Help needed with preamp circuit

[dromichet]

Hello:

I was wondering is anybody could lead me in the right direction with a rather simple preamp stage.

This preamp stage is designed by lampizator as a TDA1541 DAC tube preamp which is an I/V DAC chip, see pic Capture1. I built this for my TDA DAC and everything works great and very satisfied with the sound, however I'd like to expand it and also incorporate another USB DAC so I can toggle between the 2 using relays. The second USB DAC is Hidisz S9 which is a headphone DAC that only outputs about 168mV @ 1KHz at max volume.

While toggling between the 2 DACs, both going through the same lampizator tube preamp, the TDA1541 works the same as before however the Hidisz has a high pitch squeal. I was reading about it and I've added a 220 ohm grid stopper (see Capture2). It did lower the squeal but so did the overall output. Not sure if that's the right thing to do and I was wondering if I could do more to lower this squealing without affecting the FR. I played around with different values and 220ohm seems to be the best compromise. With this value, the output is about 1.7V compared to the TDA at 1.8V.


Thanks

 

[Bigun]

Well, that grid stopper is just creating a voltage divider with R2, move it to the other side of R2, the grid leak resistor, right up to the grid. Then it will be a grid stopper. No idea if that will fix your problem though, if it’s oscillations it could be caused by other issues such as lead dress.

Lampizator circuit is not the only option, see also: https://www.tubecad.com/2012/06/blog0233.htm

 

[rayma]

The schematics are wrong, check your source.

 

[dromichet]

You're right, the right schematic is attached as Lampizator01. This is the one I actually built except I've used 6N2P instead of 6N1P.

I have tried Bigun's recommendation by removing R2 entirely and it was louder, I guess because the voltage divider was gone. The overall output also increased because of that.

 

[rayma]

The grid resistor connects directly to the grid pin and "after" the resistor to ground.

 

[dromichet]

It turns out i made a nube move and assumed the Hidisz s9 output would be quiet. It turns out the noise is coming out of the dac itself which of course gets amplified. This is a bit dissapointing considering the rave reviews about it on the web. Its output is about 1v at 1k when connected to a laptop with max volume. Lampizator tube output is about 4.5v which is a lot more obvious.

Thanks again for all your input.

 

[lechuck]

It turns out i made a nube move and assumed the Hidisz s9 output would be quiet. It turns out the noise is coming out of the dac itself which of course gets amplified. This is a bit dissapointing considering the rave reviews about it on the web. Its output is about 1v at 1k when connected to a laptop with max volume. Lampizator tube output is about 4.5v which is a lot more obvious.

Thanks again for all your input.

Did you really use 62 ohms for R2? That value would be way too low and I don't think that your source can drive it.

What I would try in your case is this:

• Increase R2 from 62 ohms to 10k-100k.
• Add a grid-stopper between R2 and the grid of U2. I would use 1k-ish for that.
• While you're at it, also add a grid-stopper to U1. You might want to experiment with connecting it between the Anode of U2 and the grid of U1.

 

[dromichet]

For R2 I used 60 ohms as it seems to be the common value for a I/V converter for a TDA1541 current dac. I think the maximum would be 100 ohms.

I did some more testing and the high pitch noise when I connect the USB SS DAC to this circuit, with or without a grid stopper, comes from the 5V USB. I've used a few laptops as a source and some laptops are quiet and on some the squealing bleeds into the audio.

So I've modified a USB-c cable and broke off the 5V and connected to an external power supply. This improved the noise a lot.

I guess it comes down to how good the USB power the source has.

 

[basreflex]

why not feed the current output capacitively into the cathode and use that as an input. just ground the grid with 1Mohm and 1uf in paralell, plus a small series grid stopper. this is cascode operation, more linear..