Film-Tech Forum ARCHIVE: Help with sound problem

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Posted by Clive Carmock (Member # 2882) on 05-22-2005, 11:21 AM:

I wonder if anyone might be able to offer some advice with a few sound questions I have?

I have a TK 35 potable machine with original mono Sound head. The cable that comes from that sound head is a single centre core plus screen, which connects to two pins on an XLR plug on the front of the machine.

I assumed (from the XLR) that this was a balanced source. However am now not so sure given the one wire + screen I find inside.

I have a cable with an XLR socket to connect to the projector and the 1/4" jack input on a preamp.

The output of which connects to a standrad hi fi amp.

The sound quality is good but suffers from mains hum, which is at a level that it can only be heard in quiet scenes, but is districting.

The Pre Amp has both balanced and unbalanced outputs. - I am using unbalanced as I am connecting to the CD/AUX in jacks on the main amp which are unbalanced.

OK what Have I tried so far? - well the preamp has both unbalanced (1/4") and balanced (3 pin XLR) inputs. So I swapped my cable for a balanced XLR to XLR cable, connecting one core to the screen at the projector end and the other to the HOT wire. At the preamp end I have these wires conencted to + & - signal and the overall cable screen connected to pin one (ground).

I still get hum in this config, though it's less.

In the latter config I get NO hum if I disconnect from the projector itself.

When I have an unbalanced lead connected to the preamp 1/4" jack but not connected at the other end it becomes microphonic - in that if I rattle the cable itself I hear that in the speakers.

I am thinking that it's a problem with the soundhead or wiring in the projector itself.

I do have a Westrex 7000 stereo cell I plan to fit to this machine which has three (unscreened) wires coming from it.

Any help or advice will be much appreciated on how to resolved the initial (mono) problem, and how I could best connect the new stereo cell.

Regards
Clive

Posted by Brad Allen (Member # 470) on 05-22-2005, 11:34 AM:

Get some good quailty two conductor plus shield cable.
Connect the two conductors with the insulation to the solar cell leads. Do not connect the shield at the sound head.
Connect the other end of the two conductors from the solar cell to the two pins in the XLR that are currently used. At this end only, connect the shield to the ground pin in the XLR.
This will "most likely" take care of the hum.

Posted by Clive Carmock (Member # 2882) on 05-22-2005, 11:46 AM:

Brad, thanks for your help - this is in effect what I tried when I made up my custom XLR cable I think.

I used high quality twin core screened microphone cable. At the projector end I did not connect the screen at all. I then connected the two cores to the two wires from the cell - one beeing screen from the Cell. At the preamp end I wired the XLR as per standard - ie + & - signal and the cable screen to pin 1

Still produces hum, but only when the projector is connected - it's silent if I unplug that end.

Regards
Clive

Posted by Dave Macaulay (Member # 813) on 05-22-2005, 01:01 PM:

You might be getting light from a flourescent lamp hitting the projector cell, that makes a great hum source. See if it still hums in total darkness. Incandescent lamps are much less trouble but can do the same. If your exciter lamp supply is AC or (if DC) it has old and weak filter capacitors then hum comes straight from the exciter lamp. Does it hum with the exciter lamp off/removed?
Other than that your cell might be grounded internally and you're getting hum electrically. If you unplug the projector and make sure the case is floating and it gets quiet that may be your problem, you should read infinite resistance to the projector frame with an ohmeter on both cell wires. You can insert an audio transformer in the cell line to eliminate this, but you'll want a capacitor before the transformer to get rid of the DC component of the cell output and avoid distortion from saturating the transformer core.

Posted by Maddison Lake (Member # 2876) on 05-22-2005, 02:25 PM:

I know its not as complicated as the other replies, But have you tried checking to see if there is any power backing back into your TK 35? (I know it doesn't seem probably) but it has happened before. I'd check the whole circuit. 2nd, are you sure its not your pre-amp itself?

-ML

Posted by Clive Carmock (Member # 2882) on 05-23-2005, 04:42 PM:

Thanks for your suggestions. I have spent some time this evening trying a few more things.

I have removed the XLR plate and the entire solar cell and wiring from the machine and also covered the end of the cell so that it doesn't pick up any stray light.

As soon as I connect the XLR to the pre-amp I get some hum. Having tried to connect a mirophone to the pre-amp I get no hum and it works just fine, so I think this rules out a problem with the preamp.

This seems to support the view that the Solar cell itself is at fault - weird though I can see where it's getting mains hum from as it's not conenct to anything at all at present! not even bolted into the chassis of the machine! No lights or TV etc were on when I tried this as I wanted to ensure there wasn't a source of mains interference close by.

Next task - try the same experiment with my Westrex 7000 cell I think...

Posted by Dave Macaulay (Member # 813) on 05-23-2005, 09:55 PM:

Try shorting the cell wires. If that quiets the hum down maybe try a load resistor - about 1K as a guess, across the cell itself (if you're not using a cinema preamp this resistor is often required). If the hum stays, check the connection of the cell to the XLR as it may be wired wrong.

Posted by Peter Hall (Member # 695) on 05-24-2005, 03:48 AM:

HI

I'd suggest isolating the preamp - RS do a small tx for this at around £7, or pop in and see us in Herne Hill (will cost you a six pack of Stella !)

Cheers

Posted by Clive Carmock (Member # 2882) on 05-25-2005, 04:31 PM:

Dave - I tried your suggestion - shorting out the cell removes hum completely. I will try a resistor across the wires as you suggest. Thanks for that.

Also tried a lash up with my ex Westrex 7000 stereo cell - that also hums but less than the old Zeiss cell from the 1950's. Also the output level from the Westrex cell is significantly greater which helps. Perhaps that needs a resistor across it too to remove the small amount of hum

I guess the theory behind this is that the pre-amp wants to see a load across it's input?

Regards
Clive

Posted by David Graham Rose (Member # 1450) on 05-26-2005, 12:45 PM:

Greetings Peter once again!

Since you appear to deal in the rather sensible currency of 'the pint' as opposed to the Euro I would like to order a 10 screen fully kitted out multiplex from you. All theatres are to have Dolby Digital Sound (CP650's) and the two largest are to have 70mm 6-track magnetic and 35mm 4-track magnetic sound as well. The smallest theatre should seat no less than 250, and the largest 1000 (with balcony). A Compton organ would also be nice in the 'main' theatre (along with the usual spotlights SoopahTroopah's will do).

I would like all Phillips projectors, but no DP75's since I do not agree with the design of the C/O shutter circuit.

My son is currently a drayman for Greene King Brewery Ltd. Please advise on where you would like the renumeration dropped off, and on which days - he's normally in your area on Tuesdays, so I'll tell him to put a few extra barrels on for the next few years.

Also could you let me know the exact A.B.V for eacg barrel, and of course quantities. I will then forward you detail of location of new cinema etc.

With best wishes

David

Posted by Dave Macaulay (Member # 813) on 05-27-2005, 08:43 AM:

The solar cell is actually a current generating device rather than a voltage source (ie light in = current out) and has a very low effective impedance. Cinema preamps have inputs designed for this, "normal" preamps are designed for voltage sources. Voltage sources don't care much about load impedance as long as it isn't too low, common audio practice is to use very high input impedances so any source impedance can be tolerated - plus you can connect several inputs to a single source that way.
The "theory" is that the resistor load converts the solar cell current source into a voltage (E=IR). With a very high resistance (like a 100K amplifier input) the cell maximum voltage is reached with a miniscule current and the signal necessarily gets distorted because the cell can't make any more voltage. With a reasonable load resistance the cell current at 100% modulation produces a voltage below maximum and everything is as undistorted as possible. Too low a resistor will distort also if the cell's maximum current is reached before 100% modulation.
With a cinema solar cell directly connected to a high impedance input the sound is usually very "tinny", probably because the treble tends to be low modulation compared to the bass?

Posted by John Pytlak (Member # 331) on 05-27-2005, 08:50 AM:

Dave: You suggested a 1000 ohm load on the cell as a starting point. Would the optimum load be determined by looking at distortion on a scope?

Posted by Dave Macaulay (Member # 813) on 05-27-2005, 07:11 PM:

1K was just a guess.
Today I found a mono Kelmar solar cell and another unknown brand. Both had maximum voltage output (in open sun) of about 500mV - with whatever input resistance a Fluke meter has - and maximum current of 4mA - into again, whatever input resistance the Fluke has on amps input. This suggests something about the internal resistance and/or source impedance, but I don't know quite what.
Certainly the light intensity passing through a soundtrack from an exciter or LED source would be much weaker than full sun on the entire cell area, but the cell definitely has a maximum output voltage around 500mV.
With a 2K load resistor the cell managed to produce 100mVAC reading the flashing sunlight passed through my 5 blade window fan; that's enough to read the frequency so now I know it turns at 600RPM.
With a 200 ohm load it still managed to make 500mV. Two milliwatts!
In theory one could measure the output current with the cell installed in a soundhead with exciter or LED illumination threaded with clear film. If you select a resistor for close to 500mV with that current I think you should have pretty much the optimum load. I don't know if the cell will actually produce 500mV with reduced light though.
I didn't set up a soundhead with the mono cell and an exciter for any further experiments but I might get ambitious Monday.

Posted by Mark Gulbrandsen (Member # 72) on 05-28-2005, 10:58 AM:

quote: David Graham Rose
Compton organ would also be nice in the 'main' theatre (along with the usual spotlights SoopahTroopah's will do).

I would like all Phillips projectors, but no DP75's since I do not agree with the design of the C/O shutter circuit.

As long as they are the older Arc Soopah Troopahs that is fine... the new ones are genuine crap!

Yes absolutely... All FP-56's in the smaller houses and DP-70's in the large auditoriums.... [thumbsup]

You would never be down for sale of a projector problem.... thats for sure...... I think Peter is going to want the entire "Pint" distillery in trade for this though.....

Posted by Peter Hall (Member # 695) on 05-29-2005, 03:04 AM:

Hi David etc

Curious on the changeover comment on the DP75 - this machine is riddled with oddities however I thought the c/0 design was OK. At least it's not plastic - still, the two we refitted for Liverpool and the two at Mayfair run quietly and without pix movement. But yes, I think you wouldneed to equal the specific gravity of the DP75 with the SG of the Stella ! God I wish I had another DP70 to sell on this basis - would be pissed the rest of my life !

Posted by Steve Guttag (Member # 268) on 05-29-2005, 07:36 AM:

The problem I have with the C/O on the DP-75 is that it opens faster than it closes...if C/Os are not perfectly timed (frame accurate) you will double-image on c/o. At least this is true with a pair of circa 1993 units. On this pair, the async motors run a bit fast on USA 60Hz power.

Posted by Peter Hall (Member # 695) on 05-30-2005, 04:01 AM:

HI Steve

We've got three pair doing changeovers and you are right - one paid is slow, the other two are OK. Not sure of a solution but when we do new electrics for these we use a seperate supply for changeover..

Posted by Pete Naples (Member # 761) on 05-30-2005, 04:16 AM:

I've had double imaging on older ones in the past, caused by the hydraulic damper on the change over assmebly not having any oil in it, so the blade on the outgoing machine was bouncing. As some numpty in the past had seen fit to remove the damper and re fit it 180º out, the filling screw was innaccessible so it took ages to take the damper off, clean it up, put it back the right way round and fill it up. Not my favourite machine!

Posted by Peter Hall (Member # 695) on 05-31-2005, 08:43 AM:

Pete

Yes but can you name any other machine where you never have to touch the drive train ? If they are maintained well and very frequently then they run pretty good - the two in Liverpool ran 70mm on opening with no pix movement at all - you couldnt hear them running

Posted by Clive Carmock (Member # 2882) on 06-01-2005, 05:07 PM:

Thanks to everyone here for their help with working this sound problem through with me. In the end I did need a resistor across the cell - but went for a 2.2k resistor in the end as a 1k one removed too much useful signal.

However also I found that the screen in the XLR cable wasn't actually earthed at either end.

So I re-made the pin 1 connection at the projector end and ensured it had a good ground connection - that coupled with the resistor and the hum is gone.

Once again many thanks to all who offered help and advice.

Regards
Clive