PreenFM > preenfm2 and preenfm3

Audio Output from SMD version

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Xavier:
It's difficult to compare without knowing exactly what your patch is.

With the use of Velocity, multiple carrier operators, 4 instruments, several voices on your instrument, the resulting amplitude of one single sine wave can be very low.
So what you seem to hear is the granularity of the DAC.

The pfm2 DAC is 12 bits + 6 bits that are added with the usage of a divider resistor  bridge. Bad choice of resistors here can increase this effect.

Check with :
. Velocity 0
. Algo 1 (to have only 1 carrier operator).
. Instrument 1 : 1 voice
. Instrument 2,3,4 : 0 voice.
Is it better ?

This sound also should disappear as soon as you start increasing the modulator indices (IM).
If that's the case, stop focusing on a single sine wave and make real FM !  ;)

ericd:
Hi Xavier

My patch is exactly as you suggest:  only a single carrier, all other OPs turned off, velocity at 0, no IM and no modifiers assigned in the matrix. Just a pure single voice sine wave.

Is the sound in the recording as good as I would get from a PreenFM2 using just a sine wave? I understand this is not really the intended purpose PreenFM2 and can get more pure sine waves from other machines. ( I just love the sound of pure sine waves!)

But I really just wanted to know if it was an issue with the SMD version, or with my particular unit. If it is normal behaviour then I can stop worrying about it!

Xavier:

If you have only 1 voice and the other instruments have 0 voice (the DAC is shared between all instruments + all voices per instrument), this digital noise we can here is very loud.

ericd:
OK, so it is related to what you said:

"The pfm2 DAC is 12 bits + 6 bits that are added with the usage of a divider resistor  bridge. Bad choice of resistors here can increase this effect."

Can I find the information somewhere on best choices of resistors to see if this is a problem?

Xavier:
I just checked and I can hear that kind of sound with pure sound wave long release with my preenfm2.

The resistors are the 2x 115k and 1.8k on the picture here (near the mpc4922) :
http://ixox.fr/preenfm2/the-pcb-r5/pfm2_06/#main

115/1.8 = 63.88 (~64) = 2 ^6.
That's how the 6 bits of the second DAC are added.

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