Stax SRM Monitor
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SRM Monitor - Overview
The SRM-Monitor is a combination of Stax' solid-state amplifier, the SRM-1 Mk2 Professional and their diffuse field equalizer, the ED-1, in one larger package. The SRM-Monitor came to the market in 1988 and was still available in 1995, according to the Stax 35th Anniversary product brochure. The volume control of the SRM-1 Mk2 appeared to have been changed to a dB calibrated numbered type with indents, for repeatable and measurable volume variations in a professional situation, but internally the same blue ALPS RK-27 dual channel potentiometer has been used. Most of these units appear to have been sold to audiometry workplaces, as they are quite often referred to in research papers, and rarely mentioned elsewhere.
Headphone frequency response measurements, conducted with a microphone in front of the headphone driver, much as speaker measurements were conducted, observed that headphones with a measured flat frequency response did not sound as flat as would be expected from a speaker with identical measurements. Comparing the sound of a flat measured speaker with a flat measured headphone revealed extreme tonal differences that started off a whole lot of investigation into why they sounded different and how a headphone’s frequency response could be altered to make it sound like flat measured speakers. An experiment was set up as follows:
1. A loudspeaker playing a frequency sweep was recorded by a high quality, miniature microphone in one of two types of room – either an anechoic chamber or an approximation of an ordinary room (see later) – and the frequency response was charted.
2. The same microphone was inserted into a subject’s ear canal and the same speaker replayed frequency sweep was charted again.
3. It turned out there was quite a difference between the charted frequency response of the two recordings.
4. It was postulated that if the frequency response of the recording made by the microphone in the ear (see 2) could be altered by pre-equalization to ultimately match the shape of the frequency response of the recording of the same microphone when not in the ear (see 1), then replay of that in-ear microphone recording would sound the same as that of replay of the recording made by the same microphone in a room if that pre-equalization was applied.
5. This gave rise to a target measured frequency response for a headphone to sound like a flat measured speaker i.e. if the headphone had a measured frequency response that looked like the target response, it should sound flat when reproducing a recording that had been mixed with speakers in front of the mixer, and sound as if one was listening to speakers in front of him/her, rather than via headphones.
6. The concept of pre-equalization of headphones was thus born.
Pre-equalization could either be mechanical (i.e. the driver frequency response was manufactured to behave that way e.g. the AKG 240DF – not so easy) or electrical (which should be cheaper, easier and field-adjustable), and meant that although the headphones now had a frequency response that had been altered to something that looked decidedly non-flat when measured, it reproduced the sounds coming from a sound source with the same frequency response at the ear canal as if recording and replay over headphones had not been introduced into the chain i.e. the headphone replay should now sound the same as sitting in the room and listening to the speakers. Two main theories of the correct pre-equalization curve were forwarded. The first, called free-field equalization, suggested that the above experiment be conducted in an anechoic chamber (like a field, free of reflective, absorptive and refractory surfaces). So, to reiterate, a free field equalized headphone is designed to sound like the reproduction of speakers, as if a listener is sitting in an anechoic chamber. Although an anechoic chamber is more reproducible as a standard, it was argued that nobody listens in an anechoic chamber (and indeed, most listeners find even speaking in an anechoic chamber uncomfortable) and a reasonable approximation of a standard listening area be used to conduct the above experiments. This was called diffuse field equalization. There are many things that alter sound between the release from the sound source and arrival at the ear canal. Reflections, diffraction and absorption from objects in the listening environment, reflection, diffraction and absorption by the head, hair and ears all contribute to alteration of sound before it reaches the ear canal. Diffuse field equalization, as mentioned before, is an attempt to make the replay of a recording on headphones sound like you are listening to the same recording through speakers in a non-anechoic room. Experiments were also done so that headphone users were asked to equalize various sharply limited frequency bands’ playback on headphones until they had matched the loudness of the same playback through speakers and with headphones removed. A good correlation was obtained between this method and the probe microphone recording method. The direction of sound (from the front in a reverberant field) with speakers is far removed from actually injecting the sound directly into the ear canal. Naotake Hayashi (of Stax), pondering this problem, possibly because Stax couldn’t successfully mechanically create a diffuse field equalized headphone, and any electrical equalizer would have to be a custom unit, first decided to create a new headphone that coupled its own reproducible miniature room (complete with uneven diffractive, reflective and absorptive surfaces) called the Stax SR-Sigma Panoramic Earspeaker System. It had headphone drivers that fired from anterior to posterior instead of laterally into the ear canals. The sound was bounced off irregular mineral wool into the ear canals, creating a mechanical diffuse field room for each ear as well as having "speakers" that fired sound from the front, rather than straight into the canals. It was partially successful, but listeners either hate it or absolutely love it. Personally I love it, but they were inefficient headphones and they sounded quite rolled off at both ends of the frequency spectrum. They were also huge and very odd looking. Better drivers than the original Sigma drivers (which were the same as the later Lambda non-professional earspeakers) improve the frequency extremes, allowing the merit of the theory to finally shine through (e.g. the very rare Sigma/404 hybrid). Stax later decided (around 1986 – 1987), instead, to bite the bullet and build 4 custom equalizers to electrically equalize their latest headphone range to provide individual target diffuse field responses for each of its various, then-current headphones (the ED-5 for the SR5 normal bias headphone, the ED-1 and SRM-Monitor for the Lambda Professional high bias phone and the ED-Signature for the high bias Lambda Signature). The headphones could then be less bulky than the Sigma and more fashionable (see my avatar for what the Sigma looked like – it definitely had a style only a mother could love). Again, reactions to Stax engineers’ diffuse field equalized headphones literally polarized listeners into “hate it” or “love it” camps. I would guess that economically, this proved to be a dead end, and no further research into diffuse field equalization has ever been mentioned by Stax. Consequently, the rather rare equalization units sell for a premium on the used market these days.
As mentioned above, the four ED diffuse field equalizers were designed for three different Stax phones. The ED-5, ED-1 and ED-Signature were placed between the source and the headphone driver and are connected by way of RCA cables. The ED-1 matched the construction and size of the SRM1 Mk2 and was finished, like those units, in either black or silver. The ED Signature matched the chocolate brown of SRM-T1/S/W. The SRM-Monitor incorporated an ED-1 and an SRM-1 Mk2 Professional into one large package and was finished in either black or silver, and had switchable RCA or XLR inputs. The ED-5 unit was made to match the then current SRD-6 transformer unit (an interface between a speaker-driving power amp and the Stax earspeakers) in silver. The ED-Signature would most likely also match the 404 and Lambda Nova Signature. The ED-1 equalization (in my case, provided by a very rare SRM Monitor) sounds excellent with the Lambda Nova Signature and surprisingly good with the Omega 2 Mk 1, despite being the wrong equalization for the latter. The upper midrange/lower treble, in particular, sounds quite a bit flatter and the low end remains in good balance with the mid and high. As Bill Sommerweck said in his review of the ED-1 in the April 1989 issue of Stereophile, track 9 on Stax' own “Space Sound” CD changes from objectionable (without the equalizer switched in) to quite listenable with the equalization switched in. In my opinion, there is no magical out of the head experience, except when listening to the aforementioned CD, or the Ultrasone binaural tracks (i.e. binaural recordings). These are seriously spooky, but sound 3 dimensional with or without equalisation. Try them with someone who is not used to listening to headphones and see what happens when you cue up track 1 or 2 of the former, or the fireworks track of the latter. Sabine whispering in your ear - oh yes! Shower spraying on your shower cap - OMG!
Now, here is where things start getting weird. I had a listen to the Sigma/404 and the SR-007 phones with the equalizer on and they both sound great - it may just be happenstance, but I've never heard "Kind Of Blue" sound so wonderful and with plenty of lower bass (which even the SRM-717 doesn't seem to match). This is strange, as the above frequency response for the ED-1 is flat in the bass, and should not be suited to the Sigma/404 or SR-007 anyway.
Stax ED-1 diffuse field equalizer, frequency response with EQ out (top) and in (bottom) 5dB per vertical division. Maybe it's a de-emphasized treble spike? I don't know, but whatever, this pre-equalization is not just scientific theory and sounds really good. To me, the sound has gone from lots of good hi-fi parts and moved to an organic whole. To my ears, the Stax SRM-Monitor is the single best piece of equipment I have ever purchased.
Inputs consisted of:
1. 1x Unbalanced line level RCA input
2. 1x Balanced line level XLR input
Outputs consisted of:
1. 1x Unbalanced RCA loop output (with the Diffuse field equalizer output being on or off switchable)
2. 2x Pro bias headphone jacks (580V)
3. 1x Normal bias headphone jack (230V)
General Description of the Sound
The SRM-Monitor, without the equalizer engaged, is a very good pure A class FET DC amplifier with clear dynamic sound and excellent low level detail resolution (indeed, under these circumstances it is almost the same as the SRM1 Mk 2 Professional, apart from the added balanced input which appears to make improvements to the sound, smoothing the slightly spiky sound of the SRM 1 Mk2 Pro). Compared to new Stax amplifiers, such as SRM-717, the sound is slightly less refined and on the dry side, but still very impressive. Stax tube amplifiers such as SRM-T1, SRM-007t etc might offer a little bit more refinement and are more fluid in their presentation as well but are not as powerful. In my opinion, the SRM-Monitor is superior to almost all new cheaper Stax amplifiers (SRM-323, 303 and down).
However, when the Diffuse Field Equaliser is switched in, this amplifier becomes magic. A reduction in any residual Lambda series tizziness in the upper midrange and a slight filling in of the Lambda midrange trough allows the Lambda to sound much flatter while retaining the power it is capable of. The vocal reproduction is much improved overall, coming closer to the nearly perfect Sigma series in that department. The bass quality is also improved, which is odd, as the equaliser appears to leave the bass frequencies unchanged.
The SRM-Monitor also sounds great with the SR-007 Mk1, especially with the Diffuse Field Equaliser switched in, despite the equalizer being specifically adjusted for the Stax Lambda Pro. There is no hiss or hum at any setting of the volume control.
Pricing & Other Data of Interest
Used prices at the moment vary from $1000 to $1500 depending on version and condition.
Frequency characteristics: DC~20KHz/±1dB
Input impedance: 50KΩ
Amplification factor: 60dB
Distortion factor: 0.02%/1KHz/100V with Lambda Pro
Input level: 100mV
Bias voltage 230V (normal bias) 580V (professional bias)
Electric power consumption: 47W
Size (W)300mm × (H)87mm × (D)314mm
Start of production: 1988
--John Buchanan 21:59, 21 October 2009 (EDT)