Hi Pat
Its about time you said hello to the dudes down here at RAT.
And here I am!
The one I have at my site becomes too noisy if the rated
input voltage is less than 0.8 mV, if the input termination is 47k.
Maybe with less R at the input a 12AX7 could be OK, but few MC carts
have high output AND need a low R termination.
In fact I like very low R myself. I often use 10-22R with Lo-Outputs MCs,
- that will lower noise realative to 47K, but also lower output, so still
giving
no gain in S/N. So I approve of...
I have recently addopted the use of a fet in the RIAA preamp of mine
so that like Allen's design, the fet allows MM and MC.
When others copy, that is flattery.. :-) But it does work. What fet you
use?
I prefer to have all the RIAA passive eq done in the one single network,
In some cases there isn't any other choice, but when you can split
it makes for easier tweaking. I use an inverse RIAA and aim for sq
waves at 100Hz, 1KHz and 10KHz. A perfect sq wave indicates virtual
flatness a decade above and below that freq.
I must say, and I looked at your circuit, not a great admire of SRPP.
It is PP and not that great. The distortion created on the bottom tube
is fed to the top. Also it strikes me as always hunting to finds its own
equalibrium, look at the DC biasing and it looks like TWO cs in series.
The fet with a cascoded triode on top, needs less than 10V, and is
now a proven method. It sounds clean, precise and not the softness
I hear in SRPP. It's more dynamic, involving and still a lot of tube
qualities.
I don't travel much, too much work, too little money,
you know how it is.......
I don't like the traffic in Sydney, sure, where you are its not bad,
but where I usually stay when I go up is nth shore, and bleedin
terrible.
Consider you have an open invitation... when/if you do. Just call.
Tremain's Audio Encyclopedia, 1965, has a power amp schematic
where the tube driver stages were replaced by fets, using the same
supply voltages, and the fets seemed to be fairly high voltage rated
types, but with numbers I have never seen around.
The author went on to describe how IMD/THD was reduced with fets, but
methinks
it was due to the effectively higher amount of applied FB, due to the
extra fet gain. NFB was SO popular in 1965.
Almost certainly true! Betcha after IMD/THD they didn't bother listening
for differences? I suspect today anyone worth their salts would have.
So at least some things *are* better! :-)
Joe Rasmussen
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