adding a Headphone amp to the HH

[Unregistered]

I would like to add this amp http://www.headwize.com/projects/sho...=cmoy2_prj.htm
in between the HH and my headphones, this way I can increase the resistance of the volume control pot(R41) to get rid of the bad interactions when R41 is at its lower settings (higher volume). I have built the single channel version of this amp, 1 op amp, and have a question on hooking it up to the HH. I noticed the HH audio output goes to -VB. Where would I hook up the -VB connection on the HH to the amp? The amp uses a virtual ground supply, +4.5V, Gnd, and -4.5V. Any ideas on how I would hook it up?
Thank you

[DarthCarl]

Capacitor C1 is a decoupling cap, and blocks DC, so you can connect the amp ground to HH at either -VB or ground. Intuitively, you would normally connect the ground to -VB.

- Carl

[Bill]

I built a amp to boost the output of the HH audio when a larger volume control pot was used. I have had problems with the Amp behaving strangley, "high pitch noise" with the actuall detector signal way off in the backround(very quiet). I am currently going through the audio portion of the shcematic to see if I can make some improvements (impeadance matching, etc) The Cmoy amp works well with my stereo just not my HH for somereason. Anyone else have any ideas to help improve the audio on the HH?



Taking my baseline HH out for some tests this weekend on the beach!

Will

[bbsailor]

High pitch audio noise

William N

Does the high pitch audio noise change when you vary the TX frequency?

If so, you are picking up the TX pulses in the audio circuit. If not, it may be being picked up from the PI circuit itself.

To better understand a possible fix for this problem, take a few measurements and post them.

What is the P to P amplitude of the noise signal?

What is the P to P amplitude of the minimum and maximum audio signal (from a detected target) feeding your headphone amplifier?

Is the high pitch noise in the signal feeding your headphone amplifier?

What gain is the headphone amp IC set at?

What is the output impedance feeding the headphone amp?

When I use an outboard amplifier to boost the signal of my Sovereign on the beach, I pick up the signal being radiated from the 15" search coil. If the headphone amp is in a plastic box and has high gain, you may need to shield the circuit and use shielded cable to connect the headphone amp to the metal detector. Copper foil may be the best solution for adding shielding quickly. This is how I dealt with this radiated noise problem in my outboard headphone amp.

If the headphone amp is mounted inside the PI case, try moving it around and changing it's orientation and see if the noise increases or decreases. You may need some better physical isolation or even better decoupling on the shared power line. Try powering the headphone amp with a separate 9V battery just to eliminate the possible decoupling problem.

If you post more details, you may be able to get some more help.

Good Luck,

bbsailor

[Bill]

Thanks for the help! Yes the high pitch tone does vary with the main freq adj. The headphones are 30 ohm, and the headphone amp is set to a gain of 10. I will get the scope out tomorrow and see if I can get some more info for you. Thanks again for the time.
Will

[Bill]

Success! I built a new mono amp circuit and it performs excellent. It uses its own 9VDC power supply (battery) and gives a loud clean signal. For some reason now when I move an object within a few inches of the coil the detector freaks out and locks up. I think I am over loading the HH preamp. When I first hooked up the amp it worked well but I had a faint constant high pitch whine in the backround. A shielded case for the amp solved the problem. Thanks for the help guys!

Will