Transcript Digital Amps - Audio Developers

DIY Digital Crossovers
Neil Davis
July 18, 2009
Approaches

Analog
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DSP separates
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Works well, but hard to achieve flexibility
Stopped doing this ~2004
DCX-2496 (or equivalent) + amps
Difficult to integrate with loudspeaker measurement tools
Big, expensive & not DIY
Integrated: amps with embedded DSP

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Uses amps developed for home theater systems (multi-channel) or for
LCD TV’s
Can be designed as a plate amps for making active speakers
DIY: lots of flexibility and more fun
Focus Area: Integrated DSP

Audiodevelopers product range:
Amps with DSP--Example
Lots of Class D
amps have digital
biquads!
Some Amps with Biquads
Plate
Designator amp?
#
Biquads Coeff
per chan size
spdif
Delay
analog in
biquads
DAC
power
amps
Watts per
chan
power
supply
channels sub out sub amp
BCPA-1
yes
7
24
none
none
TAS3004
TAS3004 TAS3004
LM3886
40
on-board
2
analog
BCPA-2
yes
7
24
none
none
TAS3004
TAS3004 TAS3004
AD1994
20
trans or wallwart
2
no
BCPA-3
yes
7
28
CS42526
AD1953
AD1953
AD1994
20
trans or wallwart
2.1
analog TDA8920
BA-4
maybe
5
24
CS4525
CS4525
CS4525
CS4525
20
wall-wart
2.1
PWM
H-Bridge
+ PS
BA-5
yes
7
26
CS42526 TPA5050 CS42526 TAS5706
---
TAS5706
20
trans or wallwart
2.1
PWM
H-Bridge
+ PS
PPA-2
yes
4
24
CS42526
STA328
---
STA328 x
2
30
?
4
no
STA328
SA-1
no
7
26
CS42526 TPA5050 CS42526 TAS5518
---
TAS5518
50
switchers
8
SA-2
maybe
10
24
CS42526 TPA5050 CS42526
---
STA5XX
40
switchers
4 or 8
CS42526 AD1953
optional
TPA5050
none
CS42526
STA308
What’s a Biquad?

…a second-order recursive linear filter, containing
two poles and two zeros:
Implementation:
OK, So What’s a Biquad?

One biquad can be configured as:
 2-pole
low-pass filter (or one pole)
 2-pole high-pass filter
 2-pole shelving filter, low or high
 Peaking filter for EQ—variable F, Q and Gain
 Notch filter
 Bandpass filter
 All-pass filter
 Phase inverter
Cascading Biquads
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
Cascading filters allows making higher order filters
or crossovers
It takes 8 biquads to make a 4-pole 3-way L-R
crossover:
HFP
Q=.7
F=3000
LFP
Q=.7
F=3000
HFP
Q=.7
F=3000
LFP
Q=.7
F=3000
LFP
Q=.7
F=300
HFP
Q=.7
F=300
LFP
Q=.7
F=300
HFP
Q=.7
F=300
Unusual value

Apogee DDX-8001: $5
8 processing channels , with drivers for class D output stages
(30-100W ea)
 Each channel has 10 biquads
 Commonly used in many HT amps
 Similar chip: TI TAS5518, (Panasonic SA-XR series)
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TAS3103: $8
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3 processing channels, each with 12 high-resolution biquads,
plus digital delay
TAS5706: $6
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2.1 amplifier, 20W/channel; 7 biquads/channel + 2 highres biquads for sub channel
How Do You Use These Things?
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Need to calculate the biquad coefficients (b0, b1, etc)
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Need to program the biquads
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Most are controlled by I2C bus, so need a microprocessor
Need to model the response to build loudspeakers
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Requires math--trig and complex variables
Combine driver response with filter transfer functions to view
system response
Manage on-chip resources: volume, channel mapping,
etc
Software/HW Overview
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Unavoidable feature: microprocessor to control the
amplifier
Microprocessor stores amp data, so PC only needed for
designing the speaker
Load
Driver
Data
Calculate
Coefficients
Audio Interface
USB
(Analog, USB,
SPDIF)
Microprocessor
Amps
Crossover Designer
PC
Amplifier
Amps
Early version—2005 or so
LM3886
SPKR
±12V
LM3886
SPKR
Low Voltage
Power Supply
Power
Supply
Sub Out
Analog In
L
R
TAS3004
Analog LPF
& HPF (Sub)
EEPROM
PC Intfc
I2C
3.3V
Connectors
Modules
Power
Budget Computer Plate Amp Version 1
BCPA-2

Digital amps run cooler and use smaller transformers
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Easier to install into a speaker cabinet, and lower cost
Small: can still use small low-cost prototyping PCB’s
Frustrating: both TAS3004 and AD1994 are now obsolete (*sigh*)
Tweeter
TAS3004
Mid or Woofer
AD1994
SPKR
Analog In
SPKR
Connectors
Modules
I2C
EEPROM
Microcontroller
MC9S08 (QG8)
Low
Voltage
USB Intfc
USB
I2C (out)
18V Power
Supply
Power
Budget Computer Plate Amp Version 2
STA328 (DDXi2161)
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“Analog-free”: all-digital signal path
2-channel for active speakers; USB version for computer speakers
Connectors
Modules
SPDIF
SPKR
DDXi2161
Filter
Connectors
Modules
Filter
Low
Voltage
PCM2707
DDXi2161
Filter
SPKR
MC9S08 (QG8)
USB In
Microcontroller
SPKR
SPKR
SPDIF In
Filter
USB Intfc
USB
I2C (out)
MC9S08 (QG8)
SPDIF 2-Channel
Low
Voltage
USB Intfc
USB
I2C (out)
Power
USB 2.1 (40/15/15W)
Sub
Filter
Power
Microcontroller
Multichannel Amps (DDX8001)

Working on a 4+2 board—should be done soon
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50/50/15/15/15/15 with DSP: good for 3-way’s
Has digital delay (each channel) for making steerable line arrays
Picture shows a different version (eval board + control board)

Works in stand-alone mode or with PC-based software
Software
Two Versions, maybe 3
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Biquad Crossover Designer (BCD)
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Smaller amps: 2-channel or 2.1
BCD Multichannel
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3-ways to 5-way systems with flexible channel routing
AudioDevelopers # Biquads
Product
per channel
BCPA-1
7
Digital
Delay
no
Audio Processor Power amps
TAS3004
LM3886
Watts per
channel Channels
40
2
BCPA-2
7
no
TAS3004
AD1994
20
2
BCPA-3
7
yes
AD1953
AD1994
20
2.1
BA-4
5
yes
CS4525
CS4525
20
2.1
BA-5
7
yes
TAS5706
TAS5706
20
2.1
PPA-2
4
no
STA328
STA328 x 2
30
4
SA-1
7
yes
TAS5518
TAS5518
50
8
SA-2
10
yes
STA308
STA5XX
40
EQ-1
12
yes
TAS3013
---
---
Design
Software
BCD
BCD4 to 8 multichannel
3
BCD-EQ
Driver Response
charts
System Response
SystemAmp() = Sys_amp_Data
SystemPhase() = Sys_phase_Data
If biquad windows
not opened
Response File
Biquad charts
Plotdata2() =
SumTarget
System_complex()
Plotdata() =
Biquad_Data
Volume
Updates
If biquad windows
opened
Systemresponse()
Biquadresponse()
Offset and
Phase
Driver_with_offset()
Driver_complex()
Driver_data()
Biquad Updates
Tweeter
Mid
Graphs
FRD_data()
Inputs
Woofer
Bold indicates Complex
BCD (2-2.1 Amps)
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Now in 3rd major revision, (…but none complete)
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Designed to support multiple amp chips

Code for TAS3004 and STA328—others are “placeholders”
“Hardwired” for a maximum of 3 channels
 Current version uses XML structure for saving state
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Driver Input
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Reads FRD or CLIO text files for driver response
Allows adjustment of acoustic center offset
Still working on Min phase algorithm…
System Response graph updated when biquads are programmed
Biquads (one per channel)
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Easy to tweak response and hear effect in real time
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Data gets calculated and sent to the amp for any change
Has equations for many “textbook” filter types
Other Features
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Includes screens to control registers in the chips
Allows saving settings in EEPROM
Controls what the amp loads at power-on
 Allows rapidly switching loads without re-calculating
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BCD Multichannel
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Uses logical channels rather than physical channels
DDX-8001 Example
Prototypes (currently free)
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Send email via PE Techtalk for ExpressPCB files
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Be ready for the “SMD challenge” (actually fairly easy with decent
soldering tools)
Need to program microprocessor—I will do that for cost of postage
Download latest posted SW version:
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http://www.audiodevelopers.com/Software/BCD.zip
http://www.audiodevelopers.com/Software/BCD-Multichannel.zip
Need Microsoft .NET framework version 3.5, SP1
Need to download the mschart controls:


http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyI
D=130f7986-bf49-4fe5-9ca8-910ae6ea442c
Need FTDI D2XX drivers:

http://www.ftdichip.com/FTDrivers.htm
Crippled Marthas
Rushed into Service…

Uses Spherex Xbox360 amp, not fully modded
Still need to isolate digital audio lines…CPU on the board is
partially in control
 Amp is 6-channel DDX-8001 circuit, but is configured
“funny”: bridged sub & single-ended surrounds
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Software to control the amp isn’t complete
“Hardwired” (assembly code) as 5-way with 4-pole LR
crossovers at 100, 500, 3000 and 7000Hz
 All drivers arbitrarily set at same volume level
 No programmable delay control yet (circuitry done)
 No omni/dipole switching control yet (relays inside)
 Probably 2-3 weeks away from being done
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Unusual Features
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“Ring arrays” with no comb filtering due to crossover
frequencies
Super-tweeter uses 8cm headphone elements
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Approx 600 Lumens of light output
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Also designed to accommodate a circular Heil
Total of 28 1W LED’s in each cabinet, but still needs proper
resistors for higher output
3-channel color organ feature not enabled yet (but it works…)
Fully programmable channel mapping, crossover
frequencies, delays, slopes and volume levels

Could be an all-day event in itself
Proposed Follow-Up

“Biquad class” at our cabin in Western MD
 Plenty
of PC’s to design speakers (at least 4)
 I can bring lots of speakers and amps to play with, or
bring your own speakers

MarthaFest
 Compare
topologies (2-way to 5-way)
 Compare crossover frequencies and slopes
 Compare omni to dipole
 Listen to effect of time delay compensation
 Drink beer, experiment, drink beer, etc