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I used to think passive preamps were superior to active preamps given right the setup, but

my recent evaluation of a modded old SS preamp has me a little befuddled.  I've evaluated $10K+ active preamps in the past and was never impressed especially given their cost.  In general, I've found passives to do better job. I know there's ongoing debate on this.  But here's a very illuminating video on the subject by Bascom King, one of the legends of high end audio.

https://youtu.be/HHl8F9amyY4
dracule1
dracule1 OP
1,085 posts
 
George, sometimes it's just too much of a hassle to match your system to a passive preamp, although you can get some great results when all the stars align. Sometimes an active can be as or more rewarding without the hassle.  As it stands now, I like both in my system, but the active pre just does more for me overall.
georgehifi's avatar
georgehifi
10,312 posts
 

Not really, like I said over 90% of systems are a perfect match for a 10kohm passive, it's the few oddball high impedance tube output sources that are the trouble.

eg: like the well know "cottage industry" dac with a 12AX7 output tubes at 3kohms!!!, the 12AX7 was never meant to be an output tube buffer. High gain input tube for phono/preamp/poweramp stages yes but an output tube?? Really?? 


Cheers George 

mitch2
3,894 posts
 
It is not just about gain but I believe also about control of the interconnects, impedance matching and frequency variations with impedance.  The passives I have used include several resistor types (including Endler direct to amp - i.e., no interconnect) and Bent-type TVC, but not LDR.  All had excellent clarity but all missed out in the areas of bass drive and tonal depth compared to my reference, which is a no-gain, buffered preamp with a Shallco attenuator using great-sounding AN tantalum resistors, Lundhal coupling transformers and an outboard choke-filtered power supply.  My conclusion was that even with a 4V source, amps with 100K ohm input impedance, and short, balanced interconnects, the passives were not able to match the tonal saturation of the buffered preamp.

tortugaranger
9 posts
 
We'll soon have an optional solid state buffer output stage for our Tortuga preamps. We've been trying various designs for quite a while now and have settled on a JFET output stage with a servo DC offset correction and direct DC input (no coupling cap). I've been listening with and without a buffer for a while now and am challenged to discern whether having the buffer improves dynamics, bass or just gives it more "authority".  Above all trying to do no harm. It may well depend on the combination of other gear - sources/amps - that you have. Still, for those who simply will not consider a passive preamp under any circumstances, offering a buffered LDR preamp (i.e. an active version) seems to make sense. 

Best,
Morten
georgehifi's avatar
georgehifi
10,312 posts
 

Mitch you say "controlling the interconnects".

There are two parameters interconnects have that could influence the sound with a passive, resistance and capacitance. And they would have to be very high to have any effect.

Resistance is nothing to worry about as it would be so low it’s not even worth considering,, even in 5mts of interconnect.

Capacitance: A 10kohm passive has at worst output impedance of 2.7kohm, with a bad case cheap interconnect of 100pf per ft, if you had 2mts that equates to 600pf cable capacitance. This would have a HF roll of effectively at -3db at 98khz!!!

There is no having to controll the interconnects full stop with a 10kohm passive!!

That’s why you should always use a 10kohm passive, as a 20kohm passive with the above interconnects would be -3db at 49khz. A 50kohm passive would be -3db at 20khz as you can see that does cause a HF cut in the audio band.

Let me remind you what Nelson Pass said.

Nelson Pass,

We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more.

Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up.

Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.

What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.

And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp.


Cheers George

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