wrote:

Hi Patrick

Surprised to see you actually pointing people to our web site.

Its about time you said hello to the dudes down here at RAT.
Hello Joe!



The schematic to look at, if I may be helpful to our enquirer, is:

http://www.vacuumstate.com/images/FVP5A.gif

Please note, that while Patrick's is MM this one is MC capable,
it has split EQ and thus the termination impedance for the top
part of the RIAA needs to be 50K. Also would need some kind
of buffering unless built within same chassis.

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.

ppl can use a step up tranny for MC with the type of amp at my site
if they wish.

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.

At least I feel the fet can be trusted with a tiny voltage without
mauling it to death as would be the case if the fet was used in a line
stage
or power amp stage.

I prefer to have all the RIAA passive eq done in the one single network,

but for the 75 uS, I use an extra series R out from the network
used for 3180, and 318 TCs, and then have the 75 uS attenuation
separated
off a bit which gives a truer adherence to the RIAA curve.
The noise in the following SRPP triode gain stage is negligible.
The Ro of this stage in my latest amp using 12AT7 SRPP, or more
correctly, what is called a bootstrapped follower, is 2k,
low enough to allow folks to connect the rec out to sound cards,
or to just about anything with short cables, and Rin above 5k.





Joe R.



PS: Any chance of seeing you coming to Sydney, Pat? I live
near the highway BEFORE you get to Liverpool. Plenty of guys
come up from Canberra and find it easy to get here... there is
no ^*#%% Sydney traffic to deal with! Maybe get some guys
from ASoN and we'd have a good time, got some nice Canberra
wine!

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.

Maybe I will retire to Bermagui! Nice fishin, mountains, unspoiled
places,
ninety mile long beaches, hardly a soul in sight, but that's 3 hours sth
east
of here. I only need a 'puter and soldrin iron.

BTW,

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.

If I ever build a balanced preamp, and post the schemo, I won't have to
refer
ppl on to vaccum state eh...

Patrick Turner.