EA tremolo finished - mods?

[Branimir]

greets!

phew! i've been going mad for the last few days,... my ea trem didn't work, and finally today i realized not only i've put the jfet wrong (i used 2N5457), it was bad, since i took it from my burnt fet muff...

it works now and i'm not really satisfied...

yes, i tried searching the forums for mods and i saw i can replace those 1uF with 0.68uF for more speed...

but i would like to have more depth maybe...

i used the pcb and schem from the ROG site...
mosfet is BS170
jfet is 2N5457
npn trannie is BC549c

is there a point in changing 180ohm, 1k2 resistors in the mosfet's source? will i get more effect on the drain with raising that 1k2 and lowering the 180ohm?

i don't know how to explain this, maybe a bit more choppy?

i'm into that "money" sound from pink floyd... and ea is just too soft for that..

i thought of building a tremelus lune, but before that, i would like to go as far as ea goes in modding, since it's boxed and i don't really like making pedals and not using them afterwards

greets from croatia!

ps: here's the schem for quick reference:

[Branimir]

i changed 1uF caps in the LFO to 0.68uF, the speed is faster...

i ommited the 180ohm in the mosfet's source, it added a bit of depth, but nowhere near to on/off state...

pedal sounds pleasing with single notes, but chords or faster strummed anything is bad, the tremolo effect is barely audible...

any ideas how to increase the depth?

[gez]

The original article explains how to get more depth (the circuit needs tweaking from unit to unit).  Don't have the link, sorry (can someone please oblige?).

You need more of the LFO's output controlling the FET so you need to mess around with the values in the chain coming off Q3's output cap (.47u).  So, you could decrease the 120k (and possibly make the .47u cap larger), or increase the 68k (this would be my prefered choice).  Depending on how bad the problem is you might just go the whole hog and use a 470k pot instead of the one specified (can't see the value as I type).

[KORGULL]

Checking my notes from when I was building this pedal, I see that if you change the depth pot to 1M it should give you more depth. If you do this you may also need to change the 68K resistor to 100K or higher.
I never got around to trying it since I didn't have any 1M pots at the time.
I got the info on this forum, so you should be able to locate it if you want to know more details. I think "Dragonfly" posted it (this past winter).
The other thing, which I did try (successfully) is to use an MPSA18 instead of the 5088.
I'm not familiar with the transistor you used, but if it is not high-gain, then maybe replacing it with something that is will give you more depth.

[Transmogrifox]

The LFO tranny is probably one of the less important components.  The 2N5088 is the cross reference match to the MPSA18.  It typically has an hFE of around 400.

I have whipped up a modification to the schematic in MS paint, so I apologize for the crudeness:

http://putfile.com/pic.php?pic=1/016055265.jpg&s=x11

This mod adds a constant current source--you may use a 2N5088 for the extra transistor shown.  Adjust the trimpot to get the bias right at the drain of the MOSFET.  The trimpot that is used for bias on the MOSFET can be used to optimize symmetrical headroom (this is where it would be very handy to have a scope, but your ears will tell you if it's distorted).

Anyway, this is an improvement in 2 ways: 
1.  Increased headroom.
2.  The EA tremolo has a gain of about unity through the first stage when the JFET is completely off, so there is only a certain depth you can get no matter what the LFO does or does not--and amplifying too much just makes it more distorted.  The constant current source reduces the "off" gain to probably -40dB as an off-the-cuff guess.  With this, the JFET has almost complete control over the AC gain on this circuit.

The downside is that the added components here require some less-than-elegant modifications to the stripboard circuit, or even some "floating" components over the board.  You may want to point-to-point the extra components and seal them in a ball of epoxy so it looks like a 3-terminal device with the trimpot screw accessible on the outside (don't cover that in epoxy).  Anyway, using the epoxyball method, you could just connect into the pads for the resistor, then find a place to tap the 3rd terminal to 9V.

I would also recommend a bipolar 4.7uF capacitor if you're having distortion problems and bias adjusting doesn't help.  This can be done by taking (2) 10uF caps and connecting their (-) sides together.  This will result in an equivalent 5uF capacitance.

[Processaurus]

I have whipped up a modification

Bravo!  A new generation of EA builders is in your debt.  Depth is the only issue with that design I haven't been able to live with.
Do you happen to know if this will work with the bi-polar design, like at GGG and Geofex?

As for the limited LFO speed range, it works great to just make two LFOs, a faster range one, and a slower.  To do that, change the value of the 3 capacitors in the LFO section.  Smaller= faster.  There's very little added current consumption, minimal extra parts, plus you can footswitch between the two.  Its easier to get the right tempo too, without a bunch of range in one pot.

[Transmogrifox]

It would work equally well with the bipolar design.  With whipping up the schematic I should have whipped up the circuit on my breadboard to verify that it improves the depth as much as I think it should.  Maybe tomorrow if I find I need some more distraction from doing the laundry.

With the 2 LFO idea:
What would be really cool is to buffer the slower LFO and connect the output to the faster LFO "+V" connection so to modulate its amplitude.  I have done something similar by applying a tremolo effect to a signal twice in Audacity at different rates which is tantamount to modulating the LFO with a lower LFO.  It creates some more interesting sequenced type sounds since you have the beat frequencies between the 2 LFO's coming into play.

That LFO from the EA is very smooth and nice.  I think that's most of what makes the EA as nice as it is.  It is unfortunate that the range of oscillation frequency is so limited, but often one parameter must be diminished to optimize another. 

[Processaurus]

With the 2 LFO idea:
What would be really cool is to buffer the slower LFO and connect the output to the faster LFO "+V" connection so to modulate its amplitude.  I have done something similar by applying a tremolo effect to a signal twice in Audacity at different rates which is tantamount to modulating the LFO with a lower LFO.  It creates some more interesting sequenced type sounds since you have the beat frequencies between the 2 LFO's coming into play.

Man, your ideas are like my ideas, except you know something about electronics   I made a kind of variation of that, except its panning between the LFO's
http://www.elixant.com/~stompbox/smfforum/index.php?topic=35257.0
Feeding one with the other was what I really wanted to do, but couldn't figure an easy way out at the time.

[Transmogrifox]

I missed that thread somehow.  It looks pretty cool.  Nice work on the cosmetics and packing stuff into tight spaces.  Sound samples would be fun. 

Heres the "How-To" on multiplying the EA oscillators:

http://putfile.com/pic.php?pic=1/116115914.jpg&s=x11

You can actually take various outputs.  The output shown is a sum and product of the oscillator outputs.  With PSPICE simulations, I determined that the High frequency oscillator is best used as second in line. 

The other output you can use is right at the emitter of the buffer transistor.  This creates some sharper edges.

You can also take the normal output from the low frequency oscillator to use it plain.  It may be nice to use a rotary switch to make it more flexible.

For even more flexibility, you could make the 150k resistor (the only 150k in the schematic) into a 250k pot.  At 0 resistance there would be no effect from the first oscillator.  As it gets cranked, there gets to be less effect from the second oscillator.

I hope it is found useful.  I'll have to give it a try now.