1. Class D Amp Notes

2. Putzey’s Open Loop PWM Model
AES Convention Paper 6690, 2006
“All amplifiers are analogue, but some amplifiers are more analogue than others”

3. Open Loop PWM Challenges
Output is highly dependent on power supply quality
High currents modulate the signal and result in distortion products
Power supply should be a switcher with sense feedback or a linear regulator for best results
Output filter is load-dependent
No feedback from filter results in some interaction with the loudspeaker (peaking or roll-off)
Fixed clock frequency results in higher EMI
Power concentrated in fixed frequencies
Modulator linearity

4. Output Stage Feedback
Feedback taken after output devices improves Power Supply rejection (PSRR)
Most analog input Class D chips use output feedback (including Tripath)
TI and Apogee do not use output feedback, but new TI output stage uses local feedback for improved PSRR
Output feedback can improve modulator linearity
Carrier-based modulators benefit most from output feedback
Output feedback can improve output stage fidelity
This was an issue for early Class D designs with “dead-time” issues, but not a big driver for new designs

5. Modulators Two major classes in use today
Sigma-Delta
Carrier-based (triangle wave/comparator)
Lowest distortion achieved with Sigma-Delta + feedback
Analog Devices achieves .005% distortion levels
“Open Loop” Sigma-Delta achieves good results
Basis for Apogee and TI designs—distortion approx .02%
Carrier-based with feedback is also good
Feedback reduces distortion to .01% levels
Feedback difficult to optimize (power-level dependent)
See “Carrier Distortion in Hysteretic Self-Oscillating Class-D Audio Power Amplifiers”, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 24, NO. 3, MARCH 2009

6. Other Design Issues Self-oscillating vs clocked Filter feedback
Self-oscillating reduces EMI and results in simplified design for analog-input Class D
Digital input is always clocked, but some digital modulators (e.g. Apogee) use spread spectrum to reduce EMI
Filter feedback
Results in lower output impedance and less interaction with the loudspeaker
Low noise
Early Class D amps suffer from Fab process that have high noise levels in analog stages (e.g. Tripath)
Digital input
Simplifies the design for multi-channel (6-8 DAC’s with I-V converters and filters consumes a lot of board space)

7. Output Power Three distinct tiers at higher outputs:
125W: 50V H-bridge chips (STA517B or TAS5631)
300W: TDA8954 or Sanyo STK-428 (plus and minus 40V)
500W+: Discrete outputs (plus and minus 60 to 80)
Note: power ratings are for 8ohms—double for 4ohm
Tripath version at 500W:
TA0105A/TA2500 (or RA2500), based on TC2000 with drivers plus discrete outputs
More complex than newer IRF or ICE designs and with higher distortion
Poor reputation for reliability

8. Other high power options
UCD self-oscillating amps (carrier-based)
Marketed by Hypex; chief evangelist is Bruno Putzeys
Elegantly simple design
Fairly low distortion
Uses output feedback (after the filters) to work with many loads
IR IRF2092
Simple, low-distortion design
Uses Sigma-Delta modulator with output stage feedback (but before the filters)
Zetex (Diodes, Inc)
Used in NAD M2
High power and very low distortion
Direct Digital Feedback Amplifier uses DSP to control modulator with feedback after the filters.

9. Chip Comparison output feedback filter feedback Delta-sigma
self-oscillating
low-noise
Digital input
8ohm
Apogee + STA517b no
yes
125
Tripath + STA517b
sort-of
Tripath RA-0105
400
TI + old output chip 80
TI + TAS5631
Analog: ADAU1592 15
ADAU1592+booster
400?
TDA8954 (NXP)
300
IRF2092
500
Hypex UCD
Modulator + IRS20957
N/A
Zetex ZXCZM800
Yes – “digital”
Yes
Yes?