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.
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.
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| 300B Single Ended Output Transformer |
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| 300B Output Transformer |
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| 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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The sky is limit of creativity.
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