I was curious about my Corda Soul, so I measured a few things. My measurement setup is pretty basic, which limits what I can measure.

Setup

This PC has a Juli@ XTE sound card. It’s a great sound card, but it’s not professional test equipment. But it does have balanced inputs & outputs. So here’s the setup:

Source: PC playing test signals through USB output
Test Device: Corda Soul, USB input, Analog output (balanced XLR)
Measurement: PC sound card, Analog input (balanced TRS)

Update: I also used Tascam SS-R1 and DA-3000 recorders to explore distortion & noise, see below.

Baseline Loopback

I made these measurements with my sound card, so its performance is the baseline. To measure that, I used TRS cables to connect its balanced outputs directly to its inputs to measure its loopback performance.

As you can see below, the Juli@ measures quite well for a sound card. It should be audibly transparent.

Loopback Frequency Response

At both sampling frequencies, frequency response is flat with less than 0.1 dB variation through the audible spectrum. Phase response and group delay are equally flat.

Loopback Distortion

The Juli@’s distortion was the same at 44.1 and 96 kHz sampling. So I’ll show the graph for 96, measured with a -1 dB digital signal:

We can see 60 Hz power at -86 dB and its harmonics nearly as strong. Overall, this is good performance for a sound card (especially one nearly 10 years old) and should be audibly transparent. The baseline now completed, let’s look at the Corda Soul.

Frequency Response

I expected to see perfectly flat response. It was nearly so but not quite. At 96 kHz sampling with the filter in the “sharp” position, the Soul is entirely ripple-free, yet shows slow rolloff, down 0.5 dB at 20 kHz. These variations are inaudible, but worth exploring.

I measured the Soul’s frequency response at different volume settings. Why? Because it has 2 unique design features that might make its response vary with volume.

1. Its unique volume control
2. Its frequency-shaped gain-feedback

The Soul has a uniquely designed volume control. Instead of attenuating a fixed gain ratio like most preamps do, it changes the gain ratio. It has 64 discrete positions, each applying different resistors in the gain-feedback loop. As you reduce volume from full, it has less gain and more negative feedback. Theoretically, this gives lower noise and distortion, and wider bandwidth, which could impact frequency response.

The Soul’s frequency-shaped gain feedback means it digitally attenuates low frequencies before DA conversion, then it boosts them back to normal level in the final analog stage (after DA conversion and analog gain/volume control). These shaped curves are applied in separate steps, one digitally, one analog, so any imperfections in the matching of these curves should appear as variations in frequency response.

To see if the above features had any measurable impact, I tested frequency response at different volume settings:

The grey line is the sound card, for reference. I made all lines equal at about 600 Hz, which is the perceptual midrange. Note the Y scale is only 1/2 dB per division to exaggerate the differences. At lower volumes the Soul has a small lift in the bass and the treble. This is only 1 or 2 tenths of a dB, so it is inaudible. Also, it has a gradual roll-off in the treble that is down from 0.2 to 0.5 dB at 20 kHz, also inaudible. Note also that the Soul’s frequency response is perfectly smooth, free from the Juli@’s ripples.

At higher sampling rates (48, 88, 96 and 192), the Soul applies a slow rolloff that starts just above 20 kHz and it fully attenuates before Nyquist. This wide, gradual transition band provides optimally smooth amplitude and phase response in the passband.

Note: when using an external DAC, the Soul's frequency response depends on the DAC.

Below, frequency response at half volume at sample rates 44, 48, 88, 96 and 192. It’s essentially ruler flat so I zoomed the Y scale to 0.1 dB per division to see the differences. The 0.2 dB LF attenuation at 192k is the Juli@ sound card, not the Soul.

Now the same, but from 10 kHz on up. They’re identical up to 20 kHz. Each sample rate is free of ripples and uses the full available transition band to make the smoothest, gentlest attenuation – while still providing full attenuation before Nyquist.

The Soul’s digital filter has a switch to select linear phase or minimum phase. More on that subject here. Summarized in chart form:

Rate	Filter	20k (max; half)	-1 dB Fr	-3 dB Fr	-3 dB %Fs
44.1	lin	-0.5; -0.2	20,960	21,350	0.484
44.1	min	-4.4; -4.1	19,500	19,850	0.450
48	lin	-0.5; -0.2	22,750	23,250	0.484
48	min	-0.5; -0.2	21,200	21,620	0.450
88.2	lin	-0.5; -0.2	25,350	28,350	0.322
88.2	min	-0.5; -0.2	25,810	28,650	0.325
96	lin	-0.5; -0.2	27,350	30,700	0.320
96	min	-0.5; -0.2	27,850	31,050	0.324
192	lin	-0.5; -0.2	36,000	45,800	0.238

Note: the Soul’s output is non-inverting, so readers with EE knowledge may wonder: if the Soul’s volume knob changes the gain, how can it have less than unity gain? The Soul uses an inverting topology in the gain-feedback loop, so gain is simply Rf/Rin and can be less than unity. Its final fixed gain stage is also inverting, so it does not invert overall.

Noise & Distortion

The Soul’s distortion profile is rather unusual, so I spent hours digging into it in detail (link for those interested). Here’s the summary:

At max digital level, using a frequency sweep, THD measures -70 dBr (surprisingly high) and drops with the signal digital level, sinking into the noise floor of the Tascam DA3000 recorder by -12 dB. The slope is about 2:1 – starting at full scale, every 1 dB reduction in signal level gives roughly a 2 dB improvement in THD.

The Soul has significantly lower distortion with multi-tone signals that resemble music. Here is the multi-tone at 0, -6 and -12 dB.

The peaks of the grass are around -90, -100 and -110 dB respectively. It goes from mediocre at 0 dB, to exemplary at -12 dB.

Headphone Notch Filters

Many headphones have a resonance causing a bump in frequency response between 6 and 12 kHz. The Soul has a notch filter to correct this. The manual says it ranges from 6 to 11 kHz, each is -6 dB, Q=2.0. Specifically, the frequencies should be spaced 6.3% apart which is 1/11 of an octave, or slightly further apart than a musical half-step.

Here’s how they measured. The grey line is the frequency response with all controls disabled.

Here’s a closer in look:

Each measures spot-on to what the Soul’s manual says: in frequency, amplitude and width. For example, with the Sennheiser HD-800 you’d set this to 6 kHz, the lowest setting, to dampen its 6 kHz resonance spike.

Tone Controls

The Soul has 4 tone controls. Meier customized mine to be equally spaced in octaves. That is, the corner frequencies should be 80, 320, 1,250 and 5,000 Hz. All 4 are shelf controls; the bottom two are low pass, the top two are high pass. Each control has 5 clicks up and 5 clicks down, each click should be 0.8 dB. I measured each at click positions -5, -3, +3, and +5.

Note: I measured these with a digital frequency sweep at full scale / 0 dB. This should cause digital clipping when the tone controls are set in the positive range. But due to Meier’s “FF” or frequency shaped feedback, the lower frequency controls don’t clip. That is, “FF” is reducing low frequencies more than 4 dB, which is the tone control range. More on this later.

In each of the following graphs, the vertical marker is at the corner frequency.

Knob 1, low bass.

Knob 2, mid bass

Knob 3, mid treble

Aha! In the above we finally see clipping, so we get some idea of the shape of the FF response curve. To compensate, I lowered the frequency sweep to -6 dB:

Knob 4, high treble

You can see that the lowest position attenuates a lot more. Let’s zoom out a bit to see the full curve:

What we see here is that the lowest position on knob 4 triggers the CD redbook de-emphasis curve, which is a gradual cut that starts at 1 kHz and becomes -10 dB at 20 kHz. This feature was rarely used, but if you have any old CDs using it, and they sound too bright, it means your playback equipment failed to detect it. The Soul enables you to apply the proper de-emphasis manually.

Here are all the tone control knobs seen at once

You can see they are spaced symmetrically. Also, their combined effects are cumulative, which enables a lot of flexibility when setting them. Because they are shelf controls, you won’t get amplitude ripples when combining them.

Crossfeed

The Soul has DSP to narrow (for headphones), and widen (for speakers), the stereo image. This is a common feature for headphone amps, having several different implementations. Meier’s is one of the best: it reduces the “blobs in my head” effect that headphones can have, especially with recordings that have instruments hard-panned fully L or R. And it does this without any perceptual sonic side effects like changes to frequency response, which is what sets it apart from others.

I measured the Soul’s frequency response in all 10 modes (5 narrow, 5 wide), plus its frequency response with all DSP disabled. As you can see below, all 11 curves are exactly the same, even with the Y scale zoomed in to 0.1 dB per division.

For example, the crossfeed in the Headroom amps from 15-20 years ago attenuated mids & treble due to comb filter effects from their inter-channel time delay. These amps had a gentle high pass filter to compensate for this. Meier’s crossfeed is free of these effects.

This doesn’t necessarily mean the crossfeed will be perceptually transparent. Measuring the same doesn’t imply that it sounds the same, because crossfeed is mixing some L into R and vice versa, with time shifts. Percepetually, this may make it sound like the FR has changed, to some people.

Meier FF

The Corda Soul uses Meier’s Frequency Adaptive Feedback. I’ve written about this here and here. Essentially, it shapes the frequency response to attenuate low frequencies in order to “unload” the digital and analog stages of the DAC and preamp, and brings the bass level back to normal for the final output stage, so the overall frequency response remains flat. This improves the midrange & treble where our hearing perception is most sensitive.

The theory behind it seems consistent with some practical observations:

• Analog circuits (opamps & transistors) have constant gain*bandwidth, which means negative feedback gives more “correction” in the bass than in the treble. This is one reason devices often show slightly rising distortion in the treble.
• We sometimes see modern DACs measuring slightly higher distortion near digital full scale. Their best/lowest distortion is typically seen anywhere from -3 to -12 dB.
• The Soul has lower measured distortion with multi-tone, than with single tone signals. This is likely a consequence of FF.

Customizations

Meier customized my Soul’s firmware for me. These changes are:

• Auto-Mute: the Soul auto-mutes whenever the digital input signal drops below a threshold for more than a brief time. This prevents the outputs from carrying a DC offset. The threshold is just above digital zero, so digital dither won’t prevent auto-mute from triggering. Auto-Mute is a standard Soul feature, not something Meier did just for me. But he did change its behavior a little:
  • Extend the auto-mute delay
    • The original delay at 44 kHz was only a couple of seconds. This caused the Soul to auto-mute, then turn back on, on some CDs that had between-track silence. When doing this, the Soul emitted an audible “click”. The delay is in total digital samples, so it’s shorter at higher sampling rates.
    • The new delay is about 20 seconds at 44 kHz, so this never happens anymore.
  • Disable auto-mute entirely
    • The Soul has a 3-way gain switch: high, medium, low. It’s implemented digitally, so high gain can clip loud signals. I never used the high position, so on my Soul, this switch position disables auto-mute entirely (mine has no high gain mode).
    • The medium and low settings are unchanged.
  • Silent auto-mute
    • The Soul emitted an audible click when auto-mute triggered. Meier changed my firmware so this does not happen; the auto-mute is completely silent in both ways, coming on and off.
• Tone control changes
  • Space the corner frequencies at equal octave intervals (80, 320, 1250, 5000).

After doing these customizations, Meier sent me the firmware code so I can keep a backup copy, in case my Soul ever needs maintenance. From this code, I have the actual frequency response curve he uses for FF. Meier asked to keep this confidential, so I do not publish it here. Suffice to say, like the rest of the measurements above, the implementation is exactly what he says it is.