Preamp function.

Thank you all for explaining what a preamplifier is. i now have a better understanding. i appreciate the detailed responses. i had been using the droplet direct for a long while. the a3cr, while no match for the pass, did something to the sound which i find pleasing. a review of the droplet suggested that there would be very minimum improvements by using a very high quality preamp. i therefore did not look into it further. however i found significant improvements in adding a preamp that now sell for very little ( 300 uk pounds) on the used market. it just that reviews should not be taken too seriously. their writing skills are very enviable though, and have used several pieces in english classes that i teach! i am now considering a quality pre. i would imagine a pass preamplifier would be a good match. anyone got any suggestions? a forum user suggested an arc reference 5. however that is beyond my set budget. thank you kindly.

I like to think of a preamp as 3 parts - input stage, gain stage, and output stage. The input stage has 2 functions - one to select the appropriate input and two to provide a high impedance so the source can drive it easily and a low impedance to the gain stage. The gain stage amplifies the signal. The output stage presents a high impedance to the gain stage and delivers the signal for output at a low impedance. The preamp also typically has some mechanism for blocking any DC voltage from the power supply to the outputs. This can be done with blocking capacitors or other devices. Some preamps do not have such blocking devices and are known as "direct coupled".

Some general comments on impedance and on the gain of the amp.

The rule of thumb of 20x input/output ratio that Marakanetz gives helps insure that the input can drive the output easily. The input side of a preamp or power amp has a high impedance and the output side of a source or a preamp has a low impedance. The high input impedance of a preamp means the source does not have to work as hard to drive the preamp - that is, it has to provide less current. Some sources struggle to provide enough current if the impedance they are driving is too low. So, if you want to have a CD player or DAC drive an amp directly, you need to be sure it can provide enough current into the amps input impedance.

Another issue to consider is the gain of the amplifier. The typical gain of a SS power amp is in the mid to high 20's dB range - say 28 dB. Some amps, like the new Emotivas, have a gain of over 30 dB, which might sound nice, but more is not always better. If you have high efficiency speakers with an amp that has a lot of gain, then the typical 2 volt signal from a CD or DAC may produce way too much volume. As others have already noted, if you end up having to use the very lowest part of the volume control you may not have enough control over the volume and, depending on the volume control, the sound might not be optimal. You even see this type of problem with some preamps, especially when the preamp has a large amount of gain.

So you should consider both the input/output impedance ratios and also the gain of the sources and amp when putting together the system.

Dtc, than you for the detailed response. it is very useful to me in understanding what a preamp is. even the musical fidelity a3cr, now old but certainly with merits- i purchsed this unit 10 yrs ago, did something very nice to the sound. i really wish i had done this test earlier. ive seen for sale a preowned pass x2.5. anyone got any thoughts on this? thank you.