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Old 2nd July 2009, 09:42 PM   #21
Gordy is offline Gordy  United Kingdom
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I've had real-world success with a similar circuit to the cascade/cascode (whatever it is called) DN2540s as posted by dsavitsk (/ Doug). However, this is only subjective, as I have no figures or simulations to back it up.

I used 1K stoppers and a pot to set the current at about 25mA. The CCS dropped about 60V as an anode load. It seemed to be fine.
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Old 2nd July 2009, 09:56 PM   #22
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Quote:
Originally posted by ikoflexer


Second suggestion I've tried initially, when I first looked at direct coupling, but couldn't get it working well. Didn't spend much time trying to fix it though, I'll revisit the issue. It'd be nice, of course, to get rid of using a whole lot of parts.
It seems like worrying about direct coupling while leaving that electrolytic on the output is like eating at KFC and ordering a diet drink -- it is least of your worries. Regardless of what bigwill says, the cap will take away from the circuit.

So, why not drop a stage and add a coupling cap between stages and add a servo to the output,

Click the image to open in full size.

or a second mosfet to bring the ground bias up to the same level as the signal,

Click the image to open in full size.

or use a low impedance transformer.

Click the image to open in full size.

Quote:
Originally posted by ikoflexer
Regarding the DN2540 ccs; I've also tried that, but for some reason the simulated output impedance was not very good at all. I can give more details if interested.
Not really, as the actual measured results are excellent.
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Old 2nd July 2009, 10:06 PM   #23
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Quote:
Originally posted by ikoflexer
As for headphones, I have the run of the mill Sony MDR-V6, claiming 106 dB/mW, 63 ohm at 1kHz. Lack of lots of power (read loud) is not something I worry too much about.

Also, keep in mind that you can most likely leave out the tube stage as your phones only need a fraction of a volt, so you don't need gain. The tube is just a filter here. Why not replace the gain stage if your source with those tubes and just build a current source for the phones?
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Old 2nd July 2009, 10:22 PM   #24
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by ikoflexer
sheldon, interesting output stages over at tubecad. Ok, I've simplified the abomination somewhat. Have a look.



You will see the simple ccs load on the 3a5. I use double the jfets (if different Idss the source resistor needs to be adjusted offline until the voltage drop across the two jfets is roughly equal; the reason for this is that the 2sk170 has a 40V Vds max; I know it's a hack, but will look for more suitable jfet, with higher voltage).

Funny, that simple ccs has this output impedance. Over 100kOhm at 1Mhz. Of course, in the real world it's not like that



So, the simpler circuit simulates a bit better for distortion. Nice figures for the voltage on the 3a5 plate (V(dplate)).
I like the resistor load better. It's actually lower on 5th and higher order distortion - which is more dissonant than the low order distortion - assuming it models realistically. I'd still try a resistor load first and see if you like it. I agree with Doug, that the output stage is unnecessarily complicated, and that the coupling cap is not a big issue. I disagree that the output cap (even electrolytic) will necessarily sound bad. Try it, you may like it. Even if you don't want an electrolytic there, you can probably find a 100uf motor run cap or other film cap from a surplus source.

My impression is that this is intended partially as a learning project, which is great. If so, start simple. You can always add complexity, but you may find you like the simple version as well or better than more complicated versions.

Sheldon
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Old 3rd July 2009, 01:17 AM   #25
iko is offline iko  Canada
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Sheldon, the resistor load has higher number in all 9 harmonics. See below.

Code:
N  CCS        Resistor load
1  1.000e+00 1.000e+00 
2  9.492e-05 7.106e-03
3  7.066e-06 1.591e-04
4  1.275e-07 4.994e-06
5  2.602e-07 5.711e-07
6  2.488e-07 5.624e-07
7  2.417e-07 6.734e-07
8  3.246e-07 6.543e-07
9  1.828e-07 6.295e-07
As for the output stage, unless I can get a simpler version that works just as well, then the more complicated version is not unnecessary. It is a learning project for me, so I'm exploring various topologies and I know some of you disagree with simulation, but it's a great learning tool for me. The spud amp I built behaved almost exactly as I had simulated it before. It was nice to see simulation and reality be very close, and it allowed me to explore very much.

dsavitsk, I am more concerned about the large output cap, that's for sure. But until I switch to OT I'll just have to live with it. Perhaps it's just a dream, but I was thinking of direct coupling two tube stages with that p-channel mosfet schema I showed above (in the future).

About the DN2540 ccs of Gary Pimm. Something doesn't compute, perhaps I made a mistake. To satisfy my own curiosity I'll try to get some of these devices and measure their impedance myself. FWIW the models I used in simulation are from supertex.
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Old 3rd July 2009, 02:35 AM   #26
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by ikoflexer
Sheldon, the resistor load has higher number in all 9 harmonics. See below.

As for the output stage, unless I can get a simpler version that works just as well, then the more complicated version is not unnecessary.
I guess I was reading the distortion data incorrectly. I believe that tube will sound good with a resistor load. Also with a current source. Your amp. Your call.

When you say "works just as well". You mean in simulation. I've nothing against simulations at all. But just because something simulates with lower distortion, doesn't mean it will sound better to you. If the goal is to produce an amp that has the lowest possible distortion, then by all means follow the simulations. But if that's the goal, why bother with tubes at all? Heck, a 3A5 doesn't even have a visible glow to warm the psyche. You can always get a more complex solid state circuit to simulate better in that regard than a tube circuit. I'm not sure I see the point in using a tube, then very tightly regulating it's environment, to the point that you could substitute a solid state device in its place without changing the sonics.

Sheldon
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Old 3rd July 2009, 03:10 AM   #27
iko is offline iko  Canada
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Hi Sheldon, sorry if I sounded negative. It wasn't my intention to dismiss your suggestion at all. In fact I appreciate very much your input and I will try the resistor load, listen to it, and then see if I can hear any difference with a ccs load. That being said, if you had seen any of the other struggles of mine in other threads, you would have known where I'm coming from. In the initial stages of a design I tend to explore a lot of different ideas. I use the simulator to dismiss the very horrific ones; there's always the risk of dismissing something that would be good in the real world, but it's a risk I'm willing to take. I build my own models for tubes, from data sheets. The lowest thd is not necessarily what I'm after. But if I see large odd and high number harmonics I tend to think there's something wrong with what I'm doing. Phase shift, stability, are also issues I try to pay attention to. And then I start testing those ideas in reality, with real parts. It's a hobby, what can I say. In the end what doesn't sound good, will be dismissed, I'm man enough to admit when I'm wrong. But I like to convince myself of things and will always keep in mind suggestions I get from you, the more experienced guys.
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Old 3rd July 2009, 03:33 AM   #28
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Quote:
Originally posted by ikoflexer
[B]Alright, I'm back.

I should say that I built a 21LU8 based headphone amp (pentode part triode strapped), RC interstage coupling, power transformers re-purposed to OTs. Total cost not more than $50 and it sounds great. Now if I go to a cost of $100 for the DHT amp, it better be twice as good; or so I hope. You may laugh, but that's my personal challenge.

Any chance you could post a schematic for the headphone amp? I have quite a few 6/21LU8's laying around

Thanks!

M1
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Old 3rd July 2009, 03:57 AM   #29
iko is offline iko  Canada
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Of course. You'll find a whole thread following this link
http://www.audiokarma.org/forums/sho...d.php?t=163886

The basic schematic is on post #5. I modified it a little (tiny difference in operating points) but it's basically that. I think they're running it in ultralinear mode, I just triode strapped it.
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Old 3rd July 2009, 04:57 AM   #30
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by ikoflexer
Hi Sheldon, sorry if I sounded negative. It wasn't my intention to dismiss your suggestion at all. In fact I appreciate very much your input and I will try the resistor load, listen to it, and then see if I can hear any difference with a ccs load. That being said, if you had seen any of the other struggles of mine in other threads, you would have known where I'm coming from. In the initial stages of a design I tend to explore a lot of different ideas. I use the simulator to dismiss the very horrific ones; there's always the risk of dismissing something that would be good in the real world, but it's a risk I'm willing to take. I build my own models for tubes, from data sheets. The lowest thd is not necessarily what I'm after. But if I see large odd and high number harmonics I tend to think there's something wrong with what I'm doing. Phase shift, stability, are also issues I try to pay attention to. And then I start testing those ideas in reality, with real parts. It's a hobby, what can I say. In the end what doesn't sound good, will be dismissed, I'm man enough to admit when I'm wrong. But I like to convince myself of things and will always keep in mind suggestions I get from you, the more experienced guys.
Hey, no need to apologize. I was just trying to give the reasoning behind the suggestions. It is DIY, and my priorities need not be yours. My experience ain't all that much more than yours. I have learned to appreciate simplicity when it works well. That's the beauty of tube circuits. They can be made simple.

As an aside, I would think that a monotonic phase shift would be much more benign than something lower in magnitude but more irregular. That's just a guess, and may have nothing to do with anything.

BTW, here's a model for 3A5 you can try if you like: http://www.geocities.com/dmitrynizh/3a5-phono.htm

Sheldon
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