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!
That would be really cool. I would build one of those also.
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.
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!
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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Quote from: madbean on August 25, 2011, 12:56:22 AM
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.
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.
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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Might be some interesting reading for you.
http://circuitworkshop.com/forum/index.php?topic=870.0
Quote from: raulduke on August 25, 2011, 10:05:50 AM
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.
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
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.
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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Bump...as I would like to know as well.
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.
Thanks for the explanation Madbean; that makes sense to me mate.
I have done a PCB design for this anyway (dual layer). Its a bit messy but its a start.
The tonestack values can always be altered to taste I guess.
I'll upload it as a .brd file if anyone is interested. I use gaussmarkov's libraries as well so they will need to be installed.
I believe with JFETs, you only need the source resistor to set the bias, but it limits the input swing available as a large negative input will send the device into cutoff. Not necessarily a problem with guitar-level inputs.
Quote from: madbean on August 26, 2011, 08:08:12 PM
I believe the above is mostly correct. If R.G. Keen were here, I'm sure he could tell me how it's wrong :).
You rang?
It doesn't need bias resistors on the gate -other than the pulldown resistor, which does pull the gate to the proper bias voltage of 0V - because it's biased at the source. Instead of pulling the control pin (gate for FET, base for bipolar, grid for tube...) to some predetermined voltage/current/whatever, source biasing pulls the source to a voltage/current/whatever to make the grid come out right.
In the self-biased circuit, which is what that is, the inherent current that flows through a depletion mode JFET is used to create a voltage through the source resistor that raises the source above ground. The gate is a very high impedance point, so it is easily held at ground by a single high value biasing resistor.
The source rising creates a voltage in the direction that turns the JFET off with the gate held at ground. The source rises until the net off-voltage between source and gate is just enough to hold the source where it is, and that condition is then stable. It's how triodes are most often biased, too, by the way.
Quote
1) JFETs have much lower current leakage from their gates than a bi-polar does from its base and
This is correct, although I would say it differently. Both JFETs and bipolars have very similar leakage from their control pins - that of a reverse-biased silicon junction. However, a bipolar is an enhancement-mode device. That means that left all to itself, it won't conduct at all. You have to do something to make it conduct. A bipolar MUST have a voltage forward biasing the base-emitter so current goes into the base to make it conduct. JFETs are depletion-mode devices, meaning that left to itself it conducts maximum all the time. A JFETs with its gate open is a resistor between the drain and source of something like 10 to 1k ohms for most JFETs. You have to pull the gate negative with respect to the source (for N-channel; reverse for P-channel) to get it to back down towards off.
Which gets me to how I'd say it: JFETs bias differently from bipolars. Their gate has to be pulled negative with respect to the source. This circuit cleverly holds the gate at zero volts and the source pulls itself up to a stable bias voltage for the device. A bipolar has to have its base pulled higher than the emitter (analogous to source, kinda), hence external resistors are needed, because the default action of the bipolar won't obligingly self bias it.
Quote2) 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.
Again, correct, but I'd say it a little differently. In linear amplification operation, a JFET has a bigger magnitude gate-source voltage than a bipolar transistor's base-emitter voltage, but opposite polarity. An NPN bipolar base-emitter voltage is nearly always about 0.5 to 0.7V. An N-channel JFET's gate-source voltage is always with the gate negative with respecto the the source, and usually within the range of 0.5V negative to maybe 8V negative. It depends hugely on the type number of the JFET and the specific JFET within the type number.
Which gets down to how I'd say it: The stable bias voltage for an N-channel JFET in the self bias circuit requires the gate to be held at ground while the source is elevated by the normal conduction of a depletion-mode JFET. Since the gate is very high impedance and does not either leak or suck current from any signal wire connected to it, an incoming signal which has an average DC level of zero volts does not need a DC-blocking capcitor to work properly with a self-biased JFET.
Notice that if the incoming signal does have an average DC level that's not zero, it will upset the self-bias balance of the JFET and so a blocking cap may be required to protect the JFET bias from the signal.
Thanks for the amazingly thorough and enlightening explanation! And, welcome to the forum!
Heehee!
Welcome R.G.
For those of you around here who don't know, R.G. is a sort of 'grandfather' of DIY pedal building. He's a great, generous, helpful man who knows way more about this stuff than anyone else I know. He's also the guy who taught me about debugging.
R.G. - hope you're well, and it would be cool to see you around here once in a while.
Jacob
Hi RG, welcome to the forum! Love your website www.geofex.com (http://www.geofex.com) :)
Ah! The legend himself! Welcome home RG haha hope you can hang out with us more often ;) It's a real pleasure...
-Kaleb
Welcome to the forum RG Keen!
And thanks for the thorough explanation!
Quote from: k.rock! on August 27, 2011, 11:11:11 PM
Ah! The legend himself! Welcome home RG haha hope you can hang out with us more often ;) It's a real pleasure...
-Kaleb
Dido!!!
Attached are the schematic file and board file for the baxandall eq schematic shown above.
I don't know if it will be of any use to anyone but I thought I would post it just in case someone wants to modify/change the design.
It is a dual layer board but could easily be made single layer with a bit of tweaking (and a larger board area).
R4 on the schematic can be changed to a pot (say 25K) to provide a 'shift' control for the mid frequency.
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Doh!
brd and sch files attached here...
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