video amplifier

[Josh]

Hi.

Whats the deal with video signal. i want to run 2 tvs off one signal. I've tried just wiring my VCRs output to both TVs inputs, but there is a loss of quality and stuff. Is what i'm looking for a masthead amplifier? are these easy to make? can i just use an audio amplifier?

Thanks, all help appreciated.

Josh.

 

[Russlk]

You cannot use an audio amp. A video amp would help if you are using the video signals, but if you are using the RF output from the VCR to the antenna inputs, use an RF amp. But what you really need is a signal splitter. there is an impedance mis-match when you connect two TVs in parallel to one source, so you need a splitter which in most cases gives a loss in amplitude, therefore the amplifier will help.

 

[Nigel Goodwin]

Hi.

Whats the deal with video signal. i want to run 2 tvs off one signal. I've tried just wiring my VCRs output to both TVs inputs, but there is a loss of quality and stuff. Is what i'm looking for a masthead amplifier? are these easy to make? can i just use an audio amplifier?

You don't give any mention of how you are connecting them?.

But both RF and AV are 75 ohms impedance, so when you connect two in parallel you are automatically feeding a 75 ohm output into a 37.5 ohm load impedance.

Specific splitters are available to do this, you can get them for RF or for AV, both are completely different.

An RF splitter (feeding from a VCR) should be fine with a simple passive splitter, there are two types, one is a classic resistive splitter, the other uses a small RF transformer and has less loss.

For AV you really need an amplified splitter, to prevent the loss of contrast - although if you remove the 75 ohm load resistor in one TV it should be OK (monitors often have this resistor switchable!), and can just parallel the AV inputs.

 

[Josh]

Thanks, yeah i was talking about AV composite out. is there a splitter IC or something?

Josh.

 

[Nigel Goodwin]

Thanks, yeah i was talking about AV composite out. is there a splitter IC or something?

You need to amplify the signal, then you can split it with a simple resistive splitter - bear in mind you need a good frequency response from the amplifier (50Hz-5MHz or so?), and it must be able to provide a reasonable amount of power.

You can buy ready built ones quite cheaply, we used them to distribute a DVD AV output round the shop at work.

 

[Josh]

I found this schematic. Do you think it will work? it looks relatively easy to make. Basically i want to make a splitter that i can put inside my (cheap) video mixer to give me a 'monitor' outputs for each input.

 

[Josh]

i have just done some research and found that the IC no longer exists. can anyone tell me which common IC could be used in it's place?

Thanks

Josh.

 

[Nigel Goodwin]

i have just done some research and found that the IC no longer exists. can anyone tell me which common IC could be used in it's place?

The circuit looks fine, as for the opamp, you need to find one with a similar high bandwidth - you can probably find them specifically listed as video amplifiers?.

Notice that the three outputs are simply taken through 75 ohm resistors, and the input has a 75 ohm in parallel with it!.

Your existing single output will be fed through one 75 ohm, in order to give the 75 ohm output impedance required. You could simply add a second 75 ohm, connected from the input side of the existing 75 ohm and going to another socket. The only potential problem is that you are doubling the power demands on the existing amplifier?, it depends if it will supply enough power.

 

[Sebi]

Another circuit...

 

[audioguru]

Hi Sebi,
T2's base doesn't have a DC connection in your circuit. :lol:

 

[Nigel Goodwin]

Hi Sebi,
T2's base doesn't have a DC connection in your circuit. :lol:

Well spotted! - the feedback resistor is missing :lol:

 

[Sebi]

Good point! I've changed. I've redrawed it from a very bad scanned file...

 

[Josh]

in the schematic i put on, there are 3 outputs, which are just 3 75 ohm resistors in parallel. does this mean i can put as many or little of these as i want? also if i build it with 3 outputs and only use 2, will that stuff everything up?

 

[Nigel Goodwin]

in the schematic i put on, there are 3 outputs, which are just 3 75 ohm resistors in parallel. does this mean i can put as many or little of these as i want? also if i build it with 3 outputs and only use 2, will that stuff everything up?

No, that's fine - although it may be a good idea to put 75 ohm dummy loads on unused outputs? - but this shouldn't matter for video!.

The number of outputs you can feed are dependent on the power available from the amplifier, the more power it will supply, the more outputs you can have.

 

[Josh]

Hey, should i substitute the video op amp with one that operates on the same voltage? coz i found a national one that runs on +/- 5V. Also, how hard is it to make this powered off a single DC supply?

Thanks.

Josh.

 

[audioguru]

Hi Josh,
It is easy to convert a dual supply amp to work on a single supply. Just bias its input to mid-supply voltage and capacitor-couple its input, output and feedback ground connection: