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Old 23rd November 2005, 05:18 PM   #1
AdamH is offline AdamH  United Kingdom
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Default Anyone help out a newbie?

Re-posting here as I think i put the last one in the wrong place.. oops!

Firstly, hello all!

I have a few things I want to try to get done over the coming months and felt this looked like the idel place to get some advice on buying the right stuff and not making silly mistakes!

I... well I should say we (as a company) have inherited a large quantity of speakers in addition to the stuff we already run. I want to try and get this out on the road as hire PA but really dont want to be spending money on the good, expensive amps that we ourselves use.

I've been looking at some units branded "CMARK" which seem to be undeniably cheap. I listened to them in the shop and I must admit I was impressed. I'm going to need to buy in the region of 10 of them (various different models and power outputs) and am a bit wary before i stick my hand in the company "pocket"!

The big issue here is, I want to be able to drive them down to 2ohms buy hanging a set of 4 Martin Audio EM186s off 2 of them. (These are bi-wired and must be driven HF and LF separately). They're 4 ohms which is my sticking point, now these babies aren't cheap and damaging the amp aside... I DEFINATELY don't want to damage the speakers!

I do it all the time with expensive amps with dirty great big PSUs in them and no problems.

Can someone give me some advice, am I going to be committing electronic sin here and make a fool of myself or are we safe? I can cope with losing a chunk of wattage at the price i'm getting the amps for - I'll buy the bigger ones... but will they cope or are we likely to cook them do you think....... ANYONE!?

I'd be really grateful for some good advice, thank you!

Adam.
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Old 25th November 2005, 01:08 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
I just bought a Cmark (350) as an experiment to see how commercial gear compares to domestic on sound and build quality.
Not tested, awaiting speaker conns.
A look inside warned that they are not built to drive low impedance loads. +-93V PSU rails with 4 pair 2SA1943/C5200 can just about manage a not too severe 8ohms speaker.
The spec sheet calls up 4 ohms minimum but it would need to be a very low phase angle to survive hard usage.
In bridge mode I would recommend 12ohm to 16ohm speakers!
But at $320 it is extremely good value if treated accordingly.
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Old 25th November 2005, 04:45 PM   #3
AdamH is offline AdamH  United Kingdom
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Thanks Andrew,

Very helpful indeed, it's great to have someone with a little knowledge take the lid off!

I've ordered 6 of them so I can drive every channel separately - the speakers ARE 4ohms but having said that I have 2 saving graces: firstly the amps are drastically more powerful than the speakers can handle - quite deliberate, I learned that lesson a few years back - so they'll only be driving at a little over half power, and secondly all their literature DOES quote that 4ohm loads are fine and I have a 2yr warranty on them so if they cook I'll send them back.... lets hope they dont! I'm putting them in a big amp rack with 4 dirty great fans in the top of the back panel to literally "drag" the cool air through - fingers crossed.

Thanks again, I'll let you know how it goes when they all arrive!

Adam.
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Old 26th November 2005, 07:38 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
the Cmark MR2350 already has fans inside.
One to each channel blowing back to front, temp switch to shut down if too hot.
A well ventilated cabinet ( big holes opposite the inlets or open back) should do.
However I do not think cooking is the problem. I believe it is potentially using the output pairs outside their SOAR that is the problem.
If you feel like modifying the amps then a replacement transformer could be substituted to reduce the +-Vrails. Same VA rating but 50+50Vac instead of 62+62Vac would help and give a bit more PSU stiffness.
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Old 26th November 2005, 05:14 PM   #5
AdamH is offline AdamH  United Kingdom
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Back to front cooling.... ah ha, I didn't know that! I suppose I could turn the fans round, the rack is already built!

Have dropped you a mail Andrew, in response to this post however, how much do you think it would set me back to modify them?

Bearing in mind here that I haven't tried this before - I'm perfectly capable but have never yet attempted pulling an amp apart - we all have to start somewhere though and I'm very interested (I'm sure that must be why I'm here)! I'd need a little guidance....
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Old 27th November 2005, 03:17 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
at the price you obtained the whole ready built amps, a new set of transformers will cost more (possibly a lot more) than what you have already spent.
The best you can do to modify the amps, at sensible cost, is obtain second hand 50Vac transformers and add on some extra floating windings to give the high voltage supplies for the voltage amplifier stages.

There is room to add two extra pairs of output devices and this on it's own will give some additional output current ability. You will need to hard wire the connections and emitter resistors.

You would be best to reverse engineer the PCB to check that the board will work with lower voltage supplies and to ensure your additional outputs fit correctly.

The smoothing caps are a bit small for 8ohm use. I recommend a minimum of 2mF per peak amp of output current. This requires 18mF on each channel rail. For 4 ohm use this doubles to 36mF and since both channels share the caps you are up to 72mF per rail. However a stiffer PSU will exacerbate the output stage SOAR problem so I would NOT recommend this mod on it's own!!!!
You could consider this if you do the other reliability mods but I would go further and add double bridge rectifiers with their own smoothing caps for each channel.

If no modifications are carried out then just rely on the manufacturer's protection scheme and drive to domestic use levels as against commercial/PA duty. The small smoothing caps will be helping here since they will allow the PSU Vrails to collapse when heavily loaded and this in turn helps save the output stage on prolonged high level drive, to the detriment of bass level and overall linearity/accuracy.

In summary, firstly check output devices and Vrail voltages. Decide whether the layout and complexity make the modifications within your level of competence and budget. Then propose a scheme to suit your loadings and drive situation. Even 6 times 200W is heck of loud - sore eardrums!

I cannot comment on the 1200W amps.
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