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General Description

The AD24QS is a high quality Audio Analog to Digital Converter available as kit for DIY (Do It Yourself). It features:

Sample Rates		10 different ones, 16 - 192 kHz
Analog Inputs		On-board Stereo (RCA), 10 kΩ Option: External XLR balanced, 20 kΩ
Input Sensititivity @ 100% FS		2 VRMS Others or adjustable gain as option
Dynamic Range 20 Hz - 20 kHz		109 dB unweighted, typically 111 dB A-weighted typically Option "Flagship" chip-set: 115 (117) dB approx.
THD		typ. -120 dB @ -6 dB FS
Digital Audio Outputs		1 x on-board optical (Toslink, up to 96 kHz only) 1 x on-board coaxial (RCA), 75 Ω Option: External AES-3 XLR balanced, 110 Ω
Latency		192 kHz: 46 µs 96 kHz: 126 µs 48 kHz: 310 µs 44.1 kHz: 275 µs (yes, less than @ 48 kHz)
Frequency Response		2 Hz - SR / 2 @ -3 dB Except 192 kHz SR: 3 Hz - 73 kHz @ -3 dB
Power Supply Rejection Ratio		approx. 100 dB @ 100 Hz
Power Supply		Standard version: 12 V DC, 180 mA approx. Option "Flagship" chip-set: 12 V DC, 210 mA approx.
Board Size		91.44 mm x 88.27 mm (3.6" x 3.475")
Mechanical Drawing		GIF file or PDF file

The data above is measured at my samples and cannot be guaranteed, of course.

Sample Rates

The ADC is capable of 10 different sample rates up to 192 kHz, set with the switch on the ADC's front:

Setting		Sample Rate (kHz)		Format		Copyright
0		16		Professional		-
1		22.05		Professional		-
2		24		Professional		-
3		32		Professional		-
4		44.1		Professional		-
5		48		Professional		-
6		64		Professional		-
7		88.2		Professional		-
8		96		Professional		-
9		192		Professional		-
A		44.1		Consumer		Not asserted
B		44.1		Consumer		Asserted, original version
C		44.1		Consumer		Asserted, copied version
D		48		Consumer		Not asserted
E		48		Consumer		Asserted, original version
F		48		Consumer		Asserted, copied version

The first 10 sample rates, with the digital audio signal indicated as "Professional Format", are selected in positions 1 - 9.

Positions A - F are indicated as "Consumer Format", where you can select 44.1 kHz or 48 kHz only, but each with the options "no copyright asserted", "copyright asserted, original version" and "copyright asserted, copied version". In Consumer Mode the ADC's category code is "General" (00_hex). Using a decimal encoding switch would prevent access to the consumer format.

LEDs

A green "Power"-LED is provided. The red "Ovr"-LED on the front lights up, when at least one of the channels is overdriven. If both are overdriven, it is brighter.

Analog Inputs

All connections are made on the rear. The analog, unbalanced RCA inputs have a sensitivity matching the usual CD or DVD player output level of 2 V_RMS. As options, a gain switch can easily be connected externally as well as balanced XLR inputs and a selector switch between RCA and XLR inputs.

Digital Audio Outputs

Both digital audio outputs, the optical and the unbalanced RCA output, can be used simultaneously. The RCA output is galvanically decoupled by a digital audio transformer. Also as an option, a third, e.g. balanced (XLR- or AES-3-) digital audio output with its own digital audio transformer can be connected externally.

Input Levels and Other Technical Data

The preamplifiers are dimensioned for an input sensitivity of 2 V_RMS approx. at 100% FS, a usual level e.g. for CD or DVD players.

When the balanced XLR input option is used, the input sensititvity should be reduced to the standard studio level of +4 dBu (1.23 V_RMS) plus a reasonable headroom of e.g. 12 dB. I.e., 4.91 V_RMS (+16 dBu) for 100% FS might be a good choice.

With the "Flagship" option, consisting of a significantly more expensive ADC-IC which the manufacturer calls his "Flagship" and better (and more expensive) op-amps an approximately 4 dB higher dynamic range than with the standard chip-set is achieved.

For more measurements I made on the previous prototypes read the corresponding article about the AD2496.

Power Supply

The power supply is 12 V DC and the current consumption is about 180 mA for the standard and 210 mA for the "Flagship" version.

I use regulated power supplies even though unregulated ones are ok, too. Due to the ADCs' high power supply rejection ratios only very little hum from unregulated power supplies remains in the output signal so that the ADCs' dynamic range is not decreased noticeably (unweighted measurements). BTW, I measured with a full-wave rectified 9 V transformer (i.e., without buffer capacitor) the same dynamic range as with a regulated power supply. Even in the worst case with a bare 9 V AC supply (which is possible, too) and the "Flagship" chip-set the dynamic range was decreased by less than 1 dB. Only in the spectrum you can observe the effect of an unregulated power supply.

Circuit Description

For the current circuit diagram of the AD24QS, have a look either at the GIF-file or the PDF-file.

Analog Inputs

The analog stages up to and including the ADC-IC's input are designed fully balanced, though, in the basic kit version, the input jacks are unbalanced only. The first stage serves as input buffer but an additional amplification can be introduced there, too. The second stage is a balancing 3^rd order low-pass Butterworth filter with a corner frequency of 190 kHz approx..

The AD24QS can be equipped with external professional, balanced audio inputs (XLR) and also with an external switch to select between the on-board unbalanced RCA or the external XLR inputs.

For the balanced input and adjustable gain option an additional connector can be placed. This Micro-Match (or compatible) connector matches exactly the one on the new version of the high quality microphone and line preamplifier 2ChPre. It does not only carry the audio and gain adjustment signals but the power supply, too. On the photo you can see the position for this Micro-Match connector, but it is not populated there.

ADC

The Cirrus Logic CS5361 is a high quality 114 dB delta sigma A/D converter. The reference supply is a simple 78L05A, but there is no reason for concerns about its noise as reference voltage noise is very well filtered by the ADC's external filter components. The ADC-IC can be replaced by the even better and significantly more expensive 120 dB ADC CS5381. Read more in the DIY section.

Clock

The clock generation is done by an MK2703, a "PLL Audio Clock Synthesizer", which is an oscillator and PLL circuit for the 44.1 kHz and 48 kHz based oscillator frequencies derived from a single 27 MHz crystal.

S/P-DIF Transmitter

I use a Cirrus Logic CS8406 "192 kHz Digital Audio Interface Transmitter" in hardware mode. The hardware mode provides enough functionality, i.e., signal inputs for control, for my goals. Three digital audio outputs can be connected to the S/P-DIF transmitter's outputs: One optical output device (Toslink TOTX177PL), one unbalanced electrical output (0.5 V_PP @ 75 Ω terminated). Moreover, the AD24QS can be equipped externally with a professional digital audio output, either balanced (XLR, AES-3, 2 V_PP @ 75 Ω terminated) or unbalanced (BNC, 1 V_PP @ 75 Ω terminated). The decoupling transformer and the termination resistors are provided on board.

The optical output device (TOTX177PL) is specified up to slightly more than 96 kHz only, though it works on the workbench up to 192 kHz. Anyhow, I do not recommend to use it for 192 kHz.

The digital audio transformer for the coaxial signal is not specified for 192 kHz either, but when you have a look at its frequency response or its pulse transmission characteristics you understand why I unscrupulously use and recommend it. It would be fine for even much, much higher frequencies than 192 kHz.

Control Logic

In order to achieve these many sample rates I had to introduce a more complex logic than on the AD2496. It is still a static circuitry, but too complex for one or two logic gate ICs. So I was forced to use some kind of programmable logic and chose a GAL16V8, the simplest, smallest and cheapest one that is available (except that, in order to save power, I recommend the slightly more expensive quarter power GAL version). In kits, you'll get them programmed.

Power supply

The nominal supply voltage for the device is 12 V, but up to 15 V are allowed and it operates down to 10 V or less. The power input is protected against polarity reversal and buffered with an extra large, 1000 µF capacitor . The digital part of the AD24QS operates at 5 V. I use a linear regulator, a standard 78M05 in a TO220-package, to regulate the digital supply voltage.

Like in the DA24QS, the negative analog supply voltage is generated by a DC-DC voltage inverter (LT1054, up to 15 V / 100 mA) and both, the positive and the negative analog supply voltages are smoothed by one low-pass emitter follower each.

The DIY Section

For different reasons I sell these converters as kits and thus for DIYs only.

Is it difficult?

I expect sufficient experience from people to assemble the kits. I don't explain how to read resistor values, how to discriminate a 100 µH inductor from a 100 Ω resistor, which components are polarized and so on. The most difficult thing might be to solder the SO-IC packages, though I don't find this to be really difficult. You don't need much more than a fine tip and a steady hand.

What even happens to me is to mix up similar looking resistors, put components in the wrong places,rotate ICs by 180° or forget to solder all joints. Usually all this soon becomes quite obvious and can easily be corrected.

Kits

The kits I provide come with all necessary electronic and mechanical parts for the basic version of the ADC, the AD24QS-K (unbalanced inputs, no gain adjustment, no connector for balanced inputs, no 3^rd digital audio output.) The GAL is programmed. The kits come without an enclosure, front or rear panel - just as you can see on the photos.

See the BOM (bill of material) what is actually included in the kit.

Normally, all axial components, i.e., resistors, diodes and inductors, come cut and bent. This saves a lot of assembly time for you and may help that the assembly looks neater.Not all components on the board need to be populated. They are partially intended for options. Which one is to be populated and where is shown in the assembly drawings.

9 jumpers, indicated red in the assembly drawing, must be placed: 4 at ST2, 1 at SW1 and 2 each at L1 and L3. For the longer wires I prefer colored insulated ones because it looks better.

Preparing the LEDs The photos on the right show how the LEDs must be built into the LED holders, their polarities (the long leads are anode) and how the leads must be bent. Click on the photos to enlarge them. Note that both LED holders have an up- and an underside.

Preparing the TO92 Cases

The photo on the right shows how to prepare the transistors and ICs in TO92 cases. Unfortunately I cannot easily get the leads bent as originally planned, so they must be bent correctly by hand during assembly. I intended the pad layout in a triangular shape with 0.1" pitch. Today most TO92s come with all leads close to each other in one row (both left transistors) or, quite exotic, like the TL431 in the middle.

Do not force TO92s with such unsuitable leads into their holes!

Try to bend them like the rightmost ones which I bent manually. Each lead is bent twice. Do not bend the leads directly at the case because the case as well as the leads would be very much stressed and endangered. After bending, they should more or less "fall" into their three holes in the PCB.

Distance Holder for the Crystal Place the distance holder underneath the crystal so that the crystal's case doesn't have direct contact to the PCB and the leads are soldered at some distance from the case.

Gold Flash Plated PCBs Up to now I used HAL PCBs (hot air levelling, which basically means tinned). They are shown on many of the photos you see here. The newest PCBs I ordered have more expensive gold flash plated surfaces. I didn't choose that because it "sounds better" or because the board might become more reliable - I simply experienced that because of the different colors forgotton solder joints are very clearly visible. Also in the past I experienced that forgotton solder joints are one of the most common causes for malfunction. With this in mind I actually had no other choice than to use gold flash plated PCBs.

Assembly Drawing

Find here two assembly drawings:

Indicating the component names		Indicating the component types or values		Photo of the component side

Options

120 dB Dynamic Range: The CS5381 is a 120 dB ADC, pin compatible to the 114 dB CS5361. It has slightly more power consumption and is significantly more expensive than the CS5361. In order to maintain its extremely high dynamic range I recommend to use better (and again significantly more expensive) low-noise, dual op-amps, e.g. LM4562 from National Semiconductors instead of OP2134. With CS5381 and LM4562 I measured an increase of 6 dB for the dynamic range.

Balanced Inputs, Variable Gain: I can offer the particular Micro-Match connector set with the appropriate ribbon cable.

With an additional switch (2 x change-over per channel), wired to the Micro-Match connector, it is possible to switch between the on-board unbalanced RCA inputs and the external, balanced XLR or TRS inputs. The switch must be connected where in the schematics currently the 4 jumpers at ST2 are indicated. XLR:

Signal		XLR		TRS		Left In		Right In
Ground		1		Sleeve		ST2D		ST2N
Pos.		2		Tip		ST2E		ST2O
Neg.		3		Ring		ST2C		ST2M

By using additional resistors variable gain rising is available. Populating R3 and R20 and one external variable resistor each between ST2A/B and ST2K/L the gain can be increased according to the table shown in the circuit diagram. E.g., with R3 = R20 = 430 Ω a maximum gain of 15 dB is achieved. The external variable resistors may be either switched for a set of precise fixed gains or potentiometers, preferably a negative logarithmic ones.

Balanced AES-3 Output: For the balanced digital audio output another digital audio transformer, a connector set and two resistors are needed.

Different Gains: By replacing the resistors R8, R9, R10, R11 and R25, R26, R27, R28 the conversion gain can be increased or decreased. The rules are:

R_A = R8 = R11 = R25 = R28 and
R_B = R9 = R10 = R26 = R27, where
R_B = 1.1 x R_A.
R_A for a specific full-scale input voltage U_FS is: R_A = U_FS x 1.1kΩ / 1V
The full-scale input voltage U_FS is: U_FS = R_A x 1V / 1.1kΩ

Experimental switch: In the layout I provided a quad DIL-switch for

• the User-Bit U
• the Validity bit V
• the emphasis bit (which indicates that the incoming analog audio signal is emphasized) and
• turning off the digital high-pass filter inside the CS5361 (which enables the CS5361 to convert DC voltages)

EMC chokes: For an improved EMC (electro-magnetic compatibility) the board is prepared for two common mode chokes, L1 and L3. One is for the power supply input, the other one for the unbalanced, digital audio output. I pay attention to EMC performances but I am not able to test them. I prepared the chokes "just in case". Under normal circumstances they are not required.

Anyway, the ADC should always be installed into a well shielding enclosure!

Word Clock Synchronisation: I am working on a word clock synchronization module, the XS-AD24. It is not available as a "convenient" kit, but somehow I want to make it available. Should you be interested, read about it in the articlte "XS-AD24: VCXO-based Word Clock Synchronization for the Audio-ADC AD24QS".

Enclosure and Front Panels

For my units I used an SG206 enclosure from the (German) company Fischer Elektronik. Front panel, enclosure and board are directly fixed to each other using the element 5.60.422 from the (German) company Ettinger, available at Bürklin. You need not necessarily use the enclosure I used. The board can easily be built into other enclosures. For this enclosure I provided a detailed dimensional drawing, either as a GIF file or as a PDF file. You will see there how the board is fixed within the SG206 and how it can alternatively be fixed directly to other front panels. Of course you may also use four stand-offs, distance bolts or distance tubes to fix the board on the bottom of your enclosure. The drilling positions can be found in the dimensional drawing, too.

The front panel is manufactured by Schaeffer AG (in Europe) and is available in the US from Front Panel Express, LLC, too. You need the design files for the front and the rear panel, then you just have to send it to the manufacturer and you'll get a perfectly milled and engraved front panel, as the photos show. You may also modify the design file with the front panel design software "Front Panel Designer" German or English (it's free and very convenient) so that it fits to other enclosure of your choice.

All companies mentioned above have international branches, e.g. in the USA.

How to order...

... is explained on page AD/DA24QS_Order.