mini mosfet audio amp design

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

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

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

The wiper of potentiometer R4.

Bob

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.

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.

On a sunny day (Thu, 20 Mar 2008 20:02:45 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

You still need a decent transformer, forget about using PC supplies. But this one works fine, it drives a bass speaker now.

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.

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

?pgno=3D4

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

-- Bill Sloman, Nijmegen

wrote in message news: snipped-for-privacy@d21g2000prf.googlegroups.com...

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

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wrote in message news: snipped-for-privacy@d21g2000prf.googlegroups.com...

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

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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 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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

If you want to build an audio amplifier I would suggest heading on over to

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

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

This one looked really good:

until I realized I'd have some trouble getting some of the parts.

Thanks y'all,

Michael

National Semiconductor LM1875, available from Digi-Key.

The basic circuit uses 7 capacitors and 5 resistors, and that is including .1 and 100 uF capacitors from each supply rail to ground, and resistors to ground from both ends of the input coupling capacitor. No additional semiconductors are used in the basic circuit.

- Don Klipstein ( snipped-for-privacy@misty.com)

I think the original poster was looking for more of an educational experience, i.e. discretes.

If I were to roll together a power amp, I'd be inclined to try some of those Class E chips.

Yep, I was looking for the discretes, but thanks anyway. Only 5 leads on the LM1875, eh...

Class E chips? Which ones?

Mouser has several Class D chips, but they all look hard to solder.

8-)

MD

Somebody is going to have to explain page 3 of this data sheet to me. Please don't tell me it is a misprint, as National has had six years to correct.

At a supply voltage of 15 volts, the curve shows 10 watts out.

Last I looked no single-supply design can drive to below zero or above supply rail without the use of some inductive tricks that the test circuit doesn't show.

So, converting a 15 volt peak-to-peak output signal (assuming that the device can swing to both rails) gives about 5.3 volts RMS.

The data sheet specifies an 8 ohm load, so given that Power = volts^2 / load yields about 3.5 watts out.

What gives?

Jim

--
"If you think you can, or think you can\'t, you\'re right."
        --Henry Ford



"Don Klipstein"  wrote in message 
news:slrnfug3sc.gjt.don@manx.misty.com...
>
>  National Semiconductor LM1875, available from Digi-Key.
>
> http://www.national.com/ds/LM/LM1875.pdf
>
>  The basic circuit uses 7 capacitors and 5 resistors, and that is
> including .1 and 100 uF capacitors from each supply rail to ground, and
> resistors to ground from both ends of the input coupling capacitor.  No
> additional semiconductors are used in the basic circuit.
>
> - Don Klipstein (don@misty.com)

It's a dual supply-- that's what the "+/-V" means on the X axis of

00503011.

10W into 8 ohms requires about +/-9V peak.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Only thing I can think of - they're assuming (-15V) and (+15V) available.

If you go from 0-15V, you're right;

15 / SQRT(2) / 2 = 5.3V RMS; V^2/R = 3.5 W.

If we go (-15) to (+15), we take

30 / sqrt(2) / 2 = 10.6V; (10.6V)^2 / 8 ohm = 14 W out possible. With some going to heat.

Michael

To be honest, no. First, a circuit that needs potmeters where setting them wrong can cause stuff to blow up isn't my cuppa tea. Second, an audio power amp that requires a regulated supply isn't my cuppa tea either.

If I'd spend the time building an audio amp I'd go all out and build the biggest honking monster class-D amp the neighborhood has ever seen. The kind when one twang on the E-guitar causes the plaster to fall off the ceiling. That's what I did as a kid, except it wasn't class D and had tubes (and the laws came after some playing on the E-guitar ...)

--
Regards, Joerg

http://www.analogconsultants.com/