Supratek Owners Thread

Long time Supratek user here. I owned a Cabernet Single since 2010. When I got it it was used and already had several issues. I told myself it was just common with tube equipment and I would fix it one day.

After 10 years owning and comparing it to a high voltage output DAC without preamp, I finally put myself to face the fact: The supratek just doesn't sound as good as without a preamp. And I also looked into the issues the unit has for so many years.

Frankly I was disappointed when I opened the unit. The two silver cans are empty and just for show. The unit actually uses PCB power transformers as output transformers.

I investigated the many issues with an oscilloscope. The right channel always had noise that felt like ground loop. Turns out it was a loose ground wire of the XLR connector to the center tap of the output transformer, probably due to poor soldering.

The right channel always felt louder to me, regardless how I swap the tubes between channel. Surely this was verified with a voltmeter and oscilloscope. It turns out the DHT filament wires were soldered in the other way round!

I also found the input tube clips when volume knob is pass 10 o'clock, which is just one third of the maximum volume! Still after so much tuning the phase of the two channels are still a little off.

The summary: if you plan to own a supertek, be prepared for a very involved experience. Either communicating with Mick or do it yourself as a project.. 

Hi Markus,

As this thread is titled the owner thread and I thought my experience would be interesting to someone who owns a Supratek or planning to get one.

Don't get me wrong, I still like the preamp and it is good looking. And frankly my hearing could only tell a small difference between having the preamp versus not. That's why I kept it and continued to use for more than 10 years.

My intention was to present facts about my experience, which is the workmanship left a lot to be desired. And that caused actual issues in the sound. Namely loose wire and switched polarity of the electrolytic capacitor. It would be quite unthinkable that the original owner decided to solder the wires in opposite polarity. I am trained as an electronic engineer and tell the unit was not modded.

Anyway take it anyway you want, I still plan to work on the unit to further improve it. I'm looking forward to connect with fellow owners and trade ideas about tube rolling and modifications.

Last night I fixed the incorrect polarity and it corrected the volume imbalance issue on the Cabernet preamp. I plan to replace the electrolytic capacitor for the DHT filament and adds a film capacitor in parallel. Also planning to replace the TJ 101D with a set of Linlai.

I was referring to the b+ filtering cap for the DHT filament, which was soldered in reverse. This obviously would fail to work for its intended purpose of smoothing out ripple. I believe what my eyes see and you are just speculating that it was modded. These are the same electrolytic caps used for all filaments and the same as in the pictures of the supratek online. I bought this from a reputable dealer years ago.

Anyway it seems like negative comment is the minority here. 

Sorry I admit I'm not familiar with terminologies in tube circuitry. The problem I found was with the DC filament on the right channel. The DC wires from power supply are reversed. Here is a closer look https:/ibb.co/tKXVz4j
On the left channel is correct. Here is closer look for comparison. https://ibb.co/k17Zvdx
The electrolytic cap on the right channel swelled and fell off. I replaced them with Nichicon KG with the same ratings.

I am also disappointed to see power transformers used as output transformers. It is using the Talema 22VAC @ 0.8VA. It's hard to know the effect without some measurements. I might attempt to replace it as well.

I appreciate very much @jslateiv's constructive comment.

Thanks @jslateiv for your feedback. Indeed the electrolytic caps are not on the sound path as they are coming from the 5V DC regulator. These are the Panasonic 10000uf. My guess is they are used for smoothing the DC from regulator. Nonetheless I replaced them with similar equivalent.

Very interesting that you tried different set of transformers. What did you try and was it a drop in replacementm? I might try a set of Hashimoto with similar ratio.

Thanks @jslateiv. This indeed matches my guess. Let's talk separately with mod ideas. :)

I just got a set of Linlai 101D hifi series to replace my older set of TJ 101D Mesh plate. These TJ 101Ds have been on the Supratek Cabernet for more than 10 years which I used very sparingly over the years.

For the price ($100 a pair from aliexpress) they perform very well to my years. They have a very sweet and full sound for vocals and less muddy than the old set (which really should have been replaced long time ago). I like them particularly with voice and the Eva Cassidy album Live at Blues Alley had a very euphoric quality to it. The amplifier I used was Cary Audio 120s by the way. I replaced all the 6SN7 tubes with some RCA and Sylvania NOS on both units. 

Getting the tubes to run at the full potential do take some effort however. TLDR is the tubes archived -70dB THD after tuning the voltages on both channels. If I play at low volume it could even archive -80dB THD wihch mostly 2nd harmonic. This is quite good for a tube design.

I attached an analyzer to the balanced output with 1k input to RCA. I first tuned the 6SN7 bias voltage then adjusted the plate voltage of the 101D and then finally the grid voltage of 101D. There seems to be a feedback in the design and the voltage takes a little while to stabilize after each adjustment. I was watching the FFT of analyzer and voltmeter during the tuning.

After some experiments I found the highest gain setting have the lowest distortion. Lowering the gain will gradually increase distortion for the same output level. I hope this information is useful for those who wonder the effect of the gain on the sound.

Comparing the FFTs of the Linlai and the TJ mesh plate I had the appreciation of the TJ very much. The TJ had very low 60Hz noise and mostly only 2nd harmonic distortion. The Linlai however is quite a bit noiser with 120hz and higher order harmonics of the power noise. The 3rd harmonic distortion is also quite high. I should probably get a set of TJ while it is still available.

couger4u: if you turn down the gain and turn up volume, and reduces the hum and noise. That's probably because the hum is power supply related. My Cabernet has the same characteristic. I used to run the Cabernet 101D with lower gain with higher volume to minimize the noise and hum. The downside is higher level of distortion. You might like the result though. 

My particular 101D Cabernet unit has not a small noise from the power supply. The HT lines had about 20mV p-p ripple and the DHT filament DC supplies had the same amount. It was a scary to work on the power supply so I decided to work on the main unit. Fortunately after removing the remote control PCB which I never use, it has space for me to put in a 5H choke and a 80uF film cap on each of the HT lines. I made use of the empty cans and put in a 18mH choke and 10000uF for the DC filament supplies. The circuit seems to be a constant current supply for filament with a LM317 which had about 20mVp-p ripple. After the mod which does not touch the signal circuit, I cannot measure the ripples with my equipment. The 2nd, 3rd, etc harmonics of the power supply noise are all gone. The SNR improved by 16dB and I cannot barely hear the annoying humming noise from the speakers. There's still a stubborn 60Hz noise from the FFT which I suspect is coming from the cathode which is from another DC regulator. But now I'm quite happy with the results.

Like @jslateiv said the sweet spot is around 8-10 o'clock of the volume knob to get minimal distortion, mostly consisting of 2nd harmonics. The gain I got was about -6dB to 0dB.