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Old 31st October 2006, 03:01 AM   #1
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Default UcD volume control?

I'm building an amp for my dad - his equipment is limited. I need to add a volume control to one of my UcD180 power amps but I have no idea how to do it.

Stupid, I know, but could anybody give me a clue as to how to proceed? I have seen the DACT attenuators but don't know if that is what I need or how to wire one.

He's running an HK integrated receiver with a line out but I don't think it's variable.

I'd be grateful for a direction here....

Regards,
Tom
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Old 31st October 2006, 02:09 PM   #2
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Default easy solution

fundamental issue: you need to vary the maximum voltage input swing to the UCD modules.

If your power supply is designed correctly, 2V at the UCD inputs will max the output.

Hence you need to know how much maximum voltage swing the line out from your dad's amp produces. If its wired straight to an input, you need to know how much swing the CD player etc produces that is input to the amp.

Once you know the max swing coming from the HK amp (you should ideally measure this with an oscilloscope), you can design an adjustable resistor-divider network which nets an output voltage adjustable from 0V to 2V.

this is essentially what a dact does, it uses various resistor combinations to lower a given input voltage to the level you want.

if your dont know how this works, look up "voltage divider" in google, and get yourself some education

much easier than a dact is a simple potentiometer (variable resistor).

what the heck, just look here:
http://sound.westhost.com/vda.htm
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Old 31st October 2006, 07:28 PM   #3
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Default Thanks...

Dad's amp is in Colorado but I'm googling for specs.

Man that DACT attenuator seems expensive but I'm sure a $3 radio shack pot would affect the sound adversely.

Thanks again for the heads-up on volume control!

[edit] BTW thanks for the link - I read it and understand a bit more now - I have other pages on that site bookmarked but you pointed me to the right one...

Regards,
Tom
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Old 31st October 2006, 07:45 PM   #4
Bgt is offline Bgt  Netherlands
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http://www.diyfidelity.com.au/produc...products_id=32
cheaper pot, and very nice also. Have DACT also but cannot hear the difference. The DACT has direct connection on highest position, The Aussie 1 has a bit of att. at highest position.

In my passive: crosstalk >-100Db@20khz
Click the image to open in full size.



Click the image to open in full size.
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Kind regards,
Bert
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Old 31st October 2006, 08:22 PM   #5
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Default Lovely...

Thanks!

That's the solution I was looking for I reckon. Hope my fingers don't wobble too much :-)...

Nice job on the pre BTW.

Regards,
Tom
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Old 1st November 2006, 07:00 AM   #6
Bgt is offline Bgt  Netherlands
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you can order them pre assembled Like I did.
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Bert
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Old 1st November 2006, 11:18 AM   #7
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Default Bqt - What value did you get?

What value did you buy, 100K? I've seen most buying 100K but others have installed different values.

Regards,
Tom
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Old 1st November 2006, 01:25 PM   #8
Bgt is offline Bgt  Netherlands
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10k, its a passive preamp. My Dac is 100 ohm out. 10k input impedance is no problem anyway as long as there are no caps. in the signal path. Only high value biased caps. will be ok. But no 0V on caps.!
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Bert
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Old 6th November 2006, 07:48 PM   #9
Bgt is offline Bgt  Netherlands
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I've tested the passive pre against my Rotel C162 and Nad 990 pre and find it more natural sounding. Voice is awsome/clean. And absolutely not less bass. As long as everything is DC coupled you won't have less bass. The nicest attr. of a passive is the freedom of any compression/disturbance of PSU's.
Click the image to open in full size.
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Bert
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Old 7th November 2006, 01:30 PM   #10
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Default Looks fabulous!

Looks fabulous! Nice work all the way around.

I'm a big fan of NAD stuff - nice sound and good bang for the buck.

This winter I hope to have time to build a passive - just to darn busy now to do much more than finish the two UcD180's I have in process and get the 2641's done and veneered. Whew...

Thanks for the pic - I'm envious and inspired all at once...

Regards,
Tom
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