Very low frequency noise/motorboat?

Okay. It's confirmed. It likes to see a load across the secondaries between 600-1200 ohms.

My inclination is to go with the flattest response, right around 900 ohms. Any thoughts?

Dependents on what you going to connect. G9 is a classic example with similar enough ratio and requirement for output drive. 10k, 2k4, up to you.

AD converter in most cases. Perhaps an outboard compressor on occasion.

Frequency response seems best with 2.2uf cap and around 700 ohm across the secondaries. Starts rolling off at about 30Hz with the -3dB mark around 6Hz. Noise level around equals out to normal, and quite low.

Thanks everyone. I was pulling my hair out.

While the problem is gone it's not truly solved. I still want to know why it was oscillating without a decent load.

Kingston said:

While the problem is gone it's not truly solved. I still want to know why it was oscillating without a decent load.

Click to expand...

I don't think it was oscillation. This is most likely /f noise amplified by a very low frequency resonance.

Cheers

Ian

ruffrecords said:

I don't think it was oscillation. This is most likely /f noise amplified by a very low frequency resonance.

Cheers

Ian

Click to expand...

Indeed. Actually have settled on the 4.7 uF cap and 1.2k termination across the secondaries.

I thought that schematic looked familiar, that's the Hamptone LCMP that he posted in TapeOp. I've built this without the  problems you've had with both Edcor and Triad HS-54 on the output without termination.
You may want to look at your layout, I recently had low frequency motorboating issues while restoring some Gates preamp cards, fixed it with a re-routing of grounds.

pH said:

I thought that schematic looked familiar, that's the Hamptone LCMP that he posted in TapeOp. I've built this without the  problems you've had with both Edcor and Triad HS-54 on the output without termination.
You may want to look at your layout, I recently had low frequency motorboating issues while restoring some Gates preamp cards, fixed it with a re-routing of grounds.

Click to expand...

The LF hump effect is very dependent on transformer parameters.

Cheers

Ian

ruffrecords said:

I don't think it was oscillation. This is most likely /f noise amplified by a very low frequency resonance.

Click to expand...

Interesting and good to keep in mind.

After more trials with this preamp's low frequency ripple, I'm becoming convinced that it is not just output transformer loading. I bypassed the transformer and the oscillation is still there.

Here are a few videos on the oscilloscope:

Output noise: https://youtu.be/8-evNL6DuTU
B+ at circuit: https://youtu.be/EDmsDhTjOXs
B+ from PSU: https://youtu.be/Z8un8DEzhXA

Anyone ever experienced this before?

The two B+ voltage variations are typical of a plain RC smoothed HT line. What you are seeing is low frequency variations in the local mains supply voltage. How much of this appears at the output depends on the CMRR of the circuit and its very low frequency response. It can also be made worse by any very low frequency peak in response due to the resonance of the output capacitor and the the transformer inductance. How big this peak is depends on the series resistance which determines the Q of the peak. If memory serves, your output stage is a White follower. This has a very low output impedance so the only damping resistance is that of the transformer winding. My own output stages are based on an SRPP and even with negative feedback applied, their output impedance is around 150 ohms. So one thing you could try is to add a series resistor between the White follower output and the transformer primary. i would suggest trying 100 ohms or so.

The first trace has a rather strange  but regular blip on it. To me this looks like external interference. It is clearly not getting in via the HT. If it shows on two separate preamps it might be worth while looking at the output of both at the same time to see if the blips are synchronised. If they are then I think it must be an external source.

Cheers

Ian