crystal oscillators?

Keith collins · Post by **Keith collins** » Thu May 11, 2006 11:13 am

i recently got a prodigy. after playing my rogue for a year, the prodigy is pretty wonderful... with the rogue i had a lot of problems with the 2nd oscillator drifting. some live shows i couldn't get it in tune at all. the prodigy so far seems to be doing better, though i'll be surprised if it doesn't go all to hell the first muggy bar we play in.
so when i brought the prodigy in to get cleaned and checked out, i asked what tricks could be done to fix an unstable oscillator.
the tech mentioned replacing the original oscillators with "crystal" oscillators. now, i'm no expert, but i assume that would completely change the tone of the moog... still the idea of a moog that never goes out of tune is tempting. does anyone know anything about "crystal" oscillators?

analoghaze · Post by **analoghaze** » Thu May 11, 2006 11:52 am

Hey... I am going through school, studing electronics. I actually cannot tell you much but I will tell you what I know.

This is a direct quote from the Basic Electronics 9th Edition textbook (Grob/Schultz)
(there is a section just on crystal oscillators)

Crystal Oscillators
In this type, a piezoelectric crystal is used as a resonant circuit, replacing an LC circuit. The crystal is a thin slice of natural quartz or a synthetic material. The piezeelectric effect means that the crystal can vibrate mechanically when exited electrically and produce ac voltage output. The resonant frequency is fixed by the size of the crystal. The advantages of crystal is its very high Q as a resonant circuit, which results in good frequency stability for the oscillator.

analoghaze · Post by **analoghaze** » Thu May 11, 2006 12:00 pm

The crystal is a thin slice of natural quartz or a synthetic material.

If you decide to make the change, stick with real quartz. The last thing you want is a synthetic crystal.

I am deeply intrested in things like this, and will talk with an instructor tonight.

I am not sure, but replacing these crystals to quartz may just effect the timing of the oscillators, which would keep them at a set frequency, and therefore, keeping them in tune.

But really, if your keybord is drifting so bad that it is bairy usable, it is not a bad idea to get it fixed.

I should be studing for finials.

Keith collins · Post by **Keith collins** » Thu May 11, 2006 1:56 pm

thanks,
i appreciate the help. i guess i'm most interested in how this would effect the sound. i'm willing to spend the money, but if it doesn't sound like a moog when it's all said and done, then it would be pointless. has anyone done this kind of mod to their moogs? how vital are the oscillators to the "moog sound"?

Dr.T · Post by **Dr.T** » Thu May 11, 2006 2:18 pm

I don't see how a crystal osc could work in a Prodigy at all. The VCO's in analog synths are designed to cover a large frequency range (more than 5 octaves in a Prodigy) at the cost of some stability while crystal oscs do the opposite: they are extremely stable but cannot cover any substantial frequency range (typically MUCH less than a semitone). Plus they mostly operate in a very high frequecy range and need to be "divided down" to be used in the audio range ..... this also yields a sound which is not what you have in the Prodigy right now.

Crystal oscillators were used as central basic frequency sources on all these "divider" synths like the Polymoog and the Arp Quadra. All necessary audio frequencies were created in parallel at the same time by appropriately dividing the frequencies of one or two high-frequency crystal "top oscillators". These frequencies (sawtooth or square/pulse waves) could then be tapped to create as many notes as desired at the same time.

This is an entirely different principle compared to the monophonic VCO-synth like the Prodigy where the output of one or two or three oscillators are in the audio range and directly feed VCF, VCA and output.

On a slightly different note: I played a Prodigy many years ago for quite a while and never ever had any tuning problems with it - I had to give it 5 minutes after powering it up to come to terms with itself and warm up, but then ist stayed in fact quite nicely in tune. In your place I would scrap the plan to replace the oscillators and trust the Prodigy! It's really a very nice little machine!

Take care & have fun,

T.

Keith collins · Post by **Keith collins** » Thu May 11, 2006 2:55 pm

yeah, the little bit i read about the crystal oscillators, it seemed i would run into problems, but i figured the tech i talked to would know what he was talking about... guess not.

in the practice space the prodigy is running like a champ. i guess we'll see what happens at the next show we play. analog sure does keep things exciting!

misterhemi · Post by **misterhemi** » Thu May 11, 2006 9:26 pm

I'm also an electronics person.

A crystal oscillator is used to produce a FIXED frequency.

That would only apply if this were some type of frequency dividing synth (like some early organs) or if you were using the crystal oscillator to provide the clock for a microprocessor, UART, etc.

It is not applicable to analog VCOs.

rogs · Post by **rogs** » Fri May 12, 2006 8:42 am

There is the other option using crystal oscillators -- frequency synthesis using a phase lock loop --- but again not really suitable for analogue music synthesisers!

Anyone who has experimented with musical VCOs will know about the very convenient relationship between a linear change in Vbe causing a doubling of the collector current -- linear to log voltage conversion - nice and simple!

They will also have discovered just how senstive to temperature that characteristic of a transistor is --- just blowing on a non temperature compensated junction can shift the oscillator half an octave!!

The Prodigy used heated chips to overcome the problem - by using a transistor array, and then using one of the elements to heat the chip so that changes in room temperature have little or no effect.

I wonder how the LP controls its 'ultra stable' oscillators ?

MC · Post by MC » Fri May 12, 2006 9:01 pm

Yep, not only are quartz crystals at a fixed frequency, they are in the Mhz range, way out of audio range.

Keith collins · Post by **Keith collins** » Sat May 13, 2006 1:25 pm

well, then obviously that tech had no idea what he was talking about.
thanks gang.

LWG · Post by **LWG** » Sat May 13, 2006 2:42 pm

Hello,

Is it possible he may have been throwing some sarcasm at you?

Regards,

Lawrence

electrical_engineer_gEEk · Post by **electrical_engineer_gEEk** » Mon May 15, 2006 1:27 pm

well....you're not completely right when it comes to the MHz ONLY comment.......

http://www.digikey.com/scripts/DkSearch ... t=33227435

there are plenty in the kHz range.....but they are all above 20kHz & the hearing range

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crystal oscillators?

crystal oscillators?

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