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Old 20th March 2003, 05:00 PM   #1
CV is offline CV
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Default Using TVC as a filter?

I'm planning some amps for a multi-way system with crossovers built in (via interstage coupling constants)

With the S&B TVC, we have a primary inductance of about 400H.

If I use a 5nF blocking cap and parallel a 200k res with the primary, then that gives me a 2nd order Butterworth rolloff at 110Hz which is where I want to crossover.

Elsewhere in the mid/treble amp, I could have an LC coupled stage, similar to the above. Voila, 4th order Linkwitz-Reilly (cascade of 2 2nd order Butterworths) with no additional circuitry (sort of).

I'd then need another attenuator for the bass with the same steps (could go for cheap resistive atten. as less critical).

This approach would keep LF from the TVC which is another benefit.

But my concern is that the 5nF cap, having such a high impedance, might make things sound weedy. Obviously I'd need a high i/p impedance valve right by the output of the TVC...

Anyone got any thoughts as to how well this approach might work?

Cheers
cv
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Old 24th March 2003, 11:18 PM   #2
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Default Re: Using TVC as a filter?

Quote:
Originally posted by CV
Anyone got any thoughts as to how well this approach might work?
Not a clue I'm afraid, old boy.

However I thought that someone ought to respond....

7N7
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Old 25th March 2003, 01:12 AM   #3
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Default MORALLY OBLIGED TO RESPOND...

Hi,

Quote:
However I thought that someone ought to respond....
I know the feeling...

A hell of an expensive way to deal with filters I'd say.

If you ask me I'd use the things for what they were designed for if at all...

Cheers,
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Old 25th March 2003, 01:56 AM   #4
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I thought I read somewhere that the resistance(DCR?) of the TVC changes according to setting. That's why some recommend not using one after a split riaa network. If this is true, wouldn't you be creating a variable x-over?
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Old 25th March 2003, 08:54 AM   #5
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The DCR of primary cannot possibly change irrespective of secondary setting.
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Old 25th March 2003, 10:33 AM   #6
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Sorry, maybe I used the wrong term. I think it was Allen Wright who said the S&B's input impedance changes with attenuation setting, therefore making them unusable when placed directly after his phono section(needs to see a consistent 50K(?) load).
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Old 25th March 2003, 10:43 AM   #7
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Allo all,

Thanks to all who replied.

Well, it's a tricky one - I wasn't expecting too many replies as it's something that I've never seen suggested before; it's a bit at the fringe...

Frank - you're right, it's an expensive way of building a filter, but if the TVCs are already bought and paid for...

The main issues I can see are:

1) Following stage must have very high impedance
2) The inductance of the TVC is a function of frequency and level.

As regards (2), that's what air-gaps are for. Dunno how much of a gap S&B use though...

Now if we are filtering the audio signal -12db/octave at 120Hz, our ac flux on the core is much lower, which should reduce (already low) distortion, but I was thinking that this might give us some headroom to apply a tiny dc bias current to the primary (say a few microamps) which I was thinking might help linearise the inductance. I'm guessing the B-H curves are unlinear around zero crossing, so we'd get away from that. Much like gapped SE vs ungapped PP transformers.

Wish I had the test gear to check this out...

And don't forget, in addition to getting a filter for "free" it means we can use a lower value, higher quality cap.

Cheers
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Old 25th March 2003, 11:13 AM   #8
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Seems like there is no airgap. Which is of course a pity. Do you really need 400H? 40 would do me fine and i bet it would sound better too.


cheers

peter
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Old 25th March 2003, 12:11 PM   #9
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Quote:
Originally posted by analog_sa
Seems like there is no airgap. Which is of course a pity. Do you really need 400H? 40 would do me fine and i bet it would sound better too.


cheers

peter

Hi Peter,
I agree with you there; the TVC does have 2 primary windings which can be wired in parallel to give a quarter of the inductance - but it would be nicer to achieve 100H with a bigger air-gap...

The TVCs are on loan to a friend at the moment; when I get them back I'll see if Ican make some crude measurements of L vs level and frequency.

cheers
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Old 25th March 2003, 01:40 PM   #10
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hey chris... thanks for the pointer

Quote:
but I was thinking that this might give us some headroom to apply a tiny dc bias current to the primary (say a few microamps) which I was thinking might help linearise the inductance. I'm guessing the B-H curves are unlinear around zero crossing, so we'd get away from that. Much like gapped SE vs ungapped PP transformers.

its interesting to note that looking at the curves in the old texts for perm vs frequency for various values of H (dc offset) that small amounts of H while reducing the perm, do make the perm much more linear, which seems to be a very similar behavior to what an airgap does.

dave
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