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7th March 2010, 02:53 PM  #1 
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Join Date: Feb 2009

Multi tap transformers dont use the whole windings?
This is something that has been bothering me and doesnt make much sense.
The transformers with 4816ohm taps. Ive heard that they only use the whole winding at the 16 ohm tap, and that if you use the 4 ohm tap your only using a 1/4 of the iron and its going to perform like a transformer thats rated much smaller. Literally its a 16ohm coil with taps in the center and the 1/4th mark. Now in my mind i thought that these transformers just had quad secondaries. 4ohms, all 4 secondaries are wired in parallel. 8ohms, 2 sets are wired in series, then paralleled. 16ohm all 4 sets are wired in series. In this way the whole iron is used at any given time. Or am i completely wrong? 
7th March 2010, 03:00 PM  #2 
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Join Date: Feb 2009

Impedance ratio is not proportional to number of turns, it is a square of number of turns. To double impedance you only need to increase the number of turns by a square root of 2, meaning two sections of a 4R & 8R transformer are not identical (real life figures of one of my OPTs are 100 turns for 4R and additional 41 turns for entire secondary winding of 8R).
So yeah, you are completely wrong
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mod verb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it." 
7th March 2010, 03:34 PM  #3  
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Join Date: Jan 2008
Location: Eureka, CA

Quote:
You're right, it doesn't make sense and shouldn't It's completely bogus to think that fewer turns on the secondary == less use of the iron or smaller rating. The thing you heard was "madeup", possibly to explain something or more likely to sell something, or perhaps they just didn't remember the facts, so... There may be a slight disadvantage in frequency response with a multitap winding but there is no derating. What this world needs is a good factual primer on electromagnetics... If only Michael Faraday was still with us. Michael Last edited by Michael Koster; 7th March 2010 at 03:53 PM. 

7th March 2010, 03:50 PM  #4 
diyAudio Member
Join Date: Apr 2009

secondary impedance
Hi,
You are right, and you are not quite right. Let's take an example of a single ended output transformer with a 5k primary impedance, and 4, 8 and 16 ohm secondaries. This transformer can be wound using different winding techniques. In our example we have 2500 primary windings. This means that to get 4, 8 and 16 ohm secondaries we must have 71 secondary windings for 4 ohm (70,7 to be exact), 100 secondary windings for 8 ohm, and 142 secondary windings for 16 ohm (141,4 to be exact). The "cheapest" way to do this is to make a 16 ohm secondary of 142 windings, and make taps at 71 windings and 100 windings for 4 and 8 ohms. We could repeat this winding technique using several parallelled secondary layers interleaved with primary layers; we call these "sections". However using the 4 or 8 ohm tap, part of the winding is "unused". It is not correct to say that part of the core is unused, but the coupling between primary and secondary is impaired because of the unused part of the secondary windings. This will result in high frequency loss, in the worst case already within the audio range. A better way to make multiple secondary connections in the same output transformer is to wind a number of 1 ohm secondary sections. In our example 35 windings would make a 1 ohm secondary (35,35 to be exact). Then in order to have a 4 ohm connection (70 windings) we must connect two of these 1 ohm secondaries in series; for 8 ohm we need three 1 ohm secondaries in series (105 windings); for 16 ohm we connect four 1 ohm secondaries in series (140 windings). So we need at least four 1 ohm secondaries for this winding technique. Actually the 8 ohm connection is 9 ohm; we could say 4, 9 and 16 ohm (2², 3² and 4²). In case of 4 ohm two secondaries are in series, and parallelled with the two other secondaries which are in series. In case of 8 ohm the fourth secondary is parallelled with one of the three other secondaries which are in series. Quite a story but I hope it throws a bit light on the matter. Pieter Tribute Audio Transformers 
7th March 2010, 04:21 PM  #5  
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Join Date: Jan 2008
Location: Eureka, CA

Quote:
Many transformer builders recommend a single secondary winding for this reason. Maybe for the ultimate nocompromise performance. I'm still not aware of any transformers that will "perform like a much smaller transformer" whan using a lower impedance output tap. Cheers, Michael 

7th March 2010, 06:09 PM  #6 
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Join Date: Feb 2009

Well that explains alot. Ok last transformer question. Putting a 4ohm speaker on a 8ohm transformer is a bad idea. Is it also bad to put a 8ohm speaker on a 4ohm tap? Ive heard several people say it actually sounds better, but with reduced output.

7th March 2010, 07:00 PM  #7  
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Join Date: Jan 2008
Location: Eureka, CA

Quote:
Designing an amp to use a 4 ohms load on the 8 ohm tap and provide the correct anode load will still have 2 problems. The DC resistance of the coil will be excessively high for 4 ohms, limiting the damping factor, and the inductance of the primary will be too low for the primary circuit impedance, resulting in loss of low frequency power and range. Putting 8 ohms on the 4 ohm tap of an existing amplifier may be OK but might not be good for highgNFB amps. The result is a flatter load line on the output tube(s) resulting in higher voltage swing (if the circuit allows it) and improved damping factor. The DCR of the output coil will be low relative to the 8 ohms, further improving the damping factor. Maximum output power, all other things being equal, will be 1/4 (1/2 the voltage headroom). In reality you get a little more than 1/4. I have been happy with the results of doing this in the case of a zeronfb triode amp and a box speaker, even though the power is down by 6db. I suppose you end up with extra iron, but it seems to me there will be higher flux density relative to the signal level to compensate. Michael 

7th March 2010, 08:23 PM  #8 
diyAudio Member
Join Date: Apr 2009

[QUOTE=Michael Koster;2110752]Putting a 4 ohm speaker on the 8 ohm tap of an existing amplifier, specifically a tube amp, reduces the load impedance seen by the output tube(s) by 1/2. With a class A triode that will increase distortion and current draw, reduces damping factor, and might increase power output depending on the idle current and amp topology.
Michael, Increase of distortion at reduced load impedance is an oversimplification. Jean Hiraga worked on this matter and showed that reduced load impedance might increase K2 but at the same time reduce K3. Finding the "sweet spot" is important. Designing an amp to use a 4 ohms load on the 8 ohm tap and provide the correct anode load will still have 2 problems. The DC resistance of the coil will be excessively high for 4 ohms, limiting the damping factor, and the inductance of the primary will be too low for the primary circuit impedance, resulting in loss of low frequency power and range. DC resistance is important when it is "high" already. For DC resistance no more than 0,2 ohms for any load impedance I don't see much of a problem (quality transformers). Please explain why the inductance of the primary will be too low using a 4 ohm load on the 8 ohm tap. Primary inductance is a rather complex function of number of primary windings, core dimensions, air gaps, primary voltage swing and frequency. I don't see a clear connection between primary induction and load impedance. Pieter Tribute Audio Transformers 
7th March 2010, 09:09 PM  #9 
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Join Date: Feb 2009

It seems my question went the wrong way. I was asking if putting an 8ohm speaker on a 4 ohm transformer would reduce audio quality.
I have both 4ohm and 8ohm speakers that i like in the house. Some have told me to just get a 6ohm transformer. Others have said stay away from multi tap transformers. Im trying to find the best solution less having 2 sets of transformers. If the only compromise of running 8ohm speakers on a 4ohm transformer is reduced output, ill do that. 
7th March 2010, 09:33 PM  #10 
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Join Date: Apr 2009

Nightanole,
With 8 ohm load on 4 ohm transformer you reduce max power in watts by (a little less than) half, not a quarter. Presume you have a 2k5 / 4 ohm transformer. Loading with 8 ohm means that, besides the loss of power, the tube will "see" a 5k load. It depends on your type of loudspeaker how this will change audio quality. When you have low efficiency loudspeakers you might run out of power. With higher efficiency loudspeakers you might get away with the power loss and might even benefit from the higher damping factor (important in case of bass reflex loudspeakers). At the end you must try and listen. Multiple secondary impedance output transformers can be done without loss of quality. My latest 4  9  16 ohm generation single ended output transformers (also with 2k5 and 3k6 primary impedance to choose) have equal bandwidth for all impedance settings (15Hz  40kHz minus 1 dB). Pieter Tribute Audio Transformers 
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