Balanced autoformer?

discussion of magnetic volume control design for both line and speaker levels.

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drdna

Balanced autoformer?

Post by drdna »

I don't know much about electronics design, but the autoformer design was intriguing to me and i'm thinking about trying to incorporate the design into a balanced active preamplifier. Would it be possible to create an autoformer that presented a fixed impedance to the source? What about the output impedance? Also, if a resitive network can be used to present a relatively stable impedance to the output, wouldn't this obviate any advantages the autoformer presents to the signal path versus a switched resistive ladder attenuator?
dave slagle
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Re: Balanced autoformer?

Post by dave slagle »

drdna wrote: Would it be possible to create an autoformer that presented a fixed impedance to the source?
not really, if that is a main priority, i would look for some daven H attenuators.
What about the output impedance?
it decreases with attenuation.
Also, if a resitive network can be used to present a relatively stable impedance to the output, wouldn't this obviate any advantages the autoformer presents to the signal path versus a switched resistive ladder attenuator?
not in my opinion. independent of what the autoformers do right and wrong, they just sound better to my ears.

may i ask why your goal is "stable impedance"? I would modify that to be appropriatly high and stable with setting.

assuming stability for each setting i want as high an impedance as possible presented to the source and as low an impedance as possible seen by the load and this is where i feel autoformers show their true benefits.



dave
drdna

Re: Balanced autoformer?

Post by drdna »

dave slagle wrote:may i ask why your goal is "stable impedance"? I would modify that to be appropriatly high and stable with setting.
Well, I am building this preamplifier. As you can see the arrangement of the RIAA circuit demands stability in the impedence.

Do you think it would work with an inductive autoformer system for the volume control? Or is there a way to modify the circuit to achieve that goal?

Thanks!
Adrian
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dave slagle
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Post by dave slagle »

I think that circuit need the stable 50K value to keep the RIAA accurate so my guess is an autoformer would not be the proper choice.

dave
shinebox
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Post by shinebox »

I wonder if it would work to replace the riaa with a series combo of paralleled LRs (ie the conjugate circuit) after the first coupling caps... These Ls could be tapped for volume control, but would need to be on identically wound separate cores (not coupled). Taps would go to second stage grids.

Still very very jetlagged so this could be bs, but see if you can picture it... Have no way of posting pics alas til next week...
Cheers

Ps: I think Dave is right about the 50k. You could put in a balanced avc there but it would need a ridiculously high inductance I think. I also need to think if you could split the riaa - utilise a balanced avc in the second stage grid circuit for one rolloff. Impedances involved may be tricky. Just throwing some ideas out...
drdna

high inductance

Post by drdna »

Thanks for the thought. Yes, I had thought about something along those lines. The key issue is that the inductances of the coils would be rather high values, so the device would be physically very large, and the impendance, capacitance, etc. of the system would begin to have a problematic effect. Also, the interaction with the fields created by the coils may create some issues with the rest of the circuit. Maybe. What do you think?

Adrian
PET-240
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Post by PET-240 »

Why not put the AVC after the second cascade gain stage off the plate to the SLCF? I know a cap is needed there, or after the SLCF where it will have a low imp out to drive the AVC?
that way a very good matched resistor can be used to the second gain stage for the RIAA accuracy, and the AVC can be used for every input to the preamp.....

Cheers,

Drew
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