Film-Tech Forum ARCHIVE: Exciter lamp to amp wiring

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Posted by John Westlund (Member # 913) on 11-30-2002, 04:46 PM:

I have a portable Simplex SP projector that I am trying to get sound on. It doesnt have the amplifier that came with it but I am trying to hook up a Peavey 400 to it. Can I go straight from the cell wiring out of the projector to the peavey? There are 2 wires that come out of the projector from the cell. Thanks guys
John

Posted by Scott Norwood (Member # 30) on 11-30-2002, 04:49 PM:

You'll need a preamp in there somewhere. Microphone preamps work fine; the solar cell can be wired to an XLR connector and connected to a mike input on a mixing board or tape recorder or something like that.

Does this have a solar cell or a phototube?

Posted by John Westlund (Member # 913) on 11-30-2002, 04:52 PM:

I believe it does have a phototube in it. Sorry. So I could wire the phototube to an XLR connector and then plug it into an input on a stereo reciever?

Posted by Jeff Stricker (Member # 224) on 11-30-2002, 04:56 PM:

Also, load the solar cell with a 680 ohm (approx) resistor, then capacitor couple it into the preamp. A 1 MFD electrolytic should be adequate (put the "+" of the cap towards the resistor.)

Posted by Michael Schaffer (Member # 1204) on 11-30-2002, 06:36 PM:

If you have a receiver which has Pro-Logic decoding ("Dolby Surround"), you can even have 4-channel sound like in the "real cinema". Of course, you won`t have A-type or SR-type NR as the consumer format does not perform NR decoding.

Posted by Bruce McGee (Member # 131) on 11-30-2002, 07:58 PM:

You say that the SP has a phototube. You must use an amplifier that can supply the 90 volts so that the photoelectric cell will operate. You cant just hook the wires up to a plug, and hook it to your amplifier.

It is not that hard to convert it to a solar cell.

What are you doing about the exciter lamp power supply?

Bruce

Posted by Bob Koch (Member # 1062) on 11-30-2002, 08:03 PM:

If the SP has a photocell; 868 or similar you won`t get far into a mic input without providing a coupling cct, 90 volt supply and hz to nominal lo impedance. The details of the SP reproducer escape me as I havent seen one since 1940.

Posted by Scott Norwood (Member # 30) on 11-30-2002, 09:29 PM:

Jeff--what purpose is served by the resistor and capacitor? It seems to work fine without them, at least with my 16mm Elmos and the preamps in an Otari tape machine. I'm curious. Thanks.

Posted by Jeff Stricker (Member # 224) on 12-01-2002, 07:40 AM:

Hi Scott,

Well first I didn't realize this post had to do with an old phototube. So my first comment was misdirected. Sorry for confusion.

Anyway, with a modern solar cell, the cell wants to drive a resistor. I think that is because the cell sort of acts like a current source. Pumping current through the resistor, converts to current to a voltage signal which is what the input of a preamp wants to "see". I didn't know all this when I first started tinkering a few years back, until the learned folks here enlightened me. So I hooked up a resistor, voila! MUCH improvement in distortion, etc. I've used resistor values anywhere from 300 ohm to 1K-ohm and all work fine. I believe Paul Thompson has also experimentd with the same resistance values and has same good results. (Some of the commercial amp schematics I've seen, show a 680 ohm resistor at the solar cell input)

As to the capacitor, it is only needed if your preamp doesn't already have a blocking capacitor on it's input. The purpose is to keep DC voltage from getting on the first stage of amplification and upsetting the bias conditions, causing distortion. More than likely any mic preamp, etc. already has a built in blocking capacitor, so another would be redundant. Jeff

Posted by Adam Fraser (Member # 1074) on 12-01-2002, 09:44 AM:

John W.,
Could you upload a picture of the sound part of this projector? I just converted my Simplex Sh-1000 to a solar cell and is very easy. I found a pair of them in the attic of the theatre from our led conversion. I looked over my options and getting power to the phototube was more work than I wanted. And a brand new solar cell would only run you about $100.00

Posted by Scott Norwood (Member # 30) on 12-01-2002, 12:53 PM:

Thanks, Jeff. Will I damage anything if I omit the resistor? I haven't had any problems with sound quality (distortion, etc.), so I'm tempted to leave things as they are if it won't cause any problems. (Actually it sounds much better than the built-in amp.)

Does anyone know if the preamps in an Otari MX-5050BII-2 tape recorder have blocking capacitors in them?

Posted by Jeff Stricker (Member # 224) on 12-01-2002, 02:34 PM:

Scott, not loading the solar cell will not damage a thing. [Smile]

I'm sure your tape machine has blocking capacitors at it's input - I've never seen one that didn't. If you want to experiment, drop me an email and I'll send you a couple of resistors. Jeff

Posted by Frank Angel (Member # 248) on 12-01-2002, 02:47 PM:

Scott, my experience is the same as yours, for years before I got a real Dolby processor, I installed the Kelmar solar cells and connected the outputs directly to a Shure ProSeries stereo mixer with balanced inputs. It worked splendedly. Perhaps the need for the added coupling circuitry comes when inputing to consumer products.

This bastardized system outputed the mixer to an old Dolby A studio codec, then to a Sansui VarioMatrix II Studio decoder to derive the 4 channels, rear channels then going into Dolby B decoders and then to digital delay units. Then the whole thing going into a 24 channel console which also derived a center channel and processed it thru a Aphex Aural Exciter which helped out with dialog in a big 2500 seat room. The left and right channels were also processed through a psychoacoustical enhancer that spread the sound a bit outside the speaker clusters. It was a good thing we had a recording studio in our complex that I could canibalize. Then along came a Dolby CP 50 and that freed up half the soundrack! We still use the Aural and spacial enhancer plus a dbx bass enhancer for the subs.

What would we do without all these toys?! [sleep]

Frank

Posted by Gerard S. Cohen (Member # 994) on 12-01-2002, 03:15 PM:

Coming down from the stratospheric heights of hi-tech sound to cost-free level, I made a serviceable exciter lamp supply by using the plug-in power supply from a junked Walkman or other consumer appliance
(which usually outlasts the appliance itself) to restore a 16mm projector donated by a friend. Seems some dummy had soldered both leads from the amplifier to the brass shell of the exciter lamp socket, shorting it and blowing the original amp exciter supply.
I simply chose an AC to DC power supply within the voltage range of the exciter lamp, and plugged it into the projector's 110V accessory outlet. Any electronics store sells these, as well as variable voltage models, which are handy to have around.
Seems this should work for 35mm as well.

Posted by Gordon McLeod (Member # 33) on 12-01-2002, 04:20 PM:

Professional mixers are typically a low impedance input on the mic lines so the resistor is not needed and if it is a balanced input then a cap is usually not needed as there is either a balancing transformer or caps to block optional phantom powering for condensor mics

Posted by Jeff Stricker (Member # 224) on 12-01-2002, 04:49 PM:

Gerard, that's a great way to get around expensive exciter supplies. [Wink]

Any filtered DC supply with proper voltage and current ratings will do the job. I have pressed into servce your basic cheapo Radio Shack CB power supplies with great results. (Depending voltage needed, you may have to tinker with the internal regulator circuitry)

BTW: Some automotive 12V stop/tail lamps (the ones with straight filaments) make good inexpensive exciter lamps, if your willling to make a home brew lamp holder for your soundhead.