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Old 27th March 2006, 01:37 PM   #1
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Default My version of the Vacuum State FVP5

I have been working on this preamp for about 2 years now. The latest alterations have produced a significant leap in performance so I thought I would share it.
The whole project started with the Vacuum State Valve Buffered Inverted Gainclone. I built the vary basic version as described in Joe Rasmussens article. I decided to try to emulate his more sophisticated version with a Valve CCS and a MOSFET constant voltage. As explained in Joes original article, by keeping the output valve at a constant current and at a constant voltage we get all the benefits of a Cathode follower but without its main disadvantage,ie 100% cathode feedback - which is widely recongnised as robbing the sound of life and punch. I built a few versions of this at various voltages and was happy with the result - enough so that when I decomissioned the Gainclone I kept the buffer on as my main preamp.
However it had one main disadvantage and one minor one. The main one was that as a cathode follower it was unity gain which wasn't quite enough for my power amp. The minor disadvantage is that it was a fraction bass shy.
I then went back to the vacuum state website and looked at the FVP5. This inspired me to convert my split rail buffer into a single supply preamp and to add a voltage gainstage. This was a big improvement, with extra gain and increased bass punch.
I then got involved in a thread about LED bias for valves and introduced it for the output stage. This was a help. I then tried a Green LED on the cathode of the voltage Gainstage. This produced a soft and smudged sound which was due to the low current through the LED of only 4mA. Further discussion suggested apply supplementary current to bias up the LED into a more linear range. This I did with a little circuit off the heater supply. This was a signicant improvement over the unboosted LED or an unbypassed resistor.
This final design is very detailed with a very natural top end and a powerfull bass. Its only disadvantage is the it has a little bit to much gain. The voltage gain valve could usefully be replaced with a lower MU tube for a lower gain overall. The only thing to consider is that the voltage on the grid of the second stage needs to be half the +B voltage plus a few volts.

It only remains to thank Joe Rasmussen for the original inspiration and the fundamentals of the circuit design. I also would like to thank SY for his ideas and help on LED bias.

Shoog
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Old 1st April 2006, 03:33 PM   #2
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Nice one, but isn't the IRF510 a too low voltage device?
I see it just has a few volts to drop but I wonder if it could get into troubles at power up/down.

Cheers

Andrea
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Old 1st April 2006, 05:45 PM   #3
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"Nice one, but isn't the IRF510 a too low voltage device?"
I've been running mine for over a year and had no issues so far.

Shoog
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Old 1st April 2006, 05:57 PM   #4
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I have made a slight modification since posting the original circuit. I have changed the 1K resistor, which supplies the anode of the first stage, to a 660R. This is because the 1K produced a slightly bloated bass sound. As I said in the circuit, I suggest using a 1K5 pot to tune this to your exact taste. You can achieve a very full on hard sound or a very smooth sound by slight variation of this value.

Shoog
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Old 10th April 2006, 02:31 PM   #5
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Thanks for the reply,
Ive read that to sing this amp needs a good power supply, IIRC Allen Wright uses a super-regulator.

What did you use?

Cheers

Andrea
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Old 12th April 2006, 12:20 PM   #6
maikel is offline maikel  Netherlands
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Default PSU

Hi,

In my opinion is the superregged PSU form the original design from Allen a balanced PSU, as on the other hand this schema uses a unbalanced PSU with a different value of 156V DC


Is it therefore dificult to compare thes PSU's ??

Greetings
Maikel

PS: spare me, I'm a newbie :-)
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Old 13th April 2006, 10:38 PM   #7
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The power supply I used was a simple 5 stage CRCRCRC filter. IT is said in the original design that the superreg is central to the sound of the circuit.I cannot comment as I have never heard the superreg.
However in my small experience the simple RC filters often sound superior to regulated supplies.

The 156V supply can be varied but you will have to adjust the circuit to compensate. This shouldn't be to difficult.
Shoog
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Old 14th April 2006, 10:08 PM   #8
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Hi Shoog,
I made some calculations of the working point of the tubes, assuming 2V drop for the red led the SLCF works at roughly 5mA and slightly less than 80V, right?

Inspired by this thread I cooked up my version of the tube buffer you proposed time ago, using asymmetrical split supply (+100 and -50V), a C4S current source (instead of the lower tube) and a IRFD110 Mosfet for the constant voltage bootstrap.

Tested it on my scrap speakers and, tough I can't comment about the sound, it seems to work quite well.
The simple cathode follower was surely worse.

Thanks Shoog for sharing (and of course Joe Rasmussen!)

Cheers

Andrea
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Old 15th April 2006, 07:07 PM   #9
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Hi Andrea,
Good to see you gave it a go, looks good to me. The exact bias point needs to be carefully set to get the best out of this circuit- experimenting is the only way really.
If you try it on your main system please report your impressions so that others can decide whether to try it.

Just got back from a weeks holiday and my main amp is down bummer .

Shoog
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Old 28th April 2006, 03:31 PM   #10
maikel is offline maikel  Netherlands
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Quote:
Originally posted by Shoog
The power supply I used was a simple 5 stage CRCRCRC filter. IT is said in the original design that the superreg is central to the sound of the circuit.I cannot comment as I have never heard the superreg.
However in my small experience the simple RC filters often sound superior to regulated supplies.

The 156V supply can be varied but you will have to adjust the circuit to compensate. This shouldn't be to difficult.
Shoog
Hi Shoog,

Did you manage to resusitate you're amp??

Can you post the 5 stage CRCRCRC filter schema?

Did you manage to compare the sound of the FVP5 to you're design??

I never build a tube-amp, and I heard that building a phono-amp is even more difficult because of sensitivity. But I want to build a real high-end phpno-amp (MC)
That's why I think it's more realistic to build you're amp than the FVP5.

What do you think, and do you have any tips / suggentions for me?

Thanx very much for you're comments and time

Maikel
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