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.pl?f=general&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.

[Kursun]

I see that you are interested in philosophy. Good.
As long as you think and doubt in a positivist way you'll sail.
But if your thoughts and doubts are hindering you from sailing then it is bigotry.

Would you care to explain the "complex" form of drain that is on these power supplies?

[corwin99]

Ah.. okay I see. So its not that i disagree with you, its just that perhaps my perception of what Dynamic Headroom is may be wrong, or different than what you perceive it as. Thanks for the link, that gives me more insight into the stuff.

[Kursun]

I see that you have a Project II on the way.
With turntables and magnetic cartridges you'll have a whole new area of "clipping" and "headroom". Do check your amplifier's "maximum phono input level before clipping" rating before buying your cartridge. Amplifiers vary very much in this respect. Here I would want the maximum headroom.

[corwin99]

I don't even have a Phono Stage yet :) I will be getting one soon, not sure which yet but i will check what you speak, though it's never been mentioned to me before.

[lattybuck]

I am not an electrical engineer or designer but I do work in engineering and design. The one thing I have found out in 20+ years in the industry is that it is all to easy to get lost in the details of equations and loose the overall intent or real world effect of what is being said by or asked of another. So here I go stumbling in again.
My understanding of headroom is simplistic but usually works for me. Continuous power is just that, continuous. That means the constant reproduction of a source material. This does include to some degree the transients, or higher instantaneous outputs required on some material, but not all by any means. Thus an explosion, cannon firing or particularly loud drum beat containing many distinct but "louder" sounds is different from say a singer or insturment "sustaining" a note. This is what I refer to as headroom. Maybe not technically correct but that is what I am trying to clarify in my mind with todays changes in technical as well as physical changes.
It used to be indicative to see larger power supplies as meaning larger reserves of headroom as well as greater continuous power. Simply put power is converted and the more reserve you can utilize the more you can convert. This is simplistic but basicly correct with regard to various other systems, types of conversions etc., according to my understanding of the conservation of energy and matter. Thus my NAD with the old rating of 75 watts per channel and a 9db headroom could easilly perform very clean transients of well over 200 watts. I know because we used some equipment to measure it when I first baught it and I blew a few 200 watt+ rated speakers with it with very dynamic music. The cannons of the 1812 overture on a very very clean and dynamic recording I have sounded clear right up to the cones toasting on some Advents. This was one of the reasons people didn't used to mind the 10 watt ratings on some amps, the headroom more than compensated for the transients that were reaching well over 100+ watts. The old Carvers, MacIntoch, Phase Linears and many others were famous for this. They wouldn't clip or distort on very loud transients even though they may not play the constant "sounds" that loudly. The word "clip" may be outdated as well I guess since I don't see it used much anymore.
I am trying to put in focus my own experiences with todays terminology. I do apprciate what you are saying, but I suspect we are stumbling through the forest of technical jargon on different paths. Close, maybe even visible to one another, but not quite on the same path. My Taoist side there. I am curious if there is a modern equivilent of the older specification or if it has been rendered useless by better technology or abuse of the old understanding of what "headroom" was in the real world.
Your help is always appreciated.
Thx

[Kursun]

The word "headroom" is not a very definitive word. A friend of yours may consider buying a 50W amplifier and you may tell him to buy a 100W amplifier so he'll have more "headroom". The term "dynamic headroom" however is a very definitive term in amplifier design. It doesn't lose or change its meaning with time. This term, along with "clipping" and Ohm's law is still with us.

Blowing 200W+ speakers is not a clue to high power. Speakers, under dynamic conditions usually blow because of insufficient power. Besides speaker system power ratings are only very very rough guidelines. Consider a 3-way speaker system with a 200W woofer, 50W mid-range and a 20W tweeter. How should the speaker be rated? This same speaker may handle 200W continuously yet this very speaker may fry at a mere 30W. It all depends on the program material.

There has been some amplifier designs called class H and class G, which employ multilevel voltage power supply lines: a stronger low voltage line, and a higher but weak power supply line to handle the transient peaks.

I don't think people who buy 10W amplifiers depend on dynamic headroom. They just value their ears and probably say it's not quantity but quality. More important they depend on high sensitivity speakers. As you know a 10W amplifier driving 108 db/W/m speakers will match the sound pressure levels of a system of 87 db/W/m speakers driven by a 1280W amplifier.

Yes "headroom" can come in many ways. But "dynamic headroom" specification denotes only one thing: poor power supply regulation.

[entwistle101]

You blow speakers because they are drawing to high a current, whether that is because your over powering them or because your amps not up to the job, they still blow. The power consumed buy the speaker is not simply v*i, it is Re(v*i). Sound waves are complex buy nature ie p=Ae^jwt where p is the pressure, A is the amplitude, w is the angular frequency and t is the time.

We can apply the to our amp and instead of pressure we use v, for the voltage. This means that the current and voltage can be out of phase with each other. So your amp could apparently be no where near its maximum power and yet having to supply voltage and current levels very close to or exceeding its power capacity, causing your amp to clip. If your amp does not have the appropriate headroom, dynamic or other, the speakers will receive to much current and blow.

Sorry to get all 'techy' but hope that helps.

[Kursun]

Nonsense

[entwistle101]

Really? Care to explain why it is nonsense!

[Kursun]

So your amp could apparently be no where near its maximum power and yet having to supply voltage and current levels very close to or exceeding its power capacity, causing your amp to clip. If any condition causes the amplifier amplifier to clip, the amplifier is not “no where near its maximum power”.This is its maximum power. An oversized high quality power supply is crucial for supplying the currents needed for overcoming adverse load conditions.

If your amp does not have the appropriate headroom, dynamic or other, the speakers will receive to much current and blow.” You are still not able to define the factor/factors that determine dynamic headroom. Read the links I previously gave. The links state the case rationally. I think you are quarreling with your own shadow.

[entwistle101]

If any condition causes the amplifier amplifier to clip, the amplifier is not “no where near its maximum power”.This is its maximum power. An oversized high quality power supply is crucial for supplying the currents needed for overcoming adverse load conditions.
Because of the complex nature the amp could be no where near its maximum power. Power is a real component in the complex nature of a wave.

For example, The amp could be at maximum voltage at phase angle a and maximum current at phase angle b, if we multiplied the voltage and the current together we would not get power we would get the VAR’s. If the phase angles are big enough very little power would be transferred but the amp could still clip.

Whether you choose to have dynamic headroom or not is your choice so long as your happy that your amp is good enough to drive your speakers with sufficient headroom, dynamic or other wise.

Check out the following link.

http://www.audioasylum.com/audio/general/messages/132860.html

[Kursun]

Thank you for the link. It verifies my previous statements:
- An amplifier (http:///#) with low DH has VERY stiff power supply. It has a very good voltage regulation. It is expensive to manufacture.
- An amplifier (http:///#) with high DH has a power supply that greatly varies its voltage. Its power supply doesn't have voltage regulation. Its power supply produces a higher voltage but is weaker.

A 100W amplifier (http:///#) that can put out 200W (having an HD of 3db) for a brief transient can look attractive but at what impedance does it produce 200W? As you rightly state, a speaker is quite different from a 8 Ohm dummy load. It is an electro-mechanical mass accelerating in alternate directions in a magnetic flux. Its coil and other reactive components in the crossover and speaker lines result in great variations in the impedance curve. There are other factors too, such as back emf, etc.

Consider an amplifier with 3 db DH giving out 100W. It needs to give 3.5A. When it needs to give out 200W for a peak the current rises to 5A. All these figures assumes a load of purely resistive 8 Ohms. The load may dip to 2 Ohms and at that point the current will rise to 7A for an output of only 100W, which means this amplifier (#) with high DH may not be able to produce even its rated continuous power of 100W into a real load because its power supply voltage will sag even further. Current demands will be much higher with more complex loads. A soft power supply is not designed to provide high currents. In such a case its voltage will drop to levels well below its rated power.

On the other hand, a 100W amplifier with very stiff power supply and 0db DH will produce 400W continuous power into 2 Ohms. An amplifier with 0db DH looks as an unbudgable 100Watt'er only on paper. When working into real loads with wide swings of impedance it continues on producing current.

Another and probably more serious concern is whether this philosophy of cost cutting in power supply is extended to the selection of output transistors. While a 100W amplifier with high current capable stiff power supply needs to be around 85V, a 100W amplifier with a 3 db DH has to have a power supply voltage of about 120V. Though this supply varies greatly and drops as current demands rise, this overtaxes the output transistors and risks dependability. The solution is to produce a 200W/ 0db DH amplifier, but this is expensive.

This is why an amplifier with low DH is better than an amplifier with high DH, especially for critical music listening purposes. An amplifier with high DH may be desirable for HT applications where cost is a factor. It is probably desirable for mini sets too...

Some of your claims about the nature of real loads are correct, but you are defending the wrong type of amplifier.

[lattybuck]

From a real worl standpoint believe I understand what you are both saying about "speaker loading" we used to call it. Some speakers have more stable "loading" namely their input impedence didn't vary as wildly as others. I may not not be saying that right, but basicly I have seen some speakers that could eat a cheaper amp when their impedence dropped to nearly 0 while being driven hard. Some of the old electrostatics were famous for this. I have also played a klipsch, the corner horn units, on my NAD and notices the increadible difference in efficiency between those and my old maggies and to a lesser extent my old AR's as well.
On the original subject about a headroom rating your saying that Dynamic Headroom is still a reliable way, within reason, to read how great a transient the amp is capable of handling without that overreaching/overworked amp sound you can get sometimes. I recently auditioned a Rotel amp and a Krell side by side on some B&W 301's and the Rotel has a definately less open sound to it on with the material. As if it was, for lack of a better term, working much harder than the Krell. The Krell was performed effortlessly and was very open sounding. Both systems were rated close to the same output lvl's and I didn't push the amps very hard at all, maybe 1/4 to 1/3 volume. Some very clean Joe Satriani recordings I have as well as an original master of the 1812 overture by NY philharmonic really showed the differences to my ear as well as the sales guy. He was frowning before me.
So what we used to call headroom in the 70's is now Dynamic Headroom if I understand you correctly. It is also posiblethat it always was and they basterdised the term more then. I have seen more than a few examples of that over the years. I will do some more research as well.
I really do appreciate both of your opinions and knowledge. Help is always appreciated and usually the best way for me to learn is by stumbling around the reality of it a bit and someone explaining where I am wrong or where there is ties into something else, occasionally laughing while doing so as well.
Thx again
Take care all