TOPPING EHA5

[Veri]

So not a real contender for the expensive Ifi Audio iESL. Probably the best in the market if you get your speaker amp right.

Has that one been similarly stress-tested?

 

[Aokman]

So not a real contender for the expensive Ifi Audio iESL. Probably the best in the market if you get your speaker amp right.

You cannot buy the iESL anymore so the comparison is redundant. That said I doubt the iESL transformers have the capability to match Lundahl transformers.

 

[Miiksuli]

Has that one been similarly stress-tested?

I haven't seen it. My current setup limit factor is the amp. Limits how much volume I can push that bass get distorted on bass heavy songs. Could be too risky even to send my stuff to anyone. Anyway I still would love to see iESL tested and with different amps.

You cannot buy the iESL anymore so the comparison is redundant. That said I doubt the iESL transformers have the capability to match Lundahl transformers.

Yup. They did make a "rival" iCAN Phantom. https://ifi-audio.com/wp-content/uploads/2023/07/HFN-Sept-iFi-Audio-iCAN-Phantom_Reprint-LOW.pdf

 

[Aokman]

Yup. They did make a "rival" iCAN Phantom. https://ifi-audio.com/wp-content/uploads/2023/07/HFN-Sept-iFi-Audio-iCAN-Phantom_Reprint-LOW.pdf

Too expensive for what it is and cannot use better amplifers

 

[Miiksuli]

I haven't seen it. My current setup limit factor is the amp. Limits how much volume I can push that bass get distorted on bass heavy songs. Could be too risky even to send my stuff to anyone. Anyway I still would love to see iESL tested and with different amps.

Too expensive for what it is and cannot use better amplifers

More like freedom. Phantom is not that expensive but u are always limited to what u get.

 

[Aokman]

More like freedom. Phantom is not that expensive but u are always limited to what u get.

It’s very expensive for a transformer based amp, too expensive and limited in my opinion. Jack of all trades, master of none.

 

[simmconn]

What load was used for these tests ?

The loading varies depending on the test. For noise test and the internal amp test (low voltage), direct connection to the AP balanced input (200k Ohm). For the transformer impedance test, the transformer secondary has no load or was loaded with a biased SR-404 where specified. For other tests, the amp was loaded with a 100pf cap followed by a 10:1 attenuator to the AP balanced input. The equivalent total capacitance is about 120pf, and the resistance about 1.1M Ohm.

 

[simmconn]

Thanks for that indepth data, did you do any testing of the FR at higher voltages (Say 150vRMS?) I found that quite interesting when comparing the SRDs and Lundahl transformers to see if the transformers easily saturate at lower frequencies and how flat they hold at louder volumes.

The FR was done at 100Vrms. Within the internal amp's output envelope, the FR has not much dependency with the output level. 'Lundahl transformers' is too board a scope. The winding structure, core size, turns ratio and connection scheme of the primary and secondaries could all affect the performance, not to mention Lundahl has a pretty long catalog of different transformers.

 

[solderdude]

The loading varies depending on the test. For noise test and the internal amp test (low voltage), direct connection to the AP balanced input (200k Ohm). For the transformer impedance test, the transformer secondary has no load or was loaded with a biased SR-404 where specified. For other tests, the amp was loaded with a 100pf cap followed by a 10:1 attenuator to the AP balanced input. The equivalent total capacitance is about 120pf, and the resistance about 1.1M Ohm.

That will be fine and not load the transformer for lower frequencies.

 

[simmconn]

That will be fine and not load the transformer for lower frequencies.

The no-load and loaded conditions were purposely done and plotted in the same transformer impedance graph to see the impact of the loading to the resonance frequency. You are right that the headphone capacitive loading doens't have much effect at lower frequencies, where the dominating factor is the transformer's primary inductance.

 

[Aokman]

The FR was done at 100Vrms. Within the internal amp's output envelope, the FR has not much dependency with the output level. 'Lundahl transformers' is too board a scope. The winding structure, core size, turns ratio and connection scheme of the primary and secondaries could all affect the performance, not to mention Lundahl has a pretty long catalog of different transformers.

Thanks for that, the common STAX Lundahls are the LL1630 which is what I use. Sounds good at 100v, I did my tests at 150v with a 120pf load though transformers don’t seem to care really just the direct drive amps at high frequencies.
I only have limited gear to test though so I was mainly interested in transformer bandwidth limitations at higher volumes.

 

[simmconn]

The LL1630 datasheet gives the answer: Max. output level at 30 Hz (Secondaries in series) 18 Vrms (LL1630 / 5mA) or 45 Vrms (LL1630 P-P). So if you connect the secondaries in parallel and use the transformer backwards for highest turns ratio, you would want to limit the max output voltage to (7.2*2)*45/2=324V. There is not much value pushing it higher disregarding the less bandwidth and higher distortion, otherwise it'd be like the '700Vrms output level' advertisement.
Also you might want to re-check the conditions of that restored SRM-1. The top-end drops should not be much worse compared to spec (max -4dB @ 20kHz, 30V output).

 

[Aokman]

The LL1630 datasheet gives the answer: Max. output level at 30 Hz (Secondaries in series) 18 Vrms (LL1630 / 5mA) or 45 Vrms (LL1630 P-P). So if you connect the secondaries in parallel and use the transformer backwards for highest turns ratio, you would want to limit the max output voltage to (7.2*2)*45/2=324V. There is not much value pushing it higher disregarding the less bandwidth and higher distortion, otherwise it'd be like the '700Vrms output level' advertisement.
Also you might want to re-check the conditions of that restored SRM-1. The top-end drops should not be much worse compared to spec (max -4dB @ 20kHz, 30V output).

I shall revisit but seems to mirror the existing concerns of the STAX guild who claim direct drive amps struggle mostly in the high frequency department and when I increased the load with a parallel or series resistor, a corresponding drop in max voltage output was observed but had a greater impact at higher frequencies.

The SRM-1 was linear but not linear when running at maximum capabilities across the board.

 

[the_brunx]

The published measured performance of EHA5 is pretty impressive. Is it really that good? Let’s see what those published measurements didn’t tell us.

First is the ‘appalling’ SNR of 146dB. Topping calculated the SNR using the maximum output voltage divided by the zero-signal A-weighted noise. I was able to verify that the zero-signal A-weighted noise is within spec (measured at 79uV Gain=High). However, the raw measurement without A-weighting and AES-17 filter is not that great:

Vol Pot position	90k BW, no filter	AES17-20k+A-wt
Min	1.07mV	79uV
Mid (2:30 position)	5.45mV	140uV
Max	0.91mV	104uV

The SNR degradation is most likely caused by large amount of ultra-sonic noise at the output. Once the unit is powered off, those noise spikes are gone. Apparently the noise comes from the unit itself, perhaps the switching power supplies (both the wall wart and the internal switching regulators for the negative rail and the bias).

View attachment 311222

The frequency response looks pretty flat, except for the slight uptick near 20KHz. Did anyone wonder what it looks like beyond 20KHz? Here it is:

View attachment 311223

The Gain switch is set at High. A 10:1 attenuator is added at the input of Audio Precision unit to extend the measurable voltage range to cover the maximum output voltage of EHA-5. Adding the attenuation ratio of 10x (20dB) on top of the curve, the actual gain of EHA5 is about 45dB at 1kHz.

We can clearly spot the 6dB gain peaking at about 32.7kHz. Beyond the peak, the response drops quickly. The time-domain impulse response below proved the existence of a resonance peak, which corroborates with the poor square-wave response posted here.

View attachment 311224

Most people noticed the increased THD in the low end (see here). The spec shows output level of 700Vrms, which seems to surpass most direct-drive eStat amps. How do those two parameters play together? Below is THD+N vs output level at different frequencies. We can see that at 20Hz, EHA5 can only output up to 229V before the amp goes to protection, and the distortion is close to 3% already. The high end output level is also limited. In order to show the real-world distortion figures, the measurement used 90kHz bandwidth and no weighting.

View attachment 311225

Topping told us the EHA5 can output 700V, what was left out is that it can’t output 700V across the entire audio frequency range.

From the simplified block diagram, we can see this somewhat unusual low pass filter between the op amp buffer and the volume pot. Dr. KG describes the low pass filter as a slew-rate filter. I consider it having two purposes; one is to limit the high frequency component from entering the main amp (we’ll see the reason later), the second is to compensate the gain peaking of the output transformer resonance so that the frequency response is near flat up to 20kHz. View attachment 311226

The frequency response from the amp section with the transformer disconnected looks like this:

View attachment 311235

Both curves are about -1.8dB down at 20kHz and -3dB at about 27.65kHz. In other words, the amp section is pre-eq’ed to make the overall FR look flat (up to 20kHz).

Going back to the question why EHA5 uses an opamp buffer in the front like the O2 headphone amp, and a volume pot value as low as 1k Ohm. The noise measurement at the beginning of this post answered part of the question. The main amp probably has a poor current noise density spec, which means it can only work with low source impedance in order to reach the desired SNR. The THD FFT also shows a not-so-great moment (see below). This is the amp section driving a 10 ohm load with the volume knob at roughly 1 o’clock position. The second harmonic is roughly at -90dB of fundamental. This is still decent, but not as superb as you would typically see in a Topping headphone amp measurement review. That seems to indicate that the main amp’s input stage is not that linear, even slightly larger source impedance would add quite a bit of harmonics. The silver lining here is that the gain selection is done in its feedback path, making the linearity in the low gain position better than in the high gain position. That would be a relief if you don’t need the high gain position.

View attachment 311236

Speaking of the transformer, they are slightly larger than the ones in my SRD-7SB. The difference is quite small, though. The impedance measurement of the EHA-5 transformer is shown below with its output open, at 100V and 200V driving a Stax SR-404. At the low end, the impedance drops due to the limited primary inductance. The situation gets worse due to core saturation under high output level (green curve). At the top end, the impedance seen by the amp dips to only 2 to 3 ohms at around 40kHz. That’s probably why the LPF is added to prevent large ultrasonic signals from entering the main amp.

View attachment 311237

Compared to the impedance curve of the SRD-7SB below, we can see both transformers follow the same trend. The EHA5 transformer has less impedance kinks in the low end. We can see its improvements compared to the SRD-7SB, but the inherent limitation of a transformer-based solution is still there. It explains why the top end and low end output levels are reduced compared to 1kHz. Driving large amplitude into a very low impedance load is too much to ask for a headphone amp module like the ones in the EHA5.

View attachment 311238

The takeaways of this measurement session is the following: If we only test the same parameters the manufactures test their units, under the same conditions the manufactures test them with, we can expect to get the same results that the manufactures want us to see. Occasional we catch a test escape or a unit that is out of maintenance, which doesn’t necessarily reflect the majority of the units in the market. To understand the true performance of the unit, we would need to get a bit more creative.

Could you not have tested also other bass frequencies for example at 40Hz or 50Hz. It’s very rare that recordings have significant energy at 20Hz

 

[simmconn]

Could you not have tested also other bass frequencies for example at 40Hz or 50Hz. It’s very rare that recordings have significant energy at 20Hz

I could, but 20Hz to 20kHz is the commonly accepted audio and audio test frequency range.

 

[the_brunx]

The

I could, but 20Hz to 20kHz is the commonly accepted audio and audio test frequency range.

The to is what’s missing though. Anyway thanks for the data you provided

 

[kevin gilmore]

Its all about size (as in core cross sectional area) and maximum flux before saturation.
its obvious from the graphs above that the core saturation of the eha5 transformer is about 1.4 tesla indicating the cheapest of core materials.
better and much bigger transformers are available from lundahl and edcor. which will both give 20hz to 20khz +0 -1 db and will actually do 1800vpp
with a 50 watt amplifier over the full frequency band. without series resistors. without filters.

as far as i can tell lundahl does not specify maximum field strength. edcor does and its 2.01 tesla.
there are custom c cores with magic materials that can do up to 2.5 tesla. these are a bit expensive.

the ll1630 mentioned above will NOT do this. core size is too small.
you need a transformer with a minimum of a 30 watt core. The 60 watt edcor are MUCH better.

the ifi iesl which is extremely sensitive to power amplifiers is yet another example of a poor design with a tiny transformer.
does not matter how carefully you wind the core if you drive the transformer into saturation.

the result will be a box about 3 times the size of the eha5. and a weight of about 20 lbs.
cost about $600.

nope, no cheap way of doing this.

 

[fabien.56]

I ordered a a Topping EHA5 from APOS Audio last night. I'll report on it after it arrives. I have no real test gear so I can't do any real testing; I do have OmniMic and also the RightMark audio analyzer app, as well as a Scarlet A/D converter which is quite good, so I COULD do some S/N and distortion comparisons of the Carbon amp vs the Topping EHA5 acoustically using my Stax SR-007a headphones and a DIY mic coupler - not sure it would reveal much about the amps, though, as their distortion levels would have to be pretty high to show any differences between the amps through the headphones. Certainly I couldn't do any kind of meaningful FR testing with a cardboard acoustic coupler, but I think it would be possible to compare the noise and distortion through the 'phones between the two amps. And I can do a listening comparison, though actually I would be surprised if there are any audible differences . I'll post some pictures of the insides.

Hi Unless I'm mistaken, I haven't seen any response to your message???

 

[zachary80]

Looks like it is on sale down to $339 from $399, is this likely a noticeable upgrade from a Stax SRM-252S? I mostly use a L300 Limited and can't really see myself upgrading from it unless something major changes (like an 007mk2 for half price). I do have to use most of the available power from the 252S due to EQ

 

[chasefrench]

Have been thoroughly enjoying my EHA5 with L700s for a while now. However, just plugged in my new A70 Pro with the passthrough functionality of the EHA5 (switched off). The distortion added to the music makes it unlistenable. Does this sound like a fault or have I missed something in the setup?

I have confirmed the A70 Pro is working well when EHA5 is taken out of the chain.

Appreciate any experience some of you might have on this matter

Thanks,