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Old 15th January 2008, 02:23 AM   #1
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Default Transformer Coupling Questions

Hello all,

How does transformer coupling work? It seems simple enough to swap a coupling cap with a 1:1 coupling transformer but what about step up transformers? Suppose you use a 5k:10k coupling transformer, does the second stage see a source impedance of 10k? If so it seems like transformer coupling is a nice way to match input and output impedances as well as improve hi frequency response.

Also suppose my power stage requires a 20Vpk-pk input then if I use this 5k:10k transformer can I simply have my driver stage output 10Vpk-pk since the voltage at the secondry will be 20Vpk-pk? Any help or references to transformer coupling online would be helpful.

Also, where can I buy Tamura transformers and chokes in the US?

Thanks for all the help guys.
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Old 15th January 2008, 02:37 AM   #2
kevinkr is offline kevinkr  United States
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Transformation ratios are the square of the turns ratio so for 10K:20K you actually get a voltage ratio of 1:1.414 or about 3dB of voltage gain..

The impedance reflected to the secondary in the case of a triode is actually rp x the impedance transformation ratio or for the above case using an rp of 1.7K (say 45 dht) looks like 3.4K to the succeeding stage. The impedance the driving 45 will see depends heavily on what value of resistance if any is across the secondary - if 20K then it would see a 10K load.

All depends on which way you are looking. The additional voltage gain of the transformer provides additional swing at the expense of significantly higher source impedances, and lower drive current, but the trade off may be a good one in some (many?) applications. Here you might get as much as 500Vpp on a 250V rail with excellent linearity figuring on the tube swinging about 350Vpp on its own..

I'm not sure of any outfits that carry a good selection of Tamura transformers in the USA, the last time I bought any I got them from a Japanese outfit known as EIFL - the deal was costly but the customer service was excellent. (8yrs ago)

You might want to try contacting Jack Elliano at Electra-Print or Mike LeFevre at Magnequest, if you can afford the Tamura these guys can offer something competitive for somewhat less from experience.

Brief, but I hope this helps.
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Old 15th January 2008, 02:55 AM   #3
BudP is offline BudP  United States
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Transformers couple in two actions. E Field and B Field. The B Field coupling is the most commonly understood. This is the flux event within the core "window" and the amplification factor provided by the window. However, before the B Field event there is an electrostatic coupling, across the primary to secondary dielectric barriers that "sets up" the conditions for the leading edge of the B Field coupling. With poor E Field coupling you will have a loss of fine detail. Unless, as some companies do, you use very permeable core that works as a power transform component out to 18k Hertz and attempt to eliminate the E Field coupling. This is a valid approach. Others use a much less expensive core, one that ceases to pass power above 440 Hz or so, and rely upon excellent antenna event direct coupling B Field and useful E Field coupling within the window, for basically equivalent performance.

Step up interstage devices are very difficult to use as they magnify the capacitance of the driven tube back to the driver. It is actually much better to use a higher gain driver and step down with an interstage transformer. One to One is fine, but it is rare not to have a rising response out above 25 kHz or so. Step ups struggle to have flat phase to 20 kHz and step downs are easily the flattest in phase and FR, out past 40k Hz for both.

Sonically, a good non inductive cap, like multicaps, are the equivalent of good interstage transformers except in one area. The caps all have an unrealistic increase in perceived physical size of an instrument, as it increases in amplitude. The transformer raises amplitude without this artifact.

Both will provide excellent musical performance, depending upon how much money you want to spend, with the transformer usually being more expensive.

Having said that, I prefer caps in preamp interstage and transformers in power amp interstage, especially if the preamp is SE, with an OPT on the cathode of the output stage, driving a split secondary directly to a fully differential power amp's input grids, through balanced cables.

Bud
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Old 15th January 2008, 06:13 AM   #4
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Quote:
Originally posted by kevinkr
You might want to try contacting Jack Elliano at Electra-Print or Mike LeFevre at Magnequest, if you can afford the Tamura these guys can offer something competitive for somewhat less from experience.
Bud Purvine of O'Netics (see the previous post) and Dave Slagle (http://www.intactaudio.com/) should be on that list too....

dave
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Old 15th January 2008, 02:19 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by planet10


Bud Purvine of O'Netics (see the previous post) and Dave Slagle (http://www.intactaudio.com/) should be on that list too....

dave
Hi Dave,
While I don't disagree at all I don't generally recommend vendors I have not used.

One of these days I'd like to try some of Dave Slagle's volume control transformers. Have you tried them?
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Old 15th January 2008, 07:17 PM   #6
BudP is offline BudP  United States
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kevinkr

Both Lynn Olson and Gary Pimm rate Dave's tapped autoformer volume controls highly. The two I have heard were not audible in any way, in very high resolution systems. No phase change or frequency response oddities were audible.

We do not make these parts, though we are considering a fully isolated volume control. It would be quite expensive however and considering the sound of Dave's unit, not needed in most cases.

Our expertise is actually in PP OPT's, SE OPT's, SE to PP interstage, PP to PP interstage and SE to PP/SE preamp OPT's.

Those same two folks rate our level one and level three PP OPT's and interstage's as best in the world. Others like our SE OPT's and rate them as just below the best from Tamura, for our level one units, equivalent to James, Lundahl, Dave Slagle and Electraprint, all of whom are excellent manufacturers, so we are happy to keep company with them.

We do not advertise so we don't really compete in the DIY market. We are an OEM supplier in the Guitar amplifier market with THD and Soldano as our best known customers. In addition we are an OEM supplier for power transformers and have been in OEM manufacturing for 25 years.

Our DIY activities are by request and those customers seek us out due to word of mouth, both in Guitar amplifiers and audio reproduction amplifiers.

Bud
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Old 15th January 2008, 08:14 PM   #7
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One of the pleasures of DIY audio for me is that it is one of the few places where you can interact with actual skilled crafts people. Whether Electra-Print, O'Netics, Magnequest, Dave Slagle, Cinemag, or whatever, being able to call these people up and talk through designs is really a pleasure. Yes, there is cheaper stuff coming out of China, and high (not higher) quality stuff from all over, but this is such a more interesting and personal aspect of this hobby.
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