Simple inline booster schematic

[JTEX]

I thought I'd share this simple but very well-performing inline buffer/booster. It's intended to go onboard, but nothing's stoping you from using it as part of some other circuit, pedal, whatever. I made it following a request from a European luthier who found a very good sounding, but lower output pickup he wanted to use in one of his basses. The problem was that its level didn't match his other bass models, and the onboard EQ he's using doesn't have a gain trimpot.

Using the opamp shown, the circuit has the equivalent input noise of a single 5k resistor, the gain can go between unity and +20dB, current draw is <0.3mA, and the output can swing from rail to rail. For multi-pickup instruments, 2 or 3 units can be summed by simply wiring their output pins together, as if they were passive pickups in parallel.

 

[DailyDovetails]

This is a cool looking circuit I like how compact the board ends up. This would probably work well on my HSS Yamaha to even out the single coils and hum positions.

Out of curiosity have you experimented with using this circuit as a differential amplifier at all? I’m going down a bit of an active pickup rabbit hole lately and think it might be fun to experiment with making my own. I’m wondering if you could just copy C1 and C2 then feed the other coil of a humbucker into input 2 like the EMG81 below.

 

[lowpitch]

Very cool! How is C1 affecting the tone and why is it necessary? I see in your previous design (https://forum.pedalpcb.com/threads/an-idea-for-a-universal-input-buffer-booster-onboard.21235/) there is no equivalent cap.

 

[JTEX]

Out of curiosity have you experimented with using this circuit as a differential amplifier at all? I’m going down a bit of an active pickup rabbit hole lately and think it might be fun to experiment with making my own. I’m wondering if you could just copy C1 and C2 then feed the other coil of a humbucker into input 2 like the EMG81 below.

View attachment 86146

I've seen this circuit floating around. It makes sense if you're designing an active pickup from scratch, where you have control over pickup parameters such as the inductance and output level of L1 and L2, and you might want to amplify the coils separately and maybe even load them differently (as in the above circuit) to get the voicing and whatever output level that you want. My little circuit is meant to work with existing pickups, already voiced a certain way, with a predictable tone. If I tapped into the individual coils with a differential amp, who knows how that would affect the tone? The coils wouldn't interact with each other anymore, so that would change things. That's something for a pickup designer to worry about

For sure, the coils in an EMG active pickup are made differently from a typical passive pickup, with fewer windings, lower output, lower inductance and DC resistance, which is why they need to add some significant preamp gain, and also why they can get away with going into a low input impedance amp (R1=30k, R4||R5 = 68k) without sounding super dark.

BTW, LM4250 is obsolete (not to mention noisy), I wouldn't design anything new around it.

 

[JTEX]

Very cool! How is C1 affecting the tone and why is it necessary? I see in your previous design (https://forum.pedalpcb.com/threads/an-idea-for-a-universal-input-buffer-booster-onboard.21235/) there is no equivalent cap.

C1 is there mostly as a placeholder. It can be left out or changed to a different value. As shown, at 100pF, it doesn't do very much, other than some protection against RF interference. It's the equivalent of around 3ft of guitar cable. Higher values would start making the sound darker/warmer, which may or may not be what one wants. Adjust to taste, basically, or do not populate.

My other, more elaborate buffer/booster, is more universal, can also do justice to piezos (input Z can be set as high as 10M). I felt that a cap across the input would be a bad idea, since I don't know what type of pickup will feed it and how a cap would affect it. A 100pF cap across a piezo having a 100pF capacitance will cut its output in half.