Single Ended Output Transformer
In SET, often users are complaining about the lack of bass.
The single ended output transformer frequency response is part of the biggest
contributor to that due to the way the single ended output transformer design requires
it to be – air gap.
The air gap is to address the DC current present on the
transformer in order not to saturate the core. That also makes the primary
inductance of the transformer lower than those without an air gap. The primary
inductance will determine how good the bass response is for a particular
amplifier.
f = Z/(2*Pi*L) @ 3dB
·
f = freq
·
Z = impedance
·
L = primary inductance
Take for example we want to find out the single ended output
transformer frequency response for 2A3 (low frequency f -3dB). Let’s take one
famous output transformer LL1623 as example.
For LL1623@90mA, the primary inductance is 30H.
For LL1623@90mA, the primary inductance is 30H.
F -3dB = 3000 / (2 * 3.142 * 30) = 15Hz
For LL9202 @50mA used for 801A single ended amplifier, the
primary inductance is 100H.
F -3dB = 11000 / 2 * 3.142 * 100) = 17Hz
For -1dB, the Z needs a multiplier of 2, and for -0.5dB, Z
needs multiplier of 2.76.
f = 2Z/(2*Pi*L) @ 1dB
f = 4Z/(2*Pi*L) @ 0.5dB
Impressive! So now you have one more trick up your sleeve
what to do when you purchase single ended output transformers. Normally, the
manufacturers will state the primary inductance. With that, you’d be able to
calculate the bass response. If you’ve the equipment, measure it yourself to
get the REAL data instead of paper data.
If they don’t even publish that or can’t even provide that,
then don’t even bother to measure it and don’t even bother to buy it. LOL!
Since we talk about high impedance output transformer here,
there’s one thing that I’d like to bring up is the design of such transformers.
It is not easy to design high impedance output transformer due to the winding
difficulties to have ~11k Ohm on the primary. The number of turns, the winding
stray capacitance, the inductance needed, and the impedance at high frequency
versus low frequency, and etc are all working against each other. One needs to
balance the low and high frequency response and therefore there will always be
compromise.
Just how hard is it to wind high impedance output
transformer? Take 10k Ohm:8 Ohm as example. The turns ration is about 36:1.
When there is 1 Ohm change on the secondary, there will be 36 Ohm change in the
primary. Imagine that this is a high power output transformer (211/845 usually
uses this), the secondary wires will be thick to accommodate the current and
the primary will be thin to have such high impedance. So, it is very hard to
control the impedance changes over such a range.
Take for example, at 100 Hz, the impedance will be 2 x 3.142
x 100 x L = 10,000. L will be 15H.
When it goes to 10,000 Hz, the impedance kept constant, L =
1H.
When it goes to 20Hz, the impedance kept constant, L = 79H.
How easy is it to wind such a transformer with such a big
inductance change?
The hard part about tuning audio output transformer
frequency response is the low frequency. You need to have enough inductance to
have good low frequency response, but that will require more wires, more
windings and that causes high frequency response to drop. We have not taken
phase difference into consideration yet, where it will impact the resultant
sound.
We need to ensure the phase difference is low and consistent across audio frequency spectrum. We try to have the low frequency phase difference at <20 deg @ 5Hz and <15 deg @ 20Hz. That will give you very solid extended bass. For phase difference above that until 50kHz or more, it should be flat and very low. When both are met, you will get an output transformer frequency response that will satisfy your taste buds.
We need to ensure the phase difference is low and consistent across audio frequency spectrum. We try to have the low frequency phase difference at <20 deg @ 5Hz and <15 deg @ 20Hz. That will give you very solid extended bass. For phase difference above that until 50kHz or more, it should be flat and very low. When both are met, you will get an output transformer frequency response that will satisfy your taste buds.
If you have a high impedance output transformer, give it a
measure and see how big the impedance changes over the audible frequency
spectrum. I will give it a try with the one I have, from a famous brand, but I
cannot publish the data. So, you can go figure it yourself!
J&K Audio Design
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