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Quick question about the output of my amp
Quick question about the output of my amp
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Old 15th July 2006, 04:23 PM   #1
Turbo7MN is offline Turbo7MN  United States
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Default Quick question about the output of my amp

Hey, I have a Bogen M60A PA amp that I converted to use as a guitar amp and I was curious about something in the output section of the amp. This M60A schematic shows a couple RCA jacks coming off the 25V tap of the output transformer, one of them labelled for a WMT-1 (some kind of transformer) and the other one labelled as "Tape/Booster". It was my understanding that anything shown as a "tape" output was a line-level signal, is this the case here?

My problem here is that my M60A is a very early model that did not have these jacks, as well as a few other differences in the circuit. I want to be able to record straight from the amp instead of having to mic it, and I was studying the schematic and saw this. The extra part of the circuit looks nice and easy to duplicate and put in an external box to hook up to the amp, if it does in fact provide a line-level signal. A lot of the Bogen CHB series amps have an identical setup as well.


One more thing, what is the purpose of the 25V tap at all? I see a lot of Bogens have one, though not all of them have the WMT-1/tape output jacks hooked up to it. What kind of speakers do you connect to a 25V output?

Thanks in advance.

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Old 15th July 2006, 10:36 PM   #2
Klimon is offline Klimon  Belgium
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Just an idea; maybe it's intended for PA-use (long cables) - I know it is and was common to use additional opts for this purpose that transform the output to 100 volts... maybe 25v is a variation on this theme?

Good luck finding a definite anwer ,

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Old 16th July 2006, 04:28 AM   #3
Tubelab_com is offline Tubelab_com  United States
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The Bogen amplifiers were originally intended for PA amplifiers. They were often installed in office buildings, department stores and the like for background music and paging. The installations were often large. If you were to connect all of the speakers in parallel and the amplifier had fraction of an ohm tap, the system would work, except for the problem of resistance in the speaker wire. In a large installation this could be several ohms. The speakers at the far end of the building would be quiet and the ones near the amp would be loud. What if you wanted a different volume level for each speaker?

A long time ago someone discovered that if you had higher impedance taps on the output transformer, and you used a second transformer at each speaker to reduce the impedance to 8 ohms, the resistance in the wiring would not make as much of a difference. The National Electric Code requires wiring carying over 75 volts to be in conduit, so the "70 volt line" was created, so that ordinary twisted pair could be used. 70 volts refers to the maximum voltage that could be on the wires. The "line matching" transformers mounted at each speaker, have taps to control the volume of that speaker without affecting the others.

Other "voltages" exist also, 25 volt for small systems, and 140 volt for something really big, or really loud.

This setup is still used today.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 16th July 2006, 04:38 AM   #4
DigitalJunkie is offline DigitalJunkie  United States
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I don't see why it wouldn't work,except the previously mentioned voltage level...you might need to tweak R49 and R50 so that you get a true line-level output,something around 1-2V p-p out.

FC3S huh? Do you know "RotorDemon20B"? (He's a buddy of mine.)
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Old 16th July 2006, 05:00 AM   #5
Turbo7MN is offline Turbo7MN  United States
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Sweet, I'll give this a shot next week sometime. I never knew that about PA systems, that's pretty cool.

Yeah I know Rotordemon, he's over on Team FC3S a lot. I'm a moderator over there, but I sometimes need to host a quick picture so I'll post it there.
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Old 7th August 2006, 08:11 PM   #6
Turbo7MN is offline Turbo7MN  United States
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Well, that was a failed experiment. No matter whet I did, I always got distortion from my MiniDisc recorder that I plugged into. I tried tweaking both resistor values with a pot, as well as adjusting the input volume on the recorder, but I couldn't get rid of the distortion.

Would there still be a way to get a true line level signal from the output transformer, or am I screwed?
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