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Re: Question Re: C and R value/ratio on line level crossover filters

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In Reply to: Question Re: C and R value/ratio on line level crossover filters posted by Ian L on November 28, 2005 at 18:02:10:

Yes, of course. You just have to know what to optimize. Easier said than done, of course - that's where engineering comes in! :^)
If the minimum impedance (usually the resistor value is a good estimate) is too low, whatever is upstream will have a hard time driving it. If the impedance is too high, stray capacitances will modify the tuning in unplanned ways, and RFI pickup is enhanced. So, all you have to do is find out the perceived distortion of the upstream component as a function of loading, the RFI susceptibility of the circuit, and the stray capacitances, then weight those variables "appropriately" and you can calculate the optimum.
I like to stay around 100k with most tubes, but if you are using high-impedance tubes like the 12AX7 you may want to go higher - 200 or 300k. Don't forget to worry about multiple filters in parallel, which will reduce the impedance sen by the upstream stage!

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Re: Question Re: choice of tube for line level crossover filters - Ian L 16:02:23 11/29/05 (2)
Thanks Paul!
in the above post you mentioned "but if you are using high-impedance tubes like the 12AX7. . . ."
This leads to one of my other questions about this, which is, would it not be better to use a lower Rp , Higher Gm tube with a constant current source in the cathode for a circuit like this? (I think it will be relatively easy to configure the Marchand board for other tubes.)
The second question I have is a more general one about the choice of topology. IIRC, in searching on the web for ideas, most of the tube circuits I found were using cathode followers and one of the stated reasons for this was low noise (next to the benefits of low impedance). Curious about this I started looking at RIAA circuits which I presume would have a greater need for low noise design, and found that most of them use plate loaded stages. . . . What's the question? I guess it would be something like , if you were going to design a not for profit line level crossover for yourself what would your leanings be and why?
Thanks again !!!
- Re: Question Re: choice of tube for line level crossover filters - Paul Joppa 17:07:29 11/29/05 (1)
  Most use cathode followers because the Sallen & Key circuits commonly used calls for a unity gain active circuit. Actually, there are several popular circuits that call for a fixed gain, usually in the range of 1 to 3 - which is easy with opamps but kind of clumsy with tubes.
  I don't see noise as an issue at line level, so I don't know - and don't really care - what some marketing department is talking about... :^)
  For a cathode follower, gm determines the output impedance; a lower rp for a given gm will let you run more current so it can drive a lower load impedance comfortably. So yes, a tube with more "beef" would be preferable. That's what we did to the studio's Marchand. Watch out for too much transconductance though - cathode followers are (the simplest kind of) feedback amplifiers, and they can go unstable at very high frequencies if the layout, lead dress, and phase of the moon are against you.
  For profit or not, I would use a cathode follower. That's what we have been doing with the Climax development. Naturally I would load it with a C4S and shunt reg the power supply. Once we get the FP-III C4S/shunt reg kit going, I'll probably make up a Climax crossover on a FP chassis - it's kind of a natural platform for a modest 2-way.
  - Thanks very much Paul - Ian L 23:52:43 12/01/05 (0)
    I started work on the boards, deciding to start my active crossover education by following the recipe as given and then taking it from there to see how the changes in circuit affect things. Though my very first introduction to all this was with the Foreplay I wan't able to retain much of what I read about cathode followers - so this will be a chance to remedy that.
    Your friendly jibe about the engineering wasn't lost on me. I have never had a good foundation in math, something I never cared about until gettting into this. I hope to get started soon on a good looking book I picked up - Basic Mathematics for Electronics by Cooke . . . .we'll see how it goes. `,;^)
    Will post again with results (and likely more questions ) when part one is up and running.
    Thanks again for your great help! Good orientation is way more than half the Battle !!!

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