Passive - Active Pre-Amp arguments

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To Bare, and other interested parties,

I wrote this in response to a question Bare had, but perhaps others may also find this usefull.

Let us look at some technical issues when dealing with passive pre-amps and why active ones may just be better in some applications.

Occasionally there is a requirement to add some gain, reduce output impedance or convert an unbalanced signal into a balanced or visa versa. Thus some type of active stage is usually chosen to accomplish this. Even when such a stage is designed to very high standards it will still add some noise and distortion. For the most part however the added noise and distortion are extremely low and thus likely to insignificant at the line signal levels encountered at this point. There are times adding a stage will actually improve overall performance due to reduced loading on the prior stage and being able to overcome problems associated with driving long interconnect cables from fairly high impedance sources.

If a impedance translation is required to drive a fairly long set of cables for example then it may be beneficial to add a active line stage of some kind to do that job. This would reduce high frequency loss and the phase shift associated driving long cables that provide a high capacitive load significantly.

Thus to a large degree what is required for your system may come down to what your particular system requires to work properly and how your equipment is hooked together. Cable length, cable type and load impedance presented to the output end of the cables between the control console and power amplifier all enter into the equations as to what needed.

If you can avoid using an active line stage then it would be beneficial to do so. Measurements should be made on any system to see if there is a high frequency loss with the volume set at mid position. Measurements should also be made to see if the prior stage is bothered by the capacitive load that the cable running to the power amplifier may present to it. If this is the case the problem it will usually show up as an increase in distortion and or a lowering in its output level on the high end.

In other words you have to know all the facts before jumping to any conclusions as the sound of a passive control console versus an active one. Many passives roll of the high end by the time the signal reaches the power amplifier due to their high output impedances and thus also introduce phase shift if the cable capacitance is significant.

Let me provide a example of a passive preamp driving a power amplifier on the other side of the room in between the speakers.

Passive preamp has a output impedance of 10 K ohm at normal listening levels.
Power amplifier input impedance 50 K ohm.
Interconnect cable capacitance 1000pf

High frequency signal loss at 20 Khz will be down about: -3.1 db
Phase shift at 10 Khz will be around: –28 degrees.
Phase shift at 20 Khz will be around: –45.7 degrees.

Now I don’t know about you but this does not appear to be very attractive to me. How many of you use long cable lenghts along with a passive Pre-Amp?

There are two primary solutions:

1: Move the power amplifier right next to the passive preamp to reduce the lenght of the cable and thus cable capacitance.

2: Leave everything were it is and add active line stage designed to drive this length of cable.

Other solutions involve changing the passives component values to reduce loading effects to minimum, but will very likely not eliminate the problem all together.

Thus in applications were the power amplifiers are located away from the “passive preamp” the passives do not work well. So if you use a passive Pre-Amp put it and the power amp very close together to reduce high frequency loss and phase shift.

I find it an error to call a passive device a pre-amp since this would indicate that it contains active components that amplify the signal in some way. It is more correct to call such a device a control console, not a passive pre-amp. The only time I have seen a passive device act as a pre-amplifier is in RF parametric amplifiers. Perhaps there are others but I have not used them.

It is also important to know the capacitance of your interconnect cables and circuit impedances. Without this information you cannot do any calculations of signal loss and phase shift and not truly cannot properly evaluate differences between equipment.

When all is done correctly you will have a matched and properly configured system.

I hope this information is helpful and provokes though on the subject.

John Fassotte
Alaskan Audio
Link to Level and Phase Graphs

I have added a link to show the effect of amplitude and phase for the approximation I mentioned above for the 10 K ohm output impedance passive Pre-Amp with cable and power amplifier load.

This PDF file is a single page of about 38KB. The upper chart on the page shows amplitude while the lower chart shows phase versus frequency.

Being revised. It will be available again in the near future.

The -1.552 Db upper level at a referance frequency of 1 HZ is the voltage/power drop caused by the 10 K ohm output impedance of the passive Pre-Amp driving the simulated power amplifier input impedance of 51.1K. The input referance level was 0.

The charts for a active stage with 50 ohm output impedance driving this same load.

Being revised. It will be available again in the near future.

As you can see the there is a lot of potential difference between active stages driving the power amplifier and a passive device attempting to do the same thing.

So the next time you hear a passive line stage ask yourself how much of the highs are missing and how much additional phase shift is being induced due to loading and high frequency roll off. This is quite hard to judge because each system may get different results due to different cables or cable lenghts being used. A further variable is volume control position and power amplifier input impedance. If we wanted to reduce these variables to a minimum we would need some kind of active buffer on the output of the so called passive Pre-Amp and turn it into a active Pre-Amp.

To be totally honest there will be some phase shift and high frequency lost when a volume control is added to the input of a active buffer but it will be no where near as bad as with the passive setup. The buffer amplifier itself will also add a small amount of phase shift itself.

If anyone is interested I can plot some with a volume control of any value in its half point position front of the buffer amplifier.

Remember these charts only cover the differences in signals from the input of the Pre-Amps to the input of the power amplifier. They do not include any other segments of the audio path.

So if you insist on using passive Pre-Amps get the best lowest capacitance cable to run to your power amps you can find and keep the runs as short as possible. Hopefully less that 3 feet.

John Fassotte
Alaskan Audio
Plotting volume control freq response

I was snooping around the internet today and found and interesting spreadsheet on the Danish Audio Connect web site.

This excell spread sheet calculates the actual frequency reponse of passive volume controls and plots their frequency response with various resistive and capacitive loads.

In any case if any one is interested in it can be download along with its manual at:

The only bad thing is the fact that they extended the upper frequency range way way to much to see any real detail in the frequencies of interest. A 100khz upper limit would have been sufficient. The second thing is that DACT opted to password protect the xls sheet.

John Fassotte
Alaskan Audio
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