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  #11  
Old 17th October 2011, 11:22 AM
FreddieT FreddieT is offline
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Default Re: 50 or 100k

Just to be pedantic I think Philip means unity gain rather than zero gain unless he is expressing himself in dB!

I use a passive pre with a shunt wired pot into my K5881 Mk II and like the results. However, our cat is getting nervous again because I too have become curious about the benefits of a cathode follower as a buffer. A while ago I lashed up a headphone amp using my valve prototyping rig (I won't be publishing a photo) and it is time to convert it to a CF configuration. The advantage is if things don't improve I save the expense of putting it all in a box.

Any suggestions for the best valve for a CF - I note that the early WAD line stage pre used an ECC88? B9A base suggestions would be best or I'll have to drill a hole for an octal base!

F.
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  #12  
Old 17th October 2011, 12:27 PM
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pre65 pre65 is offline
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Default Re: 50 or 100k

Unity gain it is.

If you want a valve unity gain buffer perhaps the CF conversion of the pre2 (as in FAQ) would do.

Greg swears by it and so do many others.
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Everything in this post is my honest opinion based on what i thought I knew at that very moment in time.
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  #13  
Old 17th October 2011, 02:31 PM
FreddieT FreddieT is offline
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Thumbs up Re: 50 or 100k

I'm about to check the FAQ for details. Thanks for the prompt.

After my previous post I had a rummage in the garage loft for the rig - and perhaps not suprisingly the spiders had moved in. Even tag strip and electrolytics seem fair game as feeding stations this year!

F.
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  #14  
Old 17th October 2011, 06:23 PM
John Caswell John Caswell is offline
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Default Re: 50 or 100k

Hi all,
I have never seen the point of putting a volume control after a cathode follower as it tends to negate the advantages of the C/F of low output impedance. I have found when driving a high impedance destination from a low impedance source eg PreIII into a KEL 84 there is a certain something that is missing when you stick a v/c in the way and I think it is all down to impedances.
I have a preIII and I am looking at ways to reduce the overall gain and will report back here when results and time allows.
Professionally all audio devices tend to be low output impedance (600R and lower) and high input impedance (10K and up).
Also being pedantic a c/f is actually a gain loss usually 0.9 X input.

John
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  #15  
Old 18th October 2011, 08:43 AM
bob orbell bob orbell is offline
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Default Re: 50 or 100k

Hi John, I will be very interested, as I am shure others will be, in any attemped at reducing the amount of gain of the PRE3, without afecting it's performance, shunt pot mod bring's the level about right, but makes the sound too bright, for me anyway, I did think of having new transformer's made, but don't have the necessary information. At the moment I am very happy with my C/F pre, but would still like to do something with the PRE3. BOB
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  #16  
Old 18th October 2011, 10:34 AM
FreddieT FreddieT is offline
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Default Re: 50 or 100k

I'll second Bob's interest in a low gain Pre3. In the meantime I think I'll try the CF route.

Bob - I presume you are using a shunt pot arrangement with your CF (ahead of the valve of course)? As John states there is a small gain reduction with the CF but nothing like what is needed with the change from potential divider to shunt wiring to give good control.

F.
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