Re: Biasing output mosfets
That's ok for later on but not recommended when you first set up the amp. The reason for the resistors in line with the power rails is to protect the FETs in case of mistakes. If there are errors and all the FETs are fully turned on at the same time then literally a hundred amps or more will flow through them. This will result in a brief, if not spectacular failure ....and a bill for a new set of FETs.
So while the suggested resistors will help you set up the amp their real purpose is to protect it .....and we all make mistakes.
Cheers
Dusk said:
That's ok for later on but not recommended when you first set up the amp. The reason for the resistors in line with the power rails is to protect the FETs in case of mistakes. If there are errors and all the FETs are fully turned on at the same time then literally a hundred amps or more will flow through them. This will result in a brief, if not spectacular failure ....and a bill for a new set of FETs.
So while the suggested resistors will help you set up the amp their real purpose is to protect it .....and we all make mistakes.
Cheers
In fact discrete needn't be any more complicated at all. Each to their own though.
Re discreet design
Hey Dusk,
I gotta agree with the other guys. Why don't you check out my thread "power amp under development".
The complete schematics and board layouts are posted ready to go so you wont have to design your board. It also uses N-channel FETs only so it could actually work out cheaper than the opamp version.
As a bonus the 2 that I have built so far are very powerful, very quiet and proving to be very reliable. Overall it is far superior to the op-amp design.
But hey you gotta do what you gotta do ...so keep us posted.
Cheers again.
Hey Dusk,
I gotta agree with the other guys. Why don't you check out my thread "power amp under development".
The complete schematics and board layouts are posted ready to go so you wont have to design your board. It also uses N-channel FETs only so it could actually work out cheaper than the opamp version.
As a bonus the 2 that I have built so far are very powerful, very quiet and proving to be very reliable. Overall it is far superior to the op-amp design.
But hey you gotta do what you gotta do ...so keep us posted.
Cheers again.
Discrete design
guasi
I have built one all discrete mosfet power amplifier based on the old Hitachi circuit and it works fine it's a 100watt/8ohm and 150watt/4ohm. Itis used as a guitar power amplifier.
I wanted to try somthing different with the op amp design this time.....but thanks for the suggestion.
Now,back to the Elektor....project,if i change R8&R13 to 180ohm won't this decrease heat/power at idle?
2)change R5 240ohm to 22ohm
3)put a 330p in series with R6 1K on the output.
If i change R8 and R13 willthe amplifier run more like a classA/B? and less heat on the heatsink(main heatsink will be a fan tunnel).
As the values are now,is it running more class A?........let me know
Peace
guasi
I have built one all discrete mosfet power amplifier based on the old Hitachi circuit and it works fine it's a 100watt/8ohm and 150watt/4ohm. Itis used as a guitar power amplifier.
I wanted to try somthing different with the op amp design this time.....but thanks for the suggestion.
Now,back to the Elektor....project,if i change R8&R13 to 180ohm won't this decrease heat/power at idle?
2)change R5 240ohm to 22ohm
3)put a 330p in series with R6 1K on the output.
If i change R8 and R13 willthe amplifier run more like a classA/B? and less heat on the heatsink(main heatsink will be a fan tunnel).
As the values are now,is it running more class A?........let me know
Peace
1. Changing R8 & R13 will not make any difference to the heat dissipated at idle. FETs are voltage driven and these resistors simply set the turn on voltage for the FETs (approx 4 volts). It would not matter if these resistors were 100 ohms or 100,000 ohms. The difference the value of these resistors make is the capability to charge and discharge the gate capacitance of the FET. Generally it is better to have a lower value resistor because this will present a lower charge and discharge impedance to the gate. This is why I mentioned that 270 ohms would be a better value here.
2. Do not change R5 to 22 ohms. This will exceed the drive capability of the op-amp. You see the way this cct works is actually interesting. The signal delivered to the FETs is from changes to op-amps supply current draw as it delivers an unused output into R5 which is like a dummy load. The higher the voltage into R5 (equals higher current) the more current the op-amp will draw through R8 & R13. This changing current translates into a changing voltage across these resistors and hence a changing voltage on the FET gates.
3. Umm....no capacitors here please. You may be thinking about protecting the op-amp output from excessive voltage swing from the output stage (FETs) output. This point is valid so change R6 to 2k2.
This amp is a class A/B amp. By virtue that the amp is set up with 100mA or so in the output stage means that with low signal both FETs will be partially on. Under these signal conditions (probably around 1 watt) the amp is operating in Class A. As the signal gets stronger one FET will turn on harder and the other FET will turn off. This point is called the crossover point (you've heard of crossover distortion). In this mode the amp is operating in class B. The reason amps are biased is to shift the crossover point higher up the waveform (power scale) so that it is less audible.
If this amp was a true class A then none of the FETs would ever turn off. This amp could never run in class A with +/- 55 volts unless you had maybe 10 FETs (strong ones) per rail and heatsinks the size of a car radiator.
Sorry about the length of the reply.....
Cheers
2. Do not change R5 to 22 ohms. This will exceed the drive capability of the op-amp. You see the way this cct works is actually interesting. The signal delivered to the FETs is from changes to op-amps supply current draw as it delivers an unused output into R5 which is like a dummy load. The higher the voltage into R5 (equals higher current) the more current the op-amp will draw through R8 & R13. This changing current translates into a changing voltage across these resistors and hence a changing voltage on the FET gates.
3. Umm....no capacitors here please. You may be thinking about protecting the op-amp output from excessive voltage swing from the output stage (FETs) output. This point is valid so change R6 to 2k2.
This amp is a class A/B amp. By virtue that the amp is set up with 100mA or so in the output stage means that with low signal both FETs will be partially on. Under these signal conditions (probably around 1 watt) the amp is operating in Class A. As the signal gets stronger one FET will turn on harder and the other FET will turn off. This point is called the crossover point (you've heard of crossover distortion). In this mode the amp is operating in class B. The reason amps are biased is to shift the crossover point higher up the waveform (power scale) so that it is less audible.
If this amp was a true class A then none of the FETs would ever turn off. This amp could never run in class A with +/- 55 volts unless you had maybe 10 FETs (strong ones) per rail and heatsinks the size of a car radiator.
Sorry about the length of the reply.....
Cheers
The Hitachi cct..
If you are talking about the same cct that I have seen (Hitachi application note) then you may have genuine misgivings about the performance of a discreet design.
I have never built that amp but the impression I got was that amp may have been noisy (hum). This is because the input stage has poor Power Supply Ripple Rejection.
Almost all of the designs I have seen in this forum operate a constant current source in the input stage and this virtually eliminates the problem.
Let me assure you that with my amp that I mentioned earlier the output is absolutely silent (no hum, buzz, hiss etc).
I thought I would mention this if this is why you wanted to move from the discreet design.
Cheers again.
If you are talking about the same cct that I have seen (Hitachi application note) then you may have genuine misgivings about the performance of a discreet design.
I have never built that amp but the impression I got was that amp may have been noisy (hum). This is because the input stage has poor Power Supply Ripple Rejection.
Almost all of the designs I have seen in this forum operate a constant current source in the input stage and this virtually eliminates the problem.
Let me assure you that with my amp that I mentioned earlier the output is absolutely silent (no hum, buzz, hiss etc).
I thought I would mention this if this is why you wanted to move from the discreet design.
Cheers again.
Change in values
Hi quasi
Don't worry lenght of reply........iamlearnig something here.....and thats good.
I won't make the changes in values of componets.but will change the ones that u said would improve the circuit.Your reply has help me understand this circuit a little better, as i said iam no amp Guru.
I do have another question........about the IRFP9240 &IRF240 i was told that Hitachi were better because they were made for audio only and all their values are matched......butu pointed out that they (IRF)would be more efficient at full power.....anyway do i need to match IRFP9240 to each other voltage wise and the match the IRFP240 to each other as well voltage wise....i hope this make sense.
About the Hitachi application note circuit....no it did't turn me off to discrete circuits...i rather like it and i play it all the time.....i know it has a passive constant current setup and i have seen version of this circuit with an active constsnt current setup in the diff. input,but for guitar playing it's ok because the guitar is a low -frequency thing. about 5khz .and the Hitachi circuit get noisy after it passes into high freq......but like i say it's cool for guitar a low-freq. thing........i did learn that u could build 2 of these circuit (Hitachi circuits) and make one inverting and the other non-inverting .......and set them up in a bridge circuit to get more power .....that is cool........no i like that discret cause i only use it for guitar playing and it'sa easy one to build.
.......now who has a long reply......smile......if only i could type.
Peace
Hi quasi
Don't worry lenght of reply........iamlearnig something here.....and thats good.
I won't make the changes in values of componets.but will change the ones that u said would improve the circuit.Your reply has help me understand this circuit a little better, as i said iam no amp Guru.
I do have another question........about the IRFP9240 &IRF240 i was told that Hitachi were better because they were made for audio only and all their values are matched......butu pointed out that they (IRF)would be more efficient at full power.....anyway do i need to match IRFP9240 to each other voltage wise and the match the IRFP240 to each other as well voltage wise....i hope this make sense.
About the Hitachi application note circuit....no it did't turn me off to discrete circuits...i rather like it and i play it all the time.....i know it has a passive constant current setup and i have seen version of this circuit with an active constsnt current setup in the diff. input,but for guitar playing it's ok because the guitar is a low -frequency thing. about 5khz .and the Hitachi circuit get noisy after it passes into high freq......but like i say it's cool for guitar a low-freq. thing........i did learn that u could build 2 of these circuit (Hitachi circuits) and make one inverting and the other non-inverting .......and set them up in a bridge circuit to get more power .....that is cool........no i like that discret cause i only use it for guitar playing and it'sa easy one to build.
.......now who has a long reply......smile......if only i could type.
Peace
The reason most people match output FETs (and other stuff) is to try and achieve even current sharing between the output devices and better distortion figures. Achieving even current sharing can remove the need for using source resistors on the output (but I always use them). Also whilst I have matched devices myself before, I have stopped doing so because the differences achieved in terms of distortion were completely inaudible and often immeasurable.
You can match IRF FETs too, but you would need to buy 20 or so of each and then try to get 2 or 3 matched pairs. IMO ....don't bother matching the FETs. As Richieboy suggested use source resistors of around 0.33 ohms.
Information you received that the Hitachi mosfets are matched is just mumbo jumbo (unless the people you bought them from actually matched them). All semiconductors operate within a tolerance range just like any other device.
You can bridge almost any single ended amp, by inverting the input signal into its partner.
You can match IRF FETs too, but you would need to buy 20 or so of each and then try to get 2 or 3 matched pairs. IMO ....don't bother matching the FETs. As Richieboy suggested use source resistors of around 0.33 ohms.
Information you received that the Hitachi mosfets are matched is just mumbo jumbo (unless the people you bought them from actually matched them). All semiconductors operate within a tolerance range just like any other device.
You can bridge almost any single ended amp, by inverting the input signal into its partner.
Matching Fets
Hi quasi
Thanks for the information on matching the IRFs. Trying to macth 20 or more pair could hurt the pocket$$$$$$$$,so i will only buy a little over what i need and use source resistors.
On bridge the Hitachi circuit,i could use two amplifier one inverted and one non-inverted and get300watts or add more mosfets and get about 400watts. I may build this bridge set-up later but this time i will use a active constant-current source in the diff. input of each amplifier. Now, back to the op amp circuit, this will be a test bed.......meaning once it is up and running i will use other op amp in it to see what effect the have on the sound.
THANKS.....for your on-going help
Peace
Hi quasi
Thanks for the information on matching the IRFs. Trying to macth 20 or more pair could hurt the pocket$$$$$$$$,so i will only buy a little over what i need and use source resistors.
On bridge the Hitachi circuit,i could use two amplifier one inverted and one non-inverted and get300watts or add more mosfets and get about 400watts. I may build this bridge set-up later but this time i will use a active constant-current source in the diff. input of each amplifier. Now, back to the op amp circuit, this will be a test bed.......meaning once it is up and running i will use other op amp in it to see what effect the have on the sound.
THANKS.....for your on-going help
Peace
speaking of Tube Works
Hey -- can I ask whether anyone has the schematic for the MosValve RT-2100 by Tube Works (now owned by Genz Benz, who wants $10.00 for the info)? I found one and a painted-over cabinet to match; both are in sorry shape, but the amp looks salvageable. I need to know which (power transistor?) to use to replace the chipped one screwed to the heat sink. The rest ofthe electronics appear sound, and I fixed the ugly jumper he had in there because the tube socket lost one of its little leg pinchers. Also, I'll have to build a cabinet for the head -- it's only a chassis.
I haven't been back to the thread about the airplane on a treadmill for a while...hmmm....
I know this is audio, but you might try one of those little solar-powered BEAM robots you make out of old Walkman parts and stuff, for an interesting diversion.
Jonathan
Hey -- can I ask whether anyone has the schematic for the MosValve RT-2100 by Tube Works (now owned by Genz Benz, who wants $10.00 for the info)? I found one and a painted-over cabinet to match; both are in sorry shape, but the amp looks salvageable. I need to know which (power transistor?) to use to replace the chipped one screwed to the heat sink. The rest ofthe electronics appear sound, and I fixed the ugly jumper he had in there because the tube socket lost one of its little leg pinchers. Also, I'll have to build a cabinet for the head -- it's only a chassis.
I haven't been back to the thread about the airplane on a treadmill for a while...hmmm....
I know this is audio, but you might try one of those little solar-powered BEAM robots you make out of old Walkman parts and stuff, for an interesting diversion.
Jonathan
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