Hi All,

Did some tests today on the Koss EQ/30 headphone amplifier/equaliser. The detector used for the tests was the SD2200 which can benefit from a degree of audio boost. At first the results were a bit disappointing using the phones supplied with the SD. There was some degree of boost but with the volume control more than half way the audio signal would break up on loud targets. I tried a pair of fairly expensive aftermarket phones, except these were piezo, with the same result. Next I tried the pair of cheapo’s that I normally used for testing purposes and it was as if someone had opened a door on the SD’s audio. Then I realised the reason for the difference. My cheapo’s are stereo phones with a stereo jack plug while the others are just wired for mono with a mono plug. The EQ/30 being a stereo amplifier, with the sockets etc wired as such, inserting a mono plug into the output socket effectively shorts out one channel and on loud signals the poor amplifier on that side is sucking gobs of current out of the batteries which causes the breakup.

Many of the aftermarket phones for metal detectors have mono plugs which would not give a good result with the EQ/30 for the above reason. Anyway, using cheap stereo phones the results were very good; there is more than enough range in volume and the equaliser gives that fine tuning to the sound to get the best clarity. I found that the best sound was with Bass and Treble in the centre and Mid at full boost.

Anyone else had problems with phones with the EQ? or was it just me being a bit dim today? Anyway, with the EQ at full bore there was no discernable background tone from the SD2200’s transmitter. (Threshold turned back). This had been previously reported with some SD’s. There was a just perceptible hiss from the EQ but this was in no way obtrusive. Look forward to trying it in the field.

Eric.