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Old 16th January 2014, 04:38 PM   #11
PLB is offline PLB  United Kingdom
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Hi richie00boy,

Please excuse my late reply, I've been output town.
Yes, the THD of 0.005% was measured across the amplifiers output terminals with the 8R+0R1. When the THD analyzer input (Sound Technology 1701A) was reconnected across the 0R1 resistor the THD went up by a factor of 10 times to 0.05%.


Hi DF96,

In my measurements, I used the balanced input of the analyzer to avoid grounding problems. The 0R1 resistor has the following markings;

NTE5WD10
5W.1ohm5%

I also tried two 0R05 wirewound resistors in series. These resistors had no markings. They gave the same THD figures as the 0R1 resistor.
I don't recall the exact wave shape of the THD waveform, except to say that it was different from that of the amplifiers output and both spectra having predominantly low order harmonics. The amplifier used in the tests was an NAD906.

Hi Conrad Hoffman,

Both types of 0R1 resistor were attracted to a permanent magnet!
The resistive material of the 0R05 resistors appeared to have a tape rather that a round type of wire. I was not able to see if the 0R1 resistor had the same wire material since it was one of these square shaped white ceramic encapsulated type resistors.

Best regards

Peter
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Old 16th January 2014, 04:52 PM   #12
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Quote:
Originally Posted by PLB View Post
Yes, the THD of 0.005% was measured across the amplifiers output terminals with the 8R+0R1. When the THD analyzer input (Sound Technology 1701A) was reconnected across the 0R1 resistor the THD went up by a factor of 10 times to 0.05%.
It looks like a measurement method error.
Perhaps it is a very small signal across the 0R1 resistor to get a reliable THD mesurement.
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Old 16th January 2014, 06:22 PM   #13
sbrads is offline sbrads  United Kingdom
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The only way to be sure it's not your test gear is to substitute the 8R with 80 0R1 resistors in series. I wouldn't blame you for not trying!
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Old 16th January 2014, 10:27 PM   #14
PLB is offline PLB  United Kingdom
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Hi Ezavalla and Sbrads,

Perhaps if I advise you of my past experience with measuring THD it will dispel your concerns about measurement error.
1) I joined Dolby Laboratories in 1967 and worked in the R&D department for 9 years as a junior engineer where I had many occasions to make THD measurements with various analyzers like that made by B&K and Radford.
2) I joined Acoustic Research in 1976 and worked in the R&D department for 18 months as an engineer where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Amber and HP.
3) I joined NAD electronics in 1982 and worked for 8 years as Director of Research where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Amber, Tektronix and FFT analyzers.
4) I joined Weltronics Corp. in 1989 and stayed till 2010 (21 years) as Director of Research where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Advantest R9211C FFT analyzer and several PC based analyzers.
In 2012, I attained the grand title of septuagenarian, so as you can see, I have many years experience, not one years experience many times over!! BTW, I’m still hoping that somebody will post a reference or application note etc.

Peter

Last edited by PLB; 16th January 2014 at 10:32 PM.
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Old 17th January 2014, 01:40 AM   #15
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That's a lot of experience!!! but I still suggest to you to apply a larger signal across the 0R1 resistor. You know that if you apply 10 Vpp across de 8.1R, the 0R1 only gets about 120 mVpp and a THD of 0.05% is about 60 uVpp... a bit on the low side for the measurement device.
Anyway, may be the resistor's wire material, may be the electrical joints. AFAIK I have never seen a wirewound resistor which can be attracted by a magnet, but may be just a bad resistor after all....
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Old 17th January 2014, 02:52 AM   #16
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Quote:
Originally Posted by PLB View Post
Hi richie00boy,

Please excuse my late reply, I've been output town.
Yes, the THD of 0.005% was measured across the amplifiers output terminals with the 8R+0R1. When the THD analyzer input (Sound Technology 1701A) was reconnected across the 0R1 resistor the THD went up by a factor of 10 times to 0.05%.


Hi DF96,

In my measurements, I used the balanced input of the analyzer to avoid grounding problems. The 0R1 resistor has the following markings;

NTE5WD10
5W.1ohm5%

I also tried two 0R05 wirewound resistors in series. These resistors had no markings. They gave the same THD figures as the 0R1 resistor.
I don't recall the exact wave shape of the THD waveform, except to say that it was different from that of the amplifiers output and both spectra having predominantly low order harmonics. The amplifier used in the tests was an NAD906.

Hi Conrad Hoffman,

Both types of 0R1 resistor were attracted to a permanent magnet!
The resistive material of the 0R05 resistors appeared to have a tape rather that a round type of wire. I was not able to see if the 0R1 resistor had the same wire material since it was one of these square shaped white ceramic encapsulated type resistors.
Nickel chromium alloy is a common resistive material in wirewounds, metal film and bulk foil resistors. Flat tape is a common construction in the better quality resistors. Low Tc is harder to obtain below 1 ohm and I look for 1% tolerance to be sure I'm getting reasonably low Tc, ie, 100 ppm or slightly better. The cheap sandcast resistors tend to have very much higher Tc, 200 ppm and up.
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Old 17th January 2014, 07:47 AM   #17
RJM1 is offline RJM1  United States
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The last half-century: Wirewound resistors Part one | EDN
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Old 17th January 2014, 12:57 PM   #18
AndrewT is offline AndrewT  Scotland
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I bought some ceramic encased wirewound resistors.

I smashed one up to see inside.
It was an oxide film resistor ! on a round ceramic mandrill with metal end caps that had welded on copper wire ends that came to the outside.

But it was still very good at the job I had intended for it.
__________________
regards Andrew T.
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Old 17th January 2014, 02:38 PM   #19
PLB is offline PLB  United Kingdom
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Hi RMJ1,

Page 4 under the heading “Polarities” and the resistor “diode” effect, seems to shed some light -- I wonder! Many thanks for the link.

Regards

Peter
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Old 17th January 2014, 03:22 PM   #20
sbrads is offline sbrads  United Kingdom
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Quote:
Originally Posted by PLB View Post
Hi Ezavalla and Sbrads,

Perhaps if I advise you of my past experience with measuring THD it will dispel your concerns about measurement error.
1) I joined Dolby Laboratories in 1967 and worked in the R&D department for 9 years as a junior engineer where I had many occasions to make THD measurements with various analyzers like that made by B&K and Radford.
2) I joined Acoustic Research in 1976 and worked in the R&D department for 18 months as an engineer where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Amber and HP.
3) I joined NAD electronics in 1982 and worked for 8 years as Director of Research where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Amber, Tektronix and FFT analyzers.
4) I joined Weltronics Corp. in 1989 and stayed till 2010 (21 years) as Director of Research where I had many occasions to make THD measurements with various analyzers like that made by Sound Technology, Advantest R9211C FFT analyzer and several PC based analyzers.
In 2012, I attained the grand title of septuagenarian, so as you can see, I have many years experience, not one years experience many times over!! BTW, I’m still hoping that somebody will post a reference or application note etc.

Peter
I never doubted your experience one iota, you beat me (just) in years and in experience as well I dare say.

My own experience is more based in finding problems in new designs and that tells me that equipment however, no matter the cost, is never perfect.
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