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Old 8th November 2003, 03:09 AM   #1
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Default Noob Toob Question

Just wondering, I am considering building a tube preamp kit, but I want to use it with monoblock amps and need to have a volume control, is it as simple as wiring in a pot on the input or output?(what value?) Also I want to mount the tubes (12ax7) on the top plate of the chassis, off the PCB, can I just use flying leads onto a socket for the valves with no ill effects?
TIA Beau.
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Old 8th November 2003, 09:34 AM   #2
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Default Re: Noob Toob Question

Quote:
Originally posted by Beau
Just wondering, I am considering building a tube preamp kit, but I want to use it with monoblock amps and need to have a volume control, is it as simple as wiring in a pot on the input or output?(what value?)
If you're going to drive more than a couple of inches of cable, put it at the input.
But I have to ask, do you really need a linestage? Almost all systems I've ever seen have way too much gain and all you'll end up doing is attenuating it away adding noise and lowering resolution.

Quote:
Also I want to mount the tubes (12ax7) on the top plate of the chassis, off the PCB, can I just use flying leads onto a socket for the valves with no ill effects?
TIA Beau.
12AX7's are probably the worst choice for a tube for a linestage; too much gain and way too high an output Z to be useful, unless it uses CF's, in which case it will just sound lousy. A single stage 6BX7/12B4A/6H30 resistively loaded would make a much better line stage (speaking from long experience) and give a more useful gain structure.

Why are you using a PCB? is it part of a kit/project?
The tubes can be mounted to the top plate and flying leads used if required, but why not simply buy some PCB mount sockets and solder them directly to the board?

If the design you're using includes a phono stage, I'd need to see the design before I comment further.
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Old 8th November 2003, 12:38 PM   #3
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Brett: A line stage can still be a useful thing for those of us with sources that vary greatly in output (like tube tuners with 500mV outputs). But I'm with you for the vast majority of other times; line stages are a relic. Buffering is nice, though.

Your comments on the 12AX7 as a line amp are true IF you're not going to use feedback. But if you are, the 12AX7 can work quite well, its high gain being an asset.
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Old 8th November 2003, 01:25 PM   #4
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Quote:
Originally posted by SY
Brett: A line stage can still be a useful thing for those of us with sources that vary greatly in output (like tube tuners with 500mV outputs). But I'm with you for the vast majority of other times; line stages are a relic. Buffering is nice, though.
My system has 9 different sources plugged into it, and almost never do I have it turned up within 3dB of full volume (on the vol control)

Quote:
Your comments on the 12AX7 as a line amp are true IF you're not going to use feedback. But if you are, the 12AX7 can work quite well, its high gain being an asset.
Take a turd and smother it in peanut butter, and it's still a turd.
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Old 8th November 2003, 01:48 PM   #5
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Quote:
Take a turd and smother it in peanut butter, and it's still a turd.
Brett, i seldom disagree with you but to liken NFB to peanut butter is a bit extreme
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Old 8th November 2003, 02:10 PM   #6
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it's really a bit strange though...why would you want a 12AX7 as a linestage?? a guitar distortion circuit would more likely fit the bill...
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Old 8th November 2003, 04:02 PM   #7
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There are a lot of classic preamps with 12AX7 line stages and feedback. Whether they fit your personal creed is not relevant- that they work well for many, many people is attested to by the longevity and continuing high regard for Marantz 7, Dyna PAS and PAM, Audio Research SP-3 and SP-6 series...

I don't see much resemblence between these classic circuits and guitar distortion boxes, but I'm not always in search of novelty for its own sake. Boring old circuits are often that way just because they work so well and without much fuss.
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Old 8th November 2003, 11:17 PM   #8
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hmmm...you got a point there SY...but I wouldn't consider the 12AX7 and "audiophile" tube...anyways...most of the time I see them on guitar amps only...why is that so??
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Old 8th November 2003, 11:43 PM   #9
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Hi,

Quote:
There are a lot of classic preamps with 12AX7 line stages and feedback. Whether they fit your personal creed is not relevant- that they work well for many, many people is attested to by the longevity and continuing high regard for Marantz 7, Dyna PAS and PAM, Audio Research SP-3 and SP-6 series...
Mediocrity here we come....goodbye excellence.

Cheers,
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Old 9th November 2003, 12:07 AM   #10
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Default Polished peanut butter

Quote:
Originally posted by Brett
Take a turd and smother it in peanut butter, and it's still a turd.
Yuck! Can't stand peanut butter. Over here, we attempt polishing. That doesn't work either.

In a DIY system, the expensive degradation caused by a pre-amplifier is unnecessary. The only excuse is to allow power amplifiers to be near loudspeakers. As you have monoblocks, you could put them near the loudspeakers and reduce the length of (thick, expensive) loudspeaker cable, but at the expense of needing a line stage that can drive the capacitance of a long cable. 12AX7 can't really swing enough current to do this properly. You need something a bit beefier, configured as a cathode follower - ECC88, 6SN7 are possibilities, but one of the Russian muscle triodes such as 6S45 could be even better.
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