So, as part of the drive to reduce the cost of the Mark 3 mixer I have looked at alternative transformers to replace the relatively expensive Sowter, Jensen and Cinemag types used currently. There are three types of transformer we need; 10K:10K line input; 1:10 mic input and 2K4:600 output. A UK transformer manufacturer, OEP, makes a good range of modestly priced transformers
10K:10K Line Input
There are three OEP 10K:10K products we could use:
Z21807C (£15 from Canford, 50% Nickel core))
A187A13C (£25 from RS, 80% Nickel core)
K30A06C (£40 from RS, 80% Nickel core)
All are considerably cheaper than the typical Sowter 10K:10K which is around £75. The price of each OEP alternative is directly related to its core size. Normally, level handling capability, especially at low frequencies, is directly related to core size but it does also depend on core material.
The distortion, at a range of frequencies from 1KHz down to 20Hz ,was measured for each transformer. The input signal level used was +18dBu supplied by a Lindos MS10 Audio Test Set. The transformer output was loaded with 10K in the form of a Scarlett 2i2 line input which was connected the REW software for making measurements. The results of the percentage distortion created by each transformer versus frequency are given in the table below:
The performance of the Z21807C is remarkable for such a small cored transformer. Its distortion is below 0.1% down to 100Hz and quite acceptable down to 30Hz. It is only below 30Hz that its +18dBu performance falters.
The performance of the larger core A187A13C by contrast is rather disappointing. Although distortion is very low down to 50Hz, below that frequency it rises rapidly and soon overtakes that of the smaller Z21807C.
The performance of the large cored K30A06C is exceptionally good with very low distortion at +18dBu right down to 20Hz. I conducted some additional test on this transformer down to 10Hz where the distortion finally rose to 0.53%
The question is, under what circumstances is it really necessary to be able to cope with +18dBu signals below 40Hz? The lowest note on a bass guitar or orchestra instrument is 41.2 Hz. Some synths can reach lower but how often will such signals reach +18dBu?
Clearly in cost sensitive applications the Z21807C should be considered. If best quality is required then the K30A06C should be used and it will still achieve a good cost reduction.
2K4:600
There is no obvious OEP product for this job in their published catalogue. However, they sent me a sample of their K30A11C which is a 2K4:600 type. I tested this at +16dBu into a 600 ohm load. I also tested the Sowter 1461 which is the current preferred output transformer, at the same time. The signal source was again the Lindos test set but to reach +16dBu at the secondary requires at least +22dBu at the primary which is beyond the capability of the Lindos. So the Lindos was fed into one channel of a Twin Line Amplifier which is quite capable of outputting +30dBu into a 2K4 load and it is also what will normally be driving these transformers. The output signal was again fed via a Scarlett 2i2 into REW. For reference, the very large Carnhill VTB2291 used in the EzTubeMixer project was also tested at the same level.
The results of the percentage distortion created by each transformer versus frequency are given in the table below.
At frequencies above 50Hz the Sowter has lower distortion than the OEP but both are very low. The Sowter has typically 6dB less distortion than the OEP. Below 50Hz the Sowter distortion rises more rapidly than the OEP and at 40Hz equals the OEP. Below 40Hz the Sowter distortion rises more rapidly than the OEP and is 15.7% at 20Hz compared with 4% for the OEP.
At mid band frequencies there is little to choose between the two but below 40Hz the OEP is definitely the better choice.
The VTB2291 results are very interesting. You can see the difference in using an all steel core. Mid band distortion is much higher than the Sowter or OEP even though the VTB2291 core is huge in comparison. But the benefit at low frequencies is clear with distortion at 20Hz. being 28dB below the OEP and nearly 40dB below the Sowter
For completeness the Carnhill VTB2291 transformer was tested down to 10Hz at +16dBm and also tested down to 20Hz at +22dBm. The results are shown below:
Clearly there is a trade off between performance and core type. Steel cores give higher mid band distortion but much better low frequency performance. On the other hand, cores containing nickel have much lower mid band distortion at the expense of much higher low frequency distortion. The Carnhill performance should be the standard by which alternatives are judged. With this in mind I tested two more transformers, the Carnhill VTB4096 (a high level input transformer similar to the Neve 31267) and the Cinemag 2820 which is a nickel/steel cored transformer but smaller than the VTB2291. Their measured percentage distortion at +16dBm into 600 ohms, with the VTB2291 figures included as a comparison are shown in the table below:
The VTB4096 is clearly not usable. The Cinemag transformer has a 50% Nickel core which probably explains why its mid band distortion performance is about 10dB better than the Carnhill VTB2291. However, from 40Hz downwards the differences disappear and the Carhill has lower distortion. However, the Cinemag CM2028 20Hz distortion at 0.39% is far superior to either the OEP K30A11C (4.15%) or the Sowter 1461 (15.7%). Unfortunately this transformer is still too large to fit in a 35mm wide Mark 3 module but it might be a useful option if external transformers can be used.
1:10 mic input
The A187A15C is one of OEP’s mic input transformer products. They offer several different turns ratios but the A187A15C is a 1:10 ratio type so it is comparable with the Sowter, Jensen and Cinemag types used at present. It was initially tested at an input level of -20dBu as this represents a typical maximum mic input level (with a 20dB pad engaged the input could handle 0dBu). However, it soon became clear that this transformer can handle much higher input levels so it was tested at -10dBu, -5dBu and 0dBu. The results are shown in the table below.
It is quite clear that at -20dBu the performance is exceptional with 20Hz distortion only 0.2% and even at 10Hz it is below 1%. At higher level the distortion increases but even at -5dBu the distortion at 20Hz is below 1%.
Conclusions
Considering that most OEP transformers are half the price of their Sowter equivalents they turn in a very creditable performance. In some instances, especially at low frequencies and high levels, their performance exceeds the Sowter equivalents. At mid frequencies the Sowters undoubtedly perform better but the distortion levels in all the transformers tested are very low at these frequencies.
Considering that a typical channel strip would contain three transformers, using OEP types could reduce the BOM cost by over £100, so they should at least be offered as an alternative.
Both the Sowter and OEP output transformers perform badly at low frequencies and high levels in comparison to an all steel core transformer like the Carnhill VTB2291 which means we still do not have a top quality output transformer that can be contained inside a 35mm wide module. It is perhaps worth considering sourcing a smaller all steel core transformer just for the output stage.
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