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Series Pass Modulator for D.I.Y. Hi-Fi & Asymmetry

Discussion in 'CB Radio Modifications' started by LeapFrog, Jan 2, 2020.

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What is more important to you?

  1. Audio Quality

    12 vote(s)
    92.3%
  2. RF Power Output

    1 vote(s)
    7.7%
  1. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    =========================================
    Solid State "Ultra Modulation" Series Pass Modulator
    Inspired by: RevBow's "BME"
    & CBphreaker's "Wabi-Sabi Mod"
    written by: LeapFrog 1-1-2020
    =========================================
    Wideband Audio Modification W/ Controlled Carrier
    Too long, didn't read: Discussion of series pass modulator modification, not a "how to".

    In high-level modulated radios such as Cobra 19/25/29's the "communications grade"
    modulation transformer is an obvious audio bandwidth choke-point or bottleneck when Hi-Fi experimenting.
    Let's remove the audio bottleneck and Install a solid state modulator (darlington configuration used here).

    The frequency spectrum of your audio signal will determine how wide you're transmitting.
    After the Series Pass Modulator is installed, "Carrier Control" will be in your hands!
    You MUST employ some form of processing/limiting if you intend to bypass the Mic Pre-Amp (AMC/feedback circuit).

    If you wanted to stay (very) mobile you could modify the stock mic pre-amp (Goldfinger style) and simply use the 4 Pin Mic jack,
    this way you wouldn't need to add any additional switching circuits for the audio chain, it's all there.

    You'd have to get a little creative and work in some type of discreet feedback between the Audio source, Regulator & preamp/AMC.

    With enough experimenting you can get Positive Peak Expansion/Negative Peak Compression type results, without a P.C. or rack gear.

    The trade-off for achieving wideband frequency response, the radio will now make less power than before you started.

    While the transformer multiplied voltage, the Darlington Configuration functions mainly as a current amplifier.

    Maximum PEP is now determined by the collector voltage of the modulator, your audio, D.C. bias level, & the configuration of the RF output stage(s).

    The Grant XL can produce 4 Watts Carrier with less voltage than you'd find measuring collector voltage in a Cobra 29 LTD producing 4 Watts (about 1/2 or 6 Volts) The 148/Grant has an additional transistor in the PA.

    The only "work around" I've come up with is to raise the collector voltage of the modulator (about 24 Volts).
    This increases the available head-room, or "voltage swing potential"
    You can use a D.C. boost converter or separate power supply.

    After this modification a $25 Studio Mic and $50 Compressor/Limiter can get you into Hi-Fi CB Audio!


    ~~~~~~~~~~~~~~~~~~
    Pros
    ~~~~~~~~~~~~~~~~~~
    *Wideband Audio Frequency Response: 20 Hz to 16 kHz

    *Ready for Hi-Fi Studio Mic + Rack Gear (with optional buffer or attenuator circuit installed)

    *No need for expensive modulator boards, they rely on the Series Pass Modulator anyway! (Must use limiter somewhere)
    I.M.O. That kind of money would be better spent on rack gear, Effects, EQ, Mixer and so on.



    *Can use a cheap computer headset microphone and Laptop/P.C. with freeware (digital processing/D.I.)
    but latency is a concern. (even with great or multiple sound cards, there is a slight delay).

    *Carrier Control, and Asymmetrical Modulation (With proper build/configuration, & or limiting/processing).

    ~~~~~~~~~~~~~~~~~~
    Cons
    ~~~~~~~~~~~~~~~~~~
    *Voltage Drop through the Transistors.
    You can use something like the Vishay VO1400AEFTR, T.I. PGA4311, or Any analog MUX to switch your (processed)
    line level audio utilizing the factory T/R switch voltages (Transmit Audio is now PTT operated, once again!)
    ***Audio Switching Problems Arise, especially if you're not retaining use of the factory mic preamp (+ Factory T/R Switch).
    Die-Hards will say that using a mechanical relay is the only way to go.

    The Modulator Collector Voltage is a huge factor in this build, and will limit headroom if you let it.
    (Standard 13.8 Volts Supply = QRP)
    (22 Volts or Higher = More Power)

    Instead of limiting the audio going into the modulator, you could use a regulator to drop 24 Volts down to about 12 Volts; creating a Variable "Max Peak" Control by limiting the collector voltage of the modulator.

    After all, the RF Final + Driver's Collectors are only going to see what the Modulator can send.
    Honestly the RF PA section needs modifications to raise gain/PEP anyway; the transformer multiplied voltage, but the Darlington multiplies current.

    If you stick with 13.8 Volts (and no other supply) you are "stuck" with about whatever approximately PEP your "dead key" (resting carrier) was before the mod!

    So if the radio was only producing a 4 Watts resting Carrier with 13.8 Volts input before the mod,
    don't expect a lot of head room left for positive voltage swing.
    For example, a Cobra 2000 GTL with blown regulator may produce a large carrier and virtually no modulation..
    The Cobra 148/2000 has an additional transistor in the RF PA

    ==============================
    ~=[Modulator Bill Of Materials]=~
    ==============================
    =============================
    ~=[Audio Signal Path Switching]=~
    =============================
    Considerations & Solutions:

    We have to work out a solution to isolate (switch) the receive & transmit audio paths.
    The radio normally switches between TX & RX Audio switching by action of the Mic PTT.

    Factory audio chain is roughly this:
    Mic Audio ~or~ RX Audio (determined by PTT) => Pre-Amp => High Level Audio I.C. => Transformer =>Output (RF PA Section or RX Speaker)

    Modified transmit audio chain is like this:
    Studio Mic => Rack Gear/Limiter/FX/Filter => Buffer Amplifier for "Line Level" Switch => High Level Audio I.C. => Modulator => RF P.A.


    ==================================
    ~=[Choose a method to Key the Rig]=~
    ==================================
    A) Get an old hand mic and use that, I snip the audio wire off internally.

    B) Wire a female "4-Pin mic connector" to a suitable foot pedal switch, wire in rx, tx, gnd; you won't need the audio wire.

    C) Add a key-up jack on the radio (to key the transmitter, short pins 3+1 together) any momentary switch
    can operate this jack, beware of your RX audio signal path if you're not using an SDR

    D) If you plan on modifying the factory Pre-Amp (Goldfinger Style) just use your favorite 4 Pin mic.

    Optional: Add an RCA mono jack on the rear of the radio (to key an amplifier)
    This is only hot while you're in TX mode, when you unkey the RCA goes open circuit.

    =============================
    ~=[If using the factory audio I.C.]=~
    =============================

    If you bypass the factory mic preamp and input an attenuated "line level" signal to the high level audio amplifier, you will need to create an audio switch for If you don't "mute", buffer (or disconnect) this line you will hear your transmit audio chain in the speaker while you're in RX mode,
    (without muting/running a gate on your rack this "pseudo talk-back" could get ugly with feedback)

    cannot simply disconnect the output of
    the preamp => from the Input of High Level Audio I.C.
    because if we did, no RX Audio path would exist, this isn't an issue if
    you're using the radio as a transmitter only, or if you've used an SDR + P.C. for
    your receiver.

    ============================
    ~=[If using the factory receiver]=~
    ============================

    (x1) 1,000uF Cap and a jumper wire are required for the new Receive Audio Path once the transformer has been removed; our connection from "RX audio-path-to-speaker Out" no longer exists.

    We cannot connect the RX speaker directly to the base of Tr1 as we now have a D.C. Bias "Carrier Voltage" present w/ the audio!

    To remedy this we will be using a 470uF-to-1,000uF 35V 105c Electrolytic Capacitor (optionally bypassed w/ 1uF tant) to connect RX speaker to Base of Tr1 in the Series Pass Modulator. (Negative end goes..)
    Now we have receive audio again, and it's controlled by the mic PTT circuit!


    =================================
    ~=[Comments about audio switching]=~
    =================================

    You can switch the radio between factory audio chain (4 pin mic audio)
    and external source with a relay or other switching scheme.

    ========================
    ~=[Comments about SDR]=~
    ========================

    If you decide to use a computer with your receiver you can simply leave the "RX audio-to-speaker" connection out of circuit and configure switching for the
    P.C. + SDR Dongle
    If you want to receive true wideband audio, use can an SDR by itself, or you can use it with an I.F. of the radio, gets around filtering and effectively give you a "multi-mode
    receiver" that can demodulate a lot more than just A.M. it can also act as a "panadapter" or "band scope".
     

  2. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    PowerRail-8Volt_supply_Close-up.png

    Don't perform this mod on your prized Cobra 29 LTD, use a beat up & scratched radio.

    Your power output will suffer in the name of audio.
    You need to figure out a solution for limiting & switching, this is just the bare bones.

    Power output with 13.8 V correlates to your resting carrier before modification.
    You will want your D.C. bias (carrier voltage) to be no more than half of the collector voltage.
    (slightly less actually because of V drop through the modulator)
     
  3. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    LeapFrog you bin playin' possum!
    Good write up.
     
  4. Mustang 131

    Mustang 131 Sr. Member

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    Great info..where was all this 25 years ago..and piles of rusty iron!
     
  5. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Great info! Now I have something to think about, I was looking at audio transformers to direct inject my 29LTD but I may try this instead. Thanks!
     
  6. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    I see "T T" has posted a new video!


    [The Series Pass Configuration by CB Phreaker's Schematic]


    Who wants to build something that can sound like this?! :sneaky:

    It's not too hard :)
     
    Shadetree Mechanic and 500kpot like this.
  7. nomadradio

    nomadradio Analog Retentive

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    Very cool, but where do you find a CB receiver with any frequency response?

    All those highs and lows are wasted if the receiver at the other end filters them all out.

    73
     
    Rick Hicks likes this.
  8. sp5it

    sp5it Master of puppets

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    Any schematic?
     
  9. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    Sp5it, here is the basic schematic.


    Carrier supply voltage can be provided by any adjustable regulator or circuit that provides reasonable current.

    The Darlington configuration I like to use
    2N2222
    2N6487

    If going "direct" to the audio amp (rack gear or P.C.) you'll want to use a buffer or attenuation and switch the single off during RX

    Nomad, I believe using an SDR and accommodating for TX/RX switching is an ok solution.
     

    Attached Files:

    #9 LeapFrog, Jan 11, 2020
    Last edited: Jan 11, 2020
    sp5it likes this.
  10. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    Updated the image a little.

    Going to add in a circuit for the Buffer/Switch
    with a lil' creativity, and a relay
    you can add a switch to choose between
    the pre-amp built in (4 pin mic socket)
    and your external source.
     
  11. NZ8N

    NZ8N Active Member

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    The AMC feedback should be sampling the voltage output of the circuit to the Final and Driver, not the input to the Darlington Pair.
    Also, I noticed that the signal on the video was 40kc wide?

    Otherwise, looks like a neat project.

    Edit: - You forgot about R166, the 3K3 resistor that is a part of the AMC feedback circuit, the rest of it is fine.


    Cobra 148 AM Modulator .jpg
     
    #11 NZ8N, Jan 12, 2020
    Last edited: Jan 12, 2020
    Shadetree Mechanic likes this.
  12. LeapFrog

    LeapFrog Wielding Hanlon's Razor

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    For this mod to be "standalone" some form AMC/limiting feedback is necessary.

    NZ8N, I've never bothered using the diode/resistor labeled as AMC feedback, I've always left those out and do the processing/limiting externally. Following your highlight I see they are part of a feedback chain.
     
    #12 LeapFrog, Jan 12, 2020
    Last edited: Jan 12, 2020
    543_Dallas likes this.
  13. Rick Hicks

    Rick Hicks Member

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    ....wait until you hear my 60's era Neumman condenser thru a Neve 1073 pre and into an UA 1176 limiter back into a customized Demeter tube DI to bring back to line level and into audio circuit of my chromed cobra 19 using direct injection. hell yea it rocks!
     
    LeapFrog and Shadetree Mechanic like this.
  14. Robb

    Robb Yup

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    Wow; a little over the top - ya think?!?
    But yeah, it better rock!
    Beau coup expensive - too
     
  15. Low_Boy

    Low_Boy Well-Known Member

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    How come on the spectrum analyzer it looks like he is on 3 channels?
     

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