Output impedance

arduino based linestage using 12B4 tubes

Output impedance

Postby Ludwig » Sat May 23, 2015 5:36 am

Hey folks, I'm in the closing stages of my budgie build and realised I hadn't checked if the output impedance of the preamp will suit my solid state power amp (50k), has anyone measured the output impedance of the budgie? Would also be nice to have some THD figures etc if anyone has done some analysis....
Ludwig
 
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Re: Output impedance

Postby 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
seanzozo
 
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Re: Output impedance

Postby Ludwig » Mon Jun 01, 2015 2:20 pm

Thanks Sean, that's fantastic. A good result I'd say.

Any chance of being able to read the THD from your test setup?
Ludwig
 
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Joined: Tue Jan 27, 2015 8:01 pm


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