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power amp/pre amp input and output impedence question

Hello, im in the process of researching a new system, low and behold my limited knowledge has led me to try understanding the input and output impedences to better assemble my new system as to the power amp and preamp.

in all of my years as a Hometheater guy I've avoided such topic because with "most" HT gear it all works together just fine.......

Hear I am many years later "catching up" on terms I should know, I was gonna just google and read, but most of the time well versed members can get to the meat better than random google replies

Thank you KG

kgveteran
14 responses Add your response
arcticdeth's avatar
arcticdeth
1,572 posts
 
My grail preamp is :

OUTPUT

  •  Two pair of adjustable, unbalanced, outputs at line level using RCA connectors
  • One pair of adjustable, balanced, outputs using XLR connectors (Pin 2 "hot")
  • One unbalanced, fixed level output at unity gain for recording using RCA connectors
  • All outputs may be used simultaneously

 Frequency Response

-3 dB at 5 Hz and 200 kHz

Noise

Greater than 100 dB below 1 volt reference

Distortion

Less than 0.002% from 10 Hz to 40 KHz @ 5 volts peak into 600 ohms or higher, shunted by 1,000 pF or less

Output Impedance

50 ohms, non-reactive, balanced or single-ended

Phono Input Impedance  

User adjustable between 47,000 and 100 ohms. Default setting is 47,000 ohms

Phono Input Capacitance  

User adjustable between 50 pf and 1,200 pF. Default setting is 50 pF.

Input Impedance

47 kohms, balanced or single-ended

Gain  

User adjustable up to 18 dB. Default setting is +8 dB

 Maximum Output  

 10 volts peak

Crosstalk

Greater than 70 dB from 20 Hz to 20 KHz

Power Supply

Independently regulated with shielded toroidal transformer and 20,000 uF of capacitance

Power Consumption

10 watts

Voltage

Voltage is user selectable for use world-wide

 

 



great preamp!
atmasphere's avatar
atmasphere
11,778 posts
 
As long as you are north of 10:1 you're good. The only issue you might run into with amps with a very high input impedance is you might want to keep the capacitance down with your interconnect cables so as to maintain high frequency bandwidth.
dhite71
317 posts
 
Hello all,

When looking at my ARC component specs the ratio is well beyond 10:1

My Classic 120 mono block amp has an input impedance of 100,000 Ohms
My LS3B preamp has an output impedance of 500 Ohms

That ratio is 200:1

My next amp will be a REF 75, which has an input impedance of 300,000 Ohms.

That ratio matched with my LS3B will be 600:1

Am I doing my math wrong here?  This seems to be so well beyond the 10:1 target to not be of any concern.

Wondering about mixing brands, I checked a Conrad Johnson preamp.  Its output impedance was 100 Ohms that would drive that ratio even higher.

Next I went to McIntosh's website and the C2600 preamp doesn't list the output impedance.....only input at 44,000 Ohms, while my LS3B is 50,000 Ohms.

Are there some extreme examples of mixing brands where this is a concern, but most often not at all?

kgveteran OP
377 posts
 
Gotcha
atmasphere's avatar
atmasphere
11,778 posts
 
@kgveteran Yup! Sometimes the output impedance of some preamps isn't all that low (particularly tube preamps) even though on the right setup it can sound pretty nice. In some cases the manufacturer might not have even measured it.

We don't specify out output impedances either, but we **do** spec that they can drive a 600 ohm load and that the output impedance is flat with respect to frequency.
kgveteran OP
377 posts
 
I notice as i research amps and preamps, sometimes these specs are hidden away or non existent in the owners manual  :0/
 
Why would they not include these important numbers ? Is this kinda like when a manufacturer says an amp can put our 1000watts/ch @1Khz and not a full bandwidth watts/ch, not just a convenient 1Khz signal which is fairly easy for the amp

georgehifi's avatar
georgehifi
10,312 posts
 
"Same for poweramp to speaker."
????

Yes "damping factor", stretching this one a little, but the lowest impedance of the speakers curve can also have the 1:10 rule applied to it that the amp is 10 lower than it or even better.
So if the speaker is 2ohms in the bass the amps output impedance "should" be 0.2ohm or better lower at that frequency also.

Then in there’s the capacitor coupled components, these need to be looked at first to see if the cap is big enough to maintain the 1:10 rule, if not there could roll offs at the frequency extreme
atmasphere's avatar
atmasphere
11,778 posts
 
what effect can be expected from a miss match.

What does an output impedence of 1.2k ohm say to the expert......
If the ratio is less than 10:1 you might have less headroom before the preamp makes distortion. There would also be more overall distortion so less detail and probably harsher sound.
An output impedance of 1.2K ohms isn't bad. You can drive amps with 20K or higher input impedance no worries. The thing to look at is the output impedance at 20Hz. Many preamps have acceptable output impedance at 1KHz but at only 20Hz their impedance can be quite a bit higher. If at this frequency its less than the 10:1 rule of thumb you might experience less impact in the bass.
kgveteran OP
377 posts
 
What are the effects of a mismatch 
kalali's avatar
kalali
1,977 posts
 
"Same for poweramp to speaker."

????


georgehifi's avatar
georgehifi
10,312 posts
 
kgveteran

yogiboy The rule of thumb is the input impedance should be at least 10 times the output impedance!
What yogi said is correct, the ratio of 1:10 or more.

EG:
CD or dac output impedance = 100 ohms, preamp input impedance should be 1000ohms (1Kohm) or more.
Same for preamp to poweramp
Same for poweramp to speaker.

Cheers George
kgveteran OP
377 posts
 
Whats an acceptable variation so i can gauge a miss match.

what effect can be expected from a miss match.

What does an output impedence of 1.2k ohm say to the expert......
atmasphere's avatar
atmasphere
11,778 posts
 
The rule of thumb is the input impedance should be at least 10 times the output impedance!
'Input impedance' referring to the amp, 'output impedance' referring to the preamp.
yogiboy's avatar
yogiboy
5,653 posts
 
The rule of thumb is the input impedance should be at least 10 times the output impedance!
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