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Old 22nd February 2010, 06:16 AM   #1
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Default QSC RMX1450 FAMILY ( long )

GOOD MORNING LADIES AND GENTS ....

-----it is obvious that many of us clone amplifiers in many possible ways ... so in case you need an amplifier that is suposed to be profesional and at least 300w you may as well take a look at the forum and see designs like the DHR or the lynx or the Quasi or P101 and variations or others ....

----- then again none of these designs have what it takes to be called profesional ....hmmm that will be crowbars , vi limiters and generally protection methods applyied in comercial amps


---- then again people that have diferent opinion on the above may skip the all post cause they will find it of no interest.

---- now if you go for a clone of that power why not clone a Qsc rmx1450 ??? for sure there is going to be construction tips and /or construction mistakes that you are going to make while cloning and probably you may end up close to the specs of the rmx 1450 or met the specs but not 100% ...in a couple of words it is simply very possible

benefit side

looking at the schematic i have made a few thoughts that i would like to have comments for

---- the amplifier is short of sziklai that will probably mean that like all sziklai output stage is easy to drive ( just an op amp and a couple of drivers ) but then again all big sziklai amps have bootstraps, capacitors, and snubers, here and there to keep steady ( a good reference might be the HILL chameleon regarding snubers and bootstraps )


----- the rmx 1450 has nothing almost of the above !!! i wonder why .... one explanation might be that the rmx 1450 is capacitor coupled that probably means that capacitors actually provide isolation from reactive loads ...can this be correct ???


---- the rest of the design is nothing to comment about ...is based on a specific philosophy either you like it or you dont ... crowbar makes sense no real dc protection is requiered cause the capacitor coupled circuit has a chance one in a million to fail and destroy the speakers ---- VI limiters are there thermmal protections and so on and on and on

conclusion

----now there is got to be a fact to say that you like or you dont the circuit and then go ahead from there ... after that you need to think that an behringer EP 2500 which is an actuall clone of the qsc RMX 1850 is something that you can get with about 350 euros so all the above is almost pointless you will never manage to clone this at this cost

---- though there is an option to reduce the cost of the amp regarding the elctronics parts and apart from the box by using an ON LINE POWER SUPPLY ... now probably this against forum regulations and i would like the discussion not to proceed regarding the power supply since its dangerous and provides low safety margin but still its extremelly cost effective

---- thanks for your time and please comment on my thoughts if you feel like

kind regards sakis
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Old 22nd February 2010, 09:49 AM   #2
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Most people just want a good amp that plays music.

Unless your disco or a band and constantly moving the gear about there is little chance of a short circuit.

I dont like over current protection because it can clip sharp transients.

QSC while pro does have its problems, take a look at its forums, loads of people with loads of problems.
http://media.qscaudio.com/forum/
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Old 22nd February 2010, 10:26 AM   #3
wahab is offline wahab  Algeria
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So far, a friend who is bassist asked me to bring my keyboards dedicated
stage amp ,a low cost DIY mosfet amp of about 2X100 W RMS/8R ,
for comparison purpose with his QSC amp, a 2X200W model, i don t exactly
remember the model.

The qsc sounded absolutely crap, with hollow basses , a truly empty
sound that has no contour.
He was amazed by the difference between the two amps, and so i was..

The qsc are capacitve coupled amps with floating ground...
This allow to connect the collectors of T03 devices without insulation,
since they are connected to ground.

Anyway, no advantages, and a lot of drawbacks.
A terrible design, indeed..
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Old 22nd February 2010, 11:31 AM   #4
djk is offline djk
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"The qsc are capacitve coupled amps with floating ground..."

http://www.qsc.com/support/library/s...ies/MX2000.pdf

The center tap of the transformer is hooked to one side of the speaker, the other side of the speaker is hooked to the collectors (ground). That's DC coupled.

"The qsc sounded absolutely crap, with hollow basses , a truly empty
sound that has no contour.
He was amazed by the difference between the two amps, and so i was.."

Execution of the design, not the design itself.

"Anyway, no advantages,"

This allows the collectors of T03 devices to be grounded without insulation, saves labor, saves heatsink (lower thermal resistance). Vas only need swing ±5V to make the outputs swing ±100V.

Hafler Trans-Nova uses the same basic grounded output stage.

One-half of most Crown amplifiers in their grounded-bridge designs drive ground.

http://k-amps.8m.com/cgi-bin/i/Power...afler_915c.jpg
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Last edited by djk; 22nd February 2010 at 11:39 AM.
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Old 22nd February 2010, 11:43 AM   #5
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Not all pro amps are bad, I dont know the model sakis is referring to but some pro amps are very good indeed. I have modified some crest amps after they went up in smoke for some reason. Some of their amp design is as good as youll get from typical designs like you find in this forum, single differencial lin type, I even consider a certain model to be better than all the same type designs I see here, change to audio quality passive parts like power supply caps, polystyrene miller caps, non inductive resistors for outputs etc like we do in our diys and youll be pleasantly surprised.
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Old 22nd February 2010, 01:12 PM   #6
wahab is offline wahab  Algeria
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[QUOTE=djk;2094328]"The qsc are capacitve coupled amps with floating ground..."

http://www.qsc.com/support/library/s...ies/MX2000.pdf

The center tap of the transformer is hooked to one side of the speaker, the other side of the speaker is hooked to the collectors (ground). That's DC coupled.

QUOTE]

true..

the one my friend did test seemed to have a classic power supply.
Anyway, the sound was horrible when tested at high power..
And i m not a subjectivist...
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Old 22nd February 2010, 02:07 PM   #7
wg_ski is offline wg_ski  United States
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[QUOTE=wahab;2094399
the one my friend did test seemed to have a classic power supply.
Anyway, the sound was horrible when tested at high power..
And i m not a subjectivist...[/QUOTE]


In the early days of QSC, they did indeed capacitor-couple it. The modern version (RMX) is DC coupled. With class H, connecting that center tap is absoluetly required to keep the power supply balanced when the amp switches rails.

But the real difference in the new ones vs. the old are the sustained-beta driver and output transistors. With the older transistor types, the amp topology was barely able to drive 4 ohm loads. And of course, the sound quality suffered. This is still the limiting factor on SQ of these amps, but now it's 2-ohm (or sub 2 ohm) loads that push it to the limits.

A simple (even an SE!) EF stage between the opamp and the sziklais could make a world of difference in how this topology sounds. In theory, this is simple enough and could make a workable DIY solution. Until you start unraveling how QSC's protection circuits work. Then all the workarounds cause the component count to skyrocket and by then you've lost all the advantage of the topology.
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