CORDA SOUL
The mission of MEIER-AUDIO always has been to produce gear with an optimal price-performance-ratio. It never was the aim to make the best-of-all headphone amplifier but instead the intention was to produce the-best-you-can-get-for-your-money.

However, over the last decade the technical standards of the top-of-the-line headphones have improved considerably (and unfortunately prices have exploded accordingly) and with many technical ideas circulating in my head this seduced me to develop a best-I-can-make DAC/headamp combo without any limitations towards costs. I called it the SOUL.
THE CONCEPT:

The SOUL not only combines a DAC and a headphone amplifier into one device but also has a built-in DSP (Digital Signal Processor). Thus the digital input signals can be processed before they're converted into an analog signal by the DAC-section. The DSP operates with 64-bit precision and this allows to implement tone-controls, balance controls, crossfeed and a few other functions with a precision that never could be realized with any analog circuitry. Also important, the DSP makes it possible to shift the first stage of the FF-technique forward into the digital domain. Thus the DA-conversion stage as well as the analog stages of the DAC-section fully benefit from the FF-technique and this enhances sound quality considerably. Below please take a look at the design of front- and backside of the new amp.
THE REAR:

The rear of the SOUL is pretty straightforward. The DAC/amp has one USB-input (upto 24/192), three optical inputs, three coaxial inputs and one pair of balanced analog inputs. There also is an optical output that provides the processed digital signal so people can use the tone-controls and the crossfeed-controls of the SOUL but have the DA-conversion and amplification done elsewhere. The SOUL also has balanced volume-controlled outputs and can act as a preamp to drive active loudspeakers or a poweramplifier. A toggle switch allows to manually activate de-emphasis and to deactivate the left or right sound channels (to test the various settings of the crossfeed).
THE FRONT:

The front of the SOUL has a larger number of control elements which underpins the versatility of the amp.

The left-most dial controls crossfeed. In the upper (neutral) position the output is plain stereo. Turning the dial clockwise activates headphone crossfeed (5 levels), turning anti-clockwise activates loudspeaker crossfeed (5 levels). The latter widens the soundstage of loudspeakers and is especially usefull when loudspeakers are placed relatively close together.
The second dial from the left has three different functions:

- With loudspeaker crossfeed activated this dial allows to optimize the delay time between the direct signal and the crossfeed signal. The optimal delay time strongly depends on the positions (angles) of the loudspeakers and the size of the head of the listener.

- In stereo or in headphone crossfeed mode and with the switch to the right of the second dial in lower position the dial controls channel balance. People with bad hearing thus can compensate for different sensitivities of their ears.

- In stereo or headphone crossfeed mode and with the switch to the right in upper position the dial controls the central frequency of a 6dB, Q=2 notchfilter. Many (even high-end) headphones do have one or two distinct resonance frequencies in the 6..11 kHz range due to the entrapped air-volume between driver and ear. Proper application of the notch filter can lower the energy at the resonance frequency and our tests (using HD800, HE-500, T1, ...) have shown that this can result in a remarkable improvement of quality of sound.
To the right of the front four dials offer tone control.

LLF; de-/increase of frequencies below 170 Hz
LF; de-/increase of frequencies below 500 Hz
HF; de-/increase of frequencies above 1.5 kHz
HHF; de-/increase of frequencies above 4.5 kHz

All controls have Q=0.5 shelving, Stepsize 0.8 dB, range +/- 4 dB.

The effects of the separate controls are cumulative
A central dial selects the digital input source and there is a seperate switch to activate the analog inputs.

To the right of the input selection a switch allows to select between low and high gain. A three-color LED indicates the gain setting (green = low). At high volume settings the LED turns yellow/red. At low gain the signal of the optical SPDIF output is lowered by 6dB to prevent clipping that can be caused by the signal processing (tone controls, increase of the high-amplitude low frequency components).

The SOUL has a discrete volume control with 64 positions. Typical stepsize is around 0.7 dB. For visual control the ON-OFF LED of the amp changes it's color/intensity with each step.
SIGNAL-FLOW:

The SPDIF input signals are decoded and de-jittered by a WM8804 SPDIF-receiver-chip. The clock used is an extremely high-quality TCXO.

The digital audio signal is then fed into an ADAU1451 DSP for all processing concerning the FF-technique, crossfeed, channel balance, notchfilter, and tone-controls.

After processing the digital signal is converted to an analog signal using two WM8741 DAC-chips in dual-mono-mode.

Each analog signal (four signals in total as this is a balanced amp) is amplified/buffered by one single amplification stage, using an AD797 opamp (biased into class-A) that is buffered by two BUF634s (in high-current-mode). Volume is set by changing the feedback-resistance of the opamp. This results in a much lower background noise and much lower distortion then using a conventional approach with a potentiometer in front of an amplification stage with fixed amplification factor. This amp is "black", even with the most sensitive headphones.

The USB-input uses the CM6631 from C-Media to receive data in asynchronous mode from your PC. These are then send in SPDIF-format to the WM8804. The use of extremely high-quality TCXOs guarantee a very jitter-free operation.

The SOUL accepts PCM signals with samling frequencies of 32, 44.1, 48, 88.2, 96, and 192 kHz. Bit-resolutions upto 24-bit.

DSD-signals and 384 kHz signals can not be processed by the DSP and therefore are not accepted. I'm sorry for that!
POWER-SUPPLY:

The amp has three separate medical-grade switched powersupplies. One for the digital section and two for the positive and the negative supply lines of the analog amplification stage. The output of each supply is LC or RC filtered and further down-regulated with the use of analog voltage regulators. There are separate regulators for the opamps and the buffers. This, together with a total of around 70,000 uF (!!!) buffer-capacity results in extremely stable supply voltages. Sound-critical capacitors are Nichicon Fine Gold.

The switched powersupplies work with a typical switching frequency of 70 kHz. This is far beyond the limit of human hearing and thus any noise induced by the supplies is inaudible. There is no humm or any noise audible produced by the powersupplies.


DURABILITY:

Great care has been taken in the choice of the components used. Neutrik XLR-sockets, an ALPS potentiometer (although it does not carry an audio-signal) for smooth operation, LORLIN siver-plated rotary switches, sealed relais, etc.... all contribute to a high life-time-expectancy.

Although no decision has been made on the final enclosure yet, the intend is to have it completely made by sturdy aluminium (non magnetic, high heat conductivity).
PROTOTYPES:

Fully functional prototypes have been finished and were used for fine-tuning of sound and functionality. To be honest, they sound superb!

These prototypes have been presented on the recent CanJam Europe event in Berlin and were received extremely well by all who gave them a listen. By Headfonia the SOUL was elected "best amp of the show".

ANTICIPATED PRODUCTION:

The fine-tuning of the prototypes has led to quite a number of (small) modifications that require a redesign of the PCB. When this redesign is finished two new prototypes will be produced, just to make sure everything works as planned (be aware that this is a rather complex design!).

If all goes well, the amp should be ready for production in the spring of 2018. It will be produced by the German company LAKE PEOPLE.

PRICES and SALES STRATEGY:

The larger a production batch of an amplifier is, the cheaper it can be made and sold. However, the financial risks increase, since one normally has no guarantee that all devices will be sold.

To make sales-prices as low as possible and to minimize financial risks it is planned to work with a subscription list. This list will be started as soon as final production costs are known and a more exact production date has been set.

Production will only start with a specific minimum of people signed in. Shortly before production start people will be asked to pay a (relatively small) downpayment.

So this will be a kind of kickstarter project but with the difference that development and design will already be finished when the project is started.

If interested please drop me a mail.

Cheers

Jan Meier
Eight LED's indicate sampling frequency, bit-resolution, and the emphasis-status of the incoming signal. Especially with computer audio this is a great help to check the software settings.

A switch to the right of the LED's allows to change the filter settings of the DAC-chip used. One can choose between classic filtering (phase correct, brickwall) and a filter-mode without pre-ringing (with a slightly softer, more analog sound).
The amplifier has two headphone outputs, one with 120 Ohm output impedance and one with zero Ohm output impedance. The output impedance of the first output can be lowered if needed.
updated 06. Dec. 2017