Interesting G-120 amp design...

[pishtak]

It's quite old thread now, but found it few days ago.
Please allow few words about my "baby":

There might be more to this, Buffo. I seriously doubt that he could get 8 degrees at 24K from an LM675T driven by only 20 volt rails. This is the kind of thing that I would have to see with my own two eyes...

It will be pleasure for me if you can look on it :-)

On the page he mentions using Showtacle software. I don't know that software very well, but I have seen in their advertisements something like "fixed 35Hz scan rate" as an attempt to try to prevent damage to scanners. Since I don't know Showtacle software, it makes me wonder if the scan rate can really be adjusted separately. It could be that this guy thinks he is scanning at 24K but really isn't...

You are right about Showtacle software, but for testing I'm using Chan's controler mentioned on my web. This controler outputs point at specified PPS speed without any
restrictions. Anyway gento from laserfreak built this amp and tested it with
pangolin sw/hw. here are the results and these are even better:-)

Anyway, if he will be present at a future LaserFreak meeting, that might be a time when I can see this with my own eyes, and also plug in our softare and do some additional testing...
Bill

Hopefully there will be chance ... Maybe this year ?

I just don't think that this thing will really work "as advertised".

This AMP is far from perfection of TT, but it can scan ILDA test pattern
at specified speed and angle as advertised here and here and here.
Believe or not, it was proven here and it is enough for me as satisfaction/feedback.

It may be possible to tune it up to 24K for very small angle... BUT, then what will happen when you put some content in with large angles? Experience tells me that it will massively overshoot and possibly (probably) break a torsion rod.

Today I tested it with smaller mirrors and mirror mounts and it can scan 12K ILDA Test
Pattern at 18deg optical at 24Kpps. Scanning angle was adjusted to 18deg optical with input gain. That means input signal with amplitude +/-10V will drive it to max. 18deg optical. After tuning, quadrature square wave can be scanned without overshoot. If the input gain is then adjusted to 40deg optical there will be overshoot that's clear because it was not tuned to this angle. But normally one will not do this. If yes, there is still protection (over position shutdown) which will protect the galvo and it just works great. On laserfreak I mentioned that once I forget to assemble C16 responsible for damping. Servo gain was almost at maximum. Without overposition shutdown torsion bar would have been destroyed in a fraction of a second. But it wasn't.

Then, it may be possible to tune this amp up to work at larger angles (which is what the G-120 is supposed to do), but then small signal response will be crap (like 15K or below, as he shows in some of his other pictures).

I asked about max. speed of G120D at 40deg optical with TT here , but it was not answered by you. Did you tested maximum speed of G120D with TT at 40deg optical ?

This amp has no "angle intelligence", so it can either be tuned up for small signals, or large signals, but not both at the same time. This amp will also "sing", depending on the mirror/mount you put on it.

I don't know what you mean with angle inteligence. Maybe short explanation from your side ?

So, in the end, I think (gulp) Chinese scanners and amps would work better than G-120s and this amp, and probably be less expensive too.

But in any event, if someone has some money burning a hole in their pocket, sure, go ahead and get it. It never hurts to experiment and sometimes that's the best way you learn.

Best regards,

William Benner

My AMP was designed in 2005. At that time no chinese scanners were available at prices which are valid today. My AMP was designed from almost ZERO knowledge about scanning/lasershow. There is still things to be improved :-)

My Amp has also built in pre-emphasis. Maybe it is not on the correct place in
schematics. But it's there and only thanks to this pre-emphasis it can work at specified
speeds.

If I will have the chance to get again pair of G120D for 100USD,
I will redesign and implement pre-emphasis at correct place.
Also there will be bridged output stage implemented (thanks to mixedgas).
http://www.cirrus.com/en/pubs/appNote/Apex_AN05U_E.pdf
Bill was also very nice when giving this explanation.

HF damping helps to reduce the oscillative nature of galvo response during breaking after large jump. If Your acceleration and deceleration peak current is not high enough this may be true that HF won't do any visible good. Are You suffering from the dip after fast slope command like square wave ? This becoms visible when tuned for highest speed only. If not - Youur amp is probably not able to push G120 really hard, so HF-damping wont be of use. Are You sure You're running G120 coils in parallel, not in series (as LaserFX suggest as I remember ?).
I've heared that there's also a limit of G120 rotor magnetic saturation ...
Piotr.K

If you look into http://www.jrbtechnical.com/pdf/t2manual.pdf you will find out,
that HF damping is not being used for G120D. I think it's because G120D has torsion bar and CT series doesn't have. About thermal limitations of G120D look here.

Best Regards

Stefan