Preamp - Getting Sounds up

After letting the Preamp cook last night, today I broke out the heavy equipment to see if I could get sounds from the input to the output. Items needed for this leg:

• Small MP3 Player
• "Jack-leg" 3.5 mm plug to Phono jack adapter
• Denon Stereo Receiver with left channel speaker connected
• High Speed Oscilloscope

MP3 Player connection
Previously, I built a Tablet to Stereo Line-in adapter for playing music on a standard (ok, old) Stereo system. The interface I built for testing this came in handy to connect my little MP3 Player to the Phono jack input on the Preamp.

MP3 Player and "Jack-let" adapter

Connections and Heavy equipment
I have had an Oscilloscope for several years and it has come in handy on a number of projects. The Scope is a RIGOL DS-1052E which can see waveforms much faster than anything that is coming out of audio equipment, so the scope will not be taxed much by this project.

Once the MP3 Player was playing music and connected to the "Tape" input of the Preamp, I hooked the Oscilloscope probe to the first stage of the Preamp (bypassing the Phono input pre-stage), raised the Preamp Volume control to 50%, and selected the TAPE input. I was now able to see the music waveforms dancing on the scope!! Checking back on the schematic, I walked through the different amplification stages and could see the levels rise. When the scope was connected to the Preamp "Main" output, the levels looked close to what a standard "Line-in" should look like.

Scoping the Preamp Output

You can see that my Denon Receiver is powered up in the background. It took only a few seconds to cable the Preamp out to the left channel TAPE Line-in of the receiver.

Those cool sounds of summer
Once everything was connected, I slowly brought up the volume on the Receiver to hear the Jazz coming through clear! SUCCESS!! Here is a clip of the left speaker output of the Receiver playing into my computer microphone direct to MP3:

Altec Preamp - Cannonball Adderley - Waltz for Debby 

Obviously, this just shows functionality and while it sounds nice, I need to begin looking at the other inputs and measuring noise levels. Also, as I adjusted the volume control, I heard some crackling which means there is some dirt on that Potentiometer (POT).  All the Pots and switches need cleaning (I have the spray bottle in house now).

Packaging of the preamp is coming up and the possibility of replacing the tubes with new ones is on the list. But, this is a major milestone.