A hardware error in the audio line out circuit design of PreenFM2

[humax]

Hi there,

First of all I'd like to say great thank you to all those who contributed in PreenFM2 project and especially to Xavier. Carry on!

After having been playing the synth for a little while I found out that there is a fundamental error made in the audio line out circuit design of the PreenFM2 hardware. The issue appears if I plug my PreenFM2 unit to my patchbay via half-normal mode in parallel to my Roland synth then I bring the the volume all the way down on the PreenFM2 I'll get the volume of the Roland being severely suppressed as well. Moreover, rotating the potentiometer will cause erratic behavior of sound volume of both PreenFM2 and the other source of sound plugged in parallel to it. This won't happen if I do the same sort of connection with any other synthesizer I have so if I swap PreenFM2 with the other Roland none of them will affect each other at all.

I believe this happens due to rotary potentiometer of the PreenFM2 is galvanically connected right to the 1/4 line out sockets which is not good since it makes the output impedance inacceptably low. I'm not an electronics designer but as far as I know the output signal level is supposed to be controlled at the input of the output amplifier such as either transistor or OP but not after it. So the only solution for that so far is just to bypass the potentiometer unless the motherboard design is changed.

Regards, Rostyslav.

[Xavier]

Hi,

Not sure we can put the volume before the op amp beause the audio output path of the preenfm2 contains :
. The sum of 2 Dacs
. anti aliasing low pass filter integrated to the op amp.
. the op amp is single supply (0=>5v)
Adding a variable resistor here will break one of the above.
I think the meeblip use the same circuit.

So you're right, you cannot output an other synth in the same connection. Or if you do, leave the preenfm2 volume to the max.

Xavier

[humax]

No worries, Xavier! It's not a big deal for me at all. Normally volume is supposed to be controlled not on the unit but from a mixing console instead. For instance Yamaha FB-01 doesn't have direct volume control feature implemented on it's front panel so the sound level can be adjusted in there via either menu diving or MIDI CC $07. BTW I'd like to see PreenFM2 to be built as 19 rack unit cause I'm not a big fan of having a desk being crowded with multiple wires run all over it so one day I'll have it done like that. 2U front panel can easily accommodate 2 pcs of PreenFM2 mounted side by side.

Cheers!