Tecpro AD913 PMR Wiring

[sunray]

I've just been looking back at your breakout circuit.  It sounds like a really neat option.  Have you considered marketing it? Especially as Maplin's are likely to close the components could be going cheap?

Thanks for the vote of confidence but I really don't think there is likely to be enough demand for such a system, it is actually very simple and I guess that most users interested in it would have the knowledge or a colleague to sort it.

 

I am in the process of building the Bluecomm system and seriously considering making an additional station including the interface bits.

[AnthonyHunt]

I’m considering building two, to connect to a pair of iem’s at either end of a big gap. Giving full duplex into to physically separated comms systems where connecting by cable is impossible.

I’ve sourced the pots and have the 4pin XLRs in stock as well as the 3pins. Can I ask what you used for the transformer? I’d like the unit to be passive if possible. As you might have guessed already, I’m handy with a soldering iron but circuit design is not my thing!

Ant.

[sunray]

I'm considering building two, to connect to a pair of iem's at either end of a big gap. Giving full duplex into to physically separated comms systems where connecting by cable is impossible.I've sourced the pots and have the 4pin XLRs in stock as well as the 3pins. Can I ask what you used for the transformer? I'd like the unit to be passive if possible. As you might have guessed already, I'm handy with a soldering iron but circuit design is not my thing! Ant.

 

Firstly I will instantly say this is potentially not a good idea, assuming I understand your proposed application of linking 2 self contained tecpro systems together. My concern is the accuracy of the hybrid adjustment and the potential for feedback around the 2 radio links.

 

I have a selection of transformers 'in stock' and happened to chose Sowter 600:600 Mu metal enclosed and a Sowter 600:10K open frame but pretty much any reasonable audio transformer should be fine for this such as the type contained in these http://cpc.farnell.com/av-link/201-086uk/ground-loop-isolator-rca/dp/AV26879?st=ground loop in fact, If you are able to work this without volume pots then just replacing the RCA's at one end with an XLR4 may be all that is needed, loads of other versions available on CPC and elsewhere http://cpc.farnell.com/pulse/pls00549/ground-loop-isolator-3-5mm-to/dp/AV25543?st=ground loop. I also use these http://cpc.farnell.com/oep-oxford-electrical-products/a262a1e/transformer-audio-6-3-6-3-1-1/dp/TF01459?st=audio transformer fairly often. Bargain: https://www.ebay.co.uk/itm/10X-Audio-Transformers-600-600-Ohm-Europe-1-1-EI14-Isolation-Transformer-HL/192424594683?epid=20013002140&hash=item2ccd664cfb:g:DrUAAOSwMVdYIHnN

[AnthonyHunt]

Well that was a really stupid thought of mine wasn't it! As a sound engineer... you'd have have thought I've had remembered that little fact before writing it wouldn't you! Of course in my head I'd done the "yeah, it's just an in and out" without remembering that it was a bus. At least it would work 1 way (which is kinda more important).

 

Thanks for those links! I had considered the first to be one of those "wrap the cable around a ring" devices. I've already got some of those lying around so might try that. Still kinda fancy making a couple though too. As you can imagine, the next decision is what terminals to put on the box? and whether I'll need fly leads to devices..... which makes me consider the viability....doh!

 

As another "you were spot on", I checked my JNR lead that was making noise when the PMR's went into powersaver. Another thread suggested putting a 30R resistor across the speaker output of the PMR. Did the trick nicely., so I now have two systems working. And I can already see the advantage of your design with the transformer in it.

 

Ant.

[sunray]

Well that was a really stupid thought of mine wasn't it! As a sound engineer... you'd have have thought I've had remembered that little fact before writing it wouldn't you! Of course in my head I'd done the "yeah, it's just an in and out" without remembering that it was a bus. At least it would work 1 way (which is kinda more important).

Well no not stupid, they are ins and outs and if the side tone is correctly set then there will be no problem, however in practice I think the system will not be stable enough and I stress the 'I think'.

I have been on the wrong end of trying to isolate loops in audio systems far too often, particularly in the OB situations.

Thanks for those links! I had considered the first to be one of those wrap the cable around a ring devices. I've already got some of those lying around so might try that.  Still kinda fancy making a couple though too. As you can imagine, the next decision is what terminals to put on the box? and whether I'll need fly leads to devices..... which makes me consider the viability....doh!

 

I have used a number of those stereo isolators and modified many of them, in particular removing the flying lead from one end and soldering it directly on the end of a cat 2/3/4/5/6 cable for permanent systems. I've done the same thing with a 100m length of 2 pair FST on a drum. They consist of a PCB containing two transformers similar to the 'bargain' link, the case is 2 steel [Mu metal in the expensive versions] cups held together with heat shrink.

 

The interface unit I knocked up consists of a reused steel box with various existing holes, 2 pots deliberately without knobs, 2 transformer held in with double sided tape and 3 flying leads. XLR4F to belt pack, XLR3M to IEM, jack plug for radio LS socket. When [or should I say if] I remake it properly it will of course have a properly fixed XLR4F flying lead and chassis mounted sockets for the others. I'm also tempted to build it directly into a bluecom beltpack unit.

[AnthonyHunt]

Sunray, I’m back again.

Thanks for all your help on the previous interface issue. We’ve been working amazingly with our interfaces set up.

I’ve made a box similar to yours to work with a standard Beltpack but I have a slight problem. And I get the same problem with the cable/box connected to a 4wire to 2wire interface. I wondered if your electronics experience could help?

So I made a box that connect to a 4pin xlr to connect to a pack. The in and out on the tecpro side connect direct to transformers. The other side of each transformer connects to a 10k (from memory but what you spec’d) pot. Which in turn connects to 3.5mm jacks.

I’ve also since made a straight cable of the above without transformers or pots.

I have the same problem with both. The “In” to TecPro is fine. Correct attenuation and crystal clear. The problem I’m getting is the “output” from TecPro into my Radios or a radio mic transmitter seems to have a 10k tone on it. Very much like feedback screech. I’ve done the obvious in terms of connections and side tone. But I’m now slightly stumped.

You said you were using a capacitor? To which I’ll openly admit to knowing nothing...

Any thoughts?

Ant.

[sunray]

This could be the same problem I had a few years ago when I wanted to mix the output into the sound ops headphones. The solution thhere was placing a resistor across pins 3 and 4.

I cant remember the value but it was there to simulate the 32ohn headphones load.

I'll try to look at it later in the week for you.

[paulears]

Two things come to mind - you sure you're not using the speaker override system? these generate ultrasonic tones to kill mics or re-activate muted speakers, but the frequency is very high and should be filtered out by the radio anyway, but maybe there's some odd mixing?

 

The other thing is that the output of the packs is very stable, and there is no problem going from the headphone level output from the 4 pin XLR into higher impedance destinations - radios, line level inputs on mixers etc. If this produces squeal on the radio system then I'd have a look at how the radio gets put into transmit. Some detect the open circuit becoming lower resistance when the mic is connected, a few use a 3 pin jack and shorting two connections instigates TX - so it's feasible that there is voltage on the connection to the tecpro amp output which is making it oscillate - a capacitor will usually stop any dc path. I think if you have one, I'd open up a remote mic and see what actually happens when you press the PTT - direct connection, removal of a short, application of a short to a different circuit - then you might work out what is happening. One thing is certain, plugging one make of mic into a different make of handheld doesn't remotely mean they work. So this should be sortable. You mentioned transformers? What are these doing in the circuit? I have a Kenwood base station connected to a Tecpro loudspeaker station with simple direct connection and it works fine - the volume control on the Tecpro providing a perfect match to the base station's line level input, using the audio tagged off the rear of the 4 Pin XLR on the front panel.

[sunray]

Two things come to mind - you sure you're not using the speaker override system? these generate ultrasonic tones to kill mics or re-activate muted speakers, but the frequency is very high and should be filtered out by the radio anyway, but maybe there's some odd mixing?The other thing is that the output of the packs is very stable, and there is no problem going from the headphone level output from the 4 pin XLR into higher impedance destinations - radios, line level inputs on mixers etc. If this produces squeal on the radio system then I'd have a look at how the radio gets put into transmit. Some detect the open circuit becoming lower resistance when the mic is connected, a few use a 3 pin jack and shorting two connections instigates TX - so it's feasible that there is voltage on the connection to the tecpro amp output which is making it oscillate - a capacitor will usually stop any dc path. I think if you have one, I'd open up a remote mic and see what actually happens when you press the PTT - direct connection, removal of a short, application of a short to a different circuit - then you might work out what is happening. One thing is certain, plugging one make of mic into a different make of handheld doesn't remotely mean they work. So this should be sortable. You mentioned transformers? What are these doing in the circuit? I have a Kenwood base station connected to a Tecpro loudspeaker station with simple direct connection and it works fine - the volume control on the Tecpro providing a perfect match to the base station's line level input, using the audio tagged off the rear of the 4 Pin XLR on the front panel.

At this point I'm not sure what is being interfaced to, several options have been discussed.

 

I have not had too much experience with using intercoms but interfacing has been included with almost every occasion. My earliest was a simple double pole switch to enable the sound op to switch headphones (but not the microphone) between sound desk and TechPro. The moment the load was removed from TechPro it went unstable and screeched right through system. It was abandoned due to lack of time to sort it.

Next attempt we tried running o/p to a small mixer so sound op heard both simultaneously, the same shrieking occurred until a resistor was placed across the TechPro o/p and an isolation transformer was added to stop the bad hum in the headphones. My conclusion was that the output needed a load for stability.

After that I automatically include transformers to prevent earth loop type problems, looking at the way intercoms meander around the show the 'ground' of an outstation is just about as far as it can be from earth, I've measured over 4Vdc and had to look for earthed grounds (due to XLR leads with screen/shell links or grounded tie circuits or multiways) to stop hum and other noise in the intercom or sound system.