Telefunken OPUS 55 output transformer

Hello,

I am measuring DC resistance from center tap to one side as ~500 ohms, center tap to other side as ~300 ohms, so it is not centered.

I remember I got the similar unbalanced DC resistance measurements for Opus 7 output transformer as well.

Assuming EC92 phase inverter has -1 gain, I would expect center tap would be really centered on the primary of o/p transformer, otherwise output would be distorted (due to different amplitudes for each half cycles).

I am trying to understand the reason of this, it is unlikely due to an issue of o/p transformer as I see the similar measurements on two different radio models.

Thanks in advance,

Cuneyt

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The output impedance for EL84 nominally is 2*4kΩ. 

Terefore you have for one EL84 Z₁=sqrt{(4kΩ)² + (500Ω)²} = 4.031kΩ while for the other EL84 you have Z₂ = 4.011kΩ. The difference is 19.9Ω. This gives a relative difference of about 0.49%.

So, what is the problem?

Hopefully, your EL84's are better matched.

Regards, DR

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Thanks for your answer Mr. Rudolph.

Please let me summarize my understanding about this issue. Output impedance of EL84s, which is typically 4K as you mentioned, should match to load impedance which is primary of o/p transformer to maximize the power transfer to the load. Therefore, ideally center tap to two sides of primary should have the same impedance for a given frequency.

I was not able to measure the impendace of two sides to center tap of primary but i think DC resistance difference will be translated to impedance difference. Assuming wire diameter is same for both parts (center tap to two sides) of primary, one side (with 500 ohms) should have 66% more windings compared to other side. Therefore it will have 66% more inductance and for a given frequency 66% more impedance (neglecting DC resistance impact on impedance).

If we assume we have a perfect impedance matching (4K - 4K) on the side with 300 ohms, there will be 4K - 6.64K impedance mismatch on the other side of primary.

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This push-pull output transformer has a simple coil like a power transformer. Beginning inside, the first coil has a smaller diameter than the second one, which therefore needs a longer piece of wire for the same number of turns. So the outer coil has a higher resistance, but the same impedance.

If the transformer was a HiFi type, it would be wound like this.

But obviously, the output transformer in your set is not a HiFi type. In 1955 HiFi was not yet a real topic for wireless sets.

Regards, DR

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Thanks for your answer and sharing the information about HiFi transformer. This is a valuable information.

Outer coil has a bigger loop area so I think it requires less number of turns compared to inner coil to have the same inductance. Detailed geometry of the transformer needs be known to make further calculations but i do not have this information. I think the best way to verify output transformer is to apply a sine wave to the input of o/p stage then to observe the o/p waveform positive and negative half cycles to catch any asymmetry. I am going to try this.

The radio I have is US version of Opus 55 and it is written "HiFi" on the dial glass.

I do not see any particular issue with the audio performance of this radio however it is always difficult for me to judge performance of these radios as I do not know how they sound 60 years ago when they were manufactured. 

As you mentioned about HiFi o/p stages, please let me get your opinion about this. Was there a HiFi industry standard followed by manufaturers? I was thinking it is related to number of speakers, available tone / filter controls etc. which makes the audio close to real sound. But I recently found a portable Telefunken tube radio with FM band and single speaker, it is written "HiFi" on it. It is very similar design as Telefunken Elite radio except for o/p stage. It uses dual DL94, but not a classical push pull output design. On the other hand I have a Telefunken Gavotte 7 radio with single ended audio output but HiFi. So it is not clear to me what makes a radio "HiFi".

Thanks in advance,

Cuneyt

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A simple transformer has an inner coil with a smaller loop than the outer coil has. That is correct. However, the magnetic flux mainly is inside the iron core, and the core is identical for both coils. Therefore both coils need the same number of turns for a certain inductivity.

But the bigger loop of the outer coil means that this coil has a geater stray inductance than the inner coil has. This indeed gives a certain degree of asymmetry. However, the stray inductance indeed is small compared to the main inductivity. So the asymmetry mainly affects the high pitches. And in 1954/55 it was not yet usual to boost treble. So most sound did not contain strong high pitches, and nobody recognized a problem resulting from the asymmetry of the output transformer.

"HiFi" was good for advertizing. In 1935 (!) Telefunken already advertized its sets as "HiFi". These were AM sets, and the audio bandwidth they had was about 4.5 kHz. HiFi meant "better than the usual reception". And in this way it may be recognized in the early 50ies.

Later on in the 60ies, HiFi was standardisized e.g. in DIN 45500. For magnetic recorders a linear frequency response (with a tolerance of 5 dB) from 250 Hz to 6.3 kHz was stated.  For studios a linear frequency response (tolerance 3 dB) from 80 Hz to 8 kHz was claimed, and this was what broadcaters then could offer.

Nevertheless the tube sets from these days sound pretty good, but this a subjective impression. 

And keep in mind that all audio stored at a broadcaster's station now is digital MPEG with about 384 kbit/s. And it is digitally transfered to the transmitter where it is D/A converted to an analogue audio signal which then modulates your FM transmitter.  

Regards, DR

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