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Last Modified: 30 September 2012
Push-Pull Loudspeaker Output And Distributed Speakers' Line Matching Transformers
Choice of Size
For this purpose the best core lamination shape is that known as the "Wastefree". The dimensions are set out in Figure 07, referred to the width of the centre limb. The small inset diagram shows the way the laminations are stamped from the sheet so there is no waste portion. It will be seen that this method of cutting can only be employed to make laminations of the E and I type. However, from the point of view of efficiency, frequency bandwidth, etc., it is obvious that a core of the same shape constructed from laminations of the T and U type will be equally good.
Figure 07
The following table gives data for a series of easily obtainable sizes, in different stacks. It shows the turns for a 1,000 winding which give a transformer of maximum efficiency at a frequency of 400 – 1,000 Hz. The section headed "Maximum power, Watts", shows the maximum power that can be handled by the transformer under this condition at two frequencies, 50 Hz and 200 Hz without introducing serious distortion.
| Impedance/Turns Relationship, Maximum Power and LF Cut-off for Maximum Efficiency at Mid-band Condition | ||||||
| Maximum power
Watts |
||||||
| Dimension A, Figure 07 |
Core stack | Turns for
1000 n |
50 Hz | 200 Hz | Mid-band
losses |
LF cut-off
Hz |
| 0.75 | 0.75
1.25 1.50 |
750
660 500 |
(1.75)
(3.50) (5.25) |
28
56 84 |
11.5%
10.0% 9.3% |
90
80 70 |
| 1.00 | 1.00
1.50 2.00 |
770
670 500 |
(7)
(12) (17) |
110
190 270 |
8.5%
7.4% 6.8% |
65
60 55 |
| 1.25 | 1.25
1.75 2.50 |
790
700 620 |
(18)
28 45 |
280
450 700 |
6.0%
5.4% 4.8% |
53
46 43 |
| 1.50 | 1.50
2.25 3.00 |
800
720 640 |
40
70 100 |
650
1100 1600 |
4.8%
4.3% 4.0% |
39
35 32 |
The figures in the 50 Hz column in brackets are so shown because they cannot be applied at that frequency under maximum mid-band efficiency conditions, because they are below cut-off, and hence the inductive load on the output valve would introduce distortion by mismatching. However, if appropriate factors from the table below are used to reduce cut~off to 50 Hz or below, the corresponding factors from the same table may be used to obtain the maximum output at 50 Hz in conjunction with the figures in brackets.
| Factors for other impedance turns relationships and for change from standard transformer iron to Radiometal | ||||
| Turns
X |
Maximum
power X |
Mid-band
losses X |
LF cut-off
÷ | |
| Factors for turns
referred to in Table 1 |
1.25
1.50 1.75 2.00 2.50 3.00 3.50 4.00 |
1.50
2.25 3.00 4.00 6.25 9.00 12.00 16.00 |
1.10
1.35 1.70 2.20 3.20 4.60 6.10 8.00 |
1.50
2.25 3.00 4.00 6.25 9.00 12.00 16.00 |
| Factor for change to Radiometal |
1.3 | X 2.3 | 1,7 | equal |
The above shows how increased power and a lower-cut off frequency may be obtained when the impedance/turns relationship is increased above the figure given for any size in the first table, together with the increases in losses, from which may be deduced the efficiency obtainable.
The factors at the bottom of the latter table show how the figures can be improved by the use of Radiometal laminations instead of standard transformer iron.
For designs of both Class A Push-Pull Output and Loudspeaker matching transformers, the total winding space occupied by the primary winding should be approximately equal to that occupied by the secondary.
For any type of push-pull output stage, the most efficient disposition of winding space is when each half of the primary occupies about 30% of the space, and the secondary occupies 40%. Under this condition the figures given by the above tables have to be modified slightly. For maximum efficiency at mid-band, the turns for 1,000 ohms should be divided by 1.1, the maximum power in watts figure reduced by 1.2, and the mid-band losses increased by 1.2. The LF cut-off frequency will also be multiplied by 1.2.
200 Hz Cut-off Matching
With push-pull type outputs the [original] author does not recommend the incorporation of 200 Hz cut-off in the output transformer. A preferable method is to incorporate the bass cut between the output transformer and the matching transformer by means of a series condenser, which should be chosen so that its reactance at the cut-off frequency is equal to the load impedance referred to that point.
Methods of Winding and Connection
For the smaller size push-pull output transformers, the best method of winding to preserve a good balance at the higher frequencies is to wind one-half of the primary before, and the other half after, the secondary. The two ends of these two windings which are adjacent to the secondary are then connected together to form the centre tap. This method is shown diagramatically at Figure 08a.
Figure 08
For larger sizes, and especially those intended for quality push-pull type output circuits, closer coupling of the windings may be considered necessary (in fact this author would specify it as an absolute minimum to compare with modern day audio standards – M.H.). The method of winding and connection shown at Figure 08b has been proved to give very accurate balance indeed at the high frequencies.
Some authorities recommend complicated arrangements using a divided bobbin, so as to maintain geometrical symmetry. The arrangement here shown maintains just as good electrical symmetry, with a far simpler winding arrangement, and gives a wider frequency response band for given size and complexity of design. The secondaries are show as two windings connected in parallel. This arrangement preserves the best balance, especially if the secondary has a fairly high impedance.
If the secondary impedance is quite low compared to the primary, then a series arrangement will serve equally well, when the junction can be used as a centre tap, and earthed.
For loudspeaker matching transformers, a simple arrangement with the primary and secondary (each in only one section) is adequate. It is not important in this case which winding is nearest to the core, so the order of winding may be determined by convenience from the point of view of the particular wire gauges to be used.
Example
A push-pull amplifier giving an output of 10 watts, with a anode to anode load of 4,000 ohms, requires an output transformer with an efficiency of about 90% to match it to a 10 ohm speaker for music and speech.
A 1" stack of 1" waste-free laminations operating at maximum mid-band efficiency has 8.5% losses and a cut-off of 65 Hz If the turns are multiplied by 1.25, then the mid-band losses become 1.1 x 8.5 = 9.5% (or an efficiency of 90.5%), and the cut-off becomes 65 1.5 = 43 Hz. Thus the maximum output at 50 Hz can now be 1.5 x 7 = 10.5 watts. The winding for 4,000 ohms will require a total of:
√(4000/1000) x 960 or 1920 turns.
The turns for a 1,000 ohm winding will need to be 1.25 x 770 = 960. The turns, and a winding for 10 ohms will require:
√(10/1000) x 960 = 96 turns.
Thus the winding will be:
1. Half Primary, 960 turns.
2. Secondary, 96 turns.
3. Half Primary, 960 turns.
Example 2
A large amplifier, having an output of 40 watts, has an anode to anode load figure of 8,000 ohms, and requires to be matched to 250 ohms for distribution to multiple speakers via matching transformers ("100 volts line"). Give appropriate designs in standard transformer iron and in Radiometal, for use on music and speech, efficiency to be 95%.
Using standard transformer iron: Either a 21/2" stack of 11/4" waste-free, or a 11/2" stack of 11/2" waste-free will satisfy the required conditions without modification. Each gives an efficiency of 95.2%.
Using Radiometal (nowadays, "electrical steel"): A 1" stack of 1" waste-free gives a midband loss of 8.5 / 1.7 = 5%. Under this condition the cut-off frequency is 65 Hz, and maximum output without distortion would only be 7 X 2.3 = 16 watts. A 11/2" stack of 1" waste-free gives, under maximum mid-band efficiency, a loss of 7.4 1.7 = 4.35%, a cut-off of 60 Hz, and a maximum output of 12 x 2.3 = 27.6 watts.
Increasing the turns by 1.25, the maximum power is increased to 1.5 x 27.6 = 41 watts, the mid-band losses become 4.35 x 1.1 = 4.8%, and the LF cut-off will be 60 @ 1.5 = 40 Hz. Thus it is seen that a 11/2" stack of 1" waste-free Radiometal will give almost identical performance with that of either a 21/2" stack of 11/4", or a 11/2" stack of 11/2" in standard transformer iron. This results in a reduction of outside dimensions from 33/4" X 41/2" to 21/2" x 3". To complete the design in Radiometal: The turns for a 1,000 ohm winding will be 670 / 1.3 x 1.25 = 640 approx. Thus the primary will require a total of:
640 x √(8000/1000) = 1,800 turns
and the secondary turns will be:
640 x √(250/1000) = 320.
Thus, following the winding arrangement of Figure 08b, the required sections are: 1. Quarter Primary, 450 turns; 2. Secondary, 32 turns; 3, Half Primary, 900 turns; 4. Secondary, 320 turns; 5 Quarter Primary, 450 turns.
Example 3
A cabinet type speaker with a speech coil impedance of 15 ohms is required to take one-eighth of the power from Example 2. Efficiency to be not less than 80%.
One-eighth of the power is 40/8 = 5 watts. The primary impedance must be 8 x 250 = 2,000 ohms. Using a 3/4" stack of 3/4" waste-free, with 1.75 times the turns from the first table above, the maximum power is 3 x 1.75 = 5.25, the mid-band losses are 1.7 x 11.25 = 19.5%, and a LF cut-off of 90 / 3 = 30 Hz. This satisfies the conditions. Then the turns required are:-
1. Primary 750 x 1.75 x √(2000/1000) = 850 turns.
2. Secondary 750 x 1.75 x √(15/1000) = 160 turns.
