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Old 24th November 2010, 08:11 PM   #21
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Originally Posted by SY View Post
But did the output sound like the input?
Not hardly. The resulting sound was like throwing a heavy comforter over the speeks. Lyrics became noticeably harder to understand, fading into a bland background. With one song, there was a passage I knew was in there, and I didn't hear it at all. No sound stage, no stereo imaging, no fidelity. That was just plain hideous.

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Just to put things in perspective, Futterman amps had upward of 40dB of feedback.
That's probably an artifact of speaker design from those days when speaker design really was a "black art", before Theil's research into speaker design. It took lots of gNFB to get that Zo way down there to tame those ill-mannered beasts with all their impedance surprises across the audio band. These days, it's not necessary, and that level of gNFB in a hollow state amp is way too much and really compromises the sonic performance.
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Old 24th November 2010, 08:42 PM   #22
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Originally Posted by kruesi View Post
Wavebourn, Sy- Thanks a lot.
That's actually what I thought at first (said to self: "Why, all this is nothing more than local FB around the output stage!) but then I made more of it that there really is/was.

Not that it's easy to implement, impedances being what they are here.

Thanks for clarifying- I'll stay tuned to this interesting thread. -!
There were other threads about this type of local feedback, discussing the topic in great details, including driver stage that is loaded on parallel feedback.
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Old 25th November 2010, 03:56 PM   #23
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Today I have found the time to experiment some more..
I tried increasing the current in the LTP so the voltage is back at initial setting.
I tried using coupling capacitors so there is no need to change the current.
I tried to use a resistor network a la baby huey.

Results where close together with all different methods.
But the sound is better with no feedback, or just a very very little bit!

When I apply a large amount of feedback, the amp loses it speed, it doesnt have that nice punch anymore.. (I think that's what Allen described as thin)

I'm running out of ideas.. I dont want to use gNFB
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Old 25th November 2010, 05:35 PM   #24
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Originally Posted by hidnplayr View Post
When I apply a large amount of feedback, the amp loses it speed, it doesnt have that nice punch anymore.. (I think that's what Allen described as thin)

I'm running out of ideas.. I dont want to use gNFB
Then don't. Your design; you have to live with it. I don't.
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Old 25th November 2010, 06:32 PM   #25
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My small experience is that the lower the output impedance the thinner and tighter the sound. Using lots of plate to plate will do that and so give a thinner sound. I do this approach with 6080 finals with 100% local feedback - these have amazing bass control compared to any other tube amp I have heard.
Speakers tend to be optimized for either tube amps or solid state amps and will tend to sound significantly different if not properly matched to their intended amp.
I also built a version of Gary Pimms Tabor using 807's but its intrinsic distortion never went away for my satisfaction, a brittle edge to everything - never spent much time on optimizing it though.

Just my thoughts

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Old 26th November 2010, 08:02 AM   #26
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Quote:
Originally Posted by hidnplayr View Post
When I apply a large amount of feedback, the amp loses it speed, it doesnt have that nice punch anymore.. (I think that's what Allen described as thin)

I'm running out of ideas.. I dont want to use gNFB
Why not ? Somethings not right here. That relationship goes completely against the grain. All amps using global feedback, use it to extend the frequency response, as such the slewrates become faster; one has only to look at 1kHz squarewave to see this with and with global feedback. The results are dramatic !

Pic with Square wave output of 100W power amp with global nfb (upper trace) and without (lower trace) . Rise time with feedback ~3.5uS.

Without the feedback, the response -3dB @ 10kHz !!; With it -3dB @ 55kHz. BIG difference. I fail to see the sounds "thin" connection. It must be something else to blame.

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Old 26th November 2010, 09:09 AM   #27
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The driver should be run at as high current where it has its sweet spot, check curves. Go for a D3a or E280F at 150V/150V/20mA. Also add gyrators instead of anode resistors. MJK has shown elsewhere how it should be done.
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Old 26th November 2010, 09:53 AM   #28
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with a gyrator load on the driver pentode, you make the feedback degree dependent on
the internal resistance of the driver pentode ?
Is it ok to do that ?
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Old 26th November 2010, 10:30 AM   #29
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Heja Vega!

Think we donīt need to care about internal resistance especially when we should leave the cathoderesistor w/o decoupling.

One can look upon it as a current source.
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Old 26th November 2010, 10:49 AM   #30
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Originally Posted by revintage View Post
Also add gyrators instead of anode resistors. MJK has shown elsewhere how it should be done.
I tried this sometime ago using high transconductance unscreened tubes, the 6CL6,12BY7 gang, and with the minimal damping and excellent ft, self oscillation can be a problem. Beware of the layout using such gyrators with a near infinite AC load Z.
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