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Measurements and comparison of isolation transformers and DI boxes

dominikz

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Almost a year ago I posted this short IMD comparison of various transformers in a iso box review by @amirm, and promised that I'll try to follow up with a more detailed post. This will be it. :)

A few different types of devices will be measured and compared, here's a list of units under test:
  • Active DI boxes
    • Orchid Electronics Classic DI Box
      Orchid Classic DI.jpg
    • Behringer ADI2 (Note: this is actually an acoustic guitar preamp + DI)
      Behringer ADI21.jpg
    • Behringer DI100
      Behringer DI100.jpg
    • ART Tube MP (Note: this is actually a tube microphone preamp + tube DI)
      ART Tube MP.jpg
  • Passive DI boxes (basically a transformer in a box)
    • Millenium DI-E
      Millenium DI-E.jpg
    • Modified Millenium DI-E where the stock transformer was replaced with a Jensen JT-DB-E, and circuit modified accordingly (same look as above)
  • Passive 'Reamp' / reverse DI boxes (basically a transformer in a box)
    • DIY box with a Jensen JT-11P-1 transformer
      DIY Jensen JT-11P-1.jpg
    • DIY box with an Edcor WSM10K/10K transformer
      DIY Edcor WSM10K_10K.jpg
    • DIY box with a Vigortronix VTX-101-1604 transformer
      DIY Vigortronix VTX-101-1604.jpg
  • Isolation transformers
    • pro snake BJJ 301-1 isolation transformer cable
      pro snake BJJ 301-1.jpg
    • Palmer PLI-01
      Palmer PLI-01.jpg
I'll try not to post too many different diagrams as I don't plan to make a full review of each box - more like a comparison of what I feel are the main attributes of each.

Here's a list of tools used to perform the measurements:
  • Topping E50 DAC as signal generator
  • E1DA Cosmos ADC (B grade)
  • RME Babyface soundcard, first generation - measured here
  • UNI-T UT60E true RMS digital multimeter
  • REW software
Active DI boxes
Let's start with a comparison of frequency response:
Active DI boxes - frequency response.png


As we can see, the Orchid Classic DI has the flattest response and the widest bandwidth between the tested units.
The Behringer ADI21, when footswitch is engaged performs FR shaping (as it is intended for use with acoustic guitars).

Next, let's compare the distortion profile vs frequency of all tested units (Note: max allowed input level is not the same for all units so I had to vary the level between the tests):
Orchid Classic DI - THD vs frequency.png

Behringer DI100 - THD vs frequency.png

ART Tube MP - THD vs frequency.png

Behringer ADI21 - THD vs frequency.png


As probably expected, we see that the ART Tube MP exhibits the most distortion (dominated by the 2nd harmonic), while the rest of the boxes have relatively low distortion levels. However all of them perform worse than the HiZ inputs of many studio interfaces, e.g. here's how my RME Babyface HiZ input measures:
RME Babyface HiZ input - THD vs frequency.png


I measured also maximum input at 1% THD for all units at 40Hz and 1kHz - here are the results:
DeviceVoltage gain1% THD @ 40Hz1% THD @ 1kHz
Orchid Classic DI-10,9 dB ('normal/0dB' input)15,5 dBu15,5 dBu
Behringer DI100-0,4 dB (pads inactive)3,3 dBu3,5 dBu
ART Tube MP0dB (but depends on gain and level settings)2,5 dBu3,5 dBu
Behringer ADI215,0 dB (but depends on level settings)6,7 dBu6,7 dBu

As we can see, there not a lot of frequency dependence of max input level with any of the units. Note that some of the units could take higher input levels if you engage the pad switches / change input trim levels.

Active DI boxes
Again let's start with a comparison of frequency responses:
Passive DI boxes - frequency response.png

Both look pretty good.

Let's see the distortion profiles vs frequency at the same (8,6dBu) input level:
Millenium DI-E stock - THD vs frequency.png

Millenium DI-E modified with Jensen JT-DB-E - THD vs frequency.png

What we can see above is that the two transformers have slightly different voltage gain (Jensen JT-DB-E is around -21,7 dB, while the stock transformer is around -16,2 dB), and we can also see that the Jensen transformer performs much better with regard to distortion in general - but especially at very low frequencies.

I tried also to measure the maximum input at 40Hz and 1kHz but I couldn't reach 1% THD at the levels I could generate easily with devices I had on hand (around 15dBu max). This means that both of these devices can take a lot of input without saturating severely.

Passive 'Reamp' / reverse DI boxes
Comparison of frequency responses:
Passive reamp boxes - frequency response.png

All devices are very good in the audible band, but it is interesting to note how the Vigortronix VTX-101-1604 low frequency roll-off increases in frequency as level goes up. This is an indication of saturation, which will also be visible in other measurements.

Let's see the distortion profiles vs frequency now:
DIY Jensen JT-11P-1 reamp - THD vs frequency.png

DIY Edcor WSM10K_10K reamp - THD vs frequency.png

Vigortronix VTX-101-1604 reamp - THD vs frequency.png

We can see how both the Jensen and Edcor transformers significantly outperform the Vigotronix one at low frequencies - even though they are driven by much higher input levels.

Let's compare how the Jensen and Vigortronix behave as we increase the input level:

DIY Jensen JT-11P-1 reamp - THD vs frequency vs level.png

DIY Vigortronix VTX-101-1604 reamp - THD vs frequency vs level.png

As we can see, while the Jensen transformer behaves pretty consistently with increasing input level, the Vigortronix transformer saturates at low frequencies - and the frequency at which the 1% THD is reached increases together with input level.

Again, with most of these boxes I could not reliably measure the maximum input level at 40Hz and 1kHz because I could not easily generate enough clean signal level with the devices I had on hand (around 15dBu).
The exemption is the Vigortronix transformer which reaches 1% THD at 40Hz with only 0,8dBu input - at 1kHz again I could not reach the 1% THD with 15dBu input.

The voltage gain of these reamp boxes is:
DeviceVoltage gain
Jensen JT-11P-1-3,5 dB
Edcor WSM10K/10K-4,7 dB
Vigortronix VTX-101-16040,0 dB
Please note however that the original voltage gain of the Jensen JT-11P-1 transformer is actually around -2,4dB; I added a resistor divider at the output of this box to reduce the level slightly as I needed this for a specific use-case.

Next let's look at another very important performance aspect of 'reamp' boxes - ground loop and noise rejection performance.
For this test I generated a ground loop in my system by connecting the Phones output of a Yamaha THR10-II guitar amplifier to the RME Babyface HiZ input, and then put the reamp box in between to test how much of the ground loop noise is removed. Each graph compares the output vs the baseline noise spectrum of the soundcard (without input - ideal case), and the ground-loop noise spectrum.
DIY Jensen JT-11P-1 reamp - ground loop isolation and noise resistance.png

DIY Vigortronix VTX-101-1604 reamp - ground loop isolation and noise resistance.png

DIY Edcor WSM10K_10K reamp - ground loop isolation and noise resistance.png

Above we can see that both the Jensen and Vigotronix transformers reject the ground loop noise completely, but not so with the Edcor transformer. I assume this might be due to insufficient shielding of the transformer component itself.
This difference in performance can unfortunately be audible under certain conditions - in my experience using the Edcor transformer to reamp sounds into a high-gain amplifier input results in more induced audible noise vs when using either the Jensen or Vigortronix units.

Isolation transformers
Frequency responses at +4dBu input:
Isolation transformers - frequency response.png

The Palmer box has a very flat and extended frequency response, while the 'pro snake' isolation cable shows evidence of low-frequency saturation.

Distortion profiles vs frequency:
pro snake BJJ 301-1 - THD vs frequency.png

Palmer PLI-01 - THD vs frequency.png

As we can see, indeed there is a significant rise in LF distortion in the 'pro snake' unit.

Let's see how both units behave at a few input levels:
pro snake BJJ 301-1 - THD vs frequency vs level.png

Palmer PLI-01 - THD vs frequency vs level.png

We see that the Palmer unit is pretty consistent, while the 'pro snake' shows rising LF distortion levels with increasing input (similar as with the Vigortronix transformer in the previous section).

Both units have around 0dB voltage gain (i.e. unity).

As in the previous section, let me show now ground loop and noise rejection performance of these two units:
pro snake BJJ 301-1 - ground loop isolation and noise resistance.png

Palmer PLI-01 - ground loop isolation and noise resistance.png

Both units perform equally well here.

Conclusion
I've been looking into the performance of audio transformers and DI boxes for many years now, since I use them for various sound production and live performance use-cases. Here's a few takeaways based on my experience so far:
  • All transformers I tested show rising low-frequency distortion. In extreme cases of input overload/saturation we also see low-frequency FR deviations.
  • I mainly grade audio transformers based on a) maximum input, b) LF distortion and c) noise rejection performance. Usually only very expensive ones do all three really well - e.g. Jensen units are in my experience really superb, but I also find performance of the reasonably priced Palmer unit really impressive.
  • Cheaper audio transformers can still perform pretty well, but you sometimes need to be careful with input levels to keep low-frequency distortion under audible levels (e.g. see the Vigotronix unit and the 'pro snake' cable). Vigortronix transformers seem to be a solid budget choice.
  • Unshielded transformers (e.g. Edcor) may not always provide sufficient noise rejection for all use-cases.
  • Active units usually have more consistent distortion performance, but may add noise (and obviously require a power source :))
[EDIT 2022-04-25] Minor formatting corrections.

[EDIT 2022-05-09] @nagster added a comprehensive set of measurements of Countryman type 85 active DI, Radial J48 active DI and Tritech DB-LL1 passive DI in posts #24, #25, #26 and #27. Thanks!
 
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Ricardus

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I haven't shopped for DIs in a long time since I built a bunch of the Bo Hansen DI's two years ago, but I have to say I am not familiar with the Palmer at all. A quick web search tells me they seem to be pretty popular right now.

In terms of what I see when I'm working in studios, I see tons of Countryman (the studio standard for decades) Radial, Whirlwind (I suspect it's a geographic thing) and now I see some of the Rupert Neve Designs DI's showing up, mostly probably because of the name.
 

Vuki

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Edcor transformers have unbelievable price/performance ratio. Pair of tested Edcores is 5x cheaper than Jensen transformers. I used some of Edcore transformers at input in my tube amps and they always performed great.
 

Ricardus

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Edcor transformers have unbelievable price/performance ratio. Pair of tested Edcores is 5x cheaper than Jensen transformers. I used some of Edcore transformers at input in my tube amps and they always performed great.
OEP are similar with their price/performance ratio.
 

milosz

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I hate ground loops. I have a couple of Jensen transformer-based boxes on hand just in case no method of actually 'curing' the ground loop can be found.

I went crazy when I first set up my main system, which is essentially 2-channel but has some 5.1 capability also- I could NOT get rid of a nasty ground loop issue. Then I traced it down to the Comcast cable box chassis ground- the problem was caused by the shield ground on the incoming RG6 from Comcast. I bought a "cable TV isolation transformer" which isolated everything from the shield on that coax.

 
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dominikz

dominikz

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Wow, impressive analysis. Promoting to home page.
Thanks - much appreciated! :)
In terms of what I see when I'm working in studios, I see tons of Countryman (the studio standard for decades) Radial, Whirlwind (I suspect it's a geographic thing) and now I see some of the Rupert Neve Designs DI's showing up, mostly probably because of the name.
Just to add that I believe a lot of the higher-priced Radial units use Jensen transformers - no wonder Radial are well regarded! Countryman and Whirlwind I often read about online, but never used either.
Edcor transformers have unbelievable price/performance ratio. Pair of tested Edcores is 5x cheaper than Jensen transformers. I used some of Edcore transformers at input in my tube amps and they always performed great.
Indeed, I agree the price/performance of Edcor is excellent - though I have to say I was a bit disappointed by the noise rejection performance, at least when used in a 'reamp' circuit into a high-gain guitar amplifier input. Perhaps some other models they offer might be better suited for this purpose.
 
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dominikz

dominikz

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Nice! I'm curious how Radial DI units perform
Thanks! :)
Note that the Radial JDI is based around the Jensen JT-DB-EPC transformer, which is the PCB mount version of Jensen JT-DB-E I tested above. Given that passive DIs are practically just transformers in a box, performance should be the same.
 

Rja4000

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I use the Radial JDI Duplex for my mic preamp measurements.

Here are some measurements.
 
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dominikz

dominikz

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I use the Radial JDI Duplex for my mic preamp measurements.

Here are some measurements.
Thanks!
Is this with the -15dB input pad switch engaged?
 

Schollaudio

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Edcor transformers have unbelievable price/performance ratio. Pair of tested Edcores is 5x cheaper than Jensen transformers. I used some of Edcore transformers at input in my tube amps and they always performed great.
I've experimented with the 10K:10K a bunch of years ago. They do good phase splitting too. They start to saturate at about 10Vp-p at 20hz. They still need a low Z driver. The larger core models can be used in the plate circuits but have less high frequency response but well above 20Khz.
 

Vuki

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I've experimented with the 10K:10K a bunch of years ago. They do good phase splitting too. They start to saturate at about 10Vp-p at 20hz. They still need a low Z driver. The larger core models can be used in the plate circuits but have less high frequency response but well above 20Khz.
I too used them as the phase splitter
6C33 PP
2A3 PP
 

Noob

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It would be nice to see the Countryman, Telefunken, and Neve DIs measured.
 

KSTR

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It would be nice to see the Countryman, Telefunken, and Neve DIs measured.
And Lundahl.
And perhaps some cheap stuff like from Triad Magnetics.

I'm using an OEP in a product and it performs really well, but key is super low drive impedance and definitely avoid capacitive coupling.

Some audio transformers show a very strange "chaotic" overload reaction at low frequencies and increasing high levels, at some point they just flip in an instant from very acceptable output waveform to something horribly distorted ("folding over", basically). When you reduce level again, it unexpectedly stays in that mode until level is really backed off significantly, or frequency increased. It strongly depends on drive impedance and load as well.
 

GWolfman

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Was it mentioned anywhere in the article what DI stands for?
 
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