Preamp gain ???

this is the second preamplifier gain thread that has popped up in recent days...

it seems to me that preamplifier gain is generally irrelevant: what i would think should matter to you is what is the input sensitivity of the preamplifier, what is the maximum output level from the the preamplifier and what the clipping voltage is for your power amplifier. if your sources provide signal levels (typically around 500mv) to the preamplifier that hit the input sensitivity level then you can hit the maximum output level at the output of the preamplifier. in all likelihood, that maximum output level from the preamplifier is more than enough to drive your power amplifier into clipping (which i think might typically occur at around 4v or 5v).

the other topic that i have frequently seen commented concerns impedance matching between the preamplifier and power amplifier. as a practical matter, i don't know how much that really is an issue. the preamplifier specifications should tell you what input impedance it needs to see to drive maximum voltage and that should be way below the input impedance of most power amplifiers. for example, i have a bryston bp-20 preamplifier - according to the specifications, it can drive a maximum voltage (15v) into impedance loads as low as 600 ohms. my power amplifier (a bryston 4b-st) has an input impedance of around 50kohm, so you can see that impedance matching isn't really an issue in my case.

in any event, you can check the specifications on this stuff to be sure in your case but hopefully you have a better idea of what kinds of data to check.

there is the whole area of passive preamplifiers but i don't know anything about that kind of equipment so there may be a different set of considerations for which to account.

i would say this: i think it unnecessarily complicates the problem is you are trying to solve amplifer-speaker interface problems at the preamplifer. it is more manageable to decompose the problem; with regard to the preamplifier the focus should be on the input signal levels from the sources. you can then focus on the output signal from the preamplifier but most reasonable preamplifiers should be able to drive a power amplifer into clipping at maximum output voltage (such that in reality you would likely never get anywehere near the maximum output voltage). then you deal with issues of speaker sensitivity sepearately but that is more concerned with whether the power amplifier is up to snuff to drive the speaker load.

so the issue at the preamplifier is the input sensitivity of the preamplifier and the input signal levels from the soruces. i would expect that the input sensitivity of a preamplifier is going to be around 500mv, which is the line voltage signal level that you tend to get from many sources. cd players tend to be a bit higher. where you really need to watch out is when using phono sources. here you have to make sure that the phono stage provides sufficient gain for the given cartridge. ideally you want the output signal level from the phono stage to come in at around the input sensitivity of the preamplifier.

so once you have managed the input signal levels from the sources then you want to make sure that the preamplifier can drive the output voltage to the power amplifier. as i stated, the voltage amplitude should be sufficient but you probably want to check the preamplifier manual to find out what is the minimum input impedance that the preamplifier expects to see at the power amplifier, then check the input impedance of the power amplifier to make sure that the specified input impedance is considerably higher than the minimum needed by the preamplifier.

that's the analysis that you should do at the preamplifier. what you want to determine is whether the sources are providing adequate signal levels to the preamplifier and that the preamplifier is providing an adequate signal level to the amplifier. a side benefit of this analysis is that the signal being provided to the power amplifier will likely have a better signal to noise ratio, which, if the amplifier is up to snuff, will result in a better sounding system. issues related to the interface between the power amplifier and the speakers should be addressed in an analysis of the capabilities of the power amplifier.

02-21-11: Swampwalker
Paperw*- Nothing that you have said is incorrect but you have neglected the attenuation/gain (if active) provided by the pre-amp, which then is added to the gain of the amp. That can result in excessive overall system gain which results in the user having to operate his pre-amp w excessive attenuation which can have negative effects on the sound. The speaker/amp interface is not usually a decision based on max gain provided by the amp vs. speaker sensitivity, and room size, source output and volume preference all vary. Therefore, god in her infinite wisdom invented the pre-amplifier. Some have adjustable gain for each input.

what i am stating is that if you manage the signal levels at the input of the preamplifier, the gain of the preamplifier is largely irrelevant [i appreciate that there is a gain relationship between the input sensitivity level and the maximum output level from the preamplifier]. that is why i am stating that you should look at the signal levels from the sources and the input sensitivity of the preamplifier and manage the problem at the input of the preamplifier. in the case of a phono source, you want to adjust the gain of the phono stage so that the phono preamplifier provides an input voltage level that hits the input sensitivity level for the preamplifier. on the other hand, if you have a source that produces a "hot" signal (like the CD that you mentioned) that is way above the input sensitivity, then the adjustment is to reduce the signal level at the input of the preamplifier. that's what an attenuator would do. some preamplifiers have trimming resistors that allow you to accomplish the same thing on a per-input basis. for my own part, i have never made use of an attenuator but YMMV.

the problem with the focus on gain as a selection criteria for a preamplifier (and, btw, i would expect that the input sensitivity of the preamplifier would vary according to the gain) is that if you select a gain level based on a "hot" signal, then the other signal levels are likely to be below the input sensitivity level for the preamplifier. in that case you are more likely to experience degraded signal to noise ratio for those weaker signals.

as i stated, you also want to make sure that you know what the minimum input impedance the preamplifier needs to see in the amplifier, which is something that you should be able to get if the manufacturer supplies good product specifications. but, in practice, i think impedance matching is rarely an issue. yeah, i suppose you should check the input sensitivity of the amplifier for completeness, but power amplifier input sensitivities tend to run between 1v and 2v, so i would rarely expect that to be an issue either. but, of course, it is always a good idea to check this stuff just in case...

the point is, if you ensure that you provide appropriate input signal levels to the preamplifier from the sources, you are not likely to have a problem in providing an appropriate input signal level to the power amplifier from the preamplifier. then the task becomes one of whether the amplifier is up to the task of driving the speakers. but if you have to take on the amplifier-speaker interface issues from the preamplifier, then you've probably got some problems in your system in general.