• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Standard Input Design Value

Think in dB.

For microphones: -55dB (Although there is some variation in mics output level.
For consumer line: -10dB
For professional line: +4db

Personally I like 0 dB for line. And I would calibrate my VU meters to read 0dB to equal 0Vu. (Makes sense.)
The old Bell telephone standard was +8dB. (They wrote the book on audio level measurements.)
 

kevinkr

Administrator
Paid Member
So it's normal to have an amp that with some souces you won't get full output. And with other sources you will over drive the grids and clip?

Not sure where you got the above impressions, there is always a gain controlling element ahead of the gain stages in a power amplifier whether that is an internal stepped attenuator, volume control pot or an external active or passive line stage.. (Or it could be the volume control in your iPod, Hifiman or whatever you choose.)

Generally you should design your power amplifier driver stage to provide sufficient gain to allow the lowest output source you use to achieve full output at something less than the highest setting on your volume control - this is to allow sufficient available gain such that you can achieve reasonable output levels with that source for any program material available.

The volume control ahead of this stage will provide sufficient attenuation to deal with signals that would otherwise overdrive the stage.

Note that providing more gain than is sufficient based on the above criteria usually results in some compromise of the signal to noise ratio so should be avoided if possible.

I use sources in my system that have maximum signal output levels of ~700mVrms to >3Vrms without any difficulty.
 
Last edited:

artosalo

Member
2010-02-16 9:00 am
So it's normal to have an amp that with some souces you won't get full output. And with other sources you will over drive the grids and clip?

This problem can be avoided by using a real pre-amplifier.
To me pre-amplifier means such device that has several inputs with different kind of features.

There should be phono amplifier with RIAA correction. The sensitivity of such stage is typically few millivolts.

There should also be such inputs that drive the output stage full with, say 300 mV input signal.

There must also be inputs having sensitivity of about 1 to 2 V to be used with CD-players and similar-level signal sources.

Actually this input level ( 1 to 2 volts ) do not require any sort of pre-amplifier, only volume potentiometer between signal source and output amplifier.

However, it seems to me that very many pre-amplifier projects introduced here are such that both input- and output levels are from 1 to 2 volts.
I do not know what is the point to build such "amplifiers".
 
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However, it seems to me that very many pre-amplifier projects introduced here are such that both input- and output levels are from 1 to 2 volts.
I do not know what is the point to build such "amplifiers".

My motivation for such things is that I like driving my cables with a constant source impedance to minimize cable interactions, and reduce susceptibility to external fields.
 
While not disagreeing (building a tube buffer into a chip amp is popular and pointless), I should point out that many of us like to increase or decrease volume from time to time and even switch sources. Passive controls don't preserve the low source impedance of the sources, so some use buffered unity gain preamps. I do, for instance.
 
While not disagreeing (building a tube buffer into a chip amp is popular and pointless), I should point out that many of us like to increase or decrease volume from time to time and even switch sources. Passive controls don't preserve the low source impedance of the sources, so some use buffered unity gain preamps. I do, for instance.

My reasons exactly, could not have put it better (I quote SY a lot for this reason :D ) and my line stage has an output impedance of approximately 300 ohms. It provides up to 4dB of gain as well which I need with some of my all analog sources which are a bit anemic voltage output-wise compared to my digital sources. My analog sources get a lot more use these days so the system is designed to accommodate their needs first.