Pietjers
You ask about sonic benefits. My experience is that the benefits of cascode topology are in the front and mid stages of a power amp. I believe cascode topology can provide sonic benefits in all stages of a pre amp. Of course this all depends on the actual topology and devices used. This is based solely upon the amplifiers that I have designed and built for myself over the years. I have found that cascode designs are easy to listen to.
With the advent of power mosfets, I find myself agreeing with Ha, that the benefits of cascoding in the output stage would not apply any more.
Don
You ask about sonic benefits. My experience is that the benefits of cascode topology are in the front and mid stages of a power amp. I believe cascode topology can provide sonic benefits in all stages of a pre amp. Of course this all depends on the actual topology and devices used. This is based solely upon the amplifiers that I have designed and built for myself over the years. I have found that cascode designs are easy to listen to.
With the advent of power mosfets, I find myself agreeing with Ha, that the benefits of cascoding in the output stage would not apply any more.
Don
Hi Hannes,
some of Self`s statements regarding distortion are confusing to me, anyway, his lack of genuine sense for audio is obvious (in terms of designing good sounding amplifiers).
The distortion in output stages strongly depend on the output power and load characteristics, which are typically nasty. You know what I hold about static measurements.
Also, I am proposing fully balanced topology decreasing the demand for rail voltage levels.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=109229&highlight=
some of Self`s statements regarding distortion are confusing to me, anyway, his lack of genuine sense for audio is obvious (in terms of designing good sounding amplifiers).
The distortion in output stages strongly depend on the output power and load characteristics, which are typically nasty. You know what I hold about static measurements.
Also, I am proposing fully balanced topology decreasing the demand for rail voltage levels.
No. Please read about the decisive parameters for linearity. These are: L, VGSth, Gm and Crss.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=109229&highlight=
h_a said:
Properly cascoding the output stage can provide some nice benefits, as Nelson demonstrated. I do believe that the cascode is best driven from the input signal, rather than fed back from the output, and this choice can affect stability.
I think the key point about whether or not to use a cascode output stage is the price to be paid in power supply voltage headroom and in increased power dissipation, both at idle and under signal conditions.
One possibly fair way to ask the question is as follows: let's design the amplifier with the cascoded output stage to produce a given output power, say 100 W at 8 ohms, and set the rail voltage accordingly.
Now let's build that same amplifier with the same rail voltages, but without the cascode. Let's assume we have enough SOA in both cases.
Which one will sound better?
Not sure, because the output stage in the second design has extra headroom at the disposal of the output transistor doing the work. Up to the 100W power level, that transistor may not deteriorate in performance as much as the one with the cascode above it. I'm not saying I know the answer, I'm just saying that it may not be as obvious when looked at this way.
Let's now put in a clean Baker clamping circuit to clamp the second amplifier at the same 100 w maximum power level as the amp with the cascoded output stage. The output stage proper will never clip or saturate.
Which amplifier will sound and perform better?
Not sure.
The point is, if we are going to take a rail voltage and power dissipation penalty, there may be more ways than cascoding to spend that headroom in making the amplifier perform better.
Good food for thought.
Cheers,
Bob
Thanks again for the interesting replies.
Bob,
Because the cascode has very different electrical properties it will have a different sonic character, regardless headroom, maximum output power or whatever. A different circuit topology will have a different character, you will hear that right away.
What I am actually looking for is a unity gain cascode output stage to use in a hybrid power amplifier with a tube front-end. I am currently building/designing an amplifier with a triple darlington emitter follower output stage. The prototype is really wonderfully sounding with its tube front-end!! Without any doubt a lot better than the normal darlington emitter follower I used before. One of the benefits of the triple is the enormous amount of current gain. My measurement s show a current gain of 6.4M. I never tried the CFP because the emitter follower does its job so well. What I hear is the magic that tube amplifiers are so famous for.
But the fact is that I am fascinated by the unique properties of a cascode output stage.
Does anyone know an example of what I am looking for (unity gain)?
Peter
Bob,
Because the cascode has very different electrical properties it will have a different sonic character, regardless headroom, maximum output power or whatever. A different circuit topology will have a different character, you will hear that right away.
What I am actually looking for is a unity gain cascode output stage to use in a hybrid power amplifier with a tube front-end. I am currently building/designing an amplifier with a triple darlington emitter follower output stage. The prototype is really wonderfully sounding with its tube front-end!! Without any doubt a lot better than the normal darlington emitter follower I used before. One of the benefits of the triple is the enormous amount of current gain. My measurement s show a current gain of 6.4M. I never tried the CFP because the emitter follower does its job so well. What I hear is the magic that tube amplifiers are so famous for.
But the fact is that I am fascinated by the unique properties of a cascode output stage.
Does anyone know an example of what I am looking for (unity gain)?
Peter
Here is the circuit that is in my head for a while. Perhaps it is more a “totem pole” than a cascode as JCX stated in one of the previous posts.
The Vbe multiplier and variable CCS are only drawn as a function, but that won’t be the toughest part.
Would this circuit work well? What do you think?
Peter
The Vbe multiplier and variable CCS are only drawn as a function, but that won’t be the toughest part.
Would this circuit work well? What do you think?
Peter
An externally hosted image should be here but it was not working when we last tested it.
Not sure what to think about this.. so outer transistor pair will see the same signal as the inner except for a different level . wonder what happens when it's clipping from too high signal. The voltage on the collector of the inner pair will be lower than normal and it will change in the same way as the emitter(output), so inner Vce will be constant but the outer pair's Vce will change almost as much as a usual output.. hmm
Hmm anyway personally I would rather split the signal with 4 resistors from the output and feed it that way.
I did an amp quite similar in topology than the one shown earler ,high fidelity danish, but with fet output and input. I'm pleased with it (talking about powersuppy, I used vicor modules ;-)..
And a sinus signal is'nt chaning much is it? hehe in a fft view it must be very lonely for it.. )
Hmm anyway personally I would rather split the signal with 4 resistors from the output and feed it that way.
I did an amp quite similar in topology than the one shown earler ,high fidelity danish, but with fet output and input. I'm pleased with it (talking about powersuppy, I used vicor modules ;-)..
And a sinus signal is'nt chaning much is it? hehe in a fft view it must be very lonely for it.. )
Andrew,
the Leach Superamp is not a cascode output stage in the meaning that is discussed here (constant Vce), rather it could be labeled a "power splitter"/"SOA doubler" or something like that.
Personally I split minds about cascoding outputs... my feeling is that it's maybe only worth the extra circuitry and the power penalty when used as a zero/low feedback output stage "where everthing counts". Cascoding only the drivers/predrivers (and with a bootstrapped or separate supply) might be a different story.
Then again, true cascoding is appealing because it completely takes the output voltage out of the distortion picture, distortion is only determinded by output current. And speakers tend to seperate current and voltage as well (current is not very well correlated to voltage). From which it seems also beneficial to use cascoding for true class A outputs, then current induced distortion is also very low, at least for small output currents (and thermal distortion is correspondingly low).
- Klaus
the Leach Superamp is not a cascode output stage in the meaning that is discussed here (constant Vce), rather it could be labeled a "power splitter"/"SOA doubler" or something like that.
Personally I split minds about cascoding outputs... my feeling is that it's maybe only worth the extra circuitry and the power penalty when used as a zero/low feedback output stage "where everthing counts". Cascoding only the drivers/predrivers (and with a bootstrapped or separate supply) might be a different story.
Then again, true cascoding is appealing because it completely takes the output voltage out of the distortion picture, distortion is only determinded by output current. And speakers tend to seperate current and voltage as well (current is not very well correlated to voltage). From which it seems also beneficial to use cascoding for true class A outputs, then current induced distortion is also very low, at least for small output currents (and thermal distortion is correspondingly low).
- Klaus
Hi Lumba,
c'mon! A lot of people are interested in zero-feedback designs. And far more in low-feedback designs. This is a liberal place here, I'm sure all interested people will assist you in the development!
By the way, you're too kind to me, the little I know is barely sufficient to change an amp the way I like.
All the best, Hannes
c'mon! A lot of people are interested in zero-feedback designs. And far more in low-feedback designs. This is a liberal place here, I'm sure all interested people will assist you in the development!
By the way, you're too kind to me, the little I know is barely sufficient to change an amp the way I like.
All the best, Hannes
It’s about 10 years ago now that I stopped using overall feedback in my power amplifier designs. I never looked back and will never return to overall feedback designs again. Simply because good zero-feedback designs can sound so good! I solely use amplifier blocks with known distortion.
Emitter follower output stages for example, have very little distortion figures. And from my experience can have an output impedance as low as 25mOhm (5 output devices in parallel) without any global feedback. So why would one use feedback for this stage?
It’s important to have simple straightforward amplifier stages that have good properties of its own.
Peter
Emitter follower output stages for example, have very little distortion figures. And from my experience can have an output impedance as low as 25mOhm (5 output devices in parallel) without any global feedback. So why would one use feedback for this stage?
It’s important to have simple straightforward amplifier stages that have good properties of its own.
Peter
pietjers said:
Charles Hansen has demonstrated with his MX-R amplifier that it is possible to build a no-feedback amplifier with great sound AND good measured performance. However, I'm sure he would be the first to say that it is neither easy nor inexpensive.
What is the distortion performance of the no-NFB amplifier you have built?
Thanks,
Bob
Bob.
I have never measured its distortion nor wondered what it would be. Simply because I know that the tube front-end has decent distortion figures and the output stage has very low distortion. Add the two and you have the total distortion.
I have learned from my experience that often measured performance has little to do with the sound quality of the final product. The real truth of high –end performance lies in other area’s than distortion figures. In most cases feedback does more harm than good.
Would love to hear more comments on the output stage idea I posted in post #30?
Peter
I have never measured its distortion nor wondered what it would be. Simply because I know that the tube front-end has decent distortion figures and the output stage has very low distortion. Add the two and you have the total distortion.
I have learned from my experience that often measured performance has little to do with the sound quality of the final product. The real truth of high –end performance lies in other area’s than distortion figures. In most cases feedback does more harm than good.
Would love to hear more comments on the output stage idea I posted in post #30?
Peter
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