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PostPosted: 26 Aug 2018, 08:41 
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I'm assuming that a 249K 0.5W 5% resistor will work just as well as a similar 250K resistor in a pre-amp I'm building.
Am I correct ? I already have a bunch of these 249K resistors.


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PostPosted: 26 Aug 2018, 11:28 
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It depends where its used in the circuit.

If its in a RIAA or NAB equalisation network then both type and value can be critical.
So if you have a batch of resistors its would be normal to match them up with a meter.

Under general PSU or general use it again depends where its is in the circuit, but 249K in place of a 250K should be fine.

Any differences in resistor values can be cumulative because gain in one channel increase multiplied by stages can offset a systems balance between channels. So it would be better to match any gain in system stages via resistor values.

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M. Gregg

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PostPosted: 26 Aug 2018, 11:59 
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Thank you for the response. It is a very simple single stage pre-amp circuit as shown in attachment.
R4 and R5 are the 250K resistors.

R1 is 100K and R2 is 1.2K

What do you think ?


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PostPosted: 26 Aug 2018, 16:45 
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Looking at the circuit,

They will be fine. If you have a batch of the 249K you could test them with a meter so you can match two for R4 of the same value and two for R5 if its stereo Left and Right.

I.e. Left/Right R4 249.3K using the 5% variation to get matching.
I try to do this with all resistors in a stereo circuit with care on anything setting a mid point I.e. voltage set 247K and 247K is better than 250K and 249K splitting a 100V rail to 50V.

NB 250K for R4 could effect the HF and being a high resistance value may have a high inductance (number of turns of resistance film) inside the resistor. <<<but that's not the question you asked.

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M. Gregg

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PostPosted: 26 Aug 2018, 23:23 
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R5 is just the termination resistor in this circuit. The differences will cause minimal change in AC load-line rotation as this is dominated by the input impedance of the following stage. The input forms a voltage divider with R3. The difference between 249 and 250 will not really make much difference.

As for the high frequency response. The value of R4 is added to the output impedance of the driving stage (Ro). This resistance plus the upper portion of R3 is seen in parallel with the lower side of R3 by the preamp input and it's Miller capacitance. In general, this parallel combination will be dominated by the lower portion of R3. Hence the minimum high frequency response will be seen at full volume and will be dominated by the value of R3. In this case, the high frequency -3dB point will be roughly 1/(2π*Cm*R3) assuming that R3 >> R4+Ro.

All in all, the 0.4% difference in mean values will mean nothing to the real world operation of the circuit.

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PostPosted: 27 Aug 2018, 12:54 
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Thanks M. Gregg and Matt for the helpful responses.


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PostPosted: 04 Sep 2018, 21:54 
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Hi, All technically correct...but in the real world nothing that you will attach to it will have that level of precision (or need it). I have never found a source component (CD, DVD, LP etc) that has matching of channels or in most cases adherence to the standard it was supposed to emulate in the 1% range. Most seem lucky to get within 5% and many are far worse. 1% components are reasonably normal for places like RIAA circuits, elsewhere 2-5% are just fine. I probably have in the range of 50K to 100K of components in my shop and the ones with tolerance better than 1% are well under 100. So...back to the original question 249K vs 250K is not important. Either will replace the other. If I were building a spacecraft that had to function for 10 years in a vacuum then I would likely use really precision parts. For audio IMO it is an unnecessary increase in cost. :2c:

Good listening
Bruce

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