Preamps... understanding volume...

Is my understanding that preamps don't "amplify" a variable line out, but only attenuate?I'm trying to understand how preamp selection is important with low sensitivity speakers, even if you have a powerful amplifier.
Most preamps do provide gain to the signal. Passive preamps do not.

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@ozzy - While the majority of passive preamps are attenuators (with 0 dB at full clockwise position), some passive preamps (namely transformer or autoformer volume controls) do provide gain, but not a significant amount.

@sarsicism - To answer your question in simple terms you are trying to achieve the proper gain structure from your source to amp. Most digital sources put out 2V output which is enough to drive most amps to clipping. So a preamp with low or no gain might just be enough for your system (or a digital source with volume control). How low a sensitivity are we talking about with your speakers? Do you have an analog source like a phono stage as well? With analog sources you almost always need some additional gain in the system and the preamp is the most likely place to add it in. I have run analog sources successfully through passive preamps, but today I use a preamp with 20 dB of gain because my two analog sources require it.
My plan is to go Mac Mini to a DAC/Preamp to a beefy amplifier to speakers with a sensitivity of 86 db at 2.8 Volts.
If I had an all digital DAC/Preamp combo that maxed out at "line level", is that enough to reach reasonably loud levels?
Is my understanding that preamps don't "amplify" a variable line out, but only attenuate?I'm trying to understand how preamp selection is important with low sensitivity speakers, even if you have a powerful amplifier.

Well the "variable line out" is the out on a preamp. The key word to understand, what everyone above is sort of but not quite getting, is "line". Line level is around 2 volts. CD, DAC, tuner, tape decks, VHS, pretty muc everything you care to think of is line level. Even the output from a phono stage is line level. In fact the only things you are ever likely to encounter that are not line level are the output from a phono cartridge or microphone. That is WHY the thing you plug your cartridge into is called a phono stage!

Amplifiers also are designed for line level inputs. Pretty much all of them have enough gain to be able to put out full power connected directly to any line level component. In fact, almost all the time all you really need is attenuation. Like you said. So what do you need a pre-amp for anyway?

Well, to conveniently switch components. But also because the design and component selection in line stage components is pretty poor compared to a decent pre-amp. One of the more common problems is impedance matching. Your line component might work great with one amp that has a very high input impedance. This in effect is not asking the source to do much in the way of current. Another amp with lower input impedance might have your line stage sucking wind. Sucking the dynamics and life out of your music in the process.

So like everything else you just have to try and listen and compare. 
So where does an amplifier input impedance of 22K ohms land on the scale?
My plan is to go Mac Mini to a DAC/Preamp to a beefy amplifier to speakers with a sensitivity of 86 db at 2.8 Volts.
If I had an all digital DAC/Preamp combo that maxed out at "line level", is that enough to reach reasonably loud levels?

Since you are using a digital source (as opposed to vinyl, for example) more often than not the answer will be "yes," assuming the amp has sufficient power capability. But for a definitive answer to be provided you should state the make and model of the amp, or at least its sensitivity spec (which is usually, although certainly not always, defined based on the input voltage that is required to drive the amp to its maximum rated power capability).

Also, "line level" is a term that is loosely defined with respect to voltage. The voltage of an analog audio signal will of course vary with the volume of the music at any given instant, but even so the **maximum** voltage of a line-level signal can be anywhere from a fraction of a volt to perhaps 3 volts or more for single-ended outputs, depending on the component providing the signal.

Typically digital sources provide maximum voltages in the area of 2 to 3 volts, for single-ended outputs. Double those numbers for balanced outputs. In the case of phono cartridge/phono stage combinations, when a cartridge is providing its rated output (which is based on certain test conditions which represent relatively high volume but somewhat less volume than the dynamic peaks of certain recordings), the single-ended output of a phono stage providing typical amounts of gain (for example, 40 db for a 5 mv cartridge; 60 db for a 0.5 mv cartridge; 66 db for a 0.25 mv cartridge) will be about 0.5 volts. (40 db more than 5 mv is 0.5 volts; 60 db more than 0.5 mv is also 0.5 volts, 66 db more than 0.25 mv is also 0.5 volts). Again, double the 0.5 volt figure for balanced outputs if the phono stage provides them.

The input voltage required to drive a power amp to full power will often be somewhere between 1 and 2 volts for a single-ended input, but in some cases may be as little as 0.5 volts or as much as perhaps 2.5 volts. And double those numbers for balanced inputs.

As I said, the dynamic peaks of some recordings can cause a phono cartridge to output somewhat more than the cartridge’s rated output, which is based on certain standard test conditions. But even if it were putting out say three times its rating on dynamic musical peaks the phono stage would provide just 1.5 volts in typical situations, barely enough voltage to drive many power amps to full power, and not enough in some cases. So you can see why Clio09 indicated that he needs his preamp to provide considerable gain with his analog sources.

Finally, an important point to keep in mind: The gain specs of preamps almost always refer to the gain they provide when their volume control is turned up to max. And it is usually desirable to have enough gain to be able to drive a power amp to its maximum power capability without having to turn the control all the way up, assuming the power capability of the amp is not **greatly** more than would ever be used with the particular speakers. As you can see from the numbers I stated above, that is likely to be a concern only in some situations involving vinyl or other analog sources. If a digital source is being used the opposite concern is more likely to arise, especially if the source is used in conjunction with a high gain preamp, the concern in that case being having to use the volume control at undesirably low settings.

Again, though, what is the amp? And if it has been determined at this point, what is the DAC/preamp?

-- Al

The amp is an Odyssey Audio Stratos Extreme.
The DAC/Preamp combos I am considering...
Pro-Ject Audio Pre Box S2 Digital
Pro-Ject Audio Pre Box DS2 Digital
Topping D70
NAD C658
... obviously quite a range. I’d like to spend the least amount possible balancing a great DAC with remote volume control. Sub out, MQA decoding, EQ, and streaming are pluses, but not required. Since my source is the Mac Mini, I have some flexibility in software taking on those features.
Unfortunately it appears that sensitivity is not specified at the Odyssey site for any of the Stratos amplifiers. Nor is gain, which in conjunction with the maximum power ratings could be used to calculate sensitivity.

And I couldn’t find relevant specs or measurements elsewhere after doing a quick Google search. So I suggest that you contact Odyssey and ask for that information.

-- Al
Ozzy is right. 

It may help to understand that usually the first stage the signal sees is a high impedance, fixed gain stage. For historical reasons, this gain is often way more than needed.

The volume knob comes after.

Schitt Audio Fraya or Saga would let you see if you need the passive or active preamp as it can switch between both. $299-$899 
I was typing from a bar, so couldn’t put a lot of detail. The simplest active preamps I know of are perhaps the Conrad Johnson PV10s, but they have a lot of similarities with others. The structure is something like this:

Input --> Source selector --> Fixed gain, high impedance buffer --> Volume knob --> Output

The combination of the fixed gain (+ x dB) , and volume (- y dB) loss determines the final output level relative to input.

One thing to note, while these systems are super simple, and short, they also are sensitive to the volume knob setting, like you see on figure 1 in this review:

This can be improved with additional buffer stages, at the cost of more devices in the audio path.

Along with gain comes noise though, and honestly, we have too much gain in most preamps, especially tubes. We’d be better off reducing the gain, and using the volume knob closer to 12 o’clock.

Anyway, the point is, active preamps almost always have a fixed gain stage, but the output is almost always lower than that.


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they amplify, and they attenuate. 
using simple round #'s, a bit of history. Legacy equipment/terms are in use but ....

1. original mono sources (radio tuners, ceramic phono cartridges)  provided strong enough signals directly to amplifiers, let's say 1 volt. This strength is called 'Line Level'. (newer, like CD's provide more, i.e. 2 volt or more). Pre-amps did not exist.

All sources producing 'line level' can go direct to an amp without a pre-amp, the only consideration being volume control (attenuation).

2. early stereo, tapes in 1956, your system being mono to begin with, simply add another mono amp and speaker, both amps receiving 1 volt. LP went stereo in 1958.

3. enter low voltage stereo signals. a. tape head (low signal direct from the tape/tape head). b. phono magnetic cartridge signals (phono low). pre-amps were ADDED, to pre-amplify the new low signals up to 1 volt, then into the original amps. Thus they amplify.

4. both new low level signals, tape and phono ALSO needed/still need 'equalization'. Standardized recording/playback systems were developed, NAB for tapes, RIAA for phono.

Phono as example: (tape recording/playback a similar process, a different 'eq' curve).)
Phono initial physical recording cuts lows (minimizes width of groove needed for bass), AND boost's highs (gets signal further away from noise, like later Dolby does).
LP. Long playing. Cutting the bass grooves allowed more grooves, thus 'long playing'.

Phono Playback needs an equalization circuit, to boost the lows and to cut the highs, before it goes to the amplification portion of the pre-amp, to get up to 1 volt to go to the amp. 

Phono 'moving coil' came later, weaker signal, needing a pre-pre amp so to speak, to get it up to phono low, then to pre-qamp's eq and boost uo to line level for the amp.

'Tape Head', signal directly from the tape, is low level, needing both NAB eq and pre-amplification, thus separate inputs. 

'Tape' is line level, already equalized and strength boosted, by the tape recorder/player's electronics, therefore it can go in any tape input, tape loop, or straight to an amp (again, how control volume?)

5. Attenuation (volume control): various sources produce a variety of signal strength, they need to be 'adjusted' prior to going to the amp, thus a pre-amp volume control. Implementation varies, cut/boost/bypass, ....

McIntosh and others provide 'trim' controls, which individually 'pre-adjust' those low voltages 'to match each other's signal strength' minimizing the need to adjust the pre-amp volume (switch inputs-get same volume).

6. Headphones: some pre-amps have separate headphone amplifiers. Never goes to the amp.

7. Rare Pre-amps, like McIntosh C26, use the headphone amplifier circuit, say 10 wpc, to directly drive efficient speakers. Like future receivers, they offer 'main, 'remote', or both simultaneously.

8. Add a tuner to the pre-amp, like my new to me mx110z

9. Add multiple inputs, switching, volume control to the amp, get 'Integrated Amp, like my new to me Cayin A88T. (I needed MK1 to get 16 ohm taps from the transformer, rare these days).

10. Add a Remote Line Controller, inputs, volume, balance (I love squeaking the balance speck by speck from listening position), tone, .

What a complicated web I have built for myself, selling my Digital pre-amp and amp and getting tube pre-amp, tube tuner, tube amp, all with various methods of control, direct and remote.

Oh yeah, you young folks need to deal with modern digital sources, and modern headphones.

To the OP: I took a look at the specs and descriptions of the six source components you listed. Some comments regarding driving the Odyssey Stratos Extreme directly from them:

1) Note that the RME device only provides balanced outputs, and see the comment regarding the Stratos that was provided by Stereo5 on 6-4-2018, and my comment immediately following his post, in this thread.

2) The lowest of the three output level settings provided by the RME (+2 dbV balanced) may be too low to be suitable, depending on the gain and sensitivity of the amp. At least one of the other two settings should be ok, though.

3) NAD apparently does not provide a spec for output level or for output impedance when one of the digital inputs of the C658 is being used. I suspect that it would be ok in both respects, though, unless the amp’s gain and sensitivity are considerably different than average (see note 5 re "average").

4) The miniDSP device is the only one of the six for which an output impedance spec is provided, and it would have no problems driving 22K. I suspect the others would also be ok in terms of impedance compatibility, but to be certain you may want to check with their manufacturers.

5) The output levels of the Topping, miniDSP, and two ProJect models will all be fine unless the gain and sensitivity of the amp are either well below or well above average. Average gain for a power amp tends to be in the vicinity of 26 to 30 db or so; for an amp like this which is rated to provide 150 watts into 8 ohms those gains correspond to sensitivities of about 1.7 and 1.1 volts respectively.

Good luck. Regards,
-- Al

volume control of tape line level signal directly to amp:

It is not good to use the tape recorder's volume control, because, you want the strongest un-distorted signal off the tape (furthest from the noise), The meters help you do this, you can go into the red for short bursts. You get the strongest un-distorted signal from each tape to start.

My office Carver cube has no volume control, tape level too loud, so I needed to add intermediate volume control, thus Chase RLC-1.
Thanks to everyone for all the info... far more than I expected!
the relationship of strong or stronger signal going from preamp into an amp, and whether the amp can therefore produce more power for low sensitivity speakers (or any) has not been answered.

I do not know for sure, but

Isuspect the amp cannot produce more than it's maximum GUARRANTEED wpc (they often produce more power in spec than their official rating).

McIntosh preamps I have owned output 2.5v (all?). McIntosh ampsI have owned have two optional inputs (all?): alternate .75v and 2.5v to match their preamp's 2.5v. The amp only has one set of published output rating for each ohm tap (4, 8, or older equipment 16 ohms). 

They do not have lower or higher ratings depending on which input level is used. 

I always advise, whenever you buy, or switch equipment, try to find more efficient speakers you love. High power amps do not/will not damage speakers, low wattage amps can damage speakers. Efficient speakers set you up to only need/try lower power amps, especially tube amps.

Amp GUARRANTEED WPC. I owned an mc2250, rated for 250 wpc. Testing at lab day at Harveys proved it produced 320 wpc within spec. The McIntosh technician said "sir, you have an unusual unit". Printed out spec sheet showing the results, changed the McIntosh decal on the back, signed and dated it.

You never know, certainly cannot plan on it. McIntosh and NAD amps both have protection circuits to protect your speakers and themselves. They get engaged when asked (by the speakers) for more than their rating. Others probably do too, I just have no experience with others. Indicates you need more power, or more efficient speakers.
Amps are limited, in absolute terms, by

  • The voltage rails in the power supplies. These provide the maximum + and - V swing.
  • The current available from their own power supplies, before those rails sag
  • Thermally limited in the amount of power they can output before they shut off or melt.This is why peak power may be much  greater than continuous, as well as limit the ability to deliver into low impedance loads.

Amps are also a fixed voltage gain. Often around 28 dB. So:
Output = ( Input + 28 dB ) OR ( MAX)
whichever is less.

does that mean a stronger input signal, from a preamp with more gain (less attenuation), boosted by amps +28db will result in higher volume output, especially important for lower sensitivity speakers like OP is asking about?
Elliott, to answer your question just above:

The gain of the preamp cannot change the amp’s maximum power capability, of course. If the maximum output voltage of the source when boosted by the gain of the preamp is enough voltage to drive the amp to its maximum power capability, additional gain will just result in the amp providing that amount of power, or a given lesser amount of power, at a lower setting of the volume control than would be the case if the preamp’s gain were lower.

The maximum output power capability of the amp, into an 8 ohm or 4 ohm resistive load, corresponds to its rated continuous power capability into those impedances, plus whatever margin may be built into those ratings, plus whatever amount of dynamic headroom the amp can provide for limited amounts of time, plus or minus whatever presumably small differences may exist between different examples of the same make and model.

-- Al