Introduction
After testing a range of 2:1 ratio output transformers for the new 35mm wide modules of the Mark 3 mixer, it became apparent that there is no off the shelf transformer with the required performance that fits in the module. The only transformer with a satisfactory performance is the Carnhill VTB 2291 which is far too big but will now be the standard by which all others are judged.
After some discussion on groupDIY, CJ came up with a design that appeared to meet the parameters of the specification, in particular the requirement not to saturate at 20Hz when outputting +22dBm into a 600 ohm load. As no off the shelf solution seemed to exist I looked for a local transformer manufacturer who was prepared to wind a custom transformer for me. The nearest I found is Electro Mag Ltd based just outside Kings Lynne in Norfolk.
I passed CJ’s design to them, and after some further discussion they made a prototype based on a modified bobbin and an EI245 core. The results of testing this transformer are shown below.
Inductances
The primary inductance was measured as 32H at 100Hz
The primary leakage inductance was measured as 5.95mH at 1KHz
The secondary inductance was measured as 8H at 100Hz
The secondary leakage inductance was measured as 1,35mH at 1KHz
DC Resistances
The primary dc resistance was measured as 178 ohms
The secondary dc resistance was measured as 90 ohms
+16dBm Distortion
Distortion was initially measured at +16dBm into a 600 ohm load from 1KHz down to 10Hz. The driving source was a standard Twin Line Amp (6922 based SRPP stage with overall negative feedback). Output impedance is approximately 150 ohms. The results are shown in the table below alongside the VTB2291 results for comparison.
22dBm Distortion
Distortion was then measured at +22dBm into a 600 ohm load from 1KHz down to 20Hz. The results are shown in the table below alongside the VTB2291 results for comparison.
20Hz Distortion versus Level
Lastly, the effect of output level on 20Hz distortion of the Electro Mag transformer into a 600 ohm load was also measured. The results are shown in the table below.
Frequency Response
Next, the frequency response from 20Hz to 40KHz at +4dBu into 600 ohms was measured using a Lindos MS10 test set. The results are shown in the table below.
Conclusions
Initial test results indicate that this transformer basically meets the specification. Its performance exceeds that of all of the other small transformers tested particularly at low frequencies. It also compares favourably with the Carnhill VTB2291 which is several times its size. In particular:
- The primary and secondary inductances seem to be satisfactory at 100Hz. The measured frequency response indicates they are adequate also at 20Hz
- The primary and secondary dc resistance are very similar to those measured by CJ. The secondary dc resistances are identical. The primary dc resistance is a little higher but CJ did not split the primary winding into two.
- At +16dBm into 600 ohms its distortion is equal to or better than the VTB2291 down to 50Hz. Below that frequency its distortion is never more than 4dB worse than the VTB2291. A very creditable performance for a transformer a quarter the size of the VTB2291.
- At +22dBm into 600 ohms its performance is similar to that at +16dBm. At 50Hz and above it is better than the VTB2291. Below 50Hz it is never more that 4dB worse than the VTB2291. You only have to reduce the level by 1dB for it to be as good as the VTB2291.
- Frequency response is within 0.5dB from 20Hz to 31.5KHz and just 1dB down at 40KHz
The only downside is this transformer is still not quite small enough to fit inside a 35mm Mark 3 module. Initially it was thought that with some minor modifications to the mechanics it could be made to fit but this turned out not to be practical. Nevertheless, this transformer has been used in other tube projects with considerable success.
 |
| Electro Mag Production EI245 transformers |
So the search continued. I spent some time looking at some of the first Neve 35mm modules and soon realised they had encountered the same problem back in the 1970s. The earliest Neve 35mm modules did not contain an output transformer; it was mounted externally. Only later did they use an internal one and it was quite large but rather flat.
Then by chance, work in a non-related area identified possible UI laminations and bobbins that could realise a low profile transformer only 25mm high and which would also provide a lot more winding space than the earlier EI transformer. It too was rather large and flat so it looked like I was moving in the right direction. I know very little about transformer design but as the saying goes, I know a man who does. Some UI transformer components were obtained by Electro Mag, who built the original EI transformer, and some were sent to my friend Volker who had kindly agreed to build some prototype transformers.
 |
| Electro Mag EI245 vs Volker's UI prototype |
Detailed tests of the first Volker prototype and a production prototype have now been carried out and the results are as follows:
Inductances
The primary inductance at 100Hz was measured at 24 Henries. The secondary inductance at 100Hz was measured at 6 Henries. These values are about 75% of the values of the CJ design.
DC resistances
The primary DC resistance was measured as 70 ohms. The secondary DC resistance was measured as 21 ohms. These are considerably lower than the values of the CJ design principally because the much larger winding space allowed a larger diameter wire to be used
Frequency Response
The frequency response was measured at +20dBu into a 600 ohm load. The frequency response was within 0.01 dB from 1KHz to 31.5KHz. It was 0.25dB down at 20Hz. and 0.75dB down at 40KHz. This is even flatter than the response of the CJ transformer.
Distortion Tests
Distortion of Volker’s prototype was measured at various frequencies at a level of +20dBu into a 600 ohm load. A Lindos MS20 was used as the reference signal source and REW was used to analyse the resulting output spectrum. The amplifier used is a standard Twin Line Amp design operated at 340V HT for reduced distortion. The amplifier distortion is predominantly 2nd harmonic and the transformer distortion is predominantly third harmonic. Both of these along with the total harmonic distortion were measured at each frequency. The results are shown in the table below:
The second harmonic distortion remains low, increasing from 0.025% at 1000Hz to 0.049% at 10Hz. This is entirely due to the amplifier. The increase at low frequencies is almost certainly due to reduced open loop gain. The third harmonic distortion rises steadily as frequency decreases and is almost entirely due to the transformer. It is a very creditable 0.21% at 20Hz and equals that of the Carnhill VTB2291.
All other harmonics were at least 20dB lower than the second and third harmonics.
To establish maximum usable levels, further tests were undertaken at 20Hz, increasing the level in 1dB increments from 21dBu to 26dBu into a 600 ohm load. The results are shown in the table below:
Even at these levels, the THD never exceeds 1% at 20Hz. In all cases the major distortion component is the third harmonic due to the transformer. This performance is better than the Carnhill VTB2291.
Production Prototype
In October 2019, a production prototype was received from Electro Mag, built to Volker’s design. The primary distortion tests were repeated for this transformer. The original test rig is no longer available so the tests were undertaken at +20dBu into 600 ohms using an HT voltage of 250V. The results are shown below:
The performance of the production prototype can be seen to be almost identical to that of Volker’s prototype despite the HT voltage being reduced to 250V. Due to changes in the test set up it was not possible to measure the distortion at +26dBu into 600 ohms but at +24dBu the third harmonic distortion was only 0.33% at 20Hz which is better than Volker’s prototype. Frequency response was also measured and found to be virtually identical to Volker’s prototype.
Conclusions
This transformer meets all the design requirements. It comfortably fits in the space available and always meets or exceeds the performance of the Carnhill VTB2291. The next step is to incorporate it into the 35mm channel amp modules of the Mark 3 tube mixer.
 |
| Electro Mag UI transformer alongside Volker's prototype |
Great blog. vary informative and useful. keep posting such blog.
ReplyDeletePCB Assembly from India | Electronics Manufacturing Services from India