Don't put a beer on top of your Amplifier!

Here is a recent repair job I did on a B-52 AT-100 guitar amp head. The client had low volume output, two power tubes glowing really hot, and a story about spilling something on the amp. After opening it up I noticed that one of the screen resistors were blown. The screen resistor is used to control the amount of current from the bias supply to the screen of the power tubes.






Each power tube will have a screen resistor. After more inspection the underside tube socket had carbon build up all around it. This is a sure sign that something is wrong. I de-soldered the tube socket and to my surprise there was a hole burnt through the board and a substance in the hole. Apparently what got spilled was still there and it created a pseudo resistor. The resistance of this substance was much lower than 470 ohms which is the screen resistors value, and with the pseudo resistor and the actual screen resistor in parallel, the resistance would be decided by the pseudo resistor. A smaller resistance means more current.
Once I cleaned out the gunk and soldered everything back together the amp worked great! Something to take from this is don't set your beer on top of your amp when your playing a show.

Multiple Output Headphone Amp

Haven't posted in awhile with my current work load with school and all, but I came up with an amazing idea! How about I solve my headphone problems and get in an electronics design project at the same time!

Just to inform, I've been working on getting a small recording studio setup in the basement, but we don't have all the equipment we need. Sure we've got a computer, DAW, mixer, power amps, effects, headphones, and microphones, but where is the capability to power multiple headphones at once? Wouldn't everyone like to hear the track on headphones while mixing? Wouldn't everyone like to hear the music while recording group vocals or record two musicians at once?

My idea is to create a headphone amplifier that will do this! I figured I need a 4-6 headphone output that will accept a range of different headphone impedances and have a separate volume control for each output. Sure I can go purchase one off of musicians friend for like fifty bucks, but this is a perfect project! My first step is to research my options and try to design the cheapest and most effective system I can build. I'll have some schematics and design ideas in a future post.

After some research I stumbled across a distribution headphone amplifier kit from www.paia.com.





The kit can drive six pairs of headphones using two op-amps for each set. The gain of each op-amp is set to a fixed gain of ten, and each output has a separate volume control which consists of a dual ganged potentiometer. The dual ganged potentiometer is needed to control the left and right channels at the same potential so that one side is not louder than the other. This design is very simple. Seeing as I don't need six channels I'm planning on building a four output headphone distribution amp. The Paia design also uses the NE5532 op-amp which is a dual package. I have three of these currently making it easy for me to build a three output amplifier at a minimum. I also have LM386's which is another option in my design. The problem with this component choice would be that the left and right channels would need to be mixed together as the LM386 is a single package and I do not have enough LM386's to have two per headphone pair. This can be done with a single op-amp on the input to mix the left and right channels. Then the output of this amp is feed to the inputs of the LM386's. This would actually help isolate the amps from the signal source so not to load it down. This isolation can also be used to improve the Paia design by adding a buffer for both the left and right channels on the input.

I'll have more design Ideas and some schematics in the near future as I continue my research and design.