Emerson Custom Guitars E-M Drive. [traced]

[soggybag]

Looks like this is, essentially, an SHO variation, with the crackle free gain control. Might be good to add an LED from S to G to protect the MosFET.

[geiristudio]

I've got the original unit......

In the original unit: what transistor is used?

I'll have a look later today. I popped it open once before and it's covered with glue. It's clear glue though. I'll see if I can make it out.

[modman]

I had a look inside my EM-Drive and there's A LOT more glue than in the original photo at the beginning of this thread. Also the transistor was covered with black nail polish.

It's an MPSA18. I couldn't take a photo because my iPhone can't get the text to show. But yes, it's an MPSA18. There you go!

[Roast7]

my "Em-Drive"


I'm still in testing, I use it to boost volume but I need more volume ...

I tested an old 2N4401 and has given sound
The sound without the effect is more bass than with the pedal on.

Tested a 2N4401 and 2N3904
2N4401 - More bass than the 2N3904 and has more gain
2N3904 - Clean and balanced (with little gain compared to 2N4401)

[vandermann]

Hey Guys!

Any factory EM - DRIVE owner could please post the transistor voltages!?

Many Thanks!!

Cheers, Vanderson

[commathe]

Those look like really, really nice pots. Does anyone know which brand?

[mdtc]

Thanks for all the posts on this pedal. I'm about to embark upon it as my first "from scratch" build. I see there are plenty of opinions on which transistor to use. I've ordered the MPSA18, although it sounds like it's worth experimenting

[tiagojoy]

my pedal!


photo uploading

[jonnyxjuturna]

Would a 5.1k work in place of the 4.7k? Much sound difference?

Trying to use components that I already have. So I can either use a 5.1k carbon comp. Or I could use 2 2.4k carbon films in series. Thoughts?

[Nocentelli]

I'd just use the 5k1, you'll have a slightly higher maximum gain available. Different transistors will have a larger impact on the sound, worth trying out a few different types, any npn Si should work.

[jonnyxjuturna]

On a similar note I currently have two 1megs in series to get that 2 meg resistance. But I could substitute in a 2.4meg in that spot. I guess same question. Is that going to negatively impact the sound. I am using a bs170 as previously suggested in this thread for my transistor

[Nocentelli]

On a similar note I currently have two 1megs in series to get that 2 meg resistance. But I could substitute in a 2.4meg in that spot

Most people only have 2M2, and would stick that in: It's only 10% higher, some resistors are up to 5% higher with tolerance, another 10% is unlikely to make that much difference.

I am using a bs170 as previously suggested in this thread for my transistor

BS170 is a mosfet, but the original circuit uses a BJT - a bipolar junction transistor. Any medium gain npn BJT (e.g. 2N5088/9, MPSA18, 2N3904, BC547/8/9, 2N4401, BC108/9 etc...) will probably sound similar to the original.

The mosfet on the other hand might work, but it's not guaranteed. Mosfets usually have similar value resistors from drain (collector) to +9v as they have from source (emitter) to ground, and they usually have a half supply bias voltage applied to the gate (base): Have a look at the AMZ mosfet booster, zvex super hard-on, or catalinbread chili picoso to see an equivalent single stage booster using a BS170. I'd imagine a mosfet in this circuit would sound different to the original EM drive, and I would try any BJT first.

The key point here is that the design is pretty simplistic: Grounded emitter amplifiers are running at full tilt, and are not that stable, different transistors with different characteristics will behave quite differently e.g. resulting in splatty fuzz if you're unlucky. A "better" design (e.g. with an emitter resistor, and some base biasing - look at the EHX LPB-1) would be less reliant on the *right* transistor to get a good sound, but you may well be lucky and get a nice sound you like with what you've got.

[MrSparkle666]

Glad someone figured out the right transistor for this circuit. The 2n5088 noted in the layout is definitely NOT appropriate here. It has far too much gain. I did some quick spice simulations, and with a 2n5088 the amplifier is biased just above ground and basically producing lots of hard asymmetrically clipping on the low end. Not good. The transistor gain is of critical importance in this circuit if you want it to function properly at all.

The problem with these grounded emitter circuits is that they are basically DC biased by the transistors gain. Since the gain of BJTs varies so much, the bias of the circuit can be drastically affected by the choice of transistor or even somewhat by ambient temperature. The performance is also greatly affected by pickup impedance. As Nocentelli pointed out, it's a highly unstable configuration. That's fine for an unwieldy fuzz pedal, but it's a piss poor design for a so-called "transparent" overdrive. The pedal could sound different on Tuesday than it did last Friday, and there will be even more variation from pedal to pedal. And even if it does sound good at low settings, once the gain knob is cranked this thing is always going to be a splatty fart factory in the low end, like a blown speaker, not amp-like at all. You can hear this clearly in some of the youtube demos of the EM-Drive, and it matches exactly what I'm seeing in the spice simulation.

I don't think this circuit has much value other than as a passing curiosity. I can't believe that some people think this pedal is some kind of holy grail of "transparent" overdrive.

[newly]

any idea which page confirmed the transistor type? MPSA18 is like bursting my speaker....

[jay42]

I was unaware of this pedal until a few days ago. After looking into it, I did a test on my BYOC Tri-Boost. If you short the 360 ohm emitter resistor in the LPB2 section, it sounds pretty much like the demos. I already had an MPSA18 in it...must have been out of 2N5088's that day....can't recall. Kind of surprising. I thought it would gate badly. It rolls off some high end, but that's not necessarily a bad thing in this case. It's subtle.

[mildew]

I tried putting my stripboard Em drive after my vox847 wah for a boost but the wah does not "wah" when output to the em drive.

I assume its the same as the "wah output to fuzz pedal stops wah working" problem, but dont know enough to be sure...

Ideas anyone?

[mildew]

done some interwebbing and it seems that the common emitter of the em drive gives it a super low input impedance, buggering up the wah before it, just like a fuzz face.

The aim was to put the em drive inside the wah as an output booster.

im still unsure about this issue, so if anybody who knows this stuff can confirm...

[induction]

The aim was to put the em drive inside the wah as an output booster.

The EM drive is a particularly bad choice for that application. Why not use a different booster with a higher input impedance? There are plenty of op-amp or jfet boosts that will work fine.

[mildew]

ive got a SHO kit in the mail. would this be better for the output of the wah?

[induction]

The SHO has a very high input impedance. It should work fine. It adds some grit at higher settings, so it depends whether you like the coloration and how much boost you want. The AMZ mosfet booster is very similar, but has less dirt. The MXR Micro-amp is another louder-but-not dirtier type of boost that would work ok. Almost anything would be better than most of the single BJT boosters out there, especially the grounded emitter ones.