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Old 3rd January 2009, 10:01 AM   #1
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Default Lower the gain on a Pass X250.5?

Hello,

I would like to know if there is a way to lower the gain on a Pass X250.5 amplifier that would not harm the sonic character of the amplifier.

Thank you,

Alan
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Old 3rd January 2009, 06:02 PM   #2
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How much did you want to reduce it, and what input impedance
can you tolerate?

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Old 7th January 2009, 02:28 PM   #3
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Hi,

I would like to get more range out of my preamp volume control, so I imagine I would like to reduce the gain by about 6 dB. I am using a solid state preamp with very low output impedance and a very beefy power supply, so I imagine it will tolerate quite low input impedance.

Thank you for any information you can offer.

Alan
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Old 7th January 2009, 02:54 PM   #4
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Isn't it easier to increase the attenuation at the preamp input? This will increase headroom as well.
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Old 7th January 2009, 03:11 PM   #5
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Quote:
Originally posted by analog_sa
Isn't it easier to increase the attenuation at the preamp input? This will increase headroom as well.

I was going to suggest this as well. If you are using a CD as a source, then no pre-amplification is probably needed. An active buffer with little to no gain would suffice. Reducing the gain of the pre-amp would would be preferred.

What pre-amp are you using?

-David
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Old 7th January 2009, 10:00 PM   #6
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Quote:
Originally posted by dw8083



What pre-amp are you using?

-David
Hi,

I am using the H2O Audio Fire preamp ( http://www.iceh2oaudio.com/index_htm.html ).

I have played around with using no preamp with digital attenuation but no longer have the ability to do so with my current setup. I also have used a passive attenuator, but I have a tube based DAC with a rather high output impedance, and the results didn't fare well with the passive attenuator. It sounds better with the preamp in the mix in terms of dynamics.

The sound of the X250.5 and the Fire preamp is very nice together, but has the problem of excessive gain. The designer of the preamp mentioned that lowering the gain on the preamp might not be the best alternative and thought that lowering the gain of the amp might be better. Any recommendations are welcome.

Thank you,
Alan
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Old 7th January 2009, 10:22 PM   #7
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Quote:
Originally posted by aljordan


Any recommendations are welcome.

Thank you,
Alan

make B1 , any iteration ;

or Lightspeed + buffer, or whatever .
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Old 7th January 2009, 11:59 PM   #8
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I could not find any indication as to the gain of the preamp on the website; perhaps I missed something. They seem more interested in telling you what the thing weighs and how big the caps are than giving information that might be relevant to this problem. There might be more to this than first meets the eye.
I confess that I am always amused by designers who want the other guy's equipment modified, not theirs.
A fellow I know had a similar problem about a year ago. I ended up getting involved late in the process, after the owner of the preamp had made a rather ham-handed attempt to convert his balance control into a "fine" volume to counterbalance the "coarse" regular one. The designer made an offhand comment during the course of a phone call that my acquaintance was not the first person to want better control of his volume. (Switched resistive pot.) You'd think that he might take the hint and recalculate his volume steps, but...
Might I suggest a somewhat simpler solution? Make a simple resistive divider with a female RCA on one end and a male RCA on the other. Place this at the input to the amp and connect the interconnect to the other end. Two benefits: First, you're not modifying anybody's equipment. Second, if you're not sure how much attenuation you want, you're likely to end up soldering and desoldering resistors as you hunt for the right values. It is by far the preferable option to tear up some RCA plugs rather than the preamp and/or amp.
Besides, you might end up wanting to get a Pass preamp later and then you'd have to un-modify the amp (again running the risk of damaging something expensive) to get the gain back.

Grey
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Old 8th January 2009, 01:56 AM   #9
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Grey stole my thunder and beat me to the punch. No commercial manufacturer wants his stuff tampered with. This preamp looks like it's singled ended output rather than balanced. If you want to stick with unbalanced RCA's, then consider getting or building a First Watt B-1. Passive attenuation alone does not seem to meet anticipated expectations. Nelson talked about this in his excellent article on the B-1. If you're handy enough to change gain resistors, you might try building a B-1, it only has a half dozen or so parts per channel.

A couple of years ago I built a low gain BOSOZ preamp. It provides balanced XLR inputs and outputs. Just something else to consider. The BOSOZ build plans are available on www.passdiy.com.

-David
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Old 8th January 2009, 12:32 PM   #10
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Thanks to all for your suggestions. I think I'll try the resistive divider as a quick fix. Building the B1 buffer looks like a fun project so I'll probably try that soon.

Alan
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