sjc@angband.UUCP (Steve Correll) (01/14/85)
> The Morrey super buffer uses a 531 op amp, I believe. TAA ran Walt Jung's > articles on op-amp selection for critical audio uses; he thinks the 531 is > a stinker. It also requires an output transistor. > I would suggest building a simple non-inverting buffer, with a voltage gain > of 3, using an NE5534 (single) or NE5532(dual) op amp -- it directly > drives 600 ohm lines, and is very quiet. > Another choice would be the TI bifet op amps, the TL071 family (low-noise), > available for around 75cents from Action Electronics (see ads in > Radio-Electronics). Need an output transistor, however. If one designs one's own circuit, the NE5534 is certainly the amplifier of choice for low input-voltage noise and high output drive current. If one doesn't feel comfortable with analog design and wishes to avoid the NE531, I believe one can substitute the TL071 or LF351 in the Morrey circuit (I don't have a copy of the circuit with me, but both are pin-compatible with the NE531, and the LF351 is a single-amp version of the Jung-endorsed LF353). I'd like to distinguish between providing low output impedance and driving a 600 ohm load. This discussion started when the owner of a preamp with (assumed) 10k ohm output impedance asked whether he could safely drive 20 or more feet of coax terminating in the (typically 50k-100k ohm) input impedance of a power amplifier or VCR. Any of the op amps we've mentioned can do this without the aid of an output transistor; their task is simply to provide a low enough output impedance to ensure that cable capacitance and electromagnetic interference pose no problems. Only if the power amplifier or VCR had a 600 ohm input (never encountered in consumer equipment, but common in professional gear) would an output transistor be necessary to provide the extra current which a 600 ohm load draws. (As you noted, the NE5534 can do without the output transistor even then.) -- --Steve Correll sjc@s1-c.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc