[ricardojoa]

Does anyone know the advantages and disadvantages of using these two type of preamp? How would they differen sonically?

Thanks

[Big Red Machine]

http://www.audiocircle.com/index.php?topic=77978.0

[roymail]

First, if you want to know for sure how a passive would sound in your system, try it for yourself.

I copied this quote from Dick Olsher's article in Enjoy The Music (2008) regarding the Pass B1 buffer.  I don't necessarily agree with this opinion.  However, he is complimentary of the B1 buffer which is also passive.

"The so-called "passive preamp," an attempt to achieve the ideal line stage without a buffer stage, has in my experience been a sonic failure. Lifeless and comatose are two descriptors that immediately come to mind."

Notice he said, "has in my experience been a sonic failure."  Therein lies the difficulty in just taking someone else's word for how well a passive attenuator will work or sound in your system.  Some people love 'em, and some don't like 'em at all.  I suggest that this is based mostly on personal experience and not upon what someone has told them.  As you know, everyone has a different viewpoint and difference preferences.

Most passive volume controls or attenuator do have limitations.  However, under the right circumstances, it can sound very good!  Impedance matching is the primary limitation, another being that passives don't have a gain stage.  So they depend upon voltage from the source like your cd player to drive the signal which is around 2V in most cases.  So speakers with lower-ish efficiency are not good candidates for passive volume controls.  In my experience, I prefer at least 90db sensitive speakers, and higher is better.

I use a 20K Goldpoint/Elma mini-v stepped attenuator which I built into a passive attenutor.  My amp has an input impedance of 47K and my speakers are rated 91db at 8 ohms.  Also, I've used a 10K Dact stepped attenuator in the past with nearly identical results.  This setup provides extremely clear, and very dynamic output.  Nothing is rolled off.  Compared to my average-ish SS preamp, I'll take the passive every time.  Although it has a bit less output than my active preamp, it has plenty of gain to play very loud, and the sound is much more refined with a bigger soundstage and more air and space around the instruments.

The Nelson Pass B1 buffer is a passive, zero gain device designed primarily to assist with  impedance matching.  I've never read anything negative about it although it is a passive attenuator with no gain.  However, it does have an active buffer stage to overcome the limitations of impedance matching.  The diy version is easy to build and is a very popular choice.  In fact, it may be your best option.  Two others that come to mind are the Burson Buffer and the Dodd Tube Buffer.

So if you can achieve a good impedance match between your amp and passive attenuator, keeping your interconnects 1m or less, you may really like the results.  In my case, it sounds very good to my ears.  Here's some reading that may help.  http://www.goldpt.com/info.html

YMMV... best of luck! 

[ricardojoa]

Thanks for the replies.

[JLM]

Component matching is tricky with passives.  The best success I've found is in using attenuators plugged directly into my monoblock power amps (which eliminates interconnect worries).  If the source output exceeds the power amplifier rated input sensitivity you should be all set with a passive.

[Construct]

I think that Dick was painting with a broad brush out there.  There are some amps that won't gel with a passive attenuator.  They need push.  Other amps (like certain of the fire bottle variety)  thrive on the minimalism and don't need that gain stage, just a volume control.

[Steve]

Component matching is tricky with passives.  The best success I've found is in using attenuators plugged directly into my monoblock power amps (which eliminates interconnect worries).  If the source output exceeds the power amplifier rated input sensitivity you should be all set with a passive.

Good point JLM. IC capacitance becomes quite a factor, so it is important to check, or even eliminate the IC altogether with only the component input capacitance to consider.

(Caveat: I do manufacturer active preamplifiers.)

Cheers.

[Jon L]

The Nelson Pass B1 buffer is a passive, zero gain device designed primarily to assist with  impedance matching. 

Unfortunately, there is no free lunch.  In addition to the volume pot and the buffer stage, B1 requires a 1 uF and 10 uF capacitor in the signal path, which is not so trivial to get half-way decently transparent..

[rollo]

Good point JLM. IC capacitance becomes quite a factor, so it is important to check, or even eliminate the IC altogether with only the component input capacitance to consider.

(Caveat: I do manufacturer active preamplifiers.)

Cheers.

 For resistor based passives absolutely yes. For transformer based no. One can use up to 5 mtrs. However I would recco a cable with low cap. The other issue is the amp needs to have a low input sensitivity of less than 1V. The lower the better.
   My only reservation with passives compared to active tubed preamps is the weight and soul of the presentation. Now the clarity and quite background is quite addictive at the end of the day the coloration suits me well.

charles

[roymail]

Does anyone know the advantages and disadvantages of using these two type of preamp? How would they differen sonically?

ricardojoa, original poster, I'd like to clarify the differences in simple terms.

An active preamp is a volume control with a gain and buffer stage.  A passive preamp is a volume control with no gain and no buffer in most cases.

However, two recent exceptions are the Nelson Pass B1 buffer which is a volume control with a buffer stage but no gain, and the Dodd tube buffer which is a volume control with a tube buffer and no gain.

The advantage of having a buffer stage is that it solves the problems caused by impedance mismatch.  However, most cd players provide approx 2 volts output which is usually enough output (gain) to drive fairly efficient speakers to loud volume levels.  For this reason, a buffered volume control with no gain is sufficient for many users.  However, if your speakers sensitivity are below 89lb, you will probably want to choose an active preamp in order to have sufficient gain to drive lesser efficient speakers.

One other obvious advantage to use an active preamp is that most provide multiple switching options which is a must for many users.  But, if you are a purist using a single source and efficient speakers, a passive volume control can work well provided you use short, low impedance cables (1m or less), your amp has an input impedance rated at 47K or greater and your source has a low output sensitivity.  Otherwise, using a buffered volume control may be more advisable.  One exception to the above discussion is if your single source is LP recordings for which you'll need either a dedicated active phono stage or an active preamp with an integrated phono output.

Hope this information is helpful to someone.

[Steve]

For resistor based passives absolutely yes. For transformer based no. One can use up to 5 mtrs. However I would recco a cable with low cap.

charles

Hi Charles,

I respectfully disagree as the capacitance of both input and output ICs is across the primary and secondary of transformer based units. As such resonances occur, but minimized with proper precautions. The higher the capacitance the lower the resonant frequency.

I have a general article discussing the subject

http://www.sasaudiolabs.com/theory11a.htm

Cheers.

[Ed Schilling]

Well, fellas...it's a big world out there. I saw this thread and just could not help but let y'all know......I build an "active buffered pre amp". It has an unmeasurable input impedance (for normal people and normal test gear) and 3 ohm out put impedance. The bandwidth is dc to about 60mhz (yes, RF range). The slew rate is >200 v/micro second. There are no caps, inductors, resistors, or potentiometers in the signal path. What is in the signal path are 2 high speed buffers and a single photocell, it does not use optocouplers but is light controlled. It laughs at cables 50 feet long. It has no gain, but yet is an "active device". It makes the best in the world sound "broken". Sorry if this sounds like an "advertisement". It's not. It's The Truth.
Ed
edit****
So let me make a couple points.....for the most part the job of the pre amp will be in it's buffering of the source and power amp. The gain means little. It just happens that most pre amps with gain are "better buffers" than most "passive preamps". Having the input impedance vary with volume setting is a recipe for disaster. Not having a stable out put impedance of a low enough value is bad as well. So if we are talking "passive" meaning no active buffering and no gain my opinion is a good "active" with gain will be "better but not because of the gain, but rather its buffering. My opinion is that the input impedance and output impedance should never vary with volume setting. But that is the start, really.....the ability to drive long (normal) cables is an area where "passive" pre amps fall short......unless they are "active" but "passive" in the regard of no gain. Hint, actively buffering your fancy attenuator may make it much better. Just my opinions, ok.

edit#2**** my point is that the argument of "passive vs. active" is not a good comparison. Typical "passive" pre amps are crappy buffers or they are not "buffers at all". Typical "active" pre amps are generally better buffers. It's that simple. Pure passive puts all the strain on the source. Active on the other hand does not, it handles the load. Ideally, the pre amp should have infinite input impedance...a very low output impedance and low distortion...and the ability to pass a fair amount of current. Hmmmmm......

It's not about "active vs. passive", if one means passive as in "no gain". For instance, Mr. Pass's is a superb buffer. It also plugs into the wall. It is an active passive preamp and again, a superb buffer.  It's all about buffering and who's got the best scheme for it and volume control.That's where the "magic" lives.

[face]

Unfortunately, there is no free lunch.  In addition to the volume pot and the buffer stage, B1 requires a 1 uF and 10 uF capacitor in the signal path, which is not so trivial to get half-way decently transparent..

This is pretty close:

I plan on upgrading the Alps to a stepped attenuator and also upgrading the binding posts to gold plated copper Vampire units.

[Letitroll98]

Compared to my average-ish SS preamp, I'll take the passive every time.  Although it has a bit less output than my active preamp, it has plenty of gain to play very loud, and the sound is much more refined with a bigger soundstage and more air and space around the instruments.

The technical arguments mentioned after this post are interesting and valid, however, "how does it sound?" is the most critical.  Passive preamps will generally be less dynamic than active stages, but as roymail notes much more pure and refined in character.  And as he notes if you have the gain for it in your source components and either the power to drive your speakers or efficient speakers, or both, passive preamps are preferred.

However, if you play a lot of rock, techno, electronic jazz, etc., you may prefer the greater dynamics of an active preamp.  Lifeless and dead are not what I'd characterize passive units as, but you can definitely notice a lessening of dramatic contrasts.  I solved it by having preamps with both active and passive stages (PS Audio 4.6 and B&K CS113) that I can flip at the push of a button.  Granted their passive stages are not as perfectly pure as some dedicated passive preamps, but it gets me most of the benefits of both worlds.

[JLM]

As I understand, the issue of dynamics should relate to source output voltage versus amp input sensitivity (measured in volts).  In my case the source has a rated output voltage of 7 volts and the monoblocks have a rated input sensitivity of 0.6 volts, so all should be good.  Speaker efficiency shouldn't be a factor in passive versus active pre-amps.  Please educate me if I'm wrong on this point.

A separate issue is providing sufficient power to provide a "commanding grip" on the given speakers.

[rollo]

Hi Charles,

I respectfully disagree as the capacitance of both input and output ICs is across the primary and secondary of transformer based units. As such resonances occur, but minimized with proper precautions. The higher the capacitance the lower the resonant frequency.

I have a general article discussing the subject

http://www.sasaudiolabs.com/theory11a.htm

Cheers.

 Very interesting Steve. Thanks.


charles

[Steve]

Very interesting Steve. Thanks.


charles

I reread your post and missed your good comment to keep the IC capacitance as low as possible. Nice job.

Cheers.

[konut]

The perception of less dynamics with a passive was not the case with my setup. I will not generalise that this is the case with all passives, but long term experience has taught me that sometimes a component will have characteristics that, while exciting, are not realistic. I once owned a pair of Magnaplanar Tympani 1-Ds. The frequency response was very linear and, when given enough power, the bass was superb. Being a planar loudspeaker they radiated sound from front and rear and the sound was "exciting". But they weren't realistic. The problem may have been that I just never got them in the right room with the right acoustics, but the sound stage was never realistic. It was exaggerated. What has this got to to with passive vs active preamps? I have found that most actives exaggerate dynamics. Most people like this kind of presentation.  With the sorry state of most recorded music, with limited dynamics, this is completely understandable.

[Steve]

The technical arguments mentioned after this post are interesting and valid, however, "how does it sound?" is the most critical.  Passive preamps will generally be less dynamic than active stages, but as roymail notes much more pure and refined in character.  And as he notes if you have the gain for it in your source components and either the power to drive your speakers or efficient speakers, or both, passive preamps are preferred.

I believe the sound quality depends alot upon the design. But the active stage normally found in the preamplifier is simply moved to the amp, with an IC (and its capacitance etc) between the volume control and active circuit.  That is why the lifelessness sound. Take that first "amp" stage out and place it back into the "passive" preamp and the life is back.

Another problem I find in the system is the analog stages in the typical CD player/DAC. They are usually very very cheap SS designs that only give a gain of approx 2,,,, or approx 6db of voltage gain. So straight out from the DAC chip itself (with impedance protection) gives approximately 1 volt output.

While there is typically room for an OK tube section, it is not the best tube design either.

cheers

[casarman]

Hello! could someone list examples of both kinds of preamp's? also active and passive is a matter of tubes preamp's or SS aswell?
Excuses for my ignorance!
Cheers!
Armando