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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 6th Jan 2011, 12:28 pm   #1
'LIVEWIRE?'
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Question NAD 3140 Amplifier queries

Specifically to do with two features of this transistor HiFi amp. Firstly,
despite servicing/repairing radio and audio gear for almost 40 years, I'm not
sure what 'S.L.C.' stands for. Depressing the button so marked to switch on this feature makes no audible difference. Secondly, even with the circuit
diagram, I don't fully understand how the 'soft clipping' feature works.
On the example I am in the process of repairing, distortion was evident only
when the 'soft clipping' circuit was activated. Having located and rectified the fault in the 'soft clipping' circuit, the amplifier now works beautifully, except that, again, I can detect no difference in sound quality between
'soft clipping' 'on' or 'off'. Can anyone explain how these two features work,
please?
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Old 6th Jan 2011, 1:17 pm   #2
paulsherwin
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Default Re: NAD 3140 Amplifier queries

I think soft clipping was a bit of a NAD gimmick. Supposedly it gave the amp a 'valve sound' when in operation. I've never been able to hear any difference either.
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Old 6th Jan 2011, 1:19 pm   #3
dave cox
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Default Re: NAD 3140 Amplifier queries

It should only make a difference when the amp is 'near' clipping, when at which point the gain will reduce somewhat compressing the very top/bottom of the waveform. In the frequency domain, this will result in fewer high harmonics - usually a good thing from the ears point of view! Of course this technique will probably make the amplifier MORE non-linear near clipping ...

dc
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Old 6th Jan 2011, 1:55 pm   #4
Herald1360
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Default Re: NAD 3140 Amplifier queries

Doesn't S.L.C. mean Speaker Lead Compensation...... presumably a 4-wire technique for driving the speakers similar to load sensing on a DC PSU. Effectively a "Kelvin" connection.

If you don't have the second (monitoring) wire pair connected, it won't do anything anyway since the system will (should) still monitor at the amplifier end via some resistors in order to prevent complete mayhem if the switch were operated in the way you describe!

As to whether anything would be audible if you use reasonable gauge speaker wiring (rather than bell wire?) with it on or off is probably a discussion for one of the more esoteric audio fora
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Old 6th Jan 2011, 2:05 pm   #5
paulsherwin
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Default Re: NAD 3140 Amplifier queries

I don't think the 3140 had anything like this. It was a respectable but basic mid market domestic amplifier.

[later] Apparently you're right, and SLC stands for Speaker Lead Compensation. It's supposed to compensate for speaker lead lengths longer than 10 metres, though I've no idea how it's supposed to work.
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Old 6th Jan 2011, 11:10 pm   #6
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Default Re: NAD 3140 Amplifier queries

Thanks to all three of you for the explanations. I never even thought of Speaker Lead
Compensation. Must check out the circuit when I get the time. In my workshop, I have the amp connected via a couple of output meters to a pair of (semi)HiFi Speakers with ca. 3M of reasonably good speaker wire to each. Haven't tried changing this setup. Presumably the S.L.C. works by sensing the overall resistance between the o/p terminals and the speakers?
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Old 8th Jan 2011, 11:13 am   #7
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Default Re: NAD 3140 Amplifier queries

I remember these. I was working in a hi-fi department at the time. SLC does in fact stand for Speaker Lead Compensator as has been mentioned. However, it was a passive, fixed thing; there was no 'active' element to the compensator. In the manual there was a table of speaker wire gauge against total lead length. If your lead length/gauge was below the value given in the table you kept the switch Off; if it was above you switched it On. I remember we played about with test leads at the time to try it out but could detect no difference. It does have some circuitry associated with it but suspect it was more of a marketing tweak that a true technology evolution.

These comments are not meant to detract from the amplifier in general though. It was/is a perfectly good and competant machine and many continue to give highly satisfactory service to this day. I see them them as well as the contempory Denons and Naims etc. in people's homes while visiting etc.

HTH

TimR
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