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Old 29th February 2004, 08:09 PM   #1
lbenoit is offline lbenoit  United States
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Default passive attenuator design

I'm working out the specifications for passive shunt attenuator and seek advice on whether to go with a 1K or 10K I/O impedance. Most attenuator suppliers don't offer shunt controls below 10K impedance, but I found one, Electronic Visionary Systems (tweakaudio.com), that highly recommends 1k input impedance for passive attenuators used with solid state amplifiers.

I'm driving a Bryston "arc welder" 14B SST (29db gain -- 2.5v for max output of 900W into 4 ohms) with an Adcom GCD-750 CD player. The Bryston has 50K unbalanced inputs and the Adcom has 50 ohm unbalanced outputs rated at 3 Vrms. The Adcom's analogue output is full class A and seems very well suited for use with a passive volume control/preamp.

I've used cheap fixed resistors to establish my normal listening level of around 80 DB SPL (stereo mode at about 10 feet from my speakers). I might run the system another 6 DB louder, but only rarely. Accordingly, the preferred listening level was established as follows with these approximate values:

10K I/O and 750 ohm shunt
1k I/O and 75 ohm shunt

While my audition of each was limited, I really couldn't reliably hear a difference between the 1k and 10K mock-ups. I plan to use an Elma 24 step attenuator and place the ideal resistance value at step 11 and calculate resistor values to produce 1 DB steps between steps 5 through 19; and +- 3/3/5 DB steps above and below the 1 DB steps. Interconnects will be less than four feet total (two feet in and two feet out) with 17pf per foot (Belden 1505F)

What might be the disadvantages/advantages of the 1k versus 10K inuput impedance?
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Old 29th February 2004, 10:09 PM   #2
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The only disadvantage of your setup could be that the Adcom is not suitable for driving this low impedance despite it's low output resistance.
Note that a low output resistance doesn't mean it can always drive low impedances ! Check f.i. the bias current of the class A output stage and you know enough.

Another thing is if the Adcom uses a capacitor in the output as coupling device. If yes, the value could be to low for driving 1k.

Using the 1k also means that in the future if you want to replace the Adcom your choice of equipment would be very narrowed by the driving requirements.

Hope this helps,

Dick.
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Old 29th February 2004, 11:28 PM   #3
eganz is offline eganz  United States
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2.5 k or 5 k may be more flexible...or use 10 k if it sounds just as good.
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Old 1st March 2004, 10:51 AM   #4
lbenoit is offline lbenoit  United States
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Thanks very much for the advice.
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