Tube Buffer using Korg NuTube

[Ravindra Desai]

Hello all,

There is a segment of Stereophiles that love the tube signature.
This tube signature is a result of tube stage in the signal chain either in the form of tube buffer (gain =1, w/ or w/o tone/ volume control), tube pre-amp (gain stage w/ or w/o tone/ volume control), tube power amplifier, a combination of either or all of these.
It is well known that tubes 'add' a rich amount of harmonics, to the input signal, with the 2nd harmonic being the dominant, that results in a warm sound especially with the guitar and vocals.

Conventional tubes are fragile and require large voltage bias.

Korg came up with a novel tube construction similar to the Vacuum Fluorescent Display (VFD) that have a semiconductor IC package. More info on the same can be found here.

What would be your thoughts if there was a Korg NuTube based buffer stage (Gain = 1 with optional DC controlled volume that would track Lt and Rt perfectly) that one could just insert anywhere in their existing signal chain, before the power amplifier, to achieve a tube signature?
This will keep your existing signal chain intact while letting you enjoy tube signature at will.

Such a unit will also let the solid state/ semiconductor group have a taste of tube signature without any disruption to their existing setup.

Do let me know what you think.

Kindly point out if there already exists such a circuit, since all I could find were specifically tailored for guitars.

Regards,

Ravindra.

P.S. Making a NuTube phono stage will make your existing phono pre-amp redundant and also you will not be able to enjoy it with any other line level input. Hence the suggestion for buffer configuration only.

 

[amit11]

Definitely a great idea !

 

[vipulpatel]

Hi Ravindra,

Not aware of any buffer design using Korg NuTube, but there exists an Hybrid headphone amp based on Korg Nutube (see the link below). I have listened to this headphone amp and I liked the signature it imparts to the sound. Indeed its a good idea to have a buffer based on Nutubes.

http://www.pmillett.com/nuhybrid.html


Regards,
Vipul

 

[amit11]

I came across one youtube video, which has this Korg NuTube being used for guitar. The sound difference without using it and with using it gives an idea... something similar to 'solid-state' and 'vacuum tube' difference.

Whether NuTube is similar to vacuum tube or not may be debatable however this video still sounds promising.

I remember many months back how i had switched from solid state to vacuum tube. I used to listen to such videos on my system and the differences in sound were apparent even on my solid state system. So it used to give me confidence that if I introduce vacuum tubes in the chain it would definitely be a step-up. After getting it I am definitely not looking back now to pure solid state.

Korg NuTube Demo


Timings like this:
without - 0.38 till 0.56
with nutube - 1.09 to 1.27

without - 1.40 till 1.57
with nutube - 2.15 till 2.32


Amit.

 

[Ravindra Desai]

Ah! Thanks Amit!!

This will help FMs decide what they like.
Again, nothing right or wrong just the difference.....

Regards,

Ravindra.

Been there, done that. Gain is low for phono and it becomes a hybrid amp with SS gain stage. The circuit exists already and guitar guys are using it.

Never got popular with tube heads for HIFi home use.

I had seen your post on NuTube and even referenced it in one of my threads.

The difference between builds available else where and this is that I propose to build only the Buffer. So it will have unity gain. Something like a cathode follower analogous to an emitter follower. It will serve two purposes only: Impedance matching and harmonic injection.

A bypass switch would also be a good idea so people need not plug and unplug it every time.

I believe this will give everyone a chance to practically compare and contrast, in the most simplistic way, rather than doing it in words on the forums.

Regards,

Ravindra.

P.S. Found this and it should be a good read for everyone. Its a 5 part article. The links for the other parts are on the right hand side of the article.

 

[dr khanwelkar]

Dear Ravindra,
I think you will need to build a prototype so that we can hear the difference in real world.
The above video shows difference of quality ,but there is also change in the volume .

I do not know about the cost involved.Can you estimate it?
Dr khanwelkar.

 

[Ravindra Desai]

Dear Ravindra,
I think you will need to build a prototype so that we can hear the difference in real world.
The above video shows difference of quality ,but there is also change in the volume .

I do not know about the cost involved.Can you estimate it?
Dr khanwelkar.

Nice observation about the volume.
Every circuit out there is using the NuTube as an amplifier with gain greater than 1.
I intend to use it as a buffer. In electronics, a buffer is characterized by gain = 1, high input impedance and low output impedance. The impedance part of the buffer favors matching of signal stages in the input and output side. While unity gain does not allow it to play any part in signal amplification.

My estimate is around Rs. 7500/- The prototype could be slightly higher.

I will start work on this as I am interested in this, personally.

Being local, you stand to become the first to hear and review it.

Regards,

Ravindra.

 

[manniraj]

PMillet boards are good and one of our local Bangalore member is using its headphone amp design with the same tube and he is loving it

But I am waiting for this board to come up from his own NP - http://www.diyaudio.com/forums/pass-labs/313612-b1-korg-triode.html

Do check out this thread as in the last burning amp festival this was show cased by NP along with couple of other builds. B1 is a very good design and now with the addition of the tube flavor definitely it will be outstanding.

 

[Ravindra Desai]

PMillet boards are good and one of our local Bangalore member is using its headphone amp design with the same tube and he is loving it

But I am waiting for this board to come up from his own NP - http://www.diyaudio.com/forums/pass-labs/313612-b1-korg-triode.html

Do check out this thread as in the last burning amp festival this was show cased by NP along with couple of other builds. B1 is a very good design and now with the addition of the tube flavor definitely it will be outstanding.

Thanks for the reference! I will go through the article in detail.
Pass is boss!!

I am thinking of participating in this year's burning amp event. Let's see how it works out.

Regards,

Ravindra.

 

[tuff]

I can see how a guitar sound with tube effect can be used to change the tonality of it. But it would be interesting to see what it does to vocals or a violin.

 

[Ravindra Desai]

If previously the tube lovers, by any chance, were into tubes for purity of signal, then they will be in for a shock. Since, as it happens, and as mentioned in the articles and shown by the waveforms, we need to allow the input signal to distort (positive half or negative half) by adjusting the bias. The more it distorts, the more is the 2nd harmonic injection, the more warm the sound it gets.
Warmth, in come cases, is achieved by limiting the bandwidth on the high frequency side. We make the sound relatively warmer as the treble is now rolled off.

So the kits available so far and the build that Nelson Pass is coming up with all follow the same path.

Korg is making a claim that their technology is "exceptionally linear" Therefore it will be interesting to see how the tonal quality turns out with the Buffer configuration with no bias voltage adjustment made and where input signal will not be allowed to distort. How much of the tonal change this brings where signal is just 'passed through the tube technology' without distorting it remains to be seen. But I am interested.

May be for the sake of comparison, I will need to build two versions: one where the tube is amplfying and there is bias voltage adjustment to change the tonality and the other a conventional 1 : 1 buffer.

Do let me know what you think.

I can see how a guitar sound with tube effect can be used to change the tonality of it. But it would be interesting to see what it does to vocals or a violin.

Sibilance in vocals will be affected as will the sharpness in treble. No doubt, it will be rare for anyone to like both.
Sad part is that 'like live' or 'life like' is not a measure.

One downside is, Nutube is highly microphonic just like 4P1L tubes. But it is as tempting as 4P1L as well ! The tube buffer is the best thing to go with it and Korg did desig a preamp around it with SS gain stage.

Nelson Pass talked about it in Oct 2017, its nearly one year now.......

When the technology was launched, you could get the tubes only from Japan by writing to them.
Now, it is available online. So I guess easy availability and brand ambassador like Pass should make it popular. In fact, increasingly popular!
Everybody would want to give it a try to find it out for themselves.

Regards,

Ravindra.

 

[kartikay]

Pmillet has also designed a buffer using Nutube and PCB is available via his eBay store. BOM should be under $100 including PCB and Nutube cost.
http://www.pmillett.com/Nutube_buffer.html
https://www.ebay.com/itm/DIY-PCB-pl...314046?hash=item25d16c1f7e:g:9eoAAOSw32lYxB4p

I am using Nuhybrid HPA and to me its a neutral sounding hybrid tube amp.. Adds a touch of tube warmth and presents better layering of instruments with dynamic headphones, gear tested was Yamaha HP1 (modded), HD800S, HD650 (modded), HD600.

 

[Ravindra Desai]

Thanks for your inputs. Now we have a first hand user!

When tube output is taken from the anode, it becomes an amplifier and not a buffer. Same in PMillets case. The tube output is followed by MOSFET buffers.
I am looking for one that takes output from the tube cathode so that gain = 1 and no semiconductors are needed in the entire tube module.
I have written to Korg and am awaiting reply.

My worry is that I may have to use two packages for stereo since the two triodes in the package have their cathodes shorted.

Regards,

Ravindra.

 

[Fantastic]

Tubes have a different distortion spectrum compared to ss because of physics, the curve of the output vs the input. Tube circuits with low or no feedback have relatively high amounts of distortion ( which is not injected into the signal). It's the result of non linear gain of the amplifying device.With increasing negative feedback the distortion ratio with signal level keeps decreasing like all other circuits. Apart from this the tube has what they call monotonically decreasing distortion. The spectrum of the distortion components keep falling as the frequency rises. 2nd harmonic being the highest. But with negative feedback applied all these distortions fall and you cannot distinguish between a well designed solid state amplifier with a well designed tube amplifier. Note that the most objectionable distortion is odd order distortion and worse with increasing frequency.

A well designed tube pre amplifier has practically no sonic signature of its own. Designing without negative feedback or operating in a non linear portion of the gain will increase distortion and so one is actually catering to what some people seem to like, distorted sound ! The distortion story doesn't end here. Even with lowered distortion, tubes sometimes have an edge ( sometimes!). The distortion spectrum still has decreasing distortion with increasing frequency which is not the case with most ss circuits. So when the levels are audible you can distinguish between the tubed amp and the ss amp. No or low, negative feedback tube circuits have relatively higher distortion than ss circuits. This again is physics. Tubes circuits have far lower gain than solid state circuits. So when you apply feed back to get a typical gain we want, tube circuits have far more residual distortion than ss circuits. So tubes without feedback don't give 'better sound'. They distort more but in a way that some people like ( with even order harmonics). Like some people like having upma with jam or sugar ! Likes and dislikes vary a lot !

A small observation. Typically a tube buffer circuit ( gain almost = 1 ) compared to a single tube ( no external negative feedback ! ) with some gain ( say X 15 ) has much lower distortion . In fact if well designed it will NOT sound warm, just neutral ! But do remember after having said all this there are a LOT of other factors that affect the sound due to component behaviour in other parts of the circuit. That's why there is no single best device or circuit in audio. So the hunt goes on....and you get to make many new friends also in the chase of the Holy grail which may never end ! QED !

 

[Ravindra Desai]

Tubes have a different distortion spectrum compared to ss because of physics, the curve of the output vs the input. Tube circuits with low or no feedback have relatively high amounts of distortion ( which is not injected into the signal). It's the result of non linear gain of the amplifying device.With increasing negative feedback the distortion ratio with signal level keeps decreasing like all other circuits. Apart from this the tube has what they call monotonically decreasing distortion. The spectrum of the distortion components keep falling as the frequency rises. 2nd harmonic being the highest. But with negative feedback applied all these distortions fall and you cannot distinguish between a well designed solid state amplifier with a well designed tube amplifier. Note that the most objectionable distortion is odd order distortion and worse with increasing frequency.

A well designed tube pre amplifier has practically no sonic signature of its own. Designing without negative feedback or operating in a non linear portion of the gain will increase distortion and so one is actually catering to what some people seem to like, distorted sound ! The distortion story doesn't end here. Even with lowered distortion, tubes sometimes have an edge ( sometimes!). The distortion spectrum still has decreasing distortion with increasing frequency which is not the case with most ss circuits. So when the levels are audible you can distinguish between the tubed amp and the ss amp. No or low, negative feedback tube circuits have relatively higher distortion than ss circuits. This again is physics. Tubes circuits have far lower gain than solid state circuits. So when you apply feed back to get a typical gain we want, tube circuits have far more residual distortion than ss circuits. So tubes without feedback don't give 'better sound'. They distort more but in a way that some people like ( with even order harmonics). Like some people like having upma with jam or sugar ! Likes and dislikes vary a lot !

A small observation. Typically a tube buffer circuit ( gain almost = 1 ) compared to a single tube ( no external negative feedback ! ) with some gain ( say X 15 ) has much lower distortion . In fact if well designed it will NOT sound warm, just neutral ! But do remember after having said all this there are a LOT of other factors that affect the sound due to component behaviour in other parts of the circuit. That's why there is no single best device or circuit in audio. So the hunt goes on....and you get to make many new friends also in the chase of the Holy grail which may never end ! QED !

I admit, I have no experience with tubes. But the NuTube interests me.
Your detailed analysis helps me a lot. Thanks!!

Personally, I would like to design a cathode follower that will have 100% negative feedback. This will result in unity gain, highest linearity, least distortion and "Neutral" sound.
As an engineer and as an audio enthusiast, this makes a lot of sense to me. Then, I will be able to make a comparison (not grade them for superiority) of the two technologies: SS vs Tubes.

I have written to Korg about using the NuTube as a voltage follower and am awaiting their reply.

Thanks again!

Regards,

Ravindra.

 

[Fantastic]

'Cathode followers' are 100% negative feedback circuits by nature. Gain is always close to 1 but never exactly 1. Can't do anything about that ( due to physics ! ). Biggest plus point is you can skip the input capacitor which contributes to varying sound quality depending on type and you have a very high input impedance which you can determine within limits. The output capacitor is required due to the high DC voltages present there. However these have high dc potential across them and many people feel that a dc biased capacitor sounds better than one without any bias ( not just electrolytic types !). But then you can always experiment and see what works for you. Other peoples results are always guide lines. What works for you might be different.......often is !

As always, the bottom line is if you like what you have made then that's the best for you ! Sit back and enjoy the music !

 

[manniraj]

Also I have read on the diyaudio forum that this Nutube has issues with stability especially with the hiss or hum sound emanating with the shaking of the mounted PCB board unlike the regular tubes. Some of them have provided solutions but others suggest that this is a drawback of the tube design, so you need to keep this also in view.

 

[Sumanta]

A Superb article, reinforces my belief and completely enjoyable experiments with simple DHT / SE tube amps / pre-amps with no feedback and simple circuits driving high sensitivity full range drivers.......

https://www.audiostream.com/content/problem-tubes#RBTfLrlW8H0dIIjO.01

Read it in one go. Lovely way of writing those realization and their logic.
Almost like Jonathan Livingstone the seagull.

 

[Ravindra Desai]

NuTube circuits available in the market, that we have looked at so far are amplifiers and not a buffers! It has a gain (distortion) control. The control that most people will love to have but to me a gimmick that you will keep fiddling with for every track than concentrating and enjoying it.

In the buffer design that I intend to do, there will be no controls! It will only have input, output a mains supply connectors.

Then who will control the distortion/ warmth?

Only The Nutube Technology!

This is how I visualize a buffer that people can add to their signal chain to get tube signature: without the 'how much tube signature?'

A Superb article, reinforces my belief and completely enjoyable experiments with simple DHT / SE tube amps / pre-amps with no feedback and simple circuits driving high sensitivity full range drivers.......

https://www.audiostream.com/content/problem-tubes#RBTfLrlW8H0dIIjO.01

It was a good read but very biased. My reaction is similar to that of the second comment from the top with the title "It’s just a matter of taste.."

Thanks for sharing!

Regards,

Ravindra.

 

[Ravindra Desai]

The buffer is supposed to be just sitting in between and with no controls. The one I am currently using based on 6DJ8 is doing exactly that. No controls. It does definitely makes a huge difference to sound in a positive way. It’s Single Ended design based on double triode tube.

One with DHT topology can be designed around Nutube. I would be interested to see how it goes....

That's very re-assuring from you. Thanks!

You said 'double triode tube.' So is that one per channel for a total of two?
Else it could be something like this. I want to build one like only the second stage of the design in the link, per channel.
That way, like @Fantastic said, we can get rid of the input capacitor and with or without the circuit, there would be no volume change making the likes of @dr khanwelkar happy. Then you can interface anything to the input (even TT cartridge) and anything to the output. Also, there is no need for any solid state device making it a true Tube buffer stage.

In your case, the the volume drop, significantly, if you bypass the tube stage?

Regards,

Ravindra.