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This is a question that we get often:

I measured the direct current resistance (DCR) of xyz using a multimeter and the result is higher than what the data sheet says it should be!

Since most of our readers are audio enthusiasts, xyz is typically a speaker, but it could be any low resistance device such as a resistor, switch or relay contacts. When trying to make a resistance measurement of low resistance devices using a common multimeter, the results are often high and almost meaningless. The reason for this is that the test currents generated by most common multimeters are very low, typically much less than 1 milliamperes (mA).

A simple way to measure low resistance is to use a "4-wire resistance measurement method". Ohm's Law states that V=IR. Resistance (R) is what we are interested in determining. Since we already have multimeter that is pretty accurate at measuring voltage (V), all we need to do is fix the current (I) and we can calculate the unknown resistance.

Figure 1 (click to enlarge) shows a schematic for a very simple low resistance measurement jig. Basically it is a LM317 voltage regulator that is set to deliver 100 mA of test current.

In the circuit, a fixed 12 ohm resistor is used to set the current of the regulator to about 100 mA (a 12 ohm resistor actually sets it at 104 mA, use the voltage regulator calculator to check).

Building the Low Resistance Measurement Jig
The test jig is very simple to build. The photos below show the measurement jig that I built. The LM317 was recycled, a 12 ohm resistor, a couple of alligator clips, a project box (Radio Shack 270-1802) and an old 5V 300 mA cell phone charger. For the power source something in the order of 4 to 6 V capable of more than 100 mA will work fine. Batteries can also be used - as little as three AA batteries will work. Note that a 9V battery will not work as it cannot deliver enough current.

Once you have built the jig, it is a good idea to check the test current to make sure it works. Mine produces a constant current of 105 mA. Anything between 95 and 105 mA will be fine as we will see later.

Using the Low Resistance Measurement Jig
Using the jig is very simple. Set your multimeter to read voltage. The lower you can set the voltage scale, the more accurate your readings will be. To determine the unknown resistance, simply connect the constant current across the device you want to measure. Now measure the voltage across the device you want to measure. Once you have measured a voltage you can apply Ohm's Law to determine the unknown resistance.

A 100 mA current was chosen for a reason - to make the math easy. With a 100 mA test current you can multiply the measured voltage by 10 to determine the unknown resistance. If you need very accurate low resistance measurements you can use the exact test current of your jig and an Ohm's Law Calculator to determine resistance.

Here are a few examples using the DIY low resistance test jig and a Wavetek Meterman multimeter:

0.1 ohm resistor measured 0.0112V (11.2mV), multiply by 10 and you get 0.112 ohms. In contrast, using the multimeter set to the lowest scale (200 ohm) 0.4 ohms was measured.

0.22 ohm resistor measured 0.0217V (21.7mV), multiply by 10 and you get 0.217 ohms. In contrast, using the multimeter set to the lowest scale (200 ohm) 0.5 ohms was measured.

0.47 ohm resistor measured 0.0481V (48.1mV), multiply by 10 and you get 0.481 ohms. In contrast, using the multimeter set to the lowest scale (200 ohm) 0.8 ohms was measured.

As you can see, for small resistance a common multimeter is very inaccurate. Of course you can spend a lot of money on a meter that can measure low resistance, but where is the fun in that? One should be able to put this jig together for less than $10.

Until now, Vacuum Tube Amplifiers have been the domain of esoteric audiophiles with large budgets and  even larger egos.    

This is Alan's mini-tube preamplifier.

It is a very simple tube preamplifier project that will work with the 12AX7 or 12AU7 vacuum tubes, both of which are fairly common. The preamp is built using primarily parts that Alan had on hand. To make a real frugal build, the preamp can be built using 12V transformers and placed back-to-back in order to get the required high voltage and for the heater supply. Total costs for this project ran Alan about $30, with the majority of those costs going towards the enclosures and RCA jacks.

The power supply is housed in a separate enclosure, also from Hammond. For full information, see the project page: DIY 12AU7 / 12AU7 Tube Preamplifier.

Related Tube Preamplifier Projects:

• DIY 5687 Tube Preamplifier Project
• Bruce's DIY 12AX7 Tube Preamplifier Kit
• Mark's DIY 12AX7 Tube Preamplifier Kit

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Steve has sent us details on his latest vacuum tube amplifier project.

Steve tells us that this great looking vacuum tube amplifier project was inspired by the Spare Time Gizmo single-ended 6T9 tube amp kit from 2004. The circuit ideas for the push-pull output stage came from his EICO HF-81 and RCA Receiving Tube Manual. The preamp stage uses 12AX7A tubes.

All of the connections are point-to-point. For the enclosure, the combination wood and brass chassis consists of a small sheet of brass and Poplar wood. The brass sheet was polished to a shine and the Poplar wood stained with Minwax Red Mahogany stain and finished with an Amber Shellac.

Steve is very impressed with the performance of this tube amp. He notes that there is plenty of gain and a nice rich sound.

For full information about this project, see the 6T9 Push-Pull Amplifier Project page.

Great work Steve!

Related 6T9 Compactron Tube Amplifiers:

• Ultra-Linear Single-Ended 6T9 Tube Amplifier
• Single-Ended 6T9 Tube Amplifier Kit
  

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