Minelab output

[Goodrat]

Who has info on headphone output impedance and wiring?
For some reason, Minelab thinks this info is confidential.

[Goodrat]

Info needed soon

If anybody knows any info at all, please reply.

[bbsailor]

Re: Minelab output

If I remember correctly, Minelab Sovereign only uses the tip and ring of a stereo headphone connection. The ground (body of shaft connector) is not used. This puts each headphone element in series.

I believe there is a series resistor that drops the power to the headphone. Just measure it by taking a resistance measurement between one of the speaker leads and one of the headphone contacts. When you find a reading that is about 150 to 200 ohms trace that connection back to the circuit board and look for the actual resistor and read the value.

bbsailor

[Goodrat]

Re: Minelab output

I heard this about tip and ring also. I do not have a machine to test. However, I had someone plug in a mono cable in the machine. The other end of the mono cable goes to a pair of headphones. There was no sound on his Explorer II. This can't be if there is no connection to the sleeve.
About the 150 ohms, I heard that louder phones had a lower impedance around 100 ohms or less. I wonder if that series resistor is actually more like 47 ohms. To accomidate home stereo phones.

[Beachcomber]

Re: Minelab output

According to the owners manuals for both the Sov and the Explorer they recommend 32 ohm headphones. The headphones that they were originally supplying with the Explorer were 32 ohm.

HH

Beachcomber

[Sean Goddard]

Re: Minelab output

Minelab would seem to think that everything is confidential IMHO. This means they can create "truths" about how their machines work and baffle the average detectorist.

In the little world of "spin" (and in thier tiny minds)it's all about who can shovel the most bulls**t to the most people and have most of it believed at any one time. Eventually people wise up. Unfortunately, a few manufacturers, have bulls**t production departments that are rather more effective than is healthy for the average poor detector user ;-(

One day, someone will cut through the manure and produce a REAL detector as a sensible price which delivers the goods. I hope it happens here first!

[Goodrat]

Minelab output mystery solved!

A friend let me play with his Explorer II and I did a few experiments measuring true rms votage, no load measurements and tests with some known impedances.

The output impedance works out to 150 ohms!
Using the UR-30 you end up with 200 ohm phones due to the 100 ohm speakers in series. Those Koss are pretty loud.
Anyway, there is no need for 32 ohm phones and theoretically with all else being equal, 150 ohms (when in series) should be the best match. The phones I developed work quite well at 250 ohms.
Many 150 ohm rated phones are theoretically 75 ohms when connected to the Minelab. Althouth there is some extra resistance due to the pots even when set to max volume, wasting a little bit of power.

[bbsailor]

Re: Minelab output mystery solved!

Goodrat,

Your results pretty much passes the logic test from my recolletion of the typical Minelab output circuit.

The reason why 150 to 200 ohm headphones sound louder is that they impose less loading on the series resistor which acts as a higher attenuation circuit for lower impedance headphones.

This is one reason why using an outboard amplifier with a 1K ohm input impedance is a good, cheap way to match any headphones to a metal detector. The 1K input impedance offers a minimum load. Adding about a gain of 20X to 30X really gives a boost to the very low signals from very deep targets. Finding a low voltage audio IC (1.5 to 3V max) also provides some limiting action to prevent surface targets from blasting off your ears. Read up about the Depthmaster for some background information in the patent section of this forum.

This circuit might be a good addition to your headphones. If you build it small enough, you could even encapsulate it in epoxy to make headphone mounting very easy. Put the battery in the other cup and you have active headphones, capable of matching any metal detector. Use a pot to adjust the gain instead of a volume control on the speaker elements. Now you have a real sensitive set of headphones. You will need to wrap the epoxy module in copper foil to keep from picking up the radiated signal from the coil.

I have always thought that post-processing the audio signal from a metal detector is one of the last unexplored areas of improvement.

bbsailor

[Goodrat]

Re: Impedance

Well, using batteries in the phones or external box is I think, not something I want to do.

Remeber, the loudest possible signal to the phones are those that match the output impedance (series resistor in the machine).

"The reason why 150 to 200 ohm headphones sound louder is that they impose less loading on the series resistor..."
Could be misleading to those who don't know about transfer of power. Above OR below 150 ohms will give you less power to the load. Higher load, less current...but lower load, less voltage. When equal, the load and series resistor share half the total power.

[Goodrat]

More

Just something I posted on the Minelab forum:

Just to explain a little from my post below.
Since I have measured the output to be about 150 ohms, the best load will be 150 ohms.
A stereo headphone rated at 32 ohms will end up with its speakers in series (with the Minelab output jack wired the way it is)resulting in 64 ohms. That is not to far from the 150.
This type of configuration was obviously done to accommodate home stereo headphones. Why, who knows. Maybe a deal with Koss.

A typical detecting headphone with 150 ohm speakers (that are actually mono) may have a switch that many refer to as a stereo/mono switch. This actually connects to the ring rather than the sleeve(ground), just to accommodate Minelab type of output jack configuration. This is not a stereo/mono switch. These type are still in parallel which makes then 75 ohms when connected to a Minelab. Even closer to the 150. As you move away (above or below) 150, you theoretically have less power than you would if the phones were really 150 ohms. This measured with an impedance meter, not an ohm meter. A 150 ohm speaker may read much less with an ohm meter if there is a high inductive component.
Although some speakers are just loud because of the efficient design.

[bbsailor]

Re: More

Goodrat,

Go to the following link to see Dave Johnson's very good analysis of the headphone impedance issue. The whole topic thread is very informative. I jumped into the discussion and threw in my 2 cents also.

http://www.findmall.com/cgi-bin/forums/pi_classroom/pi_technology.pl?read=3816

He arrived at a conclusion similar to your conclusion about obtaining the best match.

Remember, the efficiency of the headphone element can vary greatly depending on magnet weight, magnet type and diaphram design.

The best way to test or compare headphones is to plug each on into a detector and set the level to register on a sound level meter and just change headphones and measure the sound level at the same distance (don't change any dial settings). This test takes into account both the impedance differences as well as the diffrent headphone element efficiencies. Remember, that the ear can barely detect a 3 db change in sound level. The sound level meter is less subjective than the ear when the levels are close.

If you don't have a sound level meter, you can use a microphone connected to a sensitive AC voltmeter or O'scope input.

The most sensitive headphones I have ever used are the Koss in-ear monitors with hearing protectors over them to keep-out the outside noise of the beach. Placing the active element inside your ear places the element closer and provides about 6 to 10 db more sound level. Unfortunately they are not all that comfortable for long term use.

bbsailor