B1 with Korg Triode

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#2,061
I finished my Pass Korg and is now ready for some AC measurements and a listening test. The DC characteristics are within specs. T3 = 22.8V and T4 = 9.0V. T7 and T8 I adjusted to 9.5 V as a starting point. I use a 5A Meanwell SMPS which also works with my ACAs. I changed the connector to a Neutrik to-pole SpeakOn. To filter noise I placed a common mode choke at 24 VDC input and after that an additional noise filter (kind of pi-type). On board instead of the 10R resistors I use small ferrite chokes so filter is LCLC instead of RCRC. Resistance is about 8.5 ohm in these ferrite chokes. I added a little resistance after the common mode choke to be sure not to have more than 23V at T3. Box is just an alu box which I think works well. Not as nice as other builds but I like it. It will be 100% closed so no moving air can shake the Korg. I have 0.62 VDC at T5, T6. I hope my multi-step filtering will get rid of most switching noise!
Pot used is a 10k blue ALPS.
 

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#2,063
A couple of images of my setup with Arta + Korg. As can be seen THD and THD+N is 0.47% (so not much noise). I set generator to -3 dB (input is about 0.5 Vrms at 1 kHz). Then vol is set to about 11 o'clock and Focusrite is set to max. just before clipping amp in Focusrite. It is easy to get 1.5% THD if I e.g. set vol. to 1 o'clock and re-adjust Focusrite so it does not clip. But I think the distortion pattern is as expected. Dominating 2nd harmonic…...no doubt 🙂 …..so no need for the H2 generator. Will be interesting to listen to it. Problem right now is that active power amp is MoFo so Korg will replace the BA3-pre for a listening test for a moment. After that I should know how 2nd harmonic sounds (with and without inverting phase at speaker)?
 

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#2,067
Thank you! Same to you!
It is playing at the moment with MoFo. Much better than expected. It seems Korg has about the same sensitivity as BA3-pre. Korg can deliver enough voltage swing to drive MoFo to high levels. Sound is on the "soft side" but very listen friendly (reverse phase in the moment).
I made one mistake…...after MoFo was warming up and everything connected to Korg I plugged in the 24V power connector in Korg. This causes both MoFo PSU 4A Fast fuses to burn with a nice "light up". I run 3A bias but MoFo can burn the fuse it seems if it gets a large spike as input. Noting other damaged than fuses. I replaced them and powered Korg up before MoFo and everything good. Next time I will power up both same time at the 230VAC side and see if everything turns on nicely.
 
#2,068
6L6 said:
My guess (and I could be wrong) is all those chokes are fighting one another.. try the SMPS directly into the PCB power input, if that doest fix it, swap the series chokes with the 10R resistors.
Click to expand...


Ok....I could try to experiment with this. It is same in both channels. Maybe I could also measure directly on PSU? …..may be a bit more difficult.
 
#2,070
There is some 3rd harmonic……but I won't call it significant compared to the 2nd harmonic. 2nd harmonic is 45 dB down and 3rd harmonic is about 75 dB down. Could be interesting to see other distortion measurements. It seems not many does this? ….I think it is interesting to see if the build has some "problems" 🙂 ….or everything is as expected. By listening I am not able to hear the 273 Hz spike 🙂 …..think I will order some 10R 3W resistors...….so I could experiment. Maybe if I put a scope on PSU at T3 I would be able to see a 273 Hz noise signal (if problem is PSU)? …...using a X1 probe? …..if signal is below 0.1 mV it will be difficult.
 
#2,074
Yes, probably good to switch on Nutube first. The small spike I have at 280 something Hz is probably caused by some mechanical noise going into the chassis and catched by Nutube. I suspect the fan in the laptop to go through the table and shake the chassis a little bit. I also see a 6 kHz sometimes and think it is a chassis resonans. If I knock on chassis I get a lot of spikes around 6 kHz. Bitumen inside the chassis could probably dampen these. If I put a pillow around the chassis I can get the 280 Hz spike 118 dB down. If I clap my hands then I can provoke some small spikes. When I use Arta and set the generator to External I see only the noise.
 
#2,075
Yes MEPER,


what you describe sounds like mechanical noise.


But I saw your spike in the graph at around 270 Hz. I think this showed up without any shaking the case.
The sinus out of the the wallwart is 50/60 Hz. So the 270 Hz are close to 4x
60 Hz (240 Hz)? Or a switching noise of the smps?


Only some thoughts.


Greets
Dirk
 
#2,077
Yes, 270 - 280 Hz. I have a fan in the laptop PC which stands on the table and Nutube amp is also standing on the table. So some mechanical noise is transferred to the chassis from the fan inside the laptop. It could be something inside the nutube is resonating at around 280 Hz....e.g. the anode, grid, filament? ….so if chassis is vibrated the 280 Hz frequency is "triggered" more easy than other frequencies.
 
#2,079
If I have no signal and only measure noise and place the Korg amp on a pillow I can still see a small spike around 280 Hz but I am quite sure it is mechanical generated. 120 dB down…...think I am safe.
 

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#2,080
At first listening - slowly turn up the volume knob.
But I think / hope nothing will happen.


You are taking your measurements with an a/d- converter between your

B1 NUTUBE Preamp and the laptop (the focusrite)? Could also cause some
wrong / digital influenced mismeasurement?



Measure it with an analog oscilloscope?



Only some thoughts. 😕


Greets
Dirk
 
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