What does Q605, Q451, Q447 and Q443 do?
NAD 7100 Loudspeaker Protection
- NAD 7100 – Power amplifier protection
Q605 or TA7317AP is an integrated circuit especially designed for amplifier / loudspeaker protection , pins 2 and 3 monitoring for DC offset, pin 1 from Q603 and Q601 (one channel shown) senses the output stage emiter current. The output of Q605 at pin 6 drives a protection relay E600 through Q606. Under most circumstances your loudspeakers will be disconnected if there is over current or DC on the output rails if the error is sustained detected by RC network timing circuit R617 and C607.
As mentioned previously DC offset is usually one of the first checks to be made – a double clicking of the protection relay after switch on is usually detected, but not always. Some are very quiet in operation. Often the emitter resistors R445 and R447 burn when the output transistors go faulty. These resistors may be found to be burnt already but the set works fine. Replace them as they may have gone high already and will limit power output and also cause the set to be prone to shut down at a lower volume level. Another thing, burnt relay contacts are not uncommon – relay contacts carry a huge amount of current and arcing may lead to their early demise. This is a quick continuity check. If doing a restore replace with the same component.
Power Envelope – Power On Demand – Powerdrive
Power driving the NAD 7100
Q443, Q447 and Q451 form the heart of the beast. Do you recall that these are very high voltage output transistors? Q443 feeds the output transistor Q439 with a voltage jump from +44.4V upwards, blocked by diode D433. No, the amplifier is not designed to run on +/- 72V to the output pair but rather the supply rail voltage is shifted according to demand of power power requirement. Proton came up with Power on Demand hype, not surprising since they used to build NAD under licence years back. This where things can become interesting – D443 may be prone to failure. This will cause a host of problems – protection relays may kick in because of imbalance in the power supply rail but I would be checking zeners D441 and D443 first. Replacements for D443 should be designed for a beating – fast switching and high current with a minimum 100V PIV rating. Whether they go faulty I would not know but like all things electronic I’d be looking there. This is after all a high powered amplifier, don’t be fooled by the 50/60W rating.
Some topics of interest
Do read up on the TA7317AP IC. Many amplifier designers used other circuits to protect DC from loudspeakers but this is a dedicated chip and very, very popular. Knowing why the amplifier protection is on (or loudspeaker protection) is fundamental to a good repair. Sadly, usually all pointing to the output and driver transistors (except in my case, the moisture residue left in cleaning off dust with compressed air).
Preamplifiers are known to cause problems but are usually related to capacitors and switching circuitry. For another day.