sorry if this is a silly question, but for a beginner, how do you start about designing an audio power amp?...i mean as in equations and generl rules of thumb
Design step
Desired Power output -> circuit topology -> ohm's law and diode drop to deduce values -> choose part # -> PCB layout.
There is really no straight rule. In fact, no 1->2->3 kind of rules work perfectly.
Desired Power output -> circuit topology -> ohm's law and diode drop to deduce values -> choose part # -> PCB layout.
There is really no straight rule. In fact, no 1->2->3 kind of rules work perfectly.
Decide the impedance, power handling and number of the speakers you want to drive and find build a power supply that will match. For example, if you want to build an amp to drive two 8-ohm 100w speakers, a +/-40v 300va transformer would work well. That same transformer would toast your output devices if you tried to drive a 2 ohm subwoofer (the voltage is too high). Also, your transformer sees no difference between driving four 8-ohm speakers and one 2-ohm speaker so take the number of channels into account.
Decide whether you want seperate power supplies for multiple channels. Seperate supplies eliminate crosstalk but if one channel is demanding a lot of power, the other supplies can't help out.
Make sure your output devices are rated to do their job. Low impedance loads require high current devices. High voltage rails require high voltage devices. It's a good idea to buy more than you need. For example, if you're building an amp to push 100 watts, you could use a transistor rated for 100 watts, but once that transistor warms up, it's safe power output falls due to a temperature derating curve. If you started with a device rated at 200 watts, it would be able to provide 100 watts safely while running warm.
Gain and negative feedback affect each other inversely. If I reduce the gain of the output stage on my amp by 3dB, the damping factor (a product of NFB) approximately doubles. Calculating how many dB of NFB you have is almost impossible and useless. Turning off the NFB of my amp to meaure open loop gain causes it to destabilize and is dangerous.
Decide whether you want seperate power supplies for multiple channels. Seperate supplies eliminate crosstalk but if one channel is demanding a lot of power, the other supplies can't help out.
Make sure your output devices are rated to do their job. Low impedance loads require high current devices. High voltage rails require high voltage devices. It's a good idea to buy more than you need. For example, if you're building an amp to push 100 watts, you could use a transistor rated for 100 watts, but once that transistor warms up, it's safe power output falls due to a temperature derating curve. If you started with a device rated at 200 watts, it would be able to provide 100 watts safely while running warm.
Gain and negative feedback affect each other inversely. If I reduce the gain of the output stage on my amp by 3dB, the damping factor (a product of NFB) approximately doubles. Calculating how many dB of NFB you have is almost impossible and useless. Turning off the NFB of my amp to meaure open loop gain causes it to destabilize and is dangerous.
Nothing silly about this question. But it is rather broad as it stands. Could you bound the question a little?
thank you for the reply..i know it is a bit of a broad question, but i was wondering how you work out the tail currents for the input stage. is it related to the output power? and also how do you work out the lag compensation capacitors for the VAS..
i have noticed that some topogies tend to have more caps than others..does this not slow the amp down?..how do you also work out degenration resistors for the output?..
im not sure as to which topologies are the best..i was thinking of maybe 100Wpc, with separate external PSU...i hear the double differential input is good (like the opti-mos -> very similar to the leach!)..
i have noticed that some topogies tend to have more caps than others..does this not slow the amp down?..how do you also work out degenration resistors for the output?..
im not sure as to which topologies are the best..i was thinking of maybe 100Wpc, with separate external PSU...i hear the double differential input is good (like the opti-mos -> very similar to the leach!)..
this may help
This is my opinion:
Fully symmetrical (mirror image), DC-coupled, Toshiba's JFETs 389/109 input stage driving cascoded Toshiba's 1815GR/1015GR driving 5 pairs of Toshiba's 5200/1943 emmiter folllowers with overall NFB. Use lead comp instead of lag comp. Input stage load use Toshiba's 3381/1349 as current mirrors. Use Toshiba's 5171/1930 b/w VA and OPS. Set VA bias @ 6mA.
VA cascode bjt use Toshiba's 3423/1360. Do not use caps in feedback path. Use nice quality resistors in feedback. Hope you can visualise the schematic and hope it helps!
Regards,
James
Praise! Audio
Australia
demons_wing said:
This is my opinion:
Fully symmetrical (mirror image), DC-coupled, Toshiba's JFETs 389/109 input stage driving cascoded Toshiba's 1815GR/1015GR driving 5 pairs of Toshiba's 5200/1943 emmiter folllowers with overall NFB. Use lead comp instead of lag comp. Input stage load use Toshiba's 3381/1349 as current mirrors. Use Toshiba's 5171/1930 b/w VA and OPS. Set VA bias @ 6mA.
VA cascode bjt use Toshiba's 3423/1360. Do not use caps in feedback path. Use nice quality resistors in feedback. Hope you can visualise the schematic and hope it helps!
Regards,
James
Praise! Audio
Australia
Its an excellent question, one that I do not really know the answer to in depth, but I will say what I do know.
Your design almost ALWAYS starts from where you wish to end up, ie your output.
So you have your target speakers, you need a goal, commercially, it would be the price, the features of the amp, etc., try to get some concrete ideas, thats why sometimes commercially driven can be an aide.
so say 100 watts, 8 ohm speakers, from this you can work out your voltage rails, the rest of your psu inc resevoir cap sizes, diodes, transformer.
Your output device types...bipolar, fet, bias, then your heatsink size, this will all determine the required VA stage, which will go back again and determine the input stage topology.
Then your feedback amount, which determines closed loop gain and sensitivity, they are ALL interrelated to some extent. Its quite a logical and fascinating process.
You can start from the very simple and embellish. no current source in the input, jsut a resistor, replace this with active, then cascode load, current mirrors, mirror image all the way through, parallel up outupt devices for more power(current), as to determing caps, I think it is done by scopes, playing around until the thing doesn't oscillate. Maybe with spice.
The big problem is there is so little info., it has taken me ages to learn stuff, writing is by and large sparce and a lot is VERY poor, not logical, bits omitted, you end up wondering what is what.
Even Linsley hood, god save his soul, his books were not really lucid, a bit hotch potch. Still valuable though.
A lot of it stems from valves, contrary to opinion, they are very similar to transistors, the mechanism of conduction is different, but the design principles pretty much the same, they are simpler to understand, too, transistors build on this with diff pairs, and active loads, both of which came lateish in valves.
So understand valve cascodes and you will have more insight into transistor ones. the reasons behind the topology. Audio is a MASSIVE subject, but it all boils down to amplitude, frequency, waves, phase, resistance, caps, chokes, volts and current, and making things bigger and smaller. That in a nutshell is it.
As to setting currents, I am not sure how the exact value is determined, again valve design helps, its to do with the loading imposed by the next stage.
Its actually something I have thought much about myself, hence it must be a good one😀 , a lot of designers are in the past in respect of many of the developments were done in the 1940s on, and there was loads of info. going around, this is now in old books which are getting dusty, maybe some of the designers grew up with these, now you just can't find the info., and I think some of the makers don't want you to either.
Ask the real experts, the borbelys, slone?, pass, etc. usually, these people can write well.
one thing is funny, like cars, they are all starting to appear similar, as we start to find the bits to optimise. Funny, too, there hasn't been much progress in topolgies since the late 60s.
Finally, what amazes me, though is the sheer variety of designs, even in the simple 3 stager, how many makers, how many different variations.
The last thing I have found is it may look good on paper, but sound quality IMHO is in inverse proportion to how elaborate the circuit is, ie measurement perfection kills sound.
Your design almost ALWAYS starts from where you wish to end up, ie your output.
So you have your target speakers, you need a goal, commercially, it would be the price, the features of the amp, etc., try to get some concrete ideas, thats why sometimes commercially driven can be an aide.
so say 100 watts, 8 ohm speakers, from this you can work out your voltage rails, the rest of your psu inc resevoir cap sizes, diodes, transformer.
Your output device types...bipolar, fet, bias, then your heatsink size, this will all determine the required VA stage, which will go back again and determine the input stage topology.
Then your feedback amount, which determines closed loop gain and sensitivity, they are ALL interrelated to some extent. Its quite a logical and fascinating process.
You can start from the very simple and embellish. no current source in the input, jsut a resistor, replace this with active, then cascode load, current mirrors, mirror image all the way through, parallel up outupt devices for more power(current), as to determing caps, I think it is done by scopes, playing around until the thing doesn't oscillate. Maybe with spice.
The big problem is there is so little info., it has taken me ages to learn stuff, writing is by and large sparce and a lot is VERY poor, not logical, bits omitted, you end up wondering what is what.
Even Linsley hood, god save his soul, his books were not really lucid, a bit hotch potch. Still valuable though.
A lot of it stems from valves, contrary to opinion, they are very similar to transistors, the mechanism of conduction is different, but the design principles pretty much the same, they are simpler to understand, too, transistors build on this with diff pairs, and active loads, both of which came lateish in valves.
So understand valve cascodes and you will have more insight into transistor ones. the reasons behind the topology. Audio is a MASSIVE subject, but it all boils down to amplitude, frequency, waves, phase, resistance, caps, chokes, volts and current, and making things bigger and smaller. That in a nutshell is it.
As to setting currents, I am not sure how the exact value is determined, again valve design helps, its to do with the loading imposed by the next stage.
Its actually something I have thought much about myself, hence it must be a good one😀 , a lot of designers are in the past in respect of many of the developments were done in the 1940s on, and there was loads of info. going around, this is now in old books which are getting dusty, maybe some of the designers grew up with these, now you just can't find the info., and I think some of the makers don't want you to either.
Ask the real experts, the borbelys, slone?, pass, etc. usually, these people can write well.
one thing is funny, like cars, they are all starting to appear similar, as we start to find the bits to optimise. Funny, too, there hasn't been much progress in topolgies since the late 60s.
Finally, what amazes me, though is the sheer variety of designs, even in the simple 3 stager, how many makers, how many different variations.
The last thing I have found is it may look good on paper, but sound quality IMHO is in inverse proportion to how elaborate the circuit is, ie measurement perfection kills sound.
Typical rules of thumb
The bigger the better until: the power dissipation limit of the input transistors is reached OR the input transistors become too non-linear OR the input impedance becomes too low.
The current must also be adequate to drive the following stage during worst case voltage and current slewing.
The smaller the better until the closed-loop response of the amplifier becomes too unstable. This must be verified into worst-case loads.
Related to the thermal characteristics of the BE junction. Used primarily to prevent current drift or runaway as the temperature of the output transistors changes.
Typically about 1mA.
The bigger the better until: the power dissipation limit of the input transistors is reached OR the input transistors become too non-linear OR the input impedance becomes too low.
The current must also be adequate to drive the following stage during worst case voltage and current slewing.
Typically 20pF to 100pF.
The smaller the better until the closed-loop response of the amplifier becomes too unstable. This must be verified into worst-case loads.
Depends where they are used. The voltage response of the amp needn't exceed 20kHz so slowness may be a red herring.
Typically 0.1 to 0.5 ohms for bipolars.
Related to the thermal characteristics of the BE junction. Used primarily to prevent current drift or runaway as the temperature of the output transistors changes.
You need to suck'em and see.
Re: Typical rules of thumb
very good advice.
I usually run mine 4ma for bjt input (driving mosfet vas). I have, however, ran the input stage at over 20ma (sometimes to 40ma) on mosfets. I don't detect any audible difference tho.
very good advice.
traderbam said:
I usually run mine 4ma for bjt input (driving mosfet vas). I have, however, ran the input stage at over 20ma (sometimes to 40ma) on mosfets. I don't detect any audible difference tho.
Re; - audio power amp design
Douglas Self has published articles on this subject in Electronics World magazine, and published a book called "Audio Power Amplifier Design Handbook" which is very good. It explains pretty much everything. Well worth reading if you want to know about design. There's very little maths, and loads of explanations, illustrated by graphs of simulations and measurements. It's very good. His web site;-
www.dself.dsl.pipex.com
I repeat, It's very good.
Douglas Self has published articles on this subject in Electronics World magazine, and published a book called "Audio Power Amplifier Design Handbook" which is very good. It explains pretty much everything. Well worth reading if you want to know about design. There's very little maths, and loads of explanations, illustrated by graphs of simulations and measurements. It's very good. His web site;-
www.dself.dsl.pipex.com
I repeat, It's very good.
Really really really interested in that. But afraid that I will get into wrong schematic. Can anybody help drawing that explenation? Or maybe Praise! yourself can submit a schematic picture?
"sorry if this is a silly question, but for a beginner, how do you start about designing an audio power amp?...i mean as in equations and generl rules of thumb"
If the goal is actually to have a successful physical outcome, you start on two parrallel tracks
Fisrt track, build at least a couple of cookbook designs whether the be Pass, Slone, D.Self or Rod Elliot design. Each ought to be of increasing complexity. It is no trivial matter if you are a beginner just get an existing design working.
Second track, use your library and/or Amazon to do a LOT of reading: Authors to look for include Joseph J. Carr, Paul Horowitz& Winfield Hill, Douglas Self, John Linsley Hood, G.Rand Slone. There is a lot of overlap as well as inconsistancies. This is good because it reinforces concepts but also shows where some of the uncertainties lie.
This sounds like a lot of work, and it is, but if the prospect of doing you own design is something that really appeals to you, you will get sucked in despite being overwhelmed in info.
If the goal is actually to have a successful physical outcome, you start on two parrallel tracks
Fisrt track, build at least a couple of cookbook designs whether the be Pass, Slone, D.Self or Rod Elliot design. Each ought to be of increasing complexity. It is no trivial matter if you are a beginner just get an existing design working.
Second track, use your library and/or Amazon to do a LOT of reading: Authors to look for include Joseph J. Carr, Paul Horowitz& Winfield Hill, Douglas Self, John Linsley Hood, G.Rand Slone. There is a lot of overlap as well as inconsistancies. This is good because it reinforces concepts but also shows where some of the uncertainties lie.
This sounds like a lot of work, and it is, but if the prospect of doing you own design is something that really appeals to you, you will get sucked in despite being overwhelmed in info.
thanks everyone for your replies...i did buy the randy slone book...i have only read upto the output stage section...i am thinking about buying the self book too...the thing is, i wish that there was a little but of maths, so i could see how things are cmoning about..i guess audio is a black art and there is no specific formula...can anyone sugest any simulation programs?..and also what kind of equipment would be useful to have?
"i am thinking about buying the self book too...the thing is, i wish that there was a little but of maths, so i could see how things are cmoning about..i guess audio is a black art "
Not so much as it may seem. I think that Slone has directed his book toward an audience without a strong technical background. You may find some of the JL Hood reading a bit more technical as well as anything basic to linear electronics. Part of the problem may be that if you want to go deeper you are quickly into more than can be covered in a single book.
Identify somne of the basic circuit concepts in Slone or one of the others. For example:
"current mirror"
"constant current source"
"diferential amplifier"
"miller compensation"
"Nyquist (in)stability"
Do a google search and follow where these lead. You will find a lot of university level course material, profgessional publications and even DIY discussions. Absorb as much asw you can. Most of these are not limited to audio but work the same no matter the application.
Even if you stick with Slone, you can take some of his example such as matching tail current with the miller cap values that you can layout in a spreadsheet. I've found this useful not for "cookbooking" but because if I can put some of these relationships accurately into a spreadsheet it helps reinforce my understanding of them.
There is some "black art" but it's not so much in the basics.
Not so much as it may seem. I think that Slone has directed his book toward an audience without a strong technical background. You may find some of the JL Hood reading a bit more technical as well as anything basic to linear electronics. Part of the problem may be that if you want to go deeper you are quickly into more than can be covered in a single book.
Identify somne of the basic circuit concepts in Slone or one of the others. For example:
"current mirror"
"constant current source"
"diferential amplifier"
"miller compensation"
"Nyquist (in)stability"
Do a google search and follow where these lead. You will find a lot of university level course material, profgessional publications and even DIY discussions. Absorb as much asw you can. Most of these are not limited to audio but work the same no matter the application.
Even if you stick with Slone, you can take some of his example such as matching tail current with the miller cap values that you can layout in a spreadsheet. I've found this useful not for "cookbooking" but because if I can put some of these relationships accurately into a spreadsheet it helps reinforce my understanding of them.
There is some "black art" but it's not so much in the basics.
demons_wing said:
sloan's book has detailed calculations to determine, amon gothers, operating points.
Once you understand his calculations (they are all very simple), you will be able to start to play with other circuitry or your own.
demons_wing said:
I would suggest for beginners either ltspice (by linear) or multisim (the version I have is very old). You can also download protel dxp in demo version. protel is tougher to learn tho.
demons_wing said:
I have nothing but a digital multimeter (a cheapo one too that I got over 10 years ago). I would suggest that you start with actually a power supply. and may be play with some pre amps and then go from there. I also would suggest that you start with breadboards, not PCBs. I always breadboard my ideas first before actually building them, so that I can work out the bugs.
People will suggest that you investment more in instruments but I think that's ill advised as you seem to be a beginner and it doesn't pay, in my experience, to invest too much in a hobby that you may or may not like down the road. so keep it simple until you absolutely need additional investments / investment.
What is LAG compensation and LEAD compensation?
Miller cap in VAS (C-B cap) is lag or lead?
Lead compensation (upper Collector VAS to inverting differential, about 20pf) is lead compensation, like its name?
Miller cap in VAS (C-B cap) is lag or lead?
Lead compensation (upper Collector VAS to inverting differential, about 20pf) is lead compensation, like its name?
"People will suggest that you investment more in instruments but I think that's ill advised "
I built my first two with just a DMM. I half suspect that if I had so much as put them on a 'scope I might never have completed them - too much to worry about.
I built my first two with just a DMM. I half suspect that if I had so much as put them on a 'scope I might never have completed them - too much to worry about.
is it hard to do simulations in protel DXP? this would be the program (poision??) of choice as it is an industry standard..me thinks me shall have to get the Self book too..would anyone know of any "white papers" or application/design notes online that i could read?
demons_wing said:
it has a steep learning curve but it is doable.
you still need to know what's behind the circuits / simulation so getting a good book will be hugely helpful.
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