Request electrical design help with hot-rodding a Howtek.

[ah693973]

So, I switched out the R53 resistors to 3.3k Ohms.

I had also decided to try just removing the c2 capacitors. "Filters? We don't need no stinking filters." The test scans I've done look great so I think that might have been a fine idea. There is very little noise with the upgraded PMTs.

I did run into trouble when I dropped one of my 928 PMTs during assembly. I have several 3896 PMTs and so I had to use those. They are even more sensitive so I need to turn down the "gain" even more. Without the C2 caps I should be able to decrease R37 and R38 without changing a filter frequency. I should be able to get close to the value I need by "piggybacking" a resistor. I don't want to tear up the board with too many value swaps.

Here is a screenshot of the new setup using wide gamut linear chrome on DPL:


And here is wide gamut log chrome:


The PMTs are so good that the black electricians tape I used to provide a "zero" point is actually somewhat transparent in the red. When the log amp is used the velvia underneath is visible. That is the tape going from just under the target to the bottom of the image. I was impressed.

You can see that the red and green need to come down substantially still to move the histogram to the left. I may end up using different "gains" for each channel. There is a ton of information in the blue channel of velvia that most scanners will not pick up.

Thanks again for all the help with this.

Andy

[polyglot]

You want to keep the C2 capacitors. They're there to help ensure stability of the op-amp, if you take them out you risk having ringing and oscillations where there are very sharp transitions in the image.

A bit of a change to the filter frequency is far preferable to removing the stability control entirely!

[wombat2go]

Changing R38 & R38 will not do anything to the system transfer function that can be done by only adjusting R 53.
The C2 capacitors should not be removed because the value is already down around the track capacitance value.

You could add a trim pot in place of R53
http://electronics.mcmelectronics.co...onics/Trim-Pot

I am using fast Fourier transform to boost the sharpness (reduce spread function generally) of some 35mm C41 negs scanned back in 1994. It is working well but needs a powerful computer.

Convert by Fourier Transform to complex frequency domain, subtract a variable Gaussian black hole centred on Amplitude_0 then convert back to Cartesian

http://www.pentaxforums.com/forums/3...puter-fft.html

[onnect17]

I think for small adjustments in the gain it could be better to use the high voltage power supply for each PMT. Also I am a little concern about the bandwidth in the OPA827 with that level of gain.