what is the difference between a phono pre amp and a normal amp?

[vibralux]

can some one please help me as i am a hi-fi novice i have noticed that pre amps are not full size but phono pre amps look just look normal amps. please can someone explain the difference and can you use a pre amp instead of an amp?

[hifitommy]

but some phono preamps are large, not many. an audio research will be because theres a lot of stuff in there or space or isolation from intrernal transformers etc.

mostly amplifiers are large to start with for some of the same reasons.

[E-Stat]

can some one please help me as i am a hi-fi novice i have noticed that pre amps are not full size but phono pre amps look just look normal amps. please can someone explain the difference and can you use a pre amp instead of an amp?
The job of amplifying the small signal from a phonograph cartridge, tuner, CD player, etc. is performed by a number of job-specific stages. Sometimes these stages are integrated into a single box as with a receiver, or found as separate components. The phono preamp amplifies the weakest of the signals (ranging from 0.2 millivolts for moving coil types to 5 mv with moving magnet types) to a level equivalent to that of so call "high level" sources such as tuners, tape decks, and CD players. Also, phono preamps equalize the signal to reverse the effect of the RIAA compensation curve (done to reduce surface noise). The preamp is the next stage taking the 500 mv - 2 volt high level signal to 2 - 4 volts sufficient to drive the power amplifier. Preamps also serve as the control center having the source switching capability, volume and balance controls, tape monitor loops, and in some cases, tone controls. The final stage is the power amp. While the signal gain multiple is similar to that of the preamp stage (actually the phono preamp performs the greatest signal gain), the amp deals with the largest amounts of current required to drive a loudspeaker. That is why power amps are usually the largest stages.

Indeed as hifitommy indicated there are some phono stages that look like power amps in terms of their size. Audio Research, Aesthetix, ASR Basis, Manley Labs, Passlabs to mention just a few all make pretty sizable phono preamps, some of which have power supplies like you would find in a power amplifier. Most, however, are much smaller than a typical preamp.

No, you cannot use a preamp in lieu of a power amp. The signal is too weak and not suited for driving a loudspeaker. With some high gain sources, however, you can bypass using a preamp and have the source drive the amplifier directly.

rw

[DaHaq]

Are preamps used for sources other than turntables? It sounds like the sole purpose is to compensate for the equalization on records. Do "high-level" sources need anything but a power amp?

[E-Stat]

Are preamps used for sources other than turntables?
Sure, that is the middle stage I referred to. They are commonly referred to as line stages. Older preamps combined both a line stage and a phono preamp in one box. Today, however, in the prevalent age of CDs, most preamps are line stages only.


It sounds like the sole purpose is to compensate for the equalization on records.
If you are referring to a phono preamp, then it also provides the single largest stage of gain. A typical phono preamp provides 40 db of gain. That is a voltage factor of 10,000 to 1. By comparison, line stages typically provide only about 20 db of gain (voltage factor 100 to 1). Similarly, power amps usually have about the same as a line stage - say 26 db (400 to 1).


Do "high-level" sources need anything but a power amp?
That's a somewhat complicated question. Some do, some don't depending upon a number of variables. The primary factors are: output impedance of source, input impedance of amplifier, output voltage of source, input sensitivity of amplifier, and capacitance of interconnects used between them. In addition to providing gain, line stages also "buffer" the impedance from the source to the amp. While there is some controversy as to the best answer, one rule of thumb is that you need to have a 100 to 1 ratio of source output impedance to amplifier input impedance. This is necessary to ensure maintaining signal dynamics and avoiding high frequency rolloff.

So long as the source output to amp input impedance ratio is of a high enough factor, the capacitance of your cables is low enough to prevent HF rolloff and the source can drive the voltage requirements of the amp, then the answer is yes. On the DACT website, you can download a spreadsheet that will calculate the HF rolloff given several variables.

I use passive attenuators between the CDP and amplfifier in two of my systems. In the main system, all the criteria is met. While I require the use of a preamp in that system for vinyl, I get higher resolution for the CD source by using attenuators instead. Ironically, the CD has a lower output impedance than the preamp. With the other system composed of hand-me-down components from the main system, however, I don't have quite enough voltage gain. Consequently, I cannot fully drive the power amplifier. Since it is the garage system and plays loudly enough, I haven't bothered to buy a preamp for it.

rw