I think the limiter might be there on the 25 and not on the pc68xl. So ill check. Ill try the d10. Im looking for it on this...ok i see it , so direct direct.
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Uniden pc68xl /cobra 25ltd DIRECT AUDIO INJECTION POINT?
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View attachment 39898I have some experiments to try i guess. Im using the mic input. I have just clipped the wire at mic gain and tacked it to the board.
Maybe i should just add a relay to swith it out on rx
A mauldulator or asymod is much better than the direct injection method with these rigs.
If you want a project figure out the modulation impedance of the final and driver. Let's say 1 amp at 13.6 volts.
13.6 ÷ 1 = 13.6 ohms
I don't know what you have for audio gear but say the output of the audio amp is 4 ohms. 4:13.6 reduces to 1:3.4. Now get the square root of each and we have 1:1.8. Now find a suitable transformer with a 1:2 ratio to replace the stock unit. Antek transformers work well. You'll need to take measurements and do your own math.
This information was posted elsewhere by w2btk. I've tried it with toob transmitters and had excellent results.
If you want a project figure out the modulation impedance of the final and driver. Let's say 1 amp at 13.6 volts.
13.6 ÷ 1 = 13.6 ohms
I don't know what you have for audio gear but say the output of the audio amp is 4 ohms. 4:13.6 reduces to 1:3.4. Now get the square root of each and we have 1:1.8. Now find a suitable transformer with a 1:2 ratio to replace the stock unit. Antek transformers work well. You'll need to take measurements and do your own math.
This information was posted elsewhere by w2btk. I've tried it with toob transmitters and had excellent results.
Thats above my skill set.
Ive not looked at how the mauldulator works.
I suppose i could do something with a uniden grant or a cobra 140 i have or a pc244 maybe thats more worthwhile?
Shadetree Mechanic
Delaware Base Station 808
The mauldulator connects to the rig with 2 wires. A ground and the other connects to the finals. Easy install. Then the adjustments on the mauldulator board control the carrier power, audio level and asymmetry. The asymod is the same thing. They are all single voltage now. They used to require 3 power supplies to run.
The uniden grant is a good candidate for what you're trying to do now. All you have to do with that one if I remember correctly is pull c174 and inject audio into the positive side through a blocking capacitor.
Well, I'm not trying to make this harder, but I've done this myself - I just don't have the "proof" because I used a "line level" input from a tape deck (cassette) that used MIC inputs, a LONG time ago...
- But that required me to DEDICATE the radio to the direct injection method, not a bad way to go but if you wanted to revert - well, it means you'll need to keep a shopping list of "what goes out, goes back in" sort of thing if you want to revert the effort back to stock.
- You do have the flexibility to remove the amp section that is causing this - mainly TR13's Mic amp and it's associated stuff - it is up to you to control the levels after this conversion
Anyhoo, I just back off the INPUT cap into the pin 4 but in this setup, you may need to lift C89 and use it as your tap point
TR13 is the crux of a tie point for voltage and it's Mic amp output - so the output is live with voltage into C89 - so you have to lift it out and perhaps to save headaches, remove TR13 from the board so you don't have to concern yourself with any more "loop" issues. Again, this method doesn't give you a limiter - but it gives you the line level input ability to drive the Audio Amp. Just keep capacitance low - it's fussy.
Im a big believer in a brick wall peak limiter in the audio chain for this stuff. I use an Aphex Dominator 2 720 as a modulation limiter. If you inject audio anywhere that bypasses the radio's mod limiter you need a peak limiter.
If you don't use one you'll get the occasional overshoot that over modulates and sounds shitty, softer spoken words will be down in the mud and everyone will tell you to turn up the mic gain. You'll turn it up and everyone will tell you you're over modulated.
If you don't use one you'll get the occasional overshoot that over modulates and sounds shitty, softer spoken words will be down in the mud and everyone will tell you to turn up the mic gain. You'll turn it up and everyone will tell you you're over modulated.
Wow. Right to the finals. How much are these boards?
The uniden is the grant lt. So somewhat different from the xl
I think (still) you're making this harder than it needs to be...
In light of "direct injection" you have several issues to overcome in the radio.
You did mention using the Mic amp that's already there...good, now let me help you get thru this...
This may be lengthy but it's for a reason, there's a lot to understand about fidelity, RF - bandwidth, Dynamics and making it work.
Refer back to my "drawn" schematic... I put the Mic gain circuit in there too to help someone understand how Cobra versus Uniden - feed their mic amp circuits.
In light blue of the schematic drawing - shows the highlited differences in circuit values used for the SAME component support, but why the different value.
PC-68? Uses a 10uF at C148 position.
Cobra 25's use a 47uF instead...
Why such difference, well they did it so you can hear the difference the tonal and dynamics - the punch the audio has between both models is heard differenly.
Note that Uniden sounds "flat" and more "Sharp" - because of the changes in dynamics - or compression of audio drive level between both.
It is because of how the cap works in TR13's emitter leg of it's amplifier circuit.
C89? Values used - Differs between models but does not vary by much in value - I've seen 0.001uF (102) and even 0.0022uF (222) in there but did you also look at how Cobra 29 does it?
Please study the above carefully...
You'll see that TR17 a.k.a - our TR13 - has an extra resistor R67 - see inset middle photo above - it's a 33 ohm resistor. IN-SERIES in the emitter leg, with the 1K and the 47uF cap at the bottom.
This design is actually providing LESS GAIN than the PC68/Cobra 25 has - but they can use a 0.033uF (333) cap in there?
Why? Again, your "squeal" is from loops and gain factors - Many "direct inject" for C29 use the C37 approach and remove TR17 completely.
The C-25/ U-66/68 series has more limitations because the front end inputs from the front panel are more limited.
They miss out on some other issues too, like; they can easily adapt a mixer/summer rheostat in-line and across Pin 1 and 2 of the Mic jack and get away with a lot more than using the "Can opener" approach and trying to bypass the whole mess to begin with...on both sets of chassis C29 - U78 and C25 and U68.
The Mic amp "saturates" easily in both designs, they know this. It is for the issue of Intelligibility - even though they design the amp with an asymmetrical non-linear "flattop", so the audio peaks more in the Smaller chassis versus the larger brothers - that simple 33 ohm resistor limits bandwidth GAIN but provides more FIDELITY by reducing that gain, to increase bandwidth function by a small factor as a result. The distortion you hear is less in the reduced gain, because the fidelity factor is modified thru the simple 33 ohm resistor - less gain, more bandwidth - better sounding results.
There is more than one way to flush out 'possums from hiding in your garbage pail.
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I understand some of this , I have to sit down and study it .
I abandoned the cobra. And I am going with the pc68xl only at this time.
I cant find my mixer but ive been using the turner plus 3 and it sounds very good from what i can tell. The band hasnt been open and i dont have locals here so i cant test it yet.
I abandoned the cobra. And I am going with the pc68xl only at this time.
I cant find my mixer but ive been using the turner plus 3 and it sounds very good from what i can tell. The band hasnt been open and i dont have locals here so i cant test it yet.
Hoping for some education.... why would one want to direct-inject audio and bypass the previous microphone/amplification circuitry on board? What does that bring to the table?
Well, then...maybe it's me that is making this harder...
Ok, I had to show you "cobra 29" versus the Cobra 25 - so you can see SUBTLE changes they put in their boards.
It's understandable that you "get a squeal" because of what they're trying to do back there - and make it sound like it works - and you're trying to add in a source for mixing in - even though you are not mixing in per-se, but summing in another point. You're better off to use the Mic amp itself - it's been made / designed to be somewhat stable and provide some fidelity - we can "tweak" the Mic amp to obtain a different level of fidelity or emphasis equalization.
Do your mixing before it arrives to, and at a high enough volume level to overcome limitations, to the radio. Not within - because of what the typical Dynamic element already has against it. You use a 600 ohm cartridge - basically a speaker magnet and coil - only the coil has more winds, and you are talking into a vary small "cone" or diaphragm.
Power mikes can squeal big time too, but if you INSTALL them and use the right level of drive - you don't have a bad sounding little radio.
You don't have to sum in another point if you don't feel like it - as you tried earlier trying to tap in - you'll need to "buffer" your effort, from the base circuitry in the radio and keep it from adding another loop to cause a squeal.
Most of the time, the "direct inject" method requires some understanding of how a "Phono" and Tape Monitor system works in a typical Stereo receiver.
Phono uses a turntable and a magnetic cartridge - it greatly amplifies the signal and brings it to a level that the Stereo can use that is above a noise level and can overcome the inherited traits of switching, impedance mismatching and the losses in cabling connections and to maintain an adequate Signal to Noise ratio so you're not amplifying the noise the amplifier stage has - you only want the audio signal.
Tape Monitor is similar, only it is a type of loop system designed to be strong enough in power to overcome similar limitations in both losses and in fidelity degradation. It's at a higher level of drive that keeps itself above a Noise level figure of merit used in the Audio Industry and also to provide the dynamic range, but designed not to add any, if little, distortion elements like harmonic and impedance mismatch input levels - % THD is a sum of these factors and some more - as a total weighted figure of merit to tell the consumer that you are getting this type of performance, with this much of it as "distortion".
The amplifier for the Mic is a simple amplifier - being made to do a lot. It uses several means of support (or restrictions) to keep itself from squealing. It's a lot like the PHONO amp in a typical stereo...only it's a SINGLE Transistor amplifier - being told, not asked,. to do a lot.
Ok, the above is a typical Phono preamp form a JC Penney stereo - yes it's old, but so is the technology in your radio - both are from the same era at least in LEGACY hardware and function.
It uses certain key elements that you can install in your microphone to overcome some of these limitations you are encountering.
One being - when you hook up, you need to "balance" your input to meet the expected IMPEDANCE the amplifier stage expects.
It also uses Equalization filters to offset or emphasize some of the tonal range that this amplifier is supposed to faithfully reproduce and then to use a process called De-Emphasis to reduce, a reduction of amplification in a tonal range or spectrum of sound frequencies - other spectral portions of the audio spectrum so it doesn't "color" the sound and you'll hear the artist in their normal tonal range of voice and the instruments played in their normal fundamental ranges of music.
I told you the above, so all you have to do, is get your microphone handset - wire a cable from your tape monitor loop or your Mic amp mixer - take the shield and put it to Mic Shield and Audio live goes where your other Mic wire that routes to Pin 2 - goes - you then solder a resistor ACROSS these two wires to reduce or match the expected impedance the Mic Amp stage wants.
The 820 ohm resistor above - is a good example - your mixer would also use a capacitive coupling scheme to keep it's own voltages from interfering and being interfered with so your mixer is isolated from the Mic amp thru a cap - you add a resistor across your Audio cable leads to Attenuate and help remove eddy currents that can form in the cabling - the mixer uses a scheme similar too - usually 1.5K as a factor of IMPEDANCE (A high impedance level output).
You may not need to use 820 ohm, but you should use something, even a 1.5K resistor - which when parallel'd with the other (your mixer) impedance matching effort - the two 1.5K's divide down into 750 ohms - even closer to the expected 600 ohm impedance.
It's better to start with the Egg, than to try and build a Chicken...
Ok, I had to show you "cobra 29" versus the Cobra 25 - so you can see SUBTLE changes they put in their boards.
It's understandable that you "get a squeal" because of what they're trying to do back there - and make it sound like it works - and you're trying to add in a source for mixing in - even though you are not mixing in per-se, but summing in another point. You're better off to use the Mic amp itself - it's been made / designed to be somewhat stable and provide some fidelity - we can "tweak" the Mic amp to obtain a different level of fidelity or emphasis equalization.
Do your mixing before it arrives to, and at a high enough volume level to overcome limitations, to the radio. Not within - because of what the typical Dynamic element already has against it. You use a 600 ohm cartridge - basically a speaker magnet and coil - only the coil has more winds, and you are talking into a vary small "cone" or diaphragm.
Power mikes can squeal big time too, but if you INSTALL them and use the right level of drive - you don't have a bad sounding little radio.
You don't have to sum in another point if you don't feel like it - as you tried earlier trying to tap in - you'll need to "buffer" your effort, from the base circuitry in the radio and keep it from adding another loop to cause a squeal.
Most of the time, the "direct inject" method requires some understanding of how a "Phono" and Tape Monitor system works in a typical Stereo receiver.
Phono uses a turntable and a magnetic cartridge - it greatly amplifies the signal and brings it to a level that the Stereo can use that is above a noise level and can overcome the inherited traits of switching, impedance mismatching and the losses in cabling connections and to maintain an adequate Signal to Noise ratio so you're not amplifying the noise the amplifier stage has - you only want the audio signal.
Tape Monitor is similar, only it is a type of loop system designed to be strong enough in power to overcome similar limitations in both losses and in fidelity degradation. It's at a higher level of drive that keeps itself above a Noise level figure of merit used in the Audio Industry and also to provide the dynamic range, but designed not to add any, if little, distortion elements like harmonic and impedance mismatch input levels - % THD is a sum of these factors and some more - as a total weighted figure of merit to tell the consumer that you are getting this type of performance, with this much of it as "distortion".
The amplifier for the Mic is a simple amplifier - being made to do a lot. It uses several means of support (or restrictions) to keep itself from squealing. It's a lot like the PHONO amp in a typical stereo...only it's a SINGLE Transistor amplifier - being told, not asked,. to do a lot.
Ok, the above is a typical Phono preamp form a JC Penney stereo - yes it's old, but so is the technology in your radio - both are from the same era at least in LEGACY hardware and function.
It uses certain key elements that you can install in your microphone to overcome some of these limitations you are encountering.
One being - when you hook up, you need to "balance" your input to meet the expected IMPEDANCE the amplifier stage expects.
It also uses Equalization filters to offset or emphasize some of the tonal range that this amplifier is supposed to faithfully reproduce and then to use a process called De-Emphasis to reduce, a reduction of amplification in a tonal range or spectrum of sound frequencies - other spectral portions of the audio spectrum so it doesn't "color" the sound and you'll hear the artist in their normal tonal range of voice and the instruments played in their normal fundamental ranges of music.
I told you the above, so all you have to do, is get your microphone handset - wire a cable from your tape monitor loop or your Mic amp mixer - take the shield and put it to Mic Shield and Audio live goes where your other Mic wire that routes to Pin 2 - goes - you then solder a resistor ACROSS these two wires to reduce or match the expected impedance the Mic Amp stage wants.
The 820 ohm resistor above - is a good example - your mixer would also use a capacitive coupling scheme to keep it's own voltages from interfering and being interfered with so your mixer is isolated from the Mic amp thru a cap - you add a resistor across your Audio cable leads to Attenuate and help remove eddy currents that can form in the cabling - the mixer uses a scheme similar too - usually 1.5K as a factor of IMPEDANCE (A high impedance level output).
You may not need to use 820 ohm, but you should use something, even a 1.5K resistor - which when parallel'd with the other (your mixer) impedance matching effort - the two 1.5K's divide down into 750 ohms - even closer to the expected 600 ohm impedance.
It's better to start with the Egg, than to try and build a Chicken...
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