NAD repair and an old spec question

[lattybuck]

First off, I have a 20 year old NAD reciever that still sounds fantastic but has recently started to have dead spots in the sound knobs, rotary pan style. I noticed I can play with the loudness button and that will get sound back sometimes. The power isn't going off just the sound as if it was completely muted. This is still a very nice sounding reciever even with only 75 watts per channel. The headroom on the amp more than compensates for the lower wattage for me and drives my 20 year old AR's great when it isn't cutting out. It used to drive maggies and the AR's fairly well at one time. Ok, old equipment needs some maintenance sometimes, but I cannot locate a good repair shop anywhere in the Tulsa, Oklahoma area for this. I am kind of emotionally attatched to it, my first real sound system, so I would like to repair it and keep it in my bedroom. Anyone know a quality repair shop for NAD in the southwest? Preferably one that warranties it's work.
Second off, I was curious about an old issue I was familiar with that seems to not be discussed much anymore. Namely headroom in amps. I know I am behind due to having my old equipment for so long now. But that little NAD with 75 watts per channel drove my 2 old maggies and 2 old AR's better than some 150 watt amps I hooked up. It has monster reserves of headroom and this made a tremendous difference. Especially in very dynamic music, like rock, the 1812 overture or numerous "Live" recordings for example. Maybe all amps have great headroom now. I don't know myself but I am curious. I don't seem to see this rating on literature anymore. And since this forum seems to be very knowledgable on current systems as well as old maybe you all can educate me some.
Your opinions are appreciated.
Take care all

[Kursun]

By "headroom" I assume you mean "dynamic headroom"... Actually a good power amplifier that is built like a tank has 0 db dynamic headroom. It's gives out its rated output regardless of frequencies or duration or number of channels driven.

A cheap amplifier with a cheap power supply on the other hand, can supply lots of reserve for a short period but falls short when the demand requires high levels of continuous low frequency tones. See the link (especially the Music Power section): http://sound.westhost.com/power.htm

While being a weakness actually, some companies have the nerve to promote high "dynamic headroom" as a virtue! The power supplies of amplifiers with high "dynamic headroom" are in fact no different than poor line power outlets which vary their voltage between 105 and 130 Volts according to the load.

[corwin99]

I don't think this is quite right. Dynamic Headroom is a good thing, and Well Built amplifiers with properly built Transformers will have a good amount of it. Of course when it is used to augment the ratings of an amplifier to make it look good on paper or for a sales pitch that is not a good thing, but I sincerely doubt that any good amplifier out there will have 0db of dynamic headroom. If an amplifier could only play peaks of music up to its rated power it wouldn't be very good... it would mean the transformer is cheap and built only to the minimum standards of the peice of gear without allowing for any extra.

Your pots are probably worn out if your sound is cutting out when you turn them. It should be an easy fix if that is the case.

[Kursun]

I don't think this is quite right. Dynamic Headroom is a good thing, and Well Built amplifiers with properly built Transformers will have a good amount of it. Of course when it is used to augment the ratings of an amplifier to make it look good on paper or for a sales pitch that is not a good thing, but I sincerely doubt that any good amplifier out there will have 0db of dynamic headroom. If an amplifier could only play peaks of music up to its rated power it wouldn't be very good... it would mean the transformer is cheap and built only to the minimum standards of the peice of gear without allowing for any extra.

Your pots are probably worn out if your sound is cutting out when you turn them. It should be an easy fix if that is the case.

"think" and "doubt" are not very good words in science.
A good, oversized power supply has very good voltage regulation because of its low internal resistance (heavy gauge windings, oversized iron core, etc.). Its voltage doesn't drop when high currents pass.
Voltage of a power supply: V=E-I*r
(E=nominal voltage, I=current, r=internal resistance of power supply)
For a good power supply I*r (voltage drop-current multiplied by internal resistance) is almost 0 (voltage drop almost nonexistent). A poor power supply voltage may drop considerably. In fact it may dance in modulation with the current demand, which varies with music. Hence the word Dynamic comes in.
Since the voltage output of a good oversized transformer/power supply is fixed, static, it doesn't show the "dynamic" behaviour of a cheap transformer/power supply. But it doesn't back-off either, as a cheap transformer/power supply, when it is required to give out its full power (twice output with every halving of load impedance) regardless of drive or load conditions. It won't faint at continuous 20 Hz. tones or 2 Ohm loads or speakers presenting a highly reactive load.

Surf the net. You'll see that many no-expense-spared high quality power amplifiers have 0 db dynamic headroom. If you want more headroom you simply buy a more powerful amplifier.

[corwin99]

I guess i can't argue science with you... i am not an electrical guru and can't spit out equations to impress you like you can

I'm not even sure what you are trying to say. That aside, I assume what you are saying is that a good well built high-end no expenses spared amp rated at 100WRMS will immediately start clipping once driven past 100WRMS, while a cheap amp rated at 100WRMS with say 3db of dynamic headroom will be able to maintain very short peaks of 200watts as well as continuously drive 100WRMS. Is that correct? Please no more electrical mathematical mumbo jumbo since I don't understand most of it, and i would assume that since you know that i don't do sciencespeak (using godawful words such as doubt and think) that you would stick to layman's terms please! I am always ready to learn.

[Kursun]

Forgive me for spitting out formulas. It's just that I love the Ohm's Law (V=I x R). It's so versatile, yet so simple... Along with a couple of more formulas you can solve most any electrical problem!

Regarding your question:
For one thing, I don't think a good well built high-end no expenses spared amp wouldn't be 100W.
200W or 400W would be more like it. It would be solid.

A 100W amplifier that momentarily gives out 200W would be nice. In fact many people (including people with technical backgrounds) may actually prefer such an amplifier. They are called amplifiers with "soft power supplies" (the other type is called "stiff" power supply). But they would be stressed. You may have to keep your fingers crossed while listening. It may unexpectedly run into distortion. What's more, such an amplifier probably runs into more risks in terms of dependability.

I see that many manufacturers find this term (dynamic headroom) somewhat critical. It sounds nice to the ear, as if a bonus (more power) is promised. Many find it hard to say that their amplifier has zero dynamic headroom, fearing they might be misunderstood.

Searching through my favorites archive I found an old audioasylum thread that you may find interesting (btw, no formulas.):
http://www.audioasylum.com/scripts/t...neral&m=132867

[entwistle101]

Actually "think" and "doubt" are very good words in science, they start all good debates and reasearch. If people didn't "think or "doubt" then we never investigate anything! And as for your equations you've not taken into account the 'complex' form of drain that is put on these power supplies, or the fact that without a reference, a figure in dB is meaningless.

Ste.