Educate me about gain and amp/preamp matching

These are all incredible responses, thank you. I think I misinterpreted Cleeds’ answer  because you explained how I actually believed it - the amp always puts out the same amount of power, and the preamp’s power can add gain (if it is active). Together, they are additive, and the preamp’s volume control attenuates the whole.

i’m guessing the idea of headroom then is similar to a well-exposed photograph in that it has the most amount of dynamic range and is uncompressed. Having a preamp that has a lot of gain may also add noise, depending on quality, and sending that to the amp could amplify and raise the noise, making sound have a lower range. Per @millercarbon’s post, it might be that the noise balances between the amp and preamp are what can make a preamp truly successful, as the right match may result in the right blend of 2nd order harmonics, adding body and texture without losing overall range. 
The amp also has to be able feed the speakers with the power it needs to be able to handle generally what is hungry (usually the bass) and still have enough power left over to handle the rest of the frequency range, otherwise it will clip. 

I did this type of exercise last night with a different amp, just to test, which is why I decided to post this thread. The amp in question was a 2W/channel Korneff 45 clone. Of course, I knew pretty well it wouldn’t be able to drive the Quads, but I wanted to experiment even at very low levels. Oddly, combined with the Viva Linea preamp I would have been able to get the SPL I wanted, but the sound started to clip even before the volume was at 12 o’clock. I took it off right away. 

I’m actually surprised that the input gain of a preamp doesn’t really effect the output signal gain of the amp. I thought it would be additive or multiplicative (if that’s a word). Can someone explain why that is?

Amplifiers are fixed. All an amplifier does is take whatever comes in and multiply it by some fixed amount. That fixed multiplier is the gain. 

But it can only do this to a certain point. In your case 30W. At that point it doesn't matter how much more you input, or how high you turn up the volume on the preamp, that amp is not capable of putting out any more power. 

An active preamp is really nothing more than a small amplifier with an attenuator (aka volume control) and some knobs to let you switch between sources. That small amplifier is fixed just like the power amplifier is fixed. Same deal. 

All your sources connected to the preamp, they all are at line level. Line level is a couple volts. (The exception is phono, which is in millivolts, which is why turntables need a phono stage, to bring millivolts up to line stage voltage levels.)  

Now here's the thing- all your line level components have more than enough power to drive your power amplifier to clipping, ie as loud as it will go. 

So in practical terms the gain of the preamp is irrelevant. More often than not its not amplifying anything, its attenuating. 

If you have something like a CD player with variable output you can prove this yourself by simply connecting it directly to your power amp. It will drive them just as loud just fine. It will probably not sound quite as good. Why? That gets a little more complicated to explain.

Look at it this way: assume two cars with the same engine are accelerated to 50 mph. The first car requires the gas pedal depressed 1-inch and the second car requires the pedal depressed 1.5-inch. Both have a different gain but both have the same maximum hp and both are putting out the same power. The only differences are the second car may run out of pedal before max power is reached;  the first car will still have pedal travel after max power is reached but nothing changes in that extra travel.

I’m actually surprised that the input gain of a preamp doesn’t really effect the output signal gain of the amp. I thought it would be additive or multiplicative (if that’s a word). Can someone explain why that is? Is it that the output signal gain is usually the same (3-6db depending on unbalanced / balanced) regardless of preamp?

The gain of the preamp does affect the output signal of the amp. But it does not affect how much gain the amp has. Different preamps have different gain and the same is true of amps. If you have a really efficient speaker, having a lot of gain can be problematic. But if your speaker is inefficient, that gain can be handy. Conversely not having a lot of gain is really handy with high efficiency speakers but won't work if the speakers are low efficiency.


So the equipment has to match. From a manufacturer's point of view, if the amplifier has about 25dB of voltage gain it will work on the widest range of loudspeakers. Amps with low power that are going to be driving more efficient speakers might only have 15dB of gain, and high power amps often have in excess of 30dB of gain. 


The gain of the amp is different from how much power it makes. And the source signal can be different levels too. People often associate a certain position on the volume control with a certain amount of power, but really there is no correlation. Its just a level that works in a given situation. 

The overall gain of the amp/preamp combination does in fact correspond to the sum of the gains of the two components (if the gains are expressed in db; if the gains are expressed as a ratio of voltage out vs. voltage in then the two numbers would be multiplied to get the overall ratio). Although keep in mind that preamp gain specs represent the gain they provide when the volume control is at max.

However the power capability of the amp is what it is, and is not affected by the preamp. Gain differences just affect the setting of the volume control at which the amp provides a given amount of power to the speakers.

Regards,
-- Al

Thanks all, especially @cleeds for such a succinct response. I’m actually surprised that the input gain of a preamp doesn’t really effect the output signal gain of the amp. I thought it would be additive or multiplicative (if that’s a word). Can someone explain why that is? Is it that the output signal gain is usually the same (3-6db depending on unbalanced / balanced) regardless of preamp?

@almarg - thanks for the compliments on the equipment. I got lucky over time, I guess. And I don’t ever need to turn up the volume past 11 on the loudest passages given I’m only 6’ from listening position and the speakers are about 6’ apart center to center. 
And @atmasphere - thanks for the clarification on the efficiency - not sure if you’ll be around tmw but am planning to drop by to drop the S-30 and MA-1 around 10:30. Prepping all the gear now. 

Because the Quads are a panel rather than a point source, they are actually more efficient than the specs indicate on account of how that efficiency is measured (at one meter). This is because at one meter a lot of the radiated sound simply isn't picked up by the microphone.

Its likely that the real efficiency is a few dB higher. If you are able to get satisfying levels without clipping the amp then I would say you have no worries.

everything sounds superb, ...
but  ...
the Quads need more than 50w of power to perform their best.

Clearly not.

+1 Cleeds.

Also, keep in mind that the difference between 30 watts and 50 watts is only about 2 db, in terms of voltage, power, and (everything else being equal) in terms of the resulting SPL.

Differences in preamp gain will just affect the settings of the volume control that are used (everything else being equal). And as long as you don’t ever find yourself wanting to turn the control higher than max, or having to operate it near the bottom of its range, such differences are not usually an issue.

Nice equipment, btw! Regards,

-- Al

thiefoflight

... even if the signal coming into the amp has more gain to begin with, at I still limited to small headroom due to the 30w the amp puts out?

Yes. Once an amplifier has been fed an input of sufficient gain to drive it to maximum output, increasing input gain will not increase the amplifier's output power.