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Old 15th April 2008, 08:10 AM   #21
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^^^^

White Paper by O. H. Schmitt, and personal experience from building two versions of the circuit. In both cases, the gain was half of what a single stage would give.

This also applies to the SS version.
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Old 15th April 2008, 09:44 AM   #22
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Quote:
Originally #21 posted by Miles Prower
^^^^

White Paper by O. H. Schmitt, and personal experience from building two versions of the circuit. In both cases, the gain was half of what a single stage would give.

This also applies to the SS version.
Hi Miles Prower,
Do I have to pay money to read it???
Here the asymmetrical cathode coupled is explained:
http://coupling-triode.blogspot.com/
hope it helps...

Kind regards,
Darius
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Old 15th April 2008, 01:03 PM   #23
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I think the same papers are to be found at www.aikenamps.com/TI_Other.htm No matter what, you loose 6dB in a diff amp(or Schmitt-LTP).

Brgds
Lars
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Old 15th April 2008, 01:30 PM   #24
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Default #21 #23

Hi,
I can't find an asymmetrical cathode coupled amp there. ???
You don't loose 6dB, this is wrong.
This circuit is a special triode topology, don't look for SS=
solid state(?) examples.
Please read the text in the blog! I think books will not help
you. You don't have to pay or download something.

Kind regards,
Darius
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Old 15th April 2008, 03:33 PM   #25
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Had to do "your" circuit in spice and we are right in the middle.

If we do according to Schmitt and use anoderesistors on both stages we loose ca 6dB. But in this circuit the loss is 3dB compared to a standard triode gain stage.

Think I also saw this circuit in Bruce Bs "Audio Reality" where it is called "Grounded Grid Cascode Phono Preamp.
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Old 15th April 2008, 04:34 PM   #26
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Default cathode follower calculated...

Quote:
Originally posted #25 by revintage
Had to do "your" circuit in spice and we are right in the middle.

If we do according to Schmitt and use anoderesistors on both stages we loose ca 6dB. But in this circuit the loss is 3dB compared to a standard triode gain stage.

Think I also saw this circuit in Bruce Bs "Audio Reality" where it is called "Grounded Grid Cascode Phono Preamp.
Hello Lars,
yes, I know a lot of people who saw this circuit ...

of course in a symmetrical cathode coupled amp you loose 6dB.
But this is an asymmetrical one!
A grounded chathode stage and a grounded grid stage have the same
voltage gain number. The gain in the asymmetrical cathode coupled
stage is reduced by the cathode follower at the input.
Let us calculate the gain of the follower:
R load 8KΩll 22KΩ = 5900Ω, R follower 1/mutal cond. = 400Ω
5900Ω / (5900Ω+400Ω ) = 0,94 [gain reduce caused by load]
=100 1-1/100 = 0,99 [open output gain]
0,99 x 0,94 = 0,93 [this is the voltage gain of the cathode follower]
0,93 = -0,63dB
Thus the circuit loss is 0,63dB compared to a standard triode gain stage.
Where did you get 3dB from?

Kind regards,
Darius
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Old 15th April 2008, 05:07 PM   #27
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Hello Darius,
To check I did a sim in LTSpice with your component-values. I did it at work and can publish it tomorrow. My guess is that there is some kind of feedback introduced at the grounded grid input. This might be less pronounced if you have a cascode as the secong stage.

Anyway will show it tomorrow.

Brgds
Lars
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Old 15th April 2008, 05:17 PM   #28
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Default #27

Hello Lars,
thanks, I am looking forward to see your LTSpice simulation.

Kind regards,
Darius
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Old 15th April 2008, 05:17 PM   #29
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Quote:
A grounded cathode stage and a grounded grid stage have the same voltage gain number
Sorry that is incorrect, the gain in a grounded grid amplifier is dependant on the source impedance of the preceding stage as the input impedance of a grounded grid amplifier is not infinite as it is in an ordinary cathode grounded stage. The gain of a grounded grid stage is:
gm*RL/1+RL/Rp+Zs(1/Rp+gm)

which can be compared to the gain in an ordinary cathode grounded stage which is gm*Rp*RL/RL+Rp.

As the grounded grid stage is driven by a cathode follower which has a non zero impedance the gain of the grounded grid stage will be less than what it would have been if it had been an ordinary cathode grounded stage,

The gain for "your" Asymmetrical cathode coupled amp is given in "Electrical engineers handbook" by Pender & McIlwain published 1936!

as gm^2*RL(RL+Rp)*[1+(Rp/Rk)+Rp*gm]/[2*(1+Rp*gm)+[RL+Rp)/Rk]+RL*[(1/Rp)+gm]]^2

I hope I copied that correctly!

You can't say that the gain of this stage is 3dB or some other fixed amount lower than an ordinary cathode grounded stage but it is for sure that the gain will be lower, due to the gain loss in the cathode follower and the loss due to non zero drive impedance to the grounded grid stage.

You can find analysis of most and every tubes stage someone ever thought about, you just need to know where to look!

Regards Hans
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Old 16th April 2008, 07:03 AM   #30
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Default #29

Hello Hans,
in the future pelase tell the number of the treads you quoted from, thanks.
As posted in #24 these books will definitely not help you understanding.
A grounded cathode stage and a grounded grid stage have the same voltage gain number.
(Sorry but I don't care about 80 or 81, hi hi.) Please note that gm (= s = mutal
conductance) is not the same for both triodes.

Kind regards,
Darius
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