My NC-7 noise cancelling mod for the Simmons SDS7 is for those SDS7s that didn't get this mod factory fitted.
"Surely that can't be" I hear you ask.
Pictured below, is part of the Simmons SDS7 showing the quad op-amp, part of which is used to buffer the stereo outputs. Due to the mish-mosh of analogue, digital and power tracks on the back plane, Simmons was forced to come up with an easy retrofit solution to reduce specifically, CPU noise picked up by the stereo buss. Able to take advantage of a spare op-amp stage in the TL084 and dropping an 'aerial' on the rear of the back plane, inverted noise was mixed on to the stereo buss, thereby cancelling out the 'in-phase' noise. Highlighted in red is the pre-set pot and the two 10kΩ resistors which were part of that mod.
There are however, a few SDS7s out there that didn't get the mod! Pictured below, is my test SDS7 which I used to develop Pleiades, Dark Matter and AMEP and as you can see, it didn't get the mod fitted back in the day.
You'll also notice the absence of a small green capacitor, highlighted in yellow on the second image. This 47pF capacitor which is connected between pins 10 and 12 of the TL084, is sometimes soldered on the back of the back plane PCB. More about this later.
Well, I was going to implement the mod as per the original service bulletin but as it turned out, the track layout of the back plane version in my test SDS7 was different to later revisions and I wasn't able to drill the three holes for the pre-set without busting a track.
Then I had a thought; "Why don't I knock up a little snap-in PCB to do the job?" That way, anyone with a SDS7 that didn't have the noise cancelling mod installed, could also benefit. 🙂
Snaping into the IC socket that the TL084 is in and taping the aerial on to the rear of the back plane, means that NC-7 can easily be installed in a few minutes.
Of course it's not quite that straight-forward. The noise cancelling circuit has to be 'tuned', hence the pre-set pot. Instructions on how to do that are in the installation manual and that process also takes only a few minutes.
The two through-hole resistors of the original mod, are replaced by SMD 0805 resistors that are situated underneath the IC on NC-7. That 47pF capacitor I mentioned earlier, is also superfluous as on the underside of the NC-7 PCB is a SMD 2010 47pF capacitor. Hence, if the original capacitor is indeed soldered to the rear of the TL084, it must be removed.
NC-7's aerial wire can't simply flip over the back plane PCB. That's because one of three guides guide in the SDS7 top-case secures the back plane PCB exactly where the aerial wire needs to be. The wire I chose for the NC-7 aerial wire is CAT5/6 cable strand so it's solid. If it catches into the guide of the top-case, it'll break. Hence, the aerial wire has to go to one side.
So while looking into the infamous SDS7 noise and testing my NC-7 noise cancelling mod for the Simmons SDS7, I thought it might be an idea to actually record this annoying phenomenon.
The video below shows the effect on overall noise level, as the NC-7 noise cancelling mod is gradually tuned in. Check out the VU meter on the right.
WARNING!!!! The signals have been amplified so TURN DOWN YOUR SPEAKERS
Below is a spectral capture of the noise and the effect of the NC-7 noise canceller. As can be seen, tuning in the NC-7 noise cancelling mod, reduces noise between 3kHz and about 11kHz but doesn't do anything for lower frequency noise.
There's one other thing... In fact, you may already have noticed, that instead of using a TL084, NC-7 is fitted with a newer NJM2060. This device is of a much higher specification than the humble TL084. Since two stages of this quad op-amp are used to buffer the stereo buss, having a quieter device will no doubt improve things a little. 🙂 Having said that, I'm not sure the difference will be too noticeable with all of that other stuff!
NC-7 is handmade in Hemel Hempstead, Hertfordshire, United Kingdom, only a few miles from St. Albans where Simmons used to based and where I used to work.
I do not keep NC-7 units in stock and each NC-7 is built to order.