Sjöström Audio DIY forum

Per Anders:

The oscillation goes away when the power is turned off.

Definitely coming from the preamp sections, not the RIAA stages. I lifted R50 to separate the gain stage from the emitter followers. The oscillation appears at the emitter follower section. I confirmed that C103 and C104 are 100nF.

I also am a radio ham and broadcast engineer, and work on RF on both my radio hobby and at work, but there with slightly higher power of 88kW. :-)

Dan

If you turn off the power, do you still have the oscillation? For being an oscillation with BC transistors I think the frequency is very high. 1-10 MHz seems more natural.

Hi Per Anders:

Thank you for quick replies today. This is enjoyable.

I edited my previous post as you posted, please read again.

Yes I have an older analog oscilloscope that is good for RF. The scope case is grounded to the preamp chassis case, and the scope probe ground lead is connected to the ground pin of J2. The preamp card Chassis ground connection is connected to the preamp chassis.

Both preamp channels oscillate. I checked with a spectrum analyzer too. Good transmitter at 284MHz!

Thank you,
Dan

Good that you have found a point where the servo doesn't hit the roof. The output offset in TP1 should be the value which the OP07 has, single millivolts is fine. The DC servo doesn't contribute to any noise since it's heavily filtered. Nice that one channel works. Now you have to check the other channel.

Do you have the oscillation in both channels? Do you have an oscilloscope capable of 350 MHz or more? Make sure your ground wire of the probe is wounded around the probe and connected very near the TP1. A long ground wire serves as an antenna. If possible you should use a ground needle on the probe tip.

Per Anders:

More tests on IC5:
Add -15v to IC5 socket (no IC inserted) pin 5, TP1 moves from -180mv to -190mv
Add +15v to IC5 socket (no IC inserted) pin 5, TP1 no change T -238mv.
Shorting IC5 pin 5 to ground, TP1 = -237mv.

New tests on IC1:
IC1 (OP07C inserted), input shorted to ground, IC5 socket empty:
Pin 1 = +14.975
Pin 2 = -1mv
Pin 3 = -149mv
Pin 4 = -15.011v
Pin 5 = +0.2mv
Pin 6 = -14.321v
Pin 7 = +15.033v
Pin 8 = +14.919v

With IC1 removed, TP1 is at -300mv. Adding -20v to IC socket pin 6 (no IC inserted), TP1 reduces to -170mv.

Added 22K across R29, added 100k across R27. Bench supply of -9.5v at pin 6 now creates 0v at TP1!! With IC1 (OP07) installed, TP1 is now -7.5mv. Good! Will this increase noise from OP07 to preamp?

Inserting a 1KHz sine wave to the input causes IC1 servo to oscillate at about 40hz. Adding 10uf at the output of R29 eliminates this. Using an oscilloscope, I now see crossover distortion on the test sine wave at the card output, possibly from the RIAA stage-will look at this later.

If this servo change is approved by you, we now need hints on eliminating the 284MHz RF oscillation (this is not from a Radio Sweden International transmitter, but generated in the preamp). With the input shorted, the RF signal is 70mv p-p at TP1.

Thank you!
Dan

Sorry, I was mistaken here. The opamp in mind here is IC1. Is it populated? If yes, please measure pin 2, 3, 4, 6 and 7?

Hi Per Anders!

Thank you for your suggestion and guidance.

With input shorted to ground:
Added variable 0 to +15v power supply to pin 5 of IC5 socket. No change at output at TP1 (which is at -184mv). IC5 socket (no chip inserted) pin 2 = 00.00mv. Pin 3 = 00.00mv.
IC5 socket (MAX420 inserted) pin 2 = 00.00mv. Pin 3 = 00.00mv. Pin 5 = -371mv. TP1 is at -183mv.

Where does input to IC5 come from? Where does IC5 output enter the preamp for DC correction?

Cheers,
Dan

Try to figure out if the DC servo range is too small.

If the IC5 is not populated: Apply +15V at pin 5, check output and see if it moves
Apply -15V at pin 5, check output and see if it moves

I recommend that you use a variable power supply for this test so you can adjust the voltage in pin 5 from 0 to 15 V and 0 to -15 V.

You should try to locate where you have the oscillation and try to cure it. Start with only one channel active and shorted input. If you can measure the frequency or even better having a FFT measurement you may come to the conclusion that it's a radio signal you picking up.

Allright, have some days off, yippy....
Used a 2x18VAC SSR03 is doing fine now...

Audio is passing and sounds nice, all seem to be doing well, at least to me ears...
Very dynamic, had an hour listening to various stuff, mainy listening  on headphones (neighbors thank me)
consisting of an Audeze LCD-X, was a nice hour of listening....
EDIT: I have the same oscillation on Preamp out, but it doesn't seem to affect the overall sound as some have mentionned,
so not sure what to think about that....

Still needs an enclosure, it was just dummy metal plate for now....


Thanks