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Old 30th April 2009, 10:42 PM   #111
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With no feedback there is nowhere for any subjective colorations to hide - even very small colorations are obvious and gross. In addition, with Class A PP throughout, there isn't the wonderful warm bath of 2nd-harmonic distortion of a SET amplifier. You can have a fair amount of 3rd-harmonic distortion, and if there's enough 2nd present, the 3rd just gives a pleasing "bite" to the sound. 5th, 6th, 7th, and higher just sound harsh, though, so I try and find tubes that have very low upper-harmonic content.

The combination of the two characteristics makes for an extremely transparent but also brutally unforgiving topology - fortunately, it doesn't have that effect on source material, it's actually quite forgiving that way - but small errors in circuit or power-supply design are shown in sharp relief.

It's useful to keep in mind this amplifier, with no local or global feedback, has less distortion at 1 watt than a Golden Age Class AB pentode amplifier with 10-20 dB of feedback. The analogy I use is: When the tide goes out, you see all kinds of creatures clinging to the rocks that you didn't know were there before.

PS coloration is a deep subject all by itself. With a SET amplifier, the audio-frequency current modulations appear directly on the supply, which means you hear both the noise and parts colorations unmodified. With Class A PP, the audio-frequency current modulations are multiplied twice in frequency, and essentially full-wave-rectified. This isn't such a nice-sounding modulation as the SET, which is the same as the music itself.

So despite the measured noise figures being lower - which is due to PP cancellation - the sound of the noise, and audio-frequency modulations, are much worse and much less desirable as a residual coloration. That's the real reason for the shunt regulation and putting the bypass caps on physically very short current-loop paths between cathode and plate circuits. Much of the subjective tuning of the amplifier comes down to little more than details of the power supply and its physical wiring and layout. Attention to RF-style noise rejection techniques (small loop areas) and instrumentation-style grounding provides major benefits here.

The current direction for further improvement (whenever I conclude the Beyond the Ariel project) is a better filament supply that is immune to AC line noise, while also taking advantage of the inherent 20~30 dB noise rejection of a balanced DHT tube, and also having low subjective coloration. The attempt to use an off-the-shelf 3-pin DC regulator was a complete failure, with gross colorations compared to conventional AC heating. Pre-filtering the AC going into the primaries of filament transformer, though, does give cleaner and sweeter sound.
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Old 1st May 2009, 01:51 AM   #112
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Lynn:

I filter the AC feeding the filament transformer with a Sola ferroresonant transformer. While it does show some 5th, 7th, and 11th harmonics in its spectrum, there is essentially nothing higher that I can measure. The noise rejection on these units is extremely good; 120dB common mode and 60dB transverse. The regulation is of course another benefit, which I then step down to about 108V. With this method, I can get my 120Hz residual down to about 1 mV rms, which is very quiet and unobtrusive on my system. The tweeter is almost completely silent, and the woofer just a gentle rolling rumble. I can't say I yearn for a quieter system, to be honest.

I've even considered using a clean sine wave driving a class D amplifier to produce my needed 108V, to see how a 'perfect' sine wave would improve the hum. I have the sine source and output tranny, but haven't gone ahead with the amp. I suspect the bigger problem with that idea is noise, rather than harmonic spectrum. Might be a fun experiment, at least.

A very fine design you came up with. I wouldn't expect such transparency out of sooo much iron in the signal path, but it's there. Just have to choose wisely; as you say, this IS a very unforgiving design.
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Old 1st May 2009, 08:54 AM   #113
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Kurt, very nice to see you get this working!
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Old 1st May 2009, 01:56 PM   #114
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Hi Zigzagflux

Congratulations on finishing this project!

Can you tell us how the last schematic looks like? In this thread you describe a lot of changes made, so it is a bit hard to get the overall picture of the eventual amp!

Cheers, Erik
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Old 1st May 2009, 02:26 PM   #115
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Happy to. This was more of a parts swap than actual modifications.

Not a lot of detail in my drawing, but you get the idea. Lynn's drawing is more complete, and more reliable.
Attached Files
File Type: pdf schematic.pdf (75.3 KB, 178 views)
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Old 1st May 2009, 07:33 PM   #116
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Thanks!!

I will surely refer to the original schematics of the Karna, as they also demonstrates the grounding techniques, etc.

Your schematic rose another question, referring to the CCS. I saw you apparently went with the easier solution of employing depletion mode mosfets for the CCS (instead of the more complex enhancement units as used by GP). Which chip did you use, the 10M45S, the DN2540, or other? Have you used the cascoded version, or the version with only 1 chip? I have been playing around with cascoded 10M45S, but could not achieve very high current levels (as required in the Karna). Single chip CCS will do it, however...

Cheers, Erik
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Old 1st May 2009, 08:40 PM   #117
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For those of you interested in using a Constant Voltage Ferroresonant transformer, use only a CV S rather than a CV T. The CV S puts out a sine wave and the CV T puts out a square wave. Keep in mind that both throw off an immense amount of stray field energy, very close to 5 %, rather than the typical 0.1% of an E/I transformer.

Bud
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Old 2nd May 2009, 02:11 AM   #118
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Quote:
Originally posted by ErikdeBest
I have been playing around with cascoded 10M45S, but could not achieve very high current levels (as required in the Karna). Single chip CCS will do it, however...
I have successfully used the DN2540 in cascode at currents up to about 80mA. I know some people (Tubelab, I think) really like the 10M45S. They do behave a little differently, but both seem to work well. How much current are you trying to push? I would have to do a search to find some recommendations as to a better device (lower voltage, lower Rds) for the bottom current setting device, which then drives the upper DN2540 for the voltage capability. This is supposed to give you a little more extension.

To be honest, I'm not convinced you NEED a cascode for the power supply CCS's anyway. Lynn originally used a simple resistive dropping network to feed the gas tubes, and said they worked just fine. So a "perfect" CCS probably isn't critical for the power supplies I would guess.

Quote:
Originally posted by BudP
Keep in mind that both throw off an immense amount of stray field energy
Hi Bud. They certainly do. I played around with my E/M field strength meter, and also looked at hum pickup on the IT and OPT's. I found I needed about 18 inches of clearance to completely eliminate pickup. Orientation didn't seem to matter, either. The field was just spilling all over the place. You also need to put the crazy things under a box, the radiated audible noise is so high. To be honest, they're a pain in the butt, but they do regulate and isolate very well.
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Old 2nd May 2009, 10:01 AM   #119
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Hi Zigzagflux

Thanks for the information! I have a couple of the DN2540 IC's as well, but have not played around with them, yet. It is good to know a cascoded version can handle 80mA (I don't even need that much anyway!)

I am certainly not in the position of even guessing about the need for a cascoded version in a power supply, and its possible advantages towards a simple version, so I can't comment...

Cheers, Erik
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