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Author Topic: 12 V MOSFET practice amp  (Read 16310 times)

rowdy_riemer

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12 V MOSFET practice amp
« on: August 05, 2008, 09:11:42 PM »
I've recently designed a 12v amp that I think sounds pretty good. There's more info about it at http://www.riemer.us/mufu-12v. I've attached the schematic. Keep in mind that I'm still a novice, and I'm on a low budget. I think, based on some Multisim simulations of the source follower stage that this can output 2W without much distortion and about 4 watts if driven to square wave distortion. Computer simulation is no substitute for real world measurements, but I don't yet have a working osciloscope. Well, let me know what you think. I hope to learn from any criticisms and I hope this might be useful for others.
« Last Edit: October 06, 2009, 08:25:53 PM by rowdy_riemer »

Jack1962

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Re: 12 V MOSFET practice amp
« Reply #1 on: August 07, 2008, 11:15:24 AM »
Basically , in theory , it will operate , in practice it has no controls so, until they are added it's really hard to even do a spice model to simulate, let alone prototype.

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rowdy_riemer

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Re: 12 V MOSFET practice amp
« Reply #2 on: August 07, 2008, 12:34:14 PM »
Basically , in theory , it will operate , in practice it has no controls so, until they are added it's really hard to even do a spice model to simulate, let alone prototype.

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Well, I've done a simulation of the output stage using Multisim. The student edition of Multisim unfortunately does not have any 2n7000 models. In Multisim, I generated an input signal with a simulated function generator and used a simulated oscilloscope to view the results. According to simulation with a purely resistive load, the output stage will output about 2W with little distortion and about 4W with a signal high enough to produce basically a square wave from clipping. This of course assumes I've done my math right (Vrms^2/R). I've got the circuit built on my breadboard using a car stereo speaker(yeah, I know I need to get a real guitar amp speaker). Maybe when classes start this fall at the local college, I can test out the real circuit on some real equipment. For whatever reason, I haven't even thought about a volume control. I've just used the volume pot on my guitar. I know I need some tone control, but at the moment, I don't have the needed parts and I haven't gotten around to trying to engineer a tone control with the parts I've got. I'll try to address those issues.

What spice software do you use? I love multisim, but the student edition is limited, and all the more advanced editions are EXTREMELY expensive for someone designing stuff for hobby purposes. Also, I've had very little luck trying to add components to Multisim using spice models created in other software. I've tried some opensource stuff, but I'm too spoiled by Multisim's easy to use interface.

Rowdy


rowdy_riemer

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Corrected Schematic
« Reply #3 on: August 07, 2008, 01:25:58 PM »
In the original schematic, I forgot to connect the gate of Q3 to its drain. Sorry about that.

Rowdy

J M Fahey

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Re: 12 V MOSFET practice amp
« Reply #4 on: August 07, 2008, 08:49:07 PM »
Dear Rowdy.
Nice try, specially as a first design, but I think that in this case you´ll get far better results going bipolar.
The reason is you only have 12 Volts, and MosFets, being enhancement devices, "lose" around 4 volts each, just getting forward biased , and a couple more volts getting really turned on. (Look at their Current vs. gate voltage graphs) . With two of them in series, you have *very* little useful voltage left.
Besides, the "class A" design uses a lot of DC current, just heating the devices. A TDA2002/2003/2006/LM383 or similar would give you around 6 clean Watts into 4 ohms and be a far simpler circuit , with the same 12 Volts. (check their datasheets)
Congratulations.
JM

rowdy_riemer

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Re: 12 V MOSFET practice amp
« Reply #5 on: August 07, 2008, 11:08:27 PM »
Dear Rowdy.
Nice try, specially as a first design, but I think that in this case you´ll get far better results going bipolar.
The reason is you only have 12 Volts, and MosFets, being enhancement devices, "lose" around 4 volts each, just getting forward biased , and a couple more volts getting really turned on. (Look at their Current vs. gate voltage graphs) . With two of them in series, you have *very* little useful voltage left.
Besides, the "class A" design uses a lot of DC current, just heating the devices. A TDA2002/2003/2006/LM383 or similar would give you around 6 clean Watts into 4 ohms and be a far simpler circuit , with the same 12 Volts. (check their datasheets)
Congratulations.
JM

Thanks JM,

You're certainly right about the chip route being simpler. However, I really wanted to do the engineering myself. I went with a class A design partly for simplicity, but also partly for a class A sound. With a class A design, I avoid crossover distortion, and the MOSFETs operate in a more linear manner with a class A topology. I desided to go with MOSFETs for a few reasons. I like the softer clipping I get with a MOSFET. Also, I happened to have five or six of these IRF510's laying around.

In multisim, the output stage can produce a peak-to-peak voltage of about 9v, which is a peak-to-peak loss of only 3v with a theoretical maximum of 12v. Of course in multisim, you're not dealing with real components. I'll have to wait till I can test this on real equipment for real world results. I'm sure I could get a higher output voltage with BJT's with a class AB topology, but that's just part of the trade off.

Keep in mind that, at quiescence(I hope I spelled that right), the top MOSFET, Q1, is biased at 12V. If the voltage on the input cap swings up +6 volts, then the voltage on the gate of Q1 increases to 18V. If it swings down by -6V, then the voltage on the gate is 6V, which is above the approximate 4.5V cutoff. Also, the current mirror MOSFET's are always biased above cutoff. I forget the exact numbers, but the 2n7000s have cutoff voltages between about .6v and I think 1.5V. Not too big of a deal. Also, like with the power stage, the input signal adds to or subtracts from the bias voltage. If they are already forward biased, then there is no problem, other than having to use coupling capacitors.

I probably will eventually do a BJT output stage for current gain. I know I'll have to do a class AB design if I every want to do any high power amps. If I use a FET based voltage gain stages and a BJT power stage, then I can get my softer clipping from my voltage gain stage without clipping in the power stage (I think). I'll probably go the chip route when my budget allows me to build a 50-100W amp. I've already done one chip amp, the Ruby from runoffgrove.com, which sounds pretty good.

Thanks for the reply,

Rowdy

Jack1962

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Re: 12 V MOSFET practice amp
« Reply #6 on: August 08, 2008, 04:34:07 AM »
Go to duncanamps.com , I think there is a link there to a free version of spice. However, I do agree with JM , but , you can use fet's if that is what you have in mind , I would use just plain old jfet's , I would also consider raising the voltag to 18 to 32 Volts. For tone, gain an volume controls , check out schematicheaven.com or any of the link for schematics on this forum , when I have to actually build an amp I generally build off a schematic of 1 or more amps.

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rowdy_riemer

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Re: 12 V MOSFET practice amp
« Reply #7 on: August 08, 2008, 08:28:40 AM »
Go to duncanamps.com , I think there is a link there to a free version of spice. However, I do agree with JM , but , you can use fet's if that is what you have in mind , I would use just plain old jfet's , I would also consider raising the voltag to 18 to 32 Volts. For tone, gain an volume controls , check out schematicheaven.com or any of the link for schematics on this forum , when I have to actually build an amp I generally build off a schematic of 1 or more amps.

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Well, I can't use jfets for the current gain on the output stage, but jfets would be great for the voltage gain stages, and I've got quite a few j201's. I'll definitely do a higher voltage design later on, but right now, I'm limited to the output of my ATX power supply :-\. I guess I need to just go ahead and get a decent power transformer. That being said, I'm happy with the volume I get from the amplifier right now. It's plenty loud enough for a practice amp. I'll probably put in a simple tone stack, or maybe I'll leave it as is and use it to test out different tone control pedals or some of both. Thanks for the info.

Rowdy

J M Fahey

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Re: 12 V MOSFET practice amp
« Reply #8 on: August 08, 2008, 04:54:04 PM »
Hi Rowdy. Teemuk has already been so kind as to post a link for free Spice (I think he uses that version, if I remember correctly) it is: http://www.linear.com/designtools/software/#Spice  I haven´t used it myself, but if the Master says it works ... I wouldn´t doubt it.
As Jack says: "Rock on"
Keep posting your experiments.
JM.

rowdy_riemer

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Re: 12 V MOSFET practice amp
« Reply #9 on: August 11, 2008, 06:01:33 PM »
I've made a few changes. I simplified the current source to use a voltage divider instead of a current mirror to reduce power consumption. This thing is still a power hog, but much more efficient than it was. Also, I replaced the 2n7000's with j201 jfets and biased accordingly. This increased the treble quite a bit. Still no volume or tone control yet. Volume and gain control will be easy, I just haven't gotten around to it yet. I've had a busy weekend.

Read http://www.riemer.us/mufu-12v-version-2 for more info.

btw, thanks to Jack1962 and J M Fahey for links to the spice software.
« Last Edit: October 06, 2009, 08:25:04 PM by rowdy_riemer »

Jack1962

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Re: 12 V MOSFET practice amp
« Reply #10 on: August 13, 2008, 08:58:40 PM »
nothin to it bro , that's what where all here for.

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