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Old 11th Mar 2018, 4:31 am   #1
joebog1
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Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
Default Pre-amp for Golden ears

So most of the design phase of the Laboratory Golden Ears amplifier is done!
The chassis is "mostly" made, its been designed ( for better of for worse) and is waiting for weather to improve to go further. I am still "convinced" it will work just fine.

Now I will need a Phono stage to go with it.
Ohh Noo!!!!
says my post reader and approver ( David I think) .
I am looking at using some twin triodes designed specifically for RF, in cascode format. BUT being for RF they are decidedly microphonic which is nowhere near as important, as it is for audio.
My question:
I need to isolate the valve base (s) from the chassis, so that I wont suffer physical "modulation" of the valves. (6BQ7A )
Normally when I make a shock mount valve base, I use two rubber grommets fitted into holes in the chassis ( plate), with brass sleeves inserted into the grommets, a 3mm or 6BA screw and nuts and flat washers, are used so that the valve and its base is "isolated" from the chassis. That is just one idea!!!
I am asking the hundreds of thousands of hours of experience within this forum, for other ideas.

Ohh! I have tried "spring mounting" using small coil springs, and that idea is not feasible, at least as far as my experiments go. I always end up with the microphonics sounding like a reverb spring, even when I damp the springs with either heatshrink or foam inserts into the springs.

For any who are interested, the article I am basing my design on is:
Cascode Preamp Improves Signal-to-Noise Ratio
Audio magazine October 1955
by M.V. Kiebert, Jr

With many thanks in advance,
Joe
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