Headphones cancel ground effect!!!

[Anonymous]

I cannot believe the ad I just read about AccuSound Headphones. Get a load of this RUBBISH!!!
When we said their AccuSound headphones would pick up deeper signals, we weren

 

[Anonymous]

Dave,
All those pro's down in OZ that swear by the old style communications headphones must be missing the boat. But hey, what can I say, when I was a newbie I bought a hot head coil from K***.
I hope those things have a volume control. Can you picture trying to use em on a GM3 or 4B without one. Ouch!!
Tom

 

[Anonymous]

Tom,
There is nothing wrong with the old style headphones. They may well be an excellent choice. This depends a great deal on the make of detector that you are using. Here is what gets me mad:
Many headphone manufacturers claim that the higher the Ohms, the better the headphones will work. This is absolute rubbish. Maximum power is transferred to the headphones when the amplifier's output impedance equals the headphone's input impedance.
The idea that a wider audio bandwidth can somehow cancel the ground signal is so ridiculous that I won't comment on the subject other than to again say "RUBBISH". Some detectors which use a cheap or poorly designed audio output stage may well work better with high impedance headphones. However, a properly designed output amplifier will always work best when it's output impedance matches the headphones. Dave. * * *

 

[Anonymous]

Use your old II World War high impedance magnetic headphones and feel the difference!!!

 

[Anonymous]

Most metal detectors drive the speaker with pulses up to about 6 - 11 volts. Most headphones are 8 - 32 ohms, and if you drove them with that same voltage, it's hard to say which would get fried first-- your ears, or the 'phones themselves. So, in order to protect both the ears and the headset, the manufacturers wire a resistor in series with the headphone jack, anywhere from 47 to 270 ohms, depending on the battery voltage of the machine and the manufacturer's own "rule of thumb" of how loud headphones ought to be.
In a few cases, manufacturers also wire clamp diodes across the headphone jack to limit maximum headphone loudness.
Now, if a person is hard of hearing, they may want more loudness than regular low impedance headphones provide. Maximum power transfer is achieved when the headphone impedance is equal to the value of the resistor wired in series with headphone jack. The actual impedance isn't critical-- good power transfer can be achieved over a fairly broad range of impedance. 150 ohms would be a good compromise.
How loud the 'phones are depends on many factors other than just the electrical impedance. Headphones vary widely in efficiency in turning watts into loudness. In general, the hi-fi 'phones are least efficient.
Most manufacturers drive the audio with pulses from 100 us to 300 us wide, producing a tone rich in harmonics. This makes the drive circuit more efficient and gives the sound more "punch". However, everyone's hearing is different, not just in ability to hear sounds of different frequencies, but in their psychoneurological response to different sounds. Some people prefer wide frequency response, such as one gets from hi-fi headphones, whereas other people prefer the narrower frequency response which is typical of 'phones designed for communications purposes.
So-- the best headphone on the market, is the one that you personally like.
* * * * * * * * *
If a headphone is advertised as cancelling ground effect, the advertising is not just "deceptive and misleading", it is false. Products which have merit do not need false advertising; therefore false advertising is the mark of a product which the manufacturer believes to be worthless, and who is targeting not customers who want good product, but customers who want to be lied to.
If the manufacturer of a product has designed their advertising to portray the product as garbage, chances are pretty good it really is garbage, eh?
--Dave J.

 

[Anonymous]

Dave and Others,
One of the most overlooked areas of improvements in metal detector technology is the final audio circuit. I believe that the series power dropping resistor 47 to 270 ohms causes a variable interface between headphones of differing impedances. That is why some high impedance (150 Ohm) headphones sound louder in some detectors than lower 32 Ohm headphones and vica versa.
When the total resistance of the final audio circuit is a closer match to the impedance of the headphones, there is a more efficient power transfer (meaning an apparantly louder sound). Therefore no one headphone is best in all detectors!
Now for the rest of the story. There are two ways to increase the usable preceived signal. (1) increase the signal level or (2) reduce the noise.
In metal detectors, reducing the outside noise by 24 to 29 db has almost the same effect as increasing the signal by the same amount.
On the beach, I use isolation headphones to minimize the sound of the surf and the birds. This helps get the deep ones that I would normally miss with other open type headphones.
I would like to see some truly innovative person offer a high gain amplifier that matches the wide variation in metal detector output circuits; has adjustable gain from 0 db to 30 db (1000 times) and has an ajustable compressor and limiter to protect your ears from loud surface target blasts. The compressor should be exponential to account for the wide range in signals and still preserve some dynamic range to assess target depth. The compression knee should be adjustable so the user could favor very deep targets or mid-range targets.
Samson makes a product called the "Ear Amp" which has some of these features. The Ear Amp is designed for musicians to monitor their sound from a wired input, like a mixer, to inner ear phones or external headphones. It has gain, tone controls and limiting control. The manual is on the Samson web page. Check it out. American Music Supply has it on sale in their catalog for $199.00.
I have actually found an advantage to using a product called the Depthmaster (TM) in my metal detector. It acts as an impedance matching device (1 K input impedancce), an amplifier and a hard clamping limiter due to the low 1.5 volt battery powering it. Beacuse it was designed in a plastic box, it tends to pick up coil noise but moving the in#####s of the Depthmaster to an aluminum box and using shielded cable for the input solves this problem.
Whether the detectors be PI or VLF, the post processing of the audio signal is an area that could use some creative technical attention.
Gentlemen, we have not pushed the limit in this area yet!
Comments Please?
bbsailor

 

[Anonymous]

Some manufacturers allow the user to set the quiescent frequency. Some manufacturers provide a volume control. Some Fishers (and perhaps other mfr's, I don't know) used to have a control labelled "volume" which was actually limiting level.
In general (and for sure, there are some exceptions), the buy or don't buy decision on the part of the beeper customer is made on the basis of things other than audio. Make a beeper that is most sensitive, or lightest, or has a desirable newfangled feature, and people will buy it. If a manufacturer had a ho-hum beeper and decided to pack a bunch of audio control features into it, it wouldn't affect sales much, probably not by enough to cover the additional manufacturing cost.
As beepers incorporate more function into computers and offer more features through a software-controlled interface, you'll see manufacturers giving the user more ways to control the audio, because the additional software needed to do it is relatively easy to write and it adds almost nothing to manufacturing cost.
--Dave J.

 

[Anonymous]

A couple of years ago I tried a Koss EQ30 headphone amplifier to boost the rather poor audio output of a SD2200D. This little unit transformed the detector and has a "graphic equaliser" to adjust the frequency response for personal preference or headphone characteristics. The only downside is that it is not mechanically robust enough for prolonged use outdoors. It became so popular with SD users that Koss were said to be looking into making a version for detectors.
Frequency response is 40-20000Hz, Centre Band Frequencies are 100Hz, 1000Hz and 10kHz, Boost and Cut Levels +10dB, -10dB, Crosstalk is 40dB, Distortion less than 0.2%, Maximum Power Output 20 mW per channel and cord length 48 inches.
Needs 2 AAA batteries
It does need to be used with stereo phones as a mono jack will short out one channel.
Eric.

 

[Anonymous]

The only downside to the Koss is the amplifier noise itself which is generated within IC. With a detector like the SD2200 where you have a Threshold to listen to the noise is not a problem. But when used on detectors that are of the Silent Search variety this amplifier noise is very noticeable and annoying as you hear a constant hissing sound in the headphones. It is especially when you are using quality headphones. This hissing is there even with the volume on the Amplifier turned all the way down which proves that it was being generated within the amplifier itself. Still of the options currently available the Koss Amp is about the best of the ad-on amps.
Years ago I used to hunt with a Teknetics Mark I. While this detector was of the Threshold type it had low target volume when used with headphones. All it took was a simple one transistor amplifier which ran on one 1.5v "AA" battery to correct the problem and it didn't generate any noise of it's own.
Still today's detectors audio circuits are great compared to those of the older detectors. Couple that with the newer 150 Ohm High Effiency Speakers that they are currently putting into detector headphones and you have a very good combination capable of hearing very faint target signals that you would not have heard been able to hear a few years back!!!!
HH
Beachcomber

 

[Anonymous]

Hi
I totally agree on the fact that phones do not and will never could be put in context with anything that is electrically happening in the coil part of a detector. Groundeffect influence is bullshit like Dave Emery stated.
Influence from phones will be difference on different load of the audio output stage of the amplifier.This might help some people in some cases and make thinks worse in other cases when combining different detectors with different phones.
What might be the case, is that the phones/detector combination is creating a lift of some harmonics for one persons ear that another one can not hear!!!
What might help us all was to send the audio output (tone only types) through a filter that simulate the human ears (british) average characteristic curve. Such a filter with full shematic can be found here:
http://sound.westhost.com/projects.htm
This filter is made for measurement use but I believe it would do perfect for flitering the audio so it fits the standard ear.
For other interesting audio applications see the mainpage here:
http://sound.westhost.com/projects.htm
Its a very useful collection of audio circuits.
HH
Mark
http://sound.westhost.com/projects.htm

 

[Anonymous]

[No message]

 

[Anonymous]

So why don't the detector manufacturers publish the headphone impedance range for their machines?
I have finally settled on a good quality set of phones that has an impedance of 150 ohms and a filter that cuts or limits output above 3200 Hz. They do make a difference on the 1200 series Fisher machines when operated at low disc and increase the volume on my IDX Pro. It is much easier to pick a good signal out of the noise of partially disced out iron with these phones. On the 1270 in particular it is hard for many people to distinguish a good tone amid the noise.
Certainly personal preference is what matters most, but there are a few machines different enough that they require a bit of experimenting.
Tom

 

[Anonymous]

Great info and much appreciated!! I knew there had to be a sound reason (pun intended) why I like my new phones.
Tom

 

[Anonymous]

Eric and Others,
I have tried the EQ 30 and you are right that it is light duty, with hard jack connections attached to the circuit board, not standing up to field use.
The main reason why the EQ 30 is effective is that it offers a higher impedance input (than typical headphones) to better match the series resistors in most detector audio circuits. It's 3 volt DC source also provides sort of a hard clipping limiter for the real loud signals. Some of the new low voltage audio ICs could perform the same function and offer more ear protection with a 1.5 volt supply.
The three band equalizer offers some customizing of the boost/cut band to accomodate headphone defencies well as personal hearing preferences. However, boosting the high frequencies accentuates the noise in the circuit. The real advantage is better impedance matching, gain and clipping for ear protection.
Here is what detectorists really need in a shielded rugged case!
Design a 3 or 4 stage cascading audio circuit, using transistors, with a gain of 10 in each stage that can be independently set to optimize getting the most overall gain with the least amount of noise with some hearing protection based on the hard clipping offered by the low voltage DC supply in each stage. I've seen some circuits used for earthquake sensors that must operate over a very high dymamic range (in this application there were 6 audio gain stages of 10X) use this technique. It seems that this approach could also be appropriate for metal detector audio circuits.
I'll bet that the folks on this forum could come up with a quite useful add on device from this description.
bbsailor

 

[Anonymous]

Sennheiser
......................Impedance......Max.......Max
.........................(ohms)Sens Power... SPL (dB)
.....................................(dB).(mW)
HD 400,................32.....94..200........117
HD25....................70...120..240........144
Note the Max loudness of these, Talk about having a hole in your head
HD445...................52.....97..235........121
HD25SP................85...100..235........123
HD265,525, 535,.150....94...207........117
HD455, 475...........60.....94..238........118
HD465.................100.....94..225........118
HD 570................120.....95..220........110
HD580, 600.........300.....97..145........118
Sound Pressure Level Equivalents
SPL-dB Common Example
140 Irreparable damage
130 Jet aircraft taking off
120 Threshold of pain / Thunder
110 Threshold of discomfort
100 Dirt bike / Riveter
90 Start of unsafe levels
80 Average factory
70 Kids at play
60 Normal conversation
50 Elevator music
40 Normal home background (kids asleep)
30 Studio background
20 Rustling of leaves / Quiet whisper
10 Butterfly swoop
0 Threshold of hearing

 

[Anonymous]

I went to the Koss website and ordered the EQ 50
equalizer amp. Is this just a newer version of the EQ 30 or did I order the wrong thing?

 

[Anonymous]

Art,
To save the connectors on the EQ 50 from strain, I recommend that you wire up a custom 1/8" stereo short extension cord about 6" long to match the size of your headphone plug and secure this to the wire to the EQ 50 with duct tape.
Also, you need to check that the input wire on the EQ 50 activates both channels of the headphone when going through the EQ 50. It will probably work. If not, you need to make a custom input connector shorting both left and right channels together in a way that matches your metal detector headphone jack. If you hear the MD coil signal (high pitch wine) in your headphones you need to use a shielded cable between the EQ 50 and the MD.
Boosting the equalizer gives you about 10 db extra signal. Burry a deep coin and check out the best EQ settings that give you the best target signal with the least amount of noise or hiss from the EQ 50 circuit itself. This should give you an additional inch or two in depth or let you hear the targets that were previously at the fringes and were masked by external noise or varying ground conditions.
To get the maximum value from the extra boost set the threshhold on your MD to a sound level that you can just barely hear.
I am anxious to hear how this works out.
bbsailor

 

[Anonymous]

Have the detector in a mode with a constant tone. Measure the AC output with no phones (use an opened plug or something). Put a resistor or variable resistor across the output instead of phones. Measure across the resistor.
When the AC (RMS) voltage across the resistor equals 1/2 of the reading you got with no load, that is your output impedance and will be the same resistance as the series resistor inside the machine.
Also, keep in mind how your headphones are wired. 150 ohm phones may be the speaker spec. They may end up in parallel on your machine and really be 75 ohms, for example.