Korg Nutube

[4krow]

I figured that this site would have good perspective on this mini tube, if you will. It is not so new now, but being a DHT, it made me pause when I saw it, and immediately, I wondered who might have any experience with it, or an opinion on it. I believe that model is SP1, but not sure. Sure there are kits out there to configure it to different tasks, but who cares if it doesn't measure up in terms of sound quality to begin with?

[Paul Birkeland]

This device has an incredibly limited power dissipation of 0.0018W.  The plate curves look great, but the limited dissipation really confines the performance of this device.  You would have to couple this to a follower of some sort to buffer its output.

It would potentially be marvelous in 12v sound applications.

[4krow]

Yes, I later noticed that it uses a follower. That seems to put it back in square one for me. Non the less, I will be experimenting with it, just see what effect it has on the sound. Don't know if there is a way to high performance just a tadpole such as this.

[4krow]

 Just an update. Got the 'tubes' up and running last night. This certainly a different animal. When I bias the tubes according to the spec sheet (2.5 volts) it causes severe clipping, so I have decreased the bias to 2.04 volts. That is still within the spec of a 1.7 volt minimum. Now that is the case using a 12 volt battery. I plan to try an 18 volt battery since it has some effect on the distortion. I have added a volume control to the input, as it seems that a CD player input is a bit hot. The setting that the volume control provides is at 4.7/4.5 kohms.

[Paul Birkeland]

It's funny, that buffer has a fet follower to drive the tube, and a fet follower to drive whatever is on the output...  Under low voltage circumstances, this is a good way to deal with these issues.

Pete should have very specific instructions on how to bias his PCB for lowest THD, I would shoot him an e-mail if you aren't sure how to set it.

[4krow]

FWIW, I did find the information needed to adjust the bias. It is stated that it becomes a matter of preference. Distortion is more present setting the bias towards 2.5 volts, and less likely at lower bias settings. Also, my experience has taken me to lower the audio input signal a bit so that there isn't the tendency to overdrive the circuit. About 8 db down to start with, and using an 18 volt battery supply, I am getting a feel for better results. Regardless, I also get the feeling that this circuit may be more suited to guitar pedal effect, or some thinking closer to that than audio in an audio system per se, as we use in our homes.  I will try for a bit longer, and appreciate the opportunity to experience this product.
 When the PJCCS kit comes in I will be able to compare the Quickie to the Nutube. As I recall, the Quickie is much favored to my ears.
 Here is a shot of the enclosure stuffed with the Nutube circuit along with batteries.

 

[4krow]

  I have since got the circuit up and running. Enjoyed the ride. Just one thing, if you think that the 3S4 is prone to ringing, you can't imagine how much more sensitive this tube is! I would tend to think that using this tube in a headphone amp (which I have), would be the preferred device for this very reason. Aside from that, I think that someone with more knowledge than I could bring even more respectable performance to this buffer. It seems pretty dry on top, and maybe bass heavy on the bottom. The mid range is not forward though, and that compliments my system. My single driver speakers have a shout to them that is supposed to decrease as the drivers break in. In the meantime, I listen more to headphones than ever before.

[4krow]

 Here I am at the end of the experiment. I have listened to the unit and found that with the sound, it can vary, at least from different sources. i mean to say that the unit is overwhelmed by CD input, and so a 50k volume control is used to deal with that. The computer input however is fine for it, and sounds better to my ears. The bass is increased, which serves my system nicely. The treble is a bit soft, and that may just be my hearing. I am trying to ring out as much these days as I can when it comes to music, knowing that there was more to it all even 5 years ago. Aging sucks. Finally, I will add that I like the mid range, since it is fuller than without the unit in place. I feel it necessary to add as i have in the past, that these results in the system that I own, and probably would sound quite different in other systems. So that is all for now, at least until I make time to complete my Quickie/fix project.
 this is what I ended up using for the enclosure.

[4krow]

 There is still one thing to add about this tube in general. Even though I have stated the high amount of ringing associated with the tube, I found it to be even more present when using this tube in a pre-amp role. I made another on of these tube projects, but this time as a headphone amp. Pretty good results, but I noticed that the design also had outputs so it could used as a pre-amp. Why not try it? So, I put a couple of temporary RCA connectors in place and hooked the unit into my system. Overall, the sound was just different, and i wondered why. In short, it was because the tubes were indeed acting like microphones, especially since they were sitting in front of the speakers! I will try again with a lower volume, and better placement just to see if it becomes acceptable. I am now eager to finish the Quickie project and compare the two side by side. Will make a new post for that occasion.

[Paul Birkeland]

It would be interesting to buy another one of those PCBs and just not install the NuTube, but rather just use the fet buffers that are built in and see what those sound like on their own.

[4krow]

  That is a good point. After all, they do dominate the circuit. FWIW, the tube buffer circuit is quite different from the headphone amp circuit in that where as the buffer uses the FET devices running at unity gain, the headphone amp is actually using two integrated circuits (I don't recall the model number) running to have significant gain. I know that they are high output (maybe referring to current output), and this is to have needed gain for the headphone amp as compared to the buffer unit. Either way, these devices contribute the lions share of the circuit, where as the tubes seem to be in circuit just for that 'tube sound'.
 What I may do is run the buffer again in circuit before the power just hear what differences there may be there. Want to listen for that 'enhanced hall sound'.