PassLabs X1 and UCD400 input sensitivity

m.parigi · 21st February 2006, 09:54 AM

I've just bought a Pass Labs X1 preamp and I've hooked it up to my DIY UCD-400 power amp via balanced connectors.

WOW!!!!!! What a sound! It has been a HUGE improvement compared to my previous Krell KSL preamp. The image is rock solid and I'm discovering on old CD's lots of details I didn't even think existed.
Thanks, Mr. Pass!!!

Let's get now to the downsides.
Simply put, I think that my chain has simply got too much gain for the UCD 400 input sensitivity.
As soon as I go from volume "00" to volume "01" on the X1 there is a very noticeable hiss coming from the speakers (no hum, just a hiss). This seems to be largely independent from the volume level, in that it somewhat increases with the volume setting, but not that much.
Oddly, then, each increase or decrease of the volume generates a very audible "thump" from the speakers. I'm not sure whether this is normal either.

Finally, regardless of the source (CD, tuner, cassette, MP3, etc.), I get a VERY loud playback from my speakers with the volume reading only "15" - over a theoretical maximum of 60!!!

This happens with the X1 gain to "low", but apparently there's still too much voltage going into the UCD's.

Assuming the X1 is working correctly (are there any owners on this board that can tell me if its behavior is normal?), is there a way to select a lower input sensitivity on the UCD or, alternatively, how can I attenuate the input signal without affecting the other parameters of the UCD?

jam · 21st February 2006, 03:33 PM

Congratulations! you own one of the best preamps in the world - no kidding.

My solution to your problem would to build a attenuator at the input of your power amp. a few resistors configured as a voltage diveder should do the trick.

Regards,

Jam

lucpes · 21st February 2006, 04:22 PM

Remove the 560ohm resistor near the opamps on the UcD400. This will set the overall gain to 4.55 voltage or 10,something dB. Works like a charm for me

m.parigi · 22nd February 2006, 07:24 AM

I'd like to be able to adjust the input sensitivity "on the fly", so that when we have some listening sessions at some friend's house I can match my UCD power amp to their systems.

At the same time I wouldn't like the input signal to go permanently through a switch, it seems to me that this would defy the purpose of having such a wonderful preamp in the chain!

Do you think that a voltage divider on the inputs could be inserted by "switching on" the connection to signal ground through a resistor?

I'm also concerned about altering the input impedance seen by the preamp and/or the UCD modules. Can this alter the sonic performances of either one?

21st February 2006, 09:54 AM	#1
m.parigi diyAudio Member Join Date: Jan 2002 Location: Italy	PassLabs X1 and UCD400 input sensitivity I've just bought a Pass Labs X1 preamp and I've hooked it up to my DIY UCD-400 power amp via balanced connectors. WOW!!!!!! What a sound! It has been a HUGE improvement compared to my previous Krell KSL preamp. The image is rock solid and I'm discovering on old CD's lots of details I didn't even think existed. Thanks, Mr. Pass!!! Let's get now to the downsides. Simply put, I think that my chain has simply got too much gain for the UCD 400 input sensitivity. As soon as I go from volume "00" to volume "01" on the X1 there is a very noticeable hiss coming from the speakers (no hum, just a hiss). This seems to be largely independent from the volume level, in that it somewhat increases with the volume setting, but not that much. Oddly, then, each increase or decrease of the volume generates a very audible "thump" from the speakers. I'm not sure whether this is normal either. Finally, regardless of the source (CD, tuner, cassette, MP3, etc.), I get a VERY loud playback from my speakers with the volume reading only "15" - over a theoretical maximum of 60!!! This happens with the X1 gain to "low", but apparently there's still too much voltage going into the UCD's. Assuming the X1 is working correctly (are there any owners on this board that can tell me if its behavior is normal?), is there a way to select a lower input sensitivity on the UCD or, alternatively, how can I attenuate the input signal without affecting the other parameters of the UCD?

21st February 2006, 04:22 PM	#3
lucpes diyAudio Member Join Date: Dec 2003 Location: Iasi	Remove the 560ohm resistor near the opamps on the UcD400. This will set the overall gain to 4.55 voltage or 10,something dB. Works like a charm for me __________________ Deep down inside.

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21st February 2006, 03:33 PM	#2
jam diyAudio Member Join Date: May 2001 Location: Charlotte,NC,USA	Congratulations! you own one of the best preamps in the world - no kidding. My solution to your problem would to build a attenuator at the input of your power amp. a few resistors configured as a voltage diveder should do the trick. Regards, Jam

22nd February 2006, 07:24 AM	#4
m.parigi diyAudio Member Join Date: Jan 2002 Location: Italy	I'd like to be able to adjust the input sensitivity "on the fly", so that when we have some listening sessions at some friend's house I can match my UCD power amp to their systems. At the same time I wouldn't like the input signal to go permanently through a switch, it seems to me that this would defy the purpose of having such a wonderful preamp in the chain! Do you think that a voltage divider on the inputs could be inserted by "switching on" the connection to signal ground through a resistor? I'm also concerned about altering the input impedance seen by the preamp and/or the UCD modules. Can this alter the sonic performances of either one?

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