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Old 23rd August 2011, 08:04 PM   #11
Stee is offline Stee  Italy
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Default high impedance jfet input buffer

The input buffer is the heart of the system
because it allows you to bring out the detail with great accuracy
extrapolated from a difference (the potentiometer) from very low impedance
so as to avoid a tube preamp

to test the limit of the resonance
I applied a resistance 10khom
the potentiometer output

Tomorrow I will try to apply a small capacitor
As is normally used for the feedback resistor
and I will try to lower the shunt resistance of non-inductive 0R22

the transformer is the ideal tool to perform addition (Hot and Cold)
as said before
the differential stage (for the valves too) is not suitable for audio

the best input transformer is a Tamura
600 to 300 ohm 2:1
This feature allows you to get a low susceptibility
of the source (DAC1 Little_Dot)
and also helps reduce the harmonics of even order
making the sound pleasant to listen to long

Bias is AB1 with a small fan on the radiator
not known at the time distortion
even with a full piano

still can not reach a minimum volume
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Last edited by Stee; 23rd August 2011 at 08:09 PM.
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Old 23rd August 2011, 09:50 PM   #12
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Quote:
Originally Posted by Stee View Post
The input buffer is the heart of the system

I see you avoid my simple question. May be a language issue, so let's get some numbers.

Originally, the fet buffer is loaded with a 470R resistor. At 1v input 1kHz second harmonic is around -80db and third -76db. Quite acceptable.

In your improved circuit fets are working into something closer to 30ohm load. Third harmonic is now at -41db.

Intensive indeed.
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Old 24th August 2011, 07:49 AM   #13
Stee is offline Stee  Italy
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Default About external NFB

I do not know how do you calculate the load of the JFET
because it is floating
the zero point determined by the resistance of feedback does not exist anymore
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Old 24th August 2011, 10:39 AM   #14
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The load are the common base input transistors. Why do you think they had to use an input buffer in the first place?
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Old 24th August 2011, 11:11 AM   #15
Stee is offline Stee  Italy
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Default property of global feedback

because if you put the buffer in the feedback loop
After you have a low input impedance
that's why you need a transformer

Note that with the JFET into the ring
corrects the error of the buffer itself

to calculate the load buffer
must consider the current flowing on the common base input stage
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Last edited by Stee; 24th August 2011 at 11:32 AM.
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Old 24th August 2011, 12:43 PM   #16
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Quote:
Originally Posted by Stee View Post
is important to know that the differential stage introduces emphasis
the correct method to apply the subtraction is the potentiometer

so it takes an inverting amplifier
preferably with high input impedance

is also good solution
take the feedback current
able to equalize the performance of the speaker

this is my configuration happier
provides a dynamic scene (XLR source indispensable)
detailed and intensive
I have seen ccts. using chips which introduce shunt FB. What I would call a transconductance amp. The author never really felt it was an absolute solution more of an interest to be investigated.

In the schematic show there are far too many active and passive parts. Every part buggers the music!
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Old 25th August 2011, 05:17 AM   #17
Stee is offline Stee  Italy
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Default more simple amplifier

see this solution
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Old 25th August 2011, 06:11 AM   #18
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Quote:
Originally Posted by mhouston View Post
Every part buggers the music!
It depends. The circuit above will generate more than 1% thd, mostly third and not counting the transformer. Apparently this contributes to an "intensive" sound.
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Old 25th August 2011, 06:17 AM   #19
Stee is offline Stee  Italy
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Default optimized inverting amplifier

simple version
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Old 25th August 2011, 02:18 PM   #20
Stee is offline Stee  Italy
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Default What about simple version?

Quote:
Originally Posted by analog_sa View Post
It depends. The circuit above will generate more than 1% thd, mostly third and not counting the transformer. Apparently this contributes to an "intensive" sound.
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