Parallel-86 — Rev. 1.0

2016/09/04: The boards from the final run of the Parallel-86 boards have sold. The Parallel-86 is now officially in support-only mode due to TI’s decision to discontinue the LM4780 IC. Thank you to all who supported the project.

The Parallel-86 is a composite amplifier providing true world class performance. This performance level is reached by using a high-performance audio op-amp to perform error correction on a high-power chip amplifier. The chip amplifier uses two output channels in parallel for higher output power and higher current drive capability. The Neurochrome composite amplifier topology results in a high-performance, high-power chipamp-based amplifier with vanishingly low THD and phenomenal sound quality.

The Parallel-86 is a turnkey high-performance analog subsystem. It achieves state of the art performance using readily available off-the-shelf parts (see below). The Parallel-86 uses an LME49710 precision audio op-amp to control an LM4780 power amplifier. An OPA2277 precision op-amp is used as a high-end DC servo with third order filtering for an ultra-low output DC offset and fast settling time. The DC offset is typically less than 200 µV and is reached within a few seconds of power-up. A THAT1200 differential receiver provides common-mode rejection rivaling that of a line transformer, thus, minimizing hum injection by eliminating ground loops in the signal path.

Key features of the Parallel-86:

  • Ultra-low 0.00012 % THD (1 W, 8 Ω, 1 kHz).
  • Ultra-low 0.00020 % THD (55 W, 8 Ω, 1 kHz).
  • Ultra-low 0.00043 % THD+N (55 W, 8 Ω, 1 kHz).
  • 60 W output power into 8 Ω.
  • 120 W output power into 4 Ω.
  • Easily bridged for output powers exceeding 200 W (8 Ω).
  • Ideally suited for high-power amplification.
  • Differential input with nearly 90 dB CMRR eliminates hum and ground loop issues.
  • Phenomenal power supply rejection ensures consistent, high performance even using unregulated power supplies.
  • Elaborate use of planes and copper pours to maximize circuit performance by minimizing supply and ground impedances.
  • Low-inductance signal ground connects to power ground at one point only for maximum performance.
  • On-board Zobel and Thiele networks for maximum stability even with capacitive loads.
  • On-board EMI/RFI input filter and ESD protection.
  • On-board low noise voltage regulators for the driver op-amp and DC servo.
  • Power and output terminal blocks accept wire sizes up to AWG 10 (5.2 mm2).
  • All leaded. Easy to solder. 98 × 85 mm board footprint.
  • Bill-of-materials includes a Mouser Electronics Project Link as well as Digikey part numbers.

An assembled Parallel-86 board is shown below. This is the exact board used for characterization of the circuit and measurements posted here.

Parallel-86, assembled module

The total parts cost for the board shown above is approximately $60. A reasonable build budget for a complete stereo amplifier, including chassis, heat sinks, connectors, etc. is around $450.

 

Specifications

The full set of specifications for the Parallel-86 are tabulated below.

Parameter Value Notes
Output Power 60 W 8 Ω
THD 0.00012 % 1 W, 8 Ω, 1 kHz
THD 0.00020 % 55 W, 8 Ω, 1 kHz
THD+N 0.00043 % 55 W, 8 Ω, 1 kHz
IMD 0.0012 % SMPTE, 60 Hz + 7 kHz, 4:1, 55 W, 8 Ω
Output Power 120 W 4 Ω
THD 0.00021 % 1 W, 4 Ω, 1 kHz
THD 0.00033 % 110 W, 4 Ω, 1 kHz
THD+N 0.00051 % 110 W, 4 Ω, 1 kHz
IMD 0.0017 % SMPTE, 60 Hz + 7 kHz, 4:1, 110 W, 4 Ω
Gain 20.0 dB
Input Sensitivity 2.20 V RMS 60 W, 8 Ω
Bandwidth 0.065 Hz – 85 kHz 1 W, -3 dB
Full-Power Bandwidth 77 kHz
Slew Rate 15.5 V/µs 8 Ω || 1 nF load
Noise Floor -135 dBV
Residual Mains Hum -105 dBV Power-86 with recommended power transformer.
Dynamic Range (AES17) 114 dB 1 kHz
Common-Mode Rejection Ratio 89 dB 1 kHz
Common-Mode Rejection Ratio 86 dB 20 kHz
All parameters are measured using the recommended power supply (±35 V).

 

Parts availability

As touched on previously, the LM4780 is being discontinued. In addition, the LME49710NA (8-pin DIP) IC is going away as well. Thankfully, TI have decided to keep producing the LME49710HA (TO-99 metal can package), which is pin compatible with the DIP version. As shown below, it takes a little finesse to get the TO-99 package to fit on the DIP footprint.

2111a

Performance Characterization

The performance level in the Parallel-86 is obtained by meticulous attention to detail during the circuit design process, and, just as important, careful PCB layout. The PCB layout is crucial at these performance levels. The PCB employs differential signal routing and carefully designed copper pours and planes to ensure optimal board performance. All critical connections on the PCB were modeled and optimized using simulation as well as lab experiments. The result is an amplifier that performs impeccably both at high power levels and, perhaps more importantly, at low power levels.

The impressive performance is exemplified in the harmonic spectrum shown below. This spectrum was measured using an Audio Precision APx525 audio analyzer. The Parallel-86 was operating from a ±35 V Power-86 power supply. The full-power THD measures 0.00020 %. Click on any of the graphs below for a larger view.

Parallel-86: Harmonic spectrum @ 55 W, 8 Ω

Even more impressive is the distortion performance at 1 W output power as shown below. For this measurement a precision 1 kHz oscillator was needed as the 0.0001 % THD of the APx525 source was dominating the THD measurement. The THD at 1 W output power measures to an impressive 0.00012 %, consisting mostly of second order harmonics. This distortion profile is reminiscent of that of a single-ended triode tube amplifier and is commonly considered to be the distortion profile that is the most pleasing to the ear.

Parallel-86 Rev. 1.0: Harmonic spectrum @ 1 W, 8 Ω with precision oscillator

The graph below shows the THD+N versus output power. Note that this is THD+N, hence, the ratio of the fundamental signal to the harmonics plus the noise present in the audio band.

Parallel-86 THD+N vs Power, 8 Ω, 1 kHz

Examining the THD+N versus frequency reveals a very flat distortion profile, i.e. the amount of distortion is nearly constant regardless of input frequency. At higher output power, such as the maximum output power shown below, there is a slight rise in THD towards the high-frequency end of the audio spectrum.

Parallel-86 THD+N vs Frequency @ 55 W, 8 Ω

At the first watt, where the majority of listening takes place, the Parallel-86 shows an extremely flat, low THD+N profile. This is very likely the magic behind the very melodic, open, and natural presentation of this amplifier.

Parallel-86 THD+N vs Frequency @ 1 W, 8 Ω

Finally, the noise floor is shown below. This low noise floor is responsible for the dead quiet during quiet passages in the music. The residual mains hum components are well below the audible threshold.

Parallel-86 Noise Floor

 

Circuit Topology

The block diagram for the Parallel-86 is shown below.

Parallel-86 Block Diagram

The LM4780 provides the output power and is controlled by an LME49710 precision audio op-amp. This topology results in a composite amplifier where the small signal performance is controlled almost entirely by the LME49710 precision amplifier. The composite amplifier will, thus, exhibit vanishingly low distortion, stellar power supply rejection, and have a sonic signature very close to that of an LME49710.

Observant readers will notice that this topology is eerily similar to that of the Modulus-86. The topology of the Parallel-86 is indeed derived from the Modulus-86 Rev. 2.0. The Parallel-86 uses the two channels of an LM4780 in parallel to provide higher output power and the higher output current needed to drive low-impedance loads. This also makes the Parallel-86 a good candidate for bridged or BTL amplifiers. The power section of the amplifier was optimized to ensure proper load sharing between the two sections of the LM4780. Note that the ballasting resistors are inside the global feedback loop, hence, the output impedance of the Parallel-86 is extremely low. This results in a high damping factor and firmer bass.

The input to the composite amplifier is provided by a THAT1200 differential receiver. The differential input is ideally suited for differential connections, such as the XLR connections used in professional audio, but can also be configured to accept single-ended connections, such as the commonly available RCA connectors. Differential signaling provides many advantages, the most prominent advantage being the rejection of hum. This results in complete quiet during quiet passages of the music and results in a vast, wide, and open sound stage. Another advantage of the differential input is that greatly simplifies connecting two Parallel-86 boards into a bridged amplifier. Connecting two Parallel-86 amplifiers in a bridge configuration requires two identical boards; one board having its input connections swapped. In other amplifier circuits, several component changes are required to connect the amplifiers in bridge configuration. On the Parallel-86, you just swap two wires and connect the speaker between the two channels. Done!

To avoid capacitors in the direct signal path, a DC servo was implemented. The DC servo uses an OPA2277 opamp, selected for its stellar DC performance. The DC offset on the output of the Parallel-86 is typically below 200 µV. The DC servo in the Parallel-86 uses third-order filtering. This enables a fast settling time of only ten seconds while simultaneously maintaining stellar THD at 20 Hz.

The Parallel-86 has a gain of +20 dB (10×). This value was chosen to ensure a good gain structure in the end application. Should a higher gain be needed, it can easily be modified for +26 dB (20×) at a minimal reduction in THD performance. Should a lower gain be desired for further optimization of the system gain structure, the Parallel-86 circuit supports the use of the THAT1203 and THAT1206 for a total amplifier gain of +17 dB (7x) and +14 dB (5x), respectively.

 

 

Resources

Charles Kitchin, Scott Wurcer, and Jeff Smith. Composite Audio Power Amplifiers. Electronics Now, Nov 1992: 38-44. (.pdf link).

Modulus-86 on DIY Audio: Modulus-86: Composite amplifier achieving <0.0004 % THD.

Sergio Franco. Design With Operational Amplifiers and Analog Integrated Circuits. McGraw-Hill, 2001. ISBN: 0072320842. (Amazon link).

Walt Jung. Op Amp Applications Handbook. Newness, 2004. ISBN: 0750678445. (Amazon link). Also available as a free download from Analog Devices’ website.

Texas Instruments. Active Low-Pass Filter Design. TI, 2002. TI SLOA049B App. Note.