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Old 14th July 2008, 10:48 AM   #641
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Quote:
Originally posted by Edmond Stuart


NO! I said: "using the same resistors from the same batch". That also means using resistors of the same value. So for a 10:1 divider you will need ten (10) resistors, I repeat, of the same value, and of course with the same nonlinear (if any) behavior. Then nil distortion from the resistors.

Edmond,

Wouldn't this configuration also reduce noise? Multiple resistors of the same value would add noise in RMS fashion (they are uncorrelated).

For the 10:1 ratio and a 100K ladder: Nine 10K resistors would RMS to 30K (for noise figure calculations, not resistance) while one 90K resistor would RMS to... 90K and resistor noise is proportional to resistor value.

It seems to me that your configuration would reduce noise by 9.5dB (but I might be misremembering something).


Jeff
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Old 14th July 2008, 11:22 AM   #642
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Quote:
Originally posted by PHEONIX
Hello Edmond

I once saw the inside of a Halcro DM68( I think ) and they used a combination of series and parallel resistors to construct the feedback resistor , from output to inverting input of amplifier. What struck me about it was that they used quite a few resistors rodenstein MK8 metal film types . I thought the network was large to make up a low reistance value to meet the equivalent input noise figure of the amp of 5nV/sqrt(Hz).

The M70 Yamaha (0.0005% @ 20Khz) is covered in carbon resistors which surprised me, I think this shows that leaded reistors are pretty low in THD. On the other hand thick film smd resistors have measurable distortion and should be avoided for ultra low THD products.

Regards
Arthur
Hi Arthur,

Apparently the guys at Halcro were afraid of the self heating effect and used a bunch of larges MK8 (2W) resistors.

I'm also surprised about using carbon R's. According to Cyril Bateman, who did a lot distortion of measurements on C's and R's, one specific brand of carbon film R's produce ~0.00013% distortion when loaded to 25% rated power.
Welwyn RC55 (metal film), on the other hand, performs much better: 2nd harm. -127dB, 3rd harm. -125dB.

As for SMD thick film, you are right, totally unsuitable for ultra low THD. I had to replace those 'sic film' types (603) on my ESI sound card by professional MELF types. (those ******* idiotes at ESI realy know how to spoil a good sound card)

Cheers,
Edmond.
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Old 14th July 2008, 11:44 AM   #643
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Quote:
Originally posted by Jeffin90620
Edmond,

Wouldn't this configuration also reduce noise? Multiple resistors of the same value would add noise in RMS fashion (they are uncorrelated).

For the 10:1 ratio and a 100K ladder: Nine 10K resistors would RMS to 30K (for noise figure calculations, not resistance) while one 90K resistor would RMS to... 90K and resistor noise is proportional to resistor value.

It seems to me that your configuration would reduce noise by 9.5dB (but I might be misremembering something).

Jeff
Hi Jeff,

No, it doesn't matter. It's true, noise is added in RMS fashion, but remember, the noise of a (single) resistor depends on the square root of its value.

Cheers,
Edmond.
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Old 14th July 2008, 12:07 PM   #644
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Quote:
Originally posted by Christer
Yes, that was my point. I also realized what you say, but depending on layout, you might get a considerably longer path through the resistors than otherwise. On the other hand, I guess it should rather decrease the bandwidth and improve stability, than the opposite.
Hi Christer,

Sure. Making, for example, a 30:1 divider this way is not very practical.
The only reason I put forth this trick was to show that it's theoretical possible to make a divider with far lower distortion than the distortion of the resistor itself. Also, that distortion figures below -130dB from a simulator are not necessarily fooled by the distortion of passive
components in real life.


Cheers,
Edmond.
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Old 14th July 2008, 12:17 PM   #645
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Quote:
Originally posted by Edmond Stuart

Sure. Making, for example, a 30:1 divider this way is not very practical.
The only reason I put forth this trick was to show that it's theoretical possible to make a divider with far lower distortion than the distortion of the resistor itself. Also, that distortion figures below -130dB from a simulator are not necessarily fooled by the distortion of passive
components in real life.
Yes, and even if not practical to follow to an idea to 100 %, it is still interesting to figure out ways to cancel errors. If voltage dependancy is a non-linear function, it would then still help to have a few resistors in series, than using a single larger one.

For thermal effects, using an oversized resistor is better than one based on straightforward power calculations, but that has been covered in earlier threads.
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Old 14th July 2008, 12:30 PM   #646
PHEONIX is offline PHEONIX  Australia
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Quote:
Originally posted by Edmond Stuart


As for SMD thick film, you are right, totally unsuitable for ultra low THD. I had to replace those 'sic film' types (603) on my ESI sound card by professional MELF types. (those ******* idiotes at ESI realy know how to spoil a good sound card)

Cheers,
Edmond.

Hello Edmond

The funny thing about ESI is that they own a late model Audio Precision , and you would think they would find this problem. I agree with you ESI is very good value for money.

Regards
Arthur
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Old 14th July 2008, 12:33 PM   #647
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When we build Bateman's oscillator (before Bateman) we used Dale RN55C's right out of the stock room, with no special precaution's and got -130dB on all harmonics (most much better than that).
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Old 14th July 2008, 01:20 PM   #648
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Quote:
Originally posted by PHEONIX



Hello Edmond

I once saw the inside of a Halcro DM68( I think ) and they used a combination of series and parallel resistors to construct the feedback resistor , from output to inverting input of amplifier. What struck me about it was that they used quite a few resistors rodenstein MK8 metal film types . I thought the network was large to make up a low reistance value to meet the equivalent input noise figure of the amp of 5nV/sqrt(Hz).

The M70 Yamaha (0.0005% @ 20Khz) is covered in carbon resistors which surprised me, I think this shows that leaded reistors are pretty low in THD. On the other hand thick film smd resistors have measurable distortion and should be avoided for ultra low THD products.

Regards
Arthur

Hi PHEONIX,

Do you know what the distortion mechanism is in thick-film SMD resistors? Just curious.

Thanks,
Bob
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Old 14th July 2008, 02:04 PM   #649
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25% of rated power on a 1/8th Watt 1k resistor is ~5.6V peak. Are we talking about realistic situations here? This would be pretty loud at the output of a preamp and who would use 1/8th Watt resistors in a power amp feedback network?
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Old 14th July 2008, 02:33 PM   #650
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HI Scott,

Regarding feedback networks in power amps, certainly not realistic. But the test by itself is valid. Nothing wrong with a load of 25% rated power.

Cheers,
Edmond.
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