Author Topic: N00B about digital  (Read 2766 times)

a soBer Newt

N00B about digital
« on: February 02, 2012, 10:24:31 PM »
I would like to use the PWM output of my Arduino to drive the control voltage of a THAT 2181.  I know that the PWM is 0 to 5 V and the control voltage for the THAT 2181 -180 to + 540mV.  Is there a way that I can do this?  I was thinking that I could use an op amp with a Ref voltage?

AdamM

Re: N00B about digital
« Reply #1 on: February 03, 2012, 12:41:02 PM »
use a potential divider with 8.2k at the top & 1k at the bottom. Your PWM will then have a peak voltage of 543mV...

a soBer Newt

Re: N00B about digital
« Reply #2 on: February 03, 2012, 01:12:56 PM »
use a potential divider with 8.2k at the top & 1k at the bottom. Your PWM will then have a peak voltage of 543mV...

Thank you now how do I drive it down to -180mV? Because right now at 0v from PWM I would be at unity gain from the VCA I would like to be able to get at least +10dB  

I think it might be simpler to just have one more gain stage after the VCA and then not have to worry about getting the PWM to go neg. Sweet I think I am in business!
 

PRR

Re: N00B about digital
« Reply #3 on: February 12, 2012, 06:01:22 PM »
Page 8 of datasheet.

Hold one Ec pin at +180mV. Drive the other Ec pin zero to +720mV.

Wallah! The cross-pair "feels" -180mV to +540mV.

Note though that 540mV is 90db gain/loss. You NEVER need or want that much. 60db loss is often plenty. (For trem, 20db may be ample.) And even 30db gain is often excessive.

Figure out what you really need.

The control port is exceptionally sensitive to garbage. Also wants low impedance. With a more realistic 300mV swing, and 5V available, try 1K:68 voltage divider with 100uFd to ground at the Ec pin. If you need fast change-of-gain this may be slow, but it is a compromise against PWM hash and other digi-trash modulating your precious music.

a soBer Newt

Re: N00B about digital
« Reply #4 on: February 12, 2012, 06:49:40 PM »
Page 8 of datasheet.

Hold one Ec pin at +180mV. Drive the other Ec pin zero to +720mV.

Wallah! The cross-pair "feels" -180mV to +540mV.

Note though that 540mV is 90db gain/loss. You NEVER need or want that much. 60db loss is often plenty. (For trem, 20db may be ample.) And even 30db gain is often excessive.

Figure out what you really need.

The control port is exceptionally sensitive to garbage. Also wants low impedance. With a more realistic 300mV swing, and 5V available, try 1K:68 voltage divider with 100uFd to ground at the Ec pin. If you need fast change-of-gain this may be slow, but it is a compromise against PWM hash and other digi-trash modulating your precious music.

Thanks PRR

My idea is to make an outboard summing buss/mixer. I will mainly be using it for loss at most I think I will need a gain of +10


markseel

Re: N00B about digital
« Reply #5 on: February 12, 2012, 08:57:47 PM »
Make sure you consider the required impedance of the voltage sources for the EC+ and EC- inputs.  These inputs require low-impedance drivers for best operation.  So if you're using a resister network or a variable voltage level derived from PWM then it needs to be buffered by an op-amp or similar.