The schematic for my Hammond L-102 Organ (~1963) indicates an internal 15 watt E.I.A. power amp. The output transformer is a Hammond T301 indicated as 8 ohms 1.8V AC connected in parallel to two 16 ohm speakers. (picture attached)
I would like to make a line out connection to feed an external amp. I would like to leave the internal speakers connected so I assume I don't need a "dummy" load resistor.
I assume a simple voltage divider should work. Could some please recommend values for the resistors and capacitor (if required)?
Thanks. (I only know enough about electronics to be dangerous)
I would like to make a line out connection to feed an external amp. I would like to leave the internal speakers connected so I assume I don't need a "dummy" load resistor.
I assume a simple voltage divider should work. Could some please recommend values for the resistors and capacitor (if required)?
Thanks. (I only know enough about electronics to be dangerous)
Attachments
If the amplifier is earthed/grounded to avoid shock hazard, I would use a 22k 1/2W resistor in series to the centre of your jack out. The screen or outer comes from the Earth symbol on the transformer output. Place a 1k 1/2W resistor across the jack from centre core, (22k feed or centre pole) and ground (outer or screen).
Try Jon's suggestion first, it's simple, and if it works, then you're all set.
However, there is a chance you might run into ground loop problems: hum that occurs when you plug your line-out cable into an external amp or P.A. system. This is particularly likely to happen if the external amp is powered by a mains outlet located at some distance from the outlet that the (tube) organ amp is plugged into, but can also occur even with both items plugged into adjacent outlets on the same power strip or wall receptacle.
If you do encounter ground-loop problems, the cure is to insert a signal transformer: your organ amp drives the primary, and the secondary feeds the external amp or P.A. system.
I recently used a Triad TY-250P for this, and it worked very well. It's inexpensive, too: TY-250P Triad Magnetics | Mouser
To use this transformer, first wire a 1k resistor across the primary, and another 1k resistor across the secondary (these are necessary to get a good frequency response out of the transformer.)
Then ground one end of the primary, and feed the other end via a series resistor, just as Jon described. You can adjust the value of the series resistor to set how much signal level is fed to the P.A.
Since you are dividing down from a (relatively large) speaker-level signal, you will end up using a series resistor that is a lot larger than 1k; in fact, the same 22k that Jon Snell suggested is probably a good starting point.
With the primary of the TY-250P wired as above, the two ends of the secondary feed the P.A. system. I used a balanced XLR connector, so neither end of the secondary winding was grounded. This completely cured the ground-loop problems I had previously experienced.
-Gnobuddy
Very helpful. Thank you. I did get quite a bit hum with Jon's method even though the organ and amp are on the same ac plug.
Just for kicks I took a straight parallel line from the speakers into a passive DI with a 20db pad and voila - no hum and a signal my amp can handle without distortion! I guess I should have tried that first.
Will the DI in parallel with the speakers have a negative impact on the amplifier output? (i.e. drop the impedance below 8 ohms?)
Thanks again!
Just for kicks I took a straight parallel line from the speakers into a passive DI with a 20db pad and voila - no hum and a signal my amp can handle without distortion! I guess I should have tried that first.
Will the DI in parallel with the speakers have a negative impact on the amplifier output? (i.e. drop the impedance below 8 ohms?)
Thanks again!
Excellent!
Maybe your DI includes an isolation transformer? If so, that would explain why it solved your hum (maybe ground-loop) problem.
I think most DI boxes do include ground isolation. I have a Behringer GI-100 active DI box that I used to use when recording my guitar amp. It does include a signal isolation transformer.
Most likely not - the DI boxes I've used usually place very little load on the amp driving them. They offer a "direct out" output into which you have to plug a dummy load (or speaker) if you do want a load on the amp. (With tube amps, this is absolutely necessary, but solid-state amps may be quite happy without any load on the output.)
It wouldn't hurt to check, though, if you can find any literature on the specific DI box you're using.
-Gnobuddy
Thanks. I've left the internal speakers connected so I don't think I need to add a dummy load. My DI has an iso txfr and ground lift switch and I have it set to lift.
I think you are correct that there is very minimal change in the impedance load on the output txfr.
I'm all set to connect to a my "Leslie" rotary simulator pedal and into 400w amp and 4x12 speaker cabinet!
I think you are correct that there is very minimal change in the impedance load on the output txfr.
I'm all set to connect to a my "Leslie" rotary simulator pedal and into 400w amp and 4x12 speaker cabinet!
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