New Millett/Apex Sangaku amp

[skullguise]

Got an email from Todd @ TTVJ, he has this for pre-order, looks interesting with a Korg "NuTube" triode....

http://www.ttvjaudio.com/Apex_HiFi_Sangaku_Headphone_Amp_and_Pre_Amplifier_p/aaa0000998.htm

http://www.apexhifi.com/Sangaku.htm

 

Edited August 25, 2016 by skullguise

[Voltron]

I talked with Pete about this amp when I saw him in Oregon in June. He was very excited about the prospect of using these new tubes that aren't like anything that I've ever seen (including Mikhail's tiny submarine tubes from his booby amp near the end of his run). I'd love to hear this thing and hope to demo it sometime.

[dsavitsk]

I have not heard this, and I have no idea what Pete might have done, but the specs on those NuTubes are not promising. The good is that they run at low voltages and have a mu of 12 which is probably nice for a traditional tube to buffer hybrid amp without too much gain. The bad is that they have a plate impedance of 300K (higher than most pentodes), don't draw enough current to even really push a buffer, and draw nearly as much grid current as they do plate current. Gm is 40uS (0.00004S - compared to 0.0125S for a 6922 and 1.3S for an IRF510) and the limit to how much they can be biased means that they'll clip on a lot of modern sources. If you ran about 70 of them in parallel, you'd almost be on par with a 12AU7. I assume they are designed for overdrive and other effects pedals, and they probably work fine there.

[skullguise]

I think you called it Doug.  Per Apex site (my highlight):

In the Sanguku circuitry, the NuTube device is used in a single-ended configuration to provide the needed voltage gain.  

Solid state buffers provide both unbalanced and balanced outputs with a very low output impedance to drive the headphones.  The Sangaku can drive any headphones, from 16 ohms to 600 ohms, and has enough gain for even inefficient headphones.

Does this all mean it's not fully balanced?  Thinking so.....

[Dusty Chalk]

19 hours ago, dsavitsk said:

...they are designed for overdrive and other effects pedals...

(ears perk up)

Want.

[wink]

Can't have too much distortion..........     

[luvdunhill]

You can have too much forum distortion though.

[swt61]

I resemble that remark!

[wink]

I represent that remark...!  

[jelt2359]

First post here, been lurking a while but I'm a huge fan of Pete's (Doug's too, for that matter), here's some info I got from Pete via PMs, and Todd was okay with me reposting it so I thought I'd share.

The Sangaku has more "single-ended triode" sound (mostly second harmonic distortion), for a more tubey presentation than his previous hybrids. This is a result of the way the Nutube's circuit was designed. Nonetheless, his tube amps do tend to have broadly similar sonics- in short, there is a "house sound", a certain harmonic mix that he shoots for, typically by moving the operating point of the first tube stage. Some other differences between the Sangaku and the previous Apex hybrids: The new amp has much more output voltage driving capability than any of the previous hybrids; and also the lowest output impedance. Part of this is because of the Nutube- it only uses milliwatts of voltage. At the same time, the power supply voltage on the Sangaku is much higher (+/-18V, vs. 12V or 10V) to provide bigger output swing. 

The output stage is also different from the Peak/Volcano. The Peak/Volcano had a discrete MOSFET buffer. In the Sangaku, however, Pete has moved back to using power opamps. According to him the Peak was more of an experiment to see how well he could do without an opamp, but he's since decided that power opamps are better. He knows that some will argue that this is a less 'pure' approach, but he's made his choice- they dissipate less power, are smaller, and perform better. He's implemented these power opamps with CCS loads on them to keep them in class-A operation at low signals. Nonetheless, when driving lots of current, the output still goes into class AB operation. The CCS keeps more bias current flowing, which reduces any crossover distortion, an issue most problematic with small signals. Finally, the Sangaku is not DC coupled, but it's -3dB point is very low (single-digit Hz).

[sodacose]

On 8/24/2016 at 10:11 PM, dsavitsk said:

I have not heard this, and I have no idea what Pete might have done, but the specs on those NuTubes are not promising. The good is that they run at low voltages and have a mu of 12 which is probably nice for a traditional tube to buffer hybrid amp without too much gain. 

I concur.  My thoughts after checking out the data sheet:

General specs

It's a directly heated "tube" and at 0.7V filament voltage requires DC heaters. The given transconductance figure must be microhmos because the plate resistance is listed as 330k (though Mu = Gm x Rp and it doesn't quite work out).

Charts

There are two I/V plate charts: one at low voltage with positive grid bias and another with a larger voltage scale. With the 1.7mW plate dissipation, this is definitely designed for a positive grid. The curves below the dissipation max at higher voltage (where the grid could be biased negative) are kind of ugly. The curves look most interesting on the large scale chart in the 20-60V range, but the 0V grid line is smack in the middle of it.

Apps

This all makes sense given that it is direct heated and will likely be paired with other solid state parts and low voltage regulators. The app notes have a clever bias scheme for a 3.3V supply (note this will only work with positive grid bias). The 'Basic Circuit' shows source followers on both the input and output to deal with the impedance issues of the positive grid and large plate resistance.

Frequency Response and THD

Frequency response looks good at the voltage supply levels provided (5, 12, 30) with a -10dBu input (about 0.25Vrms). THD+N is not so good at the 5V supply level, but it is not bad at the -10dBu input and below with 12V or 30V supplies. Looks like it's meant to be driven with fairly modest input signals (ie portable devices).  There's no indication of the harmonic make-up of the THD on the datasheet.

Conclusion

This is all about knowing the limitations of the device and what the trade-offs gain you. This "tube" is an interesting hybrid in terms of specs and it definitely was designed to be partnered with solid state. I associate Korg more with MI than CE and so I was expecting something destined for guitar preamps and pedals (and this might work fine for that given there are two triodes in an "envelope"). However, it also appears to be an interesting candidate for a gain stage in portable headphone amplifiers, given the low supply voltage requirement and the low output voltage limitations of many phones and DAPs.  

I don't get the desktop format in this particular amp other than that it makes for a fairly unique and interesting device.  If portability isn't a concern, there are better "tubes" to use.  On the other hand, the Nu Tube device itself may allow the manufacturer to keep some production costs down in comparison to traditional tube amps (all on a PCB, no high voltage or output transformers, consistency from device to device).  Whether it's better than a JFET in the same application would be interesting to hear.

In all cases, this guy needs assistance from ancillary solid state devices. Ain't nothing wrong with that, but given this requirement it's almost more of a Nu Transistor than a Nu Tube.

[comzee]

I'm a huge tube fan, have multiple amps in that realm, with 100s of tubes =O

I used to be SS only, (even had Carbon + 009), but slowly transitioned away from all that. 

Anyway, I needed a beater amp, I found it used for $1450 couldn't pass that up.

I previously had the loaner, so I wasn't completely in the dark. I'm not great at reviews, so I'll give my own interpenetration in simple words.

 

Why did I buy this since it's technically SS (hybrid)? Well it doesn't sound like tubes, it does sound like an SS amp, BUT...

Although it's more similar to an SS sound signature than tubes, the nutubes fix enough of the SS annoyances I generally have. For me, all my SS amps have a bit of grain, glare. The nutubes take that edge off, without sacrifices detail or exactness.

It's what I would consider the battle between tubes musical liquid sound v.s. SS sterile exacting sound. The Sangaku fits inbetween those tonal signatures, making it a great general purpose amp (imo). 

[Torpedo]

 what's wrong with this thread?

[grawk]

it's a TLDR newbie magnet

[Pars]

^ I like that phrase, Dan. Accurate


Sent from my iPhone using Tapatalk

[comzee]

Lel, I've had Spritzers Carbon + 009 with a yggy, guess I'm a noob tho. 

[grawk]

please read your welcome message again.

[mypasswordis]

Glare, grain, and sterility are the trifecta of a well-lived life. 

[Sherwood]

On 3/22/2017 at 8:21 PM, comzee said:

Lel

We're done here.