Balanced autoformer?
Balanced autoformer?
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?
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Re: Balanced autoformer?
not really, if that is a main priority, i would look for some daven H attenuators.drdna wrote: Would it be possible to create an autoformer that presented a fixed impedance to the source?
it decreases with attenuation.What about the output impedance?
not in my opinion. independent of what the autoformers do right and wrong, they just sound better to my ears.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?
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
Re: Balanced autoformer?
Well, I am building this preamplifier. As you can see the arrangement of the RIAA circuit demands stability in the impedence.dave slagle wrote:may i ask why your goal is "stable impedance"? I would modify that to be appropriatly high and stable with setting.
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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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...
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...
high inductance
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
Adrian
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
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