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Old 11th December 2012, 04:18 PM   #1091
RNMarsh is offline RNMarsh  United States
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I take it your results are good?

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Old 11th December 2012, 05:06 PM   #1092
RNMarsh is offline RNMarsh  United States
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Default Greening the HP339A -

One can get excellent results by starting with parts upgrades to an HP339A; trim pots and electrolytic. Then Opamps. I only used the LT1468 types which are internally compensated. The IC thd is plenty low (>-120db spec) and I dont want to mess with wide band, unstable IC's used in a layout designed for 100KHz.... even if their thd is even lower. Contacts are a problem still... the old brass contacts do not wear well with time and the tarnish isnt nice to thd. Clean them and keep them clean. Will add the new buffer IC when it arrives. Thx-RNMarsh

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Old 11th December 2012, 07:07 PM   #1093
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Check with Brian Elliot.. I think he still lives in Stanford area. he did origianl reserach into distortion limits/mechanisms for IBM... then when he retired from IBM, went back to Stanford and worked for HP for awhile (PHD EE Stanford). He did capacitor tests using a bridge setup and found some interesting things... one I think was that on some film caps the thd/harmonics went up with DC bias on them. The MultiCap was best brand he measured. Dont have to duplicate tests if someone else has already done them, is my thinking. Thx-RNMarsh
Hi Richard,

As I recall, when Bateman did his testing different pots had different distortion levels.
Cermet being the worst but some conductive plastics had distortion levels of 0.000x
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Old 11th December 2012, 07:09 PM   #1094
1audio is offline 1audio  United States
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I have seen big differences in pots. The Alps measured the best in available technologies and the no name china 20 dB worse.
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Old 11th December 2012, 09:00 PM   #1095
RNMarsh is offline RNMarsh  United States
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Sorceress and Demian--> You are both correct: Use conductive platsic when you can. A bargain used to be that some pots sold by Radio Shack were made by Alps and Brian Elliot said they were unmeasureable in thd. It wasnt conductive plastic, though. I used one I had in my parts stash on my headphone amp input.
Those tests did not put DC thru the pot at the same time they are tested with ac.

However, to get the fine pointed, deep null trim I needed to use a multi-turn pot.... I used a 28-turn cermet. In this application the cermet worked well and did not increase distortion to where it would be noticable ---- things were more limited by other parts/issues... [dont know brand/source/quality I used.] When not able to measure, then use conductive plastic, generally, as first choice. Thx-RNMarsh

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Old 11th December 2012, 11:52 PM   #1096
davada is offline davada  Canada
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Default Pot

Okay so where are you all getting your pots from? I'm kind of stuck with Digikey and Newark as second. Everything is mail order where I am. I spend almost as much in shipping as I do in parts. I Tend to make big orders because of this. Digikey had one pot that will work for the oscillator level control that's under $65.00 and I think it is cermet. I don't like the wire wound because of the natural detent. I can never get the level quite on.

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Old 12th December 2012, 12:16 AM   #1097
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Originally Posted by davada View Post
Okay so where are you all getting your pots from? I'm kind of stuck with Digikey and Newark as second. Everything is mail order where I am. I spend almost as much in shipping as I do in parts. I Tend to make big orders because of this. Digikey had one pot that will work for the oscillator level control that's under $65.00 and I think it is cermet. I don't like the wire wound because of the natural detent. I can never get the level quite on.

Cheers,

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WW have the lowest noise. Inductance shouldn't be an issue.
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Old 12th December 2012, 12:26 AM   #1098
davada is offline davada  Canada
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Default More 339a mods

It just keeps getting better. I've done some gain redistribution in the 339a and the result is,
well, a quantum leap.

The total attenuation and total gain through the signal chain is 80dB and this is spread throughout the stages. I mentioned before the intent to attenuate the buffer input by 10dB to normalize the output of the notch amplifier to the FS of the QA400 software. This left the analyzer 10dB short so I made up for it by adding 10dB to the x100 meter amplifier. Not only did the noise floor drop but so did the distortion. With a well tuned notch the meter dropped from the 339a residual of 0.0015% to 0.0005%.

I'm going to do more gain redistribution throughout the analyzer because with lower signal at the input amp, buffer, notch filter etc. both the noise from ingress and THD is lower. The meter amp doesn't seem to mind the extra gain. I may change this amplifier to the 1468 for extra bandwidth and lower noise. The 1468 is stable at high gains without any further compensation.

Another issue is with maintaining the notch depth. As it is the it works at the level it calibrated for but loses the depth at other levels. Taking the signal straight off the notch filter doesn't offer constant level. But we have constant level off the auto set level for sure. I going to try routing the signal from here to the input of the balance modulator. I think this should work as long as there isn't a change of phase at this point with all the variables.

If we can move most of the gain to the meter amp without losing bandwidth, I think the results will be even better than what I have now. If we could put all of the 80dB at the meter amp then all we have in the rest of the signal chain is attenuation. The only thing we have to respect is the minimum level into the RMS convertor.

I will report on the effect of both the gain redistribution and routing of the modulator input.


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Last edited by davada; 12th December 2012 at 12:44 AM.
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Old 12th December 2012, 01:17 AM   #1099
RNMarsh is offline RNMarsh  United States
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When will I be able to get a QA400 like you have? My FFT works but isnt practical to lug around an old heavy HP FFT bench analyzer to monitor residuals. I know there are other FFT (ARTA/ADC-DAC) but the one you use offers some good and useful features.
Looking forward to more changes for the better. And to think, you were about to throw in the towel on this 'boat anchor'. Thx-RNMarsh

Last edited by RNMarsh; 12th December 2012 at 01:22 AM.
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Old 12th December 2012, 03:20 AM   #1100
davada is offline davada  Canada
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When will I be able to get a QA400 like you have? My FFT works but isnt practical to lug around an old heavy HP FFT bench analyzer to monitor residuals. I know there are other FFT (ARTA/ADC-DAC) but the one you use offers some good and useful features.
Looking forward to more changes for the better. And to think, you were about to throw in the towel on this 'boat anchor'. Thx-RNMarsh
It's still a boat anchor.

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