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Getner SPH10 analog hybrid (improving null)

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I'm a veteran broadcaster who has more experience using equipments as opposed to fixing hit. I have a home recording set up that I put together around 7 years ago. Nothing fancy but does the job. I also purchased, at the time, a Gentner SPH10 analog hybrid to be used for interviews (with my Mackie Mackie 1202-VLZ Prp mixer. There it sat for several years... never used yet never abused. I now want to put it to use. I have it set up correctly. I'm using the mixer's aux as a mix minus. I'm also utilizing the correct output (caller output). Despite all this I'm getting some mic bleed from the caller output which thins the host mic's sound. I realize there are issues with the SPH10 regarding this very same problem however in the manual it it says to contact Gentner regarding null issues in the hopes of better performance. Unfortunately the unit is too old for tech support (not to mention the sale of Gentner). Is there anyone who can offer assistance?
 
Initial post should read: "as opposed to fixing it".

Yes a schematic is provided in the manual but I don't have a clue when it comes to repairing or altering electronics internally although I can muddle through it if the directions are very clear and very descriptive. I'm pretty creative when it comes to rerouting around a problem though, but I don't think that will help in this case (Gotta love those patch racks.)
 
That analog Gentner was only capable of about a 20 dB null at best, so there will always be some leakage from "send" to "receive" (i.e. "caller"), even when adjusted properly. The key to minimizing the "hollowness" is to keep the send level as low as possible, even to the point where it is a little bit difficult for the caller to hear you (but not to the point where they're actually saying they can't understand you). This not only keeps the leakage to a minimum, but also has a psychological effect on the caller which actually makes them "speak up" more, which further increases the difference between the caller audio and the leakage audio, creating a better signal-to-noise ratio.
 
Makes sense. I'll give it a try. As mentioned, in the manual it said to contact the Getner tech dept. to increase null. Are there any (easy) internal changes that would increase the separation?
 
... also, is the best way to lower the send to the caller is by potting down my aux output from my Mackie board (the aux. output is being used as my mix minus) or should I lower the send level on the SPH10? I'm guessing either one will do the same... or am I wrong?
 
I'm not aware of any internal changes that will actually increase the depth of the null. There is however a nulling capacitor which can be changed in the unit, should you find that you are unable to reach the true null point, due to the capacity of the telco line coming in from the central office, but if you are already able to attain a depth of about 20 dB then that is about the best you are going to be able to do. Keep in mind as well that the hybrid will not null properly if the handset is off the cradle at the same time (i.e. there can be no double termination), and that the unit must be connected directly to the phone line (i.e. not through a system). The digital units were later developed to improve the marginal null situation with the analog units. And, yes, you can adjust the send level either way, although I would recommend using the "send" pot, for ease of use.
 
Thanks for the reply. I think I'm at the "as good as it gets" point. I'll eventually look into a digital hybrid if I find myself doing an increased amount of phone interviews.

Thanks again.
 
Quick n dirty setting: call the house, have your wife/husband/whomever just keep the phone off hook. Put a tone - 400Hz works well - on the feed, and adjust the null control for the lowest tone in the return path. If you run out of control, you need to increase or decrease the null capacitor. If the control will show a null point with an increase on either side, you're looking at the best it can do.
One other thing you might try.... the nulling process is only affected by the copper line from you to the serving switch, and the copper from the DE switch to the caller. The DE changes with each call, which is why the null changes. Sometimes, you can take a bobbin and wind a >bunch< of small guage wire in it twice and use it to essentially make your end of the line longer. If you can't get 20 - 25 dB null on a tone, try this and see if it improves.
 
I have an SPH3, found the null was less touchy when I paralleled it with an old Model 500 QKT phone (just used a splitter at the jack where the SPH3 was plugged in).

We use it for air feed back on away games, the QKT phone has a push to talk switch so you don't have the open mike.

The same effect could be done with any kind of old phone with a removable mouthpiece
 
If you look over on the main engineering board there was a recent thread. Your best bet maybe to not spend a lot of time messing with a non-adaptive hybrid and buying something.

http://boards.radio-info.com/smf/index.php?topic=151017.0

Best of luck

littlejohn said:
Quick n dirty setting: call the house, have your wife/husband/whomever just keep the phone off hook. Put a tone - 400Hz works well - on the feed, and adjust the null control for the lowest tone in the return path. If you run out of control, you need to increase or decrease the null capacitor. If the control will show a null point with an increase on either side, you're looking at the best it can do.
One other thing you might try.... the nulling process is only affected by the copper line from you to the serving switch, and the copper from the DE switch to the caller. The DE changes with each call, which is why the null changes. Sometimes, you can take a bobbin and wind a >bunch< of small guage wire in it twice and use it to essentially make your end of the line longer. If you can't get 20 - 25 dB null on a tone, try this and see if it improves.
Click to expand...
 
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