8mm Forum: Popping GS1200

This is topic Popping GS1200 in forum 8mm Forum at 8mm Forum.

To visit this topic, use this URL:
https://8mmforum.film-tech.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=003787

Posted by Graham Ritchie (Member # 559) on June 13, 2008, 04:58 AM:

I was wondering if it would do any harm to the amp to disconect the internal speaker connection at the back of the projector to eliminate the annoying pop through those speakers when switching the projector on or off, as I never use the internal speakers.

Graham.

Posted by Kevin Faulkner (Member # 6) on June 13, 2008, 05:46 AM:

Shouldn't be a problem. I have come across others who have done this to their machines for the very same reason. To be quite honest with you it's not something that annoys me and you may find that you will still get a pop come through as you plug the external speakers in.

Kev.

Posted by David Pannell (Member # 300) on June 13, 2008, 05:56 AM:

Graham,

I am sure that Kev Faulkner can give you the definitive answer to this, if there are any special peculiarities with the GS.

If you get this popping through the internal speakers, you should get the same through any extension speakers connected.

However, there are two rules of thumb with regard to audio amplifiers which should always be borne in mind.

1. NEVER operate an amplifier with the speakers disconnected. The voltage at the output device (output transformers, transistors or ICs) can rise to levels that will damage the output device. The design load of the speaker is matched to the output impedance of the output device to prevent this.

You can, however wire in a resistor of similar resistance and wattage rating instead of the loudspeaker, which will load the output stage of the amplifier correctly. You could even arrange to switch the resistors in automatically whenever the speakers are disconnected.

2. The popping is sometimes caused by a small arc or spark in the on/off switch, and the resultant back e.m.f. producing a spike in the amplifier circuitry. This can often be cured by connecting (soldering) a 0.1uF capacitor across the CONTACTS of the on/off switch. It must be across the contacts, - NOT across the mains supply. The rating of the capacitor should be at least twice the mains voltage.

Best regards,

Posted by James N. Savage 3 (Member # 83) on June 13, 2008, 06:12 AM:

My Sankyo ST-800 would start making a loud pop after about two hours of use. And the longer I would run it, the pops were more frequent, up to about one per minute. I could usually get away with one full-length feature before the poping would begin.
I never could figure out the cause.

Any ideas?

James.

Posted by David Pannell (Member # 300) on June 13, 2008, 06:19 AM:

Hi James,

Typical of a capacitor breaking down somewhere. As it warms up, it becomes more susceptible to internal shorting (flashover), referred to amongst electrical/electronics engineers as having been 'punched through'.

You just need to find out which one it is.

Posted by Kevin Faulkner (Member # 6) on June 13, 2008, 06:45 AM:

David, the GS has a 0.1uf cap and 4.7ohm resistor in series accross the out put of the power IC. I think it should be ok. I have not seen any failures with the speakers disconnected maybe it's also worth getting Bill Parsons ideas on this one.

The popping is the inrush of power as the IC busrts into life I'm sure its not an arcing power switch. All the GS's do it as do most amps. In Hi-Fi amps they very often have a relay which connects the speakers a second or so after the power is turned on to eliminate this problem. I think that the more modern IC's have this built in so that in effect the output stage comes on slowly.

I'm sure your only too familiar with all this and yes I agree that popping will be heard through any connected extension speakers as well.

Plugging a pair of headphones in before switching on or off also offers a cure.

Kev.

Posted by Paul Adsett (Member # 25) on June 13, 2008, 06:59 AM:

Several years ago I got fed up with the snap, crackle, and pop of my version 1 GS1200, and disconnected the main speakers. So far this has not caused any problems, and since I always run the machine with the AUX output connected through a 10 band equalizer to my Sony stereo amp, I do not miss the internal speakers at all, and the projector silence is golden! Incidentally, none of my other projectors (all Eumigs) have this 'Rice Krispies' phenomenon, being totally silent when powering up or down. And I have also noted that the GS1200 is supersensitive to switching activity within my house, particularly in the same room.

Posted by Kevin Faulkner (Member # 6) on June 13, 2008, 07:16 AM:

Yes thats another trait of the GS's except for the Xenon version. Elmo put a suppressor network across the mains in the Xenon but just a capacitor on the std machines. If the capacitor is replaced with one of these suppressors the noise from other household appliances will stop. I have done this for a few people with complete success. Why Elmo didn't do this on the std machine is beyond me.

Kev.

Posted by David Pannell (Member # 300) on June 13, 2008, 07:55 AM:

Thanks for the info input, Kev.

As I said, it was just a rule of thumb; all projectors, audio amps, etc have their idiosyncrasies both in design and components used. Clearly the ones you refer to don't present the usual problems, particularly if the devices already have protection built in. Actually, it all amounts to the same thing - protection is required.

Again, thanks for the clarification.

Best regards,

Posted by Graham Ritchie (Member # 559) on June 13, 2008, 02:54 PM:

Kev and David
Thanks for your help its been very interesting to read through. The projector I was using was my spare and had not been used for a while and when I switched it off it gave me a fairly loud pop/crack, my other GS will do similar things but not so bad both projectors are wired from the AUX to a external sound system all the time. I cant recall hearing any popping noise from my external sound system when switching the GS on or off could this due to it being connected to the AUX out and thus being fed from the pre-amp and not through the projectors main amp and internal speakers where this popping seems to occur most, looks like it will be safe to disconect those internal speakers, once again thanks everyone for your advice.

Graham.

Posted by Kevin Faulkner (Member # 6) on June 13, 2008, 04:31 PM:

Yes Graham you are correct. The GS uses a different circuit for the Aux out. In fact it uses the same circuit that drives the meters. Its for this reason you don't hear the thump noise that you would otherwise hear through the headphones, and all speakers outputs including the moni sockets as these all come off the output IC's.

Kev.

Posted by Rick Skowronek (Member # 385) on June 16, 2008, 06:19 PM:

Hi Guys,

Been awhile since I've been a responder to the forum but check in regularly. Always enjoy the expertise here in one of my side hobbies, albeit not as much a hobby as I would like to have the time for.

Just thought I'd add a few cents worth to this thread if I may as a long term EE and one who has designed and built audio amps all the way from the old tube days to now.

Everyone is fairly correct but in the case of solid state devices both ICs and transistors there are some sorta misconceptions partially hung over from the "old" days. Yep, I'm older, LOL.

Anyway, in the old tube days it was pretty disastrous to the circuitry if you removed the speaker load. Without getting into the physics, it had to do with the fact these were all transformer coupled devices and the speaker was a reflected load back to the output circuit keeping it in balance. Remove it and this circuit could easily go into a runaway condition. Luckily tubes were pretty hearty so if you didn't let it go too long till it fried the tubes, no harm.

Now, along comes transistors and audio ICs. In the early days any sudden transient could easily cause the output device to cruise past it's max ratings and this would cause an immediate failure. Basically either shorting (nasty) or opening which created no output but less smoke. The difference is, however, that removing the speaker load prevents any over current transient while not bothering the circuitry a whit.

So, in the basic answer to the original question of whether removing the internal speakers from the output would hurt the projector, the answer is an unequivocal no. The output ICs would see a very small current increase on a "pop" but certainly nowhere near what they would see with either the internal speakers (pretty small load) or, more importantly, external higher load speakers. Certainly, per Kev, you should spend some small time cleaning the switches to help prevent the circuit transients especially common with older brass or copper contacts.

Regards to all in the forum.

Rick

Posted by Kevin Faulkner (Member # 6) on June 17, 2008, 04:03 AM:

Rick, Good to see your still about. Thanks for that input. The most disastrous thing to do to the GS or any other semiconductor based amplifier is of course to short circuit the output.
Although the output IC in the GS is supposed to have short circuit protection built into it believe me they die very quickly.

The ST1200 range also use an IC output stage and these will blow before you can blink given the wrong operating conditions i.e. wrong load or short circuit.

Make sure that anything you connect to the output of these machines has good, tidy connections and suitable for the job.

Kev.

Posted by Bill Parsons (Member # 244) on June 17, 2008, 06:56 AM:

Hi All, interesting stuff, I agree with everything Kevin and Rick have to say, no harm whatever will come to the amplifier in the GS or any other solid state amplifier for that matter if the speakers are disconnected, I do however see a lot of projectors with damaged output transistors and ic’s when people try to connect two or three speakers in parallel, presenting the output stage with a much lower impedance than it will tolerate.

Bill.

Posted by Maurice Leakey (Member # 916) on June 18, 2008, 04:16 AM:

Bill

The speakers could be wired in series couldn't they? This would increase the impedance at the projector outlet.

But would too much impedance also harm the output ic's etc.?

Posted by Bill Parsons (Member # 244) on June 18, 2008, 06:18 AM:

Yes Maurice it is perfectly in order to wire speakers in series, this will do no harm, just reduce the volume.

Bill.

Posted by Kevin Faulkner (Member # 6) on June 18, 2008, 06:50 AM:

I have the Elmo ES1000 extension speakers. Elmo supply these as 16 ohm units and they have a socket in the rear of one of them. This way if you are plugging them into one of their mono projectors you plug one speaker into the projector and the other into the rear of that speaker.

By using 16 ohm speakers the projector is presented with an 8 ohm load. For stereo you simply plug each speaker into its respective L & R projector sockets and as Bill says the 16 ohm load is then no problem.

Doing the above with 8 ohm speakers would present a 4 ohm load which could with some amps be enough to damage the output. Most amps like to work into an 8 - 16 ohm load.

Kev.

Posted by Tony Milman (Member # 7) on June 18, 2008, 12:47 PM:

Fascinating stuff, I can see me now with bits of broken wire dripping with solder as I attempt to attach various bits to the back of the GS [Big Grin]

I have those Elmo speakers Kev and have to say that I really do like the tone and general out put from them not to mention the true "retro look" of the beasts.

The wife does groan though as I get them out and trail the long wires around th elounge [Frown]

Posted by Rick Skowronek (Member # 385) on June 18, 2008, 06:36 PM:

Since it seems like there's a lot of interest in this topic, and there rightly should be with possible damage to irreplacable ICs possible, thought I'd throw a little more "theory" the forum user's way if you're interested.

Everything everyone has said is right on as to impedances and series or paralleling speakers. Just like Ohm's law says, putting impedances or resistance in series adds and putting them in parallel halves it. Current through the speaker and ICs also follows by doubling or halving it. One of the interesting things however are speakers. These really have less resistance than what their rated value says. In fact, any of our esteemed members that have a multimeter with the ability to measure DC resistance should, just for fun, measure a regular speaker that is rated at say 8 ohms. You'll actually find it much less.

In reality, the speaker is what is called an inductive load and it's impedance (resistance) varies by the frequency applied to it. The higher the frequency (treble) the higher the impedance. The lower the frequency (bass) the lower the impedance. Most speakers are basically rated, impedance wise, at 1000 Hz. At that frequency they roughly exhibit the stated impedance like 8 ohms.

That's why amps need to have the reserve power and capacity to handle lower impedances due to heavy and sustained bass notes. Now most amps and devices are based on that premise. If you lower the basic premise by paralleling speakers then a bass note long and heavy can actually take the impedance down to under an ohm. Pretty close to a short in anyone's book.

Certianly will exceed the design parameters. Most newer amps have built in current limiting and safety circuits. Unfortunately, the circuits used in our older but classy projectors we have really didn't have much. Sometimes as basic as a fuse which as a rule could never act faster those little pieces of expensive silicon.

Just a little added piece of theory for anyone interested. That's in case anyone wondered why one of your fuller frequency range films may have put your projector out of commission when it fed a bunch of speakers. Best solution of all is as mentioned by many here. Aux output to an external newer amp is the best of all robust solutions to more volume and sound.

Rick

Posted by Clive Carmock (Member # 347) on July 15, 2008, 05:24 PM:

My GS1200 Xenon (late series with the fan transformer) can make noises from the internal speakers.

I will often notice it when rewinding films One of the internal speakers will make a hum and a sort of pop. I notice the corresponding VU meter will move at the same time. Seems mainly to do it when idle or rewinding but not during projection.

I've tried the record switch cleaning trick but that doesn't seem to make any difference.

Is this type of noise a common problem with the Elmos?

Regards
Clive

Posted by Kevin Faulkner (Member # 6) on July 16, 2008, 07:53 AM:

The GS1200Xenon is far less prone to mains borne interference compared to the std GS because it has much better interference suppression built.

For the pop or noise to be evident on the VU meter idndicates the noise is getting into the early pre-amp stage. Does it still happen with the VOL controls at minimum. This is the sort of thing that could happen with a bad earth somewhere inside the machine.

I too have a late machine but just before they introduced that extra fan transformer however mine has the later take-up motors like yours and I certainly don't suffer from any odd noises or interference. It's a very quiet machine when idle or in rewind.
Interesting that you say it's only on one Chanel. Which one?

Kev.

Posted by Rob Young. (Member # 131) on July 16, 2008, 07:56 AM:

Just out of interest, on the Beaulieu 708, the level of the aux output to an external amp is controlled by the main volume control, so you have to run the volume at maximum when using an external amp.

Since the built in speaker is still active, the manual advice is to stick a din plug in the external speaker socket which isn't connected to anything!

On my Elmo ST1200HD, I get popping and clicking through the main speaker when using the main switch, but added a speaker switching box from Maplin between the projector and speaker. This lets you turn off the main speaker until the film is running, then you switch it on with no pops at all. This box claims to present a safe load to the amp (?) when in the off mode and certainly hasn't done any harm in all the years I've used it.

As for bangs and pops when using the aux outputs for stereo, I used to have "loads" of problems, ah-hem, no pun intented!, especially as the output from the ST is quite low in this respect (the refridgerator turning on and off would present a massive bang right in the middle of a film!!!)

I found that one of those mains suppressors / clean-up blocks(used to be common for hi-fi gear but are really popular now with modern "home-cinema" gadgets) put this right, although those things aren't cheap!

Seriously though, they genuinely do improve the performance of anything that is plugged into them by effectively isolating them from the rest of the mains in the house.

[ July 16, 2008, 05:36 PM: Message edited by: Rob Young. ]

UBB.classic^TM 6.3.1.2