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Old 6th April 2007, 02:10 PM   #1
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Default DC output in Soundstream MC245

Hi everybody,

Recently I just bought a used SS amp MC 245. It has a little problem with the pop noise ONLY during on and off. However, the popping noise come from all channels EXCEPT the subwoofer output. I noticed that the sub out has its own relay, but not so for the other 4 channels. Anyone be kind enough to guide me to trace the culprit??

The DC output are small, but still it's annoying. Across 4 channels, it puts out 0.4 - 0.5 v. Help

Thanks
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Old 6th April 2007, 03:39 PM   #2
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Simply delay the turn on of the amp by using a 1000ufd cap on the turn on wire.
This will stop that turn on pop, by giving you a few seconds turn on delay. the pop comes from all the other stuff before the amp turning on < they produce these noise issues that you amp simply amplifies and "pops" in your speakers.

That DC offset is not the culprit. Its a general condition indicator of how old that amp is. try new outputs to lower the DC offset and fresh power supply capacitors as a start to renewal of that classic amplifier.

Enjoy
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Old 6th April 2007, 04:15 PM   #3
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Are you measuring the DC directly across the positive and negative speaker terminals of each channel?

Is the pop a loud pop or a mild click as the amp turns on?

Does the amp produce a turn on noise/pop without a signal source connected to it?
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Old 6th April 2007, 04:33 PM   #4
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I think there's some good idea's and some not-so-good idea's.

One of the suggestions was to add a 1000uF cap to the remote turn on terminal. The only R in this RC network is the wire resistance. If the wire resistance is 2 ohms and the cap is 1000uF, the time constant will be RXC = 2ms That'd be a little quick. If you install a series resistor, to "feed" the cap, you'll do better. a 2000 ohm resistor and 1000uF cap will give you a 2 second time constant.

...this is only usefull if the DC pop is coming from your head unit. If the DC offset is static and coming from your amp, this delay of turn on wont work.

If the DC is from your amplifier, I doubt that replacing outputs will work. I mean, yeah, the output MOSFET's could be leaky but the offset should get back to <70mVDC. If the offset remain, AND the heatsink doesn't get hot (feedback trying to correct with opposite current), you probably have a power supply problem. I say probably power supply problem as the problem is affecting more than one channel. Can you post a schematic?
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Old 7th April 2007, 05:55 PM   #5
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To justonemoreamp:

The idea of adding a capacitor on the turn on wire doesn't appeal to me since, it's not how the amp designer intended to be. However, I would consider using such method as my last resort, thanks though

Simple question: Would changing the output on the four channels and replacing caps on the power supply eliminates the pop? Prior to having this pop noise, the sub output blew and I replaced all TIP142/147. The sub channel is working fine now and by the way, can the sub output handle 2 ohm load??

To Perry Babin:

YES, I measured the DC directly across the positive and negative speaker terminals of each channel. The pop is rather mild and it produces DC spike without any signal source connected to it. I haven't tried testing it without signal source while connected to speaker. I would assume it would produces noise due to DC spike.

To mrshow4u:

How can pinpoint the problem area in power supply (assuming it's the problem)??
I haven't been able to post a schematic, but still working on it
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Old 9th April 2007, 05:11 AM   #6
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Post #5
To justonemoreamp:

The idea of adding a capacitor on the turn on wire doesn't appeal to me since, it's not how the amp designer intended to be. However, I would consider using such method as my last resort, thanks though

Actually I stole that idea from Phoenix Gold, Its in there hand book of problems and solutions, and FAQ's. Concerning turn on thump and the use of delayed turn on for the power amp to resolve the issue.


Simple question: Would changing the output on the four channels and replacing caps on the power supply eliminates the pop? Prior to having this pop noise, the sub output blew and I replaced all TIP142/147. The sub channel is working fine now and by the way, can the sub output handle 2 ohm load?



2 ohms ? I really doubt that it will live a long and healthy life unless you respect the fact that it is running at it utmost rating. By this I mean that this amp was built in the early years of car amps, and even if it was rated at 2 ohms, you will be placing it under a fair amount of stress at 2 ohms per channel, and should you decide to bridge, Well here again your looking at 1 ohm per channel loading, and I would give this amp maybe 5 minutes on a cool day at those load levels before your removing it to go to the shop for restoration.

As for replacing aged and stressed components like caps and outputs. While my experience says this is a good idea, I don't believe it will solve your issues. A delayed speaker connection relay setup would be the best I could suggest. I know for a fact this will resolve your issue without tampering with your amp internally at all. Clean, quick, outboard of the amp, and something you could implement without too much issue, or support from a service tech.

You said the thump is present even without RCA input applied to the amp, correct? And this problem was not present before the amp was repaired ??

If the above is true, then there may still be something overlooked during its last repair...

Good luck on your quest
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Old 9th April 2007, 12:31 PM   #7
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To Bayiganteng:

To see if the power supplies aren't balanced and might be causing the pops. I'm not familiar with that amp and unbalanced supplies doesn't necessarily mean that the amp will pop. ...but

I would look at the front-end circuitry. The op-amps the filter and buffer the input signal. Look for (probably ) quad op-amps and look to see if their power supplies are symmetrical. You should have equal but opposites voltages at Vdd and Vss. If they are not, it could indicate that the power supply filters are opening. Most(all) powerful car amps have switching supplies, so an oscilloscope is handy for seeing the High Frequency ripple. The "pop" sound may indcate that one of the rails (Positive, or negative) may be coming up to it's filtered working voltage slower, causing the amp to be unstable at turn on. Perhaps the Subwoofer output doesn't pop because it has a mute circuit? ....I don't have the schematic so I don't know.

There is some possibility of some bad news here. Could it be that that model amp might pop by design? Not to say that the pop was the designer's intention, but just sloppy design??

If you can post the schematic and have an o'scope, I think this problem could be nailed in a straightforward manner (famous last words )

Good luck,

Jon
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Old 26th January 2008, 08:05 PM   #8
pugdogs is offline pugdogs  United States
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Talking DC output in Soundstream MC245

Hello,

Soundstream made a lot of MC-245's in the late 80's. It was the first 5-channel amp on the market and was made in the USA.
This was a common issue with the amp. At the end of production, Soundstream installed a small relay board that took care of this problem. You cannot buy it off the shelf. It was made by Soundstream a long time ago and only the last production units had them...maybe less than 100 of them had it installed. If you can find one...it would be a very rare find indeed!

The sub channel did have a relay but the other 4 channels did not. Most of the amps had a very slight noise.
Adding a cap to delay the turn on POP is OK, but the main defect is the amp. Way too much detail to go into now but, yes...it is the amp that is the real issue here.

Thanks
pugdogs
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Old 26th January 2008, 08:19 PM   #9
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Nice to see you back "L" < Pugdogs> ... C
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Old 29th January 2008, 04:21 AM   #10
pugdogs is offline pugdogs  United States
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Hello again,

While looking for something else in my vast "Closet of Mystery", I found a Relay board for the MC-245. I still have two MC-245's in my collection but I am willing to list one of the Relay boards on eBay. I do not ship outside of the US, but I am willing to ship it to a third party if you are interested. I have a copy of the original hand-drawn wiring diagram too. You will need a soldering iron, Phillips head screwdriver and a slotted screwdriver in order to do the modification. You will also need to secure the relay board with thin double-sided tape of hot glue to hold the relay board in place on the main PCB. You will also need to place some kind of insulator on the cover to avoid the connections from coming in contact with the cover. Mylar material is fine. It must have some kind of adhesive. It mounts upside down on the main board. (Relays down) There is very little room inside the amp, but it does fit. The wires are all cut to length and it is a very simple mod. I can send you a picture of the relay board and a PDF of the layout drawing if you are interested.
Let me know if you are interested and send me your email address.
I will not list it until I have heard back from you!

All for now....
pugdogs
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