EA Tremolo decreasing depth

[Bielorusse]

Hi!

I've built an EA Tremolo last week, but I've noticed something that I didn't expect from the pedal.

When the rate knob is roughly at the middle there's a very nice tremolo sound (although not really deep), but when augmenting or reducing the rate significantly, the depth clearly decreases. I even obtain a continuous volume when rate at minimum/maximum.

Is this normal with this circuit that I can't get a really slow or really fast tremolo sound? Outer that I'm really satisfied with the effect, but if I can arrange it I'd be happy to have a wider range of tremolo

I chose this design http://tagboardeffects.blogspot.de/2012/02/ea-tremolo-modified-rog.html with the LED status indication mod.
The transistors are BS170 for Q1, 2N3819 for Q2, and 2N5088 for Q3.

Cheers!

[Mark Hammer]

I find it can take a bit for the circuit to adjust itself.  And sometimes, you need to turn the rate up or down a bit for it to "settle in".  Annoying, but that's how it works.  I gather it has to do with the time it takes to charge up the 22uf cap.

[GGBB]

Setting the rate at max can sometimes exceed the charge/discharge speed capabilities of the three 1µF LFO caps, which results in no tremolo effect. You can either back off the speed a bit and wait for a second or two (or increase the 1k resistor slightly - e.g. 1.2k - to prevent that), or decrease the 1µF caps. Decreasing the caps will increase the entire speed range, so the min speed setting will also be faster.

Depth has mainly to do with the depth pot and connected 120k and 68k resistors and Q2. The depth pot wiper position sets a ratio of the total resistance which adjusts the voltage swing applied by the LFO to Q2's gate, which in turn causes Q2's drain-source resistance to go up and down thereby affecting the AC gain of Q1. If the min depth isn't right change the 68k. If the max depth isn't right change the 120k. Changing the pot value affects both min and max. Note that zero for the min and max resistor values won't work well.

But normally the stock values work very well with a good moderate range of adjustment for both speed and depth, noting that this circuit is not designed to be either super-fast or choppy-deep sounding. Sometimes Q2 is a problem as JFETs are notoriously inconsistent.

[duck_arse]

some thimgs: try wiring a 10k resistor from the "9V" track to the "LED -" track. replace the 5088 with a 5089 or similar higher gain type. replace the three 1uF caps with 680nF (or 560nF) for faster max speed (and faster min speed). increase the 15k lower right to 27k or 33k (which will slow whatever ranges you have). use one of the removed 1uF caps to replace the 470nF cap on Q3 collector (dumps more osc output onto the depth pot).

good luck (and hello, antonis!)

[edit :] it might be your fet is a bit extreme as well. can you replace it with a 2N5457 or K30A or even a J201 (wash my mouth with soap ....)?

[GGBB]

I used an MPF102 at Q2 without problem, and I believe the 2N3819 is extremely similar if not the same part. I suspect a J201 there will be very hit-and-miss - I've read a few build reports indicating trouble getting that to work.

[antonis]

good luck (and hello, antonis!)
[edit :] it might be your fet is a bit extreme as well. can you replace it with a 2N5457 or K30A or even a J201 (wash my mouth with soap ....)?

Hellow duck_arse..!!
(hope you've washed your mouth - especially for J201..)

<try wiring a 10k resistor from the "9V" track to the "LED -" track>
If you mean to replace the LED, better with a 15k one...

[antonis]

I suspect a J201 there will be very hit-and-miss - I've read a few build reports indicating trouble getting that to work.

I'll add another - personal - one...!!!

As for 2N3819, it's more than O.K. for the specific circuit...
(although I've noticed some lack of response at higher switching speed...)

[duck_arse]

and for Bielorusse, a question - I forgot the best diagnostic tool to hand, the rate led. does it continue to blink at both extremes of the rate pot, or does it stop? if it continues blinking when the tremming stops, the problem is between the osc output and the modulating fet. if the led stop, it means the osc is crook.

I only said the 3819 might be a bit extreme, as the spec sheet says it has Vgs(off) to 8V (similar to the mpf102). and some people not a million miles away from this thread have used and advocate the j201.

and no, I don't mean to replace the led, but to bypass it. I've found 'an' osc that worked better with a 10k from supply to collector, then any old led//clr across that (superbright red with 47k). the osc doesn't draw all its current through the led, then, and maybe gets an extra 1V5 supply to play with.

I'll happily stand for corrections. [mouth and hands soapy cleaned!]

[Bielorusse]

Hi guys!

Sorry for answering so late, first of all to answer your question duck_arse, the LED also stopped blinking at max or min speed. As for the fet, I had to stick to the 2N3819 because the electronic shop where I live didn't have any J201. I guess I'll order one on the internet next time to give it a try!
I didn't particularly want to increase the speed of the effect in general, so I didn't change the 1µF caps. But I wired a 10k resistor from the 9V track to the LED- track, and I installed a 1µF cap on Q3 collector.
Unfortunately I didn't record the sound as it was before these modifications, so I can't make a precise comparison. But now I hear clearly the effect at a reasonably high/low speed, so I'll stick to that for a while! Thought I could still replace Q3 with a 5089, and change the 120k resistor on the depth pot.

Thanks for answers!