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Thread: Logging audio from ASTRO Spectra

  1. #1
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    Default Logging audio from ASTRO Spectra

    Hi all

    I need to log audio from a digital encrypted repeater -- thus the reason for needing to use an ASTRO Spectra and not a scanner, and my resulting complication which I am relying in this post

    - I'm using pins 6/7 from the ASTRO Spectra accessory jack (dashmount);
    - I've got a 22-ohm 5W resistor across both speaker leads;
    - 100uF 25V cap in series with one of the leads on the resistor, connecting to the tip of a RCA plug.
    - The ground is shared between radio ground and RCA ring.

    This works and is not killing the Spectra audio PA chip, but the loud thud is really annoying. No ground loop issues either.

    I measure 28mV DC across both speaker leads (from ASTRO Spectra) when the audio amp is active -- even if there's no audio. It drops to 0 VDC when amp is idle/no RX. So it's not a false DC reading. Do I want to isolate this DC with a non-polarized cap in series which each speaker lead?

    Thanks


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    Using a capacitor to convert balanced->unbalanced audio, or removing DC shifts is a makeshift solution. The proper way to couple the audio would be an 8 or 16 ohm 1:1 matching transformer, or an audio output stage transformer backward, i.e. the 8 or 16 ohm winding goes to the mobile and the 10K side would go to your repeater circuitry. This will remove and isolate the DC component completely. That said, the first power-on whether using caps or a transformer will have a one-time "pop" because the slope of the 1st transient will look like AC and get through a cap or transformer. You might use a large cap to ground after the isolation transformer (like 500 uF or bigger) to initially swamp the audio line to ground and allow it to charge up slowly to "mute" the thump. The cap has to be big enough to not allow too much audio through to ground after it charges up.

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    Thanks Alpha!

    I'm heading to the parts store to grab a transformer. Will update.

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    would switch the jumpers to VRS mode on the control board and use a relay on the COR signal to gate the detector audio.
    This is a low level signal but not affected by the vol control.
    This is how I drive the overheads for the firehouse monitors

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    I used a Radio shack Audio output transformer part number 273-1380, a 1K 1/2 watt resistor, and a 100k trim pot. Without the 1k resistor the astro spectra's speaker output was saturating the transformer and causing distortion. This drives aviation intercom's line inputs quite well.

    Since my fine ascii art did not post well, here is a verbal description:
    The spectra's speaker out drives the 8 ohm transformer windings. The 1k resistor goes across the 8ohm transformer leads. This keeps the transformer from distorting.
    The transformer has a balanced output side so connect the center tap wire to one of the side windings.
    I run the transformer output thru a 100k trim pot in series with one of the output leads and set my level as needed.
    I think I run the astro spectra volume at 4 or 5.
    Last edited by slim; Oct 25, 2012 at 09:25 PM.

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    I found a better solution.

    Pin 10 on the Audio Power Amplifier (U450) is used to turn the APA off when there's no active RX. This is called the muting line. It is accomplished by feeding 12VDC (nominal...whatever your power supply is set at) to pin 10, when there's RX.

    When there's no RX, the voltage at this pin is 0 VDC.

    I just added a jumper from my VPP to pin 10, to keep the APA permanently on. Yes, this draws slightly more current, but IT WORKS. NO MORE THUD NOISES IN MY AUDIO.

    Have fun.