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Old 1st December 2003, 05:48 PM   #11
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Excuse me, I forgot

I am asking for parameter values
since I am not able in
simulating the circuit without
changing resistor values.

bye
Federico
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Old 1st December 2003, 07:39 PM   #12
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> I cannot find any discernable difference between these two line stage topologies.

One on the left has more tubes. It has to be better.

Myself, with a 6DJ8 and these values, I would not expect any significant difference in simulation or measurement. Voltages and currents are high, Mu is high. What the extra tubes do is waste supply voltage and reduce maximum undistorted signal swing, but there's tons of swing left. I happily agree that the extra tubes may clip more musically, though I doubt any normal audio level takes it near clipping on the worst transients.

Your models are of course imperfect. They probably only cover x^2 and x^3 order terms. This will perfectly model most large-scale behavior, but real devices have higher order bends. Maybe the stacking cancels (or enhances?) these high-order nonlinearities that can't be modeled at the current state of simulation (or tube production).

> I wasn't sure that I had reliable parameters for the cable.

Use 25-30pFd for cable capacitance.

What kind of fat-stuff has 100pFd/foot??? (Or was it pFd/Meter, and you mis-read?)

I doubt the inductance or resistance has any effect for audio inside the home.

6 feet is common.

You might find 200pFd parasitic on some inputs.

Round-up to 1,000 pFd. If the line may run across the room (preamp at chair, power amp at speakers), figure 2,000pFd.

I assume you have checked at high levels like 3V peak?
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Old 1st December 2003, 10:16 PM   #13
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Hans, thanks for the information. It is as I expected too. CFs don't do well into small loads, just like plate loaded topologies.

Federico, here is my modified model from Norman Koren's website:

.SUBCKT 6DJ8 1 2 3 ; P G C; NEW MODEL
+ PARAMS: MU=33 EX=1.3 KG1=220 KP=250 KVB=200 RGI=2000
+ CCG=2.3P CGP=2.1P CCP=.7P ; ADD .7PF TO ADJACENT PINS; .5 TO OTHERS.
E1 7 0 VALUE=
+{V(1,3)/KP*LOG(1+EXP(KP*(1/MU+V(2,3)/SQRT(KVB+V(1,3)*V(1,3)))))}
RE1 7 0 1G
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1}
RCP 1 3 1G ; TO AVOID FLOATING NODES IN MU-FOLLOWER
C1 2 3 {CCG} ; CATHODE-GRID; WAS 1.6P
C2 2 1 {CGP} ; GRID-PLATE; WAS 1.5P
C3 1 3 {CCP} ; CATHODE-PLATE; WAS 0.5P
D3 5 3 DX ; FOR GRID CURRENT
R1 2 5 {RGI} ; FOR GRID CURRENT
.MODEL DX D(IS=1N RS=1 CJO=0 TT=1N)
.ENDS

PRR, my conclusion is the same as yours. The extra tubes are a waste of heater power. It would be awfully nice to do a strict AB blind comparison to see if any differences are audibly discernable.

The models are somewhat imperfect against the real imperfections of actual tubes and maybe these are audible, but I am doubting it at this time until I have further information.

Regarding the cable, I selected values that I thought would be much worse than any real audio cable so there wouldn't be any room for doubt. The values for the Belden cable that I did locate were well below the values I used.

I will do a few more tests with high load cap and higher signals to see if there are any noticeable changes.
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Old 1st December 2003, 10:30 PM   #14
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OK, I created five foot cables (and then I hit the PSpice demo parts limit).

200nH, 30p, 30mR per foot and 2000p, 100K load.

1Vp input, 7Vp output.

28mV second harmonic for the FVP
25mV second harmonic for the standard CF

Or, basically, no difference.

If there is something in the real world, other than the micro-features that PRR refers to, I sure would like to know what it is.
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Old 2nd December 2003, 01:02 AM   #15
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Hi,

Quote:
If there is something in the real world, other than the micro-features that PRR refers to, I sure would like to know what it is.
If we look at the AW circuit, what do we see?

I see very high PSRR and still decent voltage swing capability into lowish loads.

Whether that's the answer I'm not too sure. Short of building everything and giving it a listen it's hard to tell.

Cheers,
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Old 2nd December 2003, 02:33 AM   #16
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Quote:
Originally posted by fdegrove
Short of building everything and giving it a listen it's hard to tell.
If you look at the evolution of this pre-amp (the 5 stands for the 5th version) over the last 20 yrs or so you will see that AW has continued to stive for better power supplies & better downward dynamic range. Miss Piggy (his term for a straight cathode follower) has always been something he hasn't liked the sound of and this is many years of work to make a low impedance output stage that sounds good... Joe is using a very similar circuit in the front end of his tube buffered gain clone.

Give me a year or 2 and i'll reposrt back, the parts for an RTP are slowly gathering (and the skill to actually assemble it)

dave
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Old 2nd December 2003, 03:00 AM   #17
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Hi,

Quote:
If you look at the evolution of this pre-amp (the 5 stands for the 5th version) over the last 20 yrs or so you will see that AW has continued to stive for better power supplies & better downward dynamic range.
As it is, I don't feel all too comfortable with the idea of anyone needing twenty years to perfect an idea.

I may well be wrong but at the the time of writing I feel the WCF with its PS I presented before are doing what I expect from a buffer.

For all things audio, I can't find fault with it.
As for dynamic range, given the PS voltages, I'm darn right sure the circuit I presented wins hands down...
I invite anyone to think it through, build it or preferably both.

Cheers,
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Old 2nd December 2003, 04:19 AM   #18
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Quote:
Originally posted by fdegrove
I don't feel all too comfortable with the idea of anyone needing twenty years to perfect an idea.
That is a pretty ill conceived statement Frank... many of the best designers in the world have been honing their craft for longer than this ... Nelson Pass' 1st Thresholds are now over 20 years old and i'm sure he was starting to work things out long before... Ted Jordan is still working on perfecting a design i 1st ran into 30 years ago, and the list goes on & on & on....

dave
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Old 2nd December 2003, 12:37 PM   #19
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Hi runeigth, thankyou

I see that your parameters are different
from those in the koren website.

Are they based on published
plate curves (which one?) or have you fitted
a real tube?

However, I think Koren model is
able to well describe tube behavior.
Surely in the negative grid region.

Federico
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Old 2nd December 2003, 12:42 PM   #20
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Hi,

Quote:
many of the best designers in the world have been honing their craft for longer than this ... Nelson Pass' 1st Thresholds are now over 20 years old and i'm sure he was starting to work things out long before... Ted Jordan is still working on perfecting a design i 1st ran into 30 years ago, and the list goes on & on & on....
Oh yes, absolutely. No problem there.

What I'd thought you meant was the polishing of Miss Piggy.

Cheers,
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