Zahl EQ1 Review & Measurements

Introduction

The Zahl EQ1 is an analog parametric EQ device originally designed for professional use in API500 series racks, but might just be the best thing I’ve added to my headphone setup!

Design

The EQ1 is a dual-width API500 series module, which means it would usually need to be installed in an API500 series rack. However, Zahl has also released the Rack 500 Dual; a dual-width chassis and power supply that provides a clean, ultra low profile chassis for either two single width units or a single dual-width unit such as the EQ1 or IM1 from Zahl themselves, as well as a separate linear power supply and XLR input/output connectivity, turning any API500 module into a standalone device.

This means that the EQ1 with the rack500 can sit very comfortably in even a compact desk or rack setup, plus both the features and the level of transparency offered by the EQ1 make it a compelling option for headphone enthusiasts seeking an analog EQ that doesn’t compromise on distortion performance or take up more space than their amplifier itself.

The rack500 has two rear connection configurations, one of which offers a pair of XLR in and XLR out ports as well as 48V phantom power supply if wanting to use it for a microphone instead of a DAC or other source device. The other offers a pair of send and receive mid/side ports in addition to the full XLR in/out, though for the use case of a headphone setup, it shouldn’t matter which option is chosen as only the XLR in/out ports will be used.

The power supply is kept in a separate silver module, allowing it to be positioned away from the EQ1 itself, which is helpful both for ensuring no interference from the transformer is picked up by nearby devices, and also for keeping the footprint small so that the EQ1 can be placed next to the user’s amplifier and easily within reach.

So why am I so excited about this product? Well there are two key reasons, the first being that even though there are plenty of excellent digital tools available for doing EQ including player software such as Roon or standalone tools such as EQ-APO, these all require a non-continuous adjustment; you have to make a change, wait for it to apply, then make another change. This means “dialling in” can be a little more work than is perhaps ideal.

If you’ve ever done work with photo editing or video color-grading, you’ll know that just typing in a number and hitting apply is far less of an ideal solution than being able to continually adjust a slider to find and dial in exactly where the perfect adjustment is, and the same goes for EQ.

There is simply something incredibly powerful about being able to adjust the parameters of your EQ and hear the change in real-time, it makes it much faster to find exactly the right spot and with this I’ve been able to rapidly get to where my ideal adjustment for a headphone is whereas with digital tools there is far more trial and error involved even when you have measurements to use as a guideline. This is a huge improvement to “workflow” in creating an EQ profile and as a result I’ve also found myself eager to use EQ far more frequently than before, simply because the time taken to dial in my sound has been cut down drastically.

But there ARE other analog EQ tools available, so why this one in particular? Well….therein comes the second reason for my excitement; this is in my opinion quite simply a BETTER analog EQ than the competition, especially for this use case.

In the headphone space we have options such as the Schiit Loki series, which provides four-band or six-band adjustment, but only the gain levels can be changed. You cannot adjust the frequency or Q-factor of the bands. If you wish to do that, you have to look to the bulkier and far more expensive professional mastering equalizer options.

Even if you have thousands to spend on an EQ though, I would STILL take the EQ1 over an SPL PQ unit, as not only is this a far more compact device than the likes of an SPL PQ or a full API500 rack, it’s also simply more transparent! Read on to the measurements section for details on that.

EQ Adjustments

The EQ1 has four fully configurable EQ bands, with the first and last also being able to switch between acting as a shelf or peak filter. Each band has a continuous adjustment that can be set to either a +/-15dB range, or +/-5dB for more precise adjustments. The EQ1 also provides an adjustable high-pass filter and preamplification/gain adjustment, though these two features are not all that beneficial for most headphone setups where a high-pass is not needed and line level from your DAC to your amp is unlikely to need much attenuation or additional gain. Still, always nice to have the extras.

Band 1 - High Frequency Peak/Shelf Filter:

The first band is the high frequency adjustment, which can be toggled between acting as a peak or a shelf, though the Q-factor is fixed.

The graphs below show the range of adjustments possible for the shelf filter option:

Quite a wide range of adjustment is possible here, able to be set to either only adjust the highest frequencies, or adjusted far lower to sit right in the middle of the frequency response. It’s worth noting that the frequency indicator on the knob doesn’t indicate the corner frequency/midpoint of the shelf but instead the point at which the shelf approaches its final adjustment.

Ie: set it to 10kHz, and the shelf will start rising around 2kHz and reach the adjustment set on the level knob at 10kHz.

Personally, given as I am also using this in conjunction with the HM1 which already has bass/treble shelf filters built in, I tended to leave this band set as a peak filter, which has a Q-Value of about 1.4 or a one-octave bandwidth. Using this with the HM1 means you now in-effect have a 6-band EQ, but I’ve also been loving this with other devices and it absolutely does not need to run specifically with the HM1 amplifier.

Band 2 - High Frequency Peak Filter:

Band 2 is a fully parametric peak filter, with the user able to adjust the frequency, Q-factor, and gain level. This gives you all the same control as you’d have in a digital EQ. Q-factor can be adjusted between 0.5 and 6, giving a very wide range of possible adjustments and allowing you to use this either as more of a broad tone control or tackle a narrow peak/dip.

It seemed that the indicators on the unit were extremely accurate to the values obtained when measuring.

Here for instance is the corresponding response with the controls set as shown in the photo:

Frequency is bang-on 2.2khz as the indicator suggests, and the gain adjustment is within 0.1dB. This means you can confidently translate results from this unit to a digital adjustment and vice versa, the limit to precision will simply be your own hands on the controls, not any form of guesswork as to how close the indicated figures might be.

The frequency knob allows adjustment between 600Hz and 6kHz, but when the “f*3” button is enabled, all of these values are multiplied 3x, giving this band an effective range of 600Hz to 18kHz.

Band 3 - Low Frequency Peak Filter:

Band 3 is another full parametric peak filter with full frequency, gain and Q-factor control. In this band the frequency range is indicated as 200Hz to 2kHz, but similar to band 2, it has an “f/5” button which divides the indicated frequency by 5, giving this band an effective control range of 40Hz to 2kHz.

Testing for parameter accuracy once again, we see that it is also extremely accurate, with the adjustment measured as matching pretty much exactly the 1khz +4dB intention from the indicator text:

Band 4 - Low Frequency Peak/Shelf Filter:

Finally for band 4 we have another filter that can be switched between a peak or shelf. The image below shows the range of possible adjustments for the shelf filter:

I’d perhaps like if this could go just a tad lower in frequency so that the adjustment when desired could be contained almost entirely to <100hz content, but in practice I’ve not found many situations where this filter wasn’t able to give me what I wanted. Even in the case of a headphone that needed only a subbass lift but already had prominent midbass, it was easy to combine this shelf filter with an adjustment to take down the 100-200hz midbass on band 3.

Once again the adjustment was precise, being maybe 0.1dB off which is basically just tolerance as to how precisely you can turn the knob itself:

If wanting to really get into the nitty gritty with EQ, some users will of course want to make complex adjustments with dozens of EQ bands, but for the vast majority of users, and if you have headphones that are halfway decent in the first place, four bands (or six if using this alongside an HM1) allows you to achieve extremely flexible adjustments, and being able to do everything with continuous audible response rather than the stop-and-start approach of most digital EQs is an absolute godsend, not only speeding up the process of dialling in your EQ but quite simply just making it more enjoyable.

Band 5 - High-Pass Filter:

This particular feature is unlikely to be all that useful for headphone EQ specifically, but if you are using the EQ1 for a microphone correction it could be, so it’s worth testing as well.

Setting the cutoff frequency to 100hz, we see the corner frequency (-3dB point) is sitting at exactly 100hz, meaning again this control is extremely precise and you can be confident in the adjustment being made.

I/O Section:

At the very bottom of the unit is the IO control section. There isn’t too much to talk about in terms of ‘performance’ here, but it’s worth noting the features offered.

The bypass button allows you to quickly switch all of the EQ1’s adjustments on or off, allowing for quick AB comparisons between stock sound and your current EQ.

The EQ and IO gain buttons allow you to toggle the adjustment ranges between +/-15dB for more significant alterations, or +/-5dB for more precise EQ and gain control.

The I/O gain can be switched between being before or after the EQ circuitry itself, allowing you to optimise signal level and SNR depending on what exactly you are doing, but I mostly left this in I/P or input gain during my time with it as I’m not feeding the device exceptionally low level or high level signals that could cause clipping or noise concerns. Given the performance of the EQ1 as you’ll see below though, it’s unlikely that noise is going to be a concern, and clipping the EQ1 with its

Performance

Test Setup

• Audio Precision APx555 B-Series analyzer
• Measurement setup and device under test are running on regulated 230V power from a Furman SPR-16-Ei and an AudioQuest Niagara 3000 Power Conditioner
• EQ1 was warmed up for 12 hours prior to testing
• Exact analyzer/filter configurations for each measurement are detailed in the full reports
• CH1 (Blue) = Left, CH2 (Red) = Right

THD+N / SINAD

1khz Sine, 4V input, 4V output (Bypass):

In bypass mode, the EQ1 behaves like a cable, there’s no active circuitry going on, and no added noise or distortion. But for an EQ device that’s pretty common, the hard part is keeping good performance when you’re actually doing an adjustment, so let’s test that!

1khz Sine, 4V input, 4V output (All bands enabled and adjusting):

I switched all the bands on the EQ1 on and made a random and significant adjustment. Now we have -104dB THD+N, which for an analog parametric EQ with all bands making varied adjustments is exceptional. To put this into context, the EQ1 even with ALL bands adjusting, is achieving 0.0006% THD+N, meanwhile, the $6000 SPL PQ mastering equalizer at a similar signal level only gets 0.002% and that is likely with all EQ adjustments set flat, over three times higher distortion. The $10,000 crane song IBIS claims 0.001% with all EQ adjustments flat.

The EQ1 is outperforming top tier mastering equalizers, and in my brief looking around, I was not able to find another analog EQ device that has lower noise and distortion than this.

It’s also a vast improvement from many of the other options in the head-fi space, with the Schiit Lokius for example getting 0.002% THD+N when all EQ bands are set to flat, and falling to 0.01% when one band is set close to max.

Schiit Lokius Measurement taken by Amir at AudioScienceReview

Michael Zahl is known for a few key design choices, namely complete avoidance of any digital or switching circuitry, instead having absolutely everything including logic circuits be entirely analog and linear, and for careful choice and binning of components. Clearly his approach is showing merit here, as the performance and level of transparency offered by the EQ1 is absolutely exceptional. Even if you needed more bands, it would be more compact, cheaper, and likely still more objectively transparent to simply chain two EQ1 units together in series than to “upgrade” to a far more expensive single product with more bands.

THIS is why I’m excited about the EQ1, not only is it a fantastic product from a functionality perspective, but when talking about quality, this analog EQ might just be the most transparent option available.

Summary

The Zahl EQ1 may be a ‘professional’ tool, but as a headphone enthusiast, it’s the best thing I’ve added to my setup in years. The ability to made EQ adjustments on the fly with full parametric flexibility has not only allowed me to EQ better than I was usually able to with stop-and-start digital EQ tools, but has also made the process itself more enjoyable and had me playing with EQ on my headphones far more frequently as a result. EQ is the single biggest improvement you can make to most headphones, and the EQ1 is a device that makes doing so easy, enjoyable, and has me confident that I’m not giving up on transparency by doing so. The fact that it’s compact enough to sit comfortably beside my amp is just the cherry on top.

If you are the kind of person that has spent quite a lot on your setup and are seeking the next big upgrade, I can wholeheartedly recommend looking at the EQ1 with the rack500 dual.