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Post by ashcatlt on Jun 11, 2009 23:32:00 GMT -5
This question was raised on another forum: I'm pretty sure I read around here a while back a reasonable-sounding historical-type account of why exactly it is that we don't have balanced outputs from our guitars. Can't find it though. **** TOMB is not quite so family friendly as this forum.
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Post by sumgai on Jun 12, 2009 0:25:51 GMT -5
ash, Yet another reason rooted in history.... back in the day, noise was just plain accepted as a necessary evil, even if it was universally detested. Engineering knew about balanced lines, of course, but the cost/performance trade-off wasn't acceptable to most folks. Just looking at the cost of DI boxes in this day and age only proves my point. Nowadays, it's worse - the "vintage tone Nazis" (no reference to Godwin intended) would immediately jump on the "It ain't real" bandwagon. Case closed. HTH sumgai
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Post by ChrisK on Jun 12, 2009 15:31:03 GMT -5
I don't think that the technology of the day understood it and the cost was too high.
One could have used a two conductor plus shield cable and wired the internal circuitry isolated from the shield.
It goes without saying that all of the safety issues with the amp chassis and mains safety ground (which generally did not exist at this time) would still apply.
There would have been a signal output and a signal return (both of which are completely arbitrary since this is AC), and both of which would have their own conductor. The amp properly would have had a differential input amplifier which would have treated both conductors as signal leads. Both leads could have had relatively high impedance to the chassis for safety reasons.
However, the relatively modern "OP AMP" structure hadn't been invented yet (I believe that Teledyne Philbrick invented it in the late '50s) and the relative cost of a deployed circuit using a vacuum tube was still quite high.
So, while additional tube stages could have been used to receive a differential signal (per channel, mind you), costs were always the issue.
Keep in mind that there was less formal design engineering and more application circuits adaptation (thank you RCA) involved in the early musical instrument industry tube amp designs. The early tube circuits, transformers, and speakers didn't have the widest frequency range (and still don't), so HF noise wasn't a big problem. After all, there's a reason why the Strat bridge pickup didn't have a tone control.
The jacks and cables came from the telephone industry as did the original Telecaster 3-way lever switch (with spring return in one position it was the talk/listen/dictate switch on all of the interoffice intercomms of the day). Once some were shipped, the 1/4" single-ended single conductor with noise shield became the legacy way and for compatibility reasons, the original archaic approach remains to this day.
And, I will again reiterate that those "wonderful" vintage components were not used because they were the best available at the time, but because they were the cheapest available at the time.
So we could "fix" all of this with a modern (it's not vintage, it's not real, it's not real....) instrumentation approach using rewired guitars, two conductor plus shield twisted pair cables, differential input amplifier modules at the amp with proper safety grounding and using medical probe instrumentation practices; and then we'd have a safe relative noise-free system.
But then them fools invented RF.
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Post by ashcatlt on Jun 12, 2009 23:00:22 GMT -5
Somebody over there had put forward the idea that we could connect one end of the pickup as the + (like the T of a TRS) and the other end as the - (the R) and end up with a balanced output. I didn't think that would work, but it seems like that's what you're saying, Chris.
Is it really that easy?
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Post by ChrisK on Jun 13, 2009 0:10:06 GMT -5
Yes, in basic concept. We would then have a differential output, floating transducer in the guitar (this presumes that the volume and tone control circuitry is also isolated from - as in not connected to, the shield ground). The shield would then be connected to the stuff touchable by the operator thru the blocking cap. There would be two separate signal conductors in the cable, neither of which would be connected to the shield in any way. The shield would be a shield, period. However, at the receiving end (at the amp), these differential signal conductors would have to be "received" by a differential amplifier, or much more preferably, an instrumentation amplifier. This is a good discussion of operational amplifiers. openbookproject.net//electricCircuits/Semi/SEMI_8.htmlNote that the OP AMP was first produced by the George A. Philbrick Research company, Incorporated, in 1952, a tad earlier than I recalled. Also note that it was indeed based on two twin-triode tubes mounted in an assembly with an octal (8-pin) socket for easy installation and servicing in electronic equipment chassis of that era. Cost is. The shield itself would be connected to a fail-safe point relative to the safety ground, OR ...one of the things that is rarely discussed (outside of the medical electronics industry) is the use of the RL (right leg) drive. This is the practice of driving the patent's right leg with a signal that is out of phase with the measured common-mode noise from the differential instrumentation inputs to reduce the sensed noise (precancelation, if you will). www.analog.com/en/medical-solutions/electrocardiogram-ecgekg/applications/technical-articles/ta_High_Performance_Electrocardiogram_Signal_Cond/resources/fca.htmlThe shield could be driven with a similar signal to cancel out the common mode noise signals on the two differential conductors. Well, I think that that's enough clues for one night...
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