2. amps-preamps
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Passive preamps: What are the relevant numbers?

I'm trying to decide whether I am going to add a passive preamp to my system, but I've been reading that system compatability is very important. What numbers should I be looking at in the specs of my CD player (Rotel 1072), Amp (DNA 0.5) and Interconnects to judge whether a passive preamp might work well in my system?

I know that there was previously a thread about this, but it seems to have been deleted.
davetroy
arni
313 posts
 
Dave, if you are going analog out of CD player, then digital output is irrelevant. It is quite standard that analog output from standard CD players is at least 2.0 volts, you should be fine with a passive between the Rotel and DNA .5
unsound
7,333 posts
 
I humbly suggest you search the archives.
pubul57
3,712 posts
 
In the context of your system, it seems the low output impedance of your CD player should be able to drive the amp. I'm not sure what your amps input impedance is, but if it higher than 40Kohms you shuld be fine. I would expect the Rotel's output voltage to be at least 2volts, more than enough to get full volume from your amps. You should try to keep the interconnect from the passive pre to the amps to less than one meter (the cable can be longer if you have low capacitance cables - less than 20pf/ft).
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drubin
3,275 posts
 
I'm not sure what your amps input impedance is, but if it higher than 40Kohms you shuld be fine
Do you consider this a general rule of thumb where passives are concerned, that they feed into loads of >40K ohms? Why is this?
pubul57
3,712 posts
 
I got that as "general rule of thumb" from the manufacturer of my passive - but I don't know the technical reasons. I do know that the higher the input impedance of the amp (the 100kohm typical of tube amps seems to be ideal) the better, it is - an easier "load", and the lower the output impedance of the source the better (my DAC is 200 Ohm output impedance). The other factor is adequate voltage from the source (CDs are typicall 2 volt and up - more than adequate for most applications) and a low input sensitivity for the amps (I have a 1.5v and 835 mv sensitivity on my two amps, the 1.5v amps are more than senstive enough). It is probably ideal to have the output voltage twice the amount of the amps input sensitivity. The you want low capacitance in the IC going from you pre to the amp - I read somewhere that less than 100pfs for the run is ideal.
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