Equalizer

[txstrat]

A friend asked me to build a one channel blackface amp for him and add an equalizer to the circuit.
At time I plan to add a boogie like EQ and think about the -30v supply for it.
I have a PT with a sec. 14vac winding I could get up to 28v with a voltage doubler circuit (to get to the -30v I would use a LM317).
Or I could get the -30v off the 50vac winding for the bias, using a voltage divider circuit.
What would be the best (and easiest) way to get the -30v.
Would anybody recommend a different way or maybe even a different kind of EQ?

[Robert M. Martinelli]

Hi Matt,
I took a look at the Mesa schematics I have, and found that this one ( caliber ) calls for a lower supply ( -13VDC, see attachment ). The first thing I would do would be to change the circuit's polarity using low noise NPN BJTs in place of PNPs ( and vice-versa ), and changing the other polarized components' polarity accordingly ( or, better yet, since the EQ section capacitors' values are not that big, use polyester or polypropylene ones ). This way you can use the 14VAC tap "as is", together with a 7812 ( adding a diode or two in series with the GND leg to raise the voltage to 12.6 or 13.2 V) or a LM317 if you wish to precisely adjust the voltage to 13 V to match the schematic.



Hope this helps

Best regards

Bob

[txstrat]

I've had a look on both schematics and found the only difference is the dropping resistor after the collector of Q1 (22K compared to 3.3K). Seems the voltage isn't that important, is it?
I could use a 7912 to get -12v. The diodes on the middle leg would be upside down then, right? Is there any other advantage to use NPN?

[Robert M. Martinelli]

Mainly the wider choice they allow...

My suggestions ( in no particular order ) BC109C, 2SC1815, 2N2484,2N4104,2N5210. Recently I have built a couple of wahs using the first two types, and they both sound good, with very low noise.( the 2SC1815 NF having a slight edge over the BC109C IMHO ).

Also, by using a 7912 you need to insulate the metal tab from GND, as the pinout is GND/IN/OUT ( from left to right ), with the input internally connected to the metal tab. ( to be honest, though, you should do the same with the "7812+elevating diodes" arrangement, while, by using a 7812 in its standard configuration, the tab is at GND potential ).

Hope this helps

Best regards

Bob

[Enzo]

If I were copying the Mesa circuit, why would I bother converting the polarity? I would assume the whole reason to make it run off a negative rail in the first place was to make use of existing voltage supply. In this case the bias supply.

I am also unclear on the ppower transformer. if your Fender transformer has the 14v winding, then do what you like. But if you are thinking of adding a second power transformer just for this, I vote no. Use your bias winding.

With the bias winding, you can tap off the bias raw voltage itself, or for that matter, add another rectifier and make a + supply if you prefer.

[Robert M. Martinelli]

Hi Matt, Hi Enzo!

My suggestion was related to the supposed use of the "additional" 14VAC winding, Enzo's a good point, because it keeps things straight and simple. It is of course possible to use the bias voltage and scale it down, using a suitable voltage divider but, if this is the case, I would advise Matt not to use a voltage regulator/stabilizer.

The rationale for this is that the bias supply, as we know, being "dedicated" to supply grids with voltage ( thus having very little current capability ) shouldn't be "loaded", while AFAICR, a 78xx/79xx needs at least to be loaded with some 30-40 mAmps to be able to properly stabilize the voltage.

JM2CW

Cheers

Bob

[olddawg]

The EQ will draw very little current. Build it on a separate board with its own regulated power supply. The cost of a small transformer is negligible and taking a modular approach has its advantages. Why doesn't he just put an EQ in an effects loop to begin with?

[teemuk]

Why not power from a separate supply. Use bipolar +-15V and a simple OpAmp instead of an OpAmp built from discrete transistors.

[Nate]

Why not rip out the inductors and use gyrators instead? Simple, Easy Parametric and Graphic EQ's, Plus Peaks and Notches

[Enzo]

There are a million options. I think the OP picked the transistor circuit because he could lift it straight from the Mesa page and drop it into his amp. Doesn;t have to redesign anything. And it runs off an easily stolen power source.