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Old 23rd May 2009, 01:28 PM   #1
flysig is offline flysig  United States
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Default Transformer reflected impedance games? Overspec'd output power?

Transformers were not my strong subject in school, which was 28 yrs ago, so here go a couple of dumb questions.

Looking at possible load lines for a particular push-pull class AB1 application, it looks like a lower output impedance would work better. But there are no transformers at that exact impedance. Let's say that the 4K load line looks really good, but the available transformers are 8K, or 10K. The speaker is 8 ohms nominal.

1) The transformers are available in several single tapped versions, including 8 ohms and 16 ohms. So, could the 8K transformer designed for use with the 16 ohm load be used with the 8 ohm speaker to result in a 4K effective load on the output tubes? Would doing this substitution likely cause distortion or reduced frequency range?

2) If the transformers are multi-tapped with 4-8-16 ohm selections, could one just wire up the 16 ohm tap to the 8 ohm speaker without any problems, or would there be distortion or frequency range problems?

3) Instead, let's say we can find a transformer with the perfect match of impedances, 4K into 8 ohm speakers. But our amp is nominally 15WRMS and the transformer is spec'd for 30W. Will this overspec'd transformer operate poorly at the lower power level? Other than weight and cost are there any inherent problems with using a higher power output transformer than we need? Is there a limit on how overspec'd the output transformer can be before problems set in?
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Old 23rd May 2009, 11:27 PM   #2
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Quote:
could the 8K transformer designed for use with the 16 ohm load be used with the 8 ohm speaker to result in a 4K effective load on the output tubes?
Yes.

Quote:
Would doing this substitution likely cause distortion or reduced frequency range?
Yes, distortion is usually not an issue, and the frequency response will be affected. Every case is different, but in most cases the changes are minimal.

Quote:
could one just wire up the 16 ohm tap to the 8 ohm speaker without any problems
Same answer as above. I have done both several times with good results. I have made frequency response measurements and found that if you stick to a 2:1 impedance ratio change or less the differences are minimal.

One of my Simple SE amps uses a 3K ohm transformer with the 8 ohm load on the 4 ohm tap for a 6K ohm load. The results are excellent. In some cases the 3K transformer may not have enough primary inductance to work well at 6K ohms causing some loss in low frequency response. In this case the low frequency response (3db point) changed from 24 Hz to 30 Hz.

I have tried to use the One Electron UBT-3 (3K ohms SE) as a 1.5 K transformer. It works but becomes somewhat lossy due to its relatively high DC resistance (235 ohms). I tried is as a 750 ohms (4 ohm load on 16 ohm tap) but much of the power is lost in the resistance of the primary.

I have some 6600 ohm push pull transformers which I routinely use as 3300 ohm transformers. They work good.

Quote:
Will this overspec'd transformer operate poorly at the lower power level?
Since many OPT power ratings are optimistic any way a 2:1 over spec is probably a good thing.

Quote:
Other than weight and cost are there any inherent problems with using a higher power output transformer than we need? Is there a limit on how overspec'd the output transformer can be before problems set in?
If the OPT is grossly oversized a good percentage of the electrical energy sent to the transformer will be spent in magnetic losses leading to loss of efficiency and possible loss of fine detials in the music. I have seen and heard this effect using 11 pound Hammond "30 watt" transformers in a SE amp operating at 1/2 of a watt.
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