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Author Topic: Amplifiers Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 03-31-2005 01:48 PM                     Ferdinando the USA is 3.353 meters deep I think inches is what you where noting. QSC/ISA amplifiers are what should be considered as a replacement for the USA series....easy to repair in the field.  |  IP: Logged Ferdinando Innocenti Film Handler Posts: 79 From: Genova / Italy Registered: Jun 2004 posted 03-31-2005 01:59 PM                     Sorry, Richard, you're absolutely right  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 04-01-2005 12:28 PM                     Can a 4 ohm load be driven with a USA amp in bridged-mono mode. The brochure for these amps does not give a power rating for less than 8 ohm bridged-mono.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-01-2005 04:49 PM                     The USA (and series 1 amps) really didn't do 2-ohm loads very well. When bridging an amp you are effectively halving the impedance it sees so a 4-ohm bridged mono is like having each half of the amp run into a 2-ohm load....it will get quite hot. Again, it isn't something I would recommend. I know a lot of people that did run their 1400 amps into 2-Ohms and I can tell you from the bench...they didn't like that very much...they ran very hot didn't not gain you as much power as you would have thought. The current amps seem to do okay into 2-Ohms (4-Ohm bridged) though if you look, amp manufacturers like to cheat things...the bridge specs are typically only for a 1KHz tone and often at 1% distortion (audible). Furthermore, if an amp can't go double the wattage when moving to half the impedance, the spec you are reading is probably better than you are going to actually realize....unless you like listening to just one tone with a bit of audible distortion. I personally, have not run across and current shallow amps that I particularly like. I wish QSC still had a shallow amp line. [ 04-01-2005, 06:59 PM: Message edited by: Steve Guttag ]  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 04-01-2005 08:23 PM                     quote: Steve Guttag the bridge specs are typically only for a 1KHz tone and often at 1% distortion (audible). That one percent distortion factor is really neglageble and your speakers usually run in the high single to double digit percent area anyway. A 1% max. distortion factor in many amps will really simply not matter one bit if the amplifier is producing even order harmonics. Also the biggest factor about amplifier distortion that is the most overlooked aspect is what harmonic the amplifier is really generating... even or odd. Is it primarily second or is it primarily third harmonic in nature.... The ear perceves both very differently and differnt people are typically more sensitive to one than the other. Some people may actually prefer an amp with primarily even harmonic distortion but it can also be the other way around, but experts generally agree that even harmonic distortion is less distracting to the average human ear. The only way to tell what an amp is really doing is by connecting a distortion analyzers distortion by-product output into a spectrum analyzer and having a look. One thing is for sure. Bi-polor based amps generally tend to run with odd order harmomics while MOS-FET based amps generally run with even order harmonics. Which sounds better to you is really purely a matter of taste. BTW: Speakers also behave and can generate high amnounts of either even or odd harminics and can somewhat be designed to primarily prodice either type. Mark  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-01-2005 11:54 PM                     Mark, While you were jumping on the 1% thing...to which I don't know anyone that would not hear 1% THD...the key I was pointing out that the spec was using a 1KHz tone and looking for 1% THD...it is a silly spec. A 20-20KHz spec that has less than .5% is much more valuable since 20-20K is the effective range of human hearing and what is likely to be played through the amp at any one instant in time and a distortion of less than .5% THD has been tested enough to show that all but the best golden ears can't hear it. So you see an amp that has .05% THD with 200-watts from 20-20K into 8-Ohms...It should make it to 400-watts into 4-Ohms 20-20K with similar distortion. Often you will see that same amp listed as 375-watts into 4-Ohms bridged with 1KHz and 1% distortion. It is an indicator that the amp really is being pushed and will not be sounding its best if run near the spec. The 1KHz tone spec at 1% THD just isn't a good spec for evaluating an amplifier, IMHO...generally a manufacturer that uses that spec is hiding something or playing "power wars".  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 04-02-2005 09:52 AM                     quote: Steve Guttag The 1KHz tone spec at 1% THD just isn't a good spec for evaluating an amplifier, IMHO...generally a manufacturer that uses that spec is hiding something or playing "power wars". Steve, Oh come on, amp manufactorers have been stating this spec for decades! Even McIntosh's first tube amp was spec'd in a similar way. What you really need to look at is the actual distortion curve across the audio spectrum and at at least 4 different power outut levels on both indictive simulated real world loads and on non-inductive. Manufacturers Always only show their specs on non-inductive loads! Bench test an amp in question yourself if you're in doubt that its fit to be sold... or if the manufacturer doesn't show things this detailed... also look at the distortion products on a spectrum analyzer.... things can get really interesting there. You will find and it has been proven over and over that amps that generate odd order harmonics sound like crap and amps that generate even order harmonics sound good and are much more musical. Of course everyone knows why those old single ended Western Electric amps sound so good... its because of the even order distortion content.... It is possible to do largely duplicate this harmonic content today in solid state thanks to large advancements in high power semiconductor technology. This allows more semiconductor devices to be biased far into Class A where it can make a huge difference as comnpared to older amps that had limited bandwidth and ran class B or low bias class AB... these amps instead relied on large amounts of feedback to achieve what seems like superior specs but the majority of these amps also sounded like crap. All in all what you need to be leary of is a manufacturer that doesn't state detailed specs across the board, then he may very well be trying to hide something.... or you can test it yourself and find out what is really going on. Mark  |  IP: Logged

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