Let's assume that "tube sound" is defined as just being the distortion spectrum of the output, without looking at impedance or any other factors. Does this depend on the topology of the tube stage? For instance, if I have a 12AU7 set up as a common cathode amplifier, and then use the same 12AU7 in a cathode follower stage. The tube is biased to the same operating point in both cases (that's doable, right?). The heater supplies are the same, everything else is the same (this is a hypothetical situation, after all). Let's also assume that the input impedance of the destination/load device is high enough that both the common cathode and cathode follower can drive it with negligible degradation (I think that's a reasonable hypothetical assumption to make too).
If someone ran a distortion analysis on these two setups, would they behave differently?
Where this question comes from: There are people who mod CD and SACD players to put tubes in their analog stages. Some people only replace the final buffer stage opamp with a tube, others replace the voltage amplifier stage too. If we restrict the comparison to just distortion spectra, and ignore the fact that the second option would remove more opamps from the signal chain (i.e., assume that the voltage amplifier opamp adds no distortion of its own), is there a difference between the two? Does a tube add more "tube sound" as a voltage amplifier than as a cathode follower/buffer?
When used as output power devices, I know things change, because then you have the transformer, you have the interaction of the tube with the reactive speaker load, you have damping factor issues, and so on. But if we restrict the comparison to 'line level stages', and compare only distortion, how much effect does the stage's topology have on distortion?
Thanks, and I hope this isn't a really dumb question.
Saurav
If someone ran a distortion analysis on these two setups, would they behave differently?
Where this question comes from: There are people who mod CD and SACD players to put tubes in their analog stages. Some people only replace the final buffer stage opamp with a tube, others replace the voltage amplifier stage too. If we restrict the comparison to just distortion spectra, and ignore the fact that the second option would remove more opamps from the signal chain (i.e., assume that the voltage amplifier opamp adds no distortion of its own), is there a difference between the two? Does a tube add more "tube sound" as a voltage amplifier than as a cathode follower/buffer?
When used as output power devices, I know things change, because then you have the transformer, you have the interaction of the tube with the reactive speaker load, you have damping factor issues, and so on. But if we restrict the comparison to 'line level stages', and compare only distortion, how much effect does the stage's topology have on distortion?
Thanks, and I hope this isn't a really dumb question.
Saurav
But that doesn't mean that there is no such thing as a tube's inherent linearity, does it? A tube's plate curves don't care what topology you use them in. But I see what you're saying, the topology affects more than just the operating point.
I have a vague understanding of how/why feedback affects distortion. How does gain affect distortion?
I have a vague understanding of how/why feedback affects distortion. How does gain affect distortion?
Well, gain is what you trade off for distortion. As you increase negative feedback, gain decreases in the same proportion as the distortion (a simplification, but 99% true).
Well, yeah, it does. let's take an easier case, a bipolar transistor versus a tube. If you run the transistor common emitter, the voltage transfer function (VTF), in the absence of degeneration or other feedback, is exponential- highly nonlinear. Take a tube and do the same thing and its VTF will be MUCH more linear. Now, put the transistor in common collector configuration and the tube in the analogous common anode, and suddenly the transistor's VTF is MUCH more linear.
So which device has better "intrinsic linearity"?
Well, yeah, it does. let's take an easier case, a bipolar transistor versus a tube. If you run the transistor common emitter, the voltage transfer function (VTF), in the absence of degeneration or other feedback, is exponential- highly nonlinear. Take a tube and do the same thing and its VTF will be MUCH more linear. Now, put the transistor in common collector configuration and the tube in the analogous common anode, and suddenly the transistor's VTF is MUCH more linear.
So which device has better "intrinsic linearity"?
Duh. Of course. Thanks.
But the distribution of that distortion across the different harmonics also changes, right?
I see what you're saying. So when we look at plate curves and try to pick a linear operating point... do the plate curves only apply for common cathode stages then? Actually, I think I know the answer to that, because the gain and output impedance predicted by the plate curves do not apply in a cathode follower topology, which means that the plate curves aren't valid any more, and so neither are their linearity predictions.
Hi,
Plate curves are only valid under the conditions they were made as SY points out.
A much more interesting approach to the use of tubes is to trace the actual topology and see how that fares on a plotter.
To give you an example, you can build an outputstage using SEPP as a topology with low µ triodes and I can assure you that for a good range of operating points it's going to be so linear you'd hardly believe it...
When you actually think it through, this is to be expected.
First, you can hardly predict Zo of a tube just by looking at plate curves....Actually I can, but that's more to do with experience and knowing thousands of tubes' data by heart.
Secondly, a CF is based on 100% local FB which is one way to achieve even higher linearity and lower Zo by the same token...but what happened to the gain?
Vanished into the FB loop...no free lunches here either I'm afraid.
As for the sound of tubes, well yes, this is another one of those myths...I can assure you that you can build tubed stages that distort as nastily as any half decent transistor with pretty much the same harmonic content.
Making transistors sound or distort like tubes is only possible to some extent, fact is a tube will distort in a gentler manner ( to the human ear) than a transistor when driven into clipping, which is one reason why people often think, just think, that tube amps are more powerful than transistor amps.
That is yet another myth that can't possibly hold water, can it?
It simply can't.
Cheers,
Plate curves are only valid under the conditions they were made as SY points out.
A much more interesting approach to the use of tubes is to trace the actual topology and see how that fares on a plotter.
To give you an example, you can build an outputstage using SEPP as a topology with low µ triodes and I can assure you that for a good range of operating points it's going to be so linear you'd hardly believe it...
When you actually think it through, this is to be expected.
First, you can hardly predict Zo of a tube just by looking at plate curves....Actually I can, but that's more to do with experience and knowing thousands of tubes' data by heart.
Secondly, a CF is based on 100% local FB which is one way to achieve even higher linearity and lower Zo by the same token...but what happened to the gain?
Vanished into the FB loop...no free lunches here either I'm afraid.
As for the sound of tubes, well yes, this is another one of those myths...I can assure you that you can build tubed stages that distort as nastily as any half decent transistor with pretty much the same harmonic content.
Making transistors sound or distort like tubes is only possible to some extent, fact is a tube will distort in a gentler manner ( to the human ear) than a transistor when driven into clipping, which is one reason why people often think, just think, that tube amps are more powerful than transistor amps.
That is yet another myth that can't possibly hold water, can it?
It simply can't.
Cheers,
Right. I was thinking of Rp, which is just one of the things that determines Zo.
Oh, but that won't be a SET, so how can it possibly sound good?
The 1 tube W = 2 SS W thing? I've seen that one before.
8 pages long, the subject line is exactly what I'm asking about, and I hadn't noticed it. I guess I should visit the other forums on this board too, every now and then.
Hi,
The only half decent one I ever heard costs close 50.000$, put out 8W into equally expensive and dreadfully coloured Lowther speakers and was still nowhere near to what can be achieved regardless of that pricetag.
I'll skip the details on the cost of the rest of that system but if you, or anyone else, think that designing a fine* SET is simple, I can tell you upfront it's not just as "simple" as building an excellent PP stage, quite to the contrary.
A mere subjective expression, not a real one.
*Let me restate that: amongst the finest amps in the world.
Cheers,
The only half decent one I ever heard costs close 50.000$, put out 8W into equally expensive and dreadfully coloured Lowther speakers and was still nowhere near to what can be achieved regardless of that pricetag.
I'll skip the details on the cost of the rest of that system but if you, or anyone else, think that designing a fine* SET is simple, I can tell you upfront it's not just as "simple" as building an excellent PP stage, quite to the contrary.
A mere subjective expression, not a real one.
*Let me restate that: amongst the finest amps in the world.
Cheers,
My attempts at sarcasm have been falling a little short lately I'm aware of the "anything SET must be magic" and "anything magic must be SET" myths. I'm also aware of the physics bending 1W of type X = 2W of type Y myths.
What if we change the conditions. Let's say I'm not trying to build a world class amp, I'm just trying to build the best amp I can. For small budgets and limited skills, is it easier to design a halfway decent amp by going SET, or PP? Or is that even a meaningful question. Which one has more things that a newbie designer could get wrong?
I decided to build a DHT SET for my first amp mostly because I just wanted to see what the hype was all about, and it looked like it would be simpler and cheaper to do. I'll probably try PP next. Many people who's opinion I respect have had favorable things to say about transformer coupled PP DHT amps, to use one specific example. But that's going to be a lot more than the $500 I spent on my current amp.
I'm aware of the "anything SET must be magic" and "anything magic must be SET" myths. I'm also aware of the physics bending 1W of type X = 2W of type Y myths.What if we change the conditions. Let's say I'm not trying to build a world class amp, I'm just trying to build the best amp I can. For small budgets and limited skills, is it easier to design a halfway decent amp by going SET, or PP? Or is that even a meaningful question. Which one has more things that a newbie designer could get wrong?
I decided to build a DHT SET for my first amp mostly because I just wanted to see what the hype was all about, and it looked like it would be simpler and cheaper to do. I'll probably try PP next. Many people who's opinion I respect have had favorable things to say about transformer coupled PP DHT amps, to use one specific example. But that's going to be a lot more than the $500 I spent on my current amp.
Hi,
Mine too it seems...
That's a very tough question, honestly....from the look of things one would be inclined to say that a SET amp would be easier to build.
Which is probably why everyone jumps on the bandwagon anyway.
Reality is however that most designers make the most blatant mistakes due to lack of inside knowledge and experience.
IMHO it takes years of experience with either PP or SE to build a real good one.
An exceptionally good one only comes about every decade...if we're lucky.
I realise it's not much help to you but it's the sad truth,
Mine too it seems...
That's a very tough question, honestly....from the look of things one would be inclined to say that a SET amp would be easier to build.
Which is probably why everyone jumps on the bandwagon anyway.
Reality is however that most designers make the most blatant mistakes due to lack of inside knowledge and experience.
IMHO it takes years of experience with either PP or SE to build a real good one.
An exceptionally good one only comes about every decade...if we're lucky.
I realise it's not much help to you but it's the sad truth,
Saurav said:
Nope, because as long as a CF isn't drawing grid current, plate current (Ip) = cathode current (Ik). Load is in K circuit, but to graph it, it needs to be in plate. Well current is the same so we can move the resistor to the plate circuit, and if there's already some load there, it adds to it (being in series).
So now our represenative circuit has no resistor below the cathode, thus no output voltage appears in series with the input, and we can graph the performance of the circuit without NFB getting in the way. NFB is always the last thing you do, you add that in last and calculate the gain and distortion loss.
Even a grounded-grid circuit can be imagined the same way I think, although it's much more difficult because the input is in series with the output (as opposed to vice versa in a CF).
Tim
Not necessarily, especially if you find an older commercial amp to rebuild. If you like, when you get your SE going the way you want, I've got a pretty nice little transformer P-P amp sitting in my garage gathering dust that you're welcome to borrow for comparative listening. It's based on Dynaco iron.
Not so clear to me. It depends on the configuration and what "thing" it is that you want to be linear.
MUDDLED WATERS...
Hi,
Same here...seems to me as if our Tim is already thinking " Outside The Box".
It was my impression we were discussing this on a tracer to tracer basis...
No gimmicks applied, no particular circuit, just what a plotter uses to trace the curves:VTF.
Triodes, as opposed to penthodes etc, have feedback built in. Now, if we allow ourselves to turn a blind eye on that, than, yes triodes are intrinsically more linear than penthodes...
Now, for a reality check, we don't listen to curve tracers...we just use them as a modicum to determine operating points in the simplest of circuits.
IOW, they're nothing more than a tool...and as it is often the case the operator of the tool needs to be creative with it not the other way around.
No point in showing a kid a scope trace and than demand that he'd get creative with it, he needs to be shown the meaning before he can see the possibilties.
\Rant off.
Cheers,
Hi,
Same here...seems to me as if our Tim is already thinking " Outside The Box".
It was my impression we were discussing this on a tracer to tracer basis...
No gimmicks applied, no particular circuit, just what a plotter uses to trace the curves:VTF.
Triodes, as opposed to penthodes etc, have feedback built in. Now, if we allow ourselves to turn a blind eye on that, than, yes triodes are intrinsically more linear than penthodes...
Now, for a reality check, we don't listen to curve tracers...we just use them as a modicum to determine operating points in the simplest of circuits.
IOW, they're nothing more than a tool...and as it is often the case the operator of the tool needs to be creative with it not the other way around.
No point in showing a kid a scope trace and than demand that he'd get creative with it, he needs to be shown the meaning before he can see the possibilties.
\Rant off.
Cheers,
Thanks SY, that's very kind of you, and it would be an interesting comparison. My SE amp is done as in it's in my system and working. I do plan to experiment with it some more.
I thought about trying to restore an old amp, but decided to build one from scratch instead. In some ways, it's easier to take someone else's known working design and implement it, than try to debug a problem. I also need a couple more tools, a variac and a function generator probably being at the top of the list for working on vintage gear.
I thought about trying to restore an old amp, but decided to build one from scratch instead. In some ways, it's easier to take someone else's known working design and implement it, than try to debug a problem. I also need a couple more tools, a variac and a function generator probably being at the top of the list for working on vintage gear.
Sounds like a plan I'm free this weekend too, FWIW. And I have an autoformer passive linestage that I could bring along, if you think you might find that interesting. I don't think there's much that can significantly better it for transparency at a $300 price point. Not that I've heard everything there is at that price point, of course.
I'm free this weekend too, FWIW. And I have an autoformer passive linestage that I could bring along, if you think you might find that interesting. I don't think there's much that can significantly better it for transparency at a $300 price point. Not that I've heard everything there is at that price point, of course.
Hi, here is a quite old article about the subject that can shed some light on it:
http://bulk.pearl-hifi.com/02_PEARL_Arch/Vol_01/Sec_1/0001_Puzzled_About_Amps_.pdf
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.