IS A PASSIVE PREAMPLIFER RIGHT FOR YOU? Simply put, where suitable, a PASSIVE PREAMPLIFIER WILL SOUND BETTER THAN ANY ACTIVE PREAMP ANY DAY, HANDS DOWN. More and more audiophiles and DIY audio tinkerers are arriving at this conclusion because it’s not subtle. You can hear the difference. Passive is better. Except when it’s not – more on that below. Still, the debate over passive vs. active preamps mushes on within the audio community, evidence of passive’s inherent superiority notwithstanding. So, what is a passive preamplifier and why is it better – except when it’s not? WHAT IS A PASSIVE PREAMPLIFIER? In its purest manifestation, a passive preamp would be a wire with variable resistance that connects your music source to your amplifier. Sadly, no such magic wire exists. Most passive designs use a potentiometer, discrete resistors or even transformers to attenuate the incoming signal for volume control. Tortuga Audio uses light dependent resistors (LDRs) which have superior sonic qualities. More on that can be found in this article. Unlike active preamps, passive preamps do not actually amplify. Thus, we refer to passive preamps as unity gain or no gain devices. Ironically, very few contemporary audio sources really need preamplification per se. Also unlike active preamps, a passive preamp does not buffer the signal between the signal source and the amp. A buffer acts as an electronic firewall between the source and the amp. A good buffer has very high input impedance making the job of the source very easy. Easy means it doesn’t take much voltage (or current capacity) to deliver the audio signal. A good buffer also has very low output impedance and high current delivery capacity making it compatible with virtually every amplifier in the world regardless of the amp’s input impedance. This broad compatibility is arguably the only benefit of active preamplifiers. Lastly, unlike active preamps, a passive preamp doesn’t need a power supply since there’s no active circuitry manipulating (buffering, amplifying) the audio signal. WHY PASSIVES? It’s really quite simple. Where they fit, passive preamps will sound better than any active preamp any day, hands down. Why? Because when it comes to audio, less is more (most of the time). And passive preamps excel at less. Less what? Less crap to get between your music and your ears. Less wire, less circuits, less manipulation, less distortion, less noise, less hum, less coloration, less cross talk, less phase shift, less compression. Less bad. More what? More clean, clear, open, articulate, fuzz-free sound. More good. THERE’S EVERY REASON TO USE A PASSIVE PREAMP AND ONLY 3 REASON NOT TO: 1) Impedance Mismatch: Using a passive preamp between a high impedance source and a low impedance amp may overwhelm the ability of the source to drive the audio signal without negative effects. Negative effects like flabby sound lacking in dynamics and punch. An impedance ratio of 100:1 or greater (amp input : source output) is almost certain to work well with passive preamps. For example, a DAC with a 500 ohm output impedance and a solid state amp with a 50k ohm input impedance are probably a very good match for a passive preamp. Passive preamps will also work well with practically any tube amp because tube amps have very high input impedances (typically 1 megaohm). 2) Low Source Voltage: Closely interrelated with the impedance issue, the source has to have enough line voltage to drive the audio signal through the added impedance of the passive preamp and into the amp. The good news is most quality audio sources today have strong enough line voltages (~2 volts) to work fine with most passive preamps. 3) High Cable Capacitance: Since a passive preamp can add considerable impedance to the audio signal, the total capacitance of the interconnect cables can conspire with the cable/preamp resistance to act as a low pass filter that rolls-off the higher frequency signals in the audible range. This can be best grasped visually. With a nominal 100k ohm passive preamp running at midrange volume levels the effective impedance can be quite low, say only 2k ohm (see graph on the left). With low impedance, cable length and capacitance are a non-issue. But as volume increases, the input impedance of the passive preamp also increases and the influence of cable length and capacitance can become quite relevant (see graph on the right). As we approach full volume the impedance can climb up to 50% of maximum. With high capacitance cable of 50 pF/ft, the cable should be no longer than 3 feet to avoid rolling cutting off audio signals below 20kHz (the upper limit of human hearing). With lower capacitance cable of say 20 pF/ft, we can use an 8 foot long cable before we start rolling off the high frequencies. Given these 3 caveats, it’s not surprising that you can find audio enthusiasts with disappointing stories about the time they tried a passive preamp. And they’ll tell you that passive preamps don’t work well with the conviction born of experience. They’d be wrong even though their experience tells them otherwise. So it goes. CONCLUSIONS? Passive preamps sound better than any active preamp any day, hands down, provided you use them in the right situation and are mindful of the cabling used. Passive preamps can work well with practically any tube amp provided your source has sufficient line voltage. Passive preamps can work with solid state amps provided the impedance differential between amp and source are approximately 100:1. This isn’t a hard and fast rule. I’ve had great results with 50:1. It depends on your particular equipment. My advice is to audition the passive preamp and see for yourself. Limit interconnect cables to 8 feet or less with low capacitance cables and even shorter with higher capacitance cables. If you experience disappointing results with a passive preamp, chances are it’s due to one or more of the issues cited above. All these impediments can be mitigated with the right equipment and cabling. In most cases the benefits of passive preamps makes it worth the effort to secure compatible equipment. You’ll be glad you did.
Posted on: Fri, 18 Oct 2013 13:08:57 +0000
Trending Topics
Recently Viewed Topics
© 2015