2. amps-preamps
  3. 1080847496

Understanding impedence matching

I am a psychologist and my eyes start crossing when the conversation gets too technical, but that being said, I am beginning to wonder what impedence matching means and what I should pay attention to as I upgrade my setup.

The two pieces in my system that are likely here to stay for a while are my SF Electra Amator II, that evidently don't drop below 6 ohmns, making them receptive to an OTL amp, and my Cary pre, that has phono impedence @ 47K (evidently this can be adjusted) and output at 800 ohms. I am in the process of upgrading my amp and TT.

What does this mean in terms of how I think about these two acquisitions?? Are there specific numbers I need to pay attention to, and/or formulas for these, or is it all in the listening?

As always, thanks for your help. This community is amazing.
ccryder
morbius396c
134 posts
 
Sean,

Well said in your explanation.

I was being "loose" in my use of the word "control". In a
way, a speaker that can modulate the amp is not under "control".

You are quite correct that in the case of the interface
between the speaker and amp - "impedance matching" is the
last thing you want to do - since as you state - it would
allow the speaker to transfer what limited energy it has
back to the amp.

It is desirable for the interface NOT to be "impedance
matched". That way - only the amp - which has more power
than the speaker - can dictate what is going on.

As you state - there is a purposeful impedance mismatch
at the amp / speaker interface so the speaker can't talk
back to the amp - but of course - the powerful amp can
make the speaker do its bidding.

Dr. Gregory Greenman
Physicist
subaruguru
1,375 posts
 
Still with us, Cc? Just be careful to home-demo ANY amp you try with a relatively high output impedence, like many tube amps. Simple.
ccryder OP
45 posts
 
Thanks for asking, Subaruguru. I am tracking, except for when I'm not! The main upshot of all of this is that I am going to want to find some coherent, organized introduction to these issues, as I would like to pull back the veil a little. Any suggestions? The links people have provided seem to start in the middle, given my starting position.

And, I also understand that there is the same kind of impedence issues with the TT/cartridge and the pre-amp. I assume the same principles are involved. Is that right?

But, whether I understand it all or not, I am really enjoying reading and appreciate people's input.
morbius396c
134 posts
 
Ccryder,

The interface between the TT/cartridge and the preamp does
have to be impedance matched. However, cartridge makers and
preamp makes have standardized this interface to be a 47k ohm
transmission line - so you don't have to do anything special.

Likewise, all your interconnects are 75 ohm transmission lines
which is also standardized - and therefore impedance matched.

However, for the power amp to speaker interface there is an
intentional mismatch - so as Sean states - the speakers
can't feedback to the amp and modulate it.

For this interface, you check the specs of the power amp
or receiver - that will tell you the minimum load impedance
that the particular amp will handle properly.

Best wishes.

Dr. Gregory Greenman
Physicist
subaruguru
1,375 posts
 
...if it's a tube amp ask for the oftentimes dirty little secret of it's output impedence. This will give you a hint as to how much relative frequency response trampolining can occur compared to another amp. Again, you have to home-demo WITH your speakers. Don't make a career of reading about this...it'll come down to how the amp mates with your speakers. If you become obsessed with amplifier output impedence just get a nice ss amp....
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