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Matching Impedance Pre to Power


Hazzzy

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Hi

I am not sure about this so I thought I would ask here.

Pre amp has output impedance of 25 ohm.

Power amp has imput impedance of 47 kOhm.

Trying to work out if these should work well together before one or the other goes. Sounds very good to me but if it is a mismatch I may have some improvement to be had.

Thanks

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1 hour ago, Hazzzy said:

Hi

I am not sure about this so I thought I would ask here.

Pre amp has output impedance of 25 ohm.

Power amp has input impedance of 47 kOhm.

Trying to work out if these should work well together before one or the other goes. Sounds very good to me but if it is a mismatch I may have some improvement to be had.

Thanks

 

The 'rule of thumb', H, is that the input impedance (Zin) of a component should be at least 10x the output impedance (Zout) of the preceding component.

 

So 25 ohms feeding 47K ... is maximally good!  :)

 

As Shane pointed out, a tube pre having a Zout of 3K ohms ... is not a good match with a ss power amp having a Zin of 20-30K ohms.  A Zin of 47K might do the trick - but a tube pre with a Zout of 3K is better matched to a tube power amp ... which typically have a 100K Zin.

 

Andy

 

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10 minutes ago, Hazzzy said:

Yes I did read that somewhere. Just couldn't get my head around the ohm and kOhm bit.

 

Quite simple, H:

  • 'Ohm' is ... well, 'Ohm'.
  • 'kOhm' is ... 1000 ohms.

 

:lol:

Andy

 

Edited by andyr
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2 hours ago, Hazzzy said:

Pre amp has output impedance of 25 ohm.

Power amp has input impedance of 47 kOhm.

Trying to work out if these should work well together

No problems at all, so long as the output to input impedance ratio is 1:10 or higher your fine and yours is 1:1880 ratio. (25 to 47000)

 

Cheers George

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10 hours ago, georgehifi said:

No, this is a blanket unwritten rule.

 

Cheers George

Except that 10x ... isn't generally optimal - merely the minimum you can get away with!  :)

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1 hour ago, andyr said:

Except that 10x ... isn't generally optimal - merely the minimum you can get away with!  :)

I think people will infer from this making the ratio higher and higher, will give better and better sound.

 

 

That isn't something I'd advise anyone to optimise.     Instead, they just need to avoid a "too low" ratio.    Once the ratio is high enough, then making it higher won't help any.

 

 

 

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13 hours ago, andyr said:

Except that 10x ... isn't generally optimal

Sorry, even though it's not written EE law, a few years back we all decided that 1:10 is the minimum you can get down to without any degradation.

We conducted a very precise listening session with around 40 "golden eared" audiophiles in Haberfield Town Hall at one of our Ason meetings on a very revealing system, all impedance ratios were checked for exact level correctness to the mV at the speaker using lab discs.

 

The amp had a special input that I made up and was switchable in increments on the fly from 100kohm down to 5kohm.

Only 2 out of the 40 members "thought" they heard a difference only once it got down to 1:10 ratio, but both  said they could never pick it if blind.

It was only when we got down to 1:5 ratio did the number come up to 10 of the 40.

 

Cheers George       

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2 hours ago, davewantsmoore said:

 

That isn't something I'd advise anyone to optimise.     Instead, they just need to avoid a "too low" ratio.    Once the ratio is high enough, then making it higher won't help any.

 

 

Correct, Dave.  :thumb:

 

But too many people regard '10x' as being all you need to worry about.

 

In my view of the world, you 'optimise' by finding the right value ... so you increase from 10x.  But once the 'x' is enough to make a difference ... you try a bit further before you stop.  (If it hasn't got any better by going further.)

 

But, in terms of the loading between different components, I would regard x30 as being the lowest i would choose - so a 3K Zout tube phono stage will mate well with a 100K Zin tube power amp ... but not a 47K Zin ss power amp.

 

And in terms of optimal MC cart loading ... so many people take the "minimum recommended" figure in the specs as the one to use (which is typically 10x the coil resistance).  So they never try a greater 'x'.  In the case of my Benz LP - which had a coil resistance of 33 ohms - the recommendation was "470 ohms or greater".  I found 3K3 was the optimum - so 100x the coil resistance.  (But then, I had a phono stage which allowed me to choose any load I wanted.)

 

Andy

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49 minutes ago, DMax said:

What if they are not matched and the ratio is less than 10x?

 What we found in in that 40 + listening demo that with 1:3 ratio (levels matched) about 10 members thought they heard a bit of dynamic compression compared to 1:10 ratio.

 

Cheers George

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37 minutes ago, georgehifi said:

 What we found in in that 40 + listening demo that with 1:3 ratio (levels matched) about 10 members thought they heard a bit of dynamic compression compared to 1:10 ratio.

 

Cheers George

so it effects more on the sound quality; but will there be any danger of damaging the amps?

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2 minutes ago, DMax said:

so it effects more on the sound quality; but will there be any danger of damaging the amps?

No damage to amp, but it can act as a voltage divider, so you have to take into account a different volume level.

 

Cheers George

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Art speaks...

 

For quite a while I used a preamp with a divider at the output instead of at the input, which provides some benefits in terms of S/N because you're dividing the total output noise voltage by the same ratio as the signal voltage.  

 

 

"Matching" in the technical sense of the word, as applies in signal transmission, doesn't really apply in audio systems, so its use is mostly out of context. 

 

While it's true that there's a 6dB penalty for the condition where Rout = Rin (Zout = Zin) in a pre-power amplifier connection, it's only of relevance if total amplifier gain is insufficient, which is rarely the case, or if Zin has a capacitive reactance (shunt) component that dominates the total impedance at a low enough frequency to roll off the treble, or alternately a (series) component high enough to roll off the bass. Typically, shunt capacitance is embedded in the power amplifier's low pass filter at its input, and the designer assumes it will be driven from an ideal preamp with a low output impedance.   

 

There are also some solid state amplifiers that exhibit higher than desirable input Miller capacitance, which is typically quite non-linear, and hence provides a new distortion mechanism if driven by a too-high source impedance.   

     

"Rules of thumb" are fine but it's also nice to know how and why they exist and where and when they're applicable.  

  

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13 hours ago, Art Vandelay said:

 

Art speaks...

 

 

"Rules of thumb" are fine but it's also nice to know how and why they exist and where and when they're applicable.  

  

 

That will usually depend on what devices are being used in the input and output and whether the gain stage is typical inverted ...  also  how the designer deals with limitations and characteristics......an example as you pointed out,  using a voltage divider at the output to gain on S/N. Etc

 

Edited by Addicted to music
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