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,

You are correct about the amp output impedance.

I know I have an amp with a damping factor of 400 into an
8 ohm load. I got my ratio upside down.

The amp's output impedance is 20 milliohms.

Let me correct some more of my previous post:

Twin lead antenna cable is a 300 ohm transmission line.

Coax cable is the 75 ohm transmission line.

The connection between an audio cartridge and the phono
preamp is the 47k ohm transmission line.

Dr. Gregory Greenman
Physicist
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
1,353 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
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