Bench LC EQ

[hylaphone]

Hi folks --
Anyone tried this LC tonestack?
L-C Based Tone Stack

I've been toying with it in the Duncan program, and it looks neat. I would like to try my hand at a 4 band version.

The transformers Bench used are no longer available, how would one calculate the inductance of a comparable audio tranny?

[mhuss]

20H is a *lot* of windings on an iron or ferrite core, this will be something the size of a small transformer. The Hammond 194E is 30H, but just to give you an idea: https://www.hammfg.com/part/194E?referer=1024

[jazbo8]

This might be of interest, there are many to choose from...

[Shadrock2]

Generally the use of inductors in a high impedance tone stack are quite susceptible to picking up hum.
I have considered doing a tone stack like this also and I have a couple of what I believe to be old telephone system transformers that are fully enclosed and shielded. I would think that you should consider inductors that are fully enclosed.
Ampeg used a toroid inductor in the tone stack of its V-4 amp I believe. A toroid might be less susceptible to stray magnetic fields.

[mhuss]

You're thinking of the SVT, which uses an unshielded toroidal inductor - this is mounted only inches/cm away from the massive power transformer! (What were they thinking?) It does induce a bit of hum.

I've wondered from time to time about using a gyrator in place of the inductor.

[Enzo]

Ever look inside a graphic EQ? See any inductors lately?

[hylaphone]

Jazbo8 - thanks for the great link, that opens up some possibilities.
I am leaning towards keeping a simple Bass/Treble stack (James?), then adding a CF driven split mid control with inductors.
Would like to keep it simpler than the SVT.
Looking at this schem:

L values appear scaled back - except for the 80hZ, they are all under 1H
Might that be an easier way to go? Take two bands and adjust to taste?
Mouser has some shielded inductors that might work in series.. or could look into the tapped ones made for Pultecs and whatnot

[Leo_Gnardo]

You're thinking of the SVT, which uses an unshielded toroidal inductor - this is mounted only inches/cm away from the massive power transformer! (What were they thinking?) It does induce a bit of hum.

I've been able to - sometimes - minimize that hum simply by loosening the toroid's mounting bolt and twisting it until hum reduces then re-tightening the bolt. In another case, with one of the more modern 2-U rack mount SVT variations, put extension leads on the inductor and remounted it in a "quiet" corner of the chassis, dropped the hum to dam' near nothing. Sometimes it pays to think outside the box.

- - - - - - - - -

Modern graphics, yes I'm sure they're all gyrator. An oldie-but-goodie, one of my favorites, the half-octave graphic SAE 2700 from mid-70's is jam packed with 'em, and no hum/buzz to speak of.

[J M Fahey]

Inductors were the only game in town in the 30's to 50's , that's why Pultecs and Altec equalizers and many others used them.
Latest significative users were Acoustic and Mesa Boogie but that was in the 70's .

Today they are an expensive bulky hummy alternative, long since replaced by much better performing active electronics.

Just sayin'

By the same token, hearing aid tubes have been obsolete for a few decades now

And in fact one of the first applications where early crude Germanium transistors fully displaced tubes was in hearing aids, go figure.

[hylaphone]

Inductors were the only game in town in the 30's to 50's , that's why Pultecs and Altec equalizers and many others used them.
Latest significative users were Acoustic and Mesa Boogie but that was in the 70's .

Today they are an expensive bulky hummy alternative, long since replaced by much better performing active electronics.

Just sayin'

Same could be said for vacuum tubes..

I've been encouraged by several bass players to try the inductor approach, so I'm taking the opportunity to learn

[g1]

I am leaning towards keeping a simple Bass/Treble stack (James?), then adding a CF driven split mid control with inductors.
Would like to keep it simpler than the SVT.

As you mentioned earlier, the V4 & V4B did also use a toroidal inductor, a bit simpler than the SVT as there are no sliders.
I'ts mid boost/cut 3 way switchable at 300Hz,1K, or 3Khz.

[mhuss]

Ever look inside a graphic EQ? See any inductors lately?

Of course. What I should have said is "a tube gyrator" ... they're easy to do with op-amps.

Aside: I have a old Fender PS300 that has discrete inductors for each of the five frequency bands.
Fender 300 PS

[hylaphone]

Ok, I'm confused by that 300PS/Studiobass EQ -- are the 50k pots blending out of phase signals from V2 anode and cathode?

[J M Fahey]

Without opening the schematic, usual action is to have pot wiper grounded through the LC resonant circuit, and ends connected respectively to cathode and anode.

So when you ground cathode (through LC) you increase gain=Boost and when you ground plate=cut .

Circuit values are chosen so action is symmetrical.

[hylaphone]

Perfect sense, thanks JM!