...... PCB files .....and revised schematic
Alex.
Hi, Alex
What's version of your GM ?
I can't see any Zetex transistor ?
Thanks
Anadigit
jam - I know exactly what "Ground Plane" is now that this term has been explained to me by Krisfr. English is my second language and I just never heard this particular term before, that's all.
As far as the amplifier being on the verge of instability, complete nonsense.
If this amplifier is poorly constructed/designed it could oscillate, just like virtually any other solid state amplifier, no matter how simple. This is a non issue.
As far as the amplifier being on the verge of instability, complete nonsense.
If this amplifier is poorly constructed/designed it could oscillate, just like virtually any other solid state amplifier, no matter how simple. This is a non issue.
Jam, it is not unstable, I don't know why you would think that. Without the heavy (75 ohms!) degeneration on both LTPs it might be. See my post:
http://www.diyaudio.com/forums/soli...mplifier-i-have-ever-heard-9.html#post2328755
- keantoken
http://www.diyaudio.com/forums/soli...mplifier-i-have-ever-heard-9.html#post2328755
- keantoken
Nagys,
Having built amplifiers for the last forty years I strongly disagree with you. I am very familar with the topology in question and can be a bitch to stabllize. OS has also pointed out as such. In high bandwidth circuits like this a whole slew of factors can cause instability from board layout to stray capacitances. So it is not nonsense any qualified Rf engineer will tell you that.
If an instability were to arise my question to you is where would you apply compensation and what type of compensation would that be. Have you measured the output of the amp into a reactive load or highly capacitive speaker cables? You still have not shed light on your engineering qualifications.
Jam
Having built amplifiers for the last forty years I strongly disagree with you. I am very familar with the topology in question and can be a bitch to stabllize. OS has also pointed out as such. In high bandwidth circuits like this a whole slew of factors can cause instability from board layout to stray capacitances. So it is not nonsense any qualified Rf engineer will tell you that.
If an instability were to arise my question to you is where would you apply compensation and what type of compensation would that be. Have you measured the output of the amp into a reactive load or highly capacitive speaker cables? You still have not shed light on your engineering qualifications.
Jam
do you have power supply diagram for this amplifier??? please send me at this email add: jimms.espinosa@gmail.com
Ok, I'm not an engineer and have no qualifications, and there's no reason for anyone to listen to me, but in simulation the circuit has very well-defined gain behavior. After billions of hours simulating, I am dead sure that this is almost solely because of the high degeneration resistors, which swamp nonlinear Gm and therefore linearize phase/pulse/HF response. Considering this, the frontend should be VERY stable, it's the power stage that is problematic. Am I right? Does this mean that the oscillation is more a problem with the FETs self-oscillating? It would take a very highly reactive network, like the high-Q stuff that comes from active components near oscillation, to throw the frontend off. Am I right?
- keantoken
- keantoken
As long as the potential builders don't mess with any of the capacitors (i.e. 47pF, 4.7pF, 10nF, etc.) and include the Zobel network and series inductor, the amp should be fine.
High capacitance speaker cables can cause instability in many different solid state amplifiers. Simply don't use them!
What else can be done to prevent potential oscillations? Installing 75ohm resistors in the RCA interconnect cables, on the SOURCE/TRANSMITTING end. That could act as a preventative method.
High capacitance speaker cables can cause instability in many different solid state amplifiers. Simply don't use them!
What else can be done to prevent potential oscillations? Installing 75ohm resistors in the RCA interconnect cables, on the SOURCE/TRANSMITTING end. That could act as a preventative method.
Wait, I didn't see your last response Jam.
I stated in my last response my observations on the the effect of the degeneration on phase behavior. While they decrease open-loop gain, they linearize phase behavior, at least for the frontend. This leads me to think they would improve imaging and soundstage. How they affect other aspects of the sound, I am not sure (hell, I'm not sure of any of this though). Due to the lowered OLG, the distortions of the output stage will be emphasized. This may not seem to be a big problem if the 2SK1058/J49 give benign distortions. If the FET parasitic capacitances are too nonlinear, they may ruin the phase behavior when played loudly.
Of course, it may be more difficult to tell how this behavior will change after the crossover threshold is reached.
- keantoken
I stated in my last response my observations on the the effect of the degeneration on phase behavior. While they decrease open-loop gain, they linearize phase behavior, at least for the frontend. This leads me to think they would improve imaging and soundstage. How they affect other aspects of the sound, I am not sure (hell, I'm not sure of any of this though). Due to the lowered OLG, the distortions of the output stage will be emphasized. This may not seem to be a big problem if the 2SK1058/J49 give benign distortions. If the FET parasitic capacitances are too nonlinear, they may ruin the phase behavior when played loudly.
Of course, it may be more difficult to tell how this behavior will change after the crossover threshold is reached.
- keantoken
It is stable provided there s something like 1R + 1uH in serial with
the output.
Otherwise, like many amplifiers, it s severly ringing once there s
more then 50nF paraleled with the load.
At more than 100nF the ringing is close to the step amplitude,that is,
plain instability..
So the output LR circuit is not to be neglected...
The amp is slightly underdamped to reach a slew rate of about
+70VuS and -80VuS respectively for the positive and negative
going of the signal..
the output.
Otherwise, like many amplifiers, it s severly ringing once there s
more then 50nF paraleled with the load.
At more than 100nF the ringing is close to the step amplitude,that is,
plain instability..
So the output LR circuit is not to be neglected...
The amp is slightly underdamped to reach a slew rate of about
+70VuS and -80VuS respectively for the positive and negative
going of the signal..
Nagys,
You are totally off base. Your assumption that if use the same parts the amp will be stable. Well, what about the board material, layout, component spacing and location and list goes on, after all we are talkingf about a high speed circuit here.
I don't think your 75ohm resistor solution applies in this case.
You still have not shed any light on your qualifications.
Jam
You are totally off base. Your assumption that if use the same parts the amp will be stable. Well, what about the board material, layout, component spacing and location and list goes on, after all we are talkingf about a high speed circuit here.
I don't think your 75ohm resistor solution applies in this case.
You still have not shed any light on your qualifications.
Jam
Wahab,
Bingo! You have identified a problem. No stability without an output inductor.
.......but many noted designers have pointed out that the output inductor can be a big compromise.
There are more problems withn the design but OS is more qualified to explain them, in fact he has already pointed a few of them out.
Regards,
Jam
Bingo! You have identified a problem. No stability without an output inductor.
.......but many noted designers have pointed out that the output inductor can be a big compromise.
There are more problems withn the design but OS is more qualified to explain them, in fact he has already pointed a few of them out.
Regards,
Jam
I don't think I'm off base at all. The RC and LR circuits are there to make sure the amplifier is stable. So are all the small value capacitors in the schematic. Yes, board layout is important, but not as important as you make it sound to be. Most logical layouts should work just fine. Alex's board will work great. I'm willing and will put my money on it.
I've built this amp before with a much larger/bulkier circuit board, without a ground plane and it worked great. No oscillations of any kind. The ones I've built had a power supply on a separate board, no protection circuit, and the MOSFETS were point to point wired. The design that we have here is superior in every possible way.
The 75ohm resistor can sometimes help keep the amplifier from oscillating. But I do understand, it's a band aid fix.
As for my engineering background, education, and qualifications: I'm not as qualified as some, but more qualified than most
I've built this amp before with a much larger/bulkier circuit board, without a ground plane and it worked great. No oscillations of any kind. The ones I've built had a power supply on a separate board, no protection circuit, and the MOSFETS were point to point wired. The design that we have here is superior in every possible way.
The 75ohm resistor can sometimes help keep the amplifier from oscillating. But I do understand, it's a band aid fix.
As for my engineering background, education, and qualifications: I'm not as qualified as some, but more qualified than most
I just occurred to me, if we're using a matched Jfet, why not put the offset control at the degeneration of the Jfet rather than at the input to the VAS, so that we're not disbalancing the jfets in order to correct offset? Or maybe the idea is that for a dual jfet, they will be most balanced when offset is zero, since they're matched? But then the degeneration resistors should be low-tolerance types.
- keantoken
- keantoken
Nagsy,
Your assumption that high slew rate amps sound better is not true. It depends on the amplifier and if if is ringing or misbehaving. Past a certain vale the term slew rate becomes academic, other factors come into play. You make blanket statements like mosfets sound better without understanding their application, is simplisic at best....it all comes down to application and use.For the record mosfets don't sound anything remotely like tubes.(Another myth)
You will find that the design rings quite badly into a reactive load. I am not saying that an output inductor is totally a bad idea for some amps into certain loads but you will find this design totally useless without the output inductor.
For the record there are many mosfet amps out there that do not use an output inductor, in fact quite a few modern amps do not use an output inductor.
I am trying to point out that this design rings badly into a reactive load, I observerd it to ring into a purely resistive load all pointing to the fact that the amp is on the verge of instability.
Jam
Your assumption that high slew rate amps sound better is not true. It depends on the amplifier and if if is ringing or misbehaving. Past a certain vale the term slew rate becomes academic, other factors come into play. You make blanket statements like mosfets sound better without understanding their application, is simplisic at best....it all comes down to application and use.For the record mosfets don't sound anything remotely like tubes.(Another myth)
You will find that the design rings quite badly into a reactive load. I am not saying that an output inductor is totally a bad idea for some amps into certain loads but you will find this design totally useless without the output inductor.
For the record there are many mosfet amps out there that do not use an output inductor, in fact quite a few modern amps do not use an output inductor.
I am trying to point out that this design rings badly into a reactive load, I observerd it to ring into a purely resistive load all pointing to the fact that the amp is on the verge of instability.
Jam
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