EQ Circuit Idea

[raulduke]

I love the eq on my faultline build with guitar and bass.

Therefore I would like to build a little eq pedal with the tonestack and without all the jfet stages. This would be really cool to use with my bass as it could be shrunk down to a size that would fit in the body cavity of a bass.

My thoughts were:

Modified AMZ Mosfet booster acting as an input buffer -> Faultline Tonestack -> AMZ Mosfet booster as output buffer and volume boost if needed.

Just three controls; treble, bass and gain/volume for the last mosfet boost stage.

This would be for personal use so I don't see any problems with using the Mosfet Booster circuit.

I could even use a SHO circuit if needs be.

Does this sound like a reasonable idea or are there better ways of doing it (Op-Amps etc.)?

Your opinions would be really appreciated!

[dwstanford]

That would be really cool. I would build one of those also.

[madbean]

The way I would do it is a JFET input buffer, TB stack and then a bi-polar output stage with volume control. That gets you the boost and requires far fewer parts than the mosfet boost. It would be a very easy design to do.

For something a little more complicated, the RunOffGroove Tonemender has a bit more flexibility.

[raulduke]

Thanks for the advice Madbean much appreciated!

So could I just take a JFET buffer (AMZ Probably) -> Baxandall -> LPB1 Circuit, and then just tweak everything to taste?

DW: Thanks mate; I'll post when I have got an idea/schematic down for this.

Much appreciated guys.

I'll have a look at the ROG Tonemender as well, but I can of fancy having a go at creating something by bolting other snippets together!

[raulduke]

Here we go (see attached)  .

If any of you guys could let me know if I am on the right track I would really appreciate it!

As Madbean suggested the circuit is:

JFet Buffer -> Baxandall Stack -> Bipolar Output Stage (LPB1 at the moment).

If this looks ok I will probably do a layout in Eagle and add it to my next PCB order with Laen.


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[shawnee]

For something a little more complicated, the RunOffGroove Tonemender has a bit more flexibility.

+1 I am going to build one of these very soon.
This may be something very useful for you raulduke. I have a couple of amps with just volume and tone so I thought that a simple EQ of some sort would be great. I like the way you think! I think the Tonemender would be your best bet but if you are looking for the challenge of building your own I can totaly understand that.

[raulduke]

Thanks for the advice Shawnee but yeah I fancy having a go at building something.

It may be a stupid idea but I would like to see how it pans out.

Plus I don't really like Fender/Marshall TMB EQ's. I prefer the sound of the Baxandall on my Faultline.

[oldhousescott]

I think you will want to modify the values of your Bax stack. Attached are plots of the response curves with the original values and then modified values. I made some assumptions about the input and output impedances around the stack based on the circuit.

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[jimmybjj]

Might be some interesting reading for you.

http://circuitworkshop.com/forum/index.php?topic=870.0

[madbean]

Here we go (see attached)  .

If any of you guys could let me know if I am on the right track I would really appreciate it!

As Madbean suggested the circuit is:

JFet Buffer -> Baxandall Stack -> Bipolar Output Stage (LPB1 at the moment).

If this looks ok I will probably do a layout in Eagle and add it to my next PCB order with Laen.

In this case, you can drop C1, R2 and R3. You don't need to provide a bias voltage to the gate. And, without a bias voltage you don't need C1 to decouple the input. Have a look at the Quadrovibe input which is very simple.

[raulduke]

Oldhousescott; Thanks for the heads up but I have just stuck in the tonestack values from the Faultline schematic.

Thought it would be a good place to start and I can always tweek from there.

I want the sound of the Faultline tonestack though, not a generic baxandall response (ie. flat mids etc.). If you simulate the Faultline tonestack in the calculator you will see it is not a perfect baxandall response either.

I may have got the input/output impedances wrong though so I will check.


Jimmybj: That thread looks interesting mate. I will have a read through.

Thanks for the help guys

[raulduke]

I must have posted as Madbean was posting at the same time; doh!

Thanks for the info Madbean; I will alter the Schematic. Then it is just a case of tweeking the Baxandall values I guess.

Again, thanks to everyone for their help and input.

[raulduke]

Updated Schematic incorporating Madbeans suggestions (see attachment).

Madbean (or anyone else); could you explain why I can get rid of the Bias resistors on the input stage?

Hopefully I can try and understand a bit more about this stuff while going about trying to bolt it all together ha ha!



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[jimmybjj]

Bump...as I would like to know as well.

[madbean]

There are two factors, I think. 1) JFETs have much lower current leakage from their gates than a bi-polar does from its base and 2) the gate has a lower turn on voltage than the bipolar, hence the bias voltage is not needed to get the transistor to operate. Without the bias voltage, and because of the low leakage, a decoupling cap is not needed since current does not leak out into the input section of the circuit.

I believe the above is mostly correct. If R.G. Keen were here, I'm sure he could tell me how it's wrong . Maybe oldhousescott has a better explanation.