Consequences of the mismatched impedances

[Dilettante]

Looking at various fet-based inline preamps, they all tend to have rather high impedance. For example, popular Klark CT-1

Input impedance​	7 kΩ​
Maximum input level​	-15 dBu​
Output Impedance​	2 k​
Maximum gain​	+25 dB, 10 kΩ load​
Maximum output level​	+10 dBu, 10 kΩ load​

Let's calculate the load loss: Loss = 20 * log (R load / R load + R source)

In other words, if such preamp feeds into 2KΩ unit, there will be -6dB reduction.
This gives us amplification of ~19dB in the best-case scenario instead of the marketed 25.

What other consequences of such pairing where source is the same or higher in impedance than the receiving part?

 

[sergeauckland]

I would question whether the 'Output Impedance' number is actually 'Output Impedance' or means minimum load impedance. 2K seems rather high for a modern SS product, but reasonable for a minimum load impedance. If you look at the specs for many power amps, they quote 'Output Impedance 4-8 ohms' when they mean nothing of the sort. They mean minimum load impedance.

If that's true, then you're right, plugging a 2k output impedance device into a 2k input impedance device will result in a 6dB drop in level.

As for consequences, other than the 6dB reduction in gain, there could be consequences for headroom and distortion, as the device may not be able to drive a 2k input impedance. Maybe the manufacturers can confirm what they meant.

S.

 

[Dilettante]

consequences for headroom and distortion, as the device may not be able to drive a 2k input impedance

So, in the situation where

Microphone (say 600 Ohm) --> Fet in-line --> Channel strip

Channel strip will exhibit additional distortions due to the higher source impedance despite the fact that the signal is now "louder"?

 

[sergeauckland]

So, in the situation where

Microphone (say 600 Ohm) --> Fet in-line --> Channel strip

Channel strip will exhibit additional distortions due to the higher source impedance despite the fact that the signal is now "louder"?

The channel strip won't have any additional distortion, the Fet-in-line may have if that can't drive the input of the channel strip. That's why above I said I doubt that the CT-1 has a 2k output impedance, I would expect it to have a much lower output impedance, say around 50 ohms, and be capable of driving a 2k channel strip.

However, having had a look at the CT-1 documentation, I'm now confused! The output levels are specified into a 10k input impedance, which is a lot higher than the 1.2-2k inputs of most channel strips. Consequently, don't know is the honest answer. Maybe Klark can advise, or have a look at what the input impedance of your channel strip is. If it's 10k, then you should be fine using the CT-1. If it's 2k or under, then who knows?

Sorry couldn't be more helpful.

S.

 

[Speedskater]

In the the audiophile world, popular doesn't connote good design. Any well designed modern pre-amp must have an output impedance of 200 Ohms or less.

 

[Dilettante]

In the the audiophile world, popular doesn't connote good design. Any well designed modern pre-amp must have an output impedance of 200 Ohms or less.

I wish the 2 KOhm was a typo, but we shall see. Created the support ticket with MusicTribe / Klark.

 

[DVDdoug]

Maybe the lesson is... Use a preamp with enough gain so you don't need an add-on band-aid. Or use a condenser mic.

Since the source and load impedance are presumably resistive, the only effect is a loss in signal and that's more than made-up by the gain. With headphone amplifiers it's more important because the load impedance varies with frequency so high source impedance creates variations in frequency response, not just a loss of signal.

Just as a random guess but the output impedance may be related to the limited current available from phantom power. It would be easy to add a buffer stage to lower impedance but a lower impedance load draws more current and if the power supply can't supply the current the voltage will drop. Or, maybe it's because any amplification stage (even a unity gain buffer) adds some noise.

 

[Speedskater]

Wait! the Klark CT-1 is a mic booster. Not an audiophile system AC powered pre-amp.

 

[AnalogSteph]

Wait! the Klark CT-1 is a mic booster. Not an audiophile system AC powered pre-amp.

Yes. And most of them are more or less like that, some being substantially worse. It's generally just a handful of semiconductors that have to make do with a few mA of phantom power while keeping input noise very low. The Klark Teknik mic boosters are by far the cheapest but definitely not the worst of their kind. That's the power of Behringer, I guess.

In other words, if such preamp feeds into 2KΩ unit, there will be -6dB reduction.
This gives us amplification of ~19dB in the best-case scenario instead of the marketed 25.

Not quite. With a spec of +25 dB into 10 kOhms, open-circuit gain would be roughly +26.6 dB. So we're talking about +20.6 dB into 2 kOhms or +22.1 dB into 3 kOhms. Not an awful lot of variability, and it's not like the exact amount of gain would be critical in this application.

Either way, this is enough gain to swamp input noise on even the most mediocre mic inputs, even if EIN with a 2 kOhm source is up a bit. Distortion may be a bit worse as well, but honestly I'd be worrying about the mic booster first.

 

[Dilettante]

Either way, this is enough gain to swamp input noise on even the most mediocre mic inputs, even if EIN with a 2 kOhm source is up a bit. Distortion may be a bit worse as well, but honestly I'd be worrying about the mic booster first.

Not sure I can comprehend what's written there fully, but CT-1 is ordered and I will be able to test empirically.
Hope to fix the noise issue by the time it arrives and then it's all set for a show!