Parts listThe table below illustrates the parts needed to populate the α24 board. You must multiply the quantity by the number of boards you're building. Choose between two configurations (line stage, voltage DAC analog output stage), and 5 different board connector options.
If the α24 is being used as a balanced-to-unbalanced or unbalanced-to-balanced converter, use the line stage configuration. If you wish to change the gain, see the Voltage gain formula section below.
This list does not include wires, hardware, enclosure-related, panel-mount items or the power supply. See the sections below for specific parts recommendations and options.
Parts recommendations & optionsTo obtain the blank α24 circuit board, visit the AMB audio shop. You can get almost everything else from the vendors listed on the AMB Audio DIY Page. The recommended US vendors are Mouser and Digi-Key. Builders in other countries may search Farnell or RS Components.
The following is a list of recommendations and options. Please read through this carefully before you order any parts.
C0G/NP0 multilayer ceramic capacitor, 1% tolerance
(C6 with LPF): 5mm lead spacing, 7.5x7.5mm rectangular
polypropylene film capacitor, 2.5% tolerance
C9, C10, C11: X7R multilayer ceramic capacitor
2.5mm lead spacing
conductive/organic polymer electrolytic capacitor
U2: Texas Instruments OPA1632D high performance fully-differential audio opamp
U3: Texas Instruments OPA1611AID high performance audio opamp
Board connectorsFive options for board connectors are listed in the parts list above, using three connector types, all of them are single-row with 2.54mm pitch. For the γ3 high resolution DAC, please use board connector option 5 (pin headers and receptacles). See the part numbers in the parts list.
Power supply considerationsThe minimum and maximum power supply voltage is determined by the ratings of the opamps. It is desirable to operate at as high a power supply voltage as possible, to achieve maximum output voltage swing capability. The maximum rating for the OPA1632 is ±16V whereas the OPA1611 and OPA1612 are ±18V. However the OPA1632 will run quite hot at such high supply voltages. For the α24 the recommended supply voltage range is ±8V to ±12V. For most configurations, ±10V is sufficient to drive just about any amplifier to maximum output with room to spare.
A high performance, dual-rail low noise linear regulated power supply is recommended, such as the AMB σ22.
To minimize magnetically-induced hum and noise, mount the power transformer away from the α24 boards, input wiring, and the potentiometer/attenuator, if equipped. Or better yet, put the power transformer in a separate enclosure, so that it could be placed even farther away. See the Wiring & ground section for more details.
Component matchingThe α24 circuit is mostly symmetrical with "positive" and "negative" halves (see schematic diagram). For best CMRR performance, the resistors and capacitors in the positive half of the circuit should match their respective negative half. Even without special matching, the specified component tolerances are good enough for excellent performance. If you wish to further optimize it, then get extra parts and measure them with a good meter to achieve the closest matches possible.
Voltage gain formulaThe α24 circuit topology contains three opamps, U1, U2 and U3, each of them could have a voltage gain (or for U2 and U3, a voltage loss). To keep matters manageable, U2 is fixed to have unity gain, so the total gain for the balanced output is determined solely by the gain of U1. For the unbalanced output, the total gain is the product of the gain of U1 and U3.
The default line stage configuration has U1_gain = 5 and U3_gain = 1, for a total gain of 5 at both the balanced and unbalanced outputs.
The default voltage DAC output stage configuration has U1_gain = 2 and U3_gain = 0.5, for a total gain of 2 at the balanced output, but effective gain of 1 at the unbalanced output.
LPF response and corner frequencyThe parts list shows two configurations -- line stage with no LPF and voltage DAC output stage with LPF. The latter has a Bessel response, corner frequency of about 100KHz and a slope of 18dB/octave.
Calculating the response and corner frequency of this multi-stage fully-differential hybrid active/passive filter is complex, affects the values of many components and beyond the scope of this document, therefore only these two configurations are offered:
If you have the knowledge and expertise in this type of filter design, then you can build the α24 to have a different LPF response and corner frequency.
Hybrid configurationIf you are building a line stage but also want the 100KHz LPF, then use the parts values from the voltage DAC output stage configuration, except those that are mentioned in the Voltage gain formula section above. Use the gain formulas to achieve the gain you desire.
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