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Old 17th March 2011, 11:51 PM   #1
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Default Stepped attenuator - getting the impedance right

Hi everyone,

First post!

Right, I'm looking into building a simple passive pre with a stepped attenuator and have done loads and loads of reading.

I've read that the impedance of my attenuator wants to be at least 10 times the output impedance of my source, and that it also wants to be at least a tenth of the input impedance of my power amp. I also read somewhere that 20 times is a good figure to aim for, so I figured that bigger the difference in impedance across each interconnect the better.

The preamp will for the vast majority of the time be fed by the soundcard on my pc. The line-out jack on my soundcard is rated at 32 Ohm. I presume this is the source impedance then?

My power amp has an input impedance of 15 kOhm. So the maximum I can achieve both pc-preamp and then preamp-power amp is about 21 times each way, and this works out as an attenuator impedance of approx 700 Ohm:

32 x 21 = 672
15000/21 = 714

Why then, are most attenuators that are sold as kits rated at 10 kOhm, 20 kOhm, 50k, etc? Also, no-one seems to talk about anything below 10k. I'm nowhere near 10k.

Another problem I have is that because I (think I) need a relatively low total impedance I'm ending up with low resistance values for the quiet-end resistors, like 0.5 and 0.75 Ohm and such. The majority of these high-end metal film resistors I want to buy start at 10 Ohms, nothing lower. I dont want to start mixing and matching brands/specs, I'm far too novice for any of that.

Am I missing something? I must be, surely. Please feel free to pick holes and basically smash to pieces anything and everything that I've said which is complete rubbish. I'm new to all this, but keen to learn.

Thanks,
Leon
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Old 18th March 2011, 12:05 AM   #2
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Quote:
Originally Posted by The Boz View Post
Hi everyone,

First post!


Why then, are most attenuators that are sold as kits rated at 10 kOhm, 20 kOhm, 50k, etc? Also, no-one seems to talk about anything below 10k. I'm nowhere near 10k.


Thanks,
Leon
Your poweramp is the problem, 15k input is stupid!!! A tube preamp would even have trouble driving that.
The industrie standard is 47k-50K with most tube poweamps at 100k to 500k, I've even seen some at 1Mohm. ie. Rogue.
Talk to the designer of your poweramp to see if the input load resistor can be changed from 15k to 50k or more. Otherwise you'll need a buffer after the passive.

Cheers George
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Old 18th March 2011, 12:23 AM   #3
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No, what you have written is exactly what a beginner should have picked up from sensible reading of the forum.

Your maths is good.

The problem is the impedances of your source and load are not cooperating. This is often the case. Ideally what you would do is build the attenuator with the values you have calculated. If your source had been 1k and your amp 100k, you'd have been home free. These aren't unlikely numbers.

What you can do is compromise.

Pick a kit higher than the output impedance and lower than the input impedance (10k).

Or you can make the effort to build with your calculated values.

The reason that kits are available in 10, 20, 50k impedances is because they're easy to do.

The OTHER reason is that the relative impedances of source and sink in audio are of comparatively small significance just so long as you follow the smaller>bigger>bigger rule.

You can jigger around with this 'till the cows come home, you can put in buffer amplifiers, but this will just be more components which other people will be telling you to take out. My opinion:- go with the 10k.



w
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Old 18th March 2011, 02:35 AM   #4
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#2 on what wakibaki says- just go with 10Kohms. That's the most common value and will work fine. There's nothing unusual about a 15Kohm input impedance on a power amp. Yes, it's lower than you might like, but the nature of solid state power amp circuitry tends to give values far less than the higher numbers associated with ye olde tube amps.

I'm donning my fireproof underwear as I say this, but the 10X and/or 20X rules guarantee nothing, and you can often violate them by huge amounts. If the input impedance of the amp changes significantly with frequency (not a common problem) then you want to drive it from a much lower impedance, lest the response be affected. If the input impedance remains ruler flat with frequency (typical) then you can drive it with near anything. The attenuator steps may not be the same as unloaded, but that's of little consequence. Using a 10Kohm attenuator with a 15Kohm power amp will probably work and sound just fine.

Any solid state preamp should be able to drive a 10Kohm attenuator with no problem. Not so with tube preamps, so watch the capabilities of the particular circuit. Again, if the response isn't affected by loading, the attenuator value becomes less important. If the preamp is affected by loading, be careful if you build a low impedance attenuator. FWIW, I think I once built an attenuator of about 7.5Kohms, but it would be rare to see one below 10Kohms or above 100Kohms.
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Old 18th March 2011, 08:34 AM   #5
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Thanks for the replies guys.

So am I right in taking from this the following points:
1. The 10x rule is a rule of thumb, and not necessarily set in stone.
2. My problems are arising because of a low input impedance on my power amp.
3. I could still go ahead as I am, and build a custom 700 Ohm att if I wanted to.

Hmmmm, what to do. I know people have suggested I just take a 10 kOhm kit, but I have absolutely no trouble calculating resistor values and designing a custom 700 Ohmer.

Would you say a 10k is a bit of a gamble with that 15k input on the power amp? If there's any gamble whatsoever I think I'll stick with my original plan and build a 700 Ohm. If not, I'll go with a 10k.

Can I run into any trouble with a very low impedance attenuator?

Cheers,
Leon
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Old 18th March 2011, 01:42 PM   #6
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Why don't you test different impedances with resistors or pots before choosing an attenuator value?
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Old 18th March 2011, 02:04 PM   #7
DF96 is offline DF96  England
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There is another potential snag. The input impedance of the amp might not change much with frequency, but it could change with level instead. If so, this would give distortion.

You said that the output impedance of your source is 32 ohms. Is this so, or is it designed to drive 32 ohms? Either could be true, and people often get the two confused. Specifications will often say "output impedance X", when what they actually mean is "required load impedance >= X".
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Old 18th March 2011, 02:07 PM   #8
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Hi,
the output impedance of the attenuator is not the quoted attenuator impedance. It varies, a lot.

The maximum output impedance is {Rs + attenuator resistance} / 4

A 10k pot and a source of 200r will have an output impedance that varies from 0r0 to 2550r.

It would be nice if that 10k could interface with 20times that maximum i.e. 50K, but 15k will do. It is only ~5.88times, but it will work.
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Old 18th March 2011, 02:10 PM   #9
maxw is offline maxw  United Kingdom
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All the more reason to just use a buffer or linestage after the attenuator
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Old 18th March 2011, 02:32 PM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by maxw View Post
All the more reason to just use a buffer or linestage after the attenuator
Max,
thanks for listening.
Someone who wants the buffer at the source and not at the receiver.
When will the rest learn?
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