by seanzozo » Sat May 30, 2015 11:55 am
After you posed the question, I decided to do a quick check using what I had on hand: a smartphone signal generator, a cheapie Rat Shack meter with suitably high input impedance, and a random 1.2K resistor. My numbers were highly repeatable, but there could be errors based on my very quick and dirty setup. For Example the meter appears to have--as expected--bad frequency response at 20Khz (about 10db down). I used a pair of RCA 12B4's, other tubes might give different results.
The short answer I got was somewhere about 500 ohms at 0db gain and 750 ohms at 6db gain. I.e. suitable to drive any commercial amplifier I know of.
The long answer is:
With the preamp volume maxed, unloaded output for 500HZ sine wave with the smartphone app maxed was just under .2V on the 0db setting and just over .45V on the 6db setting.
At the 0db setting measured output impedance ranged from ~510-565 ohm measured at 50Hz, 500hz, 5khz and 20khz with the highest impedance at 50hz and lowest at 500hz
At the 6db setting measured output impedance ranged from ~740-860 ohm measured at 50Hz, 500hz, 5khz and 20khz with the highest impedance at 50hz and lowest at 500hz
I spot checked the other channel at 500hz at the same Budgie volume level, results were within 3% at ~520 Ohm at 0db and ~750 Ohm at 6db. At ~5db lower volume level on the Budgie, the output impedance dropped slightly to ~450 Ohm at 0db and ~720 Ohm at 6db.
Sean