Inductor EQ

[eggman6]

Planning on building the mesa GEQ as a standalone unit, and i have a few questions. I'd imagine the signal will have to be boosted to be used before an amp requiring some extra stages, although this should work in the FX loop.

Now onto the questions, most of you will probably know how mesa like to add errors to their schematics to prevent people from building them, does there appear to be anything significantly wrong with this schem?

http://www.tubefreak.com/mk4-5.gif

If you do the calculations the cap inductor combinations don't quite match up with the written frequencies but they are close enough, and nothing some tweaking cant change if need be.

One of the other problems is powering this, as it comes off the bias supply. This is not a problem if you are wiring it to a amp, but otherwise is not as straight forward, i have a small transformer lying about, is it possible to take a reversed diode off one of the taps as done with the bias supply without the secondaries being connected to anything else? Or is there perhaps a better way altogether?

I'm planning on winding a pultec type inductor and tapping it at the appropriate inductance, but wondered if this circuit would require separate inductors as in the original.

I'd apreciate any infomation/advice.

[ClinchFX]

I have a soft spot for inductor-capacitor equalisers.  They sound good.  If you want to use all 5 bands at once, you'll need separate inductors.  If you have a tapped inductor like the Pultec, you'll be able to use only one band at a time.

To get a -30V supply from a transformer, you would need a 21VRMS secondary winding.  Connect the secondary to the AC terminals of a bridge rectifier and connect the + output of the bridge to ground.  The + terminal or lead of the filter capacitor is connected to ground and the - terminal connects to the - output of the bridge.

To be honest, I'd use NPN transistors and a positive supply.  The MPSA63 is a Darlington, but the others are nothing special.

Peter.

[Rob Strand]

The main problem I see is you don't really know the DC resistance of the inductors - I've never seen the resistance quote.  It may be possible to back-engineer the resistance from the physical size of the inductor but I don't know that either.

If you look at the schematic the larger inductors have smaller series resistances.  That to me means the DC resistance may be significant.   The effective DC resistance is the sum of the inductor and the series resistor and my guess is that would be around 1.1kohm

To wind you own coils, or buy them you should mimic the *total* DC resistance.  If you make your inductor out of transistors you would need to make the gyrators have effective series resistances of 1.1k (or whatever) and remove the "added" series resistances in the circuit.

Regard a supply it would be possible to switch the NPNs and PNP and the electrolytic caps to come-up with a circuit which used a positive supply.

A further mod would be to tweak the circuit to run from 9V.

[eggman6]

I dont know the impact different coil resistances have, but it should still function. As long as i get a working version i will have something to experiment with. Can anyone recomend npn transistors?

[william]

All three types of transistors are available from mouse I believe.

[chicago_mike]

All in RoHS are available at mouser yes.

[eggman6]

They are indeed on mouser, but i live in the uk, not worth it for a small order but i don't know of alternatives. Can get them off ebay though. I'll probably just build it as is and work on it from there. I did own a studio preamp some years back, no bloody idea why i sold it, an act of crazyness. I got it for £250 and now they sell for near £500.

Update:

http://www.datasheets.org.uk/

This site seems pretty good for cross referencing parts. Looks like i can use a 2N3904 and BC213  instead of the MPSA20 and 70, and radio spares stock the MPSA63 anyway, so looks like i'm set.

[ClinchFX]

I dont know the impact different coil resistances have, but it should still function. As long as i get a working version i will have something to experiment with. Can anyone recomend npn transistors?

DC resistance will affect the Q of the inductors.  Q of each inductor will determine the width and maximum boost or cut of each band.  It certainly seems that the different value series resistors are an attempt to make the coil resistances appear equal to the rest of the circuit.

I hope this makes sense.

Peter.

[eggman6]

Makes perfect sense, although i bet the q values still vary a fair bit between each section. Could be worth putting trim pots in place of those reistors so i can get them spot on. But then i doubt the difference between the .068 and .22 is 500 ohm so there must be some intention of changing the cut and boost range.

[BrianJ]

There is an amp company called Acoustic that uses an identical graphic EQ (except it is PNP with no part numbers) in a couple of their models.  The tube amp book vol 4 has the schematic and it looks identical to the Mesas.  I wonder what the relationship is (read: who copied who).  There is also a tube version on another model that I wanted to build but never got around to...

I'd be interested in a source for the inductors if you have any breakthroughs.

[eggman6]

Albert king used Acoustic amps.




Anyway I've just bought a studio preamp should have it during the week, so this will give me all the information i need. I'm going to be winding my own inductors, so i can choose the appropriate cores based on the size of the originals. And if the coil resistance differers, the difference will be consistent between the inductors since they use the same size cores so any compensation for q should be pretty straightforward.

Here's an acoustic schematic with an EQ:

http://web.archive.org/web/20060211145902/acoustic360.homeunix.net/images/schematics/Acoustic320_330.pdf