Topic: I am making a correction amp and have a question :)

[Skibber Bent]

Hello all. 


I just started building my own projector and want to make a soundcard DAC to control it. While i'm waiting for the soundcard to arrive i am making the correction amp for it. I have been looking at schematics and pictures of this and afaik they all seem to have individual offsets for each channel. I think this is due to the offset being affected by the gain.


My idea is to put a unity gain opamp in front of the amplifier and reference that to a single trimpot controlling the offset reference. This means less trimpots and should make it easier to adjust. Do you think this will work or am i missing something?


When i get this up and running and tested i will put up the schematics and PCB design for download. Maybe i will have some spare PCB's as well if anybody wants.


Thanks and keep it real 

[James]

Hi Skibber Bent.

Welcome to the LaserBoy Forum!

I think you will find that your idea will get you very close to zero volts on all channels, but there will be some differences, probably due the the tolerances of the resisters in line with the DAC channels on the sound device.

If you go with the idea of using a variable negative voltage regulator, you can probably get away with not using the trimmers on each channel input to the op-amps.

Let me know what you come up with and how well it works.

Also, remember I sell a really nice kit of the correction amp!

James.

[Skibber Bent]

Thanks!


This is what i came up with. Haven't tested it yet, so i'm not sure how well it performs. The reference is just a 7905 with a voltage divider which is referenced on all channels.


Any thoughts?

[James]

It looks like you are using 4 times as many TLO72's to get the same job done.

James.

[Skibber Bent]

Yes and no.


I agree that the first unity gain buffer is redundant and this could be done with only 2 opamps, but i wanted to make it differential so i needed 3. As there is no TL073 i just figured why not use the 4th as a buffer on the input. 


Also opamps are much cheaper than trimpots so the total cost of material is cheaper.

[James]

It depends on how many of each kind you buy and where you get them!

Simplicity has its value too.

James.