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A Tube amp without coupling capacitors? Possible?
A Tube amp without coupling capacitors? Possible?
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Old 24th August 2019, 07:03 AM   #221
jjasniew is offline jjasniew  United States
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Well, I certainly would think some level of measurement and engineering care would at least point you in the right direction, to ensure quality sound. And I would like to believe what makes sense in the experience of measuring something and engineering it has a positive correlation to the psycho-acoustic experience of listening.

In other words, as a hobby it becomes fun when these two mesh and you made it so. Including the timeless benefit of having it for, say, the rest of your life. Confident in the certainty that everything's pretty much just right. If something new and radical is happening and its for real, you can certainly try it out. You have your reference.

I swapped out my 4" Fostex I've been listening to for the 8" KEF Concentrics for a bit and when I put these Fostex back (in a new cabinet no less) I felt that I had missed them. In my heart. So, I'm talking about two electro-mechanical devices here; related to, well, levels of pleasure. The Fostex are just such a pleasure to listen to - I cant even listen to the KEFs anymore. Psychology connected to sound quality. Sound quality connected to some understanding, engineering and successful measurements. Or in my case - sheer luck. Thinking about it, perhaps not so easy to measure, but I bet you could measure it.
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Old 26th August 2019, 02:52 AM   #222
indra1 is offline indra1  Indonesia
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Quote:
Originally Posted by Wavebourn View Post
... I once observed a phenomenon when a push-pull amplifier powered from electrolytic capacitors produced intermods on top frequencies similar to balanced SSB modulator. They were pretty small, but still audible, as a smeared stereo soundstage of cymbals, triangles, etc. Shunting 2x470 uF 450V caps connected in series by 4 uF only film caps I eliminated this effect.
Seems also relevant at cathode bypass duty.
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Old 26th August 2019, 06:29 AM   #223
Lampie519 is offline Lampie519  Netherlands
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My 50 cents:
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File Type: png dc_amp_fig4.png (6.4 KB, 148 views)
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Old 26th August 2019, 06:57 AM   #224
jjasniew is offline jjasniew  United States
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Re Lampie519 - I like it. What is it? What's it sound like? Can it be made in the 17 - 35 watt output range? Or is it already there.

If you took off all the power supply components as drawn - and put those on a separate page, that would leave room for multiple ways you could generate +350, 250, 150 and -33V DC. Very cool looking circuit - a tube op-amp that can only output AC.
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Old 26th August 2019, 07:31 AM   #225
Lampie519 is offline Lampie519  Netherlands
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I just found it on the web. So i have not been able to test it.

An other one i found on the web:
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Old 26th August 2019, 09:22 PM   #226
6A3sUMMER is offline 6A3sUMMER  United States
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As to the Zener amp, post # 223 my question is how much noise? The balance control is required to make that amp work.

As to the multi tube amp, post # 225, it is dangerous to both output transformers, and dangerous to loudspeakers (especially tweeters) to switch the B+ to the output stage like that. And why does it take 10 triodes to get 10 Watts?

Both amps schematics look cute. But I wonder if anybody has recently built either.

DC coupling just for the purposes of DC coupling and eliminating capacitors, sometimes causes more problems than it corrects.

Last edited by 6A3sUMMER; 26th August 2019 at 09:27 PM.
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Old 27th August 2019, 07:09 AM   #227
EL504 is offline EL504  Netherlands
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Three schematics of tube amps without coupling capacitors. The schematic with the 45's is from an article in the French magazine "Toute la Radio", March-April 1958.

Greetings,
Robert
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File Type: png 2 x 6SN7 + 2 x 45 pp + GZ32.png (941.4 KB, 111 views)
File Type: png ECC82 + ECC83 + 2 x EL84 ul pp + EZ81.png (949.9 KB, 95 views)
File Type: png ECC82 + ECC83 + 2 x EL86 ul pp + 2 x EY82.png (946.6 KB, 96 views)
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Old 27th August 2019, 07:39 AM   #228
Keit is offline Keit  Australia
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EL504,

The second circuit has both grid-coupling caps and cathode bypasses. If you believe that coupling caps entail audible defects, then you must believe that cathode bypass caps do so as well. It makes no sense to eliminate one and not the other.

All the capacitor-less circuits presented so far have potentiometers connected to grids. This means grid current will flow in the wipers. This is a well known source of both noise and early potentiometer failure. Traditionally, pots not isolated from grids by capacitors was only done in the cheapest of radios, where the absolute minimum cost was the key design parameter.

If you want quality and believe eliminating all capacitors in the signal path is part of getting there, you must find some other way of providing a volume control. (professional-style switched attenuator (these require regular lubrication) ?? transformer isolated pot (ugh! any capacitor is heaps better than any transformer)? Use a light-dependent resistor (these cause distortion)??)
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Old 27th August 2019, 08:40 AM   #229
Lampie519 is offline Lampie519  Netherlands
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These are old articles and worth considering anyway. You could eliminate these caps very easy and use a current source instead (long tail) or whatever. We are not stuck on these circuits are we? Yes, "lightspeed" volume control would be possible and stepped as well. I think this is common practice now.
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Old 27th August 2019, 08:42 AM   #230
Brice is offline Brice  United States
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Thanks for sharing EL504. As Keit said, the second schematic has grid coupling caps. The third design is interesting. But it seems to run the power tube with high current, and must likely will dissipate quite some heat. For reference, here is the Acrosound 20 design. As said, it has only 1 coupling cap in the input. The other are bypasses, not couplings but are indeed in the signal path. I used that design with various OT with success, UL taps from 20 to 25% work. It is a deceptively complex amp, thanks to Herbert Keroes. There are a few mods to make it run the output tubes cooler. But the result is an extremely detailed and dynamic sound, very hard to beat with a classic coupling design. It is a remarkable design.
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