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Old 20th June 2002, 05:40 PM   #11
dorkus is offline dorkus  United States
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Default so are we in agreement...

... that this is a worthwhile idea? any drawbacks besides the inconvenience of having to use blocking caps at the other end?

i think i will go ahead and try this on Son of Dork. so much for this being a commercial-grade design!
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Old 20th June 2002, 10:32 PM   #12
haldor is offline haldor  United States
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Hi Dorkus,

Putting a bias on interconnect cables is done routinely in pro audio gear, it's called phantom power and is done to provide bias voltage to the FET preamp built into condensor mics.

I have never heard that doing this has a positive effect on sound quality. It can have some pretty negative effects though. It's not safe to connect or disconnect a biased cable with the amps turned up or you can blow out your drivers. Phantom power immediately shows you the quality of your connections. Any kind of intermitant connection result in huge pops and crackles anytime a cable is touched.

I think the hassle and risk of damaging gear outweighs any likely benefit.

Phil
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Old 20th June 2002, 10:42 PM   #13
dorkus is offline dorkus  United States
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haldor,

i've gone over the risk and inconvenience issues already. i'm willing to live with those.

there is the danger of a lot of popping and other noises with an intermittent connection, but it goes without saying that if i use DC biasing, i will have to take the necessary precautions to prevent such issues. this may include using BNC connections for the unbalanced output to distinguish it from the AC-coupled output of the preamp. i'm not sure yet what to do about the balanced connection - i may use a 5-pin XLR to prevent an accident.

how are you sure the hassle outweighs the benefit? have you tried it? you may want to check HPotter's original thread. i know for a fact that dielectric properties are improved with some DC bias.
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Old 20th June 2002, 10:54 PM   #14
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Default It's not safe to connect or disconnect a biased cable

I bias the shield and have DC coulpled signal and ground wires so no problems. If one hardwired the cap to the cable and preamp and used a bleeder resistor to ground where the cap connects to the amp you really should not have a problem as it will appear the same as an output coupling cap for the preamp. You can do the same thing blowing up drivers with ground loop noise plugging input cables into hot amps if the signal conductor makes contact before the gound. Power down equipment or short the inputs with a switch (for you cable swapping DIYers) when change cables. Phantom powering is not the same thing as cable biasing, but one does wonder how a patent was granted for cable biasing since phantom power seems to be a conflict be prior art.

H.H.
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Old 20th June 2002, 11:08 PM   #15
dorkus is offline dorkus  United States
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ah, good idea about the bleeder resistor by the cap harry. i was wondering how to avoid potential popping when open and closing the circuit, shoulda thought of that. duh. you think a 1 meg resistor will neutralize it sufficiently, or is that too high a value? i guess i could use 200k or something.

i never plug my cables in and out of the system with the speakers connected - i always pull the hot wires out. that's why i use bananas at the amp end. every once in a while while i'm changing interconnects, my ME amp will have its high frequency overload protection triggered. i'm assumming it was caused by a pop or something.

my system is unbalanced, so i might make a special cable with a BNC at the preamp end, and RCA w/filter cap + bleeder at the amp end. of course this will limit the ability for me to swap cables around all the time, but i'm getting sick of trying different wires anyway and will probably just settle on one kind or another. right now i have some JPS Labs Ultraconductors in the system for review, they are very nice.
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Old 21st June 2002, 04:32 PM   #16
haldor is offline haldor  United States
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Default Re: It's not safe to connect or disconnect a biased cable

Quote:
Originally posted by HarryHaller
I bias the shield and have DC coulpled signal and ground wires so no problems.
I agree biasing the shield shouldn't cause a pop assuming there is also a ground reference in the cable that is not biased (or you are using transformer isolated differential signals). The problem is when you bias the signal, which is what I thought Dorkus was talking about. By the way where did the idea of of biasing the shield come from? This is very non-intuitive to me

A bleeder resistor will take care of any self charge across the coupling cap (preventing a pop when you plug it into the input), but I don't see how it can prevent a transient from connecting/disconnecting the bias voltage. This fast rise time step change in the signal level will go right through coupling caps.

Quote:
Originally posted by HarryHaller
You can do the same thing blowing up drivers with ground loop noise plugging input cables into hot amps if the signal conductor makes contact before the gound.
I have heard some annoyingly loud hum from doing this, but never the thunderous pop you get from connecting a live phantom powered mic. The problem is the signal is so fast and of such large magnitude (phantom power is 48VDC) that it can cause your drivers to exceed their excursion limits. I use brick wall limiters in my amps but they take at least a cycle or two to engage by which time the damage can already be done.

Quote:
Originally posted by HarryHaller
Power down equipment or short the inputs with a switch (for you cable swapping DIYers) when change cables.
Very, very good advice. I always mute channels before connecting or disconnecting mics. The problem is when someone "helps out" and swaps mics without giving me a heads up first. I am shopping for a new mixer and a key requirement is individually selectable phantom power so only the channels that need it get it. My current console has a global phantom switch (Mackie) so if there is even one condensor mic every channel gets phantom power.

Quote:
Originally posted by HarryHaller
Phantom powering is not the same thing as cable biasing, but one does wonder how a patent was granted for cable biasing since phantom power seems to be a conflict be prior art.
I have given up trying to figure out our patent office. If Amazon can patent "One-Click" then anything goes. I ran into an unbelievable situation a few years back while implementing a modem with data compression/error correction. Turns out the patent office has issued two different companies patents for the exact same compression algorithm and whenever you use this industry standard protocol you have to pay royalties to two different companies for the same thing.

Phil
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Old 21st June 2002, 04:45 PM   #17
haldor is offline haldor  United States
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Quote:
Originally posted by dorkus
how are you sure the hassle outweighs the benefit?
Can't speak to that issue. I have never noticed any audible difference between biased and unbiased cables (sounds like a double blind test here). ;^). My experiance with biased cables is all from live sound which is not normally a discriminating listening experiance. Comb filtering and lobing from stage monitors interacting with the Main FOH speakers tends to mask subtle effects like the audibility of cables. Not to mention 10,000 screaming people.

P.S. Never go see Dave Mathews live if you crave a discriminating listening experiance or value your hearing. The crowd noise was at least 6 dB louder than the PA. Frightening, I have never seen musicians sticking their fingers in their ears due to crowd noise before. I was never so glad for my ear plugs (I won't go to a rock concert without them).

Phil
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Old 21st June 2002, 05:00 PM   #18
dorkus is offline dorkus  United States
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that's too bad about the dave matthews concert crowd noise. i kinda like their music myself. a friend of mine went to one of their concerts back in '96 before the craze, said it was about 1500 people in a moderately-sized college auditorium. was a lot more fun than tens of thousands of teens crammed into Madison Square Garden.
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Old 21st June 2002, 06:07 PM   #19
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Default Where did the idea of of biasing the shield come from?

I wanted the cables biased and no couplings caps in the signal path for the wire. One end of the shield is tied to ground with a cap with a battery across it. I first did this with an AES/EBU balanced digital cable I designed about 5 years ago which had a 40 volt bias. It think some of the "expensive wire" guys are doing it now also. I won't give them the plublicity of a mention by name. Biasing wire and caps is pretty common knowledge in high end audio at this point. The cable charger stuff is been around for years.

H.H.
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Old 21st June 2002, 06:15 PM   #20
dorkus is offline dorkus  United States
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harry, so do you think the benefits of biasing the shield rather than the signal itself are the same, less the coupling cap? or are we acheiving different things here? i'm assumming we get the desired improved linearity of dielectric either way.
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