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Educate me about gain and amp/preamp matching

I’ve been doing a bit of research on specifics on my second system. It comprises of a Quad ESL 2805 at 86db / 8ohm nominal driven by a Viva Linea preamp, which seems to have high gain (+12db I believe) at 150 ohm output impedance and an Atma-Sphere S-30 OTL amp at 30w/ch. The acoustically-treated room it’s setup up in is quite small - 9x20 to be exact, with the Quads about 5ft from the wall, and me only sitting about 5-6ft from the Quads, so pretty near field. Quads are ideal in this room since their dispersion is linear and minimizes room reflection. Even without the preamp gain added, an SPL calculator shows I can get up to 101db at listening position. 

Because of the size of the room and listening distance, everything sounds superb, but in many of the threads I read, people always state, including Ralph Karsten of Atma-Sphere, that the Quads need more than 50w of power to perform their best.


Does the fact that I am pairing the Atma-Sphere with a higher gain preamp help this situation? I’m not hearing any clipping or noise due to driving a stronger signal into the amp. And 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?


Happy to provide any specific information needed to answer this. Thanks for helping me understand the science behind it because aurally it sounds great.


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atmasphere
11,834 posts
 
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.
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blisshifi OP
1,191 posts
 
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. 
almarg's avatar
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almarg
9,670 posts
 
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
atmasphere's avatar
atmasphere
11,834 posts
 
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. 
gs5556
1,235 posts
 
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.
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