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 processor) 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. Nonetheless, the use of the COUNTRY can be difficult to understand, especially when a certain level of technical skills is missing with respect to the nature of sound. Below please find a short description of the various control elements.
There are very different opinions on the use of symbols instead of plain text on the front plate. The solution: The amp/DAC/DSP will be produced with symbols to be compliant with all other CORDA products. However, labels will be provided that can be attached to the front of the amp and that will give you plain text solutions. These labels will be self-adhesive but glue-less and can be removed and reused. Multiple labels will be added, maybe even in different colors.
The lower right dial controls crossfeed. In the upper position the output is plain stereo. The corresponding (blue) label symbolizes the two channels/ears/drivers L/R to be well separated. Turning clockwise activates headphone crossfeed (7 levels). The red label symbolizes blending of the two channels. Soundstage narrows. An 8th position will make the signal mono. The green label indicates full blending of both channels. Turning anti-clockwise activates loudspeaker crossfeed (7 levels). This widens the soundstage of loudspeakers as symbolized by the orange label.
The upper right dial has two different functions:
- In stereo or headphone crossfeed mode it activates and 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 notable improvement of sound quality. The filter frequency increases with clockwise rotation, as indicated by the label, and ranges from 6 kHz to 11 kHz.
- With loudspeaker crossfeed activated this dial sets the delay time between the direct signal and the crossfeed signal. The optimal delay time strongly depends on the positions (angles) of the loudspeakers. Delay time increases with clockwise rotation. Range 200..450 microseconds.
At the middle of the front a seven band equalizer is found. Each slide potentiometer allows to change the amplitude of its frequency band by +/- 6 dB (0.5 dB steps). Central frequencies of the bands: 64, 160, 400, 1000, 2500, 6200, and 16000Hz. 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 CD-recording is known to have emphasis. Shelf-filtering (instead of the more common peak-filtering) results in smooth transitions without dips/peaks at the crossover frequencies
The left upper dial allows to add reverberation. Recordings sometimes have little reverberation because our living room already adds some of its own when listening to loudspeakers. However, with headphones this reverberation is missing and sound can be a little bit dry. Adding reverberation by the DSP then definitely can have a positive effect.
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.
In its middle position the dial does not add any reverb. Turning to the right increases reverb in three steps. The red label symbolizes the multiple wavefronts of the reflected signals.
Each slide-potentiometers has a LED at the control-knob. The first five LED's indicate the sampling frequency of the incoming signal. With computer audio This is a great help to check the correct software settings. The sixth LED indicates 0/-6/-12/-18/-24/-30 dB settings of the volume control. Below -30dB the LED starts to blink. The seventh LED is the power-indicator. An internal switch allows to deactivate all LED's, apart from the power-LED.
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 called the psychoacoustic bass-enhancer and was originally published at Headwize, the site of the late Chu Moy. It 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. The left upper dial also allows to activate the psychoacoustic bass-enhancer by turning to the left. There are four positions with increased levels of reverb added. The blue label symbolizes the limited frequency response (tilted line) and the addition of the discrete frequency components of the harmonics (vertical lines).
To let you experience the effect 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:
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! :-)
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 notably 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.
A toggle switch at the left of the front allows to change the amplification factor of the analog output stage. There are three positions: LOW (red), MEDIUM (blue), and HIGH (orange).
A rotary-potentiometer allows to set the volume in the digital domain. There are 63 steps, typical stepsize is 0.75 dB. Volume reduction is done with an accuracy of 24 bit, so signal losses are minimal.
The left lower dial allows to reduce the amplitude of the left channel (left turn) or right channel upto -6dB in order to correct for balance problems.
At the outer positions only one channel is active.
The COUNTRY has a coaxial RCA-input, an USB-input, and al TOSLINK-input.
The USB-input allows sampling frequencies of 44.1, 48, 88.2, 96, and 192 kHz. Resolutions of 16 or 24 bit.
The coaxial and optical inputs also allow frequencies of 32, 64, 128, and 176.4 kHz. Resolutions 16, 20, or 24 bit.
The concept of the COUNTRY is very unusual and because of its complexity it is not a product for the masses. This complexity and the low production numbers also make this a rather expensive product.
To minimize any financial risks it was decided to handle this project like it has been done previously with the SOUL. People are able to pre-order and production will be started as soon as 30+ orders have been received.
Unfortunately, right after CanJam New York, where the prototype of the COUNTRY was well received, I had to decide to put the project on ice for at least 6 months. Not only has interest dropped dramatically due to Corona, but also supply of parts can not be guaranteed right now.
This is the more a pity then since New York a few methods were found to improve sound quality considerably without any additional costs.
It is unclear right now whether this project will be restarted or not. However, if interested or if you have any questions, please do not hesitate to send me a mail.
DA-conversion is done by a WM8716 from Cirrus/Wolfson.
Each analog output stage is made of two OPA2209 with all 4 channels in parallel. Maximum output current per channel thus is around 250 mA. Of course, like with the SOUL, the FFF-technique is applied. However, the COUNTRY has single-ended headphone outputs and preamplifier outputs only. If the COUNTRY is just to be used as a DSP, the analog section can be deactivated internally with a single switch.
A prototype of the COUNTRY (see the picture) has been presented on the recent CanJam in New York and although it's presence was overshadowed by that of the SOUL, the COUNTRY was well received by the public. The amp/DAC/DSP has a nice dynamic sound with lots of drive. Sure, it is not as smooth and silky as the SOUL, but it's a lot of fun.
And if you already do have a nice DAC/amp setup, then it's always possible to connect the COUNTRY digitally.
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. 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. DSD-signals and 384 kHz signals can not be processed by the DSP-chip and therefore are not accepted. I'm sorry for that!
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.
There is one coaxial (RCA) and one optical (TOSLINK) digital output.