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Old 31st August 2005, 10:15 AM   #1
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Default Your opinion on this VCCS heater supply please.

http://www.machmat.com/sales/kits/vccs.htm

Explanation of the VCCS circuit: (from Matthijs's site)
A voltage reference for the DC current control is made by R1. The LM317 delivers a 1.25V reference, which is copied to the plus reel by T1 (for more stable operation). The reference voltage is determined by the value of R1 and R7.
The comperator LM293 biases the FET at a current which exactly drops the same voltage over the tube heaters as the reference voltage.
Because the comperator output is made very slow by R3 and C5, it only works for low frequencies. For AC frequencies the FET acts like a pure current source, with a very high internal impedance because of its flat V/I curves.
C7 and C8 improve high frequency characteristics of the circuit. They should be mounted as close as possible to T2 and the tube heater wires respectivily.
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Old 31st August 2005, 04:52 PM   #2
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hi!

i built his AFB kits.... they have the same principal of 'slowing comparator' output...

I know it's a differenty kind of kit but it DID make my amp sound MUCH much better (wether because it's fixed bias now - or because both push and pull sides are perfectly matched)

this man has some amazing thoughts about audio but he sure knows wghat he's doing.....
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Old 31st August 2005, 06:08 PM   #3
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Quote:
i built his AFB kits
What are those?
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Old 31st August 2005, 09:23 PM   #4
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http://www.machmat.com/sales/kits/afb.htm

automatic fixed bias:

gives a fixed negative voltage to the screen...you can set the resting current and when done this will always be held constant no matter what else changes eg. line voltage variation, tube aging

Because i have a PP amp...i made 2 stereo kits .... both push tubes connected to the one kit, both pull sides to the other....
L and R is now really set equally (70ma @380v) and the 'crossover distortion <- right term??? ' between push and pull will go way down.
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