Low-distortion Audio-range Oscillator

RE Victor's amazing oscillator- It represents a lot of detailed component tuning and selection. My first gen example get -150 dB H2 and -155 dB H3. The current examples are even better. Measuring that is not easy. IVX's instincts that the impedance needs to be lowered to lower the noise is on track but there are other noise sources you need to consider. 600 Ohms limits the noise to around 3 nV or -170 dBV/1 Hz band. Most analyzers I have checked have an equivalent input noise of around 5 nV/rtHz (Approx 1K Ohms in series with the source). For THD+N that sets limits. Its around -130 dBV with a 20 KHz limit and -125 dBV with the more common 30 KHz limit.
I have seen the improved results with a 10V source. I modded my KH4400 to run at 10V RMS and it does help along with the 50 Ohm source.
Using analog frequency domain filters is the only way to get really low distortion. My CLT-1 gets -170 dB HD3 with heroic passive LC filters. Its a solution that will work for a DAC (the internal oscillator of the CLT-1 starts at .1% (-60 dB) and uses filters to get to -170). Maybe posting a simple to build circuit to filter a "generic" DAC to -150 dB distortion would be useful?
I proposed that long ago :)
However I‘m so busy with something else that I could not work on that.
Was hoping someone else does…
I think I posted schematics of post 1 kHz low pass filters…
 
(How) could I get a balanced power amplifier to oscillate as a "black box" somewhere around 1 kHz for simulation purpose ?
I want to use only + in and - in, + out and - out. No use of ground. Not touch the amplifiers feedback if possible.
I'm intrigued by your question.

First thought was a Victor oscillator, powered from batteries to avoid grounding issues, but I'm betting that somehow misses the objective.
Would you elaborate a bit? I'm guessing the amp in question isn't a generic hifi power amp?
 
I'm intrigued by your question.

First thought was a Victor oscillator, powered from batteries to avoid grounding issues, but I'm betting that somehow misses the objective.
Would you elaborate a bit? I'm guessing the amp in question isn't a generic hifi power amp?
It is a hifi amplifier with balanced inputs and balanced outputs.
My ideas behind that:

1. If simulation of a oscillator based on said amplifier shows very good results, it is a sign that the amplifier must be very good too.
2. If this is the case, a scaled down version could be used to built a low distortion oscillator.
 
Well, balanced will be 1.414 x as noisy as a single ended. So why got that route?
Because: 1. If simulation of a oscillator based on said amplifier shows very good results, it is a sign that the amplifier must be very good too.

Noise can be removed by averaging, possible higher voltage swing would give lower noise ?

Still, I believe in the passive post filter.
 
I think the logic is flawed.
A power amp will always have much less performance, more distortion and noise than a dedicated low distortion/noise oscillator.
The topologies and requirements are completely different.
Starting with a power amp and scaling down will not get you a low distortion/noise oscillator like the Viktor.

Jan
Maybe because of the high gain of the opamp used by Victor.
"scaled down" means it is not a power amplifier anymore, the necessary gain could be set.
 
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The problem is that simulation always assumes perfectly matched components, and no PCB layout effects. Also it will also greatly depend on how sensitive the design is to component variations. Victor saw that when designing his oscillators.

Then also, the op amp open loop gain is both extremely high and has enough phase angle margin to be stable at the gains used. The extra gain goes directly to lowering distortion. Today's op amps would be next to impossible to beat for noise and gain. Distortion too, looking at even a decade old op amp design.

To be honest, the PCB layout and power supply will impact thing more.
 
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I've decided to make some measurements on my restored Tek SG505. It measures below specified distortion in the datasheet at full output. The measurement is taken with ADS127L01.
 

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