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For several reasons my reply here took some time. The measurement of the loaded output voltages of the Stax amplifiers last year went smoothly. (Load was 007 resp. 009). [For safety reasons, I was more than 1 m away from all devices and open connections when I made the measurements of the high voltages. In particular, I restrained from touching the volume knob at all.] The outcome was that the loaded and unloaded output voltages agreed up to the last digit of the DMM (50 000 count). So, matching levels for a specific estat headphone with different amps can be achieved by simply matching the unloaded output voltages - at least for the tested Stax amps D10 and 353X and above mentioned headphones. It would be highly appreciated if someone could share a general reason why loaded and unloaded voltages of estat amps are always equal - if this is the case.

Safety comes first. So, thank you mwl168 for sharing your concerns. Injury/death are not acceptable options for me, so I made some precautions: The measurement equipment is rated at 600 V CAT III. I found seven sources for the Stax pin configuration (male/female) on the internet, so I am quite confident where bias, L+, L-, R+, R- are correctly identified. A sine wave is generated on the left channel and null on the right channel. L+ is unmissable on the Stax connector and even if I was subject to a temporary confusion of left and right, I would measure L+ vs R+ without damage. The bias pin is further away and to be avoided. The input to the amplifiers will be decreased before the test by 20 dB. So, the voltage on the left channel will be around 5 V. If you have further suggestions on how to improve safety, please share. I would like to seize the opportunity and bring up myself some questions on safety during regular operation of electrostatic equipment: During operation, the transducer is right next to the ear. Why are the high voltages not a concern? Some (non-Stax) amplifiers have receptacles where the golden-coloured metal inserts extend right to the white front of the receptacle. At least, this is what it looks on the product images. So, I wonder what happens if the amplifier is turned on, no headphone is connected, and a person comes close to the receptacle? What risks emenate from a micro break in a Stax headphone cable which is not immediately visible to the eye?

Thank you for you explanation on the electrical behaviour of the Stax amp + headphone. On the probes: I saw those very thin U-shaped hooks, but I thought they might slip during measurement and cause a short. The idea to bend them did not occur to me, but now that I know it, this is a very good option. Some Stax amplifiers have only one receptacle (SRM-D50), so it is helpful to also have this option as a backup. Thank you for your competent and generous feedback!

That sounds very promising if I understand it correctly. Let us consider the case that the unloaded output voltage is 100 V (for one channel) and that the loaded output voltage is due to the output impedance of the amplifier 80 V. So, when the headphone is plugged into one receptacle then the voltage at the unused receptacle is also 80 V which I can easily measure. (I found some diagrams on Stax wire configuration on the internet.) Is my understanding also correct that when another headphone is plugged into the second receptacle in parallel (with the same settings, in particular for volume control) the output voltage would drop, but still be equal for both receptacles? Thank you very much for pointing out the simplification with the 1 mm hole. This was exactly my wish for posting in this forum that someone would point out a simplification which was not obvious to me. And the ide with the clamp test probes is even better concerning cost and process. I now found the probes below which look suitable. Thank you very much for your excellent feedback. I would give two likes if I could.

Well, I have not made up my mind, yet. I try to keep my health and deal safely with electricity. I also aim to maintain the integrity of expensive devices - in particular for those Stax products that I only would have on loan. Due to Covid restrictions it will be more like months rather than weeks until I can start with it afterall. So, if you have comments on anything hazardous (either on health or product inegrity) in my description above when manipulating a Stax extension cable, please let me know. Maybe someone also comes up with an easy way to match volumes with high precision. Thank you!

Thank you ktm and mwl168 for your feedback. I guess that you are right that probing in a shop would not work. The process suggested by mwl looks practical. It is just that Stax devices can be quite pricey (e.g. 700 S/T vs. D50), so I would like to make my comparison as neutral as possible. If I narrow down the amplifiers to, say, two then maybe I might be allowed to audition them at home. Measuring the unloaded voltage and adjusting for the output impedance will probably not work with a high accuracy, at least nobody suggested an idea for it. The only way seems to be to measure the loaded voltage. And since I cannot solder, I only have one idea: For some configuration of headphone, amplfiier and input I will set the volume control to normal listening level. I guess this will be less than 50 V output. I reduce the input to the amplifier by a factor of, say, 10. Then the voltage at the driver (electrodes) is only around 5 V. I get the cheapest 5-pin Stax extension cable (probably still close to 100$), make five parallel cuts in the middle of the extension cable of 10 cm to separate all wires while keeping their insulations intact. (In pariticular bias needs to have undamaged insulation.) Of course, the cable is detached from the circuit when doing this. I remove the insulation for one channel, say, from L+ and L- for ca. 1 cm each. I connect the amplifier, extension cable and headphone and I measure the voltage from L+ to L- and set it to exactly 5.000 V for each amplifier via its volume control. When this is done for all amplifiers, I remove the extension cable and plug the headphone directly into the amplifier. In the end, I turn up the volume of the input to the amplifiers and keep it constant thereafter. Any feedback would be welcome, in particular if you share a good reason to keep me from tinkering with an extension cable.

I agree that such a setup would give matched volumes within a 3 dB range. I guess that such a range can be also achieved by simply listening one amp after the other and matching volumes by short-term memory. The only problem for me with this approach is that I tend to prefer higher SPL in direct comparison, so I would very likely buy the amplifier which gives the higher volume. A sine signal with 50 V can be measured with a moderate DMM with an accuracy of 0.03 dB, so this would neutralise my bias for higher volumes. The problem with the Stax setup is that I cannot access the wires when loaded with the 009S. And in the unloaded case the measured voltage at the amplifier output can be far off the voltage over the headphone due to the unknown and likely different output impedances of the amplifiers. Any ideas how to match the volumes within, say, 0.1 dB? Thank you.

Taking a step back and a nap is always good advice. Thank you. At the same time, my hearing is not good enough to differentiate in a shop environment between amplifiers without level matching the headphone. Before buying, I would like to be sure (to some extent) that I pick the right amplifier. Measuring the unloaded voltage in a Stax plug output up to 50 V looks safe to me with a CAT III 600 V DMM. On the other hand, bare wires in some configuration for loaded voltage measurements look less safe - for me and the devices. So, if anybody in this forum has ever tried to level match a single electrostatic headphone with several amplifiers for auditory comparison purposes then I would highly appreciate your feedback.

What is a practical way to level match an electrostatic headphone with several amplifiers? As a tool, a multimeter would be available. For a specific example, take a single 009S and three amplifiers: D10, D50, 353X. As a first attempt, I might measure the unloaded voltage for the, say, left channel at the output of each amplifier and match it to a common value, say, 50 V for a some test sine wave. But then the output impedance of the amplifier is not accounted. And the output impedance of those amplifiers is not available on the internet (at least for me). And even if it was available somewhere, the impedance of devices is usually given as a real number rather than the underlying complex number, so an accurate calculation of the voltage at the load would be not possible. To measure the loaded voltage for dynamic headphones is more practical because the cables can be rather easily handled because 3.5 mm jacks (and some creativity) give easy access to the wires while the headphone is connected. But for an electrostatic setup with a Stax cable I would not really know how to access the wires under load. And then also to ensure safety at those higher voltages. Any ideas would be welcome!

I appreciate that my question has been read and that reactions were posted. My intention was to avoid arguments on subjective hearing impressions and just ask about mechanical differences between 009, 009 BK and 009S beyond the driver. In particular I wonder if they all share the same earpads. I have listened to the 009S and liked it a lot, but never had 009 or 009 BK in my hand. I am wondering about the differences between the three versions because some special offers have appeared for the 009. There is of course the option to order 009 and 009S (and some amplifier because I have none for estats, e.g. D10, D50 or 353X) online and compare them at home. But I would like to reduce cross-country ordering and shipping of expensive items as much as possible, so any help from you would be appreciated. If there was unanimous agreement on the sound differences of 009 (equal to 009 BK?) and 009S then this would be also helpful, but the above six posts indicate that this is not the case. Thank you!

Hello, I would like to ask for some clarification on the differences between the SR-009S, 009 and 009 BK beyond the driver and colour. Inexxon.com gave the 009 the highest score (five stars), but only 4.5 stars for the 009 BK because of the impact of "the new earpads" on sound. Btw, the 009S got also 4.5 stars. But staxaudio.com states for the 009 BK: "Same as SR-009" apart from colour and the limited edition serial number. Also, staxaudio.com does not list in the "parts" section any different earpads for the three 009 versions. So, are there any differences in earpads between the three versions or in yoke, arc (head band), etc apart from driver and colour? Thank you!