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Old 8th January 2005, 04:13 AM   #1
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Default Minimal (passive?) linestage

Not sure where to put this request. This seems as good a place as any, but if not, please indulge me.

I have finally reached the state of having both analog and digital sources in my system. However, my preamp is an EAR 834P, which is phono only.

So, what I'd like to build is, basically, a switch for selecting between the EAR output and the output from my CD player. I'd like the EAR output to be routed directly to the amp (except for the switch, of course) and the CD player to include a means of adjusting its volume.

Would this require some sort of gain stage or buffer for the CD input? Also, what sort of attenuation should I use? If someone could point me to a schematic for this sort of thing, that would be most excellent.

Thanks!

Paul Ebert
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Old 9th January 2005, 02:17 AM   #2
anatech is offline anatech  Canada
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Hi Paul,
I would think you would want a buffer stage for the output for sure. The input should be buffered to drive the volume control as you can't guarante what may be plugged into the input of your preamp. Your choice of buffer circuits is up to you. A diamond buffer may be a good place to start (and possibly finish).
-Chris
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Old 11th January 2005, 12:40 PM   #3
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Mate if the CD player has enough drive (it should) a good quality dual gang pot should do fine. At a guess a 50k would be about the money most would recon a log pot (fair enough) but a linear pot will probably track more consistantly.

Try it with a cheap pot If you are happy spend up with something with a brand on it.

I think you will find the vast majority of commercial power amps (that do not have ballanced inputs) will have no buffer stage before the volume pot.
cheers
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Old 12th January 2005, 01:10 AM   #4
anatech is offline anatech  Canada
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That may be true to drive the pot, but how are you going to drive the cable??
-Chris
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Old 12th January 2005, 02:52 PM   #5
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OK, sounds like a buffer between the pot and the amp (cable) may be needed, but there's no harm in trying without to see if that works.

Since I control what's going into this linestage, I think I'll pass on the input buffer, at least, for now. I suppose, long term, I should consider replacing the EAR with something that manages more than the phono.

If I do need an output buffer, I think I'll try to route it such that the phono doesn't pass through it.

Thanks. I appreciate the responses.
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Old 13th January 2005, 01:13 AM   #6
anatech is offline anatech  Canada
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Hi Paul,
What's the output impedance of your EAR preamp? If the power supply and buffer are good, you won't hear them.
-Chris
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