Does a pre-amp have an "impedance"?
May 10, 2015 at 8:28 AM Thread Starter Post #1 of 6

NoxNoctum

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I recently wrote the maker of a power amp I bought for advice on what pre-amp he would suggest for the power amp, and he told me to shoot for "a source with a balanced output that can drive a 10 Kohm impedance". I'm new to hifi --- but I thought that the "source" would be the DAC or my PC with my flac files on it?
 
Thanks for any help.
 
May 10, 2015 at 9:01 AM Post #2 of 6

DrKC

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Your preamp would be considered the "source" for your amp. Your dac is a source for your preamp.
What's important here is the input sensitivity of your power amp.  There should be a spec (along with input impedance) that indicates the maximum voltage input for the amp.  You want a preamp that can output a voltage range that's high enough to actually drive your power amp to full power out.  Some preamps have low output voltage 3 -4 volts, some have outputs in the 5 - 7 volt range.  A tube preamp can sometimes have an output voltage as high as 20 volts.  Preamp output impedance will usually be low, 200 to 1000 ohms.  Your power amp input impedance will normally be high, 22K Ohms to 100K Ohms.  That keeps the signal transfer as a voltage level event and not a power transfer function.
 
May 10, 2015 at 1:09 PM Post #4 of 6

DrKC

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  Ok so how does that work if most pre-amps have a low output voltage and I need something with 10K output voltage? Or am I misunderstanding?

OK, let's not mix up resistance and voltage.  Your amp, I believe had a 10kOhm input impedance.  That's OK.  A preamp will normally have an output impedance of less than 1KOhms and that's good.
Find in the specs for your amp it's input range, usually expressed in Volts.  Or ask the manufacturer if it's not spec'd.  You want to find a preamp that will supply an output Voltage that's at least as high as the range of your amp.  That will allow the preamp to drive the amp (not that you'll do it) to it's full rated output - expressed as Watts.
Does that make sense?  Matching impedances is NOT what you're looking for here.
 
May 11, 2015 at 1:06 AM Post #5 of 6

NoxNoctum

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  OK, let's not mix up resistance and voltage.  Your amp, I believe had a 10kOhm input impedance.  That's OK.  A preamp will normally have an output impedance of less than 1KOhms and that's good.
Find in the specs for your amp it's input range, usually expressed in Volts.  Or ask the manufacturer if it's not spec'd.  You want to find a preamp that will supply an output Voltage that's at least as high as the range of your amp.  That will allow the preamp to drive the amp (not that you'll do it) to it's full rated output - expressed as Watts.
Does that make sense?  Matching impedances is NOT what you're looking for here.

Ok so driving the 10kohm impedance means having sufficient voltage, not trying to match impedance? I think I understand better now. 
 
I can't find a spec for the input range, but I've written the manufacturer for it. This is the amp:
 
http://www.firstwatt.com/f1j.html
 
May 11, 2015 at 9:20 AM Post #6 of 6

superjawes

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Yes, driving these impedances is about having enough voltage. You only ever want to match impedances if you're working with transmission lines.

This all comes down to electrical models. Basically, your preamp is getting simplified for circuit analysis purposes. We're using a model where it (the preamp) is just a voltage source (that's where your audio signal comes from) and a series impedance (that's the output impedance). Then, the input of your amplifier is getting simplified just to an impedance and ground, so when you connect the two, you have the voltage source->output impedance->input impedance->ground all in series.

A circuit like this is called a voltage divider, and the voltage starts high at the source (even if the highest is only 5V or so), then drops down to zero (ground is zero). The drop across each impedance is proportional to the resistance. Ideally, you want all the voltage drop to happen across your amp's input impedance, but since the world is not ideal, you just want the input impedance to be significantly higher than your output impedance. That ensures good transfer (efficiency) between the amp and preamp.

Also, this same principle applies to the output of the amp and your speakers. There's also some concern about electrical dampening, but you still want good power transfer to your driver.

This Wiki article actually might show it better. You want a big Vout, so Z1 should be much smaller than Z2. Z1 is your preamp's output impedance and Z2 is your amp's input impedance. Also, the cricuit model is called a Thevenin Equivalent if you'd like to read up on it.
 

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