Could this be fixed with a 9T:2T transformer between C63 and the mixer pad, and rebiasing Q6 for a gain of 2-3?
And/or, could someone flesh out the emitter follower and specify which parts from the emitter of Q6 should be removed? (Just C62 and R64?)
? ?Thanks!
? ? ?73, Andy, KG5RKP/MM
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On Sat, Jul 15, 2023 at 09:18 PM, Ashhar Farhan wrote:
Thanks for the analysis. There are two other significanr issues with the current mic amplifier.
1. The output impedance is 1000 ohms, the modulator needs a 50 ohms drive.
2. The gain is not less but too much. The electrer mic produces about 50mV of audio, the modulatoe needs less than 150mv drive. A gain of two or three is more than sufficient. The apparent loss is due to the mismatch between the 1000 ohm output and the needed 50 ohms drive.
These can be solved by adding an emitter follower with a 47 ohms resistor in series with the output of the emitter follower and removing the emitter bypass of the original mic amplifier.
- f
I've been working on the uBitx audio circuit for a while now and I think I've discovered a problem that I've never seen anyone mention before. If anyone has experience with this circuit, please review and let me know if I've missed anything. Here are my notes:
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?Audio Amp notes.
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The uBitx has always had challenges with the audio amp. It has insufficient gain for the condenser mic that is provided. Solutions include:
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a. Use a mic with an amplifier built in.
b. Add an additional amplification stage such as the SSM2167.
c. Increase the mic circuit gain.
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Mic amplifier circuit with suggested modifications. Also showing the load of the following stage.
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SEE ATTACHED FILE
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Analysis:
Given that the receive transistor Q70 is connected directly to the output of the microphone amplifier, the output of the mic amp must be less than 0.6v. We’ll say 550 mV to give us a bit of safety margin.
The input drive required to achieve 550mV out of the mic amp is 200mV. Most have claimed that the mic amp has a gain of around 20 due to the collector resistance of 1k divided by the emitter resistance of 47 ohms. This simple analysis is incorrect because it neglects the load impedance of the mixer circuit. Spice analysis and actual observation shows the actual gain to be about 2x or 3x.
A big problem with this mic amplifier is that it is incorrectly biased. With the originally specified bias resistor R61 set at 2.2k, the transistor runs out of emitter current at about 160mV of output voltage. In addition to limiting output, this causes distortion of the audio waveform. By increasing R61 to 10k, the circuit can easily output the 550 mV (1.1V P-P) originally suggested.
In my own uBitx, I have chosen to use the SSM2167 as a preamp in order to enjoy some voice compression. However, the simplest solution is to decrease R63 to 10 to 22 ohms. I’d advise not to go lower than 10 ohms, since that circuit should fully drive the mixer with a mic output of only 40 mV. But before doing anything else, increase R61 to 10K.
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