Adjusting Infinity IRS servo controller

I got a request from @bobterry99 about adjusting Infinity servo controllers.  Inside the controller are 4 pots:

Left side level (amplitude)

Left side DC offset

Right side level (amplitude)

Right side DC offset

See the attached photo.  I used a 2 ch oscilloscope to monitor the input and output while adjusting.  With 4 ch, you could monitor both channels...



DC offset is adjusted for minimum DC on the output. The level trim adjustments are just that.  Apply a similar signal to each input and adjust so that the outputs are equal.

Thanks for that picture, jtucker. Is it possible there is an error?

Referencing the ECU schematic seen here, it seems to me that you show adjusting potentiometer R30 for the level. But that seems like the wrong one to me.

I think the correct one is R23, since it is controlling the amount of negative feedback of the amplifier comprised of ICa, ICb, and ICc. Potentiometer R30 sets the level of the output from the amplification of the accelerometer signal.

I’m going to see what some of my electrical engineering buddies think.

I suppose it is possible that I made a mistake, but I had to re-calibrate my servo on a regular basis.  Those were the pots that used.  If you have access to a scope, it would be easy to verify.  I'm pretty sure I gave the schematic to the new owner, so I have nothing else to compare.  Also, I ended up replacing all the electrolytic capacitors in it.  As I mentioned, all this was back around 2010, so I don't recall any more details.  If I come across anything else in the next few days, Ill let you know.

Again, thanks for replying.

I ended up tweaking the R30 potentiometers just as you did. R23 didn’t offer the needed range of adjustment. Besides, in my case it wasn’t the correct one to adjust.

My issue was the right channel having much more bass than the left. Testing, I found that with an input voltage of 33mV @60Hz my right channel output was driven to 25mV; the left channel to only 12mV.

The first thing I did was dial the R30 potentiometers fully clockwise. This causes the output of each channel’s accelerometer amplifier to be grounded and eliminates the accelerometers and their circuits as a variable. Now each channel measured the same. Had there been a difference I would have tweaked the R23 potentiometers to make them equal and been done.

In my case I had already determined that the accelerometers were not the issue, since when I swapped the woofer cabinets between channels the right was still louder. So the solution was to tweak the R30 potentiometers to make the outputs of each channel equal.

It seems to me that you want the greatest amount of feedback from the accelerometers that does not cause oscillations. I maximized the feedback by by dialing the R30s fully counter-clockwise. I had no oscillations, but the imbalance remained. So I raised the output of the left channel to match the right by dialing its R30 a little bit clockwise.

If memory serves, I adjusted mine for unity gain.  After I noticed that I needed to continue to make adjustments, I started keeping a log book to track it.  Maybe I can find it, but I threw out a lot paper before my last move, so we'll see.

Unity gain seems aesthetically pleasing to me.  I made new measurements. 

For listening I have the LOW FREQ LEVEL control set to around -8.  For new adjustments I set it to zero.  With each channel being driven by 18 mV I measured 21 mV output. 

With the right-channel R30 already dialed fully CCW the only way for me to achieve unity gain was to increase the ovarall amplifier feedback via R23.  Dialing each channel's potentiometer a little clockwise gave me unity.

With this done I still have to set the LOW FREQ LEVEL to around -8 to achieve a flat frequency response as determined by sound pressure readings using an app on my phone.  I think that's odd.  It's as if the woofer towers are vastly more efficient than the high-frequency panels.

Which version of the servo and which speakers?

BTW, I am looking for a reputable shop to go over my spare servo controller (IRS Beta) to make sure its all up to design specs

I had the IRS Gammas.  Sold them back in 2010.  I did a fair amount of work on the my servo controller plus another one (anybody remember Bobby Shred?), but since I am retired now I no longer have access to an electronics lab.

Is there anyone who can provide a copy of the settings/manual for the IRS servo amp. I have a Genesis 2 system and the amp should be checked out to insure it is performing properly. I figure that it may give some insight for myself and others. Does anyone know who made the amps or can service them properly? All help is appreciated.



Here are some pictures of the Genesis Servo amp and columns. There is no service procedures on the feedback to be found. You can see the 2 adjustment pots per channel on the board. Any ideas on test procedures for this??




I don't have any specific info on that Genesis servo, but based on my experience with the Infinity servo, I would suppose that one pot is gain and the other offset.  I adjusted mine for as close to zero dc bias as I could get and the the gain for unity.  You will have to try them to see which one does what.  You will also need access to an oscilloscope and probes.