Why No Love for Toroidal OPT's ?

[Tubologic]

I don't know if this topic has been already discussed here before, thus I'm starting this new thread.
I couple of days ago I was reading an old Glass Audio article (by Satoru Kobayashi) describing a Push-Pull amplifier based on a (Plitron) toroidal output transformer. I was quite impressed by the (published) measurements of the amp, especially the incredible power bandwidth and low distorsion of the completed amp.
Now I'm wondering why toroidal output transformers (despite having excellent electrical specs) seems to be turned down by most audiophiles and rarely used in commercial units. Are there any objective (technical) reasons, other than the seemingly common consensus amongst audiophiles stating that toroidal transformers "sounds harsh and mechanical",etc...
Are they really so bad ? And why ?
Price is not a consideration in this discussion.

 

[BinaryMike]

Are there any objective (technical) reasons...

The primary criticism is that core saturation occurs with a much smaller imbalance in DC primary currents, relative to EI core OPTs. Many designers feel that special circuitry is needed to overcome this drawback.

 

[RGA]

I don't have the technical knowledge as to the whys but I met with Peter Qvortrup the head of Audio Note - the biggest High End SET amplifier manufacturer on the planet about torroids and basically called them cheap rubbish. PS before he ran Audio Note he was a European imported of top end brands including SS including ones using torroids. Audio Note also makes their own transformers and without cost constraints.

And why did I ask - because I noted that a lot of people seem to prefer vintage SS amplifiers (using EI) over new SS amplifiers almost all of which use torroids. I noted that even my current Marantz Solid State (SS) surround sound receiver sounded a little better than some integrated amps for more money (more famous 2 channel names) that use Torroids which is somewhat baffling since supposedly EI suffers more noise issues but it is perfectly dead quiet in a receiver which has a LOT of extra "stuff" packed into the case.

Almost all current tube amplifiers at the top of the heap use either EI transformers or C-Core or Double C-Core transformers. The latter are typically used in the upper most products (at upper most prices). From my subjective listening experience in almost every case - whenever I hear say a $5k SS amplifier using a torroid or a $10k amp using a Torroid or a $60k amp - etc - I can almost always point to a tube amp using an EI that I think sounds considerably better for less than half the money.

The only real gain I can see - is that the SS amp with the big torroid has more watts (considerably more) - and a lower noise floor (which can sometimes be heard and sometimes not) unless the tube amp is just badly made. And the watt advantage is worthless because all the best sounding speakers are High Efficiency and don't require more than 30 watts - many don't require more than 3 watts.

I have read some engineers discussing why EI is superior but my eyes glaze over - in the end it's decided in the listening room. As a consumer I am interested in the effect and not the cause. Although I pay enough attention to make general conclusions - after 25 years - I make a list of the 30 best amplifiers I've heard and all use EI or C-Core and only 1 uses a torroid. And that one amp - the Sugden A21a - is completely different to all other SS amplifiers out there. It is a pure class A single Ended 20 watt amp. It runs frying egg hot and operates like a Single Ended amp. And as good as it is - I would really like to hear one with an EI - just to see if it would not be even better. And as much as I like it - I still prefer amps at 1/2 the price using an EI.

 

[Steve O]

I don't have the technical knowledge as to the whys but I met with Peter Qvortrup the head of Audio Note - the biggest High End SET amplifier manufacturer on the planet about torroids and basically called them cheap rubbish. PS before he ran Audio Note he was a European imported of top end brands including SS including ones using torroids. Audio Note also makes their own transformers and without cost constraints.

<snip>

Just to be clear here, the OP’s question concerned toroidal output transformers for tube circuits whereas your discussion appears to involve toroidal power transformers. While the basic operating principles of the two subtypes are the same, the details are not. I believe BinaryMike pretty much covers the objective aspects of toroidal tube OPT issues. Yours is a subjective assessment of power transformer types (and speakers) in general.

 

[soundmotor]

I've always wondered about the Genrad 942-A. AFAIK the only commercial use with them was in the Gotham PFB-150 cutter amp, fairly well respected -

 

[BinaryMike]

Aside from the DC imbalance problem, toroids can offer much tighter coupling and reduced leakage inductance relative to EI core OPTs. Low-cost toroid power transformers can even make acceptable OPTs. For instance, a 230VCT/9V power spec yields nominally 5200 ohms CT to 8 ohms impedance ratio. A few of the folks over at diyAudio have explored this possibility in depth, with at least one design relying on cathode CCSs to maintain excellent primary DC balance.

 

[Tubologic]

Just to be clear here, the OP’s question concerned toroidal output transformers for tube circuits whereas your discussion appears to involve toroidal power transformers. While the basic operating principles of the two subtypes are the same, the details are not. I believe BinaryMike pretty much covers the objective aspects of toroidal tube OPT issues. Yours is a subjective assessment of power transformer types (and speakers) in general.

I thought it was obvious, but thank you for clarifying this. Yes, this discussion is only about output transormers (OPT's) for tube amps and I'm not really interested in subjective evaluations/arguments, which should not prevent anybody posting them anyway if so inclined. All your inputs are welcome.

 

[jaymanaa]

All you need to do is have a design that idles at a low DC current like the Behringer 232 screen drive. I built one and potted some Tomiko torrid outputs, and used 6JM6 TV sweeps (use 6JN6 if you don't want a top cap). They idle at 3ma, not a typo. You can lay your finger along the side of the output tube if you aren't rocking it. The trick to that amp IMHO is having a huge reserve in the PS to take it from 3ma to full output like right now. Kind of a simplistic way of thinking, but I think I have around 10,000uf in my supply and it's easily one of my favorite amps.

 

[BinaryMike]

All you need to do is have a design that idles at a low DC current...

The naturally tight coupling and sensitivity to primary DC imbalance would seem to make toroid OPTs the first choice for class B amp designs. As the jay man points out, you can't have much DC imbalance if you don't have much DC bias to begin with! I'm going to keep this in mind for a future class B2 amplifier project.

 

[jaymanaa]

I think the fellas name is Bernning, but I'lll be danged if I can find the drawing, and it is a hand drawing that's been copied so much it's hard to read. Seems like it was called the SA232 and made an honest 30 per side, both sides running at 1k IIRC. It uses a 6SN7 to drive the screen of the 6JM6. It's got some 12AT7s too, a high tube count for a SS power supply amp (Crees of course).
100mv at the test points gives you 3 ma idle current.
I think it's too dusty to photograph.

 

[jaymanaa]

I should be horse whipped !

 

[BillWojo]

Jay, send that poor neglected thing to me for a good cleaning. Please include the tubes as well. I'll ship it right back. Promise..........

BillWojo

 

[Dave C]

I have 2 sets of 300B monoblocks that I built. after experimenting with different OPTs they now each have Plitrons I don't think I could ever go back to a conventional transformer.

 

[Lavane]

Aside from the DC imbalance problem, toroids can offer much tighter coupling and reduced leakage inductance relative to EI core OPTs. Low-cost toroid power transformers can even make acceptable OPTs. For instance, a 230VCT/9V power spec yields nominally 5200 ohms CT to 8 ohms impedance ratio. A few of the folks over at diyAudio have explored this possibility in depth, with at least one design relying on cathode CCSs to maintain excellent primary DC balance.

I see a lot of posts on other sites where toroid OPT's and toroid power tranys used as OPT's. Most are from Europe. It may just be a cost thing? Toriods being easier and cheaper to get compared to having to import say, Hammond or Edcore OPT's. Or even Chinese OPT's when you have to fingure in import and shipping costs.

 

[steerpike2]

I built one and potted some Tomiko torrid outputs,

And amplifier with a torrid output.... hmmm..... that can't be relaxing to listen to!

 

[Pio1980]

I think I have a couple Antek(?) 6BQ5 p/p topts in my parts somewhere waiting on a project I'll never get around to making.

 

[srb02]

I have a pair of "Toroidy" 5K--8 ohm SE OPT's (confirmed, air gaped) I got several years ago, Kegger said he had a "special" schematic he wanted me to try. Alas, that didn't come to pass.
These guys are really big in Europe for tube transformers. They are in Poland.
I have stuck them into several SE amps to see how they sound, and they have stayed in my bench Spud amp. Never had any issues, and sound great.
I don't have the math/design/electronic smarts most of you have, but they work and sound great, and no apparent side affects.

My 2 cents.
Stu

 

[egellings]

Problem I see with toroids as OPTs for SE amps is how do you gap a toroid to prevent the DC supply current from saturating its core?

 

[Mike Stehr]

Then there is R core.

http://softone.a.la9.jp/english/RW20.htm

 

[Retrovert]

Problem I see with toroids as OPTs for SE amps is how do you gap a toroid to prevent the DC supply current from saturating its core?

You're correct. Any air gap would, of necessity, be approximately the thickness of a sheet of paper, a difficult tolerance for fabrication.

The toroid's core, instead, is normally increased in mass to reduce the risk of saturation. Same issue as with torroidal power transformers. The mass of a Class A output transformer is considerably higher than for a Class AB, and the reason is the larger core.

The higher interwinding capacitance of the toroid must still be considered, as that couples signal from primary to secondary.