This is a review and detailed measurements of the Ortofon ST-70 Moving Coil transformer. It is on kind loan from a member and costs around US $1,500.
Other than fancier footers, the box is ordinary as you see. It is heavy though for its size. Back side shows the simple connections:
There are apparently two version of this unit: A and B. I am not sure which one I have. Here are the specs:
I tested the unit more or less like a black box to characterize its transfer function. It is kind of difficult due to inability to match the impedances but let's see what we have.
Ortofon ST-70 Measurements
I fed the unit 0.5mv input and got this dashboard (default 20 ohm source impedance):
We get a (voltage) gain of nearly 30 dB which should be able to convert a moving magnet phono stage to one that supports moving coil (purpose of the device). At this low level of input, we are essentially noise bound other than mains noise which I could not minimize beyond what you see. There is a distortion spike that is barely visible.
Frequency response is highly dependent on source impedance. Given the choices I have in Audio Precision analyzer, this is what we get:
We are not remotely spec compliant. Even if one matched their spec exactly, how would you do that in real life?
As expected from any transformer, low frequencies is where you see distortion:
I used 1mv to slightly accentuate the effect.
Looking at pure distortion (no noise), we see that the impact is essentially the same as using electronic gain stage in our reference phono stage:
Crosstalk is very good in grand scheme of things:
I ran a test with RIAA inverse EQ active on the input and got this:
Conclusions
Without data from other products like it, it is hard to know how well the ST-70 is doing. The frequency response of note, is very poor if taken at face value. I am open to input from members on how to interpret that. Note that I am on a time crunch to send this unit back (will be boxed up today to go out tomorrow). So little time for more experimentation.
The cost is way up there. I would have thought something like this would cost a few hundred dollars.
------------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
Other than fancier footers, the box is ordinary as you see. It is heavy though for its size. Back side shows the simple connections:
There are apparently two version of this unit: A and B. I am not sure which one I have. Here are the specs:
I tested the unit more or less like a black box to characterize its transfer function. It is kind of difficult due to inability to match the impedances but let's see what we have.
Ortofon ST-70 Measurements
I fed the unit 0.5mv input and got this dashboard (default 20 ohm source impedance):
We get a (voltage) gain of nearly 30 dB which should be able to convert a moving magnet phono stage to one that supports moving coil (purpose of the device). At this low level of input, we are essentially noise bound other than mains noise which I could not minimize beyond what you see. There is a distortion spike that is barely visible.
Frequency response is highly dependent on source impedance. Given the choices I have in Audio Precision analyzer, this is what we get:
We are not remotely spec compliant. Even if one matched their spec exactly, how would you do that in real life?
As expected from any transformer, low frequencies is where you see distortion:
I used 1mv to slightly accentuate the effect.
Looking at pure distortion (no noise), we see that the impact is essentially the same as using electronic gain stage in our reference phono stage:
Crosstalk is very good in grand scheme of things:
I ran a test with RIAA inverse EQ active on the input and got this:
Conclusions
Without data from other products like it, it is hard to know how well the ST-70 is doing. The frequency response of note, is very poor if taken at face value. I am open to input from members on how to interpret that. Note that I am on a time crunch to send this unit back (will be boxed up today to go out tomorrow). So little time for more experimentation.
The cost is way up there. I would have thought something like this would cost a few hundred dollars.
------------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
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