Mosfet power amp...

[artsinbloodshed]

Hey guys!
Does anyone of you have a cool schematics of a mosfet power amp suitable for guitar? 50/100 watts would be great (even more)
I looked for it but nothing really came out of that search...
thanks!

[R.G.]

Yep, I do. They're easy to find as well.

But let me repeat my cautions on power amps. I spend decades chasing this, that or the other power amp circuits, and as you say, the bigger the better. I came to realize:

The power amp circuit is a minor wart on top of the power supply. It's much less costly than the power supply, and easier to find parts for and build. The power supply dictates what kind of cabinet or enclosure that the thing will go into, and that drives even more of the costs. So the distilled wisdom is

From the design, locate and obtain the power supply parts and enclosure first. The power amp circuit is immaterial and will never make a sound if you don't get the power supply done. Figure out what power you need (this is almost independent of the power amp circuit for solid state amps) and buy/build the power supply first.

Looking over power amp circuits without realizing this is just another way of reading fiction for fun.

[thetragichero]

once again rg drops the knowledge
i'm moving to rackmount enclosures for the rest of my preamp/power amp builds.... 100$ or less for the chassis from mouser and you have a nice portable, modular unit to add to your rig

[petemoore]

  I   Built a few small amps.
  Built some larger amplifiers.
  Hafta say Mosfets makes very nice sounding amp.
  I bought a 2x35w amp for:
  a lot less than 1/2 what I'd calculated a DIY build to cost.
  Probably 1/2 the size wieight for the same or more /power / performance/ low noise...I can't think of an aspect I could have 'outdone' by DIY-ing it.

[artsinbloodshed]

Ok then, power supply first!
@R.G: according to your experience, what would be the mosfet schemo/power supply combo that would be the better/easier for a guitar amp?
My project (and I insist on "project"!) is to build a small amp head.nothing too crazy,just 2 channels thats all.
@petemoore: I was about to buy some power amp to integrate but,where I live, fet's are way more available than mosfets...thus the diy thing!
Anyway, I think mosfets make better sounds out of their power...
@thetragichero: I'm more a "head" guy but rackmounts began to "seduce" me more and more thanks to their modularity!

Thanks guys!

[petemoore]

  Where to get a lifetime supply of "Pure DC"?
  The DC store or open your own.
  Manage and train the spazzing AC monster to hold a reference 'voltage-bar' up high, super still while our signal spin-swings from it and starts pulling down real hard on the marked reference voltage-'bar'.
  Make the reference voltage immune to any wiggle-buzz-frequencies' [upper registers] and also low-band frequencies [that may pull real hard for longer periods of time.
  For low noise and 'more perfect compromises' of performance, it must then be trained to do the work without making any 'breathing noises'.
 
http://geofex.com/Article_Folders/Power-supplies/powersup.htm
 
Nelson Pass and Steve Daniels, many others contribute excellent content at this site:
 
http://www.diyaudio.com/
 
  It took some work for me to try to understand it, referencing outside data or info made it easier to increase my comprehension without blurring the otherwise consice info-infused topics.
 
   

[nick d]

                 I used a read-built power amp module with a pre-amp circuit copied from an SS Marshall
                  when building a bass amp head for my 14-year-old son .
                  He is now 30 , and the amp still works !
                     
                  Worth a look .
[url]bkelec.com

[nick d]

             Er , link didn't work .
             Anyway , www.bkelec.com
                           
             The 100w module is £53 + p&p in UK
             
             Food for thought .

[R.G.]

As a practical matter, the simplest thing to do is to to use the LM3886 power amp chip and build a simple +/-30V power supply with it. People have reported good luck with this used as a guitar amp, and it's hundreds of times easier than doing a discrete amp. The big change is that there is exactly 1.0000 part to wire up to a heat sink.They cost about $5, cheaper than two power transistors, whether bipolar or MOSFET. You can use one per speaker for about 50W/speaker if you insist on more power. You can also series/parallel them for up to 200W per amplifier, but that's trickier and usually unnecessary.

For the power supply you need a rectifier bridge ($2.50 - $5.00 for 25A/400V or better) two filter caps (18,000uF/50V, $2.60 each, B.G. Micro) and a power transformer (Antek AN-1222, $18.00 http://www.antekinc.com/details.php?p=40). So far that's $31, of which $5 is the amplifier. These are about the best prices available for new/good parts.

And an enclosure and heat sink.That's a big deal. The mechanical/chassis construction and finding a heat sink are the toughest, most time consuming part of all this. I noticed and bought a "Rogue"  combo amplifier from craigslist just because it was cheap ($40) and working. The working part meant the power supply was good, it had a heat sink and a chassis, and the rest did not matter. I will gut the chassis and put my own amplifiers and preamp in it. Right now it's headed for being a re-creation of the UK Vox "Supreme"  amplifier.

There are other power amp schematics for MOSFET power amps. However, some of my personal observations are:
- when you go solid state, ignore trying to get "power amp distortion". Use the power amp for what it does well, a good reproduction of what you feed it. Get your distortion other places, which is easy.
- the exact type of output devices does not matter. Bipolar vs MOSFET makes almost no contribution to the final sound of a classical architecture solid state amp. In a guitar amp, you can't hear the MOSFETs.  MOSFETs may be a bit more durable, but using something like the LM3886 which self-protects from overcurrent and over temperature is a far better solution.

In my opinion.

[slideman82]

Hey guys!
Does anyone of you have a cool schematics of a mosfet power amp suitable for guitar? 50/100 watts would be great (even more)
I looked for it but nothing really came out of that search...
thanks!

There's a 60W power amp in redcircuits.com, it's really simple to design a PCB (I'll send my own design if you want), sounds great. Played at 3 hours at a gig overbiased, and still works. You'll need a 30 + 30 VAC 3A transformer for the supply.

[R.G.]

Some thoughts on power dissipation.

Using the "60W" circuit at redcircuits as an example: the power supply is +/-40Vdc, which is a nominal, and could be as high as +/- 44Vdc, depending on the AC power line tolerance and more if the power transformer choice is unfortunate.

For a class AB solid state amp, the maximum power dissipation in the output transistors is P_d = V_s²/20R_l where V_s is the total power supply, 80 or 88V in this case. So for that amp and power supply, the power wasted in the output devices will be at least
P = 80*80/20*Rl, which becomes 40W for an 8 ohm load and 80W for a 4 ohm load. It's actually higher using MOSFETs, because they do not "saturate" near the power supplies, so they burn up more power for the same output than bipolars. Call it 50W and 95W dissipated, just for a guess. Notice that music is not a continuous full power output, nor is full output power the maximum dissipation point. About 40% of full power is the point at which the power outputs get hottest in a class AB solid state amp. But you may well run the thing at 40% of max power out, so it may overheat unless you keep it cool.

You have to keep the transistors from overheating, or they die. The maximum rated junction temperature for the IRF240 and IRF9240 is 150C. Room temperature is 25C, and stage temperature is often 38C. That leaves 112C temperature difference between the transistor junctions and the room air to dissipate the heat. For an 80W heat load (don't tell me you're not going to run this into two speakers paralleled!) that's 40W per transistor, to get out with a 112C rise. You can only afford a 112C/40W = 2.8C/W total heat sink, and since there are two transistors, you have to have two heat sinks like that, or one heat sink that can do 1.4C/W.

But the transistor itself has a thermal resistance from the semiconductor chip to the case of the transistor of 1C/W.  So now you have to get the heat into the room air with a thermal resistance of only 1.8C/W per device. You have to insulate the transistors from the heat sink. A mica insulator and heat sink grease get the thermal resistance from device to sink down to about 0.5 to 0.6 C/W. That leaves only 1.2-1.3C/W that you can stand for the heat sink itself to resist heat flow into the air. That's per device, by the way. The entire heat sink for two devices has to be 0.6 to 0.65C/W to keep them cool enough.

What's a 0.6C/W heat sink look like? Look here: http://www.aavidthermalloy.com/cgi-bin/stdisp.pl?Pnum=6500b That's your standard heat sink most people think of. It's 1.7C/W, and too high. A pair of IRF9240/IRF240s on this heat sink in this amplifier will overheat and die unless they're run very carefully. In fact, they will die if you use two of these heat sinks, one for each transistor.

Looking at extrusions, here's one that works: http://www.aavidthermalloy.com/cgi-bin/exdisp.pl?Pnum=72545&LengthUnits=in&ExLength=3&airflow=57.2&CType=Natural&AirUnits=LFM  It's 14.75" wide and 2.75" tall, and I specified a 3" length. It's available in 8 foot lengths for $115.51 per foot, which makes a 3" length be $3.61 if you can get it in only 3" lengths. It's actually a little better than needed.

But you get the idea - that's HUGE. And unavailable except in bulk. Which forces us into the surplus heat sink market. You can get good enough heat sinks surplus for about $20, but it takes a lot of shopping to find them. Hence my advice to find heat sinks first.

The other thing you can do is to just build it and hope your music won't run it at full power very much. That's really how most amplifiers manage to survive.   

[Johan]

does it have to be MOSFET?..Vellemann has a kit for a 100w rms poweramp ( K8060 )and also predrilled heatsink for the kit
if you're in Scandinavia, electrokit has both the amp the heatsink and suitable toroid transformers and it's all around 120$/100Euro...just a thought..
J

[SISKO]

From the design, locate and obtain the power supply parts and enclosure first. The power amp circuit is immaterial and will never make a sound if you don't get the power supply done.

I agree that for power amps, the power supply must be strong/good enough to behave like it should, and it is an important element to consider and to think about. But I dont think this way up the point of believing that the power amp circuit is immaterial. Matter of fact, you wont make ANY sound if any of the power amp elements (Power supply, power amp circuit, speaker) is missing.  
Every elements of the system should have its own attention and planning. Not less, but not more than it should

Although, as you state its a good practice to start planning the amplifier starting from the power supply

[R.G.]

I agree that for power amps, the power supply must be strong/good enough to behave like it should, and it is an important element to consider and to think about. But I dont think this way up the point of believing that the power amp circuit is immaterial. Matter of fact, you wont make ANY sound if any of the power amp elements (Power supply, power amp circuit, speaker) is missing.  

Yes, any missing element will keep it from working. I should have been clearer.

No power amp circuit at all will work without a power supply appropriate to it. However, a single power supply suitable for, say 60W@8 ohm and 90W@4 ohm will work for almost any solid state amplifier you put with it.  Moreover, there is a much smaller difference between solid state amplifier circuits and sound performance than most people think.

It is possible to tell a poorly designed, ill-biased solid state amplifier from a well-done one, but for well-designed bipolar/MOSFET/JFET/IC power amps, it is quite difficult to tell the difference within many bounds. Not impossible, but the differences are quite small.

I was trying to make the point that the thinking in the original post is backwards, and backwards in a way that is very common and an easy mistake to make.

[artsinbloodshed]

yes, concerning the heat sink...I wanted to it to be helped with a fan to avoid overheatings.
Anyway, I knew that the differences between ic's/mosfet/Jfets located in the power section are not that obvious, but I aimed at building a mosfet stage since I played for many years with my old tube works amps (which was mosfet)
I was stunned about the reliability of the power and its loudness!
Thanks R.G! Dissipation is always the obscure point in a design for me...and no, I won't run my amp at full power! I'm an energy saver you know
I checked and it appears that there's some power supply kits available on the market (heatsink+power supply). Maybe I could combine both hand soldered/kits...I think it would be even more affordable...
@johan: Yes, I looked at the velleman kits (i've found even a 200 watt kit). I consider it a "second choice" manner to build an amp because I really don't know the quality of that thing...but it's a good plan B!

[petemoore]

  Yes, I looked at the velleman kits (i've found even a 200 watt kit). I consider it a "second choice" manner to build an amp because I really don't know the quality of that thing...but it's a good plan B!
    What is in the amplifier.
  How good are the components.
    How good is the overall design.
  Knowing these things allows the designer to exploit design improvements, should there be room for any.
  The Velleman kit goodness is quite easily assessed:
  Data sheets on the relevant components [ie from parts list], performance specifications sheet.
  They've been around a while and there are plenty of goodness reviews and independant examination measurements [ie scope pics etc.], mostly the Velleman blurbs and specsheets tell exactly what the kits are and what they do.
   The boards are nice, the designs have been through extensive testing so generally were very well designed or have seen revisions, and the components all have data sheets, and need not be soldered in if an improved component can be made available for the position.
 

[artsinbloodshed]

@petemoore: I was pretty sure that the kits were from good quality. by quality I meant "to the ear"...
Anyway, thanks to you guys,I think I will go more toward a kit than toward a diy thing. The cost will be lower.(as R.G said).
I haven't checked the data sheets yet but I will!
all in all, kits will be easier but still, i'd like to build one myself...one day!

[R.G.]

all in all, kits will be easier but still, i'd like to build one myself...one day!

When I was 15, I desperately NEEDED a guitar amp, and simply had no money to buy one. I thought "well, how hard can it be to build one?" and decided to do so. That was the defining even that got me into university, into EE, and dominated my professional life. Decades later, I'm still tinkering with guitar amps. And I've now built several for myself and others. It's a good feeling.

Go, follow your dreams.

[artsinbloodshed]

all in all, kits will be easier but still, i'd like to build one myself...one day!

Go, follow your dreams.

Know what? I have wings now!
Thanks R.G and all of you guys!
Building is fun, from traditional fuzz face to more complicated circuits.
I'll build my amp,maybe not right now but I will!

[Johan]

the velleman kits are well put together and easy to follow, with components stacked on tape the way in the same order they goes in( first resistor on the tape is R1 on the board), so even for beginners they are fairly easy to get up and running.
I guess Velleman is like an European version of Heathkit. anyway, I've used their kits many times and always been happy...there is a preamp that uses a single opamp and with only a few components added and a few values changed, becomes a mxr microamp or distortion+ and the kit costs less than I can get the components and stripboard for.....I realize my enthusiasm might seem suspicious, but I am not associated with them in any way, just a happy customer....