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In the Forum: Melquiades Amplifier
In the Thread: Super Melquiades Amplifier.
Post Subject: Re: A little war with Super Melquiades.Posted by cv on: 7/18/2005
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corladjunin.org.pe Allo,
Well, the divider may certainly be responsible for some softening of transients - in particualr, what values are you using there?
Also, the received wisdom is that silver micas are created far from equal; no experience with them myself but I believe Thorsten may have something to share in this area.
My concern is not so much the type of cap as to the value; air cap is probably the ultimate type, it should be quite easy to get some that are happy at 400V plus, but I really do wonder if the high output impedance of the 410pF/100k filter you are using is responsible.
So, it will be interesting to see how the vacuum caps compare (though from what I've seen they are probably as big as the amp). In the meantime, may I suggest trying the original big value coupling cap (the Electrocube 950) with a low impedance filter ahead of the amp? I know this changes everything, but it should provide another interesting point of reference. If your preamp is happy driving it, maybe 4nF/10k or even 10nF/2.5k?
Oh yeah, if you are going to stay at an x/o of 3.7KHz, an aircore transformer becomes even easier...
Cheers
cv
PS Btw, I think I have mentioned this before, but for my planned aircore OPT amp, I will be using a step-down input transformer wound on a tiny toroidal nanocrystalline metglas core. This will
a) provide a 1st order high pass filter (fed via a series resistor to form an LR filter)
b) provide stepdown for gain matching and better drive of the input tube
c) achieve voltage isolation so that the drive can be directly coupled to the output tube (negative supply voltage to the first stage), also battery bias of 1st stage via the secondary
no caps anywhere; the toroid is wound (tediously by hand) with almost a single layer winding: 2/3 of the way round is the primary, secondary 1/3 and it goes all the way up into the MHz.
Result: the only signal path cap is the power supply resevoir to the output valve. input stage needs VR tube regulation - has to be very quiet as the dc coupling/split supply means all psu noise is presented to the opt
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