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ulav
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Post by ulav »

Ok ,so i wasted quite some time starring at synth circuits and i got used to bipolar suply ,and now i thought i make a simple booster distortion or some like that nothing fancy just the opposite to get used on soldering again .. and i stuck with unipolar supply in all the schematics - there is only +9V and ground ,i read a bit about virtual ground and dividing with 2 resistors ,but still i want a practical help ,anyone ... does the ground refers to the negative rail of the battery (battery powered ) ?? i really appreciate any help ,thanks

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Post by ulav »

..But it still refers to the negative rail of the battery (or the positive) ...
Thanks a lot ..
.. about the tech side .. it can be said i know very complicated ,as one to be missunderstood

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Post by Greg »

ulav wrote:..But it still refers to the negative rail of the battery (or the positive) ...
Thanks a lot ..
.. about the tech side .. it can be said i know very complicated ,as one to be missunderstood
Yes.. with a divider you will have ground, virtual ground (usually half V+) and V+.
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Post by mictester »

ulav wrote:Ok ,so i wasted quite some time starring at synth circuits and i got used to bipolar suply ,and now i thought i make a simple booster distortion or some like that nothing fancy just the opposite to get used on soldering again .. and i stuck with unipolar supply in all the schematics - there is only +9V and ground ,i read a bit about virtual ground and dividing with 2 resistors ,but still i want a practical help ,anyone ... does the ground refers to the negative rail of the battery (battery powered ) ?? i really appreciate any help ,thanks

OK - some basics. Let's imagine you're powering something simple like a Big Muff. You have a gound connection that is connected to battery -ve terminal (usually through some kind of switching arrangement,to disconnect the battery when not in use). The +ve terminal of the battery will be at +9V above the ground rail. (BTW, some designers call "ground" "0V" just to confuse people!). The circuit will use the +9V to power the transistors and provide bias voltages (by means of joined resistors forming "potential dividers"). Also, "potential" is the old fashioned name for "Voltage", so don't get confused!

Still with me??

Some effects (as mentioned by another writer in this thread) use "positive ground" wiring. This is because they use transistors with different characteristics, which require the relative voltages to be reversed. Don't worry - it's not complicated! These old circuits (usually things like "mojo-filled" Fuzz Faces) have the +ve battery terminal connected to ground (usually through a switching arrangement). This means that the -ve battery terminal is at -9V with repect to earth! The circuit will then work "upside down" as far as voltages are concerned.

The above are both "Unipolar" circuits. This has nothing to do with the term "bipolar transistor" - just to confuse you a bit. A much better description would be to call them both "Single Rail supplied" circuits, because each has just one supply rail.

OK. When the mid 60s arrived, those clever people at Fairchild Semiconductor developed the "operational amplifier". This was a device originally invented for use in Analogue Computers. These computers were a bit like electronic slide-rules (look up slide rules if they're alien to you - they were the first "pocket calculators"!). The op-amp wasn't really ever meant to be used for audio work, but designers quickly realised that this particular ciircuit block could be used for amplifiers, filters, clippers, and all manner of other analogue functions. Rapidly, all the component manufacturers introduced their own versions of op-amps, often with better characteristics than the original ones - lower noise, lower distortion, wider bandwidth, faster slew-rate and so on.

The op-amp was designed to be used from a dual-supply. This would be two batteries, connected in series, with the joined connection forming the ground, thereby giving you two supplies - one positive (above ground) and one negative (below ground). If you connect your multimeter black lead to ground, you'll find two rail voltages present - +ve and -ve. The op-amp will then work "either side" of ground, and will produce output voltages that can be above or below ground. If you want a "minimum components" design, a bipolar supply reduces the need for some components, but requires two batteries.

In more modern designs, we "float" the op-amp above a ground rail, using a potential divider to provide a "half rail" voltage. This allows the op-amp to be used with just one battery. The output pin of the op-amp will sit at half-rail (with no signal applied) so we have to use a DC blocking capacitor between the op-amp and the output of our board. In the same way, the input will be at half-rail, so an input capacitor will have also to be used. What we've actually done, by use of the potential divider (normally connected to the non-inverting input of the chip), is fool the op-amp into seeing a dual supply! Take a look at the MXR micro-amp to see what they've done (though they've slightly offset the mid-rail, peculiarly).

I hope this makes sense! :shock:
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Post by lolbou »

biffa wrote:Unipolar trannies are to my knowledge,are not used in guitar fx,as they can't produce gain
mictester wrote:The above are both "Unipolar" circuits. This has nothing to do with the term "bipolar transistor" - just to confuse you a bit.
Just to clarify here and complete with mictester's explanation : "unipolar transistors" appear to be a historic term to refer to the FETs. These are used in pedals (I know you know biffa :wink: ).

Though unijunction transistors are a different kind, these used to be used in pedals for LFO purposes (see the Vox Repeat Percussion here).
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Post by Grendahl »

mictester wrote:Take a look at the MXR micro-amp to see what they've done (though they've slightly offset the mid-rail, peculiarly).
Just for illustration purposes, I've marked up that schematic and included it here.
mxr micro-amp voltage divider.GIF
Mictester, What would the advantage be, if any, to the lowering of the voltage to pin 3 by using the 10M1 resistor(s)?
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Post by mictester »

Grendahl wrote:
mictester wrote:Take a look at the MXR micro-amp to see what they've done (though they've slightly offset the mid-rail, peculiarly).
Just for illustration purposes, I've marked up that schematic and included it here.
mxr micro-amp voltage divider.GIF
Mictester, What would the advantage be, if any, to the lowering of the voltage to pin 3 by using the 10M1 resistor(s)?
They're not lowering the voltage at all - the 1M resistor sets the input impedance of the circuit. The input impedance of a modern op-amp is in the many millions of ohms. The two 100k resistors (120 and 100k on some models) set the mid-rail voltage.
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Post by Grendahl »

mictester wrote:They're not lowering the voltage at all - the 1M resistor sets the input impedance of the circuit. The input impedance of a modern op-amp is in the many millions of ohms. The two 100k resistors (120 and 100k on some models) set the mid-rail voltage.
OK, I must be confused here.

I get that R7 and R8 set the mid rail voltage so that the IC can see +4.5v and -4.5v in comparison, and that it's a way to get around having to use bipolar power supplies.

So, is the input impedance set by R2, the 10M resistor alone?
Also, does R3 - the 1K play a part in that?

Sorry for the newb questions, but I've just never understood those extra resistors when I've seen them on schematics.
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Post by mictester »

biffa wrote:Mictester,if i put a 10M instead of a 1M,will my input impedance see 10M?
And what effect will this have on my tone?
Is there an upper limit?
Say,100M?
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Yes - you probably would (because the input impedance of a modern op-amp will be higher than that), but there's a law of diminishing returns, as ever with electronics! The noise contribution of the input resistor would become more significant. The circuit would be more susceptible to radio interference and other induced noises, and the stability of the op-amp could be compromised.

All the above reasons are why I moved, some years ago, to using "bootstrapped" input circuits for my effects. They have high input impedance (that doesn't affect the guitar tone), but use relatively low value resistors.
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Post by mictester »

Grendahl wrote:
mictester wrote:They're not lowering the voltage at all - the 1M resistor sets the input impedance of the circuit. The input impedance of a modern op-amp is in the many millions of ohms. The two 100k resistors (120 and 100k on some models) set the mid-rail voltage.
OK, I must be confused here.

I get that R7 and R8 set the mid rail voltage so that the IC can see +4.5v and -4.5v in comparison, and that it's a way to get around having to use bipolar power supplies.

So, is the input impedance set by R2, the 10M resistor alone?
Also, does R3 - the 1K play a part in that?

Sorry for the newb questions, but I've just never understood those extra resistors when I've seen them on schematics.
The 10M is just there to bleed DC voltage off the "outside" end of the input capacitor, to prevent clicks when a bypass switch is operated. It will slightly load the input (so very slightly reduce the input impedance), but is so high compared to the input resistor (1M) that its effect will be negligible.

The 1k series resistor is there in an effort to protect the input of the op-amp. It used to be really useful in the days when the early BiFET input op-amps were particularly susceptible to static spikes, but these days it's just included because "that's the way everyone does it"!
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Post by Grendahl »

Thank you mictester, that was a great answer.

You know, I'd love to see a section on these forums of circuit block explanations from you. You've always got such a clear way of explaining exactly what the components are doing. I really appreciate it. Thank you again!
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Post by DrNomis »

lolbou » 10 Aug 2010, 17:23

biffa wrote:Unipolar trannies are to my knowledge,are not used in guitar fx,as they can't produce gain


mictester wrote:The above are both "Unipolar" circuits. This has nothing to do with the term "bipolar transistor" - just to confuse you a bit.



Just to clarify here and complete with mictester's explanation : "unipolar transistors" appear to be a historic term to refer to the FETs. These are used in pedals (I know you know biffa :wink: ).

Though unijunction transistors are a different kind, these used to be used in pedals for LFO purposes (see the Vox Repeat Percussion here).




Hi Lolbou,


Just a clarification on uni-junction transistors,they actually have three leads on them,one is called the Emitter,one is called the Base-1,and the other is called the Base-2,uni-junction transistors aren't designed to amplify signals,you could actually think of them as Voltage-Controlled Switches,back when Valves were the only amplifying devices in Electronics,there was a Valve device called a Thyratron,a good example is a 2D21 Valve,it was called a Thyratron after the Greek word for door..."Thyros",so a Thyratron is a "Door Valve",then,when Valves were replaced with Transistors and other semiconductors,the Thyratron was replaced with the Uni-junction Transistor,a classic example is the 2N2646.... :)


a Uni-junction Transistor can be made to oscillate by connecting a capacitor from the emitter terminal to ground,one of the Base terminals to ground,the other one to +V,and then connect a resistor from the emitter terminal to +V,what will happen is,the capacitor will charge up to a certain voltage,when this voltage is reached,the Uni-Junction will turn on,shorting out the capacitor and discharging it,then the Uni-junction will turn off,and the whole process starts again,this produces a sawtooth waveform,if you vary the charging current being supplied to the capacitor,you can change the frequency of oscillation,in fact,this is how a Uni-junction Transistor based Voltage Controlled Oscillator works,a well known example is the Moog 901B VCO.... :)
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