Driving long audio cables

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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.



Re: Driving long audio cables

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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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Re: Driving long audio cables

"Bruce Varley"
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** 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




Re: Driving long audio cables

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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



Re: Driving long audio cables

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cables
a
typical

Not strange at all. That's why they sound "less harsh" to the buyers, or at
least different from regular cables.
The fact you could do the same with a tone control or a 10 cent capacitor is
something the audiophools don't understand, and would never admit anyway.

MrT.



Re: Driving long audio cables

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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.



Re: Driving long audio cables
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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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Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
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Re: Driving long audio cables
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Isn't that exactly what a 5532/4 was designed to achieve ?

geoff



Re: Driving long audio cables


**  Sheep shagger alert.

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** 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



Re: Driving long audio cables

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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.



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