Tube parametric / graphic EQ

[joelap]

Hi, looking for some help here.  I've been looking to add a simple 3 band parametric EQ to a tube bass preamp or a multiband graphic eq. My initial idea of making a simple OpAmp based EQ isn't going to fly since the voltage coming out of the tube preamp is clipping the opamps I've been using. I figure if I'm going to end up going to a high voltage op amp I may as well figure out a tube substitute.  Besides, I'm confined to a 1U rack space and would rather not have to bring in another or a different transformer and add +-15V as another power supply to the amp.

The initial design idea for the opamp circuit was Buffer -> noninverting input, while the buffer would also feed a 10k pot, with one leg of the 10k into the non-inverting input and the wiper of the pot going to a series cap, 10kA pot for Q adjustment, opamp gyrator to simulate inductor, then another 1MA pot to sweep the frequency. Basically, it is this simple design here with minimal, if any modification: http://www.geofex.com/article_folders/eqs/parmet.gif

I've been thinking of way to implement this with tubes. Since it will be internal to the preamp, the buffer can be eliminated as it will more than likely be fed from a cathode follower I already have within the preamp itself. Now, I found an old Fender Studio 180W bass amp which uses a 12AU7 cathodyne Phase inverter with 50k pots between anode and cathode, with the wiper going to a CL network. Output is taken from the Anode (similar to the geofex circuit, in which it is an inverting difference amplifier) but the catch 22 is that the fender one seems to utilize 4-terminal potentiometers. See schematic here: http://www.albertkreuzer.com/pics/el...studiobass.gif

I would like to implement this into my preamp using a 12AX7 stage (have one half of a 12AX7 triode available) but without having to use any special 4-terminal potentiometers. Is there a need for such a potentiometer? Can a normal 3-terminal potentiometer be utilized, with a normal RLC resonant circuit off of the wiper?

I'm also concerned about the inductor portion of the RLC circuit. I know in Mesa Boogie graphic EQs they use actual inductors, would I be forced to do the same or use high voltage op amps for gyrators due to the high voltage of the tube output signals? Having a difficult time finding inductors for such a circuit, at least ones that aren't very expensive.

Also: the 25uf capacitor from the 22k Rk to the 100k Rgrid ... is this bootstrapping the circuit in some way for extra gain or higher Zin? I have not seen a cathodyne with this arrangement before.

I appreciate the help.

Joe

[PRR]

> the voltage coming out of the tube preamp is clipping the opamps

Reduce the voltage coming out of the tube stage with a resistor divider. Then boost it up afterward.

Level control is THE basic skill of the audio technician.

You posted a truncated link; StudioBass- http://www.albertkreuzer.com/pics/el/studiobass/schem/studiobass.gif

The center-tap on those pots "forces" center-rotation to be zero-effect. It is a frill.

And notice that the signal-levels through the EQ are nominal 900mV... this is well within what can be handled by 19-cent low-current chips like TL074 which could be fed from a 350V rail through a not-too-hot dropping resistor and Zener.

[joelap]

PRR thanks, I already have a voltage divider for an FX loop send which knocks down the voltage low enough to be handled by opamps but what I'm trying to avoid having to add +/- 15V regulated supplies.  I already have an extra tube stage and 300V B+ source off of a 115/230VAC transformer that I am using only half the current sourcing capabilities of.  I'm using 12.6VAC heaters off of a 12.6VAC transformer.  I'd have to switch to a less readily available transformer (16VAC or more to have at least 2.5VDC more than the 15VDC regulated output), add in three DC regulators (12.6 for heaters, +/-15 for graphic eq) or add a 3rd transformer (one for B+, one for heaters, one for +/- 15VDC).

Basically, while the opamp is certainly cheaper individually, its implications will require me to make substantial changes to the overall circuit topology I am seeking to avoid.  A voltage divider is not the the issue.

The TL074 will draw 2.5mA of current per opamp, but if we're talking about a 9-band graphic eq with opamp gyrators, we're looking at 10 opamp stages @ 2.5mA = 25mA.  I'm already drawing roughly 25mA out of a total of 50mA capacity from my Hammond 229B230 transformer, I'd rather not run it to the max and leave some room for error.  If the EQ stays 3-band parametric it can be done as that should only draw 10mA on top of the 20mA I am currently using, but its nice to have options.

[HotBluePlates]

I'm already drawing roughly 25mA out of a total of 50mA capacity from my Hammond 229B230 transformer, I'd rather not run it to the max and leave some room for error.

You have 100% margin right now. And your transformer will supply more current than 50mA, it will simply start sagging the voltage.

I've been looking to add a simple 3 band parametric EQ to a tube bass preamp or a multiband graphic eq.

Is this a preamp only? If you had output tubes, then there would likely be a bias supply tap on the power transformer which could be used to make a 30v-50v supply. Which is what Mesa did in their amps with the graphic EQ; they use 4 discrete transistors powered by 30-50v.

[joelap]

Thanks HBP.  Nope, preamp only.  I've accepted the fact I will have to have a regulated DC supply and be forced to put together a circuit board of some sort to accommodate it.  This idea was put to rest.