What is a passive preamp, exactly?

[dmbr]

And what exactly is it for/does it do?

Is it meant to be used in conjunction with a "normal" amp?

Thanks

 

[Bazile]

This is quoted from the Schiit Audio site. Part of their Sys preamp description.
 
" Sys is a simple, 2-input passive preamp. No electronics. No power. No distortion. Just a 2-way switch and a potentiometer.
Huh? Why would you want such a thing? Well, if you've ever wanted to use one of our DACs with powered monitors, Sys puts the volume control right at hand. Same goes for using two sources with one of our headphone amps—use Sys to switch between a DAC and a phono preamp, for example.
And, you don't have to use Sys with our gear at all. It's a simple, high-quality, passive preamp that works with pretty much any line-level source. "
 
It's a very quiet way to control volume and change sources.

 

[obobskivich]

Similar to what Schiit's description says - it's passive, meaning it takes in no power and doesn't have any active (powered) components (e.g. a gain stage). Generally useful if you're connecting between a source (some have switches, or you can add a passive mechanical switch) that has relatively decent line output and a straight-up power amplifier. For headphones, not so much use because most all headphone amplifiers include their own volume controls.

There are also some tricky passive devices, like the CI Audio PLC1, which is remote controlled but still passive:
http://ciaudio.com/products/PLC1MKII

How does it work? It uses motorized pots and such, but it still isn't applying any active components to the signal. Most passive devices don't have a remote though, like the Schiit SYS. There's also some components that include similar functionality built-in; it was relatively popular on CD players for a while to have a second line-out with a pot for volume control to allow direct connection to an amplifier. Again, not much use for headphones, but if you want a simpler 2ch system it can be very useful.

If you're using a turntable that needs a phono preamp, that will be an active component, but you could still run the line output from there into a passive preamp and then onto your power amplifier.

Keep in mind that the passive preamp won't add gain, so you can only go quieter/lower-in-level than what your source provides - this may or may not be sufficient depending on the source, amplifier, and application, but IME tends not to be a problem for most CD players and amplifiers at least.

 

[KeithEmo]

Most "passive preamps" are a simple volume control (potentiometer) with no active eletcronics and so no gain (Schiit SYS, Emotiva Control Freak). However, there are also passive preamps that use transformers or autoformers (a type of transformer); and, even though those contain no active electronics, they can in fact produce voltage gain.
 
Since each winding on a transformer produces a specific output level for a given input level, in order for a transformer-type passive preamp to have variable gain, it must use a transformer with lots of taps - one for each output gain setting - and a multi-stop selector switch to pick the one you want. These types of passive preamps do have several drawbacks, including having a rather high output impedance, and being sensitive to load and source components (they may actually sound different depending on what you connect them to), and tend to be somewhat expensive (relatively), but they can give you voltage gain without adding any noise (and without active electronics or having to be plugged in).
 
(While you can use a "passive preamp" between a signal source and a headphone amp which both lack their own volume controls, none of the passive preamps I've ever seen can drive headphones well - mostly because they're designed to work with much higher load impedances than headphones offer. )
 
 
Quote:

Similar to what Schiit's description says - it's passive, meaning it takes in no power and doesn't have any active (powered) components (e.g. a gain stage). Generally useful if you're connecting between a source (some have switches, or you can add a passive mechanical switch) that has relatively decent line output and a straight-up power amplifier. For headphones, not so much use because most all headphone amplifiers include their own volume controls.

There are also some tricky passive devices, like the CI Audio PLC1, which is remote controlled but still passive:
http://ciaudio.com/products/PLC1MKII

How does it work? It uses motorized pots and such, but it still isn't applying any active components to the signal. Most passive devices don't have a remote though, like the Schiit SYS. There's also some components that include similar functionality built-in; it was relatively popular on CD players for a while to have a second line-out with a pot for volume control to allow direct connection to an amplifier. Again, not much use for headphones, but if you want a simpler 2ch system it can be very useful.

If you're using a turntable that needs a phono preamp, that will be an active component, but you could still run the line output from there into a passive preamp and then onto your power amplifier.

Keep in mind that the passive preamp won't add gain, so you can only go quieter/lower-in-level than what your source provides - this may or may not be sufficient depending on the source, amplifier, and application, but IME tends not to be a problem for most CD players and amplifiers at least.

 

[obobskivich]

KeithEmo Really Said Stuff

Weird - I hit quote and your entire post vanished. Anyways, I had wanted to respond and ask a question of my own:

- I've never actually seen, or even heard of, a transformer based "passive preamp" before. :eek: Got an example? (I'm curious now)

- There are in-line volume controls that can work with headphones; Sennheiser and Koss make a few examples, and the Creative ACM that comes with some of their newer soundcards also functions as such (its just a pot between an amplifier and the cans).

 

[KeithEmo]

Weird - I hit quote and your entire post vanished. Anyways, I had wanted to respond and ask a question of my own:

- I've never actually seen, or even heard of, a transformer based "passive preamp" before.

Got an example? (I'm curious now)

- There are in-line volume controls that can work with headphones; Sennheiser and Koss make a few examples, and the Creative ACM that comes with some of their newer soundcards also functions as such (its just a pot between an amplifier and the cans).

 
A " 'former" based passive preamp can use either a transformer (which has two windings), or an autoformer (which has a single winding) - and use one for each channel. In either case, the action is stepped (each step requires a separate tap in the winding, and an input on what is basically a huge selector switch), which is why they usually offer a limited number of steps.
 
Passive preamps basd on a potentiometer (or stepped resistor) have several issues. The output impedance is often quite high - and varies depending on what you have it set to and the impedance of the source. Therefore, they are very fussy about long cables, and tend to pick up hum, and sometimes to have a treble roll off if the output cable is too long (and has too much capacitance). They also only attenuate - so no gain. This is equally true for a potentiometer or a stepped attenuator.
 
Ones that use a transformer can actually step up the voltage if they're set up that way, and MAY be more able to drive longer cables. HOWEVER, the way a given transformer acts will depend on the source and load impedance, so a transformer volume control may actually have quite different frequency response depending on the source and load impedance, and many exhibit roll off or ringing with certain loads. Some balanced transformers are relatively immune to hum, but many balanced ones, and virtually all unbalanced ones, are subject to picking up environmental hum. (If the source has a low impedance, then a transformer may hove a lower impedance output, and so MAY be able to drive long cables better.)
 
Proponents like to claim that "transformers introduce no hum or distortion" - but that's not exactly true. Being passive, they don't produce "electronic noise", but they do produce distortion (hopefully very slight amounts of it), and can suffer from resonances and anomalies in frequency response (and their frequency response can vary depending on the volume setting). Some transformers are also very fussy about placement - because they tend to pick up hum from their surroundings.
 
If you Google on "passive preamp autoformer"  or "passive preamp transformer" you'll find references to several of them, and there are several commercial units that work this way available (prices range from a few hundred $$$ to several thousand).
 
 
Here are a few I found near the top of the list (although I don't know anything specific about them)...
 
http://www.tweekgeek.com/jay-sho-preamplifier/?gclid=CNLw4MjGq8gCFdgRgQodiyIA1Q
 
http://www.stereophile.com/solidpreamps/106sonic/#8lG1ZJPsl9sPELTb.97

 

[obobskivich]

Thanks - very informative! Learn something new everyday.

 

[Mambosenior]

I'd like to put a passive preamp between my CA 752BD and an amp with 47kOhm impedance. What should be the imp. value for the passive? Thank you.

 

[KeithEmo]

Skipping all the complicated logic about choosing the optimum value to minimize noise and such....
 
10k should work quite well with most modern equipment - including yours - and is a very common value.
(Remember that you need a dual potentiometer - for stereo - and you want "audio taper" - they come in both linear and audio tapers.)
Also remember that you want to keep your interconnect cables as short as possible, and keep them away from power cables and such.
(The output impedance of a passive preamp is pretty high - so you need to be careful about keeping the cables on the output away from sources of hum.)
 
The quality of the potentiomeer itself won't matter much in terms of sound quality, but higher quality ones usually maintain accurate channel tracking better at very low settings.
 
 
Quote:

I'd like to put a passive preamp between my CA 752BD and an amp with 47kOhm impedance. What should be the imp. value for the passive? Thank you.

 

[Mambosenior]

Many thanks for the advice.

 

[Pharmaboy]

Just seeing this string now...3 months since the last post, so I'm late to the party. But if anybody's watching, I've had practical experience with 2 passive preamplifiers, one transformer based, the other potentiometer based.
 
Oddly enough, the cheaper one is transformer based (NHT Passive Volume Control), and it has always sounded pretty good in my desktop system. Not the ultimate in resolution, but compared to an active preamp, I hear almost no difference in anything that matters, particularly dynamics or bass. It seems to "do no harm." And for something that cost ~$100 in 2006, that's really saying something.
 
I believe in general, due to the electrical properties of transformers, transformer-based passives are less susceptible to impedance mismatches, long cable runs and the like than potentiometer-based passives (all else being equal).
 
By contrast, the far more expensive Goldpoint SA-2-47 doesn't sound quite as good in my system. Though it's built like a tank and the stepped pot is a beautiful thing to use, it sounds lighter & brighter, overall, w/a slight lessening of the bass. I also have to crank the pot to almost the 2 o'clock position to get loud volume (a position that would cause deafness and structural damage on the NHT device!). This is no doubt an impedance mismatch w/my system, so I set aside the Goldpoint and will try it again after I make some changes in my system (swap out the DAC & the subwoofer; insert a marchand crossover in-line before the subs & powered speakers). Those are all things that will directly affect the impedance match w/a passive.
 
I've found that a big factor in the results you get w/a potentiometer-based passive is the voltage output of the device that drives it. That's something you can easily check. As it happens, the DAC that was driving the Goldpoint is a little on the low side @1.9v output (some sources go well over 2v). With a passive--up to a point--more output voltage on the device that's driving it is better.