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TOPPING EHA5

Hal Rockwell

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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,
Got my EHA5 today and it has the same problem. It seems to be unable to handle a full 4 volts in passthrough mode. I use PC as the source and have tried it with multiple DACs. Finally turned the PC volume to 40 out of 100 and the distortion went away.
 

chasefrench

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Thanks for the feedback, I ended up just purchasing a set of xlr splitters. Seems like a pretty bad design oversight

At least this way, the D70 Octo can remain in Dac Only 5V output and you can properly A / B test between the L700 and whats plugged into the A70 Pro
 

etc6849

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Kevin Gilmore is right about these designs with small transformers. My BHSE has the same output at 20Hz as it does at 10Hz and over 20kHz (with no clipping up to its rating of over 1500V peak to peak)... imagine that: a product that actually lives up to its specs.

Thanks for measuring this. Any review not mentioning this limitation of the EHA5 (and any of the small transformer based amps) is misleading IMHO. I do wish someone would build a larger transformer based option that would let me just use an external amp like a Benchmark AHB2, and have decent 20Hz output.


BHSE Peak at 20Hz.jpg


THD+N Ratio vs Measured Level_Annotated.png

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 position90k BW, no filterAES17-20k+A-wt
Min1.07mV79uV
Mid (2:30 position)5.45mV140uV
Max0.91mV104uV

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.
 

Svperstar

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Kevin Gilmore is right about these designs with small transformers. My BHSE has the same output at 20Hz as it does at 10Hz and over 20kHz (with no clipping up to its rating of over 1500V peak to peak)... imagine that: a product that actually lives up to its specs.

Thanks for measuring this. Any review not mentioning this limitation of the EHA5 (and any of the small transformer based amps) is misleading IMHO. I do wish someone would build a larger transformer based option that would let me just use an external amp like a Benchmark AHB2, and have decent 20H

I thought about returning mine to Amazon but I got it used at a pretty substantial discount, nice backup for my 717. I don't really hear a difference but the Topping shutting itself off is too bad, I was unable to get it to shut off with music, just with YouTube videos with weird sounds and a ton of EQ
 

Miiksuli

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Some people say even Ifi iesl have too small transformers but I hadn't any problems with it what so ever. Only the stators can't handle too much sub bass when playing very loudly. I was thinking to buy this but maybe I wait something better comes to the market.
 
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Aokman

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Some people say even Ifi iesl have too small transformers but I hadn't any problems with it what so ever. Only the stators can't handle too much sub bass when playing very loudly. I was thinking to buy this but maybe I wait something better comes to the market.
Hunt down someone to make you a proper transformer box for the less money or go DIY if you’re capable. The iESL definitely isn’t worth the inflated prices these days now its out of production nor is their new all in one amp which uses those transformers also… It is nice that a retail product made it to market though…
 

Miiksuli

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Hunt down someone to make you a proper transformer box for the less money or go DIY if you’re capable. The iESL definitely isn’t worth the inflated prices these days now its out of production nor is their new all in one amp which uses those transformers also… It is nice that a retail product made it to market though…
I don't really need anything to replace iesl. I can hook every headphone to it and sound comes from speaker taps. But for the curiosity I would like to test different systems.
 

Aokman

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I don't really need anything to replace iesl. I can hook every headphone to it and sound comes from speaker taps. But for the curiosity I would like to test different systems.
Oh I misread thought u were looking for one
 

the_brunx

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Kevin Gilmore is right about these designs with small transformers. My BHSE has the same output at 20Hz as it does at 10Hz and over 20kHz (with no clipping up to its rating of over 1500V peak to peak)... imagine that: a product that actually lives up to its specs.

Thanks for measuring this. Any review not mentioning this limitation of the EHA5 (and any of the small transformer based amps) is misleading IMHO. I do wish someone would build a larger transformer based option that would let me just use an external amp like a Benchmark AHB2, and have decent 20Hz output.


View attachment 349326

View attachment 349327
Kevin Gilmore is right about these designs with small transformers. My BHSE has the same output at 20Hz as it does at 10Hz and over 20kHz (with no clipping up to its rating of over 1500V peak to peak)... imagine that: a product that actually lives up to its specs.

Thanks for measuring this. Any review not mentioning this limitation of the EHA5 (and any of the small transformer based amps) is misleading IMHO. I do wish someone would build a larger transformer based option that would let me just use an external amp like a Benchmark AHB2, and have decent 20Hz output.


View attachment 349326

View attachment 349327
Still here trying to figure out how the BHSE gets 1600V (as per specs) Peak to peak output voltage from a +-400V DC push pull supply rails, somebody help me out.
 

etc6849

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I'm pretty sure they're all like that due to math :D But yeah, it confused me for a bit before I drew out the waveform...

If you measure L+ or L- wrt audio ground:
Oscilloscope wrt Gnd.jpeg


If you measure between L+ and L- with a differential probe:
Differential Probe.jpeg


Still here trying to figure out how the BHSE gets 1600V (as per specs) Peak to peak output voltage from a +-400V DC push pull supply rails, somebody help me out.
 

etc6849

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The highest the L+ wave can be wrt ground is limited to A = 400V (actually it's a little less than that). Now, since L- (dashed waveform I drew) is the inversion of L+, the max difference between the two sine waves would be 2A or +800V (when L+ is above L-). When L- gets above L+, that means this difference becomes -800V. Thus, what a differential probe would measure is 1600V peak to peak (+800V up and -800V down).

sorry, still doesn’t make sense.
 

the_brunx

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The highest the L+ wave can be wrt ground is limited to A = 400V (actually it's a little less than that). Now, since L- (dashed waveform I drew) is the inversion of L+, the max difference between the two sine waves would be 2A or +800V (when L+ is above L-). When L- gets above L+, that means this difference becomes -800V. Thus, what a differential probe would measure is 1600V peak to peak (+800V up and -800V down).
Thanks,
after asking chatgtp as well, I fully get it now. But it seems A bit slimy to use peak to peak instead of the standard RMS because peak to peak can mean two different measurements. So their 1600V peak to peak is still 400Vrms.
 
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kevin gilmore

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1600 volts peak to peak measured stator to stator is 571 volts rms measured stator to stator.

electrostatic headphones are a balanced load and need to be measured as a balanced load.

400 volts rms measured stator to stator is 1120 volts peak to peak measured stator to stator. typical of +325v/-325v power supplies.
 
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the_brunx

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1600 volts peak to peak measured stator to stator is 571 volts rms measured stator to stator.

electrostatic headphones are a balanced load and need to be measured as a balanced load.
It’s crazy that there are no different units to differentiate the different measurements. How can anyone compare any measurements when they have the same units but mean different things. How was the measurements on the topping done?
 

the_brunx

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1600 volts peak to peak measured stator to stator is 571 volts rms measured stator to stator.

electrostatic headphones are a balanced load and need to be measured as a balanced load.

400 volts rms measured stator to stator is 1120 volts peak to peak measured stator to stator. typical of +325v/-325v power supplies.
I have never seen a balanced headphone amp for dynamic headphones ever measured in this way btw.

And even STAX themselves don’t do this.
 
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Aokman

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It’s crazy that there are no different units to differentiate the different measurements. How can anyone compare any measurements when they have the same units but mean different things. How was the measurements on the topping done?
It’s all a clusterf to be honest, especially as measuring such audio performance at such voltages is tricky / no industry standard of measurement… not just about voltages but the loads being driven. Because there is barely any current available on direct drive, using a demanding load will pull down the output voltage dramatically. When I benchmarked the SRM-1 vs the Lundahls or even an SRD-7, that became the most obvious thing and I had to settle on a 120pf load only otherwise transformer based setups with a very capable power amp behind them just wiped the floor with my STAX amp…
 

etc6849

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Not slimy at all IMHO. Any technician or engineer understands rms vs peak to peak vs peak, and it clearly states peak to peak and not RMS. The designer also posts the circuit and many other details here:
https://headwizememorial.wordpress....tatic-amplifier-for-stax-omega-ii-headphones/

The version I have is from headamp, I didn't buy it new... When you get it in great condition for 3x what just the potentiometer costs: it isn't a bad deal and I'm sure I can sell it for a profit tbh.
https://www.headamp.com/products/blue-hawaii-se

I totally understand how this is confusing to normal people though... The BHSE can actually do what it says over the audible frequencies in my simple measurements (can't measure THD), while the EHA5 cannot according to what's been posted here by others. Same goes for the iFi Pro iESL, although they don't post as detailed specs as Topping does lol.

It is strange that the EHA5 advertises 700Vrms, then goes into protection at 229Vrms at 20Hz, even though it claims Frequency response: 20Hz-40kHz (±2.5dB)!?!
-2.5dB should mean voltage is still about 75% of the voltage rating, but it clearly isn't, so what gives!?!
EHA5 Specs.PNG


However, I'm not saying the EHA5 isn't an outstanding value and likely better than anything you can get at that price point on the new or even used market. It obviously is worth trying out if you don't want to drop serious money in the hobby.

Another great alternative is to build your own transformer setup like this guy did, something I'm seriously considering doing as well:

But it seems A bit slimy to use peak to peak instead of the standard RMS
 
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kevin gilmore

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at no audio frequency can the eha5 generate 700vrms without the amp going into shutdown mode. bob katz and others have tested this.
the nfca modules are limited to about 4 watts of power output, and that is nowhere near enough to drive those transformers.

a pair of edcor transformers and a bias supply ends up about $450 in parts. really excellent for the price. you still need a good 60 watt power amp.
 

Aokman

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at no audio frequency can the eha5 generate 700vrms without the amp going into shutdown mode. bob katz and others have tested this.
the nfca modules are limited to about 4 watts of power output, and that is nowhere near enough to drive those transformers.

a pair of edcor transformers and a bias supply ends up about $450 in parts. really excellent for the price. you still need a good 60 watt power amp.
Fair enough, thought I saw someone who managed hit it but I didn’t look closely so probably wrong :)

Those Edcor transformers really hurt me to send to Australia, cost me as much in shipping as the transformers did :facepalm:
 
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