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Old 1st June 2007, 12:06 AM   #1
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Default Opinions/Experience of Toroidal valve amps

Hi to all,

I hope to build a high quality valve amp, I have looked into various designs and have sort of decided on a design by a chap called Menno Van der Veen from his book on valve amplifiers with toroidal output transformers.

I have in my possession a good number of el34 tubes and was intending to build the design that uses 8 of these tubes utilised as triodes to produce 70watts per channel.

I really cant afford to make any mistakes with my choice of design as i can just about afford to build this amplifier!!!

Therefore please could you share your experiences of these designs, good or bad or suggest any other high quality designs of the highest quality and with a minimum of about 50watt output.


many thanks
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Old 1st June 2007, 04:26 AM   #2
jarthel is offline jarthel  Australia
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I believe Van der Veen has designed toriodal power/output (maybe chokes as well) transformers for plitron.
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Old 1st June 2007, 05:09 AM   #3
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There's no need to go into an expensive toroid design to get good quality results. The standard E&I design from Sowter transformers, Majestic (UK) and so many other sim vendors are capable of very good performance.. without any need for DC current balancing.

If you can afford to build a toroid o/p stage amp,with extra current balancing circuitry trown in then I don't think financing becomes an issue.

I am not the only engineer (on technical grounds) against using toroids in o/p stages. About a year ago there was such a heated thread on this issue but needs searching.

richj
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Old 1st June 2007, 03:13 PM   #4
PeteN is offline PeteN  United Kingdom
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I looked on the search but couldn't come up with anything that looked 'heated' (judging just from title, post count, views, and optionally if it was locked, ahem)....

I thought (from what I've read, at least) that toroids were closest to being 'perfect' transformers (or at least, better than EI or C cores)?
Aside from the (and this is from memory) fact that they can be - is it magnetised or polarised - by dc, and that they're rather unsightly unless hidden in a bean-tin, how come then they seem to be frowned upon?
Admittedly this seems to be output rather than power toroids that take the brunt of the bashing, but surely if they're the 'perfect components' they'd be the choice of the professional™?
Or do their foibles present a greater technical challenge than EI/C cores so they're not worth the effort - cost vs gain or whatever?
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Old 1st June 2007, 08:36 PM   #5
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Norman Crowhurst did an amp with an o/p toroid with flux balancing thrown in. My resource ability is very slow speed up here.
The advantage with E&I core is lower permeability due to micro lamination gaps.....this is a natural advantage in subsonic LF rolloff as it actually improves the LF transient response and limits out of balance currrent/ core saturation to allow mismatched tube sets with 10% current misbalance.....This would be too high for a closed iron band toroid as the BH curve essentially goes to zero whereas a gapped core with laminations has a marked sloped hysteresis loop. Getting into 1st guadrant magnetics behaviour is unavoidable and is further complicated as nfb introduced in whatever form tends to extend b/w. So a toroid o/p tranny as it stands without a core gap will saturate easier at a lower frequency for a given excitation than an E&I with sim core area. One sees many inflated specs with toroids i.e b/w -3dB at 5Hz at 1W which indicates nothing apart from not being able to hear it and the amplifer o/p section cannot handle Bmax without running into overload and severe thd.
Having painted a diffcult picture with toroid designs, one can see the E&I laminations create an automatic design solution in LF design. If you follow the Crowhurst design, it is complicated but I follow the adage "more of less is better" esp in audio amp design.
A tube failure in a toroid o/p amp will often be dramatic, whereas with an E&I one can often just keep playing til the interval.

richj
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Old 1st June 2007, 09:14 PM   #6
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My experience of using toroidals as output is entirely different to Rich's. Mine are power toroidals so not up to the spec of Piltrons.
A few points though;
Piltrons are gapped, or so I have been lead to believe. Go check. This means they are capable of tolerating tube imbalance. They even make SE toroidals.
My power torodials happily go down to 10hz before they start to complain.
High frequency bandwidth will always far outstrip an EI. Infact ringing is the real thing to design out.
My outputs are good to about 70Khz without recourse to negative feedback.
Current balancing can easily be achieved by placing Constant Current Sinks in the cathodes. These can be as simple as LM317's. The desire to get perfect current matching brings its own sonic benefits and can relatively easily be turned into a fully differential output stage. EI transformers suffer from current imbalance even though they can tolerate it.
Once or twice th pin connection of one of the cathodes has gone dicky - the result is nothing more than distorted sound out of the speaker.

Remember in all this - mine are cheap mains transformers. I assume that you will be going for good quality Piltrons, so you can expect performance to be orders of magnitude better than mine.
Take all this as you may - but remember I am talking from experience and not prejudice.

A last though - I always thought that C-cores offered the best performance.

Shoog
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Old 1st June 2007, 10:44 PM   #7
bigwill is offline bigwill  United Kingdom
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Although not technically used as a valve output transformer, power toroids I've used for ESL step-ups have sounded good! What stood out was the treble.



Toroids Sound Good! (in MY experience anyway)
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Old 1st June 2007, 11:15 PM   #8
EC8010 is offline EC8010  United Kingdom
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In theory, a toroid is a better design, but its virtue is also its downfall. The core of a toroid is a spiral-wound continuous strip rather than a series of laminations. As other posters have suggested, this means it doesn't have a gap (not quite strictly true, but pretty close), and the lack of gap means you get enormous primary inductance with very few turns (good thing). The magnetic coupling from primary to secondary of a toroid with even windings is also very good, giving good HF response (even a power transformer can pass a good 10kHz square wave).

The problem is that the gap has a linearising effect and losing it makes primary inductance very unstable. Toroids need very tightly matched currents to avoid core saturation - it's no surprise that Menno also has a bias servo to enforce balance.

I would suggest that an amplifier with a toroidal output transformer is not ideal as a first build, a more traditional design will be more forgiving and require less post-construction fettling. I would also point out that a 70W amplifier is quite an onerous build. Unless you happen to be an electronics engineer, you would probably do better to start with something a little smaller and guaranteed to work. Something based on a Mullard 5-20 has a very good chance of success, even if it is (only) 20W.
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Old 2nd June 2007, 06:27 AM   #9
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Firstly, a big thanks for all the responses, I think i am now in a position to make a more reasoned decision on which design to go for.

I have come to a similar conclusion in that the design i was considering may be a bit OTT. Therefore does anyone have any suggestion on a well proven design, with the an idea of where to get hold of the design, specs etc???
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Old 2nd June 2007, 08:54 AM   #10
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If you are careful with your choices you can buy oversized toroidals and build a small amp. When you feel a bit more confident you can expand the amp to use the extra capacity.

Using 6080's in a push pull design is a cheap way to learn all the principles of working with toroids, and the ratio will then lend itself to upgrading with more conventional parallelled valves.

Shoog
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