Good Line Preamplifier, 600 Ohm load?

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PRR

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Does anybody remember 300 ohm TV lead-in?

W-I-D-E spacing on slim conductors.

600 ohm pair will be relatively wider spacing.

The only place you have this is the old-old telephone lines, separate conductors a foot apart on glass insulators.
 

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Yes. 300 and the wider 450 Ohm twin feeder are still available and used for radio work. 600 Ohm is also used but has to be DIY'ed.
You're right, it will be wider, as mentioned before for overhead phone lines. DIY 600R line is usually about 4" wide, depends on conductor size. A very quick search threw up the table at bottom of this page HOMEBREW LADDER LINE 600 ohm 450 ohm 300 ohm ribbon cable coax verses balanced feeder coax losses ladder line losses amateur radio qrp qsl homebrew antennas home construction
 
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Good points on the physical reality of a “600 ohm” cable. Perhaps it might have something to do with signal matching transformers that may be present in some studio equipment specially designed for 600ohms.

If you use specialized line driver IC’s like a THAT1646, it can drive 50ohm lines, so 600ohms is no problem.

I know that when I measure signals from instruments designed for 50ohms with an O-scope, it’s important to use 50ohm cables. But these are RF and microwave frequencies and audio probably not as critical.

For headphone amps, I try to design them to drive at minimum, a 50ohm load, preferably 32ohms or 16ohms stable even.
 
I personally like www.amb.org alpha20 boards which should be able to drive 600ohm loads without problems. especially in balanced configuration.

my favorite output transistors on them are unexpectedly BD139-BD140 from STM as output-stage (Less precise may be but resulting in a more "warm" sound and less clinic then the Japanese ones, at least with my configuration).

I also found improvements in the sound using 500V silver/mica capacitors for the ones in the pF range. It's hard to fit them on the board but in my opinion the effort is paid back.
Anyway, I think component choice needs to be matched with the rest of the sound system. In the preamplifier that is very important.

I found big differences changing just one passive component to make the difference sometimes.
Choose carefully the components from trusted sources and, when possible (economically talking), test both options! That is the holy rule of the DIYselfer, isn't it??!!
 

PRR

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..Perhaps it might have something to do with signal matching transformers that may be present in some studio equipment specially designed for 600ohms. ...

No, telephone line impedance was "discovered" on long lines and became critical for the first attempt at a US transcontinental phone line. This almost worked with NO AMPLIFICATION, and was put in service with Mechanical Amplifiers.

(Some of the poles for this line are still standing in Wyoming. When Mountain Bell salvaged the #8 and #6(!) copper, the Dept of Wildlife asked them to leave the poles for birds to perch.)

Balancing resistive loss, capacitance, inductance, and shunt conductance, against available materials, led to a fat wire w-i-d-e space line of "around 500" Ohms characteristic impedance in the lower midrange. This is where most of Bell's early standards and test processes were developed. This is why most older WE coils show 125/500 ohm connections. Computed impedances are:
300 Hz = 1,345
1,000 Hz = 841
3 kHz = 712
http://www.cdarc.org.uk/downloads/presentations/Transmission_Lines.pdf
So any single-number "impedance" is wrong.

600 is actually a later thing. Radio Networks got to be the bulk of the Bell System's income. They needed One Meter To Rule Them All. Simpson developed a meter, later codified as "VU". But they could not quite meet spec at 500 Ohms. (Rectifier loss.) Everybody involved agreed that impedance was not a fixed thing, and went along with a spec of exactly 600 Ohms for meter testing and calibration. (I think NBC used 150r internally, a better fit to twisted-pair, but stepped-up to 600 for the meters.) While the Wichita line might really be more like 450r at one end and 700r at the other end, if the VU meter said -6VU at both ends the gain/loss balance was pretty close.

600 Ohm termination is available on much test gear. And socket-sets still have a 13/16" sparkplug wrench, even though that has not been used in decades.
 
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When seeing many line peamplifiers, they can drive '600 Ohm'. But is this really necessary?

No, it's not. But I don't think that's what you are asking.

Perhaps you want opinions on whether you should design your preamp to drive 600 ohm for a market that wants it even though it's unnecessary.

If that be the case, I would say Yes.

Your competitors have it and you don't, you'll lose out.
 
When seeing many line peamplifiers, they can drive '600 Ohm'. But is this really necessary? I would say it is enough to be able to drive like 2k Ohm. Why should a preamplifier be able to do 600 Ohm? What is your thoughts on this? What would you say about preamplifer load? What should a good preamplifier be able to do?

(I have a project going to make a good line preamplifier.)

Murphy's law I suppose.
If it is not 600 Z tolerant then eventually you have to re-do the preamp.
So it's easier to just bite bullet and go 600 from start.
 

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