OPA627 burnout in Arcam Alpha 7 CD

[InfiniteGain]

Hi All,

I read on a few threads around this site that I could replace the cheap and nasty NJM2114D opamps with many other varieties including the OPA627 which would require mounting 2 on a Brown Dog as they are single opamps not dual.

I have a batch of APO627AU and the appropriate Brown Dogs, got them on the boards last night after some really tricky soldering. I of course, observed where PIN 1 was and ensure no shorts between the tiny pins of the opamp chips.

I had removed the NJM2114Ds from my Arcam Alpha 7 (under the name JRC2114D) and soldered in DIL sockets to make experimenting with opamps mcuh easier. Thing is, it is the OPA627 that everybody raves about that I really want to hear.

I put in the BrownDog mounted OPA627s and quickly got a burning smell. The right channel OPA627 pair burnt out. Removing it and turning up the volume until I can hear the transformer hum over my speakers indicated that I had a very low signla output from the other channel, but that it was working.

Either the gain is off compared to the NJM, which for recommended "drop in replacements" seems a bit off or (more likely) something else is taking the opamps way out of their safe operating limits. Probably the latter, but looking at the datasheets the tolerances, working voltages etc... all look fairly similar to the NJM2114D.

Has anybody ever done mods to the Arcam Alpha 7 CD player, or know offhand why this would occur?

Help greatly appreciated.

Infinitegain

[pinkmouse]

Most likely oscillation causing overheating. These chips are NOT drop in replacements, they are much faster. You need to look at extra rail decoupling at the minimum.

[InfiniteGain]

Thanks. I wouldn't know without the schematic - I just trusted the info I saw.

After hunting around the web for the schematic I thought I would actually ask Arcam and now have the schematics for the Alpha 7,8,9 and associated DAC upgrades - in fact they sent me the full service manual for the series.

[analog_sa]

Infinitegain

If i were you i'd double check that soldering job. Oscillation killing a 627 may be possible but not very likely. What is the supply voltage? The 627s have 4v less max supply than the opamps they replaced.

[InfiniteGain]

Hmmm.... the schematic confuses me. 4 diagrams for one board. Physically I only find 2 NJM2114D opamps on the board, but the schematic has 6!! 4 in the diagram and 2 on their own with different labelling but with the voltage defined as +/-18V. This is on the limit of the OPA627s - the supply is regulated.

I attach the schemat for the DAC and output section for the more experienced to look at as I am a newbie to this. My father who is now retired, used to work for Raytheon and so is a bit of an electronic wiz and I can get him to look at it and explain it all closer to Christmas. In the meantime, pointers would be good as I think I will have to rip out quite alot of the ouptut stage from the player to use the OPA627s to good effect.

[InfiniteGain]

I just want to apologize if there is too much missing from the schematic, but I can only upload a max of 1000 pixels in any direction and any smaller and the component values are lost!

[AndrewT]

Hi,
I think you need to check the pin supply voltage to ensure you don't exceed the spec. Is 627 @+-18V absolute max or recommended max?
Definitely add pin to pin decoupling and if ground is nearby also add pin to ground decoupling. Using the very shortest routes you can manage, even using the back of the PCB if needed.

Does the 8 pin 627 need any correction when operating at low gain or adjustment pin loading?

By the way, big change in roll off philosophy from the earlier Delta 70.2 which used three pole Bessel whereas the Alpha uses (almost) 2 pole Butterworth.

[poynton]

[QUOTE]Originally posted by InfiniteGain
[B]Hmmm.... the schematic confuses me. 4 diagrams for one board. Physically I only find 2 NJM2114D opamps on the board, but the schematic has 6!! 4 in the diagram and 2 on their own with different labelling but with the voltage defined as +/-18V. This is on the limit of the OPA627s - the supply is regulated.

Hi,
The schematic shows only 2 op amp chips. Don't forget they are dual op-amps - 2 per chip. The 2 blocks on the lower right are only to show power supply and are not opamps.
I too would recheck the soldering - it sounds as though there may be a short on the working channel. As for the other ...!!!!!!?
Andy

[JonHarrison]

Hi 'Infinitegain',

If you email me the service manual I may be able to help with your problems.

Could you post a close up picture of the 627's mounted on the brown dog adapters? I'm curious how you have connected them up given that there must be a fair amount of wiring to match two single opamps to a dual op-amp pinout.

Regards,

Jon

[InfiniteGain]

JonHarrison,

I could send you the full schematics but you have your preferences set not to be emailable!!! No need to worry about wiring two single op-amps together, that's taken care of by the Brown Dog PCB board care of Cimarron Technologies.

Unfortunately, the girlfriend has my digital camera at her work residence - but maybe in a few days I could post a close up of some soldered chips. For the PCB, look here:
http://cimarrontechnology.com/index....PROD&ProdID=24

Thanks.

InfiniteGain