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100KOhm Ladder Attenuator with Various Amps
100KOhm Ladder Attenuator with Various Amps
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Old 28th May 2006, 03:01 AM   #1
john65b is offline john65b  United States
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Default 100KOhm Ladder Attenuator with Various Amps

I have just bought and built a Ladder type Attenuator (100KOhms) for volume control and am looking at using it for a few of my amps as a passive pre , including a Tripath TA2020, TA2024, Gainclone (Brian GT), AMP5 (not yet complete), a 6L6GC Tube Amp, and my UCD180. Should I be concerned about using it on any of these amps? I beleive the general principle is get a attenuator LESS than the required input impedance spec'ed by your amp. The UCD is spec'ed at 100KOhms . Should I have bought a 50KOhms or less?? I have also the option of putting a 4.7 uF 400V PP capacitors after the attenuator on each channel to suppress pops and switching noise (which I really don't hear). Is this advisable?

My cd player is an older Sony 5 disc changer (CDP-C735)...Output 2V@ 50KOhms...

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Old 29th May 2006, 06:19 AM   #2
g3dahl is offline g3dahl  United States
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Hi John,

The 100K attenuator will work fine if mounted in the same enclosure with the UcD modules, assuming that you can minimize the distance between the outputs of the attenuator and the inputs of the modules (~2 inches). But it won't work as a traditional passive attenuator in a separate enclosure.

The problem is the high output impedance of the attenuator, except when near its highest and lowest volume settings. The high impedance works together with the capacitance of the wiring to form a low-pass filter at audible frequencies, and to make matters worse, increases noise pickup. Using shielded cable to help with the noise problem increases the capacitance and worsens high frequency losses.

My first recommendation would be that you place your attenuators right up next to the UcD modules, whether you use 10K or 100K. If you go with an external passive attenuator, you will need to use a 10K unit.

I don't know about the input requirements for the other units you mentioned. Usually, 100K will work with any tube amp (although the distance issue is still there), while most solid-state applications require 10K. The UcD module happens to have a buffer at its input, terminated with 100K, allowing additional options.

Gary Dahl
"So many tubes, so little time..."
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Old 29th May 2006, 06:37 AM   #3
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
that 4.7uF capacitor you mentioned is for blocking DC, from the source, from getting amplified by the power amp.

It could be fitted before the attenuator or after.
However some or all of the amps may have a cap already fitted.
4.7uF will work as a high pass filter for input impedances of 22k and above. If any of your power amps are 10k then try a 10uF blocker instead.
regards Andrew T.
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