRE 128 560 128 496
WIRE 128 560 32 560
WIRE 176 560 128 560
WIRE 256 560 256 480
WIRE 256 560 176 560
WIRE 352 560 352 544
WIRE 352 560 256 560
WIRE 480 560 480 448
WIRE 480 560 352 560
WIRE 656 560 656 304
WIRE 656 560 480 560
WIRE 176 608 176 560
FLAG 176 608 0
FLAG -576 352 Vin
FLAG 624 144 Vout
SYMBOL voltage -672 112 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 40
SYMBOL voltage -576 400 R0
WINDOW 3 -75 232 Left 0
WINDOW 123 24 44 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value SINE(0 350m 2000 1u 0 0 5000)
SYMATTR Value2 AC 1
SYMATTR InstName V1
SYMBOL nmos 432 -112 R0
SYMATTR InstName M1
SYMATTR Value STD30NF06L
SYMBOL nmos 432 352 R0
SYMATTR InstName M2
SYMATTR Value STD30NF06L
SYMBOL pmos 304 416 M180
SYMATTR InstName M3
SYMATTR Value IRF7205
SYMBOL res 336 448 R0
SYMATTR InstName R2
SYMATTR Value 1k
SYMBOL res 640 208 R0
SYMATTR InstName R8
SYMATTR Value 4
SYMBOL polcap 528 160 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName C3
SYMATTR Value 4700µ
SYMATTR Description Capacitor
SYMATTR Type cap
SYMATTR SpiceLine V=35 Irms=2.03 Rser=0.033 MTBF=2000 Lser=0 mfg="Nichicon"
pn="UPR1V472MRH" type="Al electrolytic" ppPkg=1
SYMBOL res 112 -176 R0
SYMATTR InstName R4
SYMATTR Value 2.2k
SYMBOL res 240 384 R0
SYMATTR InstName R1
SYMATTR Value 220k
SYMBOL diode 336 -32 R0
WINDOW 0 -34 32 Left 0
WINDOW 3 42 35 Left 0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode 336 96 R0
WINDOW 0 -38 -28 Left 0
WINDOW 3 40 35 Left 0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL diode 336 224 R0
WINDOW 0 -33 28 Left 0
WINDOW 3 39 -30 Left 0
SYMATTR InstName D5
SYMATTR Value 1N4148
SYMBOL res 112 400 R0
SYMATTR InstName R3
SYMATTR Value 100
SYMBOL res 16 400 R0
SYMATTR InstName R6
SYMATTR Value 100k
SYMBOL Opamps\\LT1037A -176 304 R0
WINDOW 3 9 107 Left 0
SYMATTR InstName U1
SYMBOL res -432 336 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R12
SYMATTR Value 10k
SYMBOL zener -48 512 R180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D6
SYMATTR Value BZX84C15L
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res -80 -160 R0
SYMATTR InstName R5
SYMATTR Value 4.7k
SYMBOL polcap -32 256 R0
SYMATTR InstName C4
SYMATTR Value 100µ
SYMATTR Description Capacitor
SYMATTR Type cap
SYMATTR SpiceLine V=63 Irms=900m Rser=0.1 MTBF=20000 Lser=0 mfg="Nichicon"
pn="UPH1J101MRH" type="Al electrolytic" ppPkg=1
SYMBOL res -272 0 R0
SYMATTR InstName R7
SYMATTR Value 100k
SYMBOL res -304 416 R0
SYMATTR InstName R9
SYMATTR Value 1k
SYMBOL res -400 400 R0
SYMATTR InstName R11
SYMATTR Value 10k
SYMBOL res -432 -80 R0
SYMATTR InstName R10
SYMATTR Value 1meg
SYMBOL nmos 80 288 R0
WINDOW 3 49 86 Left 0
SYMATTR InstName M4
SYMATTR Value Si4850EY
SYMBOL cap -336 32 R0
SYMATTR InstName C1
SYMATTR Value 10p
SYMBOL cap -208 304 R180
WINDOW 0 24 64 Left 0
WINDOW 3 24 8 Left 0
SYMATTR InstName C2
SYMATTR Value 5p
SYMBOL diode 336 32 R0
WINDOW 0 -33 98 Left 0
WINDOW 3 37 33 Left 0
SYMATTR InstName D3
SYMATTR Value 1N4148
SYMBOL diode 336 160 R0
WINDOW 0 -33 34 Left 0
WINDOW 3 40 94 Left 0
SYMATTR InstName D4
SYMATTR Value 1N4148
SYMBOL cap 240 128 R0
SYMATTR InstName C5
SYMATTR Value 5n
TEXT -176 688 Left 0 !.tran .5
TEXT 40 696 Left 0 !;ac oct 5 20 20000
 
R

[email protected]

If you want to build an audio amplifier I would suggest heading on
over to www.diyaudio.com Search for "gain clone". Its basically a 60-
Watt amplifier using a LM3886 from National. You can find lots of
Schematics, PCB layouts, BOM's etc.

-=Randy
 
M

[email protected]

If you want to build an audio amplifier I would suggest heading on
over towww.diyaudio.comSearch for "gain clone". Its basically a 60-
Watt amplifier using a LM3886 from National. You can find lots of
Schematics, PCB layouts, BOM's etc.

-=Randy
Click to expand...


Yep, I was seriously considering the LM3886, from Jameco (Mouser
doesn't have it) but then I wouldn't learn anything. ;-) Building
from discrete components has a certain appeal to it.

Thanks,

Michael
 
E

Eeyore

Since it's still too early or Fourth of July fireworks shows, can you
(or anyone) recommend a link to a low-parts-count, 20W or so into 8-
ohms audio amplifier? Input will be from a laptop or MP3 player, so
no preamp necessary.
Click to expand...

Build one around one of the National semiconductor 'symphony' or is it
'overture' series of IC power amps. They're quite good. I dare say there's an
app note that explains it all, or Google for 'gainclone' which was an amp
based on these chips favoured by the hi-fi fraternity. It performs rather
well.

Of course you don't get to find about the mysteries of designing with
discretes this way by copying all the bad practice shown by the poster of the
circuit you referenced.

Graham
 
G

GregS

Dittos on Doug Self.
Click to expand...


I started building amps or copying some of the design from the 1971 RCA SSPC
crap book. They were typical of some of the old crappy designs. I corraled a
couple of tigers in my time. Had to tame them a bit. I fed one some bad shit
and it fired right back at me. Finally had to opperate to get it back to
normal.

My first amp, a Knight-Kit 15 wpc germanium stereo unit. Transformer
interstage driver. Greatclear SS sound.

greg
 
M

[email protected]

Build one around one of the National semiconductor 'symphony' or is it
'overture' series of IC power amps. They're quite good. I dare say there's an
app note that explains it all, or Google for 'gainclone' which was an amp
based on these chips favoured by the hi-fi fraternity. It performs rather
well.

Of course you don't get to find about the mysteries of designing with
discretes this way by copying all the bad practice shown by the poster of the
circuit you referenced.

Graham
Click to expand...


So are you saying the Gainclone is as good as, or better than,
discretes?

(Not putting you on the spot. Not at all. :cool:

Are these circuits any better, then?

http://www.redcircuits.com/Page2.htm
http://www.redcircuits.com/Page100.htm

Or should I just give up on that site...?

Michael
 
You must log in or register to reply here.

Similar threads

D
Mini Golf Windmill
Replies
4
Views
724
Erstwhile
Erstwhile
T
Panasonic SAPM-31 Mini hifi, Tapedeck motor running all the time
Replies
4
Views
779
Erstwhile
Erstwhile
B
Google nest mini speaker linkage
Replies
8
Views
682
BAM_075
B
S
Mini reflow oven advice
Replies
2
Views
1K
StealthRT
S
K
Old Gemini P800 System
Replies
3
Views
817
hardyflower
H
Top