DIY Tube Preamp

[Brett]

Originally posted by FBLEAGH
But jsut for interests sakewhat would you consider the best preamp design ?? i know it very much to a persons tastes but just wondering

My view on amplifier design is that it should be designed with it's compromises so that they are minimally disruptive to the music, and stable. Each stage should interact with the one on each side as little as possible, and be as linear as possibly both statically and dynamically, with a good margin of drive ability.

In all amplifiers/systems, use as little gain as is required, and have as much headroom in each stage as possible. Almost all systems are the other way around.

In a poweramp, the driverstage should have several dB of headroom, so that the output stage clips first, not the whole amp at once. Even heavy clipping will be less intrusive this way.

My current preamp is evolving into this (whether it's the ultimate or not I doubt it is, but it is excellent). All stages are differential balanced.

- Phono1 (MC): VSE 2SK170/6922 cascode, passive RIAA, CCS loaded 6H30 pi-DR running at 30mA (or VSE RTP5 linestage - undecided) then to selector incorporating absolute phase selection (AP is on all main inputs)
- Phono2 (Decca): 6h30pi-DR cascode, passive RIAA, CCS loaded 12B4A then to selector
- Phono3 (Strain gauge): Win Labs current source, Lundahl 1517 balancing transformer, then selector
- CD: Balanced in to selector
- Spare balanced line level in for reel to reels
- AV inputs have own selector then via 1517 transformer to main selector.

No real linestage, and this is all in one box, except for the PSU
After main selector, then via S&B TX1022 transformer volume controls to poweramps.

PSU is using FW damper diode tube rectification, LCLC filtering, CCS and shunt regulator on all stages. Phono stages are to be shuntregged, then current sourced for the filaments. Filaments have their own transformers (EI core same as B+), as do the damper diodes in the B+.

I figure the more i ask the more i learn

<b><i>Highly</i></b> recommended approach.

Also read all of Lynn Olson's <a href="http://www.aloha-audio.com/library/index.html">Library</a> (best place to start), www.tubecad.com, Allen Wright's Tube Preamp Cookbook and Morgan Jones's Tube Amplifiers.

When you can find them, also get copies of the Radiotron Designer's Handbook v4, Valley and Wallman Tube amplifiers and an ARRL handbook from the 50's.

That should keep you busy for a while. The learning curve is steep, but it will make sense eventually. Or so I've been told.
Cheers

[FBLEAGH]

.

If your on a budget you better set your posts now so we know where to cut corners.

While i dont want to spend $2000 on a preamp, i am more than happy to spend a reasonable amount on the project.
As this sis supposed to be a learning exercise at the same time I am not planning on rushing this project. I'm not in any great hurry and will take my time building it

Again thanks for all your help guys, you are definately helping a great deal

PS. on the other hand i cant wait to hear my Kyuss and QOTSA disc's through my new preamp

[Original Burnedfingers]

I would just like to say hi and say that I have recently joined this merry group and have enjoyed all the posts that I've read. I have had a solid state background and have lately been bitten by the tube bug. I am a novice with tubes as all I can remember from childhood is they burnedfingers. The nickname burnedfingers. I am getting ready to gather parts for my first tube preamp and have come up with a thought about substituting a 5687 in place of the 12AX7 and retaining the 6SN7. Would this work and how would I have to modify the circuit to handle this change? I saw the circuit in one of the posts here. Any help or ideas would be appreciated. Thanks, Joe(burnedfingers)

[Brett]

Hi Joe, and welcome.

I'd strongly advise using the SRPP in preference to the two stage 12__7/6SN7. It's a better topology, and you don't need a fraction of the gain you'll get from using a 12ax7 as the driver, even accounting for the loss across the cathode follower. The 6SN7 is a much more linear tube than the 12__7 family too.
I think I have a 5687 SRPP design somewhere here too, and I'll try to find it and scan and post.

Power supplies have been discussed earlier in the thread, and I think Frank (fdegrove) is going to do more with the PSU design.

Cheers

[Original Burnedfingers]

Brett

Thanks for the quick reply. I am always interested in looking at more designs. I will look forward to seeing your design. Tubes no doubt will be a new challenge and I just wish I hadn't steered away from them so long. I could also use some help in the power supply design end. Oh, a copy of Valve Amplifier is on its way to me.

Regards,
Joe

[fdegrove]

Hi,


Just an example:

PATS' ULTIMATE PREAMP



Cheers,

[Brett]

Frank,

I've seen that before. Looks good, but I'd remove the cathode bypass (or at least try it without) and change that humungous output cap to about 2u2 at a guess.

The circuit I was going to post is similar, but is one of Allen Wright's and is in the TPCB, not on the website, so I'd like to get permission before I post it.

If I can work out how to get my pix scanned, I'll do a PSU for it too.

[fdegrove]

Hi Brett,

Yes,it unusual to use a small cathode bypass and then an enormous coupling cap to regain bass.
At least that's what it looks like.

I think he wanted a compromise between a bit extra gain and good transient response.

If I can work out how to get my pix scanned, I'll do a PSU for it too.

If you don't mind doing that?
My docs on PSU regs aren't here yet but it comprised a EL86 as series pass,a ECC83 as a two stage error amp and a 5661A/85A2
voltage ref.
It makes all the difference on an SRPP.
If I can find something similar I'll post it.
Have you used similar circuitry already?

BTW.I've got some more SRPP's with an 5687 if you want to see them here I'll squeeze'em in.

Cheers,

[planet10]

Originally posted by fdegrove
PATS' ULTIMATE PREAMP

More Nanaimo talent!

dhean and i are talkin' Diana Krall on the spawn of this thread.

dave

[fdegrove]

Hi Dave,

You can always count on this guy to fit the bill.

Cherio,