70 v PA question

[Randall]

I feel a bit silly with this question. I made a service call to a Masonic Lodge that looks to be from days gone past. Their relic Bogen CT-100 PA amp was not powering up. I pulled it and brought it back to my bench. Found a blown out electrolytic, but I don't think that was the problem. It powered up on the first try. I measured 120 AC on the aux power receptacle while it was still in the rack, so I'm confident it had power. I spent a while measuring, looking for the 70v and 25v taps, and not finding them. Eventually, I realized the speaker sockets they had it connected to are (I think) standard speaker outputs, and are working as such.

So, this has me thinking, if this is supposed to be a 70v amp, where exactly is the 70v, and where does it come from on this schematic? It even indicates 70v on pins 3 of the speaker sockets, but it's not there. The terminal strip connections, which were not being used indicate 25V, but I do not find it there either. I thought I understood how these amps worked with transformered speakers in the ceilings, but I don't even see from the schematic where there is 70v applied on the outputs.

What am I missing?

http://www.epanorama.net/sff/Audio/P..._Amplifier.pdf

[Tom Phillips]

It doesn't appear that there is a dedicated output terminal marked "70V"
I think the 70 V connection procedure is covered in Table 1 on page 5 of the instruction manual.
Take a look at that and let us know if it makes sense for the amp you have.

[Jazz P Bass]

Best to keep in mind that the Bogen uses an output transformer.
It can be confusing, so you really have to follow the manual.

Basically the 70V line is pin 3 & 2.

Regular speakers use pin 1 & 2.

[Randall]

I have 0v on pins 3 and 2, as well as on the green/white and black/white secondary wires feeding those pins. How does one get 70vdc from a transformer winding tap? Does anyone see where this voltage should be coming from, because I don't. What's worse, the ground/Com 1 screw lug came to me with the screw backed out, but the lug in the connect position. I don't THINK it was actually making connection, and I don't know whether this was being used for normal or 70v operation. I would just like to know if I have an issue with that part before I return to install it.

I forgot to mention that C25 is out of the circuit at the moment,but I don't see how that relates to the missing 70v. Also, it is not connected to a load at present, would this matter?

[Jazz P Bass]

For 70 volt operation pin 2 (impedance selector wire) of the speaker connection must be connected to Com 2.

How is the customer using this unit.
Straight speakers, 70 volt or 25 volt?

[g1]

I'm getting the impression you are maybe looking for DC? Are you familiar with 70V distributed speaker systems?
If you are not running signal, you will not see any AC voltage at the speaker sockets.
Constant-voltage speaker system - Wikipedia, the free encyclopedia

[Jazz P Bass]

Hint: output transformer.

[J M Fahey]

1) we are talking Audio here, hence AC volts.

2) 70V means that the amplifier at the edge of clipping puts out 70VAC audio, and distributes it all around the place with a simple parallel cable, zip cord quality.

Each ceiling speaker gets 70V (they are all in parallel) but that voltage gets converted down with an individual multitap smaller transformer , to, say, 1/2/3/4VAC audio, whatever's needed to deliver selectable wattage between 1 and 10W or thereabouts.

The idea is cool: you can run long distances of cheap cable and you can adjust power speaker by speaker; say 1W in an elevator or toilet, 10W in a noisy restaurant , etc.

That Bogen is very clever design (like all Bogens, kudos to them) and apparently has two power amps inside, each with output transformers.

Outputs are labelled in Ohms and Volts, and there's a logic behind that, it just requires simple Math or experience.

Since this kind of amps are installed by specialized Techs, not consumers, some designations can look cryptic to those from "outside" .

The amp shows 4/8/16 ohms outputs ... intermixed with 25V , 25V centertap and 70V
To be more precise, the 25V tap is also called 6/8 ohms

A little Math:
a) the amp is nominal 100W RMS
b) 25V means: 25*25/6=104W so the 25V tap is also the 100W tap into a 6 ohms speaker.
c) it's labelled 6/8 because if you have an 8 ohms speaker, it's the closest matching tap.
The 8 ohms speaker and the amp will work very well, total power will be reduced to some 80W RMS, no big deal
d) I think they really have 2 x 12.5V taps wired in series, and they call that "25V with centertap" .
I can only guess that some installations demand balanced (twisted pair?) lines for interference reduction, this way they can achieve that.
e) they also have 2 independent, floating 35V taps.
Let's see: 35VAC into 16 ohms would be: 76W`(practically the same as the 78W we had found earlier into 8 ohms)
More important: both independent, floating 16 ohms (a.k.a. 35V) windings in series would now be 70V (you didn't expect it, huh? ) into 64 ohms.

That's why the manual shows both 16 ohms taps in series, with the left one grounded and the right one floating.

Looks confusing but, really, the installer has tons of wiring options, and he has to do it just once, at installation time, then it remains untouched until replaced, sometimes 60 years later.

You just follow the manual faithfully and will not have any trouble.

That said, I guess those "quick connectors"/ plugs are internally wired to the proper taps, because i seem to see a fuzzy 70V label by them.

To test your amp, don't touch the grounding tabs (closed/open) , leave them as is, just connect an 8 ohms speaker to ground and 6/8 ohms tap and call it a day, then leave everything as it was.

[Randall]

Thank you JM for that excellent explanation! I had misunderstood how these amps worked, and I was indeed looking for DC, thinking the audio rode on it and was stepped down at the speaker/transformer. I do not know how the customer is exactly using the amp, but I do know the two quick connectors is all that was connected for the output. I am getting amplified audio from those connections, so all must be good. I'm going to leave the ground jumper open, it's a low volume situation in that building. I'll replace that blown out 500uF/75v cap when it comes in, give it a good cleaning, and re-install.

Over-thinking it again. Thanks for the help once again guys.

[J M Fahey]

Cool.

Repair the electronic problems you find , and test using whatever taps fit your shop speaker but leave all connections, including the terminal strip, including the open or closed ground tags, exactly as you found them.

And later, in one of JPB posts, I saw that the plugs and jacks are internally wired to the terminal strip, so don't modify it even if apparently "you are not using it"

FWIW I had to learn that the hard way: Argentine Churches (Catholic) generally used an Italian/Argentine made Geloso 2xEL34 or EL36 tube PA ; sometime around 1970 they switched en masse to Philips Holland/Argentina made transistor amps , around 150W, and suddenly the old tube amps were everywhere , for peanuts.

They became the PA of choice for poor kids Rock bands and I modded quite a few, mainly by replacing screw on microphone connectors

with standard 1/4"jacks and also speaker output jacks.

So far so goo, the BIG problem was finding where to connect the outputs, Geloso OT trying to be truly universal (they were), has a bunch of output taps and terminals, which in a ton of different combinations allowed any output impedance, from 1 ohm to 500 ohms , and anything in between

Among old timers it's still an internal/trade joke mumbling: "unire tra loro i morsetti ... " when something impossible to understand has to be solved.

That Italian phrase means "join such and such taps to get such impedance[/B]" preceding instructions as cryptic as any Egyptian Hieroglyphic because terminal ID was long ago lost, connection table was scratched to death, so we had to try one, test, another, test, and so on until a happy combination was found.

Taps were used in series and parallel in various combinations:



they actually meant:



So you are in good company.

[Mick Bailey]

I can only guess that some installations demand balanced (twisted pair?) lines for interference reduction, this way they can achieve that.

That's right - because the installations can be run over extended distances they can end up being wired alongside mains and telephone cable runs and pick up crosstalk or hum that ends up being distributed across the whole speaker installation. In some cases the spare pairs in multi-core telelphone cables have been used where the speaker power is fairly low.

[Randall]

You guys are the absolute best! Thank you so much. I wish I could have you all over to my SW Florida home to cook for you.

[J M Fahey]

You guys are the absolute best! Thank you so much. I wish I could have you all over to my SW Florida home to cook for you.

Cool.we'll be over there the next weekend.

Nothing fancy, just a small BBQ, Argentine style:



or if getting a large enough grill complicates matters, just stick some construction iron in the ground and do it this way: