A few years ago I bought a K8084 kit to go along a LM1875 based chip amplifier I’ve built.
It ran fine since the beggining and I decided to upgrade from the supplied TL072 opamps to National’s LM4562.
These also ran fine for about a year or so until a few days ago I sadly noticed a rather strange mafunction:
When I turned the amplifier on, an insane amount of hum was audible and the volume had decreased to an almost inaudible level.
At first I thought the LM4562s were gone and I refitted the original TL072s. As a result the hum decreased a bit and the volume was back to normal levels.
The thing is I got a new lot of 6 LM4562s and having ran throught them all the problem persists: The preamp does not work with them. It works with the TL072s but there’s a lot more hum than when I first assembled the kit.
My questions:
1- What might be causing the preamp to behave so strangely (It does no longer work with LM4562 opamps that equipped it with no problem for more than a year and even when with TL072s it hums a lot more than it did when new)?
2- For a while I had the preamp transformer connected to a switched outlet that I used to switch off every day when I left for work and on again when I came back. Could some transient phenomena have blown up something in the supply voltage regulation section of the K8084?
Any thoughts on how to troubleshoot this issue so I can make my K8084 compatible with LM 4562s and humless again would be greatly appreciated.
Sorry, we have no experience with this op-amp.
Regarding the hum, check the power supplies. Make sure there is no excess ripple.
Check your assembly and wiring for hum loops or grounding problems.
The opamp I mention is audio application specific and widely used and very well renowned in DIY projects.
Nevertheless the main thing is that these opamps worked very well on board the K8084 for a long while and now they don’t, so I suppose something must have developed a malfunction, since I dind’t modify anything in the amplifier layout (wire routings, trasformer placements, grounding points, etc…)
Anyway, I’ll dig deeper into it and let you know about my findings.
Meanwhile I’ve been analysing the circuit and I’d like confirmation if increasing C21, C22, C19, C20, C13, C14, C15 and C16 can contribute to reducing the hum.
Also, any advice on how to properly ground the K8084 PCB would be greatly appreciated
1- Through the ground of the power supply (which is connected to the chassis)
2- The pot’s body is in contact with the chassis which is a common ground to the power amplifier also
3- The grounds of the inputs and outputs are also connected to the project chassis.
Meanwhile I’ve been making some measurements last night and reached the conclusion that only about ±3,5V are arriving at the opamps rails. I think this to be a rather low voltage…
Would you care to comment? What is the projected output of the rectification/regulation section of the circuit?