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Old 22nd February 2004, 11:34 AM   #51
yoke is offline yoke  Croatia
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Thanks Cheff...
Now this maid be good idea for someone who is not total newbie as Ií am...and making circuit like this would take additional costs, and I donít have AF generator ...I was hopeing for some simple circuit with two resistors or something...
Which everybody can use...I believe that here is many newbies like me who would like and have nead to match resistors...

Thanks once again Cheff ...
Any other sugestions ???

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Old 22nd February 2004, 07:36 PM   #52
akb1212 is offline akb1212  Norway
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Default Matching Resistors

Ok, there seems to be need for an explanation on how to match and more importantly MEASURE low resistance resistors.

There are ways to measure low resistances, and one of the most efficient methods is what is called the 4-wire method. In short this method uses a precision current source and a precision voltmeter.

When measuring the current source is set up to run a precisely known current through the resistance, and the voltmeter is used to measure the voltages that builds up over the different parts of the circuit.

Normal multimeter doesn't go as low in the resistance measurements. This is because the lower values will be inaccurate due to losses in wires, connections and so on. The only way to get around these problems is to use separate circuits for supplying the current that is needed to measure the resistance and the voltage measurement. This is after all the method used, but at normal values these problems doesn't play much of a role.

To give an example on how to do this: To make calculations easy letís say we use a 1A current source. Now to measure the resistance on the parts all we need to do is to measure the voltage over the part. Now if we take a 0.33 ohm resistor as an example 0.3300V should be ideally measured across the resistors. This range is perfectly measurable with a standard multimeter with fairly good accuracy. With a 4 1/2 digit multimeter you will be able to measure resistance with better than milli-ohm accuracy! This means you will even be able to measure the resistances in the contact points! And all you have to do to measure them is to measure the voltage across them and not the resistors. Remember what you are measuring are voltages, and there are no problems with losses when measuring them, and you are using almost infinitely larger inner resistance in your multimeter so your measurements will not affect the measurement.

If you want to simplify it you can dispense the dead accurate current source if you hook in a current meter in series with the measurement. That way all you have to do is use ohm's law to calculate the resistances.

The downside of this measurement method is the high currents involved. The heat losses in the components will soon be a problem if the measurements arenít done on power resistors, especially when the resistances are increasing! If you run the same 1A through a 10 ohm 0.6W resistor you'll fry it in seconds! So this method requires knowledge and thinking before using!
Elector has at least one really good article on building a really good low resistance measurement box. This also uses the low duty cycle method to avoid the heating problems. The current is on 1/100íTh of the time, and left off the rest. This way the heat in the measured device is only 1/100íTh of what it would be without. I have it somewhere but canít find it now (and it is in Swedish...). If interested you should be able to find it if you do a search on their site.

But there are even more advantages, and that is the super matching possibilities! Letís say you have 30 resistors you want to match. All you have to do is to connect them all in series with a suitable voltage source and a current-meter. If we take the 0.33 ohms resistors from the example above you need aprox. 0.33V per resistor (if you want to test at 1A), and with 0.33V X 30 = 10V. If you want to test them at a different current (for example the actual current you are going to use in your amp) all you have to do is calculate the corresponding voltage. You have to see to it that they have the same cooling possibilities they have in the amp! And also possibly a heating period to stabilize in temperature.

Now all you do is measure the voltage across each of the resistors. And remember that the connections have resistances too, and will be taken in to the measurement if you donít remember to place the probe at the right spot! All you have to do now it to match the ones with closest voltages! And since they share the current it will be perfectly equal in each resistor. So the voltage over each resistor is a perfect indication on the resistance.

Hope this helps! Now you should be able to match the resistors even better than the transistors! And this can even be done both easy and without expensive equipment!

Anders
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Old 22nd February 2004, 07:49 PM   #53
yoke is offline yoke  Croatia
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Thanks Anders...
now I will try thet method of puthing resistors in series...Not shure that I will have 1A...but will see...

thanks

Josip
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Old 22nd February 2004, 08:09 PM   #54
Faber is offline Faber  Italy
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Default Question about Q12a/b

Hi All!
The boards just arrived yesterday (they are really great, thanks Chad) and I'm starting to look them in deep.

I have questions about Q12a\b option.
Is it right that just one of those regulators mast be used? I know is a silly question, but my english can't give me the absolute safeness for this.

In addition, do I have to make the same choice for j1a\b? Is this choice related to the previous (q12a = j1a and q12b = j1b) ?

Thank you for your support.
Bye!
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Old 22nd February 2004, 08:34 PM   #55
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I don't bother to match resistors. I have measured them
over the years, and I find that decent ones hold a tight
tolerance, tighter than Mosfets or other semi's.
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Old 22nd February 2004, 09:48 PM   #56
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Hey Brian,
If we took these and ............ These might cause a bit of hum... and then there are the fans....... Gee.... how to shield them........

Mark
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Old 22nd February 2004, 11:32 PM   #57
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Do you guys have any suggestions for starting values of C6, C7, and C8?
C7, and C8 are optional,
But c6 doesnt say it is... (Ref BOM via Hifizen)

Is it true that transformers are suggested compared to SMPS's due to hum and line noise being carried into DC?

And what ar eyou guys going for heatsinks? it'd be nice to have a group order of correct heat sinks

I'm thinking about getting a 80$ set of pneumatic tools (for use with air compressors), that has a drill, a grinder (hand), and a saw... does it sound like a good deal? (i have a craftsman air compressor) (TOols are for the aleph-x's chassis )

edit:
For R49, should this be jumpered? or not? or should an actual 0 ohm (burn-up protection?) resistor be used? Any starting value ideas for this?
For R48, any idea-rs for a startin value? (Optional resistor to reduce power dissipation in q6*)
Thanks in advance
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Old 23rd February 2004, 02:15 AM   #58
Prune is offline Prune  Canada
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I can't get the spreadsheet to work right in OpenOffice. The tables work, but the graph of power versus impedance is messed up. Anyone know how to get this to work? Not everyone can afford M$Office...
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Old 23rd February 2004, 07:07 AM   #59
Prune is offline Prune  Canada
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Can someone explain the values of R18(28) and R19(29), and why the latter are different on the PCB and the original schematic (68.1K)?
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Old 24th February 2004, 08:06 AM   #60
akb1212 is offline akb1212  Norway
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Question Value of R11 and R33

While we are on the subject on what values resistors should be.... What is the correct value for R11 and R33? In this post the values of these resistors seems to make some problems when using the recommended 47k5! When replacing with 4k75 as some of the schematics floating around seems to use the problem is solved.... Part of the problem as I see it is what values should VR1 and VR3 be set in when the initial testing is done. If they are set at maximum values the total resistance of R11 and VR1 is 147k5. Will this mean full current through the amp?

Just to clarify this, how should the variable resistances be set before testing, and is it safe to use the recommended 47k5 values (or will it possibly blow up the transistors like it did to Blitz)??

Anders
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