Line amp needed

[Lawnboy]

I am looking to switch from the headphone jack of my cd player to the line out after finding that the headphone amp clips peak white. can someone design a simple, adjustable amp to bring the signal up to 1V p-p? the output now is around 300mv p-p.

i also noticed that the output of my cd player drifts negative as it plays. for example, on an all white picture, the output starts out at 300mv but slowly drops to 100mv after 15 sec, before stabilizing at about 37mv after about 40 secs. the amplitude doesnt change, it just drifts more negative. is this caused by my oscilloscope, because the brightness is stable on my monitor.

[Steve Anderson]

Firstly what sort supply in terms of voltage and polarity is available to power the 'line-amp'? That can determine it's topography. It is probable that a simple two-transistor circuit would do the job (non-inverting) or a single transistor (inverting). Of course a simple op-amp will do either.

Is the scope DC-coupled? Even so, that's quite a long time-constant and probably as a result of large-ish output capacitors in the CD player. Assuming you're using most of the published NBTV circuits which are usually AC-coupled anyway, this shouldn't be an issue. Seeing as the monitor doesn't get upset I'd forget about it.

The average AC-coupled value (the DC mean) will rise and fall on changing picture content (average brightness). This happens in all AC-coupled TV systems, including 525/625. Hence the reason for the back porch in the waveform where that black level can be clamped to a defined voltage.

NBTV doesn't have this and the only 'constant' voltage reference is the sync-tips which most published circuits use to 'DC-restore' the waveform.

Steve A.

[Lawnboy]

thanks Steve. the cd player is your standard portable battery-operated device with the correct polarity. i'll admit i havent given any thought on how to power the amp, but it will probably be a 9v battery or a wall wart.
im not sure whether the scope was DC coupled or not when i made the measurements, but i know it gives you a choice.

[Klaas Robers]

In the NBTV handbook there is a simple 3-transistor circuit for video amplification. It has an amplification of 10x and has a normal and an inverted output.
It is especially designed to accomodate an earphone output of a CD-player to the one volt needed for the one transistor LED-driver circuit.

[Lawnboy]

Klaas, where is this located? i looked through the handbook (the old version) and couldnt find it. is it in the new handbook?

[Klaas Robers]

Yes, it is in the new Handbook. When we assembled that handbook we knew a demand for a simple amplifier including the possibility for a polarity inversion. So in cooperation with Vic Brown a circuit was developed. Until now it wasn't even published in the Newsletter.