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Can a stereo, single ended, phone preamp be used in single channel balanced mode? Consider the cambridge 651p for example since schematics are public domain and easy to refer to,For example, from one channel of the cart you could route coil plus to pre in left plus and coil minus to pre in right plus.
The single channel balanced output XLR connector would then need to be hooked up to the pre outs as follows. Shield/pre ground to XLR pin 1. Pre out left plus to XLR pin 2. Pre out right plus to XLR pin 3.
Has anyone done this? Do you get the enhanced common mode noise rejection from being fully balanced?
Obviously you'd need two stereo single ended preamps to get a stereo pair of balanced preamps if it even works...
I happen to have a couple identical Cambridge 651p's available. hmmm
Edits: 01/17/17 01/17/17Follow Ups:
First off, your CMRR (common mode rejection ratio) will be terrible! CMRR is actually pretty important in a case like this.
What is needed is at least a differential input! That would improve the CMRR quite a lot.
The problem is that the cartridge, which is indeed a balanced source, has its outputs on pins 2 and 3 of the XLR connection (if in fact that's what you plan to use). On any equipment that supports the balanced line standard, the inputs are pin 2 with respect to pin 3 and ground is ignored.
Your setup won't do that as it sits, but if the input circuit were somehow to get wired together differentially, you could make a go of it.
IMO its impractical. I think you're going run into hum problems and not much output, unless you can find an input transformer with a center tap on the secondary side feeding the preamps.
;-)
Thank for your post.Since I ordered the parts for the adapter cables, I'm all in so to speak.
I was actually hoping to improve cmrr and reduce hum.
Edits: 01/19/17
The signal coming from a phono cartridge is inherently balanced unless you use a piezo cartridge.
All you need is a phono pre which does not convert the signal to single ended.
I think if I hook it up as I described a single ended stereo pre could be used as a single channel balanced pre.
I just bought over $50 worth of connectors from Digikey to build some adapter cables so I can try it an experiment.
You would need to add two more 47k resistors in series across the mono cartridge output with their center connection tied to the ground of each phono input. That combined with the phono stages internal 47k resistors would load the cartridge with 47k. If you didn't add the second set of 47k resistors the cartridge would be loaded with 2 x 47k = 94k Therefore, by adding a second set of 47k resistors it might just work. The output from the phono stage would be in-phase for one channel and 180-degrees out-of-phase in the other channel providing a mono balanced output.
I think it might work. You can try it and see if it works.
Good luck,
John Elison
I'll have to build some adapter cables to try it.
The loading and gain might not be optimal. I can deal with those things.
I hope this turns out working like I think it might.
Any circuit needs to be complete/The coil is a direct connection of two wires and a motor generating a signal.
One wire to each side is like not making a compete circuit from the cart to the input stage of the preamp.
A cart has four wire, really two pair. You seem to want to put one wire to each of four independent inputs..
Now if you could split the signal, and have it be isolated, so one cart coil was split out to two pairs of wires, with one pair left and one pair right.. to one preamp, and ditto a second..
Maybe. But I do not know if you can do so without screwing up the coils response due to halving voltage... etc.
But I think you would really need to make a device which would act as a buffer.. between the inputs to the two preamps.. Because basically you ARE joining them together electrically input end together.
And possibly it may fry the cart coils from feedback of the preamp reacting to being cross tied electrically (this is just my guessing)
If it blows up.. the cart would act like a fuse.. and the coils fry, rather than both preamps blowing up ... at least that would be probable
There would unquestionably be output. My only question is whether I could typically expect to achieve any enhancement in the common mode interference rejection.This is why I mentioned the schematic for the 651P. Look at page 10 in the pdf in the link. It's the 640p but same thing.
Take the 640p moviong coil inputs, for example, there are 470ohm resistors off pre in left plus to ground and another 470r from pre in right plus to ground. The MM inputs are similar but with 560ks.
If you put coil plus to mc pre in right plus and coil minus to mc pre in left plus tou have effectively placed a 940 ohm in parallel with one coil of your cartridge.
That may not be your carts optimal loading, but if your stuff works, that 970 ohm resistor will get a voltage across it and cause the preamp to function when you play a record, if the premp is working and turned on.
Edits: 01/17/17
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