what is the diff between active and passive preamp

[djnagle]

My preamp blew today and I need a new one. I know what the phisical diff is between a passive and active preamp is, but I don't know the sound difference. Can anyone help me out. Thanks.

[scott fitlin]

I believe a passive pre amp has no gain, its purely passive, no active stages, offering the lowest noise spec possible, but you should really try one BEFORE you buy, and see that is can drive your amp satifactorily.

[Altec19]

Hi Igor,

Here is a long winded diatribe that I posted on another forum when folks were discussing passive pre-amps. I've used one for a while now and I'm very pleased with how my combination of stuff works together!

By definition a "passive" pre-amp introduces no active stages into the audio path. It is strictly a high quality potentiometer, attenuator, or specialized auto-transformer placed in the audio path. No coloration from active stages. If there are buffer stages being used for impedance matching between source and pre-amp or pre-amp and power amp, it ain't passive.

Buffer stages do allow for more universal compatibility between components. Unfortunately, they may also color the audio. This is the tricky part about using a passive pre-amp. It does not provide a wide range of impedance tolerance to work with. If your particular collection of components don't work well together impedance-wise, the results may not sound very good. But when all the parts are flying in the same direction it is wonderful.

The key to having good sound quality with any collection of audio components is often times less attributable to the components themselves than it is to the synergy of the system as a whole. Or, sometimes the sum is greater than the parts. With an active pre-amp it is easier to have source components (CD players, phono stages) work well with power amps because the active pre-amp stage can more readily compensate for mismatched impedances and signal levels between devices.

A true passive pre-amp really can't be singled out for sounding good or bad. The correct question to ask is how well does the source device output stage mate with the power amp input stage. If they sound good without a passive pre-amp in place (don't try this until you figure out why this is probably not good to try) they will probably sound just as good after inserting a passive. Not good to begin with? It'll be no better with a passive in place. It generally takes a fairly stout output stage in the source device to drive a power amp directly and sound good doing it. How do you know what might work well together? Ask if anyone else has tried the combination you are interested in or find a dealer with an in-home trial policy.

I use the following components:

Cambridge Audio Azur 640C CD
Creek OBH-12 Passive Pre-amp
Dynaco ST-70 power amp completely rebuilt with Curcio Audio upgrades
Altec Model 19's

Brightness, punch and volume are not a problem. Much of this may be due to using an exteremly efficient horn loudspeaker but it still comes down to synergy.

Later . . .

[djnagle]

Thanks Altec Fan for a clear understanding of the two. After reading your post, I pulled the preamp out of the loop (I am using the Rane for a crossover and can adjust the volume for each amp there) and the system sounds sooooo much better. The Advent I was using really mirkied the sound. After that, I am going to get the Gold Point SA1 and call it good.

http://www.goldpt.com

[Altec19]

It's can be very revealing, pulling an active pre-amp out and going direct. Kind of like removing a layer of grunge from a window and being able to really see the colors! Again, it doesn't always work. The source device output stage has to have enough current capability to adequately drive the input of the next device. I had an older Carver pre-amp that was nice enough until I went with the Creek. Found out that Bob Carver used his favorite op-amp from input to output and everywhere in between and that it had, unfortunately, a rather low slew rate. Resulted in very unimpressive transient response. Oh well.

The Goldpoint looks like an interesting piece. Let me know how you like it.

Later . . .

[Altec19]

Well, I'm not sure. Slew rate is usually referenced with respect to individual electronic components and not entire amplifiers. Not that it doesn't apply, it just isn't commonly revealed by most manufacturers. Here's what it means: If you were to input a perfect square wave (90 degree angles at all transitons) to a device, how accurately can the device output (and amplify) the vertical rises of the square wave? Real devices are measured in terms of so many volts per millisecond. The "ideal" device would provide up to infinite voltage in zero seconds. Unfortunately, a real device will distort the "ideal" square wave because it can't quite keep up with the instantaneous rate of rise of a perfect 90 degree vertical. There will be a slight slope added to the output waveform as the device tries to keep up.

Now, music does not usually contain square waves. But, a sound (such as a cymbal crash) with a rapid rate of rise will not sound so good when amplified through a device with a poor slew rate. Since compression drivers are very good at reproducing transients, they tend to reveal poor transient response in the electronics. This is one of the advantages of a passive preamp. It tends to have just about perfect transient response because there are no active components.

[djnagle]

Thanks for another great explanation. I think then, that maybe it is not so important in this case. I am using the Crown 150A to drive the 515G woofers and from my way of reconning, the transiants in 500hz and below are not as important as what is coming out of the mids. Thanks AF. Cheers.

[Altec19]

Hi Igor,

No doubt about it, Crown D-150 and D-300 series amps are great low frequency amps. Their very stout traditional power supplies with gobs of reserve are the secret to good bass punch. Some folks don't care for them in the mid and high frequency regions but others think they sound just fine!

Later . . .

[djnagle]

Hi AF,

As I was leaving your house way back when (37 below zero), You said "don't forget to block off some of the vent". I always remembered that but am not sure what you meant. Does it have to do with lowering the bass freq? I will post pics of my system with your cabs as soon as I figure out how to work the new camera. Cheers.

[djnagle]

Hi AF, In you last note you said "and that it had, unfortunately, a rather low slew rate. Resulted in very unimpressive transient response. Oh well." The crown I have has a slew rate of 6v per milisecond. Is that high, low or mid? Thanks.