Biased interconnects and BOSOZ

[Peter Daniel]

Couldn't notice recent posts about biasing interconnect cables, which makes me think that if somebody is using BOSOZ or any other preamp with DC output it would be actually recommended to use the coupling caps at the input of the amp. Two advantages: biased interconnects supposedly sound better (BOSOZ has 30V DC at the output), having caps at the input of the amp requires much smaller values, so better caps could be used.
I understand that approach would be prohibited in any commercial offering (preamp being lethal to subsequent stages), however, for any DIY it might be another opportunity to improve the sound of his rig.
One might even use even 2 sets of output jacks: one regular with no DC component capacitively coupled for use with any amp, second - direct output for amps with coupling caps at the input.
Any comments?

[Nelson Pass]

I once had a mc preamp that biased the phono input
circuit with a few mA, and I was under the impression
that it helped to run current through, but only if it was
a really good connection already, otherwise it generated
more noise.

[lohk]

Several people tried that approach, at least to get rid of one or more caps. But in most cases the applied DC, specially if there is a kind of supply current running through, is quite bad for the sensitive plug contacts.
It works with microphones (48V), but here we have on both of the symmetrical lines the same voltage. But it is hard to compare the sound to no DC,because the microphones need this phantom supply to work at all. But it is not very good for the contacts too, and the phantom voltage never runs via the input or pad switches.

But who said that DC "biased" interconnects sound better ?

The BOSOZ has appr. 30V offset without the output cap. I see no advantage by having two different wildly variating voltages together in a cable with all possible side effects.


Much more sense makes to me to bias the screening.

You can either bias it with a fixed DC (this is how I understood HH's approach with the batteries in his reference passive preamp - I heard it once in some setup) or with a shield driver.
Quote from the BB specsheet of the INA103 for an instrumentation amplifier:
A shield driver minimizes degradation of CMR (Common mode range) due to distributed capacitance on the input lines.

Klaus

[HarryHaller]

I knew this would confuse someone so I will try to clarify:

1. I was refering to an electrostatic bias between the shield and the signal and ground conductor in my particular application.

2. There are several wire manufactures doing this on commercial products.

3. I have designed even AES/EBU digital cables with this approach.The optimum sounding bias was around 40 volts

4. This has nothing to do with phantom powered microphone cables.

5. Go read few books on electrostatics and dielectrics if you think there is no physical rational for this. It really is some pretty basic physics.

6. One does not have to put current through contacts to do this bias, but there are advocates of that approach also. I believe the guys at Pink Triangle intentionally put a few 10s of microamps through thier contacts in the PIP preamp from said company. Go read some articles on low level relay contacts and on contact contamination. Ten millivolts of DC offset from your preamp is already putting a microamp into a 10K input impedance amp, through the contacts.

7. This is not magic hand waving stuff. A lot of this has been studied for telecom and instrumention application. Many cables are designed for low tribioelectric noise, and by real engineers and not just zany audio nerds.

8. Am I the only one that reads about this stuff?

H.H.

[Jocko Homo]

I read it! I wouldn't wamt you to feel unappreciated.

Jocko

[Peter Daniel]

Harry,

I understand you were running bias between the shield and the signal and ground wire. I know it's not the same as biasing the signal wire. However, is my suggestion any good. I don't care for contacts because I would be running the output directly to the amp. Maybe even solder the connections. Are there any side efffects? Contrary to Jocko I didn't read anything on the subject. I just wanted to take advantage of the fact that the output of the preamp was already biased.

Peter

[lohk]

HarryHaller,

if you have understood that all, you should be able - instead of showing up - explain it in a few sentences please.
I repeat myself in saying that it makes sense to me even with audio applications, but I do admit that I do not understand how it works.

HPotter,

if you solder your cables on both sides and keep them short I do not see big of a problem. This is maybe a greater advantage than having more caps in the signal path. There is - amongst others - a ZEN project doing this at the PassDIY gallery.

Klaus