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-   -   A75 without preamp? (http://www.diyaudio.com/forums/pass-labs/2008-a75-without-preamp.html)

SteveG 20th January 2002 09:32 PM

A75 without preamp?
 
I am in the process of building an A75, and was wondering if I could use the output of my SACD player to drive the inputs directly (no preamp) with a passive attenuator in between. If so, how would I go about choosing the value?

SACD player output specs:(from owner's manual)
level: 2VRMS (at 50Kohm), minimum load- 10K

Since it is spec'd into a 50Kohm load, I was thinking of using a 100K pot. What do you think? I believe that the A75's input impedance in unbalanced mode is about 75K. I'm not sure how you go about determining the minimum impedance as seen by the output... is it the combination of the pot on the output paralleled with the amps input impedance?
When it is possible to connect this way, what is the point of using a preamp anyway (besides tone control)?
It dosen't make sense to me to build an active preamp, unless your amplifier is really demanding. Is there something that I'm missing here?
Thanks
Steve

jam 20th January 2002 09:57 PM

Steve,

I would choose a pot between 10k and 20k.
Cable length will be an important consideration. Keep the length of you interconnects as short as possible.

Jam

paulb 20th January 2002 10:12 PM

Assuming the sensitivity and impedance considerations are okay you don't require an active preamp. I prefer at least a buffer after the volume control but if you keep cables short it should be okay.
Many swear by passive preamps. Especially with a CD player, it shouldn't be a problem.
The impedance seen at the output of the CD player will be (at full volume) the parallel combination of the volume pot and the amp input impedance. At lower volumes, it will be higher until at zero volume it's just the pot.

audionut 21st January 2002 02:34 AM

Aleph 3
 
I have built a number of Aleph 3 channels and use them in a Bi amped system...as in the Zen amps, I have been using air core 2mh inductors in the power supplies. I measure very low ripple on the Aleph rails, (20 mv), I wonder what other benefits there are to having these inductors in the Aleph power supplies. The inductors certainly reduced the hum in the Zens to almost nothing.
To my ears, the Aleph's are sweet and warm sounding.


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