Telephone line equalizer

Anyone got a schematic for an old-style (e.g., Western Electric) equalizer for equalizing a 15kHz line, such as a studio-transmitter link? I have a long line (dry pair) to equalize. I could design my own, but a proven design would save me the trouble (and time is at a premium these days).

Dave - didn't Symetrix have something for that?
Stephen

Probably. But I'm being paid a flat fee for this job, so I'd like to keep equipment costs to a minimum :green: . The line isn't REALLY long--about a mile, I reckon--so a simple passive EQ should do the job. Actually, I'm making a big assumption; I haven't even confirmed yet that the line isn't flat. I'm sweeping it this morning.

I think he means that symetrix publish their schems, so you could take a look in what they did... :wink:

I don't know if this is any help:
http://www.symetrixaudio.com/repository/TI101_schematic.pdf
Stephen

OK, I swept the line this morning. When terminated--as any mile-long line should be--it has a smooth rolloff, 3dB down at the top of the band, with no weird peaks or valleys. So a simple homemade equalizer should do.

That Symetrix gadget is actually a telephone interface for voice applications, not a line equalizer, but thanks for looking around.

Are you using any transformers on the line?

If so, if you have 150ohm taps available, it might help flatten out your high end a bit.

The equalizers we used years ago for "radio loops" were usually passive devices from western electric. They attenuated the lows and mids, to bring the highs up, sometimes with midband losses of 20-25dB, or more!

I *might* have a copy of a schematic somewhere in my files. I'll look.

If I have it, I'll post it. If not.....

Yes, that's the kind of EQ I had in mind, the old style brute-force approach. Tremaine's books give some examples, very simple resonant equalizers shunted across the line.

I swept the line without transformers but I will be using 600CT:600CT transformers on both ends when all is said and done. I'll sweep it again with transformers in place, of course, and if the rolloff is still there, I'll try connecting the line to the 150-ohm taps. Actually, my deadline has been moved up to this weekend, so I just might have to accept a couple of dB of rolloff at the top of the band until the client allows me the luxury of coming back to it later. If the rolloff isn't noticeable they'll probably prefer that I just leave it alone. Unlike me, they're not interested in specs :wink:

Still, I'd love to see any documentation you might manage to dig up. This won't be my last project of this type, I'm sure.

Dave, I know a guy who probably has the schematic to the old WE eq's.
Or you could just take a drive into the country and climb up some old telephone poles if you know where I'm coming from, ...those things are still around somewhere. Like old stadiums wher they used to broadcast events over ten or twenty lines for radio, before they had tv.

Hey John, how's that T4 working for you?
Can you do me a favor and send it to Dukasound?
:twisted:

Might I interject my naivetae and ask.. What is a telephone equalizer???

i know the boxes that I've used for recording VO from a phone line, but you said this is not that. so, what is your exact application of/for this ??


how is it useful, and which direction is the signal going??

cheers
-david

Check out the old-school panel I whipped up for the studio end of the link. The transformers are new; the blank panel was laying around the station.




The panel selects one of two stereo program feeds to send over the tel line.

I was going for a 1940s aesthetic, to match the technology being used for this particular STL :wink:.

I think it need some bakalite to really capture that 40's 50 thang

Three cheers for Edcor! :razz:

Cool!

Barrier screw terminals as well :thumb:

...all it needs now is some genuine 1940's dust.

Dave, are you sure you're not working for the UN building "improving" their internal audio system? (remember that link from PRR :shock: )

Mark

Dave;

I thought Jensen had a schematic on their site for this below.

From memory, 990 op amp driving a 4:1 transformer
so the output is in the 50 to 75 ohm ballpark.
With that low of output Z the cable capactiance
will be less of a problem.

I would guess that a 150 Ohm driver/receiver might reduce the HF rolloff a bit. But -3 dB at the top end can also be easily corrected by a simple shelving EQ at the source end.

bri

> When terminated--as any mile-long line should be--it has a smooth rolloff, 3dB down at the top of the band, with no weird peaks or valleys.

I would not expect significant resonance on such a short line. And on a longer line, copper-loss tends to overwhelm any resonance.

Did you terminate BOTH ends in reasonable impedances?

Did you really connect test gear to a mile of telco wire withOUT isolation transformers??? Seems foolhardy to me.

5,000 feet of cable should be 150,000pFd or 0.15uFd capacitance. Using a too-simple R-C approximation, to be at -3dB at 15-20KC you must have driven with ~60-70Ω. This is now common practice in good broadcast DAs, but if you use a cheap mixer with low-price transformer you may not get that low.

Actually, unless you happen to have been put on a #20 or larger pair, your copper-loss will be greater than 100Ω. But this is distributed: you get a slow droop.

Drive with the lowest possible impedance: 600 is too high.

Hi-Z loading gives highest bass-mid output but the treble may droop 3dB at 15KHz. Loading-down to 100Ω hurts bass-mid more than treble and is "flatter" but everywhere lower than the hi-Z loading.

Frankly, for a fixed-price job, at a mile, just tell them to crank the treble knob.

At my first job, the studio in the academic building was 1,000 feet from the transmitter in the dorm. There were no handy conduits across campus. The previous engineer ordered a telco line, and because it was only 1,000 feet, did not specify any specs. Well, after a lot of terrible audio, he learned that the academic building was wired to the CO in City 2 miles one way, but the dorm was considered residential and was wired to the CO in Town which was 4 miles the other way. To get 1,000 feet, the telco had kindly given him 12 miles of wire jumpered through two COs.

Did you really connect test gear to a mile of telco wire withOUT isolation transformers??? Seems foolhardy to me.

Click to expand...

It WOULD be, as a general rule, but I wanted to measure the line and nothing but the line, and I connected only briefly, and only after checking the line for nasties (differential AND common-mode). But I would never hook unisolated equipment to the line and walk away...

to be at -3dB at 15-20KC you must have driven with ~60-70Ω.

Click to expand...

Nominal output Z of the signal generator is 50 ohms, so you're in the ballpark.

Drive with the lowest possible impedance: 600 is too high.

Click to expand...

The output Z of the line amplifier is about 50-75 ohms; I did not add additional resistance to build it out, outside of the copper resistance of the isolation transformer.

The final configuration, with transformers, works out to be "flat enough" to please the client. Here's how the (very simplified) low-frequency AC equivalent circuit works out: