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Old 10th August 2007, 03:01 PM   #1
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Default Meridan 557 Softstart and power caps

I have a 200W/ch Meridan 557 that has an elaborate softstart protection unit. I am planning on replacing the existing 10,000uF power/filter caps with 25,000 uF caps.

Could this change possibly effect the softstart functionality? Are softstart circuits designed with the power caps ratings in mind?

Thanks for any advice you can provide!

~Twangbar
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Old 10th August 2007, 09:49 PM   #2
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~Twangbar

It's likely you will have a higher inrush current so the soft start might suffer a little.Seek advice from Meridian they are pretty helpful or just leave the amp as it is. (if it aint broke dont fix it). The 557 is a pretty amazing amp and will drive anything so why change it ?I have the Meridian 605 mono blocks at the moment I'd really love to have a 557 to drive my Kef 105.3s.

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Anthony
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Old 11th August 2007, 02:25 AM   #3
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Thanks ANTHONY2181 for your reply.

The problem is, it is broken! Two of the power caps snapped off in transit! So my plan is two have an outboard capacitor bank. I do not think the design they had was very mechanically sound as the original caps are mounted sideways on the PCBs which are mounted on the heatsinks. They even used double-sided tape to help hold them on!

I did speak with a Meridian service tech today and he said the soft start should be ok with the increase in uF. We'll see... I will report back when I am finished.

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Old 11th August 2007, 02:56 AM   #4
anatech is offline anatech  Canada
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Hi Twangbar,
Quote:
So my plan is two have an outboard capacitor bank.
If you mean outboard as in outside the amp, don't do that.

Increasing your supply capacitance is not always helpful and can actually give rise to some other unpleasant effects - like a hotter transformer. I would build it back to stock if I were you.

I agree that the soft start would probably survive. The losser element will just get hotter. It's only in circuit for a short while.

-Chris
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Old 11th August 2007, 12:01 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
the soft start can do two different functions.
1.) to reduce the current surge that big transformers, particularly toroids, suffer at start up while they establish the magnetic flux in the core. This takes just a few cycles of the mains and can be a resistor bypassed by a relay which switches over in about 200mS to 500mS.
2.) to reduce the first charging DC current pulse from the transformer through the rectifier to the discharged smoothing bank. Using the soft start to do this is not good. Better to locate this in the secondary side of the transformer. i.e. a second resistor/thermistor and bypass relay with it's own delay of upto 3Seconds.

The change to the smoothing bank will not affect version 1.)

The timing of the relay will tell you which version Meridian have chosen. I hope they have gone for 1.) only or 1.) & 2.) using separate relays and ballasts. This is by far the best. I would not think Meridian would combine the function of 1.) and 2.) in a single ballast, at least I hope they wouldn't.

The extra smoothing capacitance will increase the charging current spike into the caps and also reduce it's duration. This combination increases enormously the high frequency content of the charging pulses. This amp and the decoupling/snubbing must be designed to attenuate this HF garbage or you risk reduced performance. What you propose may work and it may work very well, but this is very much a try and see experiment and be prepared to have to reverse all your mods to bring back the performance.

If you do remotely locate the main smoothing, then consider retaining local smoothing in it's original location. You can use the interconnecting cables to deliberately build in an RCRC filter that reduces mains ripple and can give good peak currents into the amp. Very high quality local smoothing may provide a good audio mod when supplied with a DC supply from the pre-smoothing.
This is effectively redesigning the amplifier. Not to be taken lightly.
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Old 11th August 2007, 02:43 PM   #6
anatech is offline anatech  Canada
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Hi Andrew,
Quote:
This is effectively redesigning the amplifier. Not to be taken lightly.
Yes, an understatement really.

The risk of having your supply voltages running outside the original case is a big risk. Not only that, but your peak charging currents are extremely high. I don't think you fully understand the magnitude of those currents.

-Chris
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Old 15th November 2011, 09:00 PM   #7
Lenin21 is offline Lenin21  United Kingdom
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As you've already been advised, DO NOT build an external capacitor bank unless you retain the existing capacitor bank, in it's current location. An RC-RC filer is an excellent idea.
It's almost 20 years since I last examined a 557. From memory, the key design highlights were a regulated input stage (standard 317/337 3-pin regs, if memory serves) and Elna Cerafine smoothing caps (perhaps explaining the frankly inadequate supply capacitance).
Trust the Meridian techs. They will know the ratings of the soft start components and their ability to deal with higher inrush currents.
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