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Old 3rd October 2011, 08:30 PM   #31
klewis is offline klewis  United States
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Dick,

Unpacked the Krohn-Hite 4402B today. It's main output is 50ohm along with a 7 Vrms fixed 600 ohm output as well. I presume that the best way to test this is through a power amp? I can also put it through my preamp which is based upon the LME49713. Which would be better?

Ken
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Old 4th October 2011, 07:25 PM   #32
klewis is offline klewis  United States
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Quote:
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Dick,

Unpacked the Krohn-Hite 4402B today. It's main output is 50ohm along with a 7 Vrms fixed 600 ohm output as well. I presume that the best way to test this is through a power amp? I can also put it through my preamp which is based upon the LME49713. Which would be better?

Ken
Now I'm starting to answer my own questions by re-reading your prior posts... " I also think that it looks like the filter is being driven from a 600 ohm source, which will tend to cause the lopsided response -- 600 ohms is about the upper limit on source Z for decent measurements. Try driving from a lower impedance source, like the output of a power amp -- just for response checking." So, the Krohn-Hite has 50 ohm output - a lower source Z than 600 ohm - so, it should be good to go without running it through an amp.

Ken
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Old 6th October 2011, 01:10 AM   #33
richiem is offline richiem  United States
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Hi Ken -- just got back from a little fall trip to Mt. Baker. If you're basing your assessment of the notch depth by sweeping the filter using a sweep from the 8903, this will not show the true depth of the notch -- the sweep frequency resolution just isn't fine enough -- we're talking milli- and micro-Hertz here. Use a fixed frequency source and manually tune the filter while looking at the filter output on the spectrum analyzer's display.

To check the filter's HF roll-off, just put the filter in bypass mode and check the response as you normally would for any amplifier -- it's entirely a function of the filter amplifier's response -- the Twin-T has no effect on the upper bandwidth limit (at least as long as the source Z is fairly low).

Nice pots!

The 4402B has a max output of approximately 7VRMS behind 50 ohms at the main output, and behind 600 ohms at the other outputs. I'm not sure what it's max output is when actually loaded with 50 ohms, but the specs probably say. Running directly into the filter is AOK from the main output -- no amp needed.

Last edited by richiem; 6th October 2011 at 01:16 AM.
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Old 6th October 2011, 04:14 AM   #34
klewis is offline klewis  United States
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Hi Dick,

I'm going to put the new pots and tighter tolerance caps in this weekend and hopefully post spectrum results of the 4402b by Sunday.

Ken
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Old 6th October 2011, 05:08 AM   #35
richiem is offline richiem  United States
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Hi Ken -- just re-read the posts -- if your approx. -1dB at the 2nd H is correct, then you don't have too much feedback; it would be good to know those figures for a 1kHz fundamental, since I've never actually checked the harmonic attenuation at high frequencies.

I selected my caps to within about 0.5% matching, so I expect that closer matching will help your tuning, both within and across ranges.
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Old 6th October 2011, 02:43 PM   #36
klewis is offline klewis  United States
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Hi Ken -- just re-read the posts -- if your approx. -1dB at the 2nd H is correct, then you don't have too much feedback; it would be good to know those figures for a 1kHz fundamental, since I've never actually checked the harmonic attenuation at high frequencies.
Hi Dick,

Have a look at Post #30, the second plot is the 1kHz fundamental. It's at -0.19dB at 2nd H. So, better than the 10kHz. Still, the 10kHz is good enough for me.

I fired up the 4402B this morning (couldn't resist). Set at 10kHz, with it's volume pot set at 2Vrms, the fundamentals we're so good, turned the volume down to 0.6Vrms and the fundamentals went very quiet. Re-calibrated the spectrum analyzer and got 2h -113dB, 3h -123dB 4 & 5h down in the noise. Turned on the +20dB switch and got 2h -92dB, 3h -103dB 4 & 5h -123db.

The 8903a reported THD+N at .0022%, probably more noise than THD. I didn't have a thumb drive handy, but will retest and post images.

Ken
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Old 6th October 2011, 03:10 PM   #37
klewis is offline klewis  United States
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Hi Dick,

I remembered that JackinNJ had posted a comparison of the 4400 and the AP1. I found the post: DIY Analog-to-Digital Converter project.Audio measurements tool It's #41. Looks like the 4402 is a shade better.

Ken
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Old 6th October 2011, 06:06 PM   #38
richiem is offline richiem  United States
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So, the 10kHz harmonic products sum to something under 0.0003% or 3ppm -- that's very good. I suspect that the notch filter fine tuning of the 8903A may be a little off -- there are a couple of pots that fine tune it. Be sure to switch the 80kHz filter (or even the 30kHz) in when measuring at 10kHz, since there are no significant products above the 3rd in the 4402B.
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Old 7th October 2011, 01:30 AM   #39
klewis is offline klewis  United States
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Hi Dick,

I ran the 4402B at about 0.4 Vrms 10kHz and got the following plots. The first one is with the Twin-T set at 0dB gain, the second one at +20db gain. The machine was set in the 1k to 11k range- which the manual shows as the lowest distortion range.

If I turn the volume nob of the 4402B up, the Harmonics start to show up, say above 0.8Vrms. But, if I use the -20db attenuator on the 4402B, the harmonics don't show up with the volume nob way up. Seems that much more that 0.8 V overloads the Twin-T. Maybe the batteries are starting to sag.

Still, nice to the 4402B has very very low distortion.

Ken
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File Type: png 4402b_10k_0-483_v_0dB.png (53.7 KB, 276 views)
File Type: png 4402b_10k_0-483_v_20dB.png (57.3 KB, 260 views)
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Old 7th October 2011, 04:25 AM   #40
richiem is offline richiem  United States
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OK Ken, I'm a little confused -- what was the attenuation of the fundamental through the Twin-T for these plots? It would really help if you included the fundamental in your plots. Do you leave it out because of something to do with the spectrum analyzer?

I ask because with the fundamental nulled by 20dB or more, I can put 6VRMS into the Twin-T without changing the relative levels of the distortion products. I've used the HP 239 to verify this by using the 239's step attenuator -- on it's output -- which selects in 10dB steps (a convenient size) while leaving the oscillator's output amp running at full level.

Is there a headroom issue with the analyzer?
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