difference between an active and a passive preamp?

04-12-11: Almarg
I can't envision a reason why the length of the cable between source and preamp would be any more critical than with an active preamp.

05-18-11: Atmasphere
With passive controls, the cable length is quite critical as is the construction of the cable. The interaction is between the source impedance, the installed variable of the passive control, the characteristics of the cable that goes from the passive to the amplifier, and the input impedance of the amp. Its a bit complicated, but with passives what you will universally hear is that as you turn down the control, the bass and impact in the system will be diminished. It does not matter the quality of the control itself- that part is really not a variable, although the *value* of the control is.

Ralph, thanks for your comprehensive and characteristically informative response. Could you explain a little further, though, what parameters of the cable connecting the source component to a passive preamp would interact with the things you mentioned, beyond interaction with the output impedance of the source component that would also occur with an active preamp?

Best regards,
-- Al

Could you explain a little further, though, what parameters of the cable connecting the source component to a passive preamp would interact with the things you mentioned, beyond interaction with the output impedance of the source component that would also occur with an active preamp?

For a transformer-based volume control, it's possible that certain combinations of high source impedance and a high-capacitance input cable, that this could affect the transformer's self-resonance point, and maybe the in-band HF transient response. In the case of a potentiometer-type resistive circuit, the only thing I can think of is the fact that the source component will also see the output cable capacitance at the highest volume settings, like above -6dB.

But this is really grasping at straws . . .

Hi Al, The cable between the source component and the passive control plays a reduced role on account of the output impedance of the source. When the series resistance of the passive control is inserted into the circuit, the character flaws of the cable are essentially magnified. So most of the issue is downstream from the passive control.

The rather obvious solution is to install the volume control at the input of the amp, but this could be really inconvenient if you have monoblocks. A remote could solve that problem though...

I am bumping an old thread here!

I am leaning towards going with a passive preamp. Although, I am not into numbers too much because as long as you like the sound, who cares but in this case, I think it is important to check some numbers.

Is passive preamp suitable for my setup?

DAC: 

• Output Level: 3.0V RMS Max. (Analog).
• 
  
• Outputs: Stereo (RCA) 300 Ohms.

AMP:

Input Sensitivity (RMS) - @3Ω, 500W,1KHz 1.36V

Input Impedance - 33K ohms

@celo , maybe.

The real number to look at is the output impedance of the DAC at 20hz. If that is the same number then I would say your chanced are good with a passive control, if you keep the cables short.