Longer instrument cable (synth) sounds audibly worse than shorter one - why?

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living sounds

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Sort of a noob question:

I've got a beautiful sounding made in USA Rhodes Chroma Polaris (also recaped and slightly modified by me). It's got a high amplitude output, pretty much the loudest synth in my studio. The output stage (schematics below) has a 100 ohm resistor in series to counter capacitance of longer cables.

The problem is that going into my console (with the synth ground connection going to the console's inverting input and the console ground floating) with a 7m cable, transients and signal clarity are significantly worse than with a 2m cable. I cannot measure cable capacitance, but the resistance is pretty much zero ohm. Also, there is no audible interference, the noise floor is no different between cables.

So what am I missing here? And what to do? Thanks!
 

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Strange! The output impedance of the circuit is very low, so a capacitve load should not make a big difference.
I could imagine that a longer cable would make a difference with a (passive) guitar element, but I can't see a reason why a longer cable would change the sound when connected to a low impedance output like this.
 
Could this have something to do with the fact that the longer cable is close to power cables over some of the distance (magnetic interferance)? There's certainly no hum or other noise to speak of. The signal from the longer cable sounds really washed out compared to the crisp signal from the shorter one...
 
So, I did more measurements. Turns out there is ca. 1M ohm resistance between ground and signal on the synth end of the cable, but nothing between + and - on the other side of the cable. Guess this would explain the problem, wouldn't it?
 
Fixed the patch bay, turns out this - again - wasn't it. So it it's back to square one...


Could there actually be such a thing as the "sound" of a cable? The shorter one is of better quality, but should this matter at line levels?
 
2 wire or 3 wire connection?

Cable that short at that impedance should not make audible difference.

More likely some obscure connection error...short, open, ??

JR

Note: it is possible to hear difference between long/high capacitance guitar cords due to high source impedance of pickups.
 
It's a two wire connection (line out on the synth) which has the ground wired to the negative input of the receiving end (console or converter) and the receiving ground floating. I've checked all the cabling/connectors, they're fine now.

I haven't got a DI unfortunately. Guess I should build a few...
 
Have you tried each cable with another synth, guitar, unbalanced gear?

Also, have you considered "upgrading" that LM348? Supposed to be four 741's in one package. 
 
mitsos said:
Have you tried each cable with another synth, guitar, unbalanced gear?

Also, have you considered "upgrading" that LM348? Supposed to be four 741's in one package.
Click to expand...

Tried them with a Tele now, same thing. The dynamics of the short cable feel more stable, the long one sounds wooly, muffled, thin in comparison. And with the preamp cranked for the low level guitar signal there is now audible hum from the long cable. Can the magnetic field from a power line next to it compromise transient response?

The cable is taped to the floor, guess I have to take it out now for troubleshooting...

I've upgraded that LM348. Did a lot of testing and listening, and two OPA2211's on a quad adapter sounded by far the best.

 
Is the long cable a speaker cable by any chance? (Instrument cable: shield braid surrounding audio conductor. Speaker cable: two similar conductors side by side.) Sometimes it's hard to tell by looking at the outside and 1/4" plugs often get soldered to either one.
 
tubegeek said:
Is the long cable a speaker cable by any chance? (Instrument cable: shield braid surrounding audio conductor. Speaker cable: two similar conductors side by side.) Sometimes it's hard to tell by looking at the outside and 1/4" plugs often get soldered to either one.
Click to expand...

It's a shielded cable with "professional low noise instrument cable" written on it.

I've now established that lenght is not the issue, since another cable of the same lenght, but even smaller in diameter sounds good.
 
Hello,
Just to add another possible reason why the 7 meter cable makes the synth sound murky...

The fairly long 7 meter (large diameter?) cable may happen to have an unusually high capacitance value. An LCR meter would come in handy here. Not that this capacitance would necessarily roll the highs off since the output impedance of the voltage follower configuration of the output opamp would be very low, as mentioned above, but the 100 ohm isolation resistor may not be large enough in value to keep the long cable's capacitance from making the three paralleled sections of the LM358 possibly oscillate. That would certainly cause the sound to deteriorate. If you have access to a scope you could check the output opamp's stability when the 7 meter cable is connected.

Also, if by chance the longer cable has the lower quality spiral type of shielding; as opposed to a thick low resistance tightly braided shield, the 7 meter cable could act as a fairly nice antenna to pick up EMI/RFI and possibly inject it into the output of the LM358.

Just thinking out loud here...

Regards,
Frank
 
fpliuzzi said:
Hello,
Just to add another possible reason why the 7 meter cable makes the synth sound murky...

The fairly long 7 meter (large diameter?) cable may happen to have an unusually high capacitance value. An LCR meter would come in handy here. Not that this capacitance would necessarily roll the highs off since the output impedance of the voltage follower configuration of the output opamp would be very low, as mentioned above, but the 100 ohm isolation resistor may not be large enough in value to keep the long cable's capacitance from making the three paralleled sections of the LM358 possibly oscillate. That would certainly cause the sound to deteriorate. If you have access to a scope you could check the output opamp's stability when the 7 meter cable is connected.

Also, if by chance the longer cable has the lower quality spiral type of shielding; as opposed to a thick low resistance tightly braided shield, the 7 meter cable could act as a fairly nice antenna to pick up EMI/RFI and possibly inject it into the output of the LM358.

Just thinking out loud here...

Regards,
Frank
Click to expand...

It's the cable, I've had the exact same result with the Tele as I had with the synth. I've substituted the LM358 for a couple OPA2211 (+ added local power bypass caps), which are unity-gain stable, but might create problems, but since this is was reproduced with other gear I think it's the cable. So I'll investigate it more.  :)
 

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