NAD 3130 Amplifier-same fault 15 years apart!!

[Nick]

Anything by Doug Self is worth a read, "Self on Audio" especially.

The other point about the emitter resistors is they are great for implementing output over-current protection, i.e. clamping the base to clip rather than destroy the output stages...

[Old Sparky]

I have now fitted a 70 degree Thermal switch (cut-out) in series with the supply to the mains transformer primary, with the cu-out mounted in the centre of the o/p transsitors' heatsink, and, although I haven't attempted to get the temperature of the H/S up high enough to open the cut-out, the amplfier should now, hopefully, be adequately protected against future failure, at any rate, due to thermal runaway!  The Thermal cut-out is secured to the H/S with self-tapping screws, and I applied a smear of Heatsink Compound before screwing it in place.

[Mark Hennessy]

Excellent - cheap insurance

The only outstanding question mark is the loudspeaker wiring - it'll be good if you can get the chance to inspect that. I've seen some pretty horrific things over the years; often, non-technical people are - understandably - really not good at dealing with stranded wire! Personally, I always use 4mm plugs...

[Old Sparky]

(16/03/2019) As I currently have two NAD amps. in my 'to do' pile, one of which is the same model, finding this old thread again is very helpful- I 'll almost certainly at least consider the same mods. to the two awaiting my attention (Neither, AFAIK, is that in this thread.) Regarding DC offset one channel in a Cambridge A3i I've been repairing has a DC offset of around 50-60mV, whilst the other has only a few mV. That amp will be put on soak test on Monday.

[Terry]

Interesting to see in the pictures in post #22 that they've drilled large holes in the heat sinks removing material where it is most needed - next to the power devices themselves!

[Refugee]

Those holes may have been punched at manufacture.
Are there vent slots in the cabinet floor below the holes?

[Old Sparky]

Those rather large holes either side of the o/p transistors were indeed punched by the manufacturer, Refugee. As to whether there are ventilation holes in the bottom the cabinet below them, I don't remember, but can check the NAD amps. I currently have in my workshop.

[Old Sparky]

Over on diyaudio.com, in a long-running thread about a Cambridge audio A3i, someone has suggested that I'm confusing DC offset with quiescent current, but, in both that amp, and the NAD in this thread, in practice they surely amount to the same thing, since, in both cases the speaker(s) re connected between the emitters of the o/p devices and ground, thus the voltage, obviously in millivolts, measured at the o/p transistor emitters will be the same as that measured at the speaker terminals, and the current through the o/p transistors with no signal i/p is the Iq. The main differences between the two circuits are that 0.22R emitter resistors are factory fitted to the Cambridge amp., but not the NAD, and that pre-set adjustments are incorporated into the Vbe multipliers of the A3i, but the corresponding resistors in the NAD are of fixed value. All this aside, if, e.g., 50mV is measured at the junction of the o/p transistor emitters, then 50mV will also be measured at the speaker terminals. This is what I've always understood to be the DC offset, and, where emitter Rs are fitted the Iq will be that flowing through those resistors, usually ca. 30-50mA in amplifiers of this type.

[Mark Hennessy]

No, DC offset is very different to quiescent current - they are two completely separate parameters. You must measure Iq with nothing connected to the output terminals because any additional current that flows in the load will upset the measurement.

I haven't read the whole thread (via the magic of copy and paste: https://www.diyaudio.com/forums/solid-st...ds-28.html), but I see that you did indeed confuse the two in post #273 - you described the voltage across the two emitter resistors as the DC offset, when in fact this voltage pertains to the quiescent current. I didn't read all the subsequent replies, so don't know if things became clearer...

Everything you need to know about audio power amplifiers is contained in this book: https://www.desmith.net/NMdS/Data/Books%...%20Edn.pdf

[Nick]

Good grief! I didn't even know that was on my site!

It's not a legal copy - it's one I found on a Russian site and reported to Doug so that the source site could be reported to their isp.

I'll remove it this evening...

(It is a very good book though...)