SUT - electrical theory and practical experience

@rauliruegas

Btw, could be interesting that your friends can ask to EMIA its measured FR due that even that exist audiophiles that just do not care about that kind of measure is really of critical importance and the manufacturer just " dead silence about " and I wonder why because other SUT manufacturers gives that kind of measure.

it seems you are really good at scouring the web and quoting other people and less adept at researching/understanding what other people write. If you click the link provided in this very thread by @antinn on 11/16 you will see the actual measured frequency and phase response of an EM/IA SUT compared to the venerable Altec 4722.

dave

@rauliruegas 

Blindly parroting frequency numbers without clear designation of the related decibel level is an effort in futility.  In looking at the 3-200kHz number of the Technics you reference (SH-305MC) there is no designation of the attenuation level at the specified frequencies, They do state 20Hz to 100kHz ±0.2dB but fail to state the exact test conditions (source Z, Load R and C).  The Denon AU-1000 states 5Hz to 200kHz but also lists no other test conditions.  The Frequency response plot I found shows it to be -2dB @ 5Hz and -4dB @ 200kHz again without specifically listing the complete test conditions.  One could assume  that the source is 30Ω and the load 50kΩ and there is mention of a low capacitance accessory cable so that is more info available than the Technics. 

For any design it is easy to find and publish the optimal wide bandwidth conditions for great measured response but absent the full details of the test condition, the numbers become meaningless.  If the underlying numbers have no real value, then any presumptions based on those numbers also lose validity.  I also fail to see how you can consider both of these transformers to be "flat" for the bandwidth you quoted yet fail to see the ±1dB response from 10Hz to 230kHz of the EM/IA 1:20L driven from a 9Ω source and loaded with 300kΩ in parallel with 80pF of capacitance.

I do want to be clear that I do not think frequency response should be the sole dictator of "quality" of a SUT.  Bandwidth is just one parameter of many that has to be addressed in a given design. In fact assuming the frequency response safely covers the audio band,  I find core material, dielectric choice and winding wire to be far more dominant factors in the final sound. I find extending bandwidth (particularly at high frequencies) is simply more icing on an already delicious cake.

Elevating the merit of a SUT based on a single ambiguous parameter as you do is like purchasing a sports car because it is a pretty shade of red or has a lower curb weight than the competition.

dave

@rauliruegas 

I should be sure you have that RIAA deviation already measured and maybe you can share with us and your customers.

I'm hesitant to give you a measurement which you will certainly turn into a cudgel to use on me in the future but typical units are ± a third of a dB across the audio band.

dave

Whether a cartridge is a 'voltage' or 'current' generator simply comes down to the load you present it with.  Make the load an open circuit (volt meter) and it will deliver max voltage and essentially 0 current.  Make the load a short circuit (ammeter) and it will deliver max current and 0 Voltage.   Without a clearly established loading criteria, the discussion becomes one of futility.

dave

@rauliruegas 

Btw, @intactaudio  : you capacitance parameter of 80pf coukld be not real to make SUT measurements as the 300k load.

as stated in the text you pulled the above numbers from, those are the actual values of the referenced phono stage cable combo not some "made up numbers" 

Normally what sees the MM stage + SUT is a load of 47k and capacitance around 200pf. Then measures arwe different that with your " choosed " parameters.

Again this is covered in the text referenced by @antinn on the first page.  It is indeed true that 47kΩ || 200pf may be a typical number, it simply represents a situation that has a good chance of being less than ideal for use with a SUT.  Gaining a fundamental understanding of the tradeoffs of the cart /SUT/phono interface goes a long way in furthering the trend of commonly occurring mistakes that historically has given SUT usage a bad rap.

Unfortuntaelly cable manafucterer almost never gives the cable capacitance but almost all are higher than 150pf and even over 200pf.

then you need a basic understanding on how load capacitance adversely effects a SUT and you also need to find a new cable manufacturer.

dave

@o_holter 

I tested a SUT with the Io and did not like it - sounded good at first but then I noted that it subtracted from the purity, the sound became more hi-fi, for lack of a better word. But that was with a mid level SUT years ago and things could be different now.

When you delve into the waters of the external SUT there are a lot of considerations beyond the obvious.  One "gotcha" many people run into is dynamically overloading the input of the phono.  It is important to remember that the numbers thrown about for phono are all typically referenced to 5cm/sec @ 1kHz so your atlas will put out 0.56mV @ 1kHz with a 5cm/sec signal.  The important thing to remember is that the same signal level @ 8kHz will be 12dB hotter for a 2.24mV level (20cm/sec).  Now toss in a bit of cushion in the form of a 12dB of headroom and you might see a nearly 9mV dynamic peak from that cart.  Now consider what happens when you put this 0.56mV signal into a 1:20 SUT and the nominal 1kHz level is in the 10mV range and the dynamic HF peaks are in the 160mV range.  That 160mV @ ~8kHz will pretty much momentarily eat the input of any phono for lunch.  The good news is this is just a quick transient overload so it isn't a constant nagging problem but one that only pops up occasionally.  The bad news is since this overload happens dynamically it is elusive and by the time your brain senses something is amiss, it is gone.  

Now back to the situation where you move to the Atlas Lambda which basically cuts the output in 1/2.  If you add anything more than a 1:2 SUT you have to dig a little deeper into the available overload margin to make sure you don't start tickling the HF input limit with your peaks.  This overload situation happens in the very first stage so any gain adjustment beyond bypassing an active stage entirely does not change the situation.  Just because a switch allows you to get 12dB less gain doesn't also mean you now have 12dB more input headroom.  With the current trend of higher output cartridges like the Atlas being paired with the typical 1:20+ SUT intended for LOMC's I see this type dynamic overload happening al lot.  

A good example of this is comparing an EMT TSD15 (24Ω 1mV) to a denon 103 (40Ω 0.3mV)  From an impedance standpoint the same SUT can handle either but if you use the typical 1:20 on the 103 you end up at the 5mV "traditional" MM spec.  but use that same 1:20 with the EMT and you end up with a hot mess because the phono stage input is not happy with the quarter of a volt peaks it is being fed.  This results in a situation where the denon owner loves his SUT and the EMT owner curses the same device.  Drop the ratio of the SUT for the EMT to 1:7 and more often than not you will have a happy camper.  

dave

@alexberger 

I use 20 Ohm input (26dB, 1:20). The phonostage has overload points - (1% THD+N) for the 834P were 69.2mV at 1kHz, 260mV at 20kHz, and 7.85mV

you seem to have a frequency number missing from the above since you have three voltages listed and only two frequencies.  

dave

People try to lump all kinds of technical truths into simple rules of thumb which then become the norm. After being accepted, the underlying truths these norms are built upon tend to get lost.  The three main things that need to be considered when designing / implementing a SUT are Impedance (source and load), gain and loading.  While all three of these items are interrelated by math, they all need to be addressed (juggled) individually.

The source / load impedances are what determine the required primary inductance and the winding geometry required to get the desired bandwidth.  What needs to be done for a 2Ω cartridge is different than what is needed for a 40Ω cartridge.

The turns ratio is set to elevate the cartridge output to the desired input of the phono stage it is driving.  A 0.25mV output 40Ω Denon 103 roughly fits the same impedance numbers as a 24Ω 1.0mV EMT but requires a different turns ratio.

Loading is the the part that gets way too much emphasis and really should be only a minor part of the consideration. The focus on cartridge loading as a primary concern causes much confusion and turns what should be a very simple process into a black art.  There are two aspects to the reflected load, the resistive part and the capacitive part.  The resistive part sets the actual reduction of gain from the ideal. Attention must also be given to the load capacitance as the turns ratio increases since that value can quickly truncate the top end.  I always consider the reflected load to be a liability and feel that using a transformer to reflect a desired load is misguided at best.  People tend to consider the transformer as an ideal device that will perfectly reflect the terminating load to the primary.  In reality this is far from being the case. More often than not, adjusting the secondary load to reflect back a desired value to the cartridge will have both a measured and a perceived effect on the performance of the SUT. It is this change that people incorrectly attributed to the load the cartridge is seeing. All transformers will reflect a minimum load based on the math and the goal should be to keep that number as high as possible. Increasing the 47kΩ input resistor values is a good place to start.  Once the minimum load is determined, if additional loading is deemed necessary it should then be added to the cartridge directly where it will have a minimal effect on the behavior of the SUT but can have a dramatic effect on the tracking behavior (ie sonics) of the cartridge.

dave

@rauliruegas 

So, Dave hopefully you can share your know-how and experiences about.

All I can give is my subjective opinions from my experiences comparing the myriad of options out there.   In my system the SUT's are always silver wire on 80% round loop nickel cores with an emphasis on air as a dielectric.

dave

@drbond 

from rothwell...

In order to make this transformer match the cartridge with a load impedance at the primary of 5 ohms, a load on the secondary of 6480 ohms could be employed instead of the 47k normally found on an MM phonostage.

Lets say for example the goal is to load a cartridge @ its internal impedance.  I agree with Rothwell that heavily loading down the secondary of a SUT to reflect the desired load to the cartridge is a mistake.  However....  One could simply accept the 47kΩ termination on a 1:36 will reflect back 36Ω. In order to reduce the cartridge load to 5Ω, simply parallel a 5.8Ω resistor with the cartridge. (it is kinda cool how 36^3=47K)

I have done this experiment and can emphatically state that in this case 5Ω≠5Ω.  In the case with the load applied through the transformer it seems like a blanket has been thrown over the speaker.  This is the exact "thick" sound people report from this type of loading.  The error all too often made is incorrectly attributing the sound of the reflected load to the cartridge when it is actually the sonic nature of the loaded transformer.  Simply doubling the turns ratio of the SUT and then loading the cartridge at the same 5Ω nets a very different sonic result. Thick, dark and muddy are the last terms I would use as descriptors.  To complete the circle on this I have then taken a transformer loaded Jfet based head amp and compared the same cartridge above loaded at 47kΩ and at its internal impedance and I would say the overall pattern of sonic character followed that of the transformer with the additional load placed on the cartridge directly and the sound of the "traditional" heavily loaded transformer secondary is the outlier.

dave

@holmz ​​​​​​

the CBS STR-112 has a 1kHz square wave for the test you propose but be warned the results of this test from a cartridge is not for the feint of heart. And yes as you suspect due to the riaa filter the actual cut of the track appears as a triangle wave.

dave

@rauliruegas 

That gentleman that pass away was a tube lover

Art made his system choices on what gave him the most musical enjoyment and had a large readership of similar minded people.  If those choices do not align with your measurement-centric view of the audio world, maybe it is time to take out the earplugs and actually listen rather than let a device prove to you how good something must sound.

dave

@holmz 

I am only trying to find a way to load the cart for my 1:4 SUT.

This seemingly simple statement is the basis for much of the confusion that exists around SUT's in general.  People feel the need to lump similar things together hence the discussion of transformer and cartridge loading in the same breadth.

This seemingly simple statement is the basis for much of the confusion that exists around SUT's in general.  People feel the need to lump similar things together, hence the discussion of transformer and cartridge loading in the same breadth.

From a cartridge loading perspective, people consider the transformer to be ideal, at which point a simple calculation nets a "nice clean" load for the cartridge.  The problem with this approach is that transformers are far from ideal and varying both the source and load values has a profound impact on both the measured and sonic behavior of the SUT.  This gave rise to the engineering approach as shown by Rothwell, Jensen et al focuses solely on the behavior of the transformer for a given situation.  The goal here is to set the source impedance to that of the cartridge and then adjust the secondary termination (load) to get the "best" response.  The problem here is this method provides a singular "load value" for a given cartridge impedance.

What is one to do when the two approaches listed above push you in opposite directions? Menno Van Der Veen acknowledged this and noted that after the proper secondary termination is determined, the load the cartridge sees can be further increased by placing resistance across the primary (ie to the cartridge directly).   Since this only lets the load be modified to a lower value, it is a step in the right direction (and the method I suggest for additional load) but does not represent the simple solution everyone craves.

When these two lines of analysis which use all the same terms are co-mingled in a discussion like this, people tend to form "technically correct" opinions which sit on a solid foundation of bad data.   

dave

@atmasphere   I would agree 100% with you if the only thing loading of a cartridge does is effect the LC resonance you laid out.  I believe loading of a MC cartridge has a dramatic effect on the electromechanical behavior of the cartridge but that is a topic for another thread.

dave

@holmz 

But I’ll need to check the diagram to see if that built-in 1:4 SUT is loaded on the primary or secondary side

There are two possible loads that this SUT could have. The unintentional which is the typical 47kΩ input of a MM phono and the intentional which would be an additional filter on the secondary to "correct" any misbehavior of the device.   Both of these loads would be on the secondary of the device.  If there are specified cartridge loads for MC then is possible there could be an additional combination of primary and secondary loading and with the low turns ratio this is a possibility. 

I do think that the typical "dull" sound from loading a cartridge down too much is purely a function of the SUT behavior.  Since you have such a low ratio transformer an interesting experiment would be to find out the "calculated" reflected impedance and then try the following experiment.  (below I am assuming a 47kΩ load and a 40Ω Denon 103 and you can scale the values for different cartridge impedances.)

Assuming the 1:4 is terminated with 47kΩ  and a the cart should see 3kΩ as a load.  Now place a 1.6kΩ resistor across the secondary to reflect just under 100Ω to the cartridge and give it a listen.  Next remove the 1.6kΩ resistor and place a 100Ω resistor across the primary and listen again.  In a perfect world the load the cartridge sees will be the same (roughly 2.5X the internal impedance).  The gain of both situations will also be the same and my experiences tell me the sounds will be quite different.  To be clear... any differences heard will be a function of the SUT behavior and not the load the cartridge sees which puts this squarely in the purview of this discussion.

now that I am done with my airing of grievances it is on to the feats of strength.  Happy Festivus to all!

dave