CORDA COUNTRY
My latest project, the CORDA SOUL, has raised quite a lot of interest. Not only because of its superior sound quality but also because of the many options to optimize sound in the digital domain. The built-in DSP (digital signal prozessor) allowed to implement tone controls, notchfilter and crossfeed with a precision that easily outclasses any implementation in the analog domain.

The SOUL as a whole is a very sophisticated design and thereby unfortunately also is quite expensive. Therefore there were many requests to built/design a more affordable version that offers all the digital options of the SOUL but with a simpler analog section.

I have been working on this amp/DAC/DSP for quite a while now and as it happens, various new ideas popped up. The current status of the design is presented here. As there is a lot of computing going on inside this project is named the COUNT(RY).

By nature the front of the COUNTRY has a large number of control elements. Of course a more slick design could have been made with a LCD-screen, programming buttons, and a menu-structure to set all parameters, but the advantage of using separate control elements is, that one can check all settings at one glance.
Last update:
August 6. 2019
Left to the crossfeed dial a rotary-potentiometer allows to set the volume in the digital domain. Volume reduction is done with an accuracy of 24 bit, so signal losses are minimal.

The original concept of the COUNTRY was that of a digital processor only. However, on multiple requests a headphone output and RCA-outputs have been added to make the device more versatile. Of course it does add considerably to the costs since a DA-converter, amplification stages and extra +/- 15V powerlines had to be added. A cost-effective solution has been choosen but although the analog outputs can not compete in sound quality with the SOUL they are still pretty competent.

At the middle of the front a seven band equalizer is found. Each slide potentiometer allows to change the amplitude of its corresponding frequency band by +/- 6 dB (1 dB steps).

Central frequencies of the bands: 64, 160, 400, 1000, 2500, 6200, and 16000 Hz. On request other frequencies are possible.

With the rightmost slide-potentiometer set to its lowest position, the COUNTRY applies a de-emphasis filter to the digital signal. It can be used when a recording is known to have emphasis.

The 7 slide-potentiometers each have a LED at the heart of the control-knob. These LED's are used to indicate sampling frequency (32, 44.1, 48, x2, x4), -6/-12dB settings of the volume control ,as well as the ON-state of the device.

Especially with computer audio the indication of the sampling frequency is a great help to check the software settings.

The COUNTRY also has some functionality not found on the SOUL.

The left lower dial allows to change the Left-Right-Balance. If you have ear deficiencies or if your loudspeakers are placed rather awkwardly this dial can compensate.
The current design has a coaxial RCA-input, an USB-input, and al TOSLINK-input.

The USB-input allows sampling frequencies of 32, 44.1, 48, 88.2, 96, and 192 kHz. Resolutions of 16 or 24 bit.

The coaxial and optical inputs also allow sampling frequency of 64, 128, and 176.4 kHz. Resolutions 16, 20, or 24 bit.

To lower the computing power required Internally all input signals are resampled before processing.

Originally an internal sampling frequency of 192 kHz was anticipated. However, some of the algorithms (especially reverberation) are rather complex and do not "fit" within the limitations of the computing power. So it will be necessary to use a sampling frequency of 96 kHz instead or, on request, people can have a version without reverb.

There are two digital outputs: One coaxial RCA-output and one optical TOSLINK-output.
The input signals are resampled by the ADAU1463 DSP-chip by using an extremely high-quality TCXO as a reference timer.

The digital audio signal is internally processed with 64-bit accuracy.

The USB-input uses the CM6631 from C-Media to receive data in asynchronous mode from your PC. The use of extremely high-quality TCXOs guarantees jitter-free operation.

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

DA-conversion is done by a WM8716 from Cirrus/Wolfson.

Each analog output stage is made of two OPA209 in parallel. Maximum output current per channel thus is around 120 mA. Of course, like with the SOUL, the FFF-technique is applied. Amplification factor (LOW, MEDIUM, HIGH) can be changed by a switch to the left of the headphone output.

I'm currently building "final" prototypes for testing and fine-tuning! If all goes well they will be presented on the upcoming Audiovista headphone show September 14./15.

If interested in this very special project, please do not hesitate and drop me a mail.

Cheers

Jan Meier
The lower right dial controls crossfeed.

In the upper (neutral) position the output is plain stereo.

Turning the dial clockwise activates headphone crossfeed (7 levels)

Turning the dial anti-clockwise activates loudspeaker crossfeed (7 levels). This widens the soundstage of loudspeakers.


The upper right dial has two different functions:

- In stereo or headphone crossfeed mode the dial controls the central frequency of a 6dB, Q=2 notchfilter (see picture below). Many headphones do have a distinct resonance frequency due to the entrapped air-volume between driver and ear. Proper application of the notch filter will lower the energy at the resonance frequency and this can result in a remarkable improvement of quality of sound.

- 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.
The left upper dial allows to add reverberation. Especially with recordings made in relatively small venues, often very little natural reverberation is added by the sound engineer because our own living room already adds some of its own when listening to loudspeakers. However, when using headphones this extra reverberation is missing and sound can be a little bit dry. Adding some (high quality) artificial reverberation by the DSP then definitely can have a positive effect. Right now this feature only needs some fine tuning.

As an example listen to the three tracks to the right. It's a real pop-classic. The first file is the original track. The second and third file have different levels of reverb. Again, your impressions are much welcome.

In its middle position the dial does not add any reverb. Turning to the left or right will increase Reverb in three steps.
No reverb

Low reverb

Medium reverb
The left upper dial also allows to activate a psychoacoustic bass-enhancer by turning to the left.

Quite some years ago I developed a device that makes signals below 60 Hz audible in loudspeakers and headphones that normally, by their mere physical construction, are not able to reproduce these frequencies. This design was originally published at Headiwze, the site of the late Chu Moy, and now can be found at:

headwizememorial.wordpress.com/tag/bass

If interested please read carefully. It's not an easy read but an interesting one. Basically the bass-enhancer monitors all the signals below 60 Hz, calculates their harmonics, and adds these harmonics to the original signal. If properly done, these harmonics fool our brains in that they suggest the presence of the lower fundamental signal.

The original design was done using all-analog circuitry. However, with the COUNTRY it was very easy to implement the algorithm in the digital domain. To let you experience the effect of the bass enhancement I've prepared an audio-file.

The file presents 10 six-seconds sections separated by a 1 second pause. Take your headphone and give the file a listen by clicking the button below. You will hear:
1. A pure 100 Hz sinus signal
2. A 100 Hz sinus signal with bass-enhancement.
3. A pure 70 Hz sinus signal
4. A 70 Hz sinus signal with bass-enhancement.
5. A pure 50 Hz sinus signal
6. A 50 Hz sinus signal with bass-enhancement.
7. A pure 40 Hz sinus signal
8. A 40 Hz sinus signal with bass-enhancement.
9. A pure 34 Hz sinus signal
10. A 34 Hz sinus signal with bass-enhancement.
With a decent headphone it will not be difficult to hear the high frequency pure sinus signals but at 40 Hz and at 34 Hz the level of sensation will be very low. However, not so with the bass-enhancement engaged. Very suddenly these signal components become very audible.
Granted, the bass-enhancement does "colour" the sound. It is not the real thing and there is no visceral sensation. Yet it can be very helpful to improve the level of realism when we hear a recording that has deep frequency components.

Please compare the following two tracks:
TestA

TestB
I will not tell you which is the original recording and which uses the bass-enhancement. The effect of the bass-enhancement is rather subtle but you should be able to detect its effect. Please let me know your impressions and observations. Do you feel the bass-enhancement to be an interesting/valuable addition to the COUNTRY design? (Note: Quite a few people wrongly identified the bass-enhanced file. They said to prefer what they thought to be the non-enhanced file but when asking which file exactly it always has been the enhanced version! :-)