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Old 10th March 2014, 02:44 PM   #171
1audio is offline 1audio  United States
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I saw that too and did socket the opamp. But when the distortion dropped basically into the noise I stopped fiddling. In the plot below the top of the scale is approx -100dB, the fundamental is -122dB and the second is at -142dB. I am not sure what will detune or otherwise alter the performance of the notch filters. The manual is completely devoid of alignment info. This is with Viktor's oscillator, which is the best source I have. I figured I would stop while I'm ahead.
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Old 10th March 2014, 06:58 PM   #172
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Is there a source for the manual to identify where things are for the 725D? I'd do this and more... I bought certain opamps for lowering noise and what all some time ago. But without that schematic and pcb board identifier, it is just too time consuming.


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Old 11th March 2014, 04:26 AM   #173
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Quote:
Originally Posted by 1audio View Post
I saw that too and did socket the opamp. But when the distortion dropped basically into the noise I stopped fiddling. In the plot below the top of the scale is approx -100dB, the fundamental is -122dB and the second is at -142dB. I am not sure what will detune or otherwise alter the performance of the notch filters. The manual is completely devoid of alignment info. This is with Viktor's oscillator, which is the best source I have. I figured I would stop while I'm ahead.
Very

-RNM
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Old 11th March 2014, 04:34 AM   #174
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Quote:
Originally Posted by Samuel Groner View Post
It might be worthwhile to point out that the opamp which bootstraps the notch filter (QA2) does contribute to the distortion residual in a non-obvious way. While it does not see significant input or output voltage swing, its output sources significant AC current which in turn causes a distorted voltage to appear at the output terminal. Perhaps biasing the opamp into class A (see e.g. Tek SG505 schematic for a neat way to do this with a '5534) is useful here.

Samuel
Great helpful input !

Curious.... what would a dual opa in parallel here accomplish?

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Old 14th March 2014, 06:20 AM   #175
RNMarsh is offline RNMarsh  United States
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I am Still waiting and keeping an eye out for a 725D schematic/manual.

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Old 14th March 2014, 07:09 AM   #176
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At best a 3 dB reduction in noise? Paralleling opamps on a chip can work. Yamaha did it in an FM tuner with good results. No improvement in distortion however.

Can you take a picture of the first band elimination filter board in the 725D? I may be able to figure out the board. The circuits have not changed much.
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Old 14th March 2014, 06:38 PM   #177
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The pcb are not labelled as such. Just a part number on them. There are no Harris opamps any more.... seems they all got changed to 5532 etal.

Can you put up the selectable filter circuit board for us? I think that part didnt change. Maybe I can reduce the S/N on the monitor output path.

-RM
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Old 14th March 2014, 08:12 PM   #178
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Quote:
Originally Posted by RNMarsh View Post
What would a dual opamp in parallel here accomplish?
As they share the output current a small reduction in distortion can be hoped for, but crossover distortion is a complex phenomena (over a limited range reducing the output current might increase distortion a bit) so this might not work that way in this particular instance.

I'm not sure if there is perhaps also a side effect from doing this--you need (small) output resistors to sum the outputs (otherwise the different offset voltages make the opamps fight each other), and that might interfere with the frequency response of the notch filter. But the effect is probably negligible.

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Old 11th April 2014, 01:11 AM   #179
RNMarsh is offline RNMarsh  United States
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FYI --- I dropped the 725D off with Demian this week. Some tweeks he has done to an older version yielded much better performance.



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Old 14th April 2014, 04:42 AM   #180
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I have Richards 725D. Here are a few quick impressions:
Its really similar to the older units but redone from top to bottom. The boards have more functionality per board.

The key new features are the dual differential inputs etc. that allow testing a stereo source without swapping cables. It has an internal 20 KHz low pass filter probably to meet the EIA and JIS standards for testing digital amps with lots of ultrasonic junk. And it has a 2 mV distortion capability for measuring the dynamic range of a DAC per the standard tests.

Richards unit does have a noise problem, that my much older unit does not have. I think it relates to the post filter gain circuitry since its not real obvious in the analog outputs but I'm not sure yet.

The notch filter is different- instead of three boards of twin t filters there is one board with enough relays for three filters. However there are not enough opamps for three filters. I will need to do some reverse engineering to figure it out. It also has a stacked opamp that appears to be two cascaded opamps as a "fix". It still uses the open loop filter scheme of the older units, no servo for tuning.

Overall its pretty similar to the older design so reverse engineering what I need should not be really hard.

I'll try to post details as I dig into the beast.
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