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Old 22nd June 2012, 06:20 PM   #1
Hamman is offline Hamman  United States
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Default Conn Organ Post Amp to Power Amp Question

Hi, I'm new to this forum and not quite sure if this is the right place to post this question. Its rather detailed but I'll give a short version of it. Just acquired a huge Conn organ and in the amplifier section (Has 6 channels of amplified audio) I'm trying to replace the multi channel amps with higher wattage output amps because I want more umph out of this organ. And a couple of the channels look to have gone south (circa 1978) They were at some time replaced with "standerized" Allen organ amps (dime a dozen) Heres the problem. This particular custom Conn organ uses the ground shield of the inputs of the power amplifier to feed +32 vdc back to the post-amp/mixer(s) of the preamp section. So when hooking up standard power amps you cannot use the shielded ground of the post amp of the organ. These post amps, (MEEC's as Conn calls them) mix in numerous pre-amp audio channels and route them to the 6 individual amplifier channels which also includes quite a few expression pedal pots (volume control) So the post amp is fed control voltage from the power amplifier via the shield of the input. Yet those 2 amplifiers that are being used in place of the original Conn amps are just hooked up with the "core" wire via going through a 1md 200v cap. No grounding of the shield. Because of this, there is slight hum coming from those 2 channels which in reality is overcome by even the slightest of playing on the organ. Any ideas on how I could wire up standard connected amps to this set up??
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Oh, I should mention as well that I belong to the organ forum and have presented this scenario as well with no real answer....sigh
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Old 22nd June 2012, 07:33 PM   #2
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May be that try to use an operational amp like a differential amp, so cancelling the hum induced in common mode along the cable?
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Old 22nd June 2012, 09:06 PM   #3
Hamman is offline Hamman  United States
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I'll try and get some schematics up later that can help in the explanation or understanding of this situation.
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Old 25th July 2012, 04:10 PM   #4
Hamman is offline Hamman  United States
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OK I know its been a month but I'm back at this issue again. Im trying to get a pic of the "MEEC" schematic uploaded. Can I take the ground for the signal at either grounding symbol of the lower right of the drawing? As you can see the upper right of the drawing shows +32 VDC coming in to feed the MEEC...which is the ground sheild of the input of the internal Conn amp which is the supplier of the voltage as well as others. Hopefully you can zoom in on the pic.
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Old 25th July 2012, 04:24 PM   #5
Hamman is offline Hamman  United States
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Here is the same pic but cropped tighter in. The "MEEC" output on the right of the pic is what feeds the power amps "audio in" signal. The MEEC audio input on the left is what comes from the main organs preamps. Any help would be very much appreciated. Again the Top right side of the pic shows +32vdc coming in via the power amps input ground shield. I want to use "standard" amps with more power and again need to find a more conventional signal ground source to use since I'm not able to use the non conventional grounding that is currently being used for the Conn power amps. I will be leaving the Conn amps in place to continue supplying the needed +32vdc to the post amps (MEEC's)
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Old 25th July 2012, 05:19 PM   #6
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Since the amp itself has a ground connection, did you try to use a shielded cable and connect the shield to ground instead of the 32V source? You can run the 32V via a separate wire if you need it.

Do you have a schematic of whatever it is that sends out that signal with the 32V on the shield?

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Old 25th July 2012, 05:43 PM   #7
Hamman is offline Hamman  United States
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Yes I'll attach it here..hopefully you can make it out. On the right of the pic you see all the numbered circuits. # 7,9 10,12 are the auido in shielded grounds that also supply back the +32 volts to the MEEC's #'s 4, 6, 13,15 are the actual audio signal inputs from the MEEC's.
Now # 14 is the actual +32 coming in to this 2 of 2 4-channel amp. The other 4 channel amp, 1 of 2, is where the +32 vdc is supplied from along with other voltages. Can I just take the hot audio signal from its known source on the MEEC and just use the MEEc's other grounding source (symbol)?
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Old 25th July 2012, 06:20 PM   #8
Hamman is offline Hamman  United States
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OK what amp are you talking about? The pre amp, post amp (MEEC pic #1 and 2) or the power amp? If you are talking about the MEEC amp then what you have suggested is what I've been wondering if I can do sooo is that what you were suggesting? Thanks for your response
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Originally Posted by janneman View Post
Since the amp itself has a ground connection, did you try to use a shielded cable and connect the shield to ground instead of the 32V source? You can run the 32V via a separate wire if you need it.

Do you have a schematic of whatever it is that sends out that signal with the 32V on the shield?

jan didden
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