The α24 fully-differential line amplifier

Parts list

The 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.

Part Quantity Description Mouser Digi-Key Farnell RS Components
 
Common parts for all configurations
- 1 α24 printed circuit board AMB audio shop
R1, R2 2 1KΩ 1% miniature thin film or metal film resistor 594-5063JD1K000F or
71-RN50C-F-1K
PPC1.00KXCT-ND or
CMF1.00KQFCT-ND
9342400 165-0224
R3, R4 2 499KΩ 1% miniature thin film or metal film resistor 594-5063JD499K0F or
71-RN50C-F-499K
PPC499KXCT-ND or
CMF499KQFCT-ND
- -
R5, R6, R8, R9 R10, R11 6 2.49KΩ 1% miniature thin film or metal film resistor 594-5063JD2K490F or
71-RN50C-F-2.49K
PPC2.49KXCT-ND or
CMF2.49KQFCT-ND
- -
R14, R15, R20 3 33.2Ω 1% miniature thin film or metal film resistor 594-5063JD33R20F PPC33.2XCT-ND or
CMF33.2QFCT-ND
9343075 -
R16, R17 2 4.99KΩ 1% miniature thin film or metal film resistor 594-5063JD4K990F or
71-RN50C-F-4.99K
PPC4.99KXCT-ND or
CMF4.99KQFCT-ND
- -
C7, C8 2 4.7pF C0G/NP0 multilayer ceramic capacitor 810-FG18C0G1H4R7CNT0 or
810-FG18C0G1H4R7CNT6
445-181241-ND or
445-173199-1-ND
- -
C9, C10, C11 3 0.1µF X7R multilayer ceramic capacitor 810-FG18X7R1H104KNT00 or
810-FG18X7R1H104KNT6 or
80-C315C104K1R or
80-C320C104K5R
445-173265-1-ND or
399-9713-ND or
399-4264-ND
2309020 653-0181
C12, C13, C14, C15, C16, C17 6 100µF 16V conductive/organic polymer electrolytic capacitor 647-RS81C101MDN1JT P16292-ND 2466590 714-9076 or
788-6977
U1 1 Texas Instruments OPA1612AID dual opamp (SOIC-8) 595-OPA1612AID 296-24959-5-ND 1754265 812-2505
U2 1 Texas Instruments OPA1632D fully-differential opamp (SOIC-8) 595-OPA1632D 296-16679-5-ND 8467455 619-9875
U3 1 Texas Instruments OPA1611AID opamp (SOIC-8) 595-OPA1611AID 296-24958-5-ND 1754264 812-2502
 
Parts for line stage configuration (gain = 5)
R7 1 1.24KΩ 1% miniature thin film or metal film resistor 594-5063JD1K240F or
71-RN50C-F-1.24K
PPC1.24KXCT-ND or
CMF1.24KQFCT-ND
- -
R12, R13 2 wire jumper or 0Ω miniature resistor 594-5063JD0R000J - - -
R18, R19 2 4.99KΩ 1% miniature thin film or metal film resistor 594-5063JD4K990F or
71-RN50C-F-4.99K
PPC4.99KXCT-ND or
CMF4.99KQFCT-ND
- -
C1, C2 2 47pF C0G/NP0 1% multilayer capacitor 581-SR151A470FAA - 2395776 -
C3 0 not used - - - -
C4, C5 2 10pF C0G/NP0 multilayer capacitor 810-FG18C0G1H100DNT0 or
810-FG18C0G1H100DNT6
445-175534-ND or
445-173169-1-ND
1100368 -
C6 0 not used - - - -
 
Parts for voltage DAC output stage (balanced gain = 2, unbalanced gain = 1, LPF = 100KHz Bessel)
R7 1 4.99KΩ 1% miniature thin film or metal film resistor 594-5063JD4K990F or
71-RN50C-F-4.99K
PPC4.99KXCT-ND or
CMF4.99KQFCT-ND
- -
R12, R13 2 1.15KΩ 1% miniature thin film or metal film resistor 594-5063JD1K150F - - -
R18, R19 2 2.49KΩ 1% miniature thin film or metal film resistor 594-5063JD2K490F or
71-RN50C-F-2.49K
PPC2.49KXCT-ND or
CMF2.49KQFCT-ND
- -
C1, C2 2 220pF C0G/NP0 1% multilayer capacitor 80-C315C221F1G - - -
C3 1 680pF 2.5% polypropylene film capacitor 505-FKP2680/100/2.5 or
75-KP1830168013-TR
- 1005990 -
C4, C5 2 330pF 2.5% polypropylene film capacitor 505-FKP2330/100/2.5 - 2470590 484-2000
C6 1 0.015µF 2.5% polypropylene film capacitor 505-FKP20.015/63/2.5 - 2470593 108-2328
 
Board connector option 1 (Molex KK 254)
J1, J2, J4 3 3P Molex KK 254 header 538-22-23-2031 (tin) or
538-22-11-2032 (gold)
WM4201-ND (tin) or
WM2701-ND (gold)
1462950 (tin) or
2112431 (gold)
679-5587 (tin) or
679-5524 (gold)
J3 1 2P Molex KK 254 header 538-22-23-2021 (tin) or
538-22-11-2022 (gold)
WM4200-ND (tin) or
WM2700-ND (gold)
1462926 (tin) or
1865212 (gold)
679-5583 (tin) or
679-5515 (gold)
J1a, J2a, J4a 3 3P Molex KK 254 Housing 538-22-01-3037 WM2001-ND 1462838 679-5375
J3a 1 2P Molex KK 254 Housing 538-22-01-3027 WM2000-ND 1462825 679-5363
- 11 Molex KK 254 crimp pins 538-08-50-0114 (tin) or
538-08-55-0102 (gold)
WM1114-ND (tin) or
WM2312-ND (gold)
1462641 (tin) or
1462642 (gold)
172-9178 (tin) or
678-3149 (gold)
 
Board connector option 2 (Molex KK 254)
J1 1 3P Molex KK 254 header 538-22-23-2031 (tin) or
538-22-11-2032 (gold)
WM4201-ND (tin) or
WM2701-ND (gold)
1462950 (tin) or
2112431 (gold)
679-5587 (tin) or
679-5524 (gold)
J2, J3, J4 1 8P Molex KK 254 header 538-22-23-2081 (tin) or
538-22-11-2082 (gold)
WM4206-ND (tin) or
WM2706-ND (gold)
1756826 (tin) or
9730982 (gold)
679-5606 (tin) or
679-5537 (gold)
J2a, J3a, J4a 1 8P Molex KK 254 housing 538-22-01-3087 WM2006-ND 2396394 679-5417
- 11 Molex KK 254 crimp pins 538-08-50-0114 (tin) or
538-08-55-0102 (gold)
WM1114-ND or
WM2312-ND
1462641 (tin) or
1462642 (gold)
172-9178 (tin) or
678-3149 (gold)
 
Board connector option 3 (Phoenix Contact MPT)
J1, J2, J4 3 3P Phoenix Contact 1725669 terminal block 651-1725669 277-1274-ND 3041360 220-4276
J3 1 2P Phoenix Contact 1725656 terminal block 651-1725656 277-1273-ND 3041359 220-4260
 
Board connector option 4 (Phoenix Contact MPT)
J1 1 3P Phoenix Contact 1725669 terminal block 651-1725669 277-1274-ND 3041360 220-4276
J2, J3, J4 1 8P Phoenix Contact 1725711 terminal block 651-1725711 277-1279-ND 3041438 220-4327
 
Board connector option 5 (pin headers/receptacles as "daughter board")
J1 1 3P pin receptacle* 517-929974-01-03-RK S7036-ND 1668337 765-5610
J2, J3, J4 1 8P pin receptacle* 517-929974-01-08-RK S7041-ND 1668363 765-5698
J1a, J2a, J3a, J4a 1 36P pin header 517-647-01-36 S1011E-36-ND 1056427 681-2405
(*) Mount on bottom of board

Parts recommendations & options

To 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.

Resistors

All resistors are miniature 1/8W, 1% tolerance, 5mm lead spacing. The resistor body should be no more than 4mm long and 1.9mm in diameter, not including the leads. Some resistance values may not be available in all brand/type lines.

Capacitors

C1 and C2: 2.5mm lead spacing
C0G/NP0 multilayer ceramic capacitor, 1% tolerance (C3, C4, C5 with LPF): 5mm lead spacing, 4.5x7.5mm rectangular
(C6 with LPF): 5mm lead spacing, 7.5x7.5mm rectangular
polypropylene film capacitor, 2.5% tolerance (C4, C5 without LPF), C7, C8: C0G/NP0 multilayer ceramic capacitor
C9, C10, C11: X7R multilayer ceramic capacitor
2.5mm lead spacing C12, C13, C14, C15, C16, C17: 2.5mm lead spacing, 6mm diameter
conductive/organic polymer electrolytic capacitor

Integrated circuits

U1: Texas Instruments OPA1612AID high performance dual audio opamp
U2: Texas Instruments OPA1632D high performance fully-differential audio opamp
U3: Texas Instruments OPA1611AID high performance audio opamp

Board connectors

Five 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.

Connector types
Molex KK 254 Phoenix Contact MPT Pin headers and
receptacles

Power supply considerations

The 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 matching

The 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 formula

The 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.
Vout(bal) = Vin * U1_gain
Vout(unbal) = Vin * (U1_gain * U3_gain)
where, U1_gain = ((2 * R5) / R7) + 1
U3_gain = R18 / R16

R5 = R6, R16 = R17, R18 = R19
To make U1_gain unity, short circuit R5 and R6, and omit R7.

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 frequency

The 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:
  • No LPF: C3 and C6 not used, C4 = C5 = 10pF, R12 = R13 = 0Ω (wire jumpers)
  • LPF with fc = 100KHz: C3 = 680pF, C4 = C5 = 330pF, C6 = 0.015µF, R12 = R13 = 1.15KΩ
The following is the frequency response plot with the 100KHz 3rd order Bessel LPF:



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 configuration

If 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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