ON 2 3 4 1 1510/1512 Audio Preamplifier Demonstration System THAT 1510/1512-DEMO Description The THAT 1510/1512 Demo Board is a self- mismatch between C5 And C6 (the shunt paths for each input share C10, and C10 s contribution contained circuit board that simplifies the evalua- tion of the THAT1510 and THAT1512. to the effective shunt impedance of each side Completely assembled and tested, this system dominates that of C5 and C6). comes with XLR connectors for signal input and When phantom power is applied, there is the output, and 3 pin headers for standard and potential for the phantom power circuitry to phantom power. attempt to take the IC s inputs above the rail and J1 is a Female XLR connector that accepts a outside the common mode input range of the balanced (or unbalanced) input signal, while P1, THAT 1510/1512. This demonstration system a male XLR connector, is connected for an unbal- has phantom power soft start that remedies this anced output. An external 15V power supply situation. In this portion of the circuit, Q1 acts for the IC should be connected via header P3, as an emitter follower, and the time constant of along with an external +48V phantom power R6 and C12 determines the instantaneous rate at supply connected via header P2. which the phantom power rises. D7 is included to keep the base-emitter junction of Q1 from C5, C6, and C10 shunt unwanted RFI being damaged by being reverse biased. When currents to chassis ground. The arrangement phantom power is disconnected, D1 also closes of these components minimizes CMRR degrada- the path for the residual charge stored on C1 and tion at higher frequencies resulting from any C2 to discharge through R6 and R7. 4-40 Threaded Standoff (4 pl) THAT 1510/1512 Demo Board Circuit Design 2002 THAT Corporation SW1 Gain +10 GND 1 +20 + +40 C9 +60 P2 Q1 + + R13 + C2 OUT C1 R12 C12 + P1 3.500 R16 R8 3.100 U1 C3 R5 R9 TP4 TP3 C4 Y2 C24 R21 C23 C22 R20 J1 U21 TP1 TP2 R23 IN OPTIONAL + + SERVO Y1 P3 R22 C21 C7 C8 1 V- V+ .200 3.600 0.200 4.000 Figure 1. THAT 1510/12 Demonstration System Layout THAT Corporation 45 Sumner Street Milford, MA 01757-1656 USA Tel: +1 508 478 9200 Fax: +1 508 478 0990 Web: www.thatcorp.com Copyright 2005 - 2012, THAT Corporation All rights reserved. Document 600038 Rev 03 Phantom Power R7 R6 C6 C5 R1 D7 C10 R11 R2 D5 D3 D6 D4 R14 R4 R15 R3 D2 D1 R24Document 600038 Rev 03 Page 2 of 4 THAT 1510/12 Audio Preamplifier Demonstration System All integrated circuit mic preamps are larger zeners or tranzorbs will typically introduce vulnerable to phantom power faults. There are distortion due to their non-linear input two primary situations where these faults capacitance. typically occur: In the first situation, the SW1 allows the gain of the THAT1510 or the phantom power is applied but the input of the THAT1512 to be switched in discrete steps of mic preamp is unconnected and floating. There zero dB, 10dB, 20dB, 40dB, and 60dB. Values will be +48V (relative to chassis ground) on C1 for a given gain resistor (shown in Table I of the and C2. If the mic preamp is accidentally schematic) were derived using the equation connected to a line driver output in another device, the positive sides of these capacitors are 10k driven to the potential of the positive rail of the A =1 + for the THAT1510 and V R G line driver. C1 and C2 will instantaneously 1 5k behave as batteries, and attempt to drive the A = + for the THAT1512. V 2 R G inputs of an unprotected mic preamp to some minus potential, typically -25 to -30 VDC. This There is a large electrolytic capacitor in will typically result in destructive currents series with the gain resistor network which helps running through parasitic junctions in the IC. avoidthump when switching gain. This The second scenario results when a mic preamp component is one of the largest components the is connected through a patch bay. If phantom circuit, and in SMT implementations, it may take power is applied, and the tip-ring-sleeve connec- up as much as 40% of the board area. Addition- tor is either inserted or removed, destructive ally, it serves as a good antenna for picking up currents will flow as the tip and ring of the unwanted noise due to its size. We ve included connector are dragged across the contact which the circuitry (unpopulated) for an optional servo is intended to connect the sleeve to ground. which can replace this capacitor. Table II of the schematic shows the parts required to implement The THAT 1510/1512 demo board has the servo circuitry. protection diodes to address these scenarios. These diodes shunt potentially destructive To ease prototyping of specific applications, currents to the rail before they can turn on spare circuit board area is perforated with plated junctions inside the IC. R8 and R9 limit the through holes. This makes this board particu- maximum current without severely affecting noise larly useful for experimentation. performance. Note that 1N4148s and the like will not work in this application, since they are not With a THAT 1510/1512 Demo Board and rated for the peak currents that can be expected. standard audio cables, an engineer can begin to Likewise, back-to-back zeners won t work for the evaluate the performance of the THAT 1510/1512 same reason. Either the currents are too high, or microphone preamplifiers in seconds, and start R8 and R9 must be increased to the point that trying out new circuit ideas in minutes. their noise contribution becomes an issue. Using THAT Corporation 45 Sumner Street Milford, MA 01757-1656 USA Tel: +1 508 478 9200 Fax: +1 508 478 0990 Web: www.thatcorp.com Copyright 2005 - 2012, THAT Corporation All rights reserved.