Quick question about the output of my amp

Turbo7MN · 15th July 2006, 03:23 PM

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.

Schematic:
http://forum.teamfc3s.org/attachment...3&d=1146186362

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.

-Darren

Klimon · 15th July 2006, 09:36 PM

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

,

Simon

tubelab.com · 16th July 2006, 03:28 AM

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.

DigitalJunkie · 16th July 2006, 03:38 AM

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.)

Turbo7MN · 16th July 2006, 04:00 AM

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.

Turbo7MN · 7th August 2006, 07:11 PM

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?

15th July 2006, 03:23 PM	#1
Turbo7MN diyAudio Member Join Date: Feb 2006 Location: East central MN	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. Schematic: http://forum.teamfc3s.org/attachment...3&d=1146186362 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. -Darren

16th July 2006, 03:28 AM	#3
tubelab.com diyAudio Member Join Date: Jul 2005 Location: South Florida	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. __________________ Too much power is almost enough! Turn it up till it explodes - then back up just a little.

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15th July 2006, 09:36 PM	#2
Klimon diyAudio Member Join Date: Apr 2005 Location: Leuven	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 , Simon

16th July 2006, 03:38 AM	#4
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	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.)

16th July 2006, 04:00 AM	#5
Turbo7MN diyAudio Member Join Date: Feb 2006 Location: East central MN	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.

7th August 2006, 07:11 PM	#6
Turbo7MN diyAudio Member Join Date: Feb 2006 Location: East central MN	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?