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Sunday, December 22, 2013

300B Single Ended Output Transformer



300B Single Ended Output Transformer

Vacuum tube is a very wonderful device. It can tolerate a very wide range of load impedance, depending on the user’s preference. We found that for 300B, 3500 Ohm is what we would prefer (at least our ears as well as measured data). 

Our 300B single ended output transformers are measured to be 3490 Ohm. Such accuracy is not met by just purely calculation or follows the book building methodology. It requires extensive practical and theoretical know-how to achieve such a feast. It’s not just having low stray inter-winding capacitance or high inductance so simple.

Below is one of the pair of 300B single ended output transformer we just completed and delivered. Please pardon the dust on the transformer, which is from the packing foams we used for shipment. 
300B Single Ended Output Transformer

300B Output Transformer

Single Ended Output Transformer
Output transformer, in vacuum tube amplifier, is used to convert high impedance to low impedance for driving loudspeakers. That’s why we always talk about impedance and phase difference. Matching of impedance at all frequencies will ensure energy is faithfully and totally transmitted with low distortion. Having low phase difference will ensure the system is stable or else oscillation will happen and could even burn the output transformer. 

Too bad it is hard to differentiate a good and bad output transformer just by the looks. Apple to apple comparison is hard too since we must use identical amplifiers to perform the evaluation. Sometimes even the amplifier circuit design or non-optimized biasing could ruin the results. Therefore, one really needs to pay extra attention when it comes to output transformer selection. We can’t stress enough on the importance of this. For me, as an output transformer user, had previously gone through a lot of hardship purchasing output transformers and hopes that our fellow readers do not go through the same route again and invested on the wrong output transformer, especially the pricey 300B single ended output transformer.

Winding output transformers are not the same as winding power transformers. You can still runaway (temporary) by getting the right output voltage and current. For output transformer, you can’t! Bad sounding output transformer will not get better by applying feedback, circuit compensation or whatever tweaks available.

Audio output transformer requires properly aligned windings, layer by layer, no cross winding, ordered primary versus secondary arrangement for proper impedance characteristics across audio frequency spectrum. Due to bandwidth, and audio requirement, the materials, termination, and insulation need to be analyzed and properly designed to achieve required audio characteristics.
A good output transformer will require a bandwidth of 20Hz to 20kHz @ +/- 3dB. For excellent quality, that needs to be extended to +/- 1dB or even +/- 0.5dB. If it can go up to 100kHz, even better in the era of super tweeter that goes as high as 100-200kHz.

To achieve such design, we need to have sufficient inductance and low leakage capacitance. Not just that, this behavior needs to be designed according to the frequency since impedance is a function of frequency. We can’t just design for overkill as everything is about balance in audio output transformer design. All these require proper measurement equipment. A simple LCR meter will not be able to measure inductance at 20Hz and characterize the whole audio frequency spectrum. 

We need high end equipment to do so and that’s not something that every transformer winder has.
On top of that, air gap for single ended output transformer is something that is to be properly tuned. Insufficient or over wide air gap will significantly affect the output transformer core saturation point and therefore affect the output. 

Let’s use a simpler example, the inductor instead of output transformer.  Inductor only needs a single wire to route until the end. Having the right turns will generate the required inductance. Simple right? Well, yes and no. We need to consider the core material, core alignment, the winding density, the air-gap, and etc. All these will affect the inductance outcome. Needless to say, wrong inductance value and core saturation point will greatly affect the usefulness and functionality of the inductor in the circuit. This will add complexity to troubleshooting the amplifier design if it fails.

For a plate load choke, it will be more completed than just a power supply choke as the inductance need to match the audio frequency it is intended to work in to ensure frequency response of the amplifier does not vary out of intended values.  All these will be achieved by the right windings, right wire size, right core material, right air-gap and etc. 

A plate load choke is already so complicated, what about output transformer? It will be more complicated.

Output transformer involves frequency response. When an output transformer is badly made, there will be anomalies at the high and low, and anywhere in the audible frequency band, even oscillations. So, it is crucial to do a sweep along the intended bandwidth to see if there are issues along the way. If there are, apply fixes or design changes to resolve the problem. Well, that’s if one knows how to repair it. 


If they depend on luck to produce a good design, well, wish those lots of luck! We don’t. We go for consistency to design our 300B Single Ended Output Transformer so that every sample falls within the allowable specifications.   


J&K Audio Design
22/12/2013

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Iso-trans: link 1, link 2
* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.  
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Product Lists

Product Lists

Level 0: link 1

Level 1: link 1, link 2, link 3, link 4

Level 2: link 1, link 2, link 3, link 4, link 5, link 6, link 7, Link 8, link 9

Level 3: depends on type, size and complexity. Email us for details.

Iso-trans: link 1, link 2

Power-trans: link 1

* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.

* Our product lines are always improving and increasing. If you do not see what you want, contact us!

* Please email for volume discounts, distributor and OEM pricing.

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