From yahoo Sent Thu, Feb 21st 2019, 13:10
From yahoo Sent Thu, Feb 21st 2019, 13:10
Version 3 of the A-131 is based on the THAT2150 and the exponential scale is predefined by the THAT2150. The module is a plain standard application of the THAT2150 and there is nothing special. The trimming potentiometer "GAIN OFFSET/P6" is used to define the max. amplification (of amplification offset). In the factory it's adjusted so that with Gain control fully CW (and without external CV) the "amplification" is about 1. Another trimming potentiometer "OUTPUT OFFSET/P7" is used to minimize the CV feedthrough of the VCA. The control scale of the module is defined by the control scale of the THAT2150. I can send you the THAT2150 data sheet and schematics if you tell me your email address. I suppose that your "problem" has to do with the exponential scale. As a rule it's always a problem with exponential VCAs because an exponential VCA does theoretically never reach zero amplification (but in reality the signal disappears eventually in the noise floor). On has to find a compromise between the exponential scale and the minimum amplification (i.e. the amplification with manual GAIN control fully CCW): If the exponential scale is too "flat" the amplification with GAIN fully CCW is not small enough (i.e. there is still some feedthrough @Gain=0). If the amplification with manual GAIN control fully CCW is very small (e.g. -80dB or 1/10000 with Gain=0) so that no audio feedthrough is noticeable it takes a while until the audio level becomes audible because at center position (5) of the gain knob it's still about -40dB (or 1/100), and even at 75% of the rotating angle (about pos. 7-8) it's still -20dB (or 1/10). In the range 8...10 then the level increases suddenly from 1/10 (-20dB) to 1 (0dB). Hope this helps to understand the exponential VCA Dieter Doepfer > -----Ursprüngliche Nachricht----- > Von: xxxxxxx_xxxx@xxxxxxxxxxx.xxx [mailto:xxxxxxx_xxxx@xxxxxxxxxxx.xxx] > Gesendet: Donnerstag, 21. Februar 2019 13:08 > An: xxxxxxx_xxxx@xxxxxxxxxxx.xxx > Betreff: [Doepfer_a100] Re: Doepfer A-131 exponential VCA - calibration > procedure > > > I have a degree in maths, and I studied electronic engineering > for a while. I also own more synthesizers than I probably should > - so I understand the concepts involved. > What I mean is, the response curve of this one, for gain vs CV, > is far from desirable. I get a very gradual increase, then a > very sudden one. I haven't attempted to graph it. > I have seen that website page before, and read the manual, but I > haven't seen anything official about adjusting or calibrating > one. I have read that there are preset pots to adjust, but only > vague details. > It seems the one I have is version 3, and there are only two > presets / trim pots, labelled Gain Offset, and Output Offset. Is > there anything to be gained by adjusting those? > I own multimeters, basic oscilloscopes, soldering irons, and a > large heap of resistors, capacitors and various other components. > I am quite prepared to modify it, if necessary. Is there a > schematic available? > > - Andy > ------------------------------------ ------------------------------------ ------------------------------------ Yahoo Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Doepfer_a100/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/Doepfer_a100/join (Yahoo! ID required) <*> To change settings via email: xxxxxxx_xxxx-xxxxxx@xxxxxxxxxxx.xxx xxxxxxx_xxxx-xxxxxxxxxxxx@xxxxxxxxxxx.xxx <*> To unsubscribe from this group, send an email to: xxxxxxx_xxxx-xxxxxxxxxxx@xxxxxxxxxxx.xxx <*> Your use of Yahoo Groups is subject to: https://info.yahoo.com/legal/us/yahoo/utos/terms/