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Old 4th July 2013, 01:48 AM   #2871
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Quote:
Originally Posted by RNMarsh View Post
Bob -- Those are exellent numbers by any standards. I did read it when published in 1981. And a couple times more recently and just now to refresh my mind. It has passed the test of time and goes down as one of the greats. A legend.

Until a few months ago, your design was the only THD schematic plus the IG-18 circuitry that I ever saw. I would have built your design but I just could not get my head around the construction of it. Especially the chassis and all.

When I could afford something, i bought an A-P model One dual domain. That served me well until I could not measure the distortion of a simple headphone amp. So, I needed something better whithout breaking the bank again. But, still I didnt want to build from scratch. I'm just not that good with my hands. So, using low cost used commercial products and updating them got me where I am today.

The THD+N of this second 339A runs .0008% (400-80KHz filtering) to 10KHz. The first 339A I was able to get below the noise level of the AD725 analyser but over a limited frequency range [ estimated to be -160dB). One additional area that helps me get so low is the coupling caps. I changed them all to bipolar type, placed back-to-back of highest quality and 105C rating.

The K-H 4400A uses similar topology and my notes say the 'tuned' THD is .0001% to .00025% (1-10KHz). However, the newer K-H 4402B departs for a better control circuitry and I can get THD of .00005 and .0001% from it. They both used the same osc opamp.
[I didnt record the THD+N on them.] I have tried many opamps and the 1468 always works best.

More elaborate commercial brands/models are very good stock when you can find them on the used market.... in the $2k range. They are much, much better than the 339A right out of the box. For example, the VP-7722A has its internal source and analyzer combined THD+N of .0003-7% and THD only of .0001% combined. Or -120dB. I am sure they can be 'tuned' as well using techniques similar to the others.

Then we get to Victor's oscillators... a great value and performance but osc only. I havent found the FFT cards to be accurate enough at low harmonic levels compared to commercial test equipment. And, we still need to try transient recorders on power amps etc.

THX RNMarsh
Hi Richard,

Thanks for your kind words.

This is a great thread, and you deserve much of the credit. My kind of thread. Hard-core design stuff that everybody can learn from - and not a lot of arguing and grand-standing.

I also have a 339A that I picked up for free a couple years ago. I have not been inside of it, but its performance as-is has been a bit disappointing (maybe you get what you pay for). Anyway, this thread has encouraged me to get in there and tweak it when I get some time.

I owe much of my SVO inspiration to Bruce Hofer, an old friend of mine from when he was at Tek designing TM500 modules, including their oscillator/THD analyzer modules. His AES paper on the Tek SVO was what got me going. Of course, he and Rich Cabot subsequently left Tek and went on to found AP.

Cheers,
Bob
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Old 4th July 2013, 05:02 AM   #2872
davada is offline davada  Canada
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Hi Bob,

I imagine you got your 339A for between $150 and $300. Wouldn't you have choked if you had pain full price for it?

Cheers,
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Old 4th July 2013, 05:45 AM   #2873
RNMarsh is offline RNMarsh  United States
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Hi Bob,

I imagine you got your 339A for between $150 and $300. Wouldn't you have choked if you had pain full price for it?

Cheers,
[ he got it for free]
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Old 4th July 2013, 04:45 PM   #2874
RNMarsh is offline RNMarsh  United States
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We can compare our progress with AP 2722 model - their THD+N spec is for 1KHz with a 22KHz BW and higher output level (2 volts): Both generator and analyzer are -112dB + 1 uV (typical -115).

Thx-RNMarsh
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Old 4th July 2013, 05:22 PM   #2875
RNMarsh is offline RNMarsh  United States
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The ShibaSoku combo is at least as good as the best Audio Precision. Here is the combined THD+N at 2v and 1KHz.
But with 100KHz BW. Combined result is: -114dB.
AP gives typical (22KHz BW) individual numbers of .... -115 plus -115dB = ? combined. Maybe -112dB? Plus 1uV each (noise floor).

-RNM

725 THD+N.jpg

Last edited by RNMarsh; 4th July 2013 at 05:32 PM.
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Old 4th July 2013, 08:58 PM   #2876
RNMarsh is offline RNMarsh  United States
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yes, there are some small phase shift going on which cause the distortion null to shift away. It can be tried with a variable C trimmer at the appropriate place while watching the thd meter.

THX-RNMarsh
Take a look at the 339 oscillator schematic I put up, you will see a cap (C47) across R50. This is probably for that HF harmonic reduction function. I will put a variable C in place of C47 and see what I can do with it to reduce THD at higher freqs.

THx-RNMarsh

Last edited by RNMarsh; 4th July 2013 at 09:06 PM. Reason: Fine tuning -
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Old 4th July 2013, 10:00 PM   #2877
richiem is offline richiem  United States
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Note that the HP 239 and 339 oscillators deliver their excellent performance at 6.3VRMS output from the oscillator into a medium-Z load; and only somewhat worse at 3.16VRMS into 600 ohms.

I have Bob's SVO set to deliver 1.6VRMS from the output of the second integrator, and use a gain stage to get to nearly 10VRMS with 15V supplies. My measurements reported recently above have been done at 6.3VRMS output for direct comparison to the HP units.
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Old 4th July 2013, 11:54 PM   #2878
RNMarsh is offline RNMarsh  United States
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Bob was using THD+N and so was I when comparing against Audio Precision 2722 as that is how A-P spec their equipment. So, the numbers you have from Bob's design... are they THD+N or THD (using FFT)? You indicated THD.

Does anyone make PCB for Bob's design available today?

Thx-RNMarsh
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Old 5th July 2013, 12:03 AM   #2879
richiem is offline richiem  United States
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THD. ARTA seems to give different result for THD+N than the analog instruments I've used in the past -- not that it's wrong, just different. Part of the problem appears to be that the THD+N numbers don't seem to properly reflect 400 Hz high-pass filtering, which most of us use in one way or another.
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Old 5th July 2013, 12:28 AM   #2880
davada is offline davada  Canada
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Originally Posted by richiem View Post
Note that the HP 239 and 339 oscillators deliver their excellent performance at 6.3VRMS output from the oscillator into a medium-Z load; and only somewhat worse at 3.16VRMS into 600 ohms.

I have Bob's SVO set to deliver 1.6VRMS from the output of the second integrator, and use a gain stage to get to nearly 10VRMS with 15V supplies. My measurements reported recently above have been done at 6.3VRMS output for direct comparison to the HP units.
Wow. I'm doing well with my SVO at 3Vrms then. If I ran it at that level the disto would be deep in the noise.
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