I wanted a set of wireless headphones for "IEM" duty in my home studio. Minimal SQ; just need to hear myself play and sing, with the mixers effects. One less cord to trip over is always welcome. So I picked up a set of the OLD FM wireless Sennheisers for a song off epay. They work / sound just fine for the intended purpose, driven by a portable digital player or connected to my laptop PC.
I chopped the two RCA plugs and wired up a 1/4 TRS to match the Soundcrafts headphone jack and plugged in the Sennheiser wireless. It sounds like white noise is loudly mixed in with the audio signal. Of course, with the wired phones, the Soundcraft UI 24R's headphone output has no such aberration; its clear as a bell.
Ah, must be some digital hash is coming out the Soundcraft UI 24R's headphone output! So I dug up a hollow ferrite slug originally intended for a power line conducted emissions suppression, and redid the TRS plug soldering after winding 3 turns of the line cable through it. No joy, it's not going to give it to me that easily.
I have a set of microphone balanced input transformers I ripped out of a Behringer mixer. Same Z on both windings I believe. Do I have a chance at blocking whatever it is upsetting the Sennheiser by placing these in the line going to it, as an isolation attempt?
It's amazing how inanimate objects can "conspire" to thwart the one simple thing I want to do. Or at least put up a good fight.
I chopped the two RCA plugs and wired up a 1/4 TRS to match the Soundcrafts headphone jack and plugged in the Sennheiser wireless. It sounds like white noise is loudly mixed in with the audio signal. Of course, with the wired phones, the Soundcraft UI 24R's headphone output has no such aberration; its clear as a bell.
Ah, must be some digital hash is coming out the Soundcraft UI 24R's headphone output! So I dug up a hollow ferrite slug originally intended for a power line conducted emissions suppression, and redid the TRS plug soldering after winding 3 turns of the line cable through it. No joy, it's not going to give it to me that easily.
I have a set of microphone balanced input transformers I ripped out of a Behringer mixer. Same Z on both windings I believe. Do I have a chance at blocking whatever it is upsetting the Sennheiser by placing these in the line going to it, as an isolation attempt?
It's amazing how inanimate objects can "conspire" to thwart the one simple thing I want to do. Or at least put up a good fight.
You could try a makeshift low-pass filter at 20 kHz. Maybe it is some out-of-band noise that gets downconverted somehow, although I haven't a clue how.
I tried the Behringer transformers and they work to eliminate the problem. The UI24R's headphone output Z is 120 Ohms, and 500 mW after a little digging.
Behringer, for similar units as my part source as I recall, only states "servo controlled output stage (optional transformer balanced) with a 60 Ohm balanced rating. Is that for the servo output stage or the optional transformer one. I'll guess its the servo one and the optional transformer one remains unspecified.
So now I have to determine empirically the transformer impedance. The Sennheiser system will go to the garbage can before I risk damaging the UI24R's headphone outputs, by driving these transformers to solve the little "problemo" presented.
Behringer, for similar units as my part source as I recall, only states "servo controlled output stage (optional transformer balanced) with a 60 Ohm balanced rating. Is that for the servo output stage or the optional transformer one. I'll guess its the servo one and the optional transformer one remains unspecified.
So now I have to determine empirically the transformer impedance. The Sennheiser system will go to the garbage can before I risk damaging the UI24R's headphone outputs, by driving these transformers to solve the little "problemo" presented.
The input load to the Sennheiser system is ~10K, which I think is a reasonable value given an RCA line input. So I can see what these Behringer transformers look like going in, with 10K across the secondary? That shouldnt be too difficult to find out with a function generator.
I keep all this junk around, in case I need to use it to solve some problem that comes up someday. I'm sure 90% of it will never get used. Mr Murphy has an interesting take; as soon as you throw something out...
Question; if I double the length of the little antenna sticking up, will the device's range increase? Thanks.
I keep all this junk around, in case I need to use it to solve some problem that comes up someday. I'm sure 90% of it will never get used. Mr Murphy has an interesting take; as soon as you throw something out...
Question; if I double the length of the little antenna sticking up, will the device's range increase? Thanks.
Not if the designers properly matched the antenna. If you know the frequency, you can calculate whether it is a quarter wavelength monopole, but that doesn't say much, because if it isn't, it can still be matched with lumped components (probably an inductor) on the PCB.
I put 2.2k // 470 pf at the big input pads you can see above and the problem went away. Unfortunately for me, now its "developed" this constant, audible 1kHz carrier whistle, which is present on any of the 3 transmitting channels and with the device not plugged to any audio source. I dont recall hearing that previously.
Thinking perhaps it was proximity of my relatively gigantic parts to the antenna, I tore it all out and nope - still there. Put it all back together, back in its original assembly and <drum roll> I just cant use it that way, with the white noise gone but now the whistle.
I know when I'm licked. Earlier this week I saw another Sennheiser transmitting base at the thrift store for $5, of which I didnt take the opportunity hint. Perhaps I cracked one of the pepper flake sized SMD parts handling the PCB... God only knows.
Well, I suppose it was fascinating that if you pluck the little antenna, it's somewhat microphonic; you can hear the tung sound coming through the headphones. $25 fascinating, $100 worth of work to notice.
Thinking perhaps it was proximity of my relatively gigantic parts to the antenna, I tore it all out and nope - still there. Put it all back together, back in its original assembly and <drum roll> I just cant use it that way, with the white noise gone but now the whistle.
I know when I'm licked. Earlier this week I saw another Sennheiser transmitting base at the thrift store for $5, of which I didnt take the opportunity hint. Perhaps I cracked one of the pepper flake sized SMD parts handling the PCB... God only knows.
Well, I suppose it was fascinating that if you pluck the little antenna, it's somewhat microphonic; you can hear the tung sound coming through the headphones. $25 fascinating, $100 worth of work to notice.
Of course, my curiosity got me to fool with it some more. The squeal was from the OEM power supply, different wall-wart switcher made different sounds. Go Figure. An analog +5V regulated only passed a little 60 Hz hum. 12V Battery, with its internal LM7805 doing the voltage drop, sounded the best.
The receiver in the headphones was another matter. It hunted, swished, cut out as regularly not tuned to anything at all as it did when fixed on the transmitter signal! I've never seen a system that bad - even moving about slightly right in front of the transmitter causes audible reception anomalies. They had some switched capacitor deal to double the voltage of the two 1.2V AAA cells up in there.
Those circuits are now at rest in the garbage can. I installed a 1/8" TRS in the headphone part to make these on-ear phones at least usable. You cant get on-ears so easily anymore, but there's page after page after page of over-ear cups available. I guess everyone these days wants bye-bye to current reality and be in their own little undisturbed, noise and everything else cancelled world.
The receiver in the headphones was another matter. It hunted, swished, cut out as regularly not tuned to anything at all as it did when fixed on the transmitter signal! I've never seen a system that bad - even moving about slightly right in front of the transmitter causes audible reception anomalies. They had some switched capacitor deal to double the voltage of the two 1.2V AAA cells up in there.
Those circuits are now at rest in the garbage can. I installed a 1/8" TRS in the headphone part to make these on-ear phones at least usable. You cant get on-ears so easily anymore, but there's page after page after page of over-ear cups available. I guess everyone these days wants bye-bye to current reality and be in their own little undisturbed, noise and everything else cancelled world.