Film-Tech Forum ARCHIVE: Amplifiers

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Posted by Tom Wienholt (Member # 1510) on 03-26-2005, 07:34 PM:

I was curious about why certain amps are deemed "not for cinema use". For example, why is a QSC RMX amp not for cinema use but a QSC ISA amp is? Does it really matter?

Posted by Mark Gulbrandsen (Member # 72) on 03-27-2005, 08:32 AM:

Its really alot of marketing bullshit. When I was at GTS we installed tons of RMX amps in theatres. So far over the three plus years they have proven completely reliable. Actually its true that amps doing music servce at concerts may get far more abuse than they might get in ANY cinema application!! For home use they would be particuluarly fine. I opened up and checked one of each model that we installed and the construction quality is actually pretty darn good, large toroid tranny, decent amount of power supply capacitance, and plenty of keat sinking. The main difference is they're just made in Asia. Overall, they are a bargan and should be made more accessable to cinema dealers. BTW: I was told by someone at QSC that the RMX series is very similar to the old USA series of amps that was discontinued... or were they really just re-packaged.....

Mark

Posted by Steve Guttag (Member # 268) on 03-27-2005, 10:21 AM:

Several manufacturers have products they target for a particular consumer. Part of this is to keep "box movers" from engaging in specialize applications. Part of it is to ensure that their products receive the proper application.

QSC has different types of dealerships...some are for sound reinforcement, some are for cinema. The ISA and RMX actually are more similar than USA and RMX. The ISA amp can be more suited to the cinema owner since its volume knobs are in the rear and not as easily tampered with. The ISA has a dataport that can accept QSC's analog crossovers or work with the DCM monitor line. The ISA features terminal blocks for speaker out...more suited to the permanent installation. About the only thing that the ISA doesn't bring to the table in its price range is that it is 3U tall and about a mile deep. It also weighs a ton.

If you want to sractch your head a bit...compare the CX line and the DCA line...aside from the color of the name plate, they are near identical (they are identical inside). A fellow dealer that is on the pro side compared our prices on the two lines...we could sell DCAs for the same price he could sell CX amps. At the AFI/Silver theatre, they supplied the CX amps for the PA system, we supplied the DCA amps for the cinema system...one nice thing was that based on color, one knew what they amp went to....go figure.

Posted by Tom Wienholt (Member # 1510) on 03-27-2005, 10:41 PM:

So the RMX would be suitable for cinema use. What about an amp such as the Crown XLS series. Im looking for an inexpensive amp to upgrade from older Smart amps. Something that is inexpensive and not very deep (most of my sound racks are wall mount and anthing above 14 inches deep will not fit). Any suggestions. Thanks.

Posted by Gordon McLeod (Member # 33) on 03-28-2005, 09:43 AM:

which smart amps do you have there mosfet ones used the original Hafler design and sound very good

Posted by Tom Wienholt (Member # 1510) on 03-28-2005, 10:17 AM:

Smart 240's and 242's. These are not very powerful amps.

Posted by Ferdinando Innocenti (Member # 2477) on 03-28-2005, 12:40 PM:

One of the reasons why there are some amplifiers that are exactly the same as other models, but with the writing “not for cinema use”, is for avoiding concurrency among vendors. If I sell cinema equipment, cinema owners must ask prices of cinema equipment to me, not to a music/concerts/studios vendor. This is what some dealers told me.

Posted by Andy Bergstrom (Member # 360) on 03-28-2005, 03:50 PM:

Which of the Smart amps are of Haffler decent? Do you have to open them up to determine this?

Posted by Mark Gulbrandsen (Member # 72) on 03-28-2005, 07:15 PM:

quote: Ferdinando Innocenti
One of the reasons why there are some amplifiers that are exactly the same as other models, but with the writing “not for cinema use”, is for avoiding concurrency among vendors. If I sell cinema equipment, cinema owners must ask prices of cinema equipment to me, not to a music/concerts/studios vendor. This is what some dealers told me.

Fernando,
As I said above its all marketing bullshit devised by QAC to keep the cinema dealers JUST selling the (supposed) cinema line! In fact the RMX amps work very well for cimema use. If you have knob twidddlers just pull the knobs off and get some plastic plugs as I have done. Having the gains on the front actually makes setup a tad bit easier too. What type of jacks are on the back make no difference to me as one can get very good connections with either binding post or screw type terminal block. You aren't going to ever find screw terminal block type on any real high end amplification equipment......

quote: Andy Bergstrom
Which of the Smart amps are of Haffler decent? Do you have to open them up to determine this?

Thats easy... the ones that always work [thumbsup]

!!

Mark

Posted by William Hooper (Member # 71) on 03-29-2005, 05:20 AM:

quote: Mark Gulbrandsen
If you have knob twidddlers just pull the knobs off and get some plastic plugs as I have done.

You can get those little plugs in various sizes in the Specialty Hardware bins/drawers in the fastener section (where you find bolts & screws) at Loew's. Cheap, effective, fast!

Posted by Tom Wienholt (Member # 1510) on 03-29-2005, 09:32 PM:

So what is a good, inexpensive amp that isn't very deep? 14 inches or less

Posted by Mark Gulbrandsen (Member # 72) on 03-29-2005, 11:06 PM:

I would watch E-Bay for used USA 900's or 1400's. Good amps that can be has very inexpensively if you keep a watch out.

Mark

Posted by Tom Wienholt (Member # 1510) on 03-31-2005, 11:34 AM:

How long ago did QSC stop producing the USA series? How reliable are these amps?

Posted by Scott Norwood (Member # 30) on 03-31-2005, 12:31 PM:

The USA series is fairly recent--they were popular in the late '90s and possibly the early '00s.

The USA and MX series are very reliable and sound good. Most of the theatres around here use QSC and I have never personally experienced or even heard about a failure. Replacement parts are available from QSC, so buying an older model shouldn't be a problem.

I have not seen the DCA series, but assume that they are just as good.

Posted by Ferdinando Innocenti (Member # 2477) on 03-31-2005, 01:38 PM:

I think I have installed my last QSC USA in 2001.

I have a problem with one channel of a USA 1310, after 4 years and in about 300 amplifiers! reliable, isn't it?

they are deep from 10 to 11 feet, depens on the model, the biggest (and the most powerful,of course) is USA 1310.

Posted by Richard Fowler (Member # 893) on 03-31-2005, 01:48 PM:

Ferdinando the USA is 3.353 meters deep [Big Grin]

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.

Posted by Ferdinando Innocenti (Member # 2477) on 03-31-2005, 01:59 PM:

Sorry, Richard, you're absolutely right [Embarrassed]

Posted by Tom Wienholt (Member # 1510) on 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.

Posted by Steve Guttag (Member # 268) on 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 ]

Posted by Mark Gulbrandsen (Member # 72) on 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

Posted by Steve Guttag (Member # 268) on 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".

Posted by Mark Gulbrandsen (Member # 72) on 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