Electrostatic Headphone Amplifier Review & Comparison

[amirm]

You could consider paralleling a (very small) capacitance, maybe even a resistor in series with it, to compensate for the loss of higher frequencies.
The same thing is done with scope probes, here it usually is a small trimmer cap which is easily calibrated by looking at a 1kHz squarewave that is usually provided by the scope itself.

Yes, the process is the same and that is what I am planning to do. The reason it is tricky is that you need a 10:1 ratio for the cap as well for what is inside the AP which makes it just 20 pf. There is enough capacitance elsewhere in the wiring and such, together with accuracy issues that makes an exact number impossible. I have bought a few 10 pf caps so I have more flexibility. Scope probes have adjustable caps for this purpose as you know but we are talking about far higher voltages here and I did not search to see if there are any adjustable caps that way.

 

[20396]

finally see the evaluation of electrostatic headphones, even though STAX SRM-T8000 and STAX SR-009S

 

[solderdude]

Yes, the process is the same and that is what I am planning to do. The reason it is tricky is that you need a 10:1 ratio for the cap as well for what is inside the AP which makes it just 20 pf. There is enough capacitance elsewhere in the wiring and such, together with accuracy issues that makes an exact number impossible. I have bought a few 10 pf caps so I have more flexibility. Scope probes have adjustable caps for this purpose as you know but we are talking about far higher voltages here and I did not search to see if there are any adjustable caps that way.

You can determine the capacitance value by using low voltages and a ordinary trimmer cap and measuring it with an LCR meter (needs to have the resolution).
Then use the nearest (lower value) fixed capacitor in HV and 'trim' by adding a few extra pF using solid wire you twist (with enough isolation thickness.
You can do all this at safe voltages and then test on higher voltages. The used caps must be 2kV minimum.
It's a 1-time thing and once mounted in a plastic box will work fine.

 

[Cortes]

I don't remember all the amps I've tried, but at least the following, all of which sounded identical to me (unlike the tube amps):
SRM-Xh
SRM-3
SRM-717 I think?
SRM-727 Mk II

I had the same experience between the SRM-T8000 and SRM-727 Mk II, a bit different, but if blind I doubt I would be able to distinguish them in a lot of songs. I did the comparsion in a Yodabashy, noisy ambient, but given the difference of price one would expect a clear improvement.

 

[SpaceMonkey]

I had the same experience between the SRM-T8000 and SRM-727 Mk II, a bit different, but if blind I doubt I would be able to distinguish them in a lot of songs. I did the comparsion in a Yodabashy, noisy ambient, but given the difference of price one would expect a clear improvement.

Why would you expect an improvement if most people can't identify improvement in a blind test between cheaper and more expensive amp. Possibly more expensive versions can swing more voltage to get louder.

 

[the_brunx]

[QUOTE="

would be interesting to see the results of the benchmark ahb2 driving a srd7 measured before and after the srd7[/QUOTE]

I bet this will be the best STAX amp in the world.

 

[LuckyLuke575]

I laughed at this statement, which in all likelihood seems correct this being a Massdrop special "The Koss in sharp contrast could be mistaken for a give-away kids toy in McDonald's happy meal".

Sad to see how poorly the Stax high end amps perform. I demo'd a new L300 and L700 with a smaller Stax amp and it sounded the best out of all the other amp/headphone combinations that day. It seems that tubes should be avoided in all cases lol

 

[brimble]

Sad to see how poorly the Stax high end amps perform.

Exactly ... but then the cheap ones are good, so it could be worse, especially for those of us in the know.

 

[Cortes]

700t has been offered for testing as well as another unit. I will have to decide whether I am publishing the results though or just testing for the owner and myself....

@amirm,

did you test the 700t?.

 

[JohnnyHonda]

Please publish! I’d love to see how you equalize the unit. I have a 700 as well with an RME and Mjolnir amp and I’d like to try your settings.

 

[pwjazz]

You are misreading the above measurement. Only second and third harmonics are counted in the total above, not total as THD in electronic measurements show. It is also an entirely different measurement system (chirp versus steps).

Finally, no I don't remember if I used the tube amp. Most likely not as I had to go and fetch them whereas the solid state was right next to my workstation.

If I'm not mistaken, your amp measurements show 2nd harmonic to already be at -35dB, which is way off from the -60dB in the combined measurements.

 

[isiora]

Hi Amirm,

I was surprised looking at your measurements. Stax is a serious manufacturer and the 006ts spec states that distortion is less than 0.02% THD @1kHz @100Vrms. So I wanted to make some measurements too.
I measured my Stax 006ts while my SR-L700 headphone was attached to it, using the second parallel output connector. So I used the real amplifier load.
My gear is not feautered for audio (Picoscope 5444D MSO using pseudodifferential setup) but it is enough to distinguish that level of distorsion.
The result is that the system shows no more than 0.025% of THD @50Vrms and no more than 0.035% of THD @100Vrms. Part of this distorsion is due to my poor setup. So I can estimate that the effective distorsion level is better.
These electrostat systems, as you know, need very low current level, so imho there is little point in measuring without considering the amplifier + headphone system as a whole.
(P.S.: the second harmonic is almost perfectly canceled. The distortion is most of the third harmonic).

 

[isiora]

Hi,

I tuned my setup. Below is my last measurement @100Vrms. THD is 0.024%.

 

[isiora]

...This should have been clearly documented by the company so I knew to avoid them and stay with solid state.

So do you state that Stax has published fake specs?

 

[isiora]

...It looks as if the SRM 006t tube design is similar to the original transistor SRM design but with tubes in the output stage. The output devices in both forms use about 50kΩ anode resistors. The 006t is a differential output, but like the SRM-1 mk2 and its ilk, is a single ended differential topology.
The SRM 007t is a very different beast, in particular it has a push pull differential output. Hence the 007t has 4 tubes (each a dual triode) whilst the 006t has two tubes.
...

Hi,

the topology of the 006t and 007t output stage is the same. Both are differential. The difference is that the 007s uses two triode in parallel for each branch of the long tail pair. So four branch (two for each channel) give us eight triodes, or four 6CG7 tubes.
Both amplifier must be measured using differential mode, alias using a differential probe, otherwise the results will be totally false.
The "single ended differential topology" is an oxymoron, it does not make sense.

 

[John_M]

Really interesting - thank you for posting this review. I luckily avoided the Stax tube amps. I hope we'll get to see reviews of the 353X, 700S, 700T, T8000 and third party amps. If some sacred cows need to be slain, so be it. Get out the captive bolt pistol.

 

[John_M]

armir's srm-007 is a rev 0 unit (3 stax jacks on the front panel) AC filament. But that also means its a minimum of 25 years old.
If it has never been serviced, then highly likely that the power electrolytics are in serious trouble. I would think that would show up
as 60 and 120hz bumps in the spectrum. large bumps. The rating of this amplifier is 1350Vpp. If the 100k load (200k balanced) did nothing
additional bad to this amp, it would bring that rating down to 800Vpp. No one certainly tests dynamic amplifiers this hard.

Kevin, if the issues identified in the review are caused by the age of the SRM-007t, why does the graph for the SRM-006t look so similar? To a non-specialist it looks like they are two members of the same high distortion family. Or is the SRM-006t suffering from similar age-related issues?

 

[maverickronin]

Kevin, if the issues identified in the review are caused by the age of the SRM-007t, why does the graph for the SRM-006t look so similar? To a non-specialist it looks like they are two members of the same high distortion family. Or is the SRM-006t suffering from similar age-related issues?

The real issue was that Amir tested all the amps into far too low a load. In speaker amp terms it's like he tested them all into 1 ohm because that was the only high power load he had on hand.

With the solid state amps it "only" drastically reduced their maximum output. With the tube circuits it drastically raised their distortion as well.

The age of the amps, and consequently their power supply filter capacitors, is pretty much just related to the 60/120/etc hz hum and not distortion levels.

 

[John_M]

The real issue was that Amir tested all the amps into far too low a load. In speaker amp terms it's like he tested them all into 1 ohm because that was the only high power load he had on hand.

Yes, just reading the flame war now. Apparently there will be another estat amplifier review so I guess let's wait and see what comes out of that.

I did notice this...

I know for me the measurements highly correlated with clear audible distortion as you turn up the volume on the tube amps. So my curiosity is gone.

If I didn't know better, I'd say that Amir was using his subjective impression of the amps to defend his conclusions, in circumstances where the tests may have resulted in expectation bias. I hope this site isn't going to be renamed Head-Fi review.

 

[isiora]

Kevin, if the issues identified in the review are caused by the age of the SRM-007t, why does the graph for the SRM-006t look so similar? To a non-specialist it looks like they are two members of the same high distortion family. Or is the SRM-006t suffering from similar age-related issues?

The issues are related to mistakes in measurements. Nothing else.