========================================= 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".