MHz Class-D MCD-255 building question - diyAudio
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Old 1st January 2008, 11:22 AM   #1
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Default MHz Class-D MCD-255 building question

I have built a power amp with UCD180 module end of 2006 and listen to it nearly everyday. I'm quite satisfied with it. Then I attempted to built a UCD amp from the Philips application notes by referring to the thread Philips UCD application note I built one channel UCD on a breadboard and seem to be working good. Then I make some PCBs and have built several modules. But unfortunately I got stability problem. I haven't found the reason why yet but I do believe the problem is with the inductor.

After reading a thread in Vendor Bazaar forum about an MHz Class-D power amp module, I decided to have a try a Class-D amp with higher switching frequency. The modules MCD-255 are not cheap at all although it is come from China.

As a DIYer, I'm quite confused that no instruction manual was enclosed in the package. I remembered that the designer had posted the connection diagram. So, I get the diagram there and start building the amp. I'm also frustrated that the some part of the circuit is sealed as shown in the attached pictures. Considering the condition of soldering, the module seems to be soldered by hand. The PCB has not been cleaned as well.

The documentation from the manufacturer is almost NONE, so I start this thread to share with other builders.

Following are my questions,

1. What is the input power voltage range? I got two 24V AC transformers currently for testing. Before any attempt to connect a speaker, I wanted to measure the switching residue at the output with a 100MHz scope first. But I failed. I can hear two relay clicks when it was powered up. I cannot measure any switching residue at the output. It seems the amp was not yet started up or the protection circuit was in action.

2. The module comes with a built-in pre-amp, which can be bypassed. So, what are the input impedances and the gains of the pre-amp and the power stage?

3. Is it appropriate to enclose the inductor with a shrink-wrap jacket? Will the inductor get hot during operation?

4. Is the input DC or AC Coupled?

Thanks in advance.
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Old 1st January 2008, 11:50 AM   #2
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One more picture.
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Old 1st January 2008, 02:00 PM   #3
fumac is offline fumac  China
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hello gogowatch
happy new year

1.about the data sheet:
you are the very first user of mcd-255(ver 6)
and we are doing with the paper work.
so we promise that we will post the pdf at this two days.
sorry for the delay.
2.
the black one is the protection of mcd-modules.
and the res is the dead time turner,
we test every pcs of mcd modules before it go out to users.
and we will turn the dead time by hand for every pcs to make it run at his best .
sorry for the "not clean well ", i will check the product line to make it better.
thank you for your post
then i will answer your questions

rg
fumac
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Old 1st January 2008, 02:12 PM   #4
fumac is offline fumac  China
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question 1
1. What is the input power voltage range? I got two 24V AC transformers currently for testing. Before any attempt to connect a speaker, I wanted to measure the switching residue at the output with a 100MHz scope first. But I failed. I can hear two relay clicks when it was powered up. I cannot measure any switching residue at the output. It seems the amp was not yet started up or the protection circuit was in action.
------------------------------
mcd-255 need a +/-45V DC supply for 250w.
the max voltage is +/-49V dc (35V AC, will protection at about 50V),
but the input voltage of 230v(or 110v)sometimes will up to 110% or 120%.
so we need a voltage under 49v-6v=43VDC(30V AC)
===so 30-0-30v (AC) is a best choose.===

24V AC is too low, sometimes cant power up.
so this voltage is not the best choise of
so the best is raise the voltage.

but you can heard the "click" of relays , that meaning it is running.
i'm sure it is running.
because the speaker protection of mcd is checking the pwm signal ,
if working ok the relays will turn on. if not , the relays will turn off.
so if you heard one sound of "click", this is meaning : mcd is running .

mcd just output very low switching residue voltage(mV level),
not like others(V level).
so u need to turn your osc scope to lower range.
perhaps you can look at it .
also u cant just hearing the speaker to know about it is on/off.
because mcd output a very low noise.

rg
fumac
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Old 1st January 2008, 02:59 PM   #5
fumac is offline fumac  China
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the built-in pre-amp is working at +/-15v regulator.
Gain : 30.5db, (33.6x)
before pot (input stage): 6db(2x)
after pot(buffer): 9.5db(3x)
power stage :15db(5.6x)

if u input a 1v signal then you can get a 33.6v at the output pin.
so you can use most of music source .also low level output source.

input impedance :
pre-amp :15k (unbalance),30k (balanced)
buffer:18k
power stage: 6k
because mcd have a high input impedance,
so you can use a tube pre-amp to replace the pre-amp.
or even replace the buffer.


4. Is the input DC or AC Coupled?
pre-amp input is DC Coupled
pre-amp to buffer is DC Coupled too
buffer to power stage is AC Coupled (20uf cap)

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Old 1st January 2008, 03:08 PM   #6
fumac is offline fumac  China
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question 3
3. Is it appropriate to enclose the inductor with a shrink-wrap jacket? Will the inductor get hot during operation?

because the good design of mcd-255, the inductor just get a mini hot during operation.

so we enclose the inductor with a shrink-wrap jacket,
this will make the value of inductor more Stability

enjoin your new amp
rg
fumac
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Old 2nd January 2008, 12:28 PM   #7
fumac is offline fumac  China
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here is the minisystem of mcd-255
when you plug the power into it , it will run
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Old 2nd January 2008, 04:13 PM   #8
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Thanks for your reply, Fumac.

I have just made one module start running. You guess what I've done wrong? .... I haven't supply the DC15V. I just thought the DC15V is used for the pre-amp circuit that I don't want to use at the moment. After I supplied the DC15V, I =got a single relay click, instead of two previously. When I power it down, I got another relay click. That's more reasonable. Does the module need high quality DC15V power supply?

I haven't got all the components to build a complete amp. So I did some measurements before I go ahead. Attached is switching residue with power stage input shorted to ground and no load connected at speaker output. Scales are 0.2us/div and 50mV/div. It shows a ptp 220mV residue and switching frequency ~900KHz. The trace shows some high order frequencies ripples. Is that normal?

Regards,
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Old 2nd January 2008, 04:30 PM   #9
fumac is offline fumac  China
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Quote:
Originally posted by gogowatch
Thanks for your reply, Fumac.

I have just made one module start running. You guess what I've done wrong? .... I haven't supply the DC15V. I just thought the DC15V is used for the pre-amp circuit that I don't want to use at the moment. After I supplied the DC15V, I =got a single relay click, instead of two previously. When I power it down, I got another relay click. That's more reasonable. Does the module need high quality DC15V power supply?

I haven't got all the components to build a complete amp. So I did some measurements before I go ahead. Attached is switching residue with power stage input shorted to ground and no load connected at speaker output. Scales are 0.2us/div and 50mV/div. It shows a ptp 220mV residue and switching frequency ~900KHz. The trace shows some high order frequencies ripples. Is that normal?

Regards,
cheers and happy new year
i like to hear about your mcd-255 is running
the 15V sub power is for control circle and low side driver
there are a 12v regulator inside mcd-255.
so not need to add a extension regulator for mcd-255
and the caps of the sub-power must be under 1000uf.
if large than it , it will be make err to the soft-star circle.

about the PWM frequency, the frequency is depend on the power rail ,if you use higher rail, (+/-43v), then you can get the higher pwm frequency, also when the mcd-255 warm up,the frequency will also goto 1mhz(+/-60K)

and about the 220mV residue , when the pwm frequency higher, the residue will be lower.
you can test about your another low frequency class-d amp, then you will find our residue is very very lower than others

another side, we can make the residueunder 10mV, but this will make the frequency respond jst up to 50kHz(-3db), but we have test about it , up to 150khz(-3db) is quite better than the 50k.
so take it easy

rg
fumac
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Old 2nd January 2008, 04:31 PM   #10
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One more trace. This one is from the power amp output pin without speaker loading with triangular signal at power stage input pins. Scales are 0.2ms and 10v/div.
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File Type: jpg mcd-6-0.2ms-10v.jpg (54.5 KB, 1141 views)
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