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.

Preamp - On a Power Trip

Where were we?? Oh yeah, preparing for power up.

Since most of the parts replacement in the Preamp has been in the power supply sections, I wanted to make sure that these sections were powered up first while disconnected from the main circuitry to ensure any voltage issues would not burn out the audio sections.

Power Supply
I was expecting the power voltages to be higher than the specs, mainly due to the fact that I replaced the Selenium rectifiers with silicon diodes (M1, M2, and M3)

Preamp Power Supply Circuit

Tube Filaments (12.5V - To FILS)
All tube based equipment I have worked on in the past used AC voltage to supply the Tube Filaments (called "Heaters"), so I was somewhat surprised to see rectifiers in the Heater supplies (M3). After some research, I learned that this was done in some older audio equipment to reduce noise. Of course, almost all tube equipment I have worked on in the past was for Ham Radio and those levels of noise were never an issue. Anyway, noise levels in the Preamp will be a project for later, once I am ready to put sound through it and have broken out the Oscilloscope.

The two Electrolytics in the heater supply (C5A, C5B) were not replaced so I will keep a close eye on these (see the earlier post).

High Voltage Output
All of the Electrolytic capacitors in the High Voltage sections were replaced and I was pretty confident that there would be no issues with this section, short of the higher voltages. The new Electrolytics were rated at 450 VDC as opposed to the originals which were rated at 300 VDC.


And then there was light!
After I was convinced that there would be no fire, I plugged the Preamp in to the AC and got the front lamp to light, always a good sign. I made some cursory voltage measurements with my trusty Micronta Multimeter (I probably bought this sometime in the 1990's) then pulled the power and connected the power supply output to the rest of the unit for the real acid test.
 

  Preamp powered up with Lamp glowing and the Multimeter showing voltage

A close look at the meter, for the sticklers...

Burn Rate
Once I was convinced the voltages were reasonable, I left the preamp powered up for about 3 hours for a good burn-in test. Next will be to introduce some waveforms into the Preamp input and check the output of the different stages on the scope.

If this checks out, I might actually plug the Preamp output into one channel of my Denon DRA-345R Stereo Receiver...the Receiver is only about 30 years old, unlike the Preamp which is closer to 60 years old!

Until then, stay cool!