Can a preamp offer too much gain?

[trvale]

The tube preamp I am building is going to have about 22dB of gain. Is that too much to send to an amp that only requires a minimum of 0.4V of output signal? Everything that I would be hooking up to this preamp has a 2.0V output. Can somebody explain the noise floor concept and headroom? Are they the same thing? I just don't quite understand the benefit of voltage gain. If anybody also knows of any websites or books that explain preamps that would be appreciated.

[lynnm]

I am no expert but I don't think that 22 db of gain in a preamp is excessive.

IF the preamp were to present the power amp with an excessively high signal the output tubes would go into "saturation" and the sound would be grossly distorted.

That said I think that 22 db. is not a particularly high gain factor and that you will be unlikely to have a problem.

There are several members here that will be able to provide a more complete answer I am sure.

[WMcD]

The 22 dB gain sounds like more than you need. Except perhaps if it refers to the phonograph input. Both moving coil and moving magnet phonograph cartridges have a very low output. They really do need amplification before the main amp; pre amplification. In any event a tube may well be capable of that when the sitution requires. I've not examined this.

On the other hand, things like tuners, tape machines, and CD players have respectable output. Some times this is called "line level". The level may be up to 1 volt, though I think that is high. 0.5 volts or less may me more typical as a max on the loud passages of music.

Putting aside the phonograph issue, the major function of "pre amp" hardware units is to be a switch to select different inputs (tuner, CD, tape, TV) and also as a volume control. There are often other functions such as tone controls, loudness, and the ability to monitor tape, send the inputs to the tape input, etc.

Overall, though, there is not need for large amounts of "gain" or amplification in the pre-amp. You may have seen mention of "passive" pre amps. That means it does not amplify at all. There are just switches and volume control. This addresses the theory that amplifiers in pre amps are a relatively useless piece of gear which adds some sort of distortion, and therefore can be done without. The purist approach. This relies on the fact that the output of the CD player, for example, is enough to drive the main amplifier. In some applications it is.

Voltage gain is a term for increasing the voltage of the signal. At most stages of signal processing or control, there is no need to increase current or power. It is all low power and low current. The voltage moves in time to the music. It is only when we need power to drive a speaker does current and therefore, power come into the picture. E.g. you see 100 watt amplifiers (I'd call them "final" amplifiers) but no 100 watt pre amps. Again, power is not an issue until you try to drive a speaker.

Any amplifier has many limitations. One is frequency response where it may work from 20 Hz to 20,000 Hz. Technically, the frequency domain.

Another has to do with the "level" of the output. Driven too hard, it can't put out the requested voltage on peaks and the peaks are flattened at the max. This is called clipping because the tops of the peaks are clipped off. This causes distortion. New frequencies are created, even though this is an issue of proper level.

Any amp also makes some noise, or hissing, with no input, or with input for that matter. Hopefully it is at a very low level. This is though, the noise floor.

Let me point out that any transmission or storage channel has the same problems of having a maximum level where there is distortion from clipping, and a minimum level where there is noise. If you predetermine in your analysis what an acceptable amount of distortion and noise will be, then you have "dynamic range." The noise may be, say 50 or 90 or whatever dB, below clipping.

The result is that you want to adjust the input to the amp (or anything else, like a recording device, or radio transmitter) to let it work in the favorable range. If the input is too high, there is clipping on peaks. If the input is too low, the output signal you want to amplify is contaminated by the noise. The latter is like listening to a weak, distant a.m. radio signal buried in the noise.

Headroom goes to the clipping issue. I'm not totally up on any industry spec to describe this with uniformity. Looking at the etimology, it goes to the concept that while there is a noise floor, there is also the ceiling set by clipping level. We're trying to adjust the input signal to fit between the floor and ceiling. Headroom is the distance between the predicted peak (your head) and the ceiling.

There is the problem that music and voices has peaks. It is sometime difficult to know ahead of time just how high a peak will be. So you want some insurance, or headroom.

For example, you may say, I'm pretty certain that the max level is going to X dB. But I'm not sure. Let me set things so that clipping does not occur until X plus 10 dB. Then you have 10 dB of headroom.

I've seen this more commonly discussed in the power amp. You may say to yourself, I have a 200 watt amp. An oversimplification is that this is where clipping is certain, or "hard", but enough for discussion. You believe the peaks will drive the amp to 100 watts of output to the speaker. The math shows that this is only 3 dB of headroom.

As folks will point out, Klipsch speakers are so efficent, or sensitive, that 1 watt is putting out a lot of sound. 10 watts of power puts the sound output 10 dB higher. 100 watts puts it 20 dB higher, and 200 watts puts it 23 dB higher. Lots of headroom.

Ya. But what about the noise floor? The problem is sometimes that the sensitivity of the speaker allows us to hear the noise from the amp. Also, sometimes, the digital signal processors in HT receivers have high noise floors.

Gil

[Deang]

I've been trying to access what the 'gain' controls do on my Cary SLP-90. There are two knobs, one for the left channel and one for the right. Are these really changing the the gain of the preamp, or do they just change the voltage going to the volume control? Or is that what 'gain' really is?

If this sounds like a retarded question, I apologize -- I'm tired as heck.

[lynnm]

Dean

The gain controls on that preamp will be controlling the voltage being applied to the grid of the driver tubes and thereby raising or lowering the voltage supplied to the power amp.

Gain is a measure of the increase (or negatively decrease ) in the amplitude of a signal as it passes through a device such as a preamp. A lame illustration follows:

Signal in ----> Signal out = negative gain

Signal in-----> Signal out = unity gain

Signal in----> Signal Out = gain

The amount of gain is expressed in db.

The gain controls regulate the gain of the preamp and thus the power amp.

[Deang]

"The gain controls on that preamp will be controlling the voltage being applied to the grid of the driver tubes"

Doesn't this mean that when I turn the knobs down, I'm changing what the driver tubes were originally spec'd to run at? I mean, if a preamp is designed to deliver 20db of gain, I would imagine the amount of gain isn't really controlled the same way my gain controls do (changing the voltage being applied to the driver tubes).

Did I miss the import of your explanation?

[lynnm]

Think of the gain control as being like the accelerator in your car. The more gas you feed the more power the engine develops, (up to the limit of its capabilities - a 200 Hp. engine develops 200 H.p. only at maximum).

The drivers in your preamp are capable of providing up to x db. of gain but will only do so when the signal fed to them is sufficiently strong to drive them to that point.