Inductor based mid boost via cathode

[Gtr_tech]

Anyone played with this idea? The only commercial example I know of is the second revision Sunn Model T. I had one of the early ones, but not the "red" version, so I don't know what it sounds like. Or if there's problems with hum pickup.

Here's the Sunn implementation:
http://www.angelfire.com/blog/sunnmo...delt2schem.gif

Now my Dean Markley Signature amp has an op amp based active mid that works really well, but they derive a dual rail supply via the bias tap and I don't like that. Makes me nervous that the bias could get dragged down in the event of op amp or zener failure.

[Steve Conner]

I did it, using a bunch of op-amps to simulate the inductor.

If bias failure worries you, fuse the connection from the bias tap to the op-amp power supply. Something like a 63 or 100mA fuse. That's what I did. It's been 12 years and the fuse hasn't gone yet.

[g1]

Ampeg V4 ?

[Gtr_tech]

From my experience the V2-4 is quite prone to hum pickup.

[Chuck H]

Yup. Neat circuit. Hummy. I've wanted to use an inductor for the same type of circuit but have avoided it for this reason. I may still try it with careful layout. But I won't be surprised if it hums anyway. Shielding the inductor is also a possibility. But if the shield, which should probably be copper or steel, is too close to the inductor you'll change the semiconductive properties. So you'd need a little real estate to make it work.

[Tage]

I once built the a similar control into and orange amp clone. although I now understand why it hummed so bad... It should ahve made sense then that the inductor might put up a lot of noise

http://www.orangefieldguide.com/OFG_...0_72output.jpg


I believe this is pretty simelar to the control in those Model T's

Does anyone have a formula for calculating this kind of controls response?

[mooreamps]

Does anyone have a formula for calculating this kind of controls response?

Ya well, simple enough... It's resonant frequency equals 5.1911e+3 Hertz.
Normally I'd punch it out on my HP 11C, but I found this link for others to refer to.

http://www.1728.org/resfreq.htm



I would need the DCR of the coil to calculate the circuit Q, to determine the circuits 3db bandwidth..


Also, as a side note, the inductor could be shielded or wound on a toroid to help abate any induced hum issues...
-g

[g1]

Ya well, simple enough... It's resonant frequency equals 5.1911e+3 Hertz.
Normally I'd punch it out on my HP 11C, but I found this link for others to refer to.

[url]http://www.1728.org/resfreq.htm

Seeing the 5 and hertz threw me off, but I see now how they are showing power of 3, so 5.1911e+3 Hertz is 5.191Khz.
The part I don't get is where they say "the formula also holds true if used with the units of microhenries and microfarads."
How can 2000x.47 used within the formula give the same result as .002x.00000047 ?
I think they omitted that the solution will then be in Mhz, not hz.

[mooreamps]

Seeing the 5 and hertz threw me off, but I see now how they are showing power of 3, so 5.1911e+3 Hertz is 5.191Khz.
The part I don't get is where they say "the formula also holds true if used with the units of microhenries and microfarads."
How can 2000x.47 used within the formula give the same result as .002x.00000047 ?
I think they omitted that the solution will then be in Mhz, not hz.

Well, then work the solution yourself on a calculator, even using the scientific calculator provided to you from Windows.. You should see the same result..

[g1]

Forget it, I notified them and they have corrected the wording on the website calculator.

[mooreamps]

I'm really starting to like this circuit... I've cobbled up different values than the one's shown here. It can be switchable between either a mid-boost or a mid-scoop.
I'll also make the pass-band selectable between two different ranges.... "Sweet"....

-g

[balijukka]

if you dc-lift bias the triode and use a larger cathode resistor (I've been using 10k..33k) you can make the boost appear even more big.
This has an added effect that the inductor can be smaller. I made the inductors by winding around an inch length of an plastic felt tip pen tube about 3mm thick layer of wire. Then glued a 6mm nut on one end.
Inserted a 6x40 mm screw and a locking nut, drilled a 6mm hole to chassis, and got myself a tweakable inductor.
Works like a charm and does not pick any hum.

[Chuck H]

Wait... The inductor value can be smaller for a higher impedance circuit?!? Isn't that backwards?

[Tage]

Wont the impedance only affect the Q of the control? not the frequency it is centered at? I wouldn't think the impedance would affect the equation presented earlier by mooreamps

[Chuck H]

I was a little confused when I looked at that calculator and found no impedance parameter. In other SPICE type calculators impedance does make a difference. But I'm formula impaired so I could be missing the point of something here.