Smooth Tremolo

[Rapidrory]

Two years ago I attempted to build a tremolo pedal for a friend. I tried various different designs that I found online, but found they distorted the signal and/or added a throbbing sound to the signal when there was no input signal. I ended up with various half working designs, but none that I was really happy with. Last weekend I decided to re-visit the problem, however I still couldn't find a design which sounded right. It may just be that I failed to get the designs to work correctly, but either way none of them gave the effect I wanted. So instead I sat down and worked out my own. It's a very simple circuit so it may well have been done before, but having googled around I couldn't see one the same it. It's based off the Trill Tremolo circuit, however uses a JFET to avoid distortion, and adjusts the input signal rather than the base current to give a more linear depth adjustment. Small changes, but I found they make a big difference to the sound.



The depth goes from completely off to full cut, and the fact it isn't modulating the gain and that it's using a sine wave LFO means it gives a very smooth and natural sounding effect. The balancing amp is set so that when the depth is set to zero, the input is at the same volume as the output. It also has almost no throbbing sounds on the output.

I dug out the original pedal from two years ago and replaced the insides with the new circuit; it's looking a little worse for ware having been kicking around the bottom of a box in my uni room :L



Below is the old circuit (left) compared to the new one on the right. I mostly use surface mount components these days so it was gonna be neater anyway, but even so it's a big improvement on size and sound.



But anyway, I hope someone finds this useful; I'll hopefully get some proper test audio from it on Sunday.

[amptramp]

It seems similar to the Univox U-65RN amplifier that has a bipolar transistor in place of the JFET:



You can get a larger readable image here as a .pdf file:

http://www.schematicsunlimited.com/u/univox/univox-u-65rn-amplifier-schematic

It shows the tremolo oscillator on the lower left  feeding a bipolar transistor that is connected directly to the input.  I have one and it works.

[duck_arse]

looks interesting, I've just given my BB 2 sideways glances .... looks similar principle as the vico vibe. any reason for choosing a j201?

[Rapidrory]

Any reason for choosing a j201?

It was just the only JFET I had in my parts box at the time.

The reason for using a JFET instead of a bipolar transistor as I found that it distorted the signal as can be seen below (the shape and voltage varies with base bias voltage and input voltage, but this gives an idea) :

 

The JFET by comparison didn't have any of that.

In reality, if it's set up right then the distortion only occurs when the signal amplitude is being pulled down by the transistor, so it's not very audible. But the JFET removes any problems with setup and just made for a cleaner effect. I had spent several hours attempting to get the bipolar version to work before swapping it out for a JFET.

[~arph]

Maybe a buffer in front to raise the input impedance?

[Rapidrory]

Could do, though it might be easier just to increase the input resistor values proportionally; again, they were picked based on what I had in my parts box. It's not too low so is working fine atm, though it's possible that some of the finer detail of the signal is being lost.

It could also do with a 1 Meg pull down on the input on the schematic; it's there in the pedal, i just forgot to add it to the diagram.

[duck_arse]

hmmmmm .......

I've never used the fet as the modulator except with great gobs'o gate bias. I'll be interested in the envelope shape, any pics of yours?

[Rapidrory]

It works because the coupling capacitor lets it bias itself, as can be seen in the simulation below:



It says it takes 30 seconds in the simulation, but I've never actually noticed much change in the sound as it 'warms up'.

The image below is taken from an actual recording showing that it settles to about a 50:50 ratio:

[duck_arse]

mmmmm, I rekon it might be biasing on leakage from that 10uF. what happens when you put a 1uF poly there, trubble?

I was thinking about this trem and attempting to "harmonic-ise" it. interested? possible you think?

[Rapidrory]

It won't be from capacitor leakage, as the capacitor used in the simulation is an ideal one so has no leakage. I believe it's actually gate leakage in the JFET, as can be shown in the image below; this is taken from the gate of the JFET (The previous simulation was showing the other side of the 330k resistor).



Definitely interested; how'd you suggest going about it? I've already given this pedal to the guy it was made for, but I can easily check out new designs in LTSpice..

[duck_arse]

well, except for having no idea, I do have an idea, which doesn't involve a dual pot. I'll have a swing at something tomorrow, maybe rough a circuit diagram. see what goes. I don't have any circuit sim stuff, I don't think I have the processor power/download speed/operating system/stomach/need for one, really, but could use all the help I can get.

[Rapidrory]

Well I can simulate most discrete component (including vacuum tube) analog circuits and run audio samples through them to see how they sound with LTSpice, so if you can work out a rough circuit diagram I can draw it up and see how it'll sound :L

[Rapidrory]

Ok, so I actually had a little go myself and this is what I came up with:



This is literally the very first draft, and the filter values were picked almost at random, so a lot of work is probably still needed, but this gives a decent skeleton with which to work (the type of JFETs in the diagram are only used as I don't have models for J201s).

It sounds pretty decent too; tremolo starts about 6 seconds in:

https://drive.google.com/file/d/0BzfEd3tVJdxRS2xONHpHRkxGWkU/view?usp=sharing

[tubegeek]

It sounds pretty decent too; tremolo starts about 6 seconds in:

Delicious!

You're saying you built with J201's though the schematic says 2N4117, right?

Is the track a sim or the actual circuit you built?

EDIT: I'm a dork. You built one, you sent it on, then you redesigned it for the harmonic trem and sim'ed that.

Reading comprehension FTW!

2nd EDIT: I would tend to agree with the comment above - you'll want to load a guitar with a higher impedance than the 50K or so you have there. Consensus seems to be that 1M is a better value for a guitar straight in. So either scaling the filter sections or a buffer would seem to be a good idea. The reason it's not showing up in the sim is probably that you aren't modeling the guitar pickup ahead of the input.

Take a look at the front end of Gus' sim here:
ASDF

Everything to the left of the spot marked "input" is a simulation of the guitar pickup's parameters.

[Rapidrory]

Yeah, I know input impedance should be ~1M, but I just hadn't scaled it for the sim; as I say it's a very rough first draft atm. I'm planning to increase the input resistance rather than add a buffer as I'm trying to keep parts count to a minimum with this one... though i guess it may add a lot of Johnson noise with the high resistance. Have to see how it goes.

I'm also gonna see if it's possible to switch back to 2N3904s instead of JFETs for the modulators just to try and make this build as simple as possible for other people.

[duck_arse]

no, no, no, stay with the jfets. you need a new name, though, it has about the steepest, squarest envelope on a trem I've ever breadboarded. real deep. you -can- round it up a bit, if you want to tune the fet to the osc signal with a voltage divider.

the phase splitter simplifies to C2-4u7, R22-56k, drop R23 and C11, R1 about right, R20 to 68k, and R2 and R4 to 10k. 1M gate resistors means C1 and C8 can be much smaller.

as the first diagram stands, the (my) balance amp distorts the bottom half of the signal. the input Z of this stage is around 38k, which is a bit low, but workable with C7 increased maybe. perhaps an R to ground like in the big muff will improve.

as for the modulator, the j201 works surprisingly well straight off. I tried a 2n5457 (yes) and mpf102 (NO!) because they were at hand, I'll run through a bunch of others as time permits. (and the depth pot seems to need either a reverse log taper, or a log taper wired backwards at the very least.)

[how big is your audio file?]

[Rapidrory]

Yeah, tried it with bipolars and it was pretty terrible so definitely sticking to the JFETs :L

R22 and R23 were set up as a potential divider to lower the signal so as not to distort the output, but I'm guessing that you can combine the two functions using the bias resistors as the other half of the potential divider.

I've updated the sim to your suggestions and it works quite a bit better with those values Sounds roughly the same, but is better electronically

The balancing amp's gain needs adjusting down to match the input values a bit better, which may help with the distortion. Not much more I can do on this till after tomorrow unfortunately; apparently family want to see me at some point over Christmas   

The output audio file is a 2MB .wav file. Also, if you have any better audio samples feel free to send them over; I only have ones I can find online :L

[duck_arse]

if you gaze upon RG's ersatz fender-pro circuits, or the pareidola (what now?) circuit, or the cardinal, you might get some more splitting/filtering ideas. the high pass/low pass aspects baffle me, they might come clear on a sim, tho.

[edit :] and merry xmas, everybloody!

[electricco]

Very good sounding sample !  ..and nice project too !!

some thought... 
in Q3 could be used a 2N5089 like in some other design for compressor...
while for Q2 using 2N3565 should improve the sound contents.

Looking forward for updates.
cheers

[midwayfair]

I'm going to echo the suggestion for an input stage of some sort, though. A buffer should be sufficient in this design since you're using the make-up gain stage instead of a gain stage for each band.

I've had terrible luck getting FETs to work without added distortion, though. Is there something in this one that helps with that?

(Also, you may find that you want to lower R25 some to keep the low band from overpowering the treble band.)

Also: Can R23 and R22 be your depth control?