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Old 25th September 2007, 01:36 PM   #1
Cassiel is offline Cassiel  Libya
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Default EL84 SEP schematic. Some questions.

Hi,


I do have a SEP stero amplifier but i'm not very happy with the feedback schemes i've tried so far.

Putting the feedback loop at the cathode of the input tube makes for a better sound, more natural etc. but unfortunately detail is loss and there's less "soul" in the music. I would say overall sound is inproved but music it is not.


I am going to try the output scheme of this schematic but i don't understand it. Please could someone tell me what having the negative side of the bypass cap at the + side of the OT does?

Also why those diodes?
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Old 25th September 2007, 06:27 PM   #2
Klimon is offline Klimon  Belgium
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Quote:
Please could someone tell me what having the negative side of the bypass cap at the + side of the OT does?
Interesting question..... anyone?
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Old 25th September 2007, 06:41 PM   #3
Svein_B is offline Svein_B  Norway
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I believe this feedback is called cathode feedback.
It is [ideally] implemented by a separate section of the OPT between cathode and ground. The one shown is a simplified solution. It is sometimes done with both AC and DC from cathode going through the OPT, but it is better to just connect the bypass cap to the output. This way the cathode AC will follow the output and generate negative feedback to the EL84.

Another alternative of local feedback is described in the thread RH ECL86 Amp: Dave?

The diodes serve no real purpose, and I think the cathode resistor for the EL84 look a little on the low side.

SveinB.
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Old 25th September 2007, 07:00 PM   #4
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I would try local feedback as well. It seems to leave the overall sound more lively than cathode feedback. Its simple to try with a 100K resistor between the two plates. the only potential issue can be high frequency roll off - but I very much doubt it would effect this design.

Shoog
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Old 25th September 2007, 09:47 PM   #5
Cassiel is offline Cassiel  Libya
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Thanks for the answers.

Well, i have tried it and i like it. You can hear how distorsion goes down but does not make the music less lively. Good. Gain is only a little less so i presume there's little feedback going on.

Shoog you're talking about Alex Kitic RH amp, aren't you? Yes i was going to try that but since my input tube is a 6sl7 it would need a different resistor, and i don't know how to calculate the proper resistance.
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Old 26th September 2007, 06:42 AM   #6
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Quote:
Shoog you're talking about Alex Kitic RH amp, aren't you? Yes i was going to try that but since my input tube is a 6sl7 it would need a different resistor, and i don't know how to calculate the proper resistance.
Yes.
The analysis of how to calculate it is a bit mind bending as this alters the way that the circuit works in a very fundamental way. Still in most situation just plugging in a resistor in the range 68K - 470K will do the job and its not that critical, ie tune by ear.

Shoog
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Old 5th January 2013, 06:25 AM   #7
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Quote:
Originally Posted by Cassiel View Post

Also why those diodes?
I am resurrecting the old thread, for my own sake mostly...
These diodes look very much like the ones used in original MiniWatt S1 diode mod. The function and purpose of these diodes was explained on one of the horn shoppe forums: VolleynBike's MiniWatt mods - The Horn Shoppe Forum
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Old 5th January 2013, 10:03 AM   #8
DF96 is offline DF96  England
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The diodes will add a little distortion. Probably not enough to notice. Imagination/placebo will do the rest. Some people interpret any change in sound as an improvement.
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Old 6th January 2013, 07:42 PM   #9
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Tried diodes on my amp, no improvement really, maybe just the opposite - added sibilance and HF stridency.
I am trying various feedback variants and can't find one that I like. Will definitely try aforementioned "cathode feedback". I suppose that none of the SEP variants will be as pleasing to my ear as triode strapped el84.
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