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Direct injection question.

Discussion in 'CB and Export Equipment and Accessories' started by Grogan, Oct 25, 2018.

  1. Grogan

    Grogan W9WDX Amateur Radio Club Member

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    was wondering about direct injection or a radio that can be modded for 3K wide receive and transmit. What would be the best radio. Thank for your input.


     

  2. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    For AM and SSB, probably a Cobra 148 or 2000. For AM only, a Cobra 29LTD Classic. The 29 in particular is a good candidate because it's transformer modulated.


    ~Cheers~
     
    Shadetree Mechanic likes this.
  3. Grogan

    Grogan W9WDX Amateur Radio Club Member

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    thanks. I was going to start another project. This might be it for the winter months.
     
  4. 543_Dallas

    543_Dallas Sr. Member

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    3k isn't very wide. Are you wanting am or ssb? 4khz (8k wide AM) can sound very good if set up correctly. 5khz (10k wide AM) is kind of the point of diminishing returns. Not many people are hearing that wide and more doesn't help much anyway.

    The 148 works pretty well but it will not transmit on ssb the way I've always done it. I believe we were pulling out c174 and injecting audio into the + hole....it's been a while. It will take a good amount of audio drive unless you run a low carrier. Anything over 2 watts is too much. The thing are only good for 8 to 10 watts pep before you start running out of headroom and sounding like poo.

    I have had decent luck with the 3 diode ultramod circuit on the 148 and tr296. It was used on plate modulated rigs of the day to keep the negative peaks from hitting baseline and to protect the mod iron. You can't over use it. It's just a safety to keep the negative peaks in check. You'll still need a peak limiter in your audio chain, this is basically your modulation limiter.

    For AM only thise asymod boards work very well. The board will run cooler with a cobra 25 or 29 with a bipolar final. SSB rigs and mosfets are biased and the old 25/29 with the bipolar is ran in class C and more efficient....no flames, this is not an amplifier, class C is kosher for high level modulation. The asymod a knock off of the motor mouth maul mauldulator. It will let you fine tune your modulation levels easier than the direct inject thing.

    I would rather have an HF rig than a cb, especially if you want to use ssb. You connect your audio through an isolation transformer to the data port on the back. On modern rigs that port usually will give you enough bandwidth to sound really good but is still filtered. Direct injection or an asymod will pass 20k of audio or more. You have to roll off the high frequencies with your equalizer or build a filter so you don't splash on adjacent channels.

    I spent a lot of time and money on this crap and now use an Apache Labs Anan. The software has more audio processing than you'll ever need and all it needs is a mic plugged into the front. They seem expensive but it would have been cheaper if I bought one to begin with. I don't regret buying the other stuff because I learned a lot.

    I still use my outboard gear with the anan. After a very long time I finally got to where I was completely satisfied with the sound earlier this year. The 200 to 500hz range was not doing good things for my voice. Sorry for the long winded response. I need to get some radio time in this weekend. :LOL:
     
    Shadetree Mechanic likes this.
  5. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    You may know all this already - so I will warn the reader that this is posted for LEGACY reasons, not just specific...it's meant for others that follow this thread...I don't want to bore you if this doesn't suit you...

    There are a lot of radios' you can try to do this to, but will they hold up to your scrutiny?

    You ask two questions, and both deserve a lot to cover - but you don't have a lot of time, winters' are bad enough...you do have to shovel the driveway and go out to get mail and groceries every now and then....

    One is - Direct Injection.

    I'm presuming you're setting up a station with a better setup than a simple mic and cord hookup to a radio - you want a more dedicated system for it.

    Well, are you using a computer to process the audio for transmission or are you making a system that is yours and not specifc to a software driven program?

    I'm thinking direct - as in dedicated - directly into and specifically for a radio.

    So to answer that, you need a little background of differences in the two most common types of radio modulation used...
    • One is Reactance modulation - that is simply a transformer that passes battery voltage thru a coil...
      (power supply) DC power thru it and after "about so many turns" into the coil winds of the transformer, they place a tap and add a capacitor to it - so they can "inject" amplified audio into the transformer. Then as the power from DC as well as Audio "melds" together thru these winds - a reactive transformation takes place where power is changed - some is converted to audio frequency via a change in phase relationship - yes, it kinda acts like AC - but uses audio frequencies. Eventually it's applied to the driver and final stages. To protect those stages a "spike diode" is used to keep the power DC and audio moving in a positive direction.
    • Another is AM Regulation...the audio is amplified thru a mic amp circuit as above, but instead of placing audio thru a transformer it is sent into a transistor amplifier used to generate this power using DC pre-set to a level of DC - I call BIAS - a level of power for the Final and Driver stages to produce a carrier - and audio is added in thru a transistor amplifier set up like a mixer - which uses, or looks at the pre-set level of BIAS as a zero -start-point. Meaning that the audio envelope and all volume starts from this DC level - volume can go up or down - and the amplifier attempts to keep DC BIAS stable (it uses a regulated voltage to set this reference point - AM PWR) but frequencies in the Audio signal are applied thru the mixer that induce a swing of power thru a range of frequencies - this is generally known as the "envelope" - it is a range of frequencies that the mixer ties in the BIAS power and sends it to the amplifier - arranged in a fashion called "Darlington" that takes battery or power supply DC voltage and converts this into that signal that powers the Driver and Final. In this type of circuit, Spike diodes are not normally needed for the DC BIAS present in the audio signal helps keep the Driver and Final forward biased - does not mean that it's fail proof, but it is not designed for a wide audio bandwidth nor a large swing in power at the DC BIAS level to affect power drive for the output stage - if too much is altered and no protection provided - well, it pretty much self-destructs.
    If you're going direct injection - you can alter the mic amp to help work as a buffer stage to the audio amp - or you can bypass the mic amp and apply the audio directly.

    Both have pluses and minuses - Mic amp can be tailored for you requirement of the 3kHz bandwidth by simple filter and bypass / feedback techniques, but it is not a cure-all for all. Some limitations are the power level as well as the distortion products and artifacts the amp can introduce to the signal because of it's own power and bandwidth limitations due to the drive level the amp will have to amplify and pass thru.

    Direct coupling to the audio (or mic) amp is ok, but also remember there are Equalization limits as well as bandpass in, thru and onto the next stages of amplification.

    Have you thought about Parametric Equalization? This method is a little more adjustable and allows you to tailor more for the needs of you to your radio. Not for the feint of heart though...

    Graphic Equalization can work. But, you have "fixed" bandwidth and frequencies - and due to the nature of the filter beasts inside - they are not narrow enough and can expand the sound beyond the capabilities or your needs to make the system work like you want it too. They can be adjusted - but not able to trim off the outer edges sharply enough, which can again - introduce distortion and artifacts that color your audio and you may not like the results.

    Parametric EQ systems run several filters in "bands" like Bass Treble and Midrange - they are the three basic building blocks for any type of EQ system. You can use one, but you limit yourself in several areas to tailor your effects as well as offset limitations to the radio or any number of radios you wish to use this on.

    Not just 3 bands to play thru and amplify, but Parametrics' also have the ability to center in on a range of frequencies within that band to peak out and also adjust the "Q" or the sharpness of the filter as you peak in that range.

    I went here,
    https://buildyourownclone.com/products/paraeq

    Wish I had this back in 2008 before the "Crash" when, I too, lost a lot of stuff I was just beginning to play with...

    Secondly - receiver - if you use a CB radio - pretty much any CB (AM only are more preferred) would provide the bandwidth you seek, but quality varies - so purchase a good quality radio with several noise reducing features is better than a simple receiver - even though the simpler the receiver - the better that bandwidth is - but it comes at the price of higher noise levels too...

    Now as far as radio - Receive? Pretty much AM-only types can fit this scenario well, some SSB types that have "conversion" in several stages before "deconversion" also can work. SSB - base radios have to compromise performance to bandwidth so they are pretty much ruled out for any true level of quality based upon the single or dual conversion principles. AM Regulation based injection provides the easier level playing field but Reactance modulation if performed correctly can inject a level of quality and punch in signals' drive that is hard to rival.

    So what's your game?
     
    Shadetree Mechanic likes this.
  6. Shockwave

    Shockwave Sr. Member

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    The transformer based modulator in the 29 and the transistor modulator in the 148 have both been discussed. Both have the ability to sound excellent when properly modified but the transistor series pass modulator has more bandwidth capability than the transformer. The difference is small and mostly in the low end bass because the transformer will distort the lowest frequencies before the transistor.

    Even the best modulation transformers struggle to get down to 20 cycles and many inexpensive ones start introducing distortion under 80 cycles. That's because the DC current feeding the final will saturate the core under the load of heavy low end bass. If we were to drive a series pass transistor modulator with a DC coupled AF amplifier, you could control the carrier level by the amount of DC injected in the mic line! Meaning the transistor modulator can be designed to have a bass response down to DC.

    In fact, the series pass modulator used in the 148 does go down to DC and it does control the carrier level through that modulation transistor. The only difference here is that all of the audio stages in front of that Darlington transistor pair are AC coupled through a 1 uf electrolytic. Modifying the audio stages in front of that modulator and adding one band of passive, parametric equalization inside the radio (copy of old electric guitar tone control) to "fix" the 200 to 500 cycle range makes these rigs sound broadcast quality with just a good mic plugged in the front.
     
    #6 Shockwave, Nov 2, 2018
    Last edited: Nov 2, 2018
  7. 543_Dallas

    543_Dallas Sr. Member

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    It took me a long time to figure out the 200 to 500hz range was not flattering. Eventhough everyone told me it sounded good but there was just something not quite right.

    I have not attempted it with the cobra 29 but believe the low range distortion can be improved with a diode string between the mod transformer and the driver and final. That should lower the dc current through the xformer. Basically remove d8 and install more diodes in series like the old school swing mod. This will also lower the carrier level so you can only go so far.
     
    Shadetree Mechanic likes this.

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