Ibanez LF-7 Mic Preamp Overdrive/Fuzz

[lioonel_dg]

Hi everyone !

I have an Ibanez Lo-Fi LF-7 pedal and noticed that, even when the pedal is bypassed, the signal can still be routed through the Mic Preamp with the mode selector. Apart from the huge boost in volume, this preamp also creates a very cool overdrive/fuzz sound that is quite different from the drive on the pedal.

I would like to recreate this mic preamp part of the circuit as a standalone effect pedal, adding a output volume knob.

Could someone help me identify the Mic Preamp circuit on the schematics ?

Thanks a lot !

[ElectricDruid]

I think it's basically just the first op-amp stage. When it's switched to "MIC" mode, R128/4K7 and C9/10u are connected, so it gives some gain. The crucial tone shaping is C9 and C10, followed by TR2.There's also an output buffer around TR3 left in, so I've marked that too, but it might not make much odds to the sound. You might be able to take the output direct from C44 without losing anything important.

[swamphorn]

Ahoy-hoy,

The mic preamp functionality looks to come primarily from the mode switch (top right of the schematic) re-configuring the preamp circuitry. In guitar/drum mode, this is connected to the output terminal of the same op amp which sets it to unity gain. But in mic mode, the negative terminal of U1A is grounded through the series RC network formed by R18 and C9 (which confers a broadband gain of 22 or 27 dB). You may need the stage formed by U1B for the full effect - with the U1A's output voltage only constrained by its capability, it is probable that the second stage is bumping against the rails as well.

So, you would start off by copying the circuitry around U1A and U1B with the switch in the mic mode; it seems there's little additional equalization applied in the mic mode besides a high cut, but this is applied after the filters.

[ElectricDruid]

U1B is only part of the circuit when the effect is on. OP said they liked the sound of just the mic preamp alone, which is left in the circuit even when the pedal is off (which is clear from the schematic too). TR2 is still in though, so maybe with the preamp boosting the level, there's some good distortion coming from TR2.

Incidentally, it looks to me like the 510K resistor from TR2's base should go down to the Vref supply, but it's actually shown connected to the next wire up, which is the op-amp output - that doesn't make sense since it then shorts out the DC-blocking cap C66/104.

[lioonel_dg]

Thank you for your answers !

To be honest I am really new to this. I've build a couple pedals from kits but this would be the first one I do from the ground up !

How should I go about drawing the schematics for this ? And would there be a way to adjust gain and well as output volume ?

Thanks again, and sorry for being clueless on most things.

[antonis]

Incidentally, it looks to me like the 510K resistor from TR2's base should go down to the Vref supply, but it's actually shown connected to the next wire up, which is the op-amp output - that doesn't make sense since it then shorts out the DC-blocking cap C66/104.

510k resistor could't be considered  "short", could it..??
(their parallel combination could be considered as a frequency variable (20Ω and downstream) signal series impedance..)

In the expense of TR2 input HPF, C66 could be omitted..

[ElectricDruid]

My annotations seem to have gone missing, so I'm having another go!

[Baran Ismen]

I also might have a question related to this pedal. As its kind of recent, I didn't want to create a new topic.

Is it possible to increase the output volume to an extent around %15-20? I normally use the drums section, because on guitar mode, total volume is rather low, drums is also low but not as much as guitar, but on Mic, as above stated, it becomes something completely different.

[antonis]

Make R86 12k to 15k (10% to 25%)..

[Baran Ismen]

Make R86 12k to 15k (10% to 25%)..

Much appreciated, komşu!