Difference between OPT - diyAudio
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Old 12th March 2012, 03:59 AM   #1
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Default 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.

Click the image to open in full size.

Testing equipment: HP3563A analyzer, FU-50 SE tube amplifier
Input signal: 1Vp-p
Output signal: about 3.6V RMS
Load: 8Ohm
Testing Freq: 10Hz - 100KHz

Frequency response (SPEC1 for right OPT and SPEC2 for left OPT)
1. Freq response between 8db - 12db
Click the image to open in full size.

2. Freq response between 4db - 12db
Click the image to open in full size.

3. Freq response between -4db - 12db
Click the image to open in full size.

4. Freq response between -12db - 12db
Click the image to open in full size.

Wave form for square wave input
Yellow for left, green for right
1. 1KHz
Click the image to open in full size.

2. 5KHz
Click the image to open in full size.

3. 10KHz
Click the image to open in full size.

4. Detail at rising/fall edge
Click the image to open in full size.
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Old 12th March 2012, 05:11 AM   #2
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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.

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Old 12th March 2012, 06:02 AM   #3
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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.
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Last edited by siliconray; 12th March 2012 at 06:04 AM.
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Old 12th March 2012, 11:37 AM   #4
DF96 is offline DF96  England
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Two issues: stray capacitance, magnetic coupling. Interleaved windings affect both.
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Old 12th March 2012, 02:22 PM   #5
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Location: Alps:Tube amp designs over 150W, SMPS guru.
Quote:
Originally Posted by siliconray View Post
Both transformers are made in the lab of our company. We just want to determin how those factors affect the performance.

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.

richy
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Last edited by richwalters; 12th March 2012 at 02:22 PM. Reason: punctuation
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Old 12th March 2012, 03:00 PM   #6
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Quote:
Originally Posted by Richard Ellis View Post
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.
Rick..........
Rick, its not a black art, its just set of formulas. You can take for example winding layout schematic from Dynaco or Quad and wind your own the same way (PM me if you want these drawings). The only things you will need to recalculate is the number of primary turns according to your output power, tubes Zout, desired frequency range and of course type and dimensions of core material.

Last edited by LinuksGuru; 12th March 2012 at 03:02 PM.
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Old 12th March 2012, 03:17 PM   #7
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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.
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Old 12th March 2012, 03:43 PM   #8
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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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Last edited by siliconray; 12th March 2012 at 03:48 PM.
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