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Thursday, December 19, 2013

Single Ended Output Transformer

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.

               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. 

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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