Adding second oscillator to EA tremolo HELP!

[davidjnichols]

Hey guys, I've been trying to figure out how to do this for a while now.  I want to add a switch to my EA Tremolo that will switch between the nice sine wave of the pedal to a choppy, square wave sounding tremolo and still use the same depth, volume, and speed controls for both.  I was originally looking to do a ramp up mod, but I think this will produce better the sound I'm looking for.  I don't know enough about the EA Tremolo circuit to identify the sine wave oscillator section, therefore I have no idea where I would put the switch to toggle between the two oscillators or where to place the additional square wave oscillator (or sawtooth, or first order low pass filter - someone said the square wave would give me popping in the audio signal).  Has anyone one this before?  Could someone help me figure it out?  Here's the EA Tremolo schematic I used:  http://www.generalguitargadgets.com/pdf/ggg_eat_sc_improved.pdf

Thanks!

[duck_arse]

the ea trem uses a phase shift oscillator, which is not a charge/discharge type setup, so the diode ramp idea won't work on this osc. the parts to the right of C4 form the oscillator itself, C4 removes the offset and applies just the signal to the resistors connected to the gate of the fet.

I've played with the oscillator a fair bit lately, but I have't actually connected it as per the trem circuit, so I'm not sure about the popping effect. if your squarewave voltage was of a similar level as the sinewave, it shouldn't be a problem. I don't think the same controls for 2 oscillators is workable.

[davidjnichols]

the ea trem uses a phase shift oscillator, which is not a charge/discharge type setup, so the diode ramp idea won't work on this osc. the parts to the right of C4 form the oscillator itself, C4 removes the offset and applies just the signal to the resistors connected to the gate of the fet.

I've played with the oscillator a fair bit lately, but I have't actually connected it as per the trem circuit, so I'm not sure about the popping effect. if your squarewave voltage was of a similar level as the sinewave, it shouldn't be a problem. I don't think the same controls for 2 oscillators is workable.

Thanks!  I was afraid the same controls might not be doable.  If the same controls are not workable, is there a mod I can do to change back and forth between the regular sound and a choppier one via SPDT or DPDT switch?  I already have one built and installed in an enclosure that I'd like to mod this way (already drilled for only 3 knobs).  It doesn't necessarily have to change the wave form, but something that will give a choppier sound.

[GGBB]

Did you try just maxing out the depth as discussed in a previous thread?

There's another version of the EA Tremolo that claims to have improved depth which might allow you to get a more choppy sound: http://www.diystompboxes.com/smfforum/index.php?topic=43902.0.

In theory it would be simple to switch C4 and the speed pot between two oscillators (since they are the two ends of the oscillator), but in practice in would only work if the pot values, voltages etc. are compatible.

[duck_arse]

you *could* try reducing R6 slightly and increasing R5 slightly, then switching C3 between its current position and the junction of R5 and the emitter. this may produce more distortion with the gain though.

otherwise, you can build a single transistor clipping stage, and feed the signal from C4 into it, and connect the output through another cap to R7. it may need trimming the output level, it might even work. do you have a breadboard? if not, when are you getting one?

I can try this for you tomorrow if you need, and get back to you in a couple of days.

[davidjnichols]

Did you try just maxing out the depth as discussed in a previous thread?

There's another version of the EA Tremolo that claims to have improved depth which might allow you to get a more choppy sound: http://www.diystompboxes.com/smfforum/index.php?topic=43902.0.

In theory it would be simple to switch C4 and the speed pot between two oscillators (since they are the two ends of the oscillator), but in practice in would only work if the pot values, voltages etc. are compatible.

Well that was my original idea, so being as gung-ho as I am, I drilled the enclosure and built the thing before breadboarding and experimenting, only to find out that the max depth mod didn't really do what I was hoping.  It gave the same sound as just having the depth pot turned all the way up.  I was hoping for a real different sound.

[davidjnichols]

you *could* try reducing R6 slightly and increasing R5 slightly, then switching C3 between its current position and the junction of R5 and the emitter. this may produce more distortion with the gain though.

otherwise, you can build a single transistor clipping stage, and feed the signal from C4 into it, and connect the output through another cap to R7. it may need trimming the output level, it might even work. do you have a breadboard? if not, when are you getting one?

I can try this for you tomorrow if you need, and get back to you in a couple of days.

Haha, yeah I do have a small breadboard, but I jumped the gun on this project and built the thing before breadboarding and experimenting with the mod.  I will rebuild the circuit in a breadboard today and try these mods.  Thanks for the help!

[duck_arse]

also have a look at the tremface circuit over at geofex. the way the fet is set up lends itself to sticking any waveshape you like into it, maybe with a few small mods ....