Impedance Matching; what am I looking for?

[MWalt]

I am currently running an EL34 Tetrode tube amp and a tube preamp that puts out around 40 watts a channel. I want to buy a high powered solid state amplifier to use with my tube preamp for times when I want more volume or headroom or in the summer months when heat is an issue.

I have a concern with impedence matching and how it affects interaction between the tube pre and SS amp. What numbers do I need to look for when deciding on a SS amp that will match up well with my tube pre?

 

[Jon_Logan]

Some tube preamps have very high output impedance. This can be problematic, even when connting to a tube power amp. With a high source impedance, the cable's capacitance forms a low pass filter even if the pwer amp is not connected. The connected amplifier's input impedance can work as a voltage divider when the source impdeance is high. Sometimes there are frequency dependent attenuations.

Tube preamps with cathode follower outputs are preferred for lower output impedance. Keeping interconnects as short as possible buys you bandwidth margin. Using a solid state amp with high input impedance is safest (>100kohms).

 

[dumptruck]

What's the output impedance of the preamp? I think going tube-to-SS you'd generally just look for amps with highish input impedance. You could also get a line level buffer and never have to worry about it again.

 

[Renzor213]

As already mentioned, you're needing to keep preamp output impeadance lower than power amp input impeadance, or gain's and lower bass frequencies will be rolled off. In my all solid state set up, preamp is 100 ohm's out, with with power amp 47K ohm's in, so certainly not a problem. A ratio like this is rather over kill, but think you get the idea. Also, a 47K ohm SS power amp impeadance input is pretty common.

 

[MWalt]

I have a Dared SL2000a preamp and have no idea what the output impedence is. I got voice mail when I called DARED Tech, so I am awaiting a call back. I will post the numbers once I get them.

 

[whoaru99]

If you have some basic goodies you can...

Hook up a potentiometer to the output of the preamp. Use a pot that has a maximum value at least 10 times the "guestimated" output impedance, 25K ohms should be enough.

Feed the preamp a fixed level signal such as a 1kHz tone that you recorded/burned onto a CD, generated from computer, or signal generator, etc. Turn up the preamp volume to 1/2 way or so.

Turn the pot to max resistance and measure the AC voltage across the pot. Then slowly turn down the pot until the voltage drops to 1/2 the initial value. Disconnect the pot from the preamp and measure the pot's resistance. The measured resistance is equal to the output impedance of the preamp.

 

[MWalt]

If you have some basic goodies you can...

Hook up a potentiometer to the output of the preamp. Use a pot that has a maximum value at least 10 times the "guestimated" output impedance, 25K ohms should be enough.

Feed the preamp a fixed level signal such as a 1kHz tone that you recorded/burned onto a CD, generated from computer, or signal generator, etc. Turn up the preamp volume to 1/2 way or so.

Turn the pot to max resistance and measure the AC voltage across the pot. Then slowly turn down the pot until the voltage drops to 1/2 the initial value. Disconnect the pot from the preamp and measure the pot's resistance. The measured resistance is equal to the output impedance of the preamp.

This would be a little over my level of expertise. I don't have a potentiometer laying around so I'll have to do this the hard way and find out from the manufacturer the output impedence. Too bad you don't live close by.