Magnavibe question

[midwayfair]

Lots of info in here about what happens when you go from one stage to another. (Blue Warbler.)

https://docs.google.com/document/d/1aM2U7PL7NiMd_RaaV-7s5epvqRBPM0ZSbdbKHx96tRw/edit

You can just leave off the envelope section if you don't want it. There are single- and two-stage versions in the build doc. The LFO is also better than the original's IMO. I also give you instructions on how to decrease the pitch modulation aspect to get a slightly different effect than the Magnavibe.

Also, CultureJam's Duo Vibe uses a different LFO and has input and output buffers, but good luck finding it. It was part of a fundraiser for their pedal company.

[Kipper4]

Thanks Leo I'll check it out.
Jon you really put the work in on that one. I love the transistor envelope detector. That's something I haven't tried using yet.
All this is gonna keep me busy for weeks once I get over this new wah experiment that's hogging my breadboard.
Brilliant stuff.

[Kipper4]

Leo I found another thread over there from Fred Briggs. With contributions by culture jam, Bajaman and even you.
Thanks
I'm not sure it ever came to a finished product but it's some good food for thought.
Where my shovel. Stand clear I'm gonna dig some more.

[Kipper4]

After a little more digging.
I found a schematic for the vibrochamp that uses a similar arrangement to the magnavibe pedal albeit with tubes.

Here it is


http://vibrochamp.org/schematics

Then I go look how the magnetone 280 looks

Here

http://www.magnatoneamps.com/schematics/magnatone_280B.png

So this one is a stereo tremolo I believe.

One PSO driving two concertina splitters.

Question.
Would it not be possible to drive two concertina splitters with one PSO (that one PSO using a single led and dual ldr) then sum the two concertina splitters via a mixing stage to a mono output?

I guess I just want a more intense vibrato. I'm fine with a subtle tremolo, but like Olly Twist I want more, vibe.

Does having a slightly wobbly sine wave help? Or can this be reconciled with playing with the vactrol led to ldr fitting distances ?

Am I just treading water here or catching a wave with some umph?

[midwayfair]

the vibrochamp isn't doing remotely the same thing as the Magnavibe/Easy Vibe etc. Magnatone made an amp with the same type of tremolo, but that's NOT the Magnatone amp that people are imitating. I forget which model it is, but it uses varistors. That Magnatone is just stereo.

In the vibrochamp, a tube creates an oscillating voltage which then wiggles the bias of a particular tube to rhythmically reduce gain. It's amplitude modulation only, and there is only ever one audio signal involved.

The Magnavibe/Easy Vibe/whatever-the-amp-is uses two audio signals: one in-phase from the emitter/cathode, and one out of phase from the collector/anode. They are mixed together in a rhythmic manner and the frequencies cancel imperfectly, resulting in a pitch-shifting phase artifact.

If you want more intensive vibrato, your only option is more stages in series operating roughly equivalently. The waveform is mostly irrelevant as that only effects where the notches fall in the cycle -- what matters is how deep the notches are. The only way to deepen the notches is by cancelling the signal further. This is why I (and others) recommended two-stages.

[Kipper4]

Thanks mate.
I'll be back home tommorow so I can get to looking at some of this.

[Kipper4]

Ive breadboarded the Magnavibe tonight.
Some of my findings. the vibe is too subtle. I know you told me so
i modded it with some tweeks from the wobbletron.

The mpsa18 transistor seems critical to operation.
I would like to know if theres a good reason why the transistor is critical?

I figured its just a jelly bean, a 2n5088 would cut it, after all its in the same gain range as the MPSA18 according to the datasheet. but no dice.
Why ?
I'm not done with it yet.

[Craiz]

As a further question on the Magnavibe - is there a way to smooth out the waveform to not be so choppy? It does what I'm guessing is a triangle wave and I find the peak very abrupt and a little disconcerting in an otherwise very nice sounding pedal.

[PRR]

> transistor seems critical

Critical how? Doesn't like your shirt?

Does it have any voltages on it? Normal, and with one of the timing resistors shorted so it is stalled.

What LED in it? Lower-Voltage (Red) is better.

[Marcos - Munky]

a 2n5088 would cut it

I've used a 2N5089 and it worked pretty well.

[Kipper4]

My transistor hates my Batman pants.

The 5088 wouldn't start up let alone stall.
I've since tried it again. It ain't having it with these values.
I'll google it up and have a look I'm sure I saw some a while back. Rick holt did one.
Least ways MPSA18s ain't a problem.

I have a bunch of made up roll my own vactrols in my breadboard box, not all are marked up plus a couple of nsl32.
I'll make up a red one later to try.
Thanks Paul.

Craiz from all I've read on tinternet phase shift oscillators with 3 RC filters produce a sine wave.

[Kipper4]

a 2n5088 would cut it

I've used a 2N5089 and it worked pretty well.

I'll give that a whirl too
5089s have a slightly higher hfe 1800 max as opposed to hfe 1500 on the 5088
IIRC

[Kipper4]

Yep. duhhh
It's official I'm a bell end.
I wrote the pinout back to front on my schematic printout

[Kipper4]

I'm still messing on the breadboard.
I tried several shrink wrapped roll my own vactrols with varying succes.
Tried a massive old ldr from an alarm system which was a smidge better than alot of them
Tried every GL55 series i had but im still not getting that wobble i want.
tried varying distances between led and ldr, I bet mrs Kipper thinks im bonkers keep switching the room lights on and off between test..
I'm starting to wonder if the PSO is too perfect and it needs a sine with a kink in it if that makes sense.
Ive yet to look into how the Maggie does it. I'm sure I read Jon Patton commenting on it some where.
Then I think I'll do some trivibe experiments and easyvibe stuff.
thanks guys
Rich

[samhay]

You can play with the value of the 100n cap too - remember, it is forming a filter with the LDR, so you want to find a value that puts the corner frequency somewhere you can hear it when the LDR is in the middle of its range.

[Kipper4]

Thanks Sam I'll do that.
I've messed with it once and made it 220nf maybe I should have gone smaller.
Don't know why but I thought it would make the effect a richer tone. Not noticibly though so I replaced the 100nf.
Oh I remember now where I saw that..... The wobbletron.
Apart from breadboard experimentation it would be difficult to determine where the ldrs middle range is.
In my experience when the sales data say an ldr goes to 1M dark this can vary somewhat from piece to piece. Although I could guesstimate it. As a starting point.
I remember in Fred Briggs paraphrase he uses 22nf on one of the concertina splitters and 1nf in the other.
Cheers man

[midwayfair]

Low resistance causes more cancellation. The caps control the notch to some extent. Your LFO's ability to drive the LED is also a bottleneck for depth.

I talk about this iin the BW document and include notes on about a dozen LDRs and vactrols.

[Kipper4]

In my ldr trials I found that a higher dark resistance didn't fair to well once I got up to 3M dark and beyond it started to fail. The 10M dark really didn't work almost like the lfo stalled, which of course it didn't.
I'll go look that doc up Jon.
Just so you know I replaced the 1.5M in the PSO with a 2M2 and the speed pot with a 50k I found it helped the speed range of the PSO.
I really appreciate your patience and ultimately I think I'm persisting with something you have been through and come out the other side of with a better understanding and knowledge of than I may ever have.
I wasn't even aware of the phase relationships of RC filters before I started this.
Although I can get it going, passing sound and tremoloing it just doesn't have that thing that I've heard other get.

Ergo

http://youtu.be/uE6lj29CJsA


So I guess it's do able.....

Next stop in the mornings trials will be the cap.
Cheers dude.

[Kipper4]

Good job you said that guys. I've been wondering why i wasnt getting the wobble turns out i bypassed the 100nf cap with all the messing on the breadboard. its sorted now im getting a nice wobble.
i tried a 20nf not as effective maybe i'll go bigger and see how that works out
cheers

edit the vactrol im using has the std Bitsbox ldr (1M dark) and super bright yellow 5mm led.
now i can retrial all the other i made for the BB too. see what works best for me.

edit edit
that didnt take longSuper brights work best. Ill get some super bright clears and try them before i perf ,box it

[Ben Lyman]

Thanks for all the help here guys, especially you kipper, this pedal is so quick and easy to make and it sounds great. I used 1 yellow superbrite, filed flat, super glued to an LDR from Tayda and heat shrink tubed. Both transistors are 2n5088's and a couple of very minor resistor changes.

EDIT: I forgot about my most notable change  I used a 10n on Q1c instead of 100n, I think it made it sound better, more shimmer, less thump