[mophead]

A newly tube convert & very much a newbie diyer at this moment in time.
Hoping fellow music loving diy-ers and veterans can share your thoughts and experience on Transformer based Volume control.

I stumbled upon this while I was researching on autoformer + Tube buffer
J&K Audio Design: OPTVC

Very intrigue by the write up.
Seemingly extremely minimalist in concept, less components = less pollution to signal integrity.
I have read as much as I can on the web, cross referencing various write-up on TVC/ Autoformer, but still can't figure out what are the sonic benefits an OPTVC can bring to the table.

How does this work vs a intact audio Autoformer?
Best of both worlds between a pure AVC & Active pre?
I hope the kind folks here can enlighten me or help to ask the designer more relevant/ pertinent questions.

 --------------------------------------------------------------------
Posted a qn to J&K:
 how do they maintain the 600 ohms secondary output impedance at various volume settings or is the 600 ohms being referred too at max volume ? If that's the case then we would need to find out whats the impedance like at lowest volume setting.

J&K's Reply:
 On OPTVC, or TVC, 600 Ohm quoted is for the maximum volume (highest step). The lower the volume step, the lower the output impedance (which is equal to higher driver strength, therefore, the lower the better). At lowest step, the output impedance is only a few Ohms. For AVC, input impedance = output impedance. The range of attenuation is about -60dB for standard design. The steps can be chosen by -2dB or -3dB steps, depending on whether you are taking 23/24 steps, or 33 steps. Other attenuation / steps can be chosen, depending on user requests.
 ------------------------------------------------------------------

Anyone had experience working with J&K Audio Design?
What's their rep amongst the DIY community?
Can't find alot of review on them.

[LordCloud]

I've not heard of these guys. Though I have a TVC preamp, and would be interested in their approach, but a passive version, without the tubes.

[Docere]

I have found J&K to be sincere, helpful, professional and on time. I've had a brace of their top level custom transformers and chokes (lots of nano-c and super hi-B) sitting here for what must be close to two years - very solid and well made. Photos of their windings also show very high quality.

Oh, the project is moving slowly but is about to pick up pace (if I can find a custom chassis fabricator!)... 

Cheers,
Ray

[Norman Tracy]

....if I can find a custom chassis fabricator!

https://www.landfallsystems.com/index.php

[Docere]

....if I can find a custom chassis fabricator!

https://www.landfallsystems.com/index.php

Thanks Norman - I'll keep them in mind if I can't find someone locally.

I prefer a classic look: 14 - 12 gauge (stainless) steel, laser-cut, bent, welded and smoothed/cleaned for coating. Solid.

Cheers,
Ray

[FullRangeMan]

Thanks Norman - I'll keep them in mind if I can't find someone locally.

I prefer a classic look: 14 - 12 gauge (stainless) steel, laser-cut, bent, welded and smoothed/cleaned for coating. Solid.

Cheers,
Ray

LandFall is too expensive. Try these fold in a single piece at RFPARTS:
https://www.rfparts.com/cases/cases-heavyduty.html
Use aluminium, steel is magnetic, interferes with transformers and all electrical signals and even dissipates less heat than aluminum.

[Mark Korda]

Hi Mophead,
  I made this volume control, transformer implemented passive preamp in a Dyna PAM-1, 1957 chassis. All your holes are drilled already. I don't want to throw a wrench in but I'm going to quote what Jack Elliano from ElectraPrint transformers  said about this little design in AudioXpress mag. You should use a steel chassis for best results. Steel will absorb stray magnetic fields from power transformers and chokes in other equipment nearby. I never put any switching in yet but the holes are there for a future project. I use mine with a Dyna ST-70 or 35. ....Mark Korda

[Mark Korda]

Mophead, I used old time shellac for the finish...Mark Korda

[Mark Korda]

Sorry...try this..Mark.

[Docere]

Use aluminium, steel is magnetic, interferes with transformers and all electrical signals and even dissipates less heat than aluminum.

I'm not a fan of aluminium. It's advantages are lightness, easy to machine, and looks contemporary. People I trust that have built using different materials prefer the sound of steel variants. I'm not technically convinced though - some claim it is due to the resonance/ vibrational characteristics; others that the type of capacitive coupling between components and an alu chassis are unpleasant... I'll just have to trust my own ears.

Edit: Mark provided another possible benefit, per Jack Elliano: "should use a steel chassis for best results. Steel will absorb stray magnetic fields from power transformers and chokes in other equipment nearby". That is more convincing than the two I provided from the depths of murky memory. 

My builds address the issues mentioned. Components mounted on Delrin and suspended by nylon posts, signal-carrying wires and transformer cores are distanced from the metal... For my next build, even connectors are mounted to Delrin to avoid being positioned near metal sheet. I may even go so far as to use a 300 series austenitic stainless steel (low magnetism) for my next build... otherwise plain ol' 250 grade.

I might discuss this more in a build thread, if I post one here. I'm going to stop posting to this thread though - I don't want to hijack any more than I have.

Cheers,
Ray

[FullRangeMan]

Thanks for your impressions. In this book Morgan Jones rec alu and to avoid steel, maybe in the end its all personal opinions:
https://www.amazon.com/Morgan-Jones/e/B001HCUX52
But Eliano made transformers and want favor his products.

[Docere]

I think personal experience, preferences, systems and goals matter.

I own two editions of Morgan's "Valve Amplifiers" and his "Building Valve Amplifiers" books. They contain comprehensive and detailed technical advice... I value and appreciate Morgan's books; I refer to them often but understand that their technical content does not assure a system that I would enjoy listening to.

Not sure how Jack saying that he prefers steel would favour his products. Most transformer manufacturers use steel frames/ bell-ends/ cases... Maybe steel showcases them under the most favourable conditions? 

Cheers,
Ray

[randytsuch]

Personally, I'd probably go with Dave Slagle's autoformers if I was going to make a TVC
http://www.intactaudio.com/

This is based on his reputation for winding iron, haven't used his stuff in anything.

I consider winding iron to be an art form, and you need to find someone who knows what they are doing for good transformers.

J and K may be great, but I haven't heard of them, but honestly I haven't look for iron in a few years.

Randy

[Mark Korda]

Hi Docere,
  The paragraph I quoted from Jack Elliano about a steel enclosure was why I used the Dyna PAM chassis plus the already drilled holes for standard 3/8 inch gold plated jacks. My mainstay passive preamp uses an aluminum chassis from Hammond.
    I made a lot of mistakes with making a decent looking chassis with a drill. What gave me a major advance was buying this little tool called a metal punch. You can put that punch where ever you want accurately, turn the screw to keep it tight to the chassis and then turn with a ratchet wrench until it bores a hole thru.....there like laser perfect.....now that I learned to send a picture here's another one to (bore) you with.........ALPS Black Beauty 100k pots, silver wire, and silver toggles....Mark K.

[Mark Korda]

Hi Docere,
   the front...Mark.

[Docere]

Looks nice Mark.

Never tried a punch - only used drills and a nice Japanese hole-saw for tube sockets. Worked well... but I bet those punches work very well without the need for a drill press, esp. on alu.

I don't love the metalwork part of a build even though I have access to some decent equipment/ knowledge (my dad was a boilermaker, but now getting on). These days I draw up a design by hand then send to a fabricator who does CAD, laser, folding welding etc. It costs more, but the quality is excellent and can achieve accurate cut-outs for mounts etc. Unfortunately my fabricator has not gotten back to me and I have to find another willing to do a small, detailed job. 

Nice to see your posts here Mark... Your name is familiar - do I remember you from AA or diyAudio?

Cheers,
Ray

[FullRangeMan]

Very nice cast enclosure

[mophead]

Wow.... a tvc thread quickly evoling into a custom chasis discussion.

Hope more sharing on Transformer based attenuation control and feedback from ppl who have engaged the services of j&k. Would like to know how proficient theysince their core business is in winding trnny

[Mark Korda]

Hi Docere and Full Range,
    thanks for the comments!....Mophead, once you get into tubes you will never go back. I'm sorry for getting off the subject because I don't know without a Google search what the J&K transformer volume control is. I'm supposed to be working right now but I find these forums a real good tool for learning.
    One site you should check out is (Blueglow) electronics. This guy puts a camera on his head and sits you through vintage tube rebuilds. He goes real slow which is great for me and I do go on Audio Asylum alot.
   If you have time could you send me the J&K link....thanks Mark Korda.

[randytsuch]

Wow.... a tvc thread quickly evoling into a custom chasis discussion.

Hope more sharing on Transformer based attenuation control and feedback from ppl who have engaged the services of j&k. Would like to know how proficient theysince their core business is in winding trnny

Sometimes you have to whip these guys to stay in line