Output impedance vs input impedance

kappclark

kappclark

AK Subscriber
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connecting Preamp to power amp question - Have read about importance of keeping output impedance low on preamp, and input impedance high on power amp ..

is there a simple explanation for why this is ?? I recall with antennas the impedance needs to match as closely as possible .. is that because there needs to be high % of power transferred (antenna) but not with preamp - power amp ? What wld be the typical result if the impedances were the same with the pre and power amps ??
 
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kappclark said:
I recall with antennas the impedance needs to match as closely as possible .. is that because there needs to be high % of power transferred (antenna) but not with preamp - power amp ? What wld be the typical result if the impedances were the same with the pre and power amps ??
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I guess I'll take a stab at this with a real-world answer, just in case some not-so-real-world replies come along later :).

I'd say you are exactly right about the antenna case being concerned with maximizing power transfer. In the pre/power amp case, the power being transferred is generally negligible, and the idea is to transfer as much as possible of the preamp's output voltage.

If the pre/power amp impedances were the same, then only half of the preamp's output voltage would get to the power amp. In decibels, that would be a 6dB loss. At first glance, this is probably not a problem - there is probably enough voltage gain available in the system as a whole that the system would still be able to produce the power amp's maximum output.

I think the main second-order concern is about whether the impedances are really the same at all audio frequencies - if they are not, then this sort-of-matching case could have noticeable frequency response aberrations. Whereas, in the typical case where the preamp's impedance is much lower at all audio frequencies, any aberrations would be negligible.

HTH, and HH (happy holidays!)
 
kappclark said:
.. is that because there needs to be high % of power transferred (antenna)
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Exactly. You match impedances for maximum power transfer (MPT). You want the most power, (ie., signal), you can get from your antenna. You want MPT for amp to speaker, and from transmitter to antenna. Most other applications, your sending signal, or data. You want a "light" load. Therefore, you want low output impedance into a high input impedance.
 
kappclark said:
is there a simple explanation for why this is ??
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Think information transfer. I.e., pretty much only the voltage is relevant for the purpose, and a lower input impedance of the sink would only require a more powerful output stage at the source without a really good reason for that.

Well, and another aspect is bandwidth. I.e., together with the cable capacitance as well as the input capacitance of the sink the output impedance of the source forms a low-pass filter, the corner frequency of which drops with rising output impdance and rising capacitance. So that's another reason, why low output impedance is desirable.

kappclark said:
What wld be the typical result if the impedances were the same with the pre and power amps ??
Click to expand...

As chazix already wrote above, only half of the output would arrive at the sink, as the other half would already drop at the output impedance of the source.

Greetings from Munich!

Manfred / lini
 
The output impedance and the input impedance form a potential divider - ie like a fixed volume control. If the output impedance and the input impedance were the same you'd get only half the voltage from the source getting to the power amp. A low output impedance and high input impedance gets you closer to maximum voltage transfer. The ultimate would be a zero output impedance and an infinite input impedance. In practice a ratio of 1:10 gets you close enough with just under 1dB of voltage lost. That's easy with most modern gear. Output impedances are often 200 ohms or lower, and input impedances are often 10k or more, so the 1:10 rule is easily satisfied.
 
Old tube gear is where you often really have to pay attention to this stuff. Some tube preamps have no output buffer stage, so they end up with output impedance in the several thousand ohms range. That works when feeding to a tube amp with a 500k input impedance but it doesn't work into a solid state amp thats probably under 50k. Tube preamps that were made in the solid state era usually don't have this issue, but stuff from the 60s very well might.
 
gadget73 said:
Old tube gear is where you often really have to pay attention to this stuff. Some tube preamps have no output buffer stage, so they end up with output impedance in the several thousand ohms range. That works when feeding to a tube amp with a 500k input impedance but it doesn't work into a solid state amp thats probably under 50k. Tube preamps that were made in the solid state era usually don't have this issue, but stuff from the 60s very well might.
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Yep, what he said ^
 
The output impedance of a preamplifier will form a low pass filter in parallel with the capacitance of the interconnect. This can roll off high frequencies within the audio range, when the output impedance of the preamplifier is excessively high.

Also, if the input impedance of an amplifier is too low, it can load the preamplifier - which would not really be a problem if the preamplifier's output impedance is a constant value across the frequency spectrum, but this is seldom the case (especially in AC coupled designs), so it can also create rolled off high and low frequencies. It can also increase harmonic distortion.

The output impedance of a preamplifier cannot really be too low, ideal is 0 ohms - similarly, the input impedance cannot really be too high. Practical acceptable values for output impedance are anything under about 600 ohms, and for input impedance anything above about 47k.
 
There is also usually a high pass filter formed by the output coupling caps and the load of the amplifier, so the bottom end goes away too if things are sized very wrong.
 
Thank you all for your detailed comments ... I am always appreciative of the learned responses from this fabulous community ..

I was considering this 'issue' as I looked at the specs of a certain tube amp, and wondered if it were compatible with the preamp I am currently using (Emotiva PT-100) .. I figured it unlikely to be a plm, and, as I just discovered, the Emo has output impedance of 1K, while the Jolida (which I just ordered today ) has input impedance of 100K -- so looks like no trouble in paradise.


As predicted, the Fisher 400 has become a gateway drug to tube audio ...
 

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