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Old 29th November 2003, 03:07 AM   #1
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Default Vacuum State Electronics FVP5A

Folks, I would very much appreciate the benefit of everyone's expertise on a problem that puzzles me.

In another post someone referred to VSE's FVP5A preamp (with built-in phono section). So, naturally, I went to Mr. Wright's web pages to see for myself. I discovered alot of very interesting information and since I'm a simulation type of guy, I decided to simulate the line stage portion of the FVP5A. After I did this, and not noticing any particular special features of this topology, I decided to compare it to a plain old unbypassec grounded cathode coupled to a CF. I've shown my simulation diagram below. The GK and CF stages are biased identically for both amps.

Now, here is where I would really like your help to understand what I'm seeing.

But first, let me admit that simulations are just that. OTOH, I do believe that while they cannot always do a great job of predicting absolute values of certain things, like THD, they can do an excellent job of comparing two topologies.

I've compared these two designs for THD, frequency response, and phase shift (from 10Hz to 10MHz), and Zo. Folks, I cannot find any discernable difference between these two line stage topologies. Absolutely none, except that the plain CF has a slightly lower Zo and drops off a little faster above about 1MHz.

Since I know there are many things that I don't understand and not meaning to in any way disparage a highly respected supplier of tube equipment, I would very much like to know what I am missing. Thanks very much for the help.

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Old 29th November 2003, 04:52 AM   #2
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I did basically the same experiment, but in hardware, ie, I built both and measured and listened a while back. I expected them to measure the same (modified AWA THD meter), and sound the same. They measured near enough to identical with a resistive load, but add a few metres of cable, and they sounded nothing alike with the SLCF being very good, much to my surprise. I wasn't expecting what I found, expecting they'd both be pretty poor.

FTR there's still a modified diff/bal version of this circuit in my pre chassis, but it isn't in use as I found something else I prefer more. I still use the phono section for my Decca.

So far, with all I've seen, I'm yet to share your confidence with modelling, so I'm not sure what you're asking. Lots of different circuits measure closely enough IME and sound completely different.
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Old 29th November 2003, 05:06 AM   #3
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Quote:
so I'm not sure what you're asking.
Actually, you've begun to answer my question already! I was hoping that someone had done a hardware comparison.

I did wonder if the FPV would be better at handling cables, since there seems to be otherwise no real difference between it and a plain design. But, I don't really see how, looking at the circuit. Not that I don't see what the design is, but I don't see how it handles the characteristics of an audio cable better than an ordinary CF.

So, I would really like to understand what it is about this topology that makes it better able to handle "transmission lines".

Thanks for the help.

BTW, if the answer is on VSE's website, just point me there.
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Old 29th November 2003, 05:49 AM   #4
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Maybe it would show some difference if you add a simulation of an interconnect cable with a load. The LCR components of the interconnect could be the part of this that is not in your simulation but is allways there in the real world.

Later BZ
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Old 29th November 2003, 03:11 PM   #5
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Thanks for the suggestion. I had already tried that, but I wasn't sure that I had reliable parameters for the cable. So I did some searching this morning on cable parameters and found some LRC for Belden shielded cable which were lower than the numbers I was using.

But, here's what I did. I modelled three feet of cable with:

200nH per foot
30mOhms per foot
100pf per foot to shield and other wire
100K termination load at the input to the next device
50pf capacitance at the input to the next device

I think most good audio cables are better than this.

In any case, I see no difference except that the FVP5A begins to show a resonance above 1MHz and so instead of tailing off after 1MHz the signal begins to rise a bit.

I'm happy to post these results if anyone wants to see them or to check my work.
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Old 30th November 2003, 10:12 PM   #6
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Hi,

Quote:
But, I don't really see how, looking at the circuit. Not that I don't see what the design is, but I don't see how it handles the characteristics of an audio cable better than an ordinary CF.
Current drive ability into lowish imedances and highish capacitance is the key of a good CF.

What you want to kick the grid of a big capacitive bottle is much alike to what you want from a buffer; low Zout and plenty of juice.

Cheers,
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Old 1st December 2003, 12:02 AM   #7
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Fair enough. In my comparison, both output buffers are idling at over 10mA and the Zo for both is less than 100R. Either of them can work into fairly difficult loads, as witnessed by my simulations of transmission lines with higher than normal inductances and capacitances.

The FVP does have a CCS at the bottom reducing current swing in the tubes, but this hardly linearizes it much more than the ordinary CF and does not reduce distortion noticeably.

So, I am still puzzled as to why this topology is better than a regular CF.
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Old 1st December 2003, 12:12 AM   #8
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Hi,

Quote:
So, I am still puzzled as to why this topology is better than a regular CF.
Yes.
This is not you're fault at all but music is dynamic not static.

If sims were to simulate music under dynamic instead of static conditions it would all be that much easier to see the picture.

Cheers,
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Old 1st December 2003, 03:07 AM   #9
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Code:
So, I am still puzzled as to why this topology is better than a regular CF.
A regular CF with low value cathode resistor and grid leak tied to ground is not very linear but if the cathode resistor value is increased either by using negative voltage supply or by increasing grid potential linearity improves considerably.

There is a interesting comparison of CF linearity in Walley & Wallman, "Vacuum tube amplifiers" in the section of DC amplifiers where the linearity of an ordinary CF is compared to a CF using negative voltage supply with a large value cathode resistor and also compared to a CF with penthode CCS. The result speaks for itself, the ordinary CF is worst and the neagtive supply CF and the CCS CF is quite similar especially for relatively low voltage swing.

In your comparison you compared a CF with relatively high value cathode resistor with a CCS coupled one and the result is as expected I think.

I have built different kind of CF's using -100V cathode power supply and either resistor or CCS using pentodes or FET's and I have also compared this with an ordinary CF with grid tied to ground, my experience is that pentode CF or large cathode resistor CF measure almost the same and also give the best sound, sorry but although I tried I dont like the sound of FET based circuits.

My current line stage is based on a SRPP DC coupled to a cathode grounded stage followed by a CF using -100V for cathode resistor supply, I use DC and AC feedback to control gain, (I believe that feedback used correctly can give excellent results both sonically and in measurements)

Regards Hans
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Old 1st December 2003, 04:34 PM   #10
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Hi,

Im am very interested to simulate
and perform this circuit comparison.

Which parameters are you utilizing
in the 6dj8 and, what model are you using?


Federico
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