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32-ohm earphone speaker as a dynamic microphone
Hi,
Silly question… I've been experimenting with a radio station on the bicycle for HF operation and one problem I've been facing is that of RF feedback getting into the microphone. I've tried a few things to try and eliminate the RF. The set is a Yaesu FT-857D, and I use runs of CAT5 cabling to hook the head unit and my headset up to the radio which is mounted in a motorcycle topbox behind me. I've ensured that the microphone + and - signals are on their own dedicated pair in the CAT5, as are the +5V and GND signals. The microphone biasing is done near the head unit of the radio, so maybe 1.5m away from the radio, and a short lead then plugs into the (helmet-embedded) headset. My biasing circuit looks like this: (please excuse the ASCII art) .-----. .---------------~~----. +5v ---o------------o----| 1k |---o ..100nF | | | '-----' '---||--o---o Mic + +| . ----- ----- '' --- ( )| Electret .-'-. 100uF ----- 100nF ,. --- 100nF -| ' Insert | | .-----. .---||--o---o Mic - | (headset) 0v----o------------o----| 1k |---o ''100nF | '-----' '---------------~~----' The capacitors are all (with the exception of the 100uF electrolytic) small monolithic capacitors: http://www.jaycar.com.au/productView.asp?ID=RC5490 My intent is that the capacitors across the 5V rail should suppress the RF on the 5V rail, and the capacitor across the output of the bias circuit should suppress any common-mode RF on the balanced feed to the radio. I think there's one across the microphone in the headset too. Originally I contemplated a phantom-power arrangement, but I'd need to sit down and design the bias circuit as most of my stuff is in the 3-5V range, not the 12-48V range that phantom-power normally assumes. I note I do not get the RF feedback if I use the stock Yaesu handmic that came with the set, which is a 600ohm dynamic microphone. Currently I use an electret insert in the headset, one out of the junk box often salvaged from old mobile phone "headsets" (those annoying earphone things with an inline microphone). The lack of problems with the Yaesu handmic got me thinking: I have amassed a large quantity of earphone speakers from those mobile phone headsets (I hate the bloody things: can't stand putting anything *in* my ears). These are speakers with dimensions not too dissimilar to small dynamic elements with an impedance of 32ohms. Question is, how well would these work as microphones? Has anyone tried this and can share any insights? |
32-ohm earphone speaker as a dynamic microphone
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32-ohm earphone speaker as a dynamic microphone
On Sun, 27 Jul 2014, Stuart Longland wrote:
Hi, Silly question? I've been experimenting with a radio station on the bicycle for HF operation and one problem I've been facing is that of RF feedback getting into the microphone. I've tried a few things to try and eliminate the RF. The set is a Yaesu FT-857D, and I use runs of CAT5 cabling to hook the head unit and my headset up to the radio which is mounted in a motorcycle topbox behind me. I've ensured that the microphone + and - signals are on their own dedicated pair in the CAT5, as are the +5V and GND signals. The microphone biasing is done near the head unit of the radio, so maybe 1.5m away from the radio, and a short lead then plugs into the (helmet-embedded) headset. My biasing circuit looks like this: (please excuse the ASCII art) .-----. .---------------~~----. +5v ---o------------o----| 1k |---o ..100nF | | | '-----' '---||--o---o Mic + +| . ----- ----- '' --- ( )| Electret .-'-. 100uF ----- 100nF ,. --- 100nF -| ' Insert | | .-----. .---||--o---o Mic - | (headset) 0v----o------------o----| 1k |---o ''100nF | '-----' '---------------~~----' The capacitors are all (with the exception of the 100uF electrolytic) small monolithic capacitors: http://www.jaycar.com.au/productView.asp?ID=RC5490 My intent is that the capacitors across the 5V rail should suppress the RF on the 5V rail, and the capacitor across the output of the bias circuit should suppress any common-mode RF on the balanced feed to the radio. I think there's one across the microphone in the headset too. Originally I contemplated a phantom-power arrangement, but I'd need to sit down and design the bias circuit as most of my stuff is in the 3-5V range, not the 12-48V range that phantom-power normally assumes. I note I do not get the RF feedback if I use the stock Yaesu handmic that came with the set, which is a 600ohm dynamic microphone. Currently I use an electret insert in the headset, one out of the junk box often salvaged from old mobile phone "headsets" (those annoying earphone things with an inline microphone). The lack of problems with the Yaesu handmic got me thinking: I have amassed a large quantity of earphone speakers from those mobile phone headsets (I hate the bloody things: can't stand putting anything *in* my ears). These are speakers with dimensions not too dissimilar to small dynamic elements with an impedance of 32ohms. Question is, how well would these work as microphones? Has anyone tried this and can share any insights? IN the old days, endless cheap 100mW walkie talkies would use the speaker for the microphone on transmit. For that matter, endless intercom systems used the same speaker as a speaker and as a microphone. Endless articles in the magazines showed taking a speaker of some sort and using it as a microphone. Of course, those were speakers, most seemed at least 2". An earphone may not offer the same level of sound collection that a speaker with a larger cone allows, you may have to play with things. I remember taking cheap dynamic earphones and taking the bit that went in your ear off, and using that as a contact microphone for various things. The other issue is the output may not be so great, mostly because the speaker is arranged for sound reproduction, not sound pickup. It may need a preamp, depending on the actual output voltage. It may also need impedance matching, which would also help boost the output, use an output transformer in reverse so the speaker winding is fed from the speaker, and the now secondary winding will have more voltage to feed the higher input impedance of the rig. Michael |
32-ohm earphone speaker as a dynamic microphone
On 7/27/2014 6:21 AM, Stuart Longland wrote:
Hi, Silly question… I've been experimenting with a radio station on the bicycle for HF operation and one problem I've been facing is that of RF feedback getting into the microphone. I've tried a few things to try and eliminate the RF. The set is a Yaesu FT-857D, and I use runs of CAT5 cabling to hook the head unit and my headset up to the radio which is mounted in a motorcycle topbox behind me. I've ensured that the microphone + and - signals are on their own dedicated pair in the CAT5, as are the +5V and GND signals. The microphone biasing is done near the head unit of the radio, so maybe 1.5m away from the radio, and a short lead then plugs into the (helmet-embedded) headset. My biasing circuit looks like this: (please excuse the ASCII art) .-----. .---------------~~----. +5v ---o------------o----| 1k |---o ..100nF | | | '-----' '---||--o---o Mic + +| . ----- ----- '' --- ( )| Electret .-'-. 100uF ----- 100nF ,. --- 100nF -| ' Insert | | .-----. .---||--o---o Mic - | (headset) 0v----o------------o----| 1k |---o ''100nF | '-----' '---------------~~----' The capacitors are all (with the exception of the 100uF electrolytic) small monolithic capacitors: http://www.jaycar.com.au/productView.asp?ID=RC5490 My intent is that the capacitors across the 5V rail should suppress the RF on the 5V rail, and the capacitor across the output of the bias circuit should suppress any common-mode RF on the balanced feed to the radio. I think there's one across the microphone in the headset too. Originally I contemplated a phantom-power arrangement, but I'd need to sit down and design the bias circuit as most of my stuff is in the 3-5V range, not the 12-48V range that phantom-power normally assumes. I note I do not get the RF feedback if I use the stock Yaesu handmic that came with the set, which is a 600ohm dynamic microphone. Currently I use an electret insert in the headset, one out of the junk box often salvaged from old mobile phone "headsets" (those annoying earphone things with an inline microphone). The lack of problems with the Yaesu handmic got me thinking: I have amassed a large quantity of earphone speakers from those mobile phone headsets (I hate the bloody things: can't stand putting anything *in* my ears). These are speakers with dimensions not too dissimilar to small dynamic elements with an impedance of 32ohms. Question is, how well would these work as microphones? Has anyone tried this and can share any insights? Don't forget that you may need to match the acoustic aspects of the earpiece to that of a mic meaning you will need at minimum to open up the opening and preferably add some sort of cone or horn. Poor acoustic coupling will diminish the signal level just as easily as poor electrical coupling. -- Rick |
32-ohm earphone speaker as a dynamic microphone
Michael Black wrote in
news:alpine.LNX.2.02.1407271413320.23911@darkstar. example.org: An earphone may not offer the same level of sound collection that a speaker with a larger cone allows, you may have to play with things. I remember taking cheap dynamic earphones and taking the bit that went in your ear off, and using that as a contact microphone for various things. Could be so. When I asswered I overlooked the detail of headphone type. The little bud types might not be so good, but the small on-ear types that came out when cheap Walkmans werre new, are a very good candidate, because the construction of those is almost identical with that of many cheap dynamic mics, and the sound fidelity is also very good with a diaphram about 0.75'' to 1'' wide. Mylar too, so no degrading with humidity. I haven't tried to work out the implications of matching impedance for gain, but these days it is likely easier not to do it, just use a high resistance input with low noise and high gain. Cheap op-amps that will do it are easily had. Dynamic mics with transformer matching might work but even if immune to RF pickup they will catch magnetic fields as if intended to do so! I also wonder if fully balanced feed is needed in either case, dynamic or electret. If one wire is firmly grounded, and is part of a twisted pair with the wire that carries the DC feed and the AC signal out, then it ought to cancel out any incoming HF anyway. It's possible that trying to make it fully balanced might make it more vulnerable, not less, because it has no firm ground on either pole. The main thing is to get the gain up as close to the electret as possible, and that will want very small parts. Transistor rather than transformer.. |
32-ohm earphone speaker as a dynamic microphone
Lostgallifreyan wrote in
: I also wonder if fully balanced feed is needed in either case, dynamic or electret. If one wire is firmly grounded, and is part of a twisted pair with the wire that carries the DC feed and the AC signal out, then it ought to cancel out any incoming HF anyway. It's possible that trying to make it fully balanced might make it more vulnerable, not less, because it has no firm ground on either pole. The main thing is to get the gain up as close to the electret as possible, and that will want very small parts. Transistor rather than transformer.. Bit more thought on that... If the circit to be fed by this has two grounds, one for frame, the other for local AF input signals, then instead of CAT5 pairs, use a twisted pair cheap 3mm thick cable with a screen. Screen to frame ground, twisted pair for AF line and signal ground. That should shunt HF pickup to ground in the equipment where it is likely already done, and allow at least a metre of signal line to work well. |
32-ohm earphone speaker as a dynamic microphone
On 28/07/14 04:18, Michael Black wrote:
IN the old days, endless cheap 100mW walkie talkies would use the speaker for the microphone on transmit. For that matter, endless intercom systems used the same speaker as a speaker and as a microphone. I do remember those, in fact I've got one gutted somewhere. Had a crystal for 27.145MHz. I once tried wiring up an electret element, not knowing there was a difference, and was disappointed when it didn't work. An earphone may not offer the same level of sound collection that a speaker with a larger cone allows, you may have to play with things. I remember taking cheap dynamic earphones and taking the bit that went in your ear off, and using that as a contact microphone for various things. In the interest of science, I gave it a shot just then. With a 100nF capacitor in series to block the DC, I wired it to a DIN5 plug (all my radios have been set up with adaptors to DIN5 headset jacks) and tried it. It did work, but without any amplification or impedance matching, the modulation is well down. I might try winding a small transformer and see what that does. |
32-ohm earphone speaker as a dynamic microphone
Stuart Longland wrote:
On 28/07/14 04:18, Michael Black wrote: IN the old days, endless cheap 100mW walkie talkies would use the speaker for the microphone on transmit. For that matter, endless intercom systems used the same speaker as a speaker and as a microphone. I do remember those, in fact I've got one gutted somewhere. Had a crystal for 27.145MHz. I once tried wiring up an electret element, not knowing there was a difference, and was disappointed when it didn't work. An earphone may not offer the same level of sound collection that a speaker with a larger cone allows, you may have to play with things. I remember taking cheap dynamic earphones and taking the bit that went in your ear off, and using that as a contact microphone for various things. In the interest of science, I gave it a shot just then. With a 100nF capacitor in series to block the DC, I wired it to a DIN5 plug (all my radios have been set up with adaptors to DIN5 headset jacks) and tried it. It did work, but without any amplification or impedance matching, the modulation is well down. I might try winding a small transformer and see what that does. In the days when speakers were used as microphones in walkie talkies and intercoms, it also was quite popular to have transformers between the final stage transistors and the speaker. The circuits of such devices were often very cleverly designed, re-using many components between receive and transmit (using a multipole switch). It is quite likely that the output transformer was used as a step-up transformer while the speaker was used as microphone. |
32-ohm earphone speaker as a dynamic microphone
On Sun, 3 Aug 2014, Stuart Longland wrote:
On 28/07/14 04:18, Michael Black wrote: IN the old days, endless cheap 100mW walkie talkies would use the speaker for the microphone on transmit. For that matter, endless intercom systems used the same speaker as a speaker and as a microphone. I do remember those, in fact I've got one gutted somewhere. Had a crystal for 27.145MHz. I once tried wiring up an electret element, not knowing there was a difference, and was disappointed when it didn't work. An earphone may not offer the same level of sound collection that a speaker with a larger cone allows, you may have to play with things. I remember taking cheap dynamic earphones and taking the bit that went in your ear off, and using that as a contact microphone for various things. In the interest of science, I gave it a shot just then. With a 100nF capacitor in series to block the DC, I wired it to a DIN5 plug (all my radios have been set up with adaptors to DIN5 headset jacks) and tried it. It did work, but without any amplification or impedance matching, the modulation is well down. I might try winding a small transformer and see what that does. I recall a couple of articles where people moved those cheap walkie talkies to 10 or 6metres, and they both mentioned adding an audio transformer to improve the audio on transmit. Michael |
32-ohm earphone speaker as a dynamic microphone
On Sat, 2 Aug 2014, Rob wrote:
Stuart Longland wrote: On 28/07/14 04:18, Michael Black wrote: IN the old days, endless cheap 100mW walkie talkies would use the speaker for the microphone on transmit. For that matter, endless intercom systems used the same speaker as a speaker and as a microphone. I do remember those, in fact I've got one gutted somewhere. Had a crystal for 27.145MHz. I once tried wiring up an electret element, not knowing there was a difference, and was disappointed when it didn't work. An earphone may not offer the same level of sound collection that a speaker with a larger cone allows, you may have to play with things. I remember taking cheap dynamic earphones and taking the bit that went in your ear off, and using that as a contact microphone for various things. In the interest of science, I gave it a shot just then. With a 100nF capacitor in series to block the DC, I wired it to a DIN5 plug (all my radios have been set up with adaptors to DIN5 headset jacks) and tried it. It did work, but without any amplification or impedance matching, the modulation is well down. I might try winding a small transformer and see what that does. In the days when speakers were used as microphones in walkie talkies and intercoms, it also was quite popular to have transformers between the final stage transistors and the speaker. The circuits of such devices were often very cleverly designed, re-using many components between receive and transmit (using a multipole switch). They were really complicated switches, for the sake of a few transistors. It is quite likely that the output transformer was used as a step-up transformer while the speaker was used as microphone. That's what I would have thought, but I recall articles about modifying those cheap walkie talkies and they added transformers to step up the output from the speaker on transmit. You're right, in that era, the audio amplifiers were using an output transformer. But it wasn't just to match impedance, it was part of the amplifier, and thus needed on transmit too. Michael |
32-ohm earphone speaker as a dynamic microphone
On 03/08/14 11:10, Michael Black wrote:
The circuits of such devices were often very cleverly designed, re-using many components between receive and transmit (using a multipole switch). They were really complicated switches, for the sake of a few transistors. Yep. This one I gutted, I recall de-soldering the switch and then reverse-engineering the pinout so I could replace it with a relay, which I did. A 4-pole double-throw relay IIRC. I had some hair-brained idea (this was when I was in primary school) to hook the thing up to the PC-speaker output of the computer (since I knew how to make tones) and then use some circuit interfaced to the game port (since I knew how to read the switches on those) and try to send data using AFSK. Exactly what data rate I'd achieve, given the whole lot would be implemented in QBasic I have no idea. I doubt it'd outpace PSK31. Not that I knew what AFSK was back then. Or that to do what I wanted to do, I really should have a radio license which I didn't back then. |
32-ohm earphone speaker as a dynamic microphone
Stuart Longland wrote:
On 03/08/14 11:10, Michael Black wrote: The circuits of such devices were often very cleverly designed, re-using many components between receive and transmit (using a multipole switch). They were really complicated switches, for the sake of a few transistors. Yep. This one I gutted, I recall de-soldering the switch and then reverse-engineering the pinout so I could replace it with a relay, which I did. A 4-pole double-throw relay IIRC. Not only that the switch has many poles, the circuit is often very tricky. It is not a receiver and a transmitter with a switch to toggle the power, antenna and speaker/mike to connect to one of them, no it is a blob of electronics that morphs between being a transmitter and being a receiver when the PTT switch is switched over. In those days I sometimes tried drawing the schematic by looking at the PCB traces and components, and it is very difficult to draw a schematic that makes any sense... It is completely contrary to the electronics world today, where one would prefer having a thousand extra transistors to save a single mechanical component (like an extra pole on the switch). The times have changed... |
32-ohm earphone speaker as a dynamic microphone
On 03/08/14 09:11, Stuart Longland wrote:
In the interest of science, I gave it a shot just then. With a 100nF capacitor in series to block the DC, I wired it to a DIN5 plug (all my radios have been set up with adaptors to DIN5 headset jacks) and tried it. It did work, but without any amplification or impedance matching, the modulation is well down. I might try winding a small transformer and see what that does. Well, I did some math, to transform a 32ohm load to ~600 ohms, I need a turns ratio of ~4.3:1. math.sqrt(600.0/32.0) 4.330127018922194 I played with this a little, and so I thought I'd add a couple more, make it a nice round number. 4.5:1 would give me a 648ohm load on the HV side. I had a few L8 toroid ferrite cores laying around, these to be precise: http://www.jaycar.com.au/productView.asp?ID=LO1230 Not being sure about the number of turns, I made a guess at 45:10, and started winding. Just tried it and now the audio is completely dead. Transformer loss is too high for the feeble signal out of the "microphone". My guess is that the turns count is waay too small, and that maybe 450:100 might be closer to the mark, but I really don't feel like winding that many turns on a toroid. 45 felt like a marathon. So for this to work, I need an in-line amplifier of some sort. My challenge is to RF-harden it, and have it compatible with both dynamic microphone inputs as well as electret: as the same headset will probably be used with both. (If the phone rings and I'm on the bike, I will sometimes plug the helmet into the phone to answer it. Right now all headsets are interchangeable with all devices, a feature I'd like to keep.) I'm thinking possibly a FET, since that's what's embedded in the electret capsules, however I suspect this will be prone to the same problem. Looks like I might be getting out the oscilloscope and making some measurements with the PTT down, try to figure out where the RF is getting in. |
32-ohm earphone speaker as a dynamic microphone
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32-ohm earphone speaker as a dynamic microphone
Stuart Longland wrote:
My guess is that the turns count is waay too small, and that maybe 450:100 might be closer to the mark, but I really don't feel like winding that many turns on a toroid. 45 felt like a marathon. Don't you have an ancient defective portable AM radio (you know, the kind that proudly mentioned "6 transistors" on the case)? The have those small E-I core transformers (soaked in wax) that are suitable for the job. |
32-ohm earphone speaker as a dynamic microphone
Lostgallifreyan wrote in
: But if you handle the twisted pair as usual, referenced to signal ground, catch the RF on the screen Meaning a screened mic cable. I said it badly there, thinking of the context in my earlier post which did mention it directly.. |
32-ohm earphone speaker as a dynamic microphone
Stuart Longland wrote in news:kea1bb-
: Looks like I might be getting out the oscilloscope and making some measurements with the PTT down, try to figure out where the RF is getting in. Did you try ferrite beads? To stop propagation along a screen? No-one seems to have mentioned them yet so I thought I should... |
32-ohm earphone speaker as a dynamic microphone
On 03/08/14 19:12, Lostgallifreyan wrote:
Looks like I might be getting out the oscilloscope and making some measurements with the PTT down, try to figure out where the RF is getting in. Did my post earlier not help? If your equipment has a frame ground and a signal ground, or perhaps an analog plus a digital ground, that might help. I did see your post… basically the frame is purely for the antenna counterpoise, and nothing else. Yaesu do connect their negative supply to the antenna ground, which yields a negative earth, however I do not use this for any kind of DC path. Not intentionally anyway. (It did happen by accident one day: +12v came in contact and nearly started a fire. Oopsie!) Unfortunately, I do not get provided with separate analogue and digital grounds. I suspect in the set they are one in the same. Yaesu do supply a separation kit: it basically consists of a mounting bracket for the head unit, a RJ11-RJ11 round cable for the head unit, a RJ45-RJ45 flat cable for the microphone and a RJ45-RJ45 adaptor so you can plug the handmic in. For the RJ11, I have no idea what the exact pinout is. On that cable would be serial data (tx/rx), power (probably 5V), speaker output, the "power/fast tune" button and a signal ground. It'd be nice to know which one is which but I'm guessing Yaesu probably want to keep that secret. For the RJ45, it's documented in the handbook. There is +5V, signal ground, open-collector inputs for PTT, Up, Down and Power/Fast Tune, then microphone + and -. The microphone input is a nominal 600ohm impedance, intended for a dynamic microphone. For convenience, I run 3 lengths of CAT5 with DB25 connectors at each end. At the ends I then plug in a suitable break-out cable which maps pins on the RJ11/RJ45 connectors to pins on the DB25. This is because the cables are frequently connected and disconnected, particularly on the front basket as I park my bike outside. I found RJ11 and RJ45 connectors became unreliable when they were used in this manner. DB25s have been good and reliable however. I did try wrapping this triple-CAT5 cable in adhesive aluminium tape and grounding that to the frame, but this did not help. I also have fairly sizeable ferrite cores clipped on. I could try running a separate screened cable just for the microphone, I'd probably have to have a separate connector too. I'll have to procure parts to do this. |
32-ohm earphone speaker as a dynamic microphone
On Sun, 3 Aug 2014, Rob wrote:
Stuart Longland wrote: On 03/08/14 11:10, Michael Black wrote: The circuits of such devices were often very cleverly designed, re-using many components between receive and transmit (using a multipole switch). They were really complicated switches, for the sake of a few transistors. Yep. This one I gutted, I recall de-soldering the switch and then reverse-engineering the pinout so I could replace it with a relay, which I did. A 4-pole double-throw relay IIRC. Not only that the switch has many poles, the circuit is often very tricky. It is not a receiver and a transmitter with a switch to toggle the power, antenna and speaker/mike to connect to one of them, no it is a blob of electronics that morphs between being a transmitter and being a receiver when the PTT switch is switched over. Yes, the switch would be so much simpler if they were just switching audio and power. I get the feeling these were the solid state equivalent of the one tube transcievers used to homestead the higher bands. They were a modulated oscillator on transmit, a superregenerative receiver on receive, and a common audio amplifier. There the space and cost of a tube meant they switch it between the two functions, but since it was a modulated oscillator, it was a simpler arrangement than switching between a superregen and a crystal controlled transmitter. Those single tube transceivers were certainly simple, and got people onto the higher bands. ONce a band got busy, there'd be a rule put in that you had to use crystal control (or have equivalent stability) on that band. So these rigs would start off at the "UHF" 10metre band, then move to 5metres, then up to 2.5Metres. Even fifty years ago, they were being used on the 420MHz band. SImple and cheap, you didn't get much range, but they helped get people on the band. In those days I sometimes tried drawing the schematic by looking at the PCB traces and components, and it is very difficult to draw a schematic that makes any sense... Expecially when you were a kid without much ability to figure out what the switch contacts were doing. All these circuit board traces would go into what amounted to a black box switch, crtainly beyond my skill at the time to trace out. It is completely contrary to the electronics world today, where one would prefer having a thousand extra transistors to save a single mechanical component (like an extra pole on the switch). The times have changed... I guess it makes sense at the beginning, but transistor prices dropped fast, yet the same scheme was used into the seventies. I assume when cheap walkie talkies moved to 49MHz, they didn't add transistors but still used that complicated switch (but I've never looked at a superregen 49MHz walkie talkie). Considering that "transistor radios" at the time were certain to tell you that they had "X transistors" you'd think the cost of adding a transistor for transmit (and thus be able to say "four transistors" or whatever) would increase sales enough that it would offset th cost of the extra transistor. It is a lesson worth repeating, adding transistors may nominall make the circuit more complicated (and expensive), but often results in the overall design being simpler. Of course, once ICs came along, that took the idea to the extreme, endless transistors in the IC, but you never see them. Michael |
32-ohm earphone speaker as a dynamic microphone
"Michael Black" wrote in message
xample.org... I get the feeling these were the solid state equivalent of the one tube transcievers used to homestead the higher bands. They were a modulated oscillator on transmit, a superregenerative receiver on receive, and a common audio amplifier. There the space and cost of a tube meant they switch it between the two functions, but since it was a modulated oscillator, it was a simpler arrangement than switching between a superregen and a crystal controlled transmitter. Nowadays when transistors are almost ten-a-penny, it is the switching that is expensive, so otherwise than as a novelty, there's not much to be said for single transistor rigs. I have in my museum pieces a boxed PM2A valve and the conditions of sale printed on the bottom say that it must not be sold to the public for less than 8 shillings and 9 pence, which pre-war was about 10% of the weekly take-home pay, but imagine paying £$40 today for each active device ! |
32-ohm earphone speaker as a dynamic microphone
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