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Old 8th September 2013, 06:34 PM   #3131
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
Another experiment is forcing the op amps to operate in class A by pulling the output to one of the rails with a current source.

Could that have a small affect/improvement on the overall distortion out of the oscillator?

-RM
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Old 8th September 2013, 06:56 PM   #3132
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
Could that have a small affect/improvement on the overall distortion out of the oscillator?


-RM

It's something Samuel suggested to me a while back with the 1468's. I was chasing sources of distortion in my SVO. Had a third H that just wouldn't go away. It didn't help for this particular circumstance and I haven't pursued the effect further but it worth looking into.
It may help with the distortion caused from loading of the stages. SVO's have a tremendous amount of loading from multiple sources. the Ring's summing and tuning, the multiplier inputs the AGC inputs and output loading. It's a large demand on the op amps. Buffering the outputs to the AGC and possibly to the multiplier help with this.

Other op amps like the 5534 might do better with class A loading. The 1468 hasn't done well with the op amp tricks. Paralleling or this. It needs more attention in an isolated circuit before trying to apply the tricks to an SVO.

The compensation used on the 5534 in the AP sys1 is interesting. The manual describes it as a two pole compensation.
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Last edited by davada; 8th September 2013 at 06:59 PM.
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Old 8th September 2013, 09:17 PM   #3133
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Originally Posted by davada View Post
Another experiment is forcing the op amps to operate in class A by pulling the output to one of the rails with a current source.
This brings up another question.
What is the distortion level of the current source.
In operation we have several volts p-p and we are looking for distortion in the -140dB range or lower.
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Old 8th September 2013, 09:19 PM   #3134
davada is offline davada  Canada
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Originally Posted by myhrrhleine View Post
This brings up another question.
What is the distortion level of the current source.
In operation we have several volts p-p and we are looking for distortion in the -140dB range or lower.
The current source can be something predictable like resistor.
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Old 9th September 2013, 02:21 AM   #3135
1audio is offline 1audio  United States
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The KH4400 has an actively modulated current source on the key opamp in the oscillator. I figured it was to lower the distortion of the opamp. Jim Williams published a design using a buffer chip on the output of the oscillator to improve the distortion.
I just got a small improvement in the KH4400 replacing the key opamps with LME49990's. The 2nd improved a little and the noise more.
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Old 9th September 2013, 08:31 AM   #3136
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Quote:
Originally Posted by myhrrhleine View Post
This brings up another question.
What is the distortion level of the current source.
In operation we have several volts p-p and we are looking for distortion in the -140 dB range or lower.
Nearly irrelevant, as it is driven by a low-Z node. Just make sure it doesn't saturate during normal operating condition (which might rule out JFET solutions).

A pure resistor will usually result in more distortion than an active current source because it causes higher output current variation.

Samuel
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Old 20th September 2013, 11:10 PM   #3137
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Default Is a group buy for a PCB available for this oscillator?

Is there a group buy for a PCB and parts for this ocillator? If not, are Gerbers
available? Let me know. Take care.
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Old 21st September 2013, 06:02 AM   #3138
RNMarsh is offline RNMarsh  United States
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Originally Posted by 1audio View Post
The KH4400 has an actively modulated current source on the key opamp in the oscillator. I figured it was to lower the distortion of the opamp. Jim Williams published a design using a buffer chip on the output of the oscillator to improve the distortion.
I just got a small improvement in the KH4400 replacing the key opamps with LME49990's. The 2nd improved a little and the noise more.
Demian --- do you know what the THD and THD+N are on the KH4400 now?

[BTW/FYI - the A-P 2722 is showing me .0003% THD+N (80KHz BW) or -110 dBV using its internal analog osc+analyzer.]

-Richard

Last edited by RNMarsh; 21st September 2013 at 06:07 AM.
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Old 22nd September 2013, 06:02 AM   #3139
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What drive level are you testing at? With 80 KHz bandwidth you are mostly measuring noise, unless the harmonics are pretty strong.

I am getting about -112 dB with 80 KHz and 10V drive with the 590AR. The KH4400 (at approx. 7V) is about -109 but there is some significant low frequency noise I need to explore. Both of those get about 2-3 dB better with 30 KHz bandwidth. I don't see any significant harmonics above -135 dB past no. 3 with the 590AR and about 7 with the KH4400.
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Old 22nd September 2013, 06:06 AM   #3140
1audio is offline 1audio  United States
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Originally Posted by mrsavage View Post
Is there a group buy for a PCB and parts for this ocillator? If not, are Gerbers
available? Let me know. Take care.
There isn't an oscillator yet to have a design. There are a number of approaches depending on different aspects. It has been mostly teaching/learning about the frontiers of ultra low distortion oscillators.
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