API 2520 problem

[AK79]

Hi Folks,
I friend of mine recently blew up two of his 550a EQs.

He did some experiments with 1200Ohm termination. As you know theres much talk on the net about doing that to get that sound.
It worked like 30 hours or so when one of the 550 got stinking hot and the signal drowned in noise.

He gave me that one and I figured out that the output 2520 went dead. There was also the 500u Cap in the output blown up.
a second 550 does crackle now once in a while. After the incident her removed the 1200Ohm terminators.

The other 4 550A he has work as good as before.  the unit tha blew up was recapped reportedly 10 years ago.

Now the question is : Is this a coincidence ? The original datasheet for the 2520 says that it can drive up to 75ohms and that 600Ohms are recommended.  I asked API and they said that the 1608 desk indeed uses 600Ohms behind the EQ. So 1200 Ohm should not be much of a problem to drive.

I checked the terminators he used and theres nothing wrong with them. 

the guy is devastated now because he lost a least one Melville 2520 (and I´m the lucky guy to fix it now ).

any thoughts on this are welcome. Thanks-

br
Arne

 

[Majestic12]

Hi,

I don't think that you can damage a 2520 with an additional 1K2 load resistor, but the additional load can put more thermal stress on it which makes it more likely to fail over time when it is an old 2520.

I've seen lots of 550 with blown output stages, usually it's either the 2520 or one of the 2501 voltage followers that go bad and put out lots of DC that will finally burn the fuse resistor in front of the output transformer.

Best,

Markus ( API service tech from ES-Pro Audio)

 

[AK79]

ok thats what I wanted to hear - I did not recommend the additional load but all people and datasheets tell me it can do that.

Thanks
br
Arne

 

[Walrus]

My burn-in ACA test bed uses 330 Ohm resistors as loads for 2520's, and I leave them running all day at +8dB output.
Probably as Markus said, an aging Melville finally gave up the ghost.

 

[Tubetec]

I have a buddy with the four channel api with stereo mix outs and direct outs , a few of the DOA's have died over the years on him ,If you google there were some batches of the op amps that werent all that reliable ,the symptoms were always more or less the same ,no or very poor sound and a noticable temprature difference between the working and broken doa's ,Im sure better air flow and heatsinking might make a difference but as the doa's are encapsulated all you can really do is replace them with third party units of the same style ,lots of good options out there though might be worth experimenting especially with the output drivers

 

[AK79]

yes I am aware of the general heat problem. Replaced dozens of 2520 over the years.
I think API at some point moved to SMD which makes temperature issues even worse.

but the Melville 2520 seemed to me to be more reliable given the fact that the guy has 6 550a with those  that are all 40+ years old.

 

[JohnRoberts]

+1 heat issues are common with DOAs.

Setting the internal operating currents can vary with rail voltage so check for proper rail voltages (more is not always better).

JR

 

[gridcurrent]

the internals of 2520's that I have seen, exhibit no thermal tracking (other than the epoxy) of the output transistors to the forward biasing diodes.
2520's can obviously work for decades minus that feature, but seems pretty iffy.
one of the high profile 2520 cloners (he markets his product under a different part number) had multiple failures until he reworked the layout to include thermal tracking.

 

[AK79]

we are going to replace it with non-potted clone and see what happens.