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SQLGuy 26th March 2013 04:38 AM

Noise in output from Orion 2500D
Hi folks,

I've got a couple of 2500D's in for repair. Both had blown outputs. One was blown for one phase, and the other for two phases.

I've replaced all the blown outputs, and a bunch of gate drive resistors, and a couple of burned up gate pulldowns, and rebuilt a couple of the driver boards.

On the driver boards, there were a number of blown KTC1027's and KTA1023's as well as 1Ohm SMD resistors and a Zener.

I replaced the outputs with same parts, but I subbed the KT's with KSA1220A's and KSC2690A's, since I had some on hand, and I wasn't finding the KT's from my regular sources (also, they looked like better parts)...

Anyway, with them back together, both now run, but one of them has about .15V of DC offset and a decent amount of 8KHz or so AC noise. The other seems normal. The noise is there regardless of gain, and regardless of the setting of the low pass filter. Googling around, it seems like a few people reported similar noise issues with these amps, but either didn't find a cause, or had them swapped out by Orion. Anyone here familiar with such a problem? Worst case I'll be doing a lot of scope comparisons between the two amps, but I'm hoping someone's seen (and solved) this before.


SQLGuy 26th March 2013 05:02 AM

I just did a bit of comparison.

The main difference I found was a current draw difference between the sides of the drivers using the KT parts and the the ones using the KS (Fairchild) parts. The two sides of the driver boards each have a 330 Ohm resistor on their supply. In the good amp, each resistor drops about 10.8V. In the bad amp, the KT side is dropping about 6.9V and the KS side is dropping more like 11.5V.

At this point I'm wondering whether the Fairchilds are maybe not suitable subs, even though they meet or beat all characteristics of the originals. Either that or I need to use them on both sides of the driver boards.

Anyone know a source for small quantities of the KT (Korea Electronics) parts?


Perry Babin 26th March 2013 05:14 AM

That would make it appear that the 12v Zeners are leaky or out of tolerance in the defective sections. This will typically make the output transistors run hotter than they should at idle.

SQLGuy 26th March 2013 01:24 PM

The 12V Zeners on the higher voltage drop sides are new. I was under the impression that these Zeners were there to protect the output gates from being overdriven - no?

Perry Babin 26th March 2013 01:34 PM

They are the voltage regulators for the driver transistors. The resistor and zener produce a 12v supply. The drivers use that voltage to drive the outputs.

SQLGuy 26th March 2013 02:09 PM

OK. I didn't measure the voltage drop across the Zeners, just across the series resistors. I was thinking they were gate protectors because the original Zeners on the blown sides of the drivers were shorted. If they're just shunting current for regulation, I don't see what would have shorted them.

I'll probably take a closer look at this circuit this evening, but I still think the subbed transistors are the issue. Best I can find so far is ordering them from China through eBay. Someone mentioned having bought them from MCM, but I don't find them on their site now.

Perry Babin 26th March 2013 02:15 PM

You could swap the old/new drivers to see if the problem follows the new drivers.

SQLGuy 26th March 2013 02:25 PM

I could, but the evidence is pretty compelling. I have two driver boards, each with one side rebuilt with the subbed drivers, and the inbalance is the same in both boards.

seankane 26th March 2013 05:18 PM

Not sure where these are located in the amp. If you don't need the hole for a screw to mount it to a heat sink, there are a lot of options available for subs.


I found about 4 other matches for each but these looked good and are readily available in the USA.

SQLGuy 27th March 2013 01:20 PM

I didn't have much time to look at the amp yesterday... maybe more today, but what i did see doesn't yet make sense to me. The "supply" side of the 330Ohm resistors seems to come from the end pins of the driver boards where they connect to the main board, and the other side feeds to collectors of a couple of the small transistors. Thing is, the "supply" voltages are about +/- 22V on the good amp, and the other sides are +/-33V. I am a bit confused as to why this looks backwards from what I would expect.

On the non-working amp, I still have -33V and -22V on the repaired side of the driver boards, but 36.8V and 30V on the side where I didn't find any damage. Maybe there is some damage there and the unrepaired side is actually the problem?

I think I'm going to need to draw this circuit out so I can better understand what's happening (unless someone has a schematic they could share?).

For the subs Sean suggested, unless these are other numbers for the exact same parts, I would still be concerned, as the KS's that I used should be a good sub too.

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