Pickup simulator using gyrator

[PBE6]

Just thinking about other useful utility pedals to have around, and my thoughts drifted to a pickup simulator. Is it possible to use a gyrator instead of an actual inductor here? The gyrator would have to be in series with some resistance and in parallel with some capacitance to for the RLC circuit, but the only configurations I've seen for gyrators in GeoFEX's parametric EQ page and the Elliott Sound Products page have the gyrator being grounded.

Is it possible to wire a gyrator in series? If so, where's the "out"?

[PRR]

> Is it possible to wire a gyrator in series?

Google "floating gyrator".

One hit:
"Fully floating (not earth referenced) gyrators are possible, but are far more complex than the traditional types and will not be covered in this article."
http://sound.westhost.com/articles/gyrator-filters.htm

Pease:
www.youtube.com/watch?v=AEJtajaRj_s   -- around 6:50

Patent from 1981:
http://www.google.com/patents/US4245202

[dschwartz]

Gyrators are too noisy.. I experimented with twin tees band pass filters and they are good enough for simulating pickups..each pickup tone is a peak frequency, research pickup resonant peaks and reproduce them..also use a lpf to "neutralize" the original pickup resonant peak

[PBE6]

Thanks Paul! Great resources.

The reason I was thinking of building a pickup simulator using a synthetic inductor was to be able to vary the inductance easily. I know that the guitar electronics and cable form an LRC circuit that causes a resonant peak before the roll-off, and I have done some fun experiments using my "magic box" (a passive box with a selection of caps to ground to simulate cable impedance and a 1M pot to ground to simulate loading effects of different magnitude pots). I was hoping to be able to manipulate the inductance directly with this design, and hopefully to simulate both the frequency response and the output impedance of a guitar with different pickups.

dschwartz, how noisy are the gyrators you've built?

Also, how much offset voltage is too much of using Bob's double-gyrator design? I have some TL2272s and a few OPA2134s but they're only listed as having offsets between 0.3 mA and 3 mA. Is the LMV2011 a necessity here?

[Tony Forestiere]

Doesn't the EPFM Passive Tone Control act as a simple RLC using a cheap transformer/inductor? Could be a baseline for tweaking a pickup sim. Luck. 

[PBE6]

I just looked it up, and yes it does use a real inductor in an RLC circuit but it goes to ground in series with a capacitor and a resistor so it creates a notch at a given frequency. The pickup simulator should have the inductor in series and the capacitor after it in parallel going to ground.

I wonder though, a simple passive solution might be to pan between two inductors (one big and one small) to get a range of inductances. I found this from Craig Anderton right now:
http://www.harmonycentral.com/articles/-panpot-guitar-rewiring

Maybe I can give that a try if the active version has issues.

[duck_arse]

swinging inductor, one opamp, and a rev counter ....

http://forum.allaboutcircuits.com/attachments/pages-from-elektor-1974-12-2-pdf.56875/

[PBE6]

Awesome!! Thanks duck. 

Just looking at the app note, it specified resistances to ground for the input and the output. It seems like a buffer at the input and an inverting buffer at the output with resistors matching the value of P1 would meet the requirements, am I understanding that right?

I was trying it out in simulation and it seems to be doing something, but at certain settings it seems to be acting as an RLC circuit all by itself (well, with a 10k resistor to ground after the output buffer). Is there some capacitive effect here too? I expected it to simply roll off high frequencies.

[R.G.]

At the risk of seeming too simplistic, REAL replacement pickups make good replacements for "synthetic pickups", and you can feed small sine waves through them as a series component.

Cheap too.

[MR COFFEE]

A 2-pole Sallen and Key low pass filter with variable frequency (requires  a dual element pot) and variable Q (3-position switch or another pot will be very low noise and will allow you to emulate virtually any pickup. This is what Ron Wickersham did with fairly flat medium impedance pickups in the Alembic Guitars back in the 70's. The circuitry works quite well, and is even quieter with modern low noise op amps. They buffered the pickup coils with a jfet mu-amp and that drove the LPF circuit.
You do need a pickup with good high end if you want to get really bright fenderish tones. Turn the frequency down and the Q up and it will push an overdrive into really inspiring tone territories. Think smokyt blues to ZZ Top.

mr coffee

[PBE6]

Creating a well-behaved resonant circuit with opamps to mimic a guitar's frequency response is fairly straightforward, but what about the output impedance? Is there a simple way to mimic that short of using an actual inductor? Does anyone know if a floating gyrator would do a good job or a bad job of it?

[Fp-www.Tonepad.com]

RG, your wisdom seems to never dry out. I'll have to try that, I've been toying with a DIY Ebow