Active pickup build

[nakedzen]

Following the schematic here: https://www.freestompboxes.org/viewtopic.php?p=230195#p230195

Just swapped the pinout to work with a LM386. At least it's making noise when I wiggle a string over the pickup.

Now I'll just need to transfer this mess to a guitar, hopefully later today if I have time.

 

[nakedzen]

Well as I thought it would go, once I transfer the pickup, it stops working. Back to troubleshooting

[curtisa]

Neat! I guess this is a little bit like what EMG do with their Retro Active line, essentially just 'activating' a passive pickup.

Any particular reason you went with the LM386? Seems a slightly left-of-field choice.

[nakedzen]

I'm just messing with components, nothing is set in place. Also considering NS4110B or RC4558. Just what I have readily available. EMG is afaik LM4250?

[curtisa]

1 minute ago, nakedzen said:

EMG is afaik LM4250?

Could be, not sure what is buried in the epoxy. If the schematic on freestompbxes is to be believed it looks like it has a gain of only 5x. With that much negative feedback you could get away with all sorts of different ICs in that situation. You'd probably be more interested in battery longevity.

LM386 would be...colourful to try?

[nakedzen]

Noisy at least! But I'll see if it's usable.

I have some 4250 in the mail too.

Btw. all active pickups are just passive humbuckers plus a preamp board. Where the point lies is that you can get away with lower wind counts and other features that would not work with a passive pickup, resulting in a very low output.

[curtisa]

7 minutes ago, nakedzen said:

Btw. all active pickups are just passive humbuckers plus a preamp board

Correct. I guess EMG just made it a 'thing' by squeezing the preamp into the pickup housing itself to make it more appealing to the masses. By that definition I suppose you could call any guitar plugged into a booster pedal an active guitar with an extension cable

Another link to some useful info, including suggestions for tweaking the response of the pickup without butchering the windings:

https://www.electrosmash.com/emg81

[nakedzen]

Yeah I've seen that.

We'll leave that to semantics as to how far away from the pickup can the preamp be for it to still be an active pickup.  

[henrim]

Cool. I keep on thinking about this subject every now and then. But then, I hate batteries in my guitars so there is always a booster in the signal chain before anything else.

Possibly a battery powered preamp shielded close to pickups is less prone to all the noises you can get from cables and cheap wall mount power supply. My current PSU is ok so I keep on trying/building different booster pedals. 

With on-guitar preamp you can obviously have controls close the hand if you need to adjust something during playing. But for me it is not something I do. I do fiddle with the settings by it's more like a different day, different settings. 

I made this pedal test rig that makes it relatively easy to change between different circuits. It has all the off-board stuff in it, that would otherwise take a lot of time to wire, and you can just drop in a circuit board for testing.

[mistermikev]

looks like that particular circuit is booster... on the sm site you can find baja's direct clone of an emg... .different animal - a differential amp.  that would use the lm4280 but more because of it's low battery drain than specific sound qualities... or so I'm told.  

afa to active or not to active... for me... I also hate batteries in a guitar... but since I play bass I have given into the dark side because active eq up front is so useful there.  

[nakedzen]

I don't really understand what difference is there with a booster vs. an amplifier? Afaik a differential amp looks at the input signal voltage and the 9V supply voltage and tries to make the output a difference of those? Ie. boosting the signal.

 

But, I'm not trying to copy emg's since, why would I? They've already been made for decades with great success. I'd just buy EMG's then without all this hassle.  Just having fun with circuits mostly to see what comes out of it.

[mistermikev]

34 minutes ago, nakedzen said:

I don't really understand what difference is there with a booster vs. an amplifier? Afaik a differential amp looks at the input signal voltage and the 9V supply voltage and tries to make the output a difference of those? Ie. boosting the signal.

 

But, I'm not trying to copy emg's since, why would I? They've already been made for decades with great success. I'd just buy EMG's then without all this hassle.  Just having fun with circuits mostly to see what comes out of it.

well no expert here... but I can relay what I've read, and that is that the emg version of a difference amp actually is running the two coils of a humbucker into sep inputs... then dropping voltage for anything they have in common, and amplifying anything they don't have in common.  This keeps the noise floor really low.  it requires some really underwound singles.

the schematic you linked to... that's just a garden variety booster.  anything at the input is being boosted -noise included.  Just like a pedal running after the guitar.  Nothing wrong w that... just isn't anything like what is going on in an emg pickup (not being critical here... just observation).  you could, essentially take any booster circuit and put it in a guitar.  I've done it with micro booster, linear power booster, mid boosters, sho boost... and enjoyed it quite a bit.  IMO it's a very useful and cool thing to do.

[nakedzen]

Here's the EMG 81, I can't really tell where the big difference is? It's missing the diode at the 9V connection but that doesn't affect the operation, it's just a safety feature.

 

Or the electrosmash version, still essentially the same circuit? You feed the humbucker coils to pins 3 and 2 and feedback the output to pin 2.

[nakedzen]

Here's a basic differential amp circuit from the LM4250 spec sheet

 

[curtisa]

The EMG circuit is different to your average opamp 'booster'. It's configured as a differential amp, ie it amplifies the difference between its '+' and '-' inputs.

The Electrosmash analysis of the circuit pretty much explains it all, but in a nutshell the EMG circuit is relying on the two coils of the humbucker to be wound anti-phase and the voltages at the 'tops' of the two coils L1 and L2 to be moving in opposite directions as the string wiggles in the magnetic field. The opamp then amplifies the difference between the two signals and out pops 2x string signal multiplied by gain.

Say the top of L1 goes down to -1V and the top of L2 goes up to +1V. The opamp is going to try and work out the difference between the two, so you get -(-1V) + (+1V) = an output of 2V. If the opamp also has gain (which the EMG does, 5x) you can multiply the 2V by 5 to get an eventual output of 10V.  Consider also that there might be 0.1V of induced noise at the same time (hum, buzzes etc). In that case the noise appears on both coils in phase with each other, ie noise signal on the top of L1 goes up to +0.1V at the same time as noise signal on the top of L2 goes up +0.1V. The opamp will take the extra noise voltage and do the math on it again and give you -(+0.1V) + (+0.1V) = 0V of noise.

(Gross oversimplification of what's actually going on, but you hopefully get the idea).

A regular garden variety (pedal) booster will just take the output of whatever it is fed and boost it. If the pickup is noisy as well as carrying guitar signal, naturally all you get is a bigger version of signal+noise at the output. All the linked versions of the schematics discussed so far are not this.

[nakedzen]

Great post, most of that is familiar already to me. But it didn't answer my question, I don't see where this "booster circuit" comes from in the first circuit compared to the other three?

Maybe I've overlooked something, but to me they look like basically the same thing?

[curtisa]

Quote

I don't see where this "booster circuit" comes from

It's not...as such.

Yes, it boosts the signal which I guess makes it a 'booster' in the traditional sense. But it's not configured to act as a regular pedal-style booster.

It's what it's doing with the two inputs that sets it apart.

Quote

 

but to me they look like basically the same thing?

 

Yep, they're all the same (or near enough to be considered the same).

[nakedzen]

Success finally with the circuit, I was missing the pickup side connections.  With the red and green from your usual 4c cable to ground/common it now works quietly even from a breadboard.  

Now I'll need to mess around with the resistor values, I didn't have that low values at hand that the schem asks for. Probably getting not so optimal voltage than the ic needs.

[mistermikev]

i stand corrected... looking at orig schem I was seeing a std booster with a 2nd input for 'buffered bypass'... or so I thought.

[nakedzen]

Got the LM4250, maybe it's placebo but I think it sounds a bit more emg-like now.  

Swapped the resistor values and the ceramic cap to an electrolytic one on C2.

R4/R3 = 6.8 so not over the top, Vout = R4/R3 (Vin1 - Vin2)

.

 

 

[curtisa]

24 minutes ago, nakedzen said:

Vout = R4/R3 (Vin1 - Vin2)

Close. Gain on the inverting input is R4/R3 = 6.8x, but gain on the non-inverting input is 1+(R4/R3) = 7.8x. There's deliberate mismatch on the two inputs that EMG have put in there; the Electrosmash analysis mentions this as well.

You can experiment more with the mismatch if you like by adding the missing resistor on the non-inverting input as shown in the LM4250 datasheet and changing the value to be bigger or smaller:

 

[henrim]

I don't know much about active pickups but as far as I have understood some people run their EMG's with 18 volts. I reckon the higher voltage has some effect on tone/headroom. That in mind, I noticed you have a 1N4148 for reverse polarity protection. If higher voltage is a desirable feature, I think 1N4148 has somewhat significant voltage drop in 9V circuit. 0,7V or so. I guess it is good to have protection but maybe you could use a Schottky diode with smaller voltage drop instead? A MOSFET solution is probably an overkill.

[nakedzen]

Thanks for the tip Henri! I've tried the 18V "mod", wasn't very impressed with the results, went back to 9V. You get a slightly extended low end and somewhat less compression. Pretty small difference, and not to a direction I liked.

[nakedzen]

Almost done now, I have got couple pcb prototypes done. Now all I'm trying to figure out how EMG managed to make their so immune to EMI. I'm guessing at least some sort of faraday cage inside the pickup.

Ground planes need work too, I'm guessing it would make sense have it as a star ground on the component side of the pcb?

But would adding another ground plane on the pcb bottom side make sense? Only connect it to the output ground. Would this copper layer work as added shielding, or make things worse?

[curtisa]

The bulk of the noise cancellation in the EMGs will be largely due to the differential configuration of the integrated opamp. You should see similar noise immunity just by building your version of the preamp.

You can experiment with additional shielding if you want but I suspect the difference won't be as much compared to the inherent noise rejection of the preamp. It certainly wont make it worse. Whatever shielding you do chose to experiment with, connect it to ground.