Difference between OPT
A HongKong tube guru told me sound quality of tube amplifier is depend on the opt. I hadn't realized this before the testings we did recently.
The testing compared 2 different OPT. Both are 4.2k to 4 and 8Ohm transformers with same core size. But the core material and wiring method is different. The left one use z11 as core material, 4 + 3 wiring. The right one is another unknown brand material, wired as power transformer. We just want to find how the material and the wiring method affect the performance.
Testing equipment: HP3563A analyzer, FU-50 SE tube amplifier
Input signal: 1Vp-p
Output signal: about 3.6V RMS
Testing Freq: 10Hz - 100KHz
Frequency response (SPEC1 for right OPT and SPEC2 for left OPT)
1. Freq response between 8db - 12db
2. Freq response between 4db - 12db
3. Freq response between -4db - 12db
4. Freq response between -12db - 12db
Wave form for square wave input
Yellow for left, green for right
4. Detail at rising/fall edge
I doubt it very much the transformer on the right was originally wired/designed as a plain power transformer as the laminations seem to be set-up with a gap in the stack....as we know gapped TXs are used solely for SE applications.
Yes OPT windings are a bit of a black art ...as it seems no-one wants to reveal 'secrets' or prime winding scenarios/patterns. The info is shaking loose from the winding community slowly , but info is sketchy & tightly held.
Both transformers are made in the lab of our company. We just want to determin how those factors affect the performance.
The wiring of the right is exact the same as power transformer: Wiring primary coil first, then second coil. No interlacing at all. The capacitance seems to be high, that's why there's a resonance peak at about 60KHz.
We also have another transoformer, same wiring method with the right one, but used good material. It performance good at high freq, but the resonance peak is still exist.
Two issues: stray capacitance, magnetic coupling. Interleaved windings affect both.
The guiding principles are all in the Radiotron handbook 4th ed, sect 5.3 around p. 211, and all there in imperial to swallow.
Siliconray doesn't refine the study; how many sections on each side, type of interleave materials and tightness, type of wire and the sequence of windings. Every single parameter effects performance.
As for iron, M6 is the cheapest & common in p-p o/p & mains trannies, with the E&I alternatively laminated.
Remember Fouriers criteria, for a 15Khz bandwidth, a designer for a good tranny one doesn't want to see the 3rd harmonic droop by -3dB at 3x15 i.e = 45Khz and that's where sectionalising issues start getting tough and most tube amps give up trying with whatever loop gain combined with global nfb, to force a 10Khz sinewave at power with low distortion. Just isn't possible unless piles of feedback is used.. The conventional output stage distortion characteristic will end up as the famous "bathtub" curve, hence thd unquote figures are usually at 400Hz or 1Khz, as shown.
In my p-pp 250+250W amp which I am currently doing the documentation; with 13kg cores and lower primary impedances the interwinding area coupled with the L/C parasitics is simply too lossy to force the issue, hence I don't bother doing power tests above this 10Khz frequency as o/p stage efficiency drops and snubber losses rise. THD vs.frequency reveals this.
Siliconray, from the pictures you posted I see sharp frequency response roll-off starting from 10 KHz (correct me if I'm wrong). It basically means both units have very high leakage inductance. Additionally, one of them have dip & spike in range of 50 - 60 KHz, which related to resonant frequency caused by leakage inductance & stray capacitance.
If you try to use transformers with such design in PP amplifier for example, it will oscillate.
If my assumption of frequency response characteristics is right, design is very basic and primitive, and unless used for 50-year old grandma's black & white tube TV, must be trashed.
Hi linuksguru, I believe the roll-off start from 10KHz is acceptable at least for the left one. It drops less than 3dB up to 30kHz. Plese note it's not a linear coordinate. Logarithmic coordinate make it looks very sharp.
Actually it's quite good for a OPT. This roll-off can be reduced by NFB. The resonace at 60kHz is a real problem, it ruins the sound quality. You can see the left one doesn't has the resonat peak.
We tested the leakage inductance, only 4.5mH for the left one, but 110mH for the right, huge different isn't it? I agree with you the turns of coil it's a set of formulas, but the material and the method of wiring make huge difference.
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