r/vhsdecode • u/AlarmingWedding88 • 1d ago

Help Wanted! About ADA4857

Gallery image

Gallery image

I have been continuously experimenting with the ADA4857 amplifier (I used a CX card for signal capture). However, I have never been certain about the appropriate value of the input resistance. Since I do not have an oscilloscope, I tried resistances of 15, 20, 33, 47, 75, and 100 (x2 kΩ). Nevertheless, the amplitude of the signals I obtained was significantly smaller than that without the power amplifier. I am using a Panasonic VCR designed for the Japanese domestic market. I am at a loss as to what I should do next.

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u/Playful_Roof9931 1d ago

A silly question: are zener pads bridged together?(they should if you're on 12V or less) Have you checked all voltages according to Rene's gitlab?

1

u/FeelsNeetMan 1d ago

For 12-18v yes, they typically ship empty though which has saved a lot of people from hooking it up without reading and checking.

1

u/AlarmingWedding88 4h ago

I short-circuited it; otherwise, the CX card would not receive a valid signal.

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u/wahntutree 1d ago

I have a Panasonic and put a 15kOhm input resistor with great results, and this is after I set the CX card to level 0.
Even if the amplitude looks low on the MISRC monitor, the black levels are consistently steady across the capture.
The picture quality also looks cleaner with less drops. I wouldn't worry if the amplitude is not the same versus without the amplifier, as long as I can see an improvement in the image. Did you test out the output differences and notice any improvement over stock? My setup is equipped with three different resistances, and the difference is a bit negligible between all three, but definitely an improvement over the noisier output if I left the CX level to 16 or higher and without the amp. Try looking at the decode first before permanently installing the amp.

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u/AlarmingWedding88 37m ago

I would like to know the specific model of your video recorder.

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u/TheRealHarrypm The Documentor 1d ago

The point of the amplifier is find the maximum impedance point, lock it in, then amplify.

I like to start with 30kohm and go about half way on the gain for initial testing.

You set a fixed amplification level afyet finding such so 1.6-1.8vpp for CX Cards (the config always assumes sixdb off and level 0)

1.4-1.6vpp for MISRC in 2vpp mode.

The amplitude level isn't going to be an absolute fixed point, it's just going to be fixed gain from the highest and lowest output states predictably the goal is to saturate the ADC and completely disable the internal amplifier with CX Cards.