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Old 2nd January 2013, 11:19 AM   #11
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Anything after the final filter capacitor or affecting a ground reference is more likely to introduce noise. In the main power circuit heat dissipation is probably going to be a major selection criteria, that is easily calculated. Metal cased wire wound resistors are compact with moderate temperature rise for their dissipation rating and OK for the voltages usually found in audio amplifiers. High voltage power supplies often use long ceramic carbon film or carbon composition resistors for voltage division. Precision voltage references often use constantan wire wound resistors for any precision voltage dividers or gain setting. For Audio metal film for the signal handling and Metal cased wire wound for any power handling should cover most applications. You should always read the manufacturers data sheets and any application notes before using any component in a design. For a resistor, dissipation and voltage rating would cover most applications. Case isolation voltage and capacitance to case are things to look out for with the metal cased resistors. Inductance should be minimised in the supply lead to a voltage regulator and these devices should have bypass capacitors in accordance with manufacturers recommendations. There is so much to cover I can only scratch the surface. Needless to say you will end up reading many documents in any electronic design process and these are usually the best source of information.
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Old 3rd January 2013, 07:51 AM   #12
MGH is offline MGH  United States
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Quote:
Originally Posted by metalsculptor View Post
Anything after the final filter capacitor or affecting a ground reference is more likely to introduce noise. In the main power circuit heat dissipation is probably going to be a major selection criteria, that is easily calculated. Metal cased wire wound resistors are compact with moderate temperature rise for their dissipation rating and OK for the voltages usually found in audio amplifiers. High voltage power supplies often use long ceramic carbon film or carbon composition resistors for voltage division. Precision voltage references often use constantan wire wound resistors for any precision voltage dividers or gain setting. For Audio metal film for the signal handling and Metal cased wire wound for any power handling should cover most applications. You should always read the manufacturers data sheets and any application notes before using any component in a design. For a resistor, dissipation and voltage rating would cover most applications. Case isolation voltage and capacitance to case are things to look out for with the metal cased resistors. Inductance should be minimised in the supply lead to a voltage regulator and these devices should have bypass capacitors in accordance with manufacturers recommendations. There is so much to cover I can only scratch the surface. Needless to say you will end up reading many documents in any electronic design process and these are usually the best source of information.
Wow, thanks for the info! I will try to digest all this. Please see your PM box.
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Old 4th January 2013, 09:17 AM   #13
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I got the PM but for whatever reason the messenger could not find you user name anyway the reply: That is fine, I grew up with tube circuits so I know my way around them. I do need to know relevant voltages and design current. If you do not know the total current, the total maximum plate dissipation of the tubes will do
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Old 4th January 2013, 09:26 AM   #14
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I guess one way of approaching the question of whether matching matters, is that It may not matter at all, but it certainly won't hurt if you do so if it is not going to cost very much to do it then why not

for example some suppliers offer a matching service at reasonable cost ( I got some matched pairs of caps and resistors for my preamp at a cost of $1 extra per matched pair). Which in the long run is probably cheaper than buying 50 or more and matching myself when it is only a small quantity.

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Old 4th January 2013, 09:40 AM   #15
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Matching is important in any circuit that has to track with another circuit, differential amplifiers is one example, push pull amplifiers another. The gain and bias setting resistors would be the most critical.
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Old 4th January 2013, 12:49 PM   #16
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if you are going to match resistors, make shure you dont get thermal mismatches between the matched resistors or all your matching will nulled depending on the change of resistance do to thermal issues. This is a layout , build issue, but I have seen matched circuitry where some resistors were nearer a heat source than there respective resistors on the other channel. This buggered things up a bit! they were 0.1% tolerance resistors though, and in cuitrity that had to work -40/+85. But it is worth noting if you are going to o to the effert to match components.
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Old 4th January 2013, 04:02 PM   #17
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Quote:
Originally Posted by metalsculptor View Post
I got the PM but for whatever reason the messenger could not find you user name anyway the reply: That is fine, I grew up with tube circuits so I know my way around them. I do need to know relevant voltages and design current. If you do not know the total current, the total maximum plate dissipation of the tubes will do
Hi, I sent you PM with my email address.
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Old 4th January 2013, 05:05 PM   #18
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Originally Posted by metalsculptor View Post
Matching is important in any circuit that has to track with another circuit, differential amplifiers is one example, push pull amplifiers another. The gain and bias setting resistors would be the most critical.
Hi, by bias setting resistor do you mean grid blocker? gain setting resistor meaning plate resistor? I have a feeling I'm wrong about these.
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Old 4th January 2013, 05:11 PM   #19
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Quote:
Originally Posted by marce View Post
if you are going to match resistors, make shure you dont get thermal mismatches between the matched resistors or all your matching will nulled depending on the change of resistance do to thermal issues. This is a layout , build issue, but I have seen matched circuitry where some resistors were nearer a heat source than there respective resistors on the other channel. This buggered things up a bit! they were 0.1% tolerance resistors though, and in cuitrity that had to work -40/+85. But it is worth noting if you are going to o to the effert to match components.
I try to get resistors with low temperature coefficient.
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Old 5th January 2013, 07:43 AM   #20
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A grid blocker AFAIR refers to a capacitor, you probably mean grid leak Bias setting resistors can be a resistor in series with the cathode, a voltage divider in a dedicated bias power supply or in a less direct way, a grid leak resistor. Gain setting resistors can be a feedback voltage divider or to a lesser extent a plate or cathode load resistor. Note this only applies to sections of an amplifier which use differential or complimentary amplification stages, for example most of the McIntosh MC75. If you are planning on building an amplifier consisting entirely of single ended gain stages none of this applies. Metal film and wirewound resistors have reasonably low temp coefficients. Marce's point was more about layout, for a differential circuit it is good practice to put the components of both halves of the differential pair in close thermal proximity.
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