mikek, I wouldn't dispute that there are big differences between mosfets and the bjts.
However, care to share with us 1) how the difference between the two bjts isn't big and 2) why the big differrences between the mosfets and the bjts will for sure cause the vise mentioned in the original post?
However, care to share with us 1) how the difference between the two bjts isn't big and 2) why the big differrences between the mosfets and the bjts will for sure cause the vise mentioned in the original post?
millwood said:
Differences do exist between the two BJTs certainly......for instance, the 21194's ustain beta at high cuurents more effctively than 15003/4s........different ft's......etc...
....but these differences are no where as large as the difference between these devices and any given MOSFET.
millwood said:
i am not sure i understand the question?
mikek said:
why did you say that, mikek?
mikek said:i am not sure i understand the question?
we all know that those devices are different. so there is really no value in pointing that out. What would have been a lot more value-added is if you could articulate how the differences can lead to audible differences, and more specifically, all the deficiencies mentioned in owdeo's orginal posts.
For example, if you can walk us through on how the Leach amp was designed to handle OPS capacitance, and its practical limit in handling capacitive loads and pointing out, "Hi, those MOSFETS have higher g-s junction capacitance, per their datasheet, than what Prof. Leach had allowed for in the design. But the MJs, per their datasheet, don't exceed the design limit of the leach amp", I would agree with you. Simply telling us that the MOSFETs have more capacitance than the BJTs, then it must have exceeded the design limit of the Leach amp is stretching.
Again, the mere fact that they are different isn't really helpful in answering that question.
millwood said:
Actually, i din't comment on whether or not the leach amp. can handle MOSFET's.....
This should not be a problem, provided the complementary drivers are not removed together with the output stage BJT's....they are capable of sourcing as much current as is desired by your MOSFET.
As for 'sound quality', a well designed MOSFET power amp. should sound indistinguishable from a well designed BJT amp. both driven within spec.
Thank you for answering some of Millwood's queries Mikek. The articles you refer to look interesting but I didn't have time to read them thouroughly enough to see whether it is really new or just marketing hype 
What do you think?
I agree that driving the capacitance of MOSFETs with the second drivers in the triple output stage of the Leach shouldn't be a problem, but as I mentioned earlier the Leach is a low-feedback design (has low open loop gain) and is therefore optimised for bipolars. MOSFETs are much less linear than bipolars (about 50 times more distortion) and the Leach woundn't have enough open loop gain to allow enough feedback to correct for the non-linearity of the MOSFETs. If you look at good MOSFET amplifiers (ie low distortion) you will see that they are designed with high open-loop gain to allow more feedback to correct for the MOSFET non-linearity.
I think it is fruitless arguing with your point about output device differences Millwood, as I think it shows a lack of understanding of the devices concerned. I think the point is that it is possible to make good amplifiers using bipolars or MOSFETs, but to take an amplifier that has been designed for one device and simply swap the outputs for the other device is reckless and disregarding of the designer's hardwork (not that I haven't done it...;-). And the results are extremely unlikely to be good....probably will make an oscillator
Mikek, I can't agree with your comment about sound quality - I think I can always tell a MOSFET amp from the tonal quality produced....but let's not get into a subjective vs objective debate!
What do you think?I agree that driving the capacitance of MOSFETs with the second drivers in the triple output stage of the Leach shouldn't be a problem, but as I mentioned earlier the Leach is a low-feedback design (has low open loop gain) and is therefore optimised for bipolars. MOSFETs are much less linear than bipolars (about 50 times more distortion) and the Leach woundn't have enough open loop gain to allow enough feedback to correct for the non-linearity of the MOSFETs. If you look at good MOSFET amplifiers (ie low distortion) you will see that they are designed with high open-loop gain to allow more feedback to correct for the MOSFET non-linearity.
I think it is fruitless arguing with your point about output device differences Millwood, as I think it shows a lack of understanding of the devices concerned. I think the point is that it is possible to make good amplifiers using bipolars or MOSFETs, but to take an amplifier that has been designed for one device and simply swap the outputs for the other device is reckless and disregarding of the designer's hardwork (not that I haven't done it...;-). And the results are extremely unlikely to be good....probably will make an oscillator
Mikek, I can't agree with your comment about sound quality - I think I can always tell a MOSFET amp from the tonal quality produced....but let's not get into a subjective vs objective debate!
The reason I did this in the first place was that I had 16 of these Mosfets from the scrapped 6000 amps. I decided to build a biamp system and was trying to save money by reusing these. The redesigned boards I made worked fine in terms of they didnt oscillate or anything, which was a major achievment after the 6000, but I agree that the dumping of 15 year old mosfets into the Leach was a bit stoopid. The amps sound a bit thick and dont play loud well. If I was to do it again I would build the Leaches and use maybe a single pair of the TOP3 transistors without any emitter resistors or output protection. For Biamp or Triamp systems the cost blows out fairly soon!
Hi Whitetrash,
Don't blame you for trying to re-use those expensive MOSFETs. But try the Leach with bipolars and I'm sure you will like it. The "thick" sound you describe is surely due to high levels of distortion caused by the MOSFETs. Even with Bipolars the Leach is not a low-distortion amp!
BUT, don't try and build the Leach without emitter resistors! You won't be able to stabilise the bias - the resistors are not that expensive (I used 3 x 1ohm 1W in parallel for each)! Guess it is fair enough to omit the protection circuitry and use one pair of outputs if you want to save money, but you might be better off building a good lower powered amp if you don't need the Leach's power. Besides, it's nice to a have an amp that is blow-up-proof (the Leach really is!) BTW, because of the triple output stage, the Leach is not overly critical of output device performance so you can use cheap (= old) output bipolars provided they can handle the voltage and current (the specified ones are 2MHz fT). (Note to Millwood: this is not the same as saying you can use another TYPE of device such as a MOSFET in place of the bipolars)
Don't blame you for trying to re-use those expensive MOSFETs. But try the Leach with bipolars and I'm sure you will like it. The "thick" sound you describe is surely due to high levels of distortion caused by the MOSFETs. Even with Bipolars the Leach is not a low-distortion amp!
BUT, don't try and build the Leach without emitter resistors! You won't be able to stabilise the bias - the resistors are not that expensive (I used 3 x 1ohm 1W in parallel for each)! Guess it is fair enough to omit the protection circuitry and use one pair of outputs if you want to save money, but you might be better off building a good lower powered amp if you don't need the Leach's power. Besides, it's nice to a have an amp that is blow-up-proof (the Leach really is!)
BTW, because of the triple output stage, the Leach is not overly critical of output device performance so you can use cheap (= old) output bipolars provided they can handle the voltage and current (the specified ones are 2MHz fT). (Note to Millwood: this is not the same as saying you can use another TYPE of device such as a MOSFET in place of the bipolars)owdeo said:
yet, you still went ahead and blasted the idea of using MOSFETs in the Leach amp. A little bit of inconsistency?
owdeo said:
then you better tell Mr. Pass how screwed up his MOSFET modification of the Citation 12 is. and how reckless and disregarding he has been to the H/K designers,
Listen, this is a place for people to share ideas and if you think you are so above the rest of us (and Mr. Pass) that you don't need to provide any facts and scientific before blasting others, why don't you find a place of your to do that?
Millwood, I'm not posting here because I want an argument (but I will give you a bit.... So, for the record:
All of my comments on using MOSFETs in the Leach have explained the reasons why I believe this to be the case - your comment that I haven't offered reasons is completely untrue.
I certainly don't believe that I am in any way superior to you or anyone else - I am simply expressing my opinion and offering my reasons for why I have said opinion. I have every respect for Mr Pass and I think his knowledge of amplifier design is much greater than mine, but I'm sure he would agree that using MOSFETs on the Leach without modifying the circuit in any other way will not give good results. His modification to the Citation12 presumably changed the circuit in other ways to compensate for the use of MOSFETs, and wasn't a blind "swap devices and see" exercise.
Incidently, Whitetrash has said that his Leach sounds bad with MOSFETs. What more evidence do you want!?
Perhaps then you can offer some reasons why you think MOSFETs WILL work well on the Leach?
So, for the record:All of my comments on using MOSFETs in the Leach have explained the reasons why I believe this to be the case - your comment that I haven't offered reasons is completely untrue.
I certainly don't believe that I am in any way superior to you or anyone else - I am simply expressing my opinion and offering my reasons for why I have said opinion. I have every respect for Mr Pass and I think his knowledge of amplifier design is much greater than mine, but I'm sure he would agree that using MOSFETs on the Leach without modifying the circuit in any other way will not give good results. His modification to the Citation12 presumably changed the circuit in other ways to compensate for the use of MOSFETs, and wasn't a blind "swap devices and see" exercise.
Incidently, Whitetrash has said that his Leach sounds bad with MOSFETs. What more evidence do you want!?
Perhaps then you can offer some reasons why you think MOSFETs WILL work well on the Leach?
owdeo said:
with due respect, I think all you offered are "possibilities". Things that can possibly go wrong. Take the following as an example:
we know that the leach amp is a low-feedback design. And we also know that MOSFETs are more non-linear than BJTs. Those are facts.
However, you provided no concrete prove that the leach amp's open loop gain is so low that it would not correct the non-linearity of the MOSFETs.
You are simply stretching the turth in concluding the above. If you want to convince us, go through the numbers and show us the money.
owdeo said:
Rather than guessing, why don't you just ask him before putting words in his mouth?
owdeo said:
Again, don't ***-u-me. go pick up Mr. Pass' schematics and study it before your next post and you will be 100% sure what changes he has made to the schematics.
owdeo said:
It may very well be that MOSFETs don't sound good. But that doesn't mean that you took the right route getting there.
owdeo said:
I don't know if it will work well or not on the leach. I can list quite a few reasons why and why not, tho.
For starters, the MOSFETs tend to have gs capacitance and that could cause problems with pre-drivers (but the leach amp used pretty heafty pre-drivers so that may not be a problem). the mosfets also are far less efficient in linear applications so output power suffers and one may have to deal with more power dissipation. The use of the T-topology in the output isn't new, but I haven't seen MOSFETs used in a T-topology so I am not 100% confident as if will work (I see no reason why it wouldn't).
But foundamentally, a Mosfet or a Bjt is just a device that controls the output. If you can, think of them as a varistor. Mr. Pass' article on Citation 12 goes into some details between similarity of both devices and I would suggest that you read it before your next post as well. It was helpful to me.
I don't know this. A BJT has an exponential gm, a FET has a square-law gm. Which is more linear? It all depends how you use the device.
I understand the analogy but sometimes thinking of a transistor as a variable resistor is too inaccurate and can lead to a misleading mental model of what is going on. Think diodes, think charge.
Millwood,
Your disection of my posts is starting to seem a bit vindictive. I only really wanted a 5-minute argument...
I really don't understand why you want me to offer proof or even evidence for my opinions. I really couldn't care less whether you agree with me and I am not out to convince anyone. I'm simply offering my opinions on the subject based on my knowledge and experience.
You seem to be arguing that I can't express an opinion without backing it up with measurements or calculations. If that is the case, what is the point of this forum? Do you require scientific papers as posts on this thread?
BTW, my knowledge of device theory is based on studies of semiconductor physics. Your comments imply that your knowledge is based on an old article by Mr Pass that compares transistors to varistors. I certainly understand that idea in relation to MOSFETs where the D-S channel "resistance" is controlled by the gate voltage, but bipolars are current controlled devices and here lies one of the fundamental reasons why they are not directly interchangeable without considering other factors of the design.
Does anyone out there have any views on amplifier topology along the lines of the original post on this thread? This egotistical criticism of reasonably expressed opinions is getting extremely tedious
Your disection of my posts is starting to seem a bit vindictive. I only really wanted a 5-minute argument...
I really don't understand why you want me to offer proof or even evidence for my opinions. I really couldn't care less whether you agree with me and I am not out to convince anyone. I'm simply offering my opinions on the subject based on my knowledge and experience.
You seem to be arguing that I can't express an opinion without backing it up with measurements or calculations. If that is the case, what is the point of this forum? Do you require scientific papers as posts on this thread?
BTW, my knowledge of device theory is based on studies of semiconductor physics. Your comments imply that your knowledge is based on an old article by Mr Pass that compares transistors to varistors. I certainly understand that idea in relation to MOSFETs where the D-S channel "resistance" is controlled by the gate voltage, but bipolars are current controlled devices and here lies one of the fundamental reasons why they are not directly interchangeable without considering other factors of the design.
Does anyone out there have any views on amplifier topology along the lines of the original post on this thread? This egotistical criticism of reasonably expressed opinions is getting extremely tedious
owdeo said:
owdeo, if you are offering your opinion, please present them as such, not under the disguise of "facts".
owdeo said:
in which article did Mr. Pass compare transistors to varistors?
owdeo said:
if that's your understanding of semiconductor physics, I would be seriously questioning how good the school you went to learn semiconductor physics really is,
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