• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6J1 China preamp thoughts

I was thinking about making a hole in the tin case, but I just cut the switch and threw it away.
 

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Hello,
I'm trying to use it as a portable headphone amplifier.
I don’t know anything about headphones and headphone amps but I wonder if the output impedance might be an issue.

Someone in the massive FX thread on AK took some measurements at different frequencies and reported that:

For the Tube 01- Output impedance: 2.87K (100), 2.64K (1K), 2.64K (10K)

Wouldn’t this indicate that, ideally, the impedance of the headphones should be 10x higher or ~26k? I was under the impression that they are typically 600 ohms or less. Not sure how that might affect the sound quality.
 
I don't know much about impedance (I'm studying), but the output is very weak to use as a headphone amplifier.
Korea(where I live) has the same smartphone earphone volume regulation as the EU, so the output is 1/2 compared to the US. So I'm thinking about adding an additional D-class amplifier board.
 
After buying something that cost 10x as much and after weeks of trying to get rid of serious hum (without success), I grabbed one of the cheap FX Audio Tube01 pre's.
It is single stage so inverts absolute phase so swapped P and N at each speaker.
Using a 50va 12vdc linear supply with it, but was not happy yet so proceeded to make some little changes.

Re-housed it and bypassed the input and output caps with copper foil caps and silver foil caps, and used a 21 step SMD attenuator (the stock one was a little plastic wiper pot but unsuitable being a linear pot). These were all parts I already had in part draws here.

I tried replacing the first caps that are after the volume pot with wire links, but that gave me a bit of DC on the pot.

Popped some Voskhod 6J1P-EVs in there and now I'm happy with it.
IMG_20250201_215126_116.jpgIMG_20250214_160851_456.jpgIMG_20250205_203354_460.jpgIMG_20250205_203404_560.jpg
 
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