difference between an active and a passive preamp?

For example, if amp X has an input sensitivity of 1.25 V, 1.25 V from the source (eg,CD player) is need to achieve full rated power from your amp X. So if amp X is rated at 100 watts, it needs 1.25 V to produce 100 watts. If amp Y has input sensitivity of 2.8 V, then your standard CD player output of 2 V is not enough to get full power out of your amp. In this case you need active preamp, to provide that extra output to get full power out of your amp. Most passive transformer or autoformer preamp can provide an extra 6 dB of gain (stepped resistor, pot, or LDR based passives can't), so you can mitigate some of this. I'm not too certain on about how the length of the interconnects affects this. But I think longer interconnects will decrease the amount of effective volts the amp sees from the source. So I try to keep my interconnects as short as possible which also helps to prevent significantly losing high frequency response. If anyone more technical can find any error in my explaination, please correct me. I'm not that technical.

@audioflyer67
+1 for the Luminous preamp. It's about $150 with the better caps and they'll use your systems measurements to get the right resistor. You can't beat it for the money.

@dracule1
What am I missing? My amp has a sensitivity of 775mV (VTL compact 100), and I've been running 20 foot interconnects between the amps and various passive pre's with great results. Not sure of the numbers for my dac (Levinson360s). For cables I've used Apogee, Discovery, and Crimson Musiclink. My current, favorite, and last passive is a Bent NOH based on the music first design and transformers. I've also used the Luminous and the Prometheus passives.

Go figure, it sounds great!

Larry to tower...get ready for the flames...here comes a statement which will be trounced on...

Personally, I've NEVER liked passives...ever.
They sound 'washed out' to me. It's almost as if the input signal to the pre is somehow absorbed more than passed and that the result is an anemic output--dynamically.
I suppose that someone on the other side of the ledger would simply claim the opposite--more pure no 'changes made by poor circuitry in the gain stage.'
I've heard several along the years and just don't get what I like.
For example--growing up in audio, my favorite preamps were,

Conrad Johnson's, (I know they use lower case) Premier 3
Gryphon's LS1 (Model may be wrong) $9000. in the late '80's.
They were both different, but both incredible.
The cj was a beautiful sounding, (though colored) preamp, with space within space along with beautiful textures.
The Grypon was a work of art, both asthetically and musically (electrically) employing two outboard power supplies. It DROVE, DROVE, DROVE, the amplifier like no other up until that point...by a wide margin.
One day, slow day in retail, a guy came in from New Jersey...he'd been to every store in North America, by his own admission...anyway, he came in to hear what we were doing. For giggles, I said, I'll bet you've never heard the Adcom GFA 535 (60wpc). It was a $299. amp back then as I remember.
I hooked it up to the THIEL CS5..(which dropped to 1.5 OHMS in the bass, very tough load to drive).
I played it with the Adcom, probably GTP500 Preamp/Tuner combo piece, that it was normally paired with.
He yawned...of course so did I.
Then, hooked the Grypon to it and he was open mouth stunned. WOW!
He said, I've never heard a preamp that controls like that.
'Me neither'.
Gryphon sent 13 into the United States that year...I sold 11 of them in Louisville, KY, the Mecca of High End Audio.
lol.
Buy a regular preamp not a passive...all things being equal, you'll be happier.

Good listening,
Larry

I never met a passive preamp I could live with...dynamics are severely squashed

04-12-11: Dracule1
I'm not too certain on about how the length of the interconnects affects this. But I think longer interconnects will decrease the amount of effective volts the amp sees from the source. So I try to keep my interconnects as short as possible which also helps to prevent significantly losing high frequency response. If anyone more technical can find any error in my explaination, please correct me. I'm not that technical.

04-12-11: Ts0711
@dracule1
What am I missing? My amp has a sensitivity of 775mV (VTL compact 100), and I've been running 20 foot interconnects between the amps and various passive pre's with great results.

With resistive and LDR-based passive preamps, the need for short interconnects derives from their high output impedance, and applies to the cable between preamp and power amp. I can't envision a reason why the length of the cable between source and preamp would be any more critical than with an active preamp.

The reason cable length on the output side matters is that its capacitance forms a low-pass filter in conjunction with the output impedance of the preamp. Cable capacitance, besides being dependent on the cable type, is proportional to cable length. If it is too high, the bandwidth of the low-pass filter will be low enough to fall within the audible spectrum, resulting in some amount of roll-off of the upper treble.

The degree to which that is significant will also depend on where the volume control is set, since the volume control setting will have a major effect on the preamp's output impedance. For a passive preamp consisting of a potentiometer or a resistive stepped attenuator, the worst case output impedance will occur at a setting corresponding to 6db of attenuation, which will typically be very loud if the source is a cdp or dac.

As was noted, it is also important for the output impedance of the source to be low (in relation to preamp input impedance), and for the input impedance of the amp to be high (in relation to preamp output impedance). Otherwise frequency response irregularities will result, to the extent that the impedances that are involved are not constant as a function of frequency.

TVC's are a completely different story, that I'm not particularly familiar with. But since a transformer provides an impedance transformation corresponding to the square of the turns ratio (with attenuation or gain being directly proportional to the turns ratio), I would expect that as long as the volume control position that is used corresponds to a significant amount of attenuation (i.e., it is not near the top of its range), cable length and component impedance matching would be much less critical than for a resistive passive preamp.

Regards,
-- Al