a variac will have enough inductance me thinks, being a toroid...worth a try i believe...i mean how small can a variac get?
The buffer's needed because trafos - whether autotrafo or true trafo - work best being driven by a low source impedance. Also an inductive load draws more current at LF than a typical resistive load so the buffer relieves the source component of this loading.
the impedance of tvc or avc is only low at the lowest frequency, it rises as signal frequencies rises...so the thing needed enough mass at the lowest frequency contemplated, say 30hz...
likewise, the voltage levels will also be very low, in the hundred millivolts levels....so impedance may not be such a real problem...
likewise, the voltage levels will also be very low, in the hundred millivolts levels....so impedance may not be such a real problem...
The biggest weakness I can see is - it'll be linear law whereas we want log law for a volume control.
Hmmm... So that's it?
The Intact Audio AVCs I have "come with approximately 140hy's of inductance ,.. The core is 80% nickel..."
How would one figure how much current needs to be sunk into the inductive load at 20Hz?
I think I have that covered. The RIAA preamp has a mu-follower output using a DN2540 as the plate load. While that can't sink more than the 6mA it's set to, it does have low Zout.
Not too hard to calculate that as an inductor has jwL impedance. I make it around j18kohm impedance at 20Hz. 160uA peak at 2VRMS input voltage.
Thanks. I'm afraid I'm not familiar with that equation. What is j? What is w? (I know L is inductance.)
160uA? That seems trivial. Even a 12AX7 cathode follower could sink that current into the load, with a Zout of about 1k ohms.
An LM4562 opamp can sink maybe 3mA into the load, which seems like plenty in this situation. No?
160uA? That seems trivial. Even a 12AX7 cathode follower could sink that current into the load, with a Zout of about 1k ohms.
An LM4562 opamp can sink maybe 3mA into the load, which seems like plenty in this situation. No?
j is there to indicate that the current and voltage are 90 degrees out of phase with each other. In math its the square root of minus one. w is my approximation to the Greek letter 'omega' which is the symbol for angular frequency (2*PI*f).
Yes to your other questions - I build AVCs with less inductance than that nickel cored one you mentioned so they draw even more current. I just make sure I have enough current on tap and a low enough driving impedance in the driving circuit, I don't add an extra buffer.
Yes to your other questions - I build AVCs with less inductance than that nickel cored one you mentioned so they draw even more current. I just make sure I have enough current on tap and a low enough driving impedance in the driving circuit, I don't add an extra buffer.
This is the smallest variac I could find on Ali, $28. Its still large, shipping costs more than the trafo itself : https://www.aliexpress.com/item/4000530337392.html?spm=a2g0o.productlist.main.1.221a2933FJ6HfJ&algo_pvid=4c0887e4-0b35-426c-b22b-068360197c1f&algo_exp_id=4c0887e4-0b35-426c-b22b-068360197c1f-0&pdp_ext_f={"sku_id":"10000002698271328"}&pdp_npi=3@dis!USD!28.0!28.0!!!!!@211bf4c516793649752542991d0705!10000002698271328!sea!HK!2609685283&curPageLogUid=jA0UmIO4SsQD
I wonder if there's a substantial audible difference between a 400hz (military/aviation) variac, and a run of the mill 50/60Hz unit. I know you can use a 400hz unit on 60hz, if you derate it substantially, but we're talking full range audio (hopefully, haha). Fun stuff!
400Hz trafo will be a lot lighter than a 50/60Hz one for the same VA. Lower primary inductance probably too.
BTSB is a an op-amp buffer for SE/BAL buffer conversion. I have parts for my sources, but haven't put them together yet.
Salas Ultra FSP phono preamp with a MC cartridge.
AD1862 DAC with Patrick's xen IV (jfet based)
Sony DVD player for spinning discs. Sounds decent.
I might have the spice file Dave helped with. He actually gapped mine a little more so inductance is slightly less. The extra gap avoided saturation from the output of tube I was using.
Seiden switches are gorgeous. I built my own AVC balanced unit years ago and also upgraded them to Dave Slagle’s silver versions. Very nice sound. That being said, I sold it last year once I moved to an all digital setup and implemented Roon with HQ Player on a diy custom built server.
Best,
Anand.
Best,
Anand.