acme bass

Discussion of Stewart Amplifiers, Speaker Impedances, and Power Conditioners

Dear Andy....

Thank you for promptly getting yet another replacement woofer to me, and here is another fried one for you to have re-coned. Thanks also for discussing what may have predisposed me to blow these speakers, other than just too much power/volume from my Stewart 2.1 power amp.

It may interest you to know that after we last spoke, I contacted Stewart Audio to ask about the fact that in the moments leading up to my (most recent episode of) blowing a woofer, that the amp had shut down/cut off twice. In the owner’s manual for my amp, it describes the shut down feature thusly:

Absolute Protection:

Stewart Audio Inc. power amplifiers employ circuits which fully protect the amplifier while avoiding any sonic compromises. First, a unique circuit monitors the current draw from the power supply. The amplifier’s output is not affected in any way until the load impedance drops below 1/2 ohm, indicating the presence of a short. Under these conditions the power supply will simply shut down until the short is removed, at which point the power supply will slowly ramp up to full power, protecting the load from a surge of high power. In the event of an amplifier malfunction this circuit will also protect the speakers, since the power supply is immediately disconnected from the amplifier. This operation is provided independently for each channel; therefore a fault condition in one channel will not shut down the other.

“Most other amplifier designs require the presence of relatively high levels of signal in order to detect a short circuit on the output. At low levels these amplifiers will continue to drive a short, causing rapid heat buildup, thermal runaway, and finally amplifier failure. The current sensing circuitry in all Stewart World Series amplifiers are capable of detecting a shorted output even at low program levels, increasing its reliability under conditions which would cause other amplifiers to fail.”

The rep at Stewart also informed me that if a speaker is “dimed”, to the point where it moves to the limit of its possible excursion, that this can be seen by the amp as the type of brief impedance drop/short, that would cause the amp to shut down. Most importantly, I was told that because I had my amp plugged into a Furman Power Conditioner that there may have not been enough current available on demand to meet the needs of this amp’s “High-Frequency Switch Mode Power Supply”. Because the amp has neither large capacitors nor toroidal transformers, the Furman may well have acted as a bottleneck to the flow of needed current. And I was told that when this happens the power output of the amp drops significantly, and it clips at a lower power output. To avoid this, it was recommended that I only plug the Stewart directly into a power outlet, instead of into the power conditioner. I was also told that many Stewart owners before me have asked for advice about this same sort of trouble, to the point where Stewart is planning to include a section about this in the next revision of the owner’s manual.

In hindsight, I believe this current bottleneck is a big part of the trouble I have been having with blown speakers. I have noticed in the past that on loud peaks in my playing, the light within the power switch of the Furman would dim momentarily (like a cheesy disco light pulsing to the beat). Unfortunately, I was not intuitive enough to think about why that was happening and how it could affect the function of my amp. So anyway, I share this with you so that in the future, if you have customers that experience troubles similar to mine, you can relay my experience and recommend that they plug their Stewart (or QSC) amps directly into a power outlet (a 20 amp circuit is recommended) and not into (through) a power conditioner.

Thanks again and best wishes.


Andy’s Reply:

Hi Curt,

Several things.  

1. A speaker's instantaneous impedance can't fall below its DC resistance, 2.5 ohms or so in the case of a 4 ohm Acme unit. Nowhere near the .5-ohm threshold. 

2. The points relating to the power conditioner make perfect sense to me. I can't imagine any decent power supply in an amp needing such a device. I think he gave you good advice.

In fact, although my customers nearly universally love the lightweight Stewarts, Carvers, QSC's, Acoustic Images, and to a lesser degree the Carvins, most people also agree that there's no real substitute for a huge transformer and capacitors when high current is needed. I don't believe the lightweight power supplies can be made quite as intrepid as the heavy ones yet. It's a matter of massive energy storage or enormous current from the mains. You have to have one or the other. I suggest the Stewart is the last amp you'd want to "choke" on the AC side.

I hope his advice has helped to make your life easier.

By the way, we don't recone the returned woofers.



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