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Can Someone Explain SWR's Limiter Circuit?

Discussion in 'Amps and Cabs [BG]' started by Bongolation, Sep 7, 2005.

  1. Bongolation


    Nov 9, 2001
    No Bogus Endorsements
    For the life of me, I can't make heads or tails of the explanation in the SWR 750X manual (SWR's copywriter baffles me every time, like magic; it's amazing).

    Cut to the chase: How could the limiter LED come on at bedroom levels, and how could it come on without the preamp clip coming on? Is it limiting the input from the final stage of the preamp into the power stage irrespective of power stage output? I put the limiter knob at about 8 or 9 o'clock and every time I dig in on a Precision, the LED comes on...but the preamp gain is low enough not to trip the preamp clipping LED.

    Is this normal? It seems like it might be, but the docs have me confused.

    Bonus question: Is SWR's "OverDrive" effect supposed to sound pretty lame and fuzz-bassy?

    As always, thanks for any help.
  2. Not sure about the limiter, but yes, the distortion is super weak on a 750X.
  3. Bongolation


    Nov 9, 2001
    No Bogus Endorsements
    Goes with the SubWave doodah, I guess. :rolleyes:

    Still, this limiter thing is starting to worry me a bit. I don't ever get the limiter light to come on on other amps with non-adjustable limiters, and this is at super-low volumes, remember.
  4. fast slapper

    fast slapper

    Dec 11, 2001
    Fresno, CA
    It's normal.

    The limiter has nothing to do with the power stage. It acts like a compressor not a clip protection circuit.
  5. I've heard that if you change the tube to a higher quality tube, you can get a great "tubey" clean sound. But this does basically eliminate the use of it as a distortion boost. Something to consider if you think it sounds like a toy when it's engaged.
  6. Bongolation


    Nov 9, 2001
    No Bogus Endorsements
    OK, so this comes from someone who has the 750X?

    If so, cool, but the docs (seem to) say exactly the contrary:
    Variable Limiter Control

    The 750X's soft knee-type limiter is there to help you protect your power amp from harmful states of operation. The circuit is located after the Master Volume and before the power amplifier, so it’s driven by the Master Volume control. Its threshold (starting point) is preset by the factory so that you can get maximum overall apparent volume without unduly overdriving (or “clipping”) the power amplifier—which, over time, can eventually damage the unit’s internal circuitry, as well as damage speakers connected to your amp.

    Using the control is simple. Turning the Limiter Control clockwise (toward “MAX”) increases the amount of limiting on your signal, while turning it counter-clockwise decreases the amount. When trying to find the highest possible level of power amp operation under clipping, simply use the Master Volume and Limiter Controls together to find: a) The highest setting possible on the Master Volume; and b) The lowest possible setting of the Limiter… all while hearing no power amp distortion whatsoever.
    Here's an old thread that's utterly confusing.
  7. fretlessrock

    fretlessrock Supporting Member

    Aug 8, 2002
    A) You can assume that the description is correct. In that case you are setting the limit control threshold too low. That means that you are turning it up too much. Because the more you turn it up the lower the threshold of the limiter is set = more limiting. You may be experiencing a case of "don't do that". If it bugs you, turn it down. It is there for protection of the PA section, not as an effect. So if you can hear it working, then you are using too much of it. And if you are playing at low levels, just turn it all the way down.

    B) The limiter is actually before the Master and the manual is wrong. In that case you could turn the master off and the limit light would still come on when you dig in. Try it.
  8. Bongolation


    Nov 9, 2001
    No Bogus Endorsements
    Well, (B) is almost what's happening, as it's coming on with as low an audio output as the amp is capable of producing without being dead off.

    In that case, it seems the manual would be wrong, which would be pretty weird...except that this identical description has been used with totally different heads for years in different SWR manuals.

    Or...the manual's right and something's definitely wrong with the amp, which is what I'm trying to determine.

    I know that a lot of people have the 750X here, so I'd be very grateful if one would straighten me out on this. :meh:
  9. Bongolation


    Nov 9, 2001
    No Bogus Endorsements
    OK, I just went in and tried it with the Master turned completely off.

    The limiter LED light still comes on when I dig in, exactly as in (B), above.

    OK, so...normal or broken?
  10. KJung

    KJung Supporting Member

    As other's have said, that's perfect then. If your preamp gain clip light is not coming on (or only when you really dig in) then just turn the limiter circuit down so that it only lights up when you are playing very loud passages.... for the SWR circuit (assuming the 750 is similar to my old 900), the level might be just barely on... maybe at 7 o'clock). At this level, it will just take the real edge off your loudest notes, and will also reduce the power amp clip light from coming on. Also, if you 'post' clip light is coming on a lot, you might just have too much low end dialed in, which really sucks up the power.

    PS.... you should be able to set the appropriate limiter level with the master volume off... this is actually the best way to do it at the start (i.e., totally focus on what the preamp and limiter lights are telling you)... then slowly bring up the volume to make sure it sounds good to you (i.e., not overly compressed, etc.).
  11. Bongolation


    Nov 9, 2001
    No Bogus Endorsements
    There is no power amp clip light. Presumably (according to the docs), the 750X's Limiter LED is effectively the power amp clip light. The Limiter is post-master (supposedly).

    That's what makes absolutely no sense here.

    How could SWR/FMIC keep an error this gross in their documentation for so long? It's still in the downloaded manual on the new and updated site.

    Otherwise, it seems to act as you describe. :confused:
  12. i have an old SWR studiopro220 that seems to have the same limiterknob. there are 3 different "cliplights" on it.
    nr 1 responds on high gain settings (reducing gain)
    nr 2 on limiter settings (using it to flash when i digin)
    nr 3 on power-amp clipping (reducing bass or mastervolume)

    i´d say the limiter works before the mastervolume as it flashes with the volume all the way down.
    with higher gain-settings the limiter starts working pretty fast, and with low gain settings you need to put the limiter really high to start working.
  13. KJung

    KJung Supporting Member

    Interesting... so that's a different circuit than on the original 'pro' series (500 and 900). I know some amps have 'protection' circuitry that limits output power, so maybe that's what's going on. Since it does seem to be a different circuit than the 900 limiter that I'm familiar with, I will drop out of the thread... good luck!
  14. Bongolation


    Nov 9, 2001
    No Bogus Endorsements
    LED # 3 is not on this amp. LED #2 is supposed to be doing this.

    Again, normal and SWR's docs are completely wrong or busted and I'm stuffed?

    Bottom line: Private party deal and I have to move fast if this is a busted amp.
  15. boogiebass


    Aug 16, 2000
    There are a number of differences between the old pro series SWR's (excellent amps, IMO) and the new Fender-owned stuff...unfortunately! :(
  16. syciprider

    syciprider Banned

    May 27, 2005
    Inland Empire
    This puzzled me with my WM210C also. But I also observed that with my preamp gain @ 1300 to 1400, and the limiter knob @ 1000 to 1100 my limiter light doesn't come on. So this is where I keep it when playing in the house.
  17. geshel


    Oct 2, 2001
    What you said contraticts the manual. They said that the threshold is fixed, and pre-set at the factory. The "Limiter" control only changes the ratio (how much the signal is compressed above the threshold).

    Given the manual's description, it seems wrong that the light would come on at bedroom levels, no matter what the setting of the Limiter knob.
  18. Bongolation


    Nov 9, 2001
    No Bogus Endorsements
    C'mon, SOMEbody here must have one of these amps!

    At this point, I'm thinking the manual is just wrong, but I need some confirmation.

    I'd check with SWR, but they have never, ever even responded to a customer-assistance e-mail from me.

    They've taken down the SWR Forum from the old site when they "upgraded" recently...so, I don't have anyone else to ask about this.
  19. I also have an old SWR SM900 and I reeeally think that this is the same type of circuit (and it's position in the signal chain) that you have in your SWR 750X. I personally think the manual is wrong.

    Now, after looking into the SWR site:

    "In most SWR models, the limiter circuit is located after the Master Volume and before the power amplifier section, thus being driven by the Master Volume Control. The Limiter threshold (starting point) is preset by the factory so that the user can get maximum overall apparent volume without overdriving (clipping) the power amplifier."

    I got this from the FAQ on the web site. MOST SWR Models - like my WM12 combo. That limiter in that amp is preset and I can only cause the limiter light to light up when I turn up the overall volume (past 12 o'clock noon) with the master volume control and really thump.

    I think that they did a copy and paste with the manuals. I personally think that your Limiter Control is before your master Volume control. If you want the limiter off, then turn it off. If you want the limiter to just limit the top peaks of your sound then set it so that it occasionally flashes (this can be done with the Master Volume in any position including off). And for sure, if it sounds good then don't worry about it. I think that you've got a great amp and a s long as you're listening to what's coming out of the speakers then you'll be fine.

    What speakers are you running? What basses are you using? How about FXs? Could you fill out your profile please?

    I run my SWR SM900 (800 watts in bridge mono @ 4ohms) into 300 watt Yorkville 2x10 and 1x15 cabs. I usually run the limiter at 12o'clock but lately I've been running it off and giving it the full dynamic range of my passive OLP Music Man copy 5 string. All controls set at 12o'clock including the Gain & Aural Enhancer, Bass EQ @ 2-3o'clock and master usually at 11 - 12 o'clock in a three piece rock band with a screamming 4x12 Marshall cab guitar player. Try running your limiter off for awhile and use your ears and fingers to go from there.

    Hmmm,... sorry that last bit was sort off of topic. I think the manual is wrong and that sucks for my SWR loyalty. :meh:
  20. mad.mick


    Nov 21, 2004
    I have the 750x. Great amp. Limiting is influenced by whatever is in or before the effects loop, but not by the master volume knob, and since everything else in this amp's preamp section is before the effects loop, the only possibility as to where the limiter is placed within the signal chain is last before the master volume. Which figures, since the 750x is not a combo, and it's possible to attach cabs to this amp, that may not be able to cope with it's output. That way you can to use the limiter to protect those cabs from being pumped to death without first having to turn the master volume to eleven. Combo amps: another story. There the designer of the amp knows well the maximum output, at which the speaker would recieve damage, so setting a limiter to protect the speaker can be (and is often) done in the factory. BTW, the preamp clip LED has got nothing to do with the limiting, nor should it have to. The Limiter is there to protect your power amp and your speakers, not your preamp. Damaging your preamp with an input signal would most likely require feeding it with the output of a power amp, or by plugging it's output directly to it's input and thereby creating a feedback loop, or plugging it directly to an AC outlet. You get the picture. Preamps are typically destroyed by mechanical force, like dropping it onto the floor, or by moist, like dropping a beer into the preamp, but usually not by an input signal. The preamp clip LED instead should help you avoid distortion that is sometimes inaudible, e.g on a loud stage, or if the bass player is deaf, like John Entwistle.

    Bonus answer: the sound of the distortion depends heavily on your settings, your bass signal itself, your cab, and whatever you plug into the effects loop. I have a Tech21 SansAmp RBI in there, and without it, what I get is mostly fuzz, but with the same setting and the RBI engaged, it's this grunt, that you almost can't recognize as distortion, but pushes everything else off the stage. Quite nice, actually.