Stereo Magnavibe - sweet

[ljudsystem]

I've seen schematics of stereo magnatones (the 280) and those only use one LFO as far a i can tell. Don't know how they got both channels to vibe out of phase with each other. Did they flip the phase of the LFO and send it to the second channel?

[duck_arse]

referencing what we'd call "a magnavibe pedal" - the Magnatone M9 circuit shows one oscillator feeding a phase splitter. there is two magnavibe stages in series, equal values throughout. each stage has two swept elements [varistors instead of our ldr's], in parallel, driven anti/phase. the output of the second stage feeds a single output stage.

[thermionix]

I've seen schematics of stereo magnatones (the 280) and those only use one LFO as far a i can tell. Don't know how they got both channels to vibe out of phase with each other. Did they flip the phase of the LFO and send it to the second channel?

Going from memory, the stereo Magnatones I've worked on (M15) had a dry side and a wet side.

[Mark Hammer]

Okay, this is driving me nuts.  I'm trying to install the stereo Magnavibe into a lunchbox amp, that uses a pair of TBA820 power amp chips with a circuit VERY close to this one from the datasheet.  Differences are that C5 is 470uf, C8 goes between pins 1 and 7, and the speaker is 4ohms, rather than 8-16.  As shown, the board includes a pair of 100k trimmers.



The front end is a ROG Tonemender, feeding an op-amp active splitter.  Sending the splitter directly to the amp board, it works just fine.  Using the Magnavibe on its own, to a larger amp, works fine.  Feeding the splitter to the dual Magnavibe and then to the amp results in instant 100db+ chaos, howling, and buzz that does not respond to changes in volume control.  I am at my wit's end.  Sooooo close, yet so far away.  I can't see anywhere that DC might leak through from one circuit to the others.  The 1M5 terminating resistor on the Magnavibe output is somewhat redundant with the 100k input trimmer on the amp, but that shouldn't interfere with anything.

[anotherjim]

Maybe power supply stability is an issue? The TBA820 datasheet shows x1 100uF and x1 100nF caps on the +Vs for a single amp chip. Your board only has 1uF total? Actually, even 100uF might not be big enough unless the power supply or battery is only a short wire run away.

[Mark Hammer]

I'll try that out.

[pinkjimiphoton]

As the graphs show, however (and thanks for that JC), the impact of the phase shift on the audible portion of the guitar signal, is not especially severe for the upper frequency content of the guitar.  One needs to keep in mind that one sort of runs out of frequency content from a guitar above 5-6khz.  So, while there IS treble loss, it has to stand at the back of the line, relative to pitch change.  But the slight modulated treble content doesn't hurt.

add fuzz, and get more like 12k

mark, if building an amp, maybe ya need to decouple each power supply stage separately? like in a fender... use bigger caps, and resistors between them so ya run your preamp stages with the most filtering, and the power amp with less, etc etc

as to speed, i think marcos is right, ya may wanna play with the lfo from a different circuit, as i recall, below a certain point this circuit seemed to stall when i built it a few years ago.

love the stereo idea.. would love to hear it, but i understand what it is to be a hosta...on vacation.

happy holidaze, btw, to you n all... peace

[Mark Hammer]

Right back at ya, Sir James.  We left snow in our city, but down here near Lake Ontario, it's snowless.

I think I will try sticking a buffer stage between the Magnavibe and the power amp.

[pinkjimiphoton]

i would think that to be wise, mark.
but from my limited experience,  building that lunchbox deal for wags, it came down to buffers between stages and decoupling power stages... i'd imagine if the chipamps are running class d, they are gonna suck down a LOT of current and may overwhelm less thirsty stages.
petey twofinger is the guy for the chip amp stuff. he does gigs with all stuff he built, all battery powered out in the woods n stuff, he knows his stuff on that way more than me.
but yeah, impedance mismatches can mess ya up too... so buffering should help that, i'd imagine.
best of luck brother, making popcorn and praying ya can get us some video. sounds like its amaZING!

[reddesert]

The input impedance of the chip amp circuit is no more than 10 Kohms (from R1), possibly less. That could be loading the output impedance of the Magnavibe by a lot. The DC level is blocked by the Magnavibe's output capacitor, but the audio signal that is supposed to be separated from ground by a large value of the LDR in its off state is getting dragged down to ground through the input impedance of the chip amp. A buffer stage in between should clean that up.