Difference between passive and non-passive preamp

[nightflier]

I know this may sound like a stupid question, but what is the difference between a passive and non-passive preamp? Hypothetically, if there were two identical preamps but one was passive, would there be an audible difference?

[Geoffcin]

I know this may sound like a stupid question, but what is the difference between a passive and non-passive preamp? Hypothetically, if there were two identical preamps but one was passive, would there be an audible difference?

A passive preamp has no gain producing circuitry i.e.; if your CD player outputs 2v, the maximum signal that can come out of a passive preamp attached to it would be 2v. A passive preamp is basically a switchbox with an attenuator (the volume control). Some passive preamps take it even further, and don't even have any switching capabilities. These are "purist preamps" for the audiophile with an extreme minimalist approach.

A non-passive preamp, and nearly ALL preamps on the market are, is one that has a gain stage (amplifier), and can take a weak signal and boost it before sending it to the amp. This is sometimes absolutely necessary, and sometimes not, depending on the strength of the incoming signal. All amplifiers modify the signal to some degree, and a powered preamp will modify the signal to some degree.

As to the hypothetical question; Yes, I can hear a difference. After doing some exhaustive testing, I prefer a passive preamp for my audio setup. Will it be that way for you? I can't say, but I will say that if you intend on getting an audio setup that has very high resolution, you owe it to yourself to at least audition a passive preamp.

[kexodusc]

There's some damn good and affordable passives out there. Whether they'd sound good in your setup is as Geoffcin mentioned, hard to say.

Sometimes a passive pre-amps minimalist treatment won't have any added benefit at all depending on the gear it's connected to. If done right, the gain at the pre-amp stage in an active pre-amp shouldn't introduce anything negative at all. In practice, this is probably hit and miss, though I'll be damned if I'm gonna sit here and say I can hear the gain in the pre-amp ruining things...I think the passive approach makes sense though, if you don't need it why have it?

This isn't a whole lot different in concept than the stereo vs. pure direct modes on most HT receivers these days...by pass a whole stage of electronic stuff.

[hertz]

There's some damn good and affordable passives out there. Whether they'd sound good in your setup is as Geoffcin mentioned, hard to say.

Sometimes a passive pre-amps minimalist treatment won't have any added benefit at all depending on the gear it's connected to. If done right, the gain at the pre-amp stage in an active pre-amp shouldn't introduce anything negative at all. In practice, this is probably hit and miss, though I'll be damned if I'm gonna sit here and say I can hear the gain in the pre-amp ruining things...I think the passive approach makes sense though, if you don't need it why have it?

This isn't a whole lot different in concept than the stereo vs. pure direct modes on most HT receivers these days...by pass a whole stage of electronic stuff.

So, how do you increase the volume in a passive preamp setup ?

[Geoffcin]

So, how do you increase the volume in a passive preamp setup ?

You can only attenuate it.

[Resident Loser]

...if your source has an output of 2V all the passive unit will do is attenuate that level...going down? But it it's relative, if you start out with max attenuation and turn the knob clockwise you are raising the level by lowering the resistance(as all pots do) allowing more of the 2V through to the powe ramp...precision, stepped attenuators that rely on discrete resistors can be quite expensive, but seem to be the preferred(possibly only?) methodology.

Problems will arise if you have sources that will not drive the amplifier by meeting it's minimum input requirements...S/N ratio can go in the cr@pper, etc.

Anyone intent on using a passive will have to do some homework, look at the specs and play with the numbers...OR you can use your ears and audition any number of components just to get in the ballpark and then fine tune it from there...chuckle...

jimHJJ(...numbers don't count?...)

[E-Stat]

I know this may sound like a stupid question, but what is the difference between a passive and non-passive preamp? Hypothetically, if there were two identical preamps but one was passive, would there be an audible difference?
I'll add a few comments as I have enjoyed great success by using passive attenuators in two of my systems. (they're really not preamps).

It is critical to understand they are not suitable in all situations, but the benefits can be well worth it IMHO. In addition to having enough gain to drive your amplifier, there is an impedance matching issue. In order to maintain proper dynamics, the output impedance of the source (such as CDP, tuner) must be low and the input impedance of the amp high. Though there is no consensus as to the exact difference, one rule of thumb is 10:1. Another related factor has to do with the capacitance of the cables going from the attenuators to the amplifier as they act as a low pass filter. High cap cables can roll off the high frequencies. Consequently, systems employing long ICs are not good candidates. The amount of rolloff depends upon the capacitance of the cable along with the input impedance of the amplifier. With my system, the rolloff occurs well beyond the capabilities ot the source. DACT offers a free spreadsheet that calculates those values.

Originally, I created a DIY unit using $15 worth of Radio Shack parts to drive my (then) office system. Just for grins, I took it downstairs to my main system. Naturally, I was kinda surprised when this $15 unit bettered the sonics of my $2500 ARC preamp when using my CDP! (One of the ARC knobs costs more than the attenuators) I later built another unit using better parts and stepped attenuators for convenience.

I have listened to a number of $10k+ preamps and I find none of them to be sonically perfect. If your source can drive your amp, then there are benefits to be found. In my case, the soundstage widened and overall resolution improved. Today, I use the preamp solely for phono use. Fortunately, my CDP has a very low 75 ohm / high 4 volt output that lends itself very well to this application. With the lowest level recordings, I listen no higher than -10 db down. On the negative side, I must manually change the interconnects when I change sources from CD to phono or vice versa.

I use the original unit in my vintage system where the gain is not sufficient to fully drive the amplifier. I find that to be no problem since the speakers I use really don't need the full 400 watt output of the amp. Staying under 10 watts / channel keeps my amp in cascode Class A for the best sonics anyway.

rw

[hertz]

So you mean to say that the passive pre will have a knob which will attenuate the level by adding the resistance. Does this not introduce the same amount of distortion as the knob in a active pre-amp with does the opposite job ?

[theaudiohobby]

In an active pre, there is some amplification circuitry in addtion to the resistance of the volume knob in the signal path, and hence the phrase preamplification..

[hertz]

okay.....thanks

[E-Stat]

So you mean to say that the passive pre will have a knob which will attenuate the level by adding the resistance. Does this not introduce the same amount of distortion as the knob in a active pre-amp with does the opposite job ?
Yes, but that's all there is in the signal path with the attenuators. The sins are cumulative. Active preamps start with attenuators and then add active devices and a bucket full of capacitors, resistors, etc. Let's see if you can spot the preamp vs. the passive attenuators:

<img src="http://home.comcast.net/~ralphwallace/audio/sp9main.jpg">

<img src="http://home.comcast.net/~ralphwallace/audio/se2.jpg">

rw

[kexodusc]

Let's see if you can spot the preamp vs. the passive attenuators:


LOL!
This of course begs the question why passives often cost as much or more than actives?

[E-Stat]

LOL!
This of course begs the question why passives often cost as much or more than actives?
Fair question. Perhaps a breakdown of the costs associated with my DIY unit will shed some light:

Par-Metals anodized aluminum case: $50
(2) DACT stepped attenuators: $270
(4) Cardas connectors: $32
JPS Labs wire: free
Silver solder: $5
Sand (for ballast): Nil
(2) ARC knobs: $60 (actually left over from a silver-to-black preamp faceplate change)
(2) Vibrapod feet: $10

That's over $400 in parts, some of which certainly is not related to the sound quality. I just like the JC-2 look! On the other hand, this first prototype using Radio Shack parts cost around $15 (plus the other two ARC knobs!). It, however, has undergone some later improvements with leftover JPS wire and Cardas connectors (since I can't add and bought too many). I use it with my vintage Double Advent / Threshold system.

<img src="http://home.comcast.net/~ralphwallace/audio/se1.jpg">

rw

[kexodusc]

Hmmm, you got me really curious now...that actually shouldn't be too hard a project at all to build.

I have a few old amps I've used that have level controls for each channel. Between that and my CD players volume control I can get by without a pre-amp at all. Would this be more passive than a passive pre-amp?

AudioSource actually makes some very good and very inexpensive small amps like this, I have one of those too. It's funny when you see stores market them as "integrateds".

[E-Stat]

Hmmm, you got me really curious now...that actually shouldn't be too hard a project at all to build.
Not difficult at all for those with basic soldering skills. Here's the recipe if you like. Looks like I overstated the cost of those knobs - I had to replace four of them originally.

http://db.audioasylum.com/cgi/m.pl?forum=tweaks&n=57638&highlight=passive+E-Stat&session=


I have a few old amps I've used that have level controls for each channel. Between that and my CD players volume control I can get by without a pre-amp at all. Would this be more passive than a passive pre-amp?
I avoid the term passive preamp since I simply use attenuators-in-a-box (and short, low capacitance cables!).

Edit: Here's the link to the calculator I mentioned earlier to see if there will be any HF rolloff

http://dact.com/html/ac_calculator.html

rw

[kexodusc]

Thanks E-Stat, soldering's not a problem.
Low capacitance cables???? What value? You worried about the cables filtering out low frequencies?

[E-Stat]

Low capacitance cables???? What value? You worried about the cables filtering out low frequencies?
The concern is the reverse: too much capacitance rolls off the top.

As for value, I use a meter having 80 pf. It doesn't necessarily have to be that low. I suggest downloading the spreadsheet I referenced which will also factor in the input impedance of your amplifier. Then you can see the effect graphically by using different parameters.

If you're willing to listen to a crazy audiophile like me and throw away $15, then I encourage that you try it for yourself. My original objective was to save money by not buying a preamp for my original office system. My experiment ended up changing my main system as well. You may be surprised at the results as was I.

:D

rw

[Geoffcin]

I noticed a very sophisticated mix of SS & Tube in there, an ARC hallmark.

[kexodusc]

The concern is the reverse: too much capacitance rolls off the top.

Really? That's counter-intuitive - to a non-engineer diy speaker building hack like myself who uses capacitors as high-pass filters in crossovers. Looks like I need to dig out my old physics textbooks again... :confused:

[corwin99]

LOL!
This of course begs the question why passives often cost as much or more than actives?

Some Passives cost a lot more also because they aren't not simply Attenuators in a box... there are autoformer/transformer based passives that solve some of the impedance matching problems that happen with tradional passives. A couple examples are the Bent Audio TAP and the Sonic Euphoria PLC. Both are quite pricey as far as passives are concerned.

[E-Stat]

I noticed a very sophisticated mix of SS & Tube in there, an ARC hallmark.
SP-9 MK III hybrid using (2) 6922 dual triodes and (2) JFETs for 67 db of gain through phono. SS power supply. Matches well with my low output Dynavector.

rw

[E-Stat]

Really? That's counter-intuitive - to a non-engineer diy speaker building hack like myself who uses capacitors as high-pass filters in crossovers. Looks like I need to dig out my old physics textbooks again... :confused:
Actually, I had the same thought in reverse when I started tinkering with the crossover in my Advents. I added a 1 uf Solen polyprop bypass cap and learned it dropped the crossover frequency somewhat.

rw