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Old 5th May 2004, 01:40 PM   #11
Jay is offline Jay  Indonesia
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Default Re: tolerances of 1% metal film resistors

Quote:
Originally posted by Wagener
How accurate do I need to have them? Some of them are precisely the value they should be. I suppose if Dr. Leach specified 1% values then using a resistor which are 1% out is ok? right?
In most cases, it is not the tolerance that matters, but the matching value between certain pair of resistors. (So you know which pairs should go to the other channel/PCB)

So if you get 4 resistors that ALL fall at exactly +10% that is probably okay, especially if the 4 are used to divide the current in the differential stage.
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Old 5th May 2004, 03:27 PM   #12
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Quote:
Originally posted by Circlotron

The meter was a HP34401A. It does have 4-wire resistance measurement facility but I wasn't using it at the time. It also has a null button for zeroing out the ~0.038 ohms (YMMV) of the test leads. It not mine - it's a work one.
Click the image to open in full size.

the HP34401A sells for around $500 on EBay.

the HP3478A and HP3468A will sell anywhere from $70 to $200. Of these the 3478A has an additional digit. the advantage of the 3468A is that it connects to the HP-IL loop in case you want to connect it to a HP Calculator.

The 3478A is also a very good RMS meter -- while it uses the AD536 chip for true RMS it is almost as acurate as my thermal RMS reading meter. The bottom meter shown below should be reading current -- I was using it to match some transistors yesterday:
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Old 5th May 2004, 03:50 PM   #13
sam9 is offline sam9  United States
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The tolerance required in a particular amp (or other device) can span the range from very critical to surprisingly uncritical. Sometimes it is the ratio between two resistors rather than the absolute value that matters. Where you hace an RC network and are using caps with a 20% tolerance, it seems dubious that either 1% or even 5% resistors are critical.

On the other hand, in differential inputs and current mirrors (though I don't think the Leach uses CMs), matching better than 1% can be useful. I think Self mentioned that even 1% de3generation resistors could have a difference of up to 2% which was enough to degrade performance.

All that said, I have on occasion, deliberately use only 5% resistors and selected transistors for CMs and differential pairs without any attempt at matching, just to see what one could get away with. The results were a functional amp. It probably just didn't perform quite as well as one might like. I say probably, because you probably couldn't hear anything wrong unless you did a very well controlled bind listening comparrisson.

Despite this, I suggest you follow the advice for the Leach and even match some resistors closer if possible. It might be more sensative to exact values somewhere and you will feel better afterwards. This last is not intended to be silly, for amps I intend to listen to and not just expiriment with, I use all 1% resistors, match CM and differential pairs, and even try to sort caps to a better tolerance than claimed. Then, at least I know that there is now issue with component tlerance. Besides it feels good - like driving on Pirelli's even if you are only going to buy groceries!
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