Crest CA9 distortion problem - advice needed.

Discussion in 'Amps and Cabs [BG]' started by secretdonkey, Jan 7, 2005.

  1. secretdonkey


    Oct 9, 2002
    Austin, TX
    Okay, I've gone down a few blind alleys thinking I had this problem identified, but now I think I've got my finger on it...

    My Crest CA9, bought used off of eBay, has some bad, ugly distortion problems when the amp's attenuators are turned down low. With the attenuators cranked up, the problem disappears.

    I know that it is typical practice to run amp attenuators wide open all of the time, but my Alembic F-1X preamp seems to change in tone depending on its volume setting (a quality inherent to its design, if I understand correctly), so I'd like the opportunity to use the amp's attenuators to control volume.

    It took me awhile to figure this out because when I encountered distortion with the attenuators down, the last thing I was inclined to do was to turn the attenuators up, and risk damaging my speakers!

    Any of you amp gurus have ideas as to whether there's a problem with the Crest amp or whether I might be dealing with an unavoidable issue?

  2. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Most power amps actually have a gain stage before the volume control. There isn't any control over that gain, so you could simply be distorting it (clipping).

    When the volume control is way down, there may be 20 dB or more attenuation. That's a loss of 10 to 1.

    That input stage typically can only put out about 12V peak, and may have a gain of 2. So if the pre exceeds 6V peak output, you may distort the input stage of the power amp.

    With power amp turned up more, you probably also don't crank the pre as less output, less distortion/no distortion.

    If so, it is "inherent" but not unavoidable....just turn down far enough to stay out of trouble with it, and hopefully that's far enough.

    If you still need to deal with more output from the pre for your sound, making a simple attenuator isn't too difficult. Doesn't the pre have an output volume control?
  3. secretdonkey


    Oct 9, 2002
    Austin, TX
    Wow, thanks!

    Forgive me, because I'm sooo not technically oriented, but I am keen on understanding the ramifications here. I have found that at a given volume setting on my preamp (around 7) I can easily coax distortion with the amp attenuators down low (yep, around -20db), but the sound will be totally clean with the preamp at 7 and the attenuators opened up fully. I think that your explanation accounts for this phenomenon, but I want to double check. Am I correct in my thinking?

    The Alembic pre has only a single volume control. The Crest amp *does* have input sensitivity jumpers inside, though they have to be set by a factory authorized shop.

    Thanks again so much for the input, Jerrold! :hyper:
  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    If you DON'T change preamp gain, just turn up the power amp, and distortion goes away, that's a bit odder. Could be a distortion, maybe crossover notch, maybe just scratchy attenuators.

    The jumpers set for minimum gain might help.

    I figured you were turning the pre up when the attenuators were down farther. That would be as I suggested.

    Maybe you need the "Crest Insider" instead of me!
  5. NJL


    Apr 12, 2002
    San Antonio
    yeah, Jerrold is the bomb!!


  6. secretdonkey


    Oct 9, 2002
    Austin, TX
    Well, getting to the bottom of this is going to be fun...

    I've called Crest service a few times and gotten voicemail every time, so *that's* going to be a phone tag proposition, and the closest authorized repair center (100 miles away) took 5 min. on hold to verify that they were "maybe" an authorized repair center and to call back on Monday.


    Thanks again, Jerrold!
  7. emjazz

    emjazz Supporting Member

    Feb 23, 2003
    Boston, MA
    If you email one of the tech guys listed on the Crest site than they'll get right back to you. I've dealt with them before and when you figure out how to get them than their service is fabulous.