Driving long audio cables

Hi, I use NE5534 opamps in custom mixers and preamps, they drive power audio power amps (high impedance inputs, level around 0dBu). The distance between my gear and the power stuff has suddenly increased to somewhere around 20 metres. I can run screened cable, but I'm wondering whether the capacitance involved in that length of screened lead could lead to instability in the opamps. Is there anything I should be considering, like maybe a parallel RL network, such as is used on some power amp outputs. If I should, typically what values would be appropriate.

BTW, this is band sound, it just has to sound OK, silly specs for THD and frequency response don't apply.

Reply to
Bruce Varley
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Is this single ended, or balanced? Several designs I've seen used 10 to 100 ohm resistors from the op amp output to the output terminal block or connectors to eliminate the stability issues. This was audio in TV broadcast and CATV head ends.

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Reply to
Michael A. Terrell

"Bruce Varley"

** A 100ohm resistor from the output pin of the 5534 will ensure stability with capacitive loads.

Will be OK up to 100nF of cable capacitance.

Equates to about 500 feet a garden variety cable.

.... Phil

Reply to
Phil Allison

One technique is to use a guard screen - an "inner screen braid" that is driven by a signal very similar to the one you send down the inner conductor. This effectively cancels out the capacitance between the inner conductor and (inner) screen braid, the second (outer) screen braid is earthed and serves the usual purpose.

For audio purposes it should be sufficient to drive the inner braid with a voltage follower fed by the wanted audio signal - it doesn't have to match the exacting standards of the signal you send down the central conductor.

Reply to
ian field

Look at the specs for the NE5534. From memory they are designed to drive 600 ohm loads, so a few hundred pf of cable is insignificant at audio frequencies. At most a series resistor should overcome any stability issues.

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Adrian Jansen           adrianjansen at internode dot on dot net
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Reply to
Adrian Jansen

Isn't that exactly what a 5532/4 was designed to achieve ?

geoff

Reply to
geoff

Well, 100nF is a lot of capacitance for an audio lead. With a 100 ohm source driving a cable with that capacitance the top end of the frequency response would be rolled off making the combination into a low pass filter with a roll off point at about 15.9kHz. Fortunately the OP only wants to drive 20 metres of cable and unless the cable capacitance is excessive for that length I can see no problems at audio frequencies. Strangely enough some unbalanced "audiophool" cables have excessively high capacitance. For example I was given a pair of Apature by Accusound cables to try out on my system. The inner conductor to screening capacitance was a whopping 700pF for a short 1.5m cable. A figure of 1/10 of that is typical of most quality audio leads of the same length.

Cheers, Alan

Reply to
Alan Rutlidge

"geoff"

** Sheep shagger alert.

** Low input voltage noise is the main distinguishing feature of NE 532/4s - about 4 nV/rt Hz - making it still one of the best op-amp for use as an RIAA pre-amp for MM cartridges.

Previous types like the LM301A and RC4558 were well able to drive 600 ohms with low THD - as will the TL 071/2 bi-fet series as they all have 20 mA output current ability.

All the above have bandwidths in the hundreds of kHz, when used with low gain factors.

..... Phil

Reply to
Phil Allison

Thanks for the info, guys. Most useful. I considered this because on one of my earlier trials I had a device take off in the tens of KHz while it was connected to a power amp in my workshop for testing, the result was smoke. Having that happen at a gig is unthinkable.

Reply to
Bruce Varley

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