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Old 5th August 2004, 06:19 AM   #1
mjh77 is offline mjh77  Finland
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Default passive preamp

Hi all!

I found recently a couple of passive preamp projects using 10kohm DACT CT2 stepped attenuator. Both of those seemed to be very successfull and got me interested. Now I'd like to hear comments about good/bad sides of passive preamplification and especially if someone has experience with DACT CT2.

I know that passive preamp is a lossy device but it's sooo much more simpler to make by a newbie like me than any solid state version that it might compensate for the lossiness...

I hope you can help me with this one, I really appreciate all the comments.

Cheers,
Mikko
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Old 5th August 2004, 07:10 AM   #2
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Default Re: passive preamp

I know that passive preamp is a lossy device but it's sooo much more simpler to make by a newbie like me than any solid state version that it might compensate for the lossiness...

------------------------------------------------------------------------------------They are fine if you have input impedance of 50 kOhm at the amp.
The DAXCT I find to have too few steps. Ideally this should be 60 or so to set the right volume.

Don't use too long a cable, 1.5m low capacitan is fine.


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Old 5th August 2004, 07:22 AM   #3
dhaen is offline dhaen  Europe
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For such a simple thing, there are a surprising number of things to get right for it to work well. Here are some that I consider the most important:

* There should be sufficient system gain.

* It's impedance should be high compared with the source impedance.

* It's impedance should be low compared with the load impedance.

* The cables to and from, should be short.
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Old 5th August 2004, 08:43 AM   #4
mjh77 is offline mjh77  Finland
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Oookay, I guess the only problem will be the power amps imput impedance. I'll have to think about that issue...

Thanks for the info everyone.
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Old 14th August 2004, 10:06 PM   #5
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Hi,

If you don't need any gain, have the attenuator followed by a buffer stage.

That should take care of the Zout, will drive relatively long and capacitive cable and interface much better with both the source and the Zin of the amplifier.

The DACT are series type devices using a 24 step Elma switch.
There are better ways of doing this as in the case of the DACT you'll always have a number of resistors in series at most positions of the attenuator.

For inspiration on alternative ways check the Goldpoint site at:


GOLDPOINT

Cheers,
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