EarMax Pro Amp Review - audioXpress
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Discovery

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The Sept. 2002 issue of audioXpress magazine has an excellent review of the EarMax Pro amplifier. It not only includes the usual listening tests, but also has a good description of the circuit topology, as well a number of independent measurements including bandwidth, power output, load and output impedance, etc. A detailed response to the review by the manufacturer is also present.

The do have a website (http://www.audioxpress.com), but it appears that the only reviews present on-line are greater than a year old.

If you are interested in this amp, this is a good resource to consider.
 
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Tomcat

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Great. Can you summarize the review and the comments by Stefan Brocksieper? Are the specs measured by audioXpress the same as the ones given by the Audiophile Club UK?
 
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Discovery

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The product review (construction, topology, and measurements) were performed by Charles Hansen. Unfortunately, this information doesn't lend itself well to a summary.

The listening test was conducted by Muse Kastanovich using Sony MDR-CD999 and Grado SR80 and SR225 phones. He concludes that "The EarMax Pro is an extraordinary sounding headphone amplifier." ... "It's not perfect, but it has no major sonic flaws, and I highly recommend it."

Stefan Brocksieper's response was included, as well as comments from Hart and Beth Huschens of Audio Advancements LLC.

Stefan's comments included minor circuit description corrections and addressed several of the measurements taken. Stefan's measurements for the bandwidth matched the measurements taken in the review very well. Possibilities for differences in both channel separation and output impedance measurements (between Stefan's and the reviewers) were addressed. Stefan spent three paragraphs explaining the maximum power output specification of 150mW and how it was set at this level to prevent hearing damage from excessive listening levels. Note that the reviewer arrived at the same conclusion at Stefan: this amp has plenty of power to drive a wide range of headphones. I am inferring that some have criticized this amp for apparent lack of power (going by specs or measurements and not actual listening tests). Overall, Stefan was impressed with both the measurement and listening tests.

The comments from the Huschens were in praise of the excellent review and they noted that they were proud to be representing this product.

Specs:
-The measured bandwidth matched manufacturer's rating well. It should be noted that Stefan notes that his measurements were taken at 600ohms (which is not stated on the link you provided). The reviewer measured bandwidth at both 35ohms and 1kohm.
-Output impedance was measured by the reviewer to be 85ohms at 1kHz
-Power outputs were considerably different when measured by the reviewer: .32mW @ 35ohms, 1.2mW @ 100ohms, 6.3mW @ 600ohms, and 7.1mW @ 1kohm (all at 1% THD + N). This is likely why Stefan spent so much time discussing output power.


Hope this helps.

edit: corrected spelling of names
 
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Discovery

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After posting, I see that my response may have been confusing on the issue of output power. Hansen theorized that the output power stated by Stefan may have been a mistake and that it should read 150mV instead of 150mW, as 100mV into the 32ohm Grados he was using was uncomfortably loud. Stefan stated that he will likely switch to giving an output power/THD diagram instead of a "mW into ohms" output power specification.

edit: spelling of names corrected
 
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Tomcat

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Thank you very much, Discovery! Those output power figures Charles Hansen measured are surprisingly low. I guess they would be higher if one disregarded the limit of 1% THD and noise. If the EMP's distortion spectrum contained primarily second order harmonic distortion, a much higher distortion figure would be tolerable. Second order harmonic distortion tends to be audible only well above 10%, some experiments say 14 or 15%. Has this been the reason why Brocksieper stated this very high figure of 150mW as the maximum output power: he didn't worry about the rising THD?
 
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Discovery

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Tomcat,

I am confused about where the original power output specification came from. Stefan indicates that it was a mistake. He seems to imply that it was quoted as a maximum limit (100mW for EarMax and 150mW for EarMax Pro) to say that no risk of hearing damage was possible. In reality, given how little power is really necessary to drive headphones, the actual figures are much lower. I am guessing that the figure wasn't originally measured as Stefan indicates it wasn't a critical parameter: "...the most unimportant spec of all, except for people who want to ruin their ears..." I am assuming that the original figure was a very conservative estimate and was not intended to indicate that power actually capable of being delivered, but rather a maximum limit that would never be exceeded. However, I could be wrong. Given that Hansen indicated that 100mV into the 32ohm Grados was uncomfortably loud, I don't think that 150mW would be a safe guarantee of no hearing damage.
 
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Tomcat

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Discovery,

You could help with another question. Many DIYers at Headwize (including Chu Moy) have speculated that the Earmax Pro employed some form of feedback to lower the output impedance to usable levels. This might be necessary because the Earmax is a single-ended design without an output transformer. Is this feedback theory true?
 
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Discovery,please post more about what they say about the topology and listening tests
 
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I like Grados (have the 325), but it is hard to believe that the Grado 225 could bring out the best in the EMP..... Isn't it odd that they didn't try RS-1 or Ety or Senn 600?
 
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Quote:

Originally posted by daycart1
I like Grados (have the 325), but it is hard to believe that the Grado 225 could bring out the best in the EMP..... Isn't it odd that they didn't try RS-1 or Ety or Senn 600?


Believe me, daycart1, while I use the 325s, HD 600s/Equinox, Beyer 931s, and K501s with my EMP, the 325s are amazing with this amp. I can't comment on the 225s, but hey, the 325/EMP combo kicks bigtime butt!

 
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daycart1

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Thanks, RickG, I'd love to hear the emp someday!
 
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Discovery

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Tomcat,

No schematic was given in the article. Hansen described the circuit based on his observations of the amplifier when disassembled. The summary I'll give below is based on Hansen's description and minor corrections given by Stefan. (A verbatim copy probably exceeds fair use). I have little knowledge of tube amplifier tologies so forgive any inaccuracies I may have inadvertently introduced.

Components: Tubes are one 12AT7/ECC81 dual triode and two 6DJ8/ECC88 dual triodes. The potentiometer is a dual 100kohm Alps Black Beauty. Capacitors are Wima film caps and Philips electrolytics. Resistors are 1% metal film types.

Signal path: The input signal is connected to the Alps pot. The wipers of the pot are AC coupled to the grids of the ECC81 to eliminate a potential noise source when moving the volume control once. Apparently, with tubes age and potentiometer wear it would otherwise be possible for the grid to induce a small voltage on the pot's resistor. The plate of the ECC81 is "coupled" (when I see coupled I think AC via a capacitor, but Stefan states elsewhere that the AC coupling of the pot's wiper is one of the limited places where caps should be in the signal path) to the ECC88's grid. The ECC88 is connected in a shunt regulated push-pull configuration. The output signal of this stage is AC coupled to the headphone output jack via a large value electrolytic with an output resistor load. No feedback is mentioned in either Hansen or Brocksieper's notes, unless it is implied by the SRPP configuration (I'm not familiar with it). So there are at least two and possibly three AC coupling junctions in this circuit.

Support circuitry: The three tube filaments are connected in series across the 19V AC power input supplied by the power supply. This 19VAC signal is doubled by another voltage (seems to be 19VAC 180degrees out of phase with the primary power also provided by the cable from the external supply) before it feeds a voltage doubler circuit . Note: it is unclear to me if the voltage across the tube heaters is 19VAC or 38VAC as Stefan corrected the author on the input voltage to the voltage doubler but I'm not clear if that applies to the voltage applied to the filaments, as well. The voltage doubler consists of diodes and large reservoir caps. This would result is a plate supply voltage of nearly 80VDC but this peak voltage isn't reached because of the tube currents and high impedance of the power supply. As a result, the plate voltage is reported to be less than 40V to increase tube life.

That's about all I could glean from the article on the amp configuration.

Rob N.,
That's all I can write now. I'll post more info on the listening tests a little later.

Regards...
 
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Tomcat

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Quote:

That's about all I could glean from the article on the amp configuration.


Discovery,
That's a lot! Once again: thank you very much!

As far as I know, there are SRPP designs without any feedback. And from your description, it seems quite certain that the EMP doesn't use global negative feedback. But I guess it's debatable whether any SRPP amp can be called truly single-ended.
 
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A couple of lazy questions:

I seem to recall a DIY Earmax(P?) clone at Headwize. Is any of this info available over there?

Wouldn't it be great if someone could dissect a Moretto HA3 and compare the innards to the EM(P)?
 
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Discovery

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Rob N,

My summary of the listening review of the EarMaxPro (EMP) conducted by Muse Kastanovich follows. I will refer you to the original article for exact details as many of the terms used in describing the sound are often subtely shaded and my comments may unintentionally give the wrong impression. I have quoted where needed to attempt to avoid distorting the results.

Sony MDR-CD999: With the EarMaxPro, these phones had a "mellow sound that gave voices and drums a nice genuine feel, but guitars and rasp were a little too soft. This particular combination had a wonderful presence and lively dynamics that kept the music very involving." The high-end of the frequency response was slightly attenuated, although the author's use of Silver Streak interconnects was reported to help this somewhat.

Grado SR80: Very detailed and well porportioned. The treble was slightly "too hot" when using the Silver Steaks, so the author recommended the use of copper interconnects, such as the Kimber PBJs that he used. "Stunningly good, almost a match made in heaven."

Grado SR225: Mostly general comments about these phones compared to the others. Reportedly "far more detailed and refined" than the Sonys. Similar in sound to the SR80s, but with smoother bass, as well as more precision, detail, and dynamics.

Although the Sonys were reported to have better frequency balance then the Grados, the Grados were reported to be better in every other area and, as a result, the rest of the tests were conducted solely with the Grados.

The EMP was then compared to several other amplifiers: Amphony model 1000 headphones with integrated head amp; NAD 1300 to provide a reference to a receiver or preamp; and Parasound Zone AMPlifier (or ZAMP).

Amphony: Author states that the ZAMP amplifier sounded better than you would think when judging it by its low $129 price. Still, the Earmax was superior in transparency, spaciousness, pace, rhythm, smoothness, and timber, and fidelity.

NAD: EarMax reported to better separate different instruments and provided a superior richness of sound. NAD mushed the instruments together and had overall graininess and thinness. The EMP was superior in every quality except for "perhaps the top octave, where the NAD had just a touch more of the pleasing shimmer."

Parasound: Dynamics were good, but not quite up to the standard set by the EMP. Frequency balance was reported to be nearly the same as the EMP. The Parasound had better bass response than the NAD and less graininess in the treble. But the NAD couldn't match the EMP in smoothness or transparency.

EarMaxPro summary: "overall balanced sound" with a "natural realism that allowed listening right down into the music." Though expensive, it is said to be "extraordinary sounding"

After this review, I am sorry I have not had the opportunity to hear this amp yet. I hope this information is useful to you.
 
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