Smooth Tremolo

[Rapidrory]

It might be possible to make active bass cut and treble cut filters to split the signal into the high and low parts more effectively, and also double them up as high impedance buffers. A JFET buffer would be pretty easy and only add one extra component, but if it could be done with two and get a better filter setup then that'd be worth the extra component. I'll have a look adding them after tomorrow..

I'm still attempting to come up with a solution that uses as few different components as possible to keep it easy to build, so I'm trying to stick to just J201s and 2N3904s if I can without it affecting the tone too much.

Might also be good to make R30 and R31 into a single potentiometer to adjust the mix between high and low tremolo tones.

[duck_arse]

I tried with yr last set of values and a fet input buffer, found that reducing R21 to 33k (just throwing resistor at it, really) made the effect MUCH more pronounced. the jfets without bias turn the lfo into square waves, so plenty of distortion in that respect.

I'd originally thought of running the low section with a single gang depth as per the original circuit, and the high section full depth, then mixing the high into the low after the balance amp, but my understanding of the workings of this circuit is not real good just yet.

un-balancing the upper and lower resistors in the phase splitter was an idea in a thread on the ersatz fender pro, might be worth a pot or fiddle.

[PRR]

> It works because the coupling capacitor lets it bias itself

The plan in post #1 is "flawed" because there is *no* bias to the JFET gate, it does not "know" where to sit. Bad engineering to leave stuff floating.

In fact I am sure it works OK (JFET resistance goes up and down). The un-biased Gate is pretty sure to leak toward the S and D pins, which means toward ground. JFET resistance will be low. When LFO signal goes negative, JFET resistance goes high. When LFO signal goes positive, first JFET resistance gets a little less then JFET Gate starts conducting current. In this type of one-way rectifier, the *average* level on JFET Gate will tend to go negative, toward about half of the peak-peak LFO signal. If LFO level is well-choosen, this may be just where we want it.

Depending on LFO and JFET parameters, it may be "self-biasing".

If the fit is not so good, LFO amplitude adjustment might be wanted.

This is shown in post #9. The positive LFO peaks are "clamped" to +0.55V (gate diode drop) and the negative peaks driven more negative.

This "should" produce a change of trem-depth in the first few seconds. In post #7 this shows as a change in trem step width.

I'm not sure the 330K to the Gate is critical or even needed. Zero added resistance (just the 10K of the LFO) would diode-bias 33 times faster, but we don't care, but it saves a resistor, but that resistor may be a helpful layout jumper.

If the LFO is left running all the time, all this re-biasing is done before you have the guitar in hand ready to play.

[Rapidrory]

I do have a 1meg pull down resistor on the JFET gate in the physical version, however I forgot to add it to the diagram in post 1 (I tried to update it but for some reason wasn't able to edit the first post). added it as I didn't want to leave it floating; bad engineering, as you say.

You're right about the 330k being unnecessary, but it's the first time I've ever used JFETs so I wanted to err on the side of caution in regards to running positive voltages through the gate; I didn't know how much current it could handle so just went for basically non. In it's present setup the time taken for it to bias itself is pretty irrelevant by the time the pedal's in use.

In case anyone's interested; here's the modulated signals before mixing; (high frequency in red, low frequency in green)

[Rapidrory]

Here's the latest update including the points from the last few posts. Missed anything?



EDIT: It should also work if R20 is put back to 68k, R22 is removed, and the Depth pot R27 is replaced with a 1 meg variable resistor. This'd create a potential divider between R27 and R20 without affecting the bias of the signal splitter.

[tubegeek]

What made you put in an input buffer? That's totally not necessary!

(Ducking, running)

[PRR]

> wasn't able to edit the first post

Some forums allow edits "forever".

This can be abused, so some forums don't allow edits.

This forum gives you 24(?) hours. Enough to patch-up your typos.

For a lost resistor, just mention it later. If you are in a real jam (wrote something you regret) and missed the edit cut-off, use "report to Moderator" and ask that the distressing words/post be removed.

[duck_arse]

I tried the depth pot today with the j201's, and it didn't really work. the output level just followed the pot down. and I swapped the j201's for j210's, just for the grabbings, and it completely changed the tonal workings of the trem. so it seems different fets will sound different, a note for the schem.

haven't tried the mix pot yet, but I did try modified filter values, based on midwayfair's cardinal values. they worked quite well, but need fiddle.

[Rapidrory]

Hmm, the simulation seemed to show the depth pot working, but then it's hard to test as you can't change it whilst the sim is running so direct comparisons are tricky... Could use a double potentiometer along side each fet (like in the original schematic for the basic tremolo), but that means you need a double pot.. alternatively, separate pots could be used, which would allow you to adjust the depth of the high and low tones separately which could be cool..

Something else I want to try is higher order or active filters to give a sharper roll off. Should make the effect more pronounced.

What component values did you end up with in the filter? I've just been going for matching values so it's not missing too many frequencies in the middle.

[duck_arse]

currently, the scribble shows 10k at R21, 5n6 at C13, 1n5 at C12, 1M at R24 and a 1M added across the channel of the other fet as well. my calcs suggest the low pass is 2800 and the high pass is 106Hz. I have no R13 or R14 fitted, or R16. and a source follower for the buffer.

I'm also switching like the cardinal between trem, vibe, and harmonic modes, just to hear. the pots under the fets means the trem shape will always be square, but it might end-up all the level control we get.

[Rapidrory]

This is the bode plot for the filter values you gave; pretty good overlap actually. The red and green lines show the frequency response of a second order version of the filters (just stacked two filters in series).



I ran some tests with both 1st and 2nd order filters using your values at maximum depth:

1st order: https://drive.google.com/file/d/0BzfEd3tVJdxRN0xWdXFnaklNQ3c/view?usp=sharing

2nd order: https://drive.google.com/file/d/0BzfEd3tVJdxRd2hqVnZIdEVGOU0/view?usp=sharing

I actually prefer the 1st order version; the 2nd order version is a bit too pronounced...

EDIT: I found a spice model for the J201 JFET online and installed it (previously I was just using a 2N4117). I see what you mean about it being a very square tremolo; very different from the subtle tones of the 2N4117. For comparison, here's the audio using a J201 (1st order filter): https://drive.google.com/file/d/0BzfEd3tVJdxRdGFFTlVrLV9Kcjg/view?usp=sharing

[duck_arse]

I only listened to your first wav, due to googoo and download speeds, but I hear nothing like that sound when I "test". (any chance I can get those files without going to googoo.drive or whatever?) nothing near as subtle. I did more shuffling of values today, and added the mix, and swapped fets. prr has some curves in "another thread" that explain a lot of what I'm hearing.

C13 can vary greatly, I'm back at 10nF with R21 at 12k, f=1300Hz, and C12 at 560pF, R24 1M, f=290Hz. the fender originals are set to 1300Hz and 72Hz. I've got R30 back at 56k and R31 back at 47k, both following 100nF caps, and 100k as the mix pot. it works much like a tone control (see prr's curves), and helps cover the current lack of depth control. it goes with the trem-vibe-harm switch quite well.

fets. as paul noted above, some aren't going to work. the j201 doesn't care, it just goes square. as you get to higher Vgs's, the trem turns to smaller peaks and wider troughs. with mpf102's in, you get a very good helicopter impression. no guitar tones, just heli whoop. 2n5486, mpf106 also don't work. 2n5457 and 2sk30a-Y dropped straight in, the 5457's did a nice watery thing, and good depth. J210's were chop-chop, no middle fill, and often sounder like 2 trems and still a trough between. they also had a strange spot mid mix, where nothing seemed to be happening, but at a fast rate. your voltage levels from your phase splitter will make all my observations moot on your breadboard, with your fets.

and because of the mix control, you can use a choppy 5457 in the low pass stage, and a smoother j201 in the high pass, and mix between. or your choice. but only if you are prepared to do endless breadboarding. it is becoming quite versatile. [I really need to find someone who can play guitar, tell me if any of these sounds are useful.]


and people thought I'd waste my 3000'th. pah!

[amptramp]

I ran into a trick with a modulator for a signal generator where a Wein bridge oscillator had to be stabilized by a JFET while minimizing distortion.  The first schematic here:

http://www.ham-radio.com/k6sti/st-1000a.htm

shows a JFET level control that uses equal resistances going into the gate and connected from the gate to the drain.  It is a different application, but it may be ideal for what you want - tremolo without much modification of the waveform and the resulting harmonic generation.