Value of Stepped Attenuator for Passive Amp. - diyAudio
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Old 17th October 2012, 03:35 PM   #1
xtaljo is offline xtaljo  United Kingdom
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Default Value of Stepped Attenuator for Passive Amp.

I am completely confused about how to select the right value resistance for a stepped attenuator in my diy passive preamp. Please help! I have read a lot of theory but understand very little of it (brain worn out).

I am driving two Mullard 3+3 valve power amps from a Marrantz CD player and a DAB radio. I have tried a 100K attenuator and it works ok but all the useful volume levels are at the top end of the 23 steps (the last 6). Is 100K too high for this set up? Should I go for 50K or lower?

Each Mullard 3+3 has a 470K volume control (to an EF86) but I don't know the input impedance. The input sensitivity is about 100mv.

Last edited by xtaljo; 17th October 2012 at 04:00 PM.
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Old 17th October 2012, 06:29 PM   #2
DF96 is offline DF96  England
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The 3-3 has a fairly high input impedance so 100k should be fine. Is it possible that there is a wiring error?
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Old 17th October 2012, 06:48 PM   #3
pdul is offline pdul  Denmark
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Or maybe linear pots
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Old 18th October 2012, 11:03 AM   #4
xtaljo is offline xtaljo  United Kingdom
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I can't find a wiring error and the attenuators are log taper. Perplexed!
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Old 18th October 2012, 11:22 AM   #5
pdul is offline pdul  Denmark
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try a cheap potmeter at around 20ko just to get a feeling of what you are dealing with
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Old 18th October 2012, 12:08 PM   #6
DF96 is offline DF96  England
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Measure the resistance from attenuator output to ground for each switch setting for two cases: attenuator input open circuit, input shorted to ground.
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Old 18th October 2012, 12:22 PM   #7
AndrewT is offline AndrewT  Scotland
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there is absolutely nothing wrong with having to use the top six steps of your attenuator to acheive the sound level you need.
this indicates that the system gain is pretty near spot on for the listening you do.

One of the worst things you can do to a "system" is build in too much gain and then attenuate the over amplified signal back down to listen-able levels.
Read What is Gain Structure?

Now back to the attenuator resistance.
I recommend that the source output impedance be ~ 1/20 of the receiver impedance.
A 100k attenuator sees the source equipment as it's source and would require the source equipment to have an output impedance <=5k. No problem there are virtually no source items that have Rs>=5k

The attenuator also acts as the source for the following power amplifier.
The maximum output impedance of a 100k attenuator is ~25K.
Using that 20:1 ratio you need a power amp with Rin >=500k. Valve amps may be the only gear that can successfully match up with a 100k attenuator.

You need a 10k attenuator for interfacing modern gear.
This has absolutely nothing to do with using the top six steps. That is a system gain issue, not a input/output impedance suitability issue.
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regards Andrew T.

Last edited by AndrewT; 18th October 2012 at 12:31 PM.
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Old 22nd October 2012, 03:13 PM   #8
xtaljo is offline xtaljo  United Kingdom
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Many thanks to all for your replies. My mind is a rest to know there isn't really a problem with 'high' settings of the attenuator.
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