Monday, 25 March 2013

Class D Audio amplifier - IRS2092S

The problem with traditional amplifiers are the amount of heat generated per unit of consumed energy. Class A amplifiers have the worst efficiency. At best as much as 70% of the energy consumed is wasted as heat and only 30% of the energy consumed is used to drive the speakers. Some hardcore audiophiles swear this is the best topology since it results in the least amount of distortion, due to the fact that the "switches" are always on.

Class D theory has been around for decades. However it's only the availability of recent technology of cost effective semiconductor switches MOSEFTS that operate at high frequency that have made this topology a reality for commercial based applications. Class D audio amplifier topologies are typically around 90% efficient.



Class D - Block Diagram (IR,2003)




Basic PWM Operation (IR,2003)

Pulse width modulation works by a carrier frequency that is fed into a comparator with the desired input signal.The result is a square wave pulse train that is a digital PWM representation of the analog input signal. The square wave pulse train at the amplifier output is then converted into an analog representation by a low pass filter.

The Class D topology is not a silver bullet. It has its own unique problems.

1. The output low pass filter has a poor rolloff typically second order 20dB per decade. This makes EMI a potential nightmare. This EMI/RF propagates through the speakers cables. The cables effectively act as an antenna radiating this as RF energy, potentially causing interference with radio equipment. It would be interesting to look at the radiated emissions using a Spectrum Analyser.
2. Inherently lower damping factor, than class AB amplifiers. The IRS2092S reference design  damping factor is rated at 120 @1kHz & 4 ohm.
3. Bus pumping in half-bridge stereo mode (IRS2092) . Most common at low frequency input signals. This puts energy back into the supply rails. This is a reason why this class D reference design requires a good regulated power supply. In addition more capacitance on the supply rails reduces this effect.
Note there is no bus pumping effect when operating the amplifier in mono full-bridged mode.


Low Cost high performance

Thanks to International rectifier there are now many Class D amplifier modules based on the IRS2092 available at a very modest cost. This gives the DIY'er great performance for the cost.
After looking at buying a Class D kitset, I was tempted to purchase a complete kit from www.classdaudio.com. Their kit and build quality looks top notch. Their kits are supplied with large mains toroid transformers, and rectifier and smoothing board.
I am led to believe these designs are based off the International Rectifier IRS2092 reference designs. So how much better can they be than a reference design module? well the SDS kits offer balanced XLR inputs. I don't see the advantage of using an XLR input if you are running a short distance under 1m from the pre-amplifier to the amplifier. For long runs like an event or a music studio sure but for home use its overkill.

This got me thinking I can build one myself if I can source a suitable power supply, Class D amplifier module and enclosure.

Amplifier Module


I settled on a Sure Electronics Model AA-AB32321 125W x 2. Conservatively rated @ stereo 125W x 2 into 8 ohms or 250W into 4 ohms.
It appears this is a direct copy of the IRS2092 reference design. However Sure Electronics have  used International Rectifier AMPS-200 model, with IRFI4019H-117P MOSFETS.
The gain of this module is 39. 
  • The ON-OFF switch has been bypassed and wired on.
  • The bridge switch is set to non-bridge mode.
  • Only the blue LEDs are present one for each channel. These indicate that the IRS2092 is operational, and that the PWM driving signal is present to the MOSFETS.

NOTE:
  1. Potentiometer P1A and P1B have been set to zero ohms. This is configured as a 2 terminal variable resistor. This should be set to 200 ohms. This will give a nominal operational frequency of 400kHz for the amplifier module.
  2. Check the tightness of the devices on the heatsink. 
One other point to note,  this particular amplifier is suitable for speakers with impedance's between 4-8ohms. With a preference towards lower impedance's. Higher impedance's may cause stability issues.

Board changes

1. The IRS2092 reference design is made for 1 VRMS full scale input signal. This is usually too much for most sources, and as a result full power is reached too soon.
2. Solution change the voltage gain of the amplifier. Stock voltage gain is 39. I changed R7 from 3k to 6.2k. This changes the gain to 19.9 (just over half the gain). What this means in that the input signal can now handle a full scale signal of 1.96V RMS (lets say 2V RMS) line voltage levels. The other advantage of this is that it effectively doubles the input impedance to the IRS2092 (6.2kohm). Make sure that your pre-amplifier has a output impedance of 600 ohms or lower using the factor of 10 rule of thumb.
3. DC blocking capacitor CP1 can be removed (and replaced by a wire link) from both channels if you are certain, you have no DC offset at the output of your pre-amplifier / source.




Power Supply


I settled on a Connexelectronic 500W SMPS, -/+60VDC regulated power supply.
The major advantage of using a SMPS unit instead of traditional mains to secondary transformer then rectification and storage / ripple smoothing capacitor banks is the WEIGHT and SIZE!
The SMPS measures 100 x 100 x 40mm high. At only 400grams.
  • The supply can be adjusted to -/+10% of the nominal voltage. The small adjustment trimpot is between both output capacitors.
  • This power supply has a soft start, I like this feature since it puts less stress on the mains switch (inrush current) upon  power on.
  • Also the power supply has a common mode choke EMI filter circuit built in so you don't need to add one.
  • To get near the distortion figures the specifications hint at, the IRS2092 reference design demands a good regulated power supply (PSRR) that can supply enough current at the set voltage.



A slightly larger enclosure would have been better. The power supply has been orientated this way as the high frequency switching section of the board to the back plate, mains section closest to amplifier this keeps the high frequency switching transients as far away as possible from the amplifier module.

Fairly simple to put together the hardest part is drilling out the holes for the connectors, cut out for the power switch and PCB mounting holes and chassis feet, plus some extra holes to allow some air through.



Make sure that the power switch is mains rated and is switching the phase (live)! and also that the chassis is connected to the mains earth. If you have not had any exposure to working with mains voltages, this is not a project you should be attempting.


Minus the top plate screws. Testing still in progress.

The verdict

For 250NZD I was able to build this amplifier. This is my first experience with a Class D audio amplifier.
Surprisingly it sounds great! definitely worth my time and effort. My impression comparing it to my Modified Marantz class AB amplifier - The lower end bass appears to be ever so slightly tighter and hit a little harder. Otherwise the sound quality appears to be of a very similar nature, I can't hear any other differences.

I like when there is no input signal,music being fed into the amplifier, it goes dead silent.

Running the amplifier at modest but comfortable volume setting for 45mins resulted in a heat sink temperature of 41 degrees C, room temperature 25C.
If the heatsink temperature becomes a problem I could look to build a thermal conduction bridge (an aluminium channel of sorts) from the amplifier heatsink to the enclosure heatsink. 

I really dig the temperatures from this amplifier, compared to a Class AB amplifier. You get great quality sound with better energy efficiency, and if using a SMPS power supply a lightweight solution. The weight of the finished amplifier is around 3kg. Most of it is the rather solid aluminium enclosure sporting left and right side heat sinks and a solid front aluminium panel.

The only drawback so far - Sometimes there is a small but noticeable "pop" on power up of the amplifier, and a small but noticeable "pip" on power down. I emphasize small, but I can live with that.

Friday, 8 February 2013

Improving the Zero DAC

The Zero DAC is a low cost Asian designed and manufactured digital to analog converter pre-amplifier. Can be bought on ebay for under USD130 plus shipping for the OPA627 version plus USB, less for the OPA2604 op-amp version.








The Zero DAC uses the Analog Devices AD1852 24bit, 192kHz max as its DAC IC. The unit is most useful interfacing to digital sources such as:
  • CD player
  • PC

This fits the bill exactly on what I want. The PC connectivity is very handy. The Zero DAC uses the TI PCM2704 USB to S/PDIF integrated circuit (IC) which makes the Zero DAC look like a soundcard. Unfortunately this IC is limited to 16bit resolution and 48kHz sampling rates. However for digital music files that have been ripped from CD's this is sufficient. CD quality is typically 16bit, 44kHz.
I intend on using this unit for both as a digital sources pre-amp interfacing to a class d amplifier.

The unit also has a headphone amplifier however this is the weak link of the package see Headphone Amplifier schematic .  Do NOT buy a Zero DAC if you intend on using it solely for headphone output.

Pros

  • Digital coaxial input
  • Digital optical input
  • Generic USB soundcard, supported by Win7&8, and latest versions of LinuxMint (Ubuntu?)
  • AD1852 DAC IC
  • Volume control (both a pro and a con!)
  • Line voltage output
  • Toroid transformer
  • Op-Amp upgradable via DIP socket
  • The use of ferrite suppression on PCB layout
  • Able to be modified

Cons

  • Headphone circuit sub-standard (refer to headphone circuit upgrade)
  • No unbalanced line outputs. Personally I don't see the need for this unless you need a long run cable between the pre-amp and the amplifier. I plan on using at most a 1m RCA cable! 
  • High quality volume control will need a new headphone amplifier circuit. Since I'm not planning on using headphones I'm going to simplify the existing circuit to use just a pot (stepped attenuator 10k) and an op-amp.  This can be done by utilising the existing headphone PCA. You will need local decoupling capacitors hung off the supply pins of the op-amp say 47nF, I had oscillation at the output. Also a sizable low ESR capacitor should also be hung off the supply pins. The below schematic is of 1 channel. Make sure you use an audio op-amp, the NE5532 is cheap and cheerful that is unity gain stable.


Modifications
  1. Remove(snip) 2 x 22pF ceramic capacitors  from line input.
  2. Increase capacitance on digital supply add small bypass capacitor on rear of PCB (33-100nF)
  3. Increase capacitance on analog supply add small bypass capacitor on rear of PCB (33-100nF)
  4. Change AC rectification diodes 1N4007 to UF4007.
  5. Upgrade supply capacitors on AD1852, bypass with low value capacitor on rear of PCB (33-100nF).
  6. Upgrade supply decoupling capacitor on TCXO
  7. Modify Amplitude / phase response of Op-amp (refer to Bode Plots)
  8. Remove 4 x AC coupling capacitors from inputs of op-amp. These are not required as any DC is common to both inputs and thus rejected (refer to AD1852 datasheet).
  9. Add local low ESR capacitors to supply rails directly off Op-Amp supply pins.
  10. Change to stepped attenuator if potentiometer begins to cause issues. 


Et voila. If you are handy / know what you are doing when it comes to electronics and want a low cost digital source DAC pre-amplifier with potential this is it. The modifications are tedious without a solder sucker desoldering station. However it can be done with patience desolder wick, and using long component leads to help clear the through holes of solder.

I suggest buying the OPA627 version with USB. For around $20 less the OPA2604 version is probably okay too.
Interestingly the 2012 version of the Zero DAC includes the opa637 as an op-amp and more expensive?! The problem is this op-amp requires voltage gains of at least 5 times to be stable. The op-amp circuit in the Zero DAC is at unity gain!

Photo of Toroid transformer

 


Saturday, 12 May 2012

Getting better sound out of the Marantz SR7500

My goal is to improve the audio fidelity of my Marantz SR-7500 AV receiver. The aim is to reproduce sound in the same way as it was engineered in the recording studio (without spending hundreds of dollars).

1. The Marantz SR7500 has the ability to switch off the video decoding circuitry. Marantz calls it Video OFF. In theory by turning off non-required circuitry will reduce conducted and RF emissions.
In 2 channel stereo mode I couldn't hear any perceived improvement.

2. Upgrading the capacitors in the speaker crossovers, from electrolytic to Metallised Polypropylene film MKP capacitors. This should improve the quality of the output from the speakers. MKP capacitors have much lower distortion. The drawbacks, they  are physically much larger than an electrolytics (for the same capacity and voltage rating), and much more expensive.

Older speakers are more likely to benefit from a capacitor upgrade.



3.Other modifications

  • Replace low voltage rail decoupling capacitors with long life low ESR, 105C units. Mainly United Chemi-con, and Nichicon brands.
  • Removal of muting transistors. These are bipolar transistors (BJT) placed across the audio signal path and ground. Its purpose is to mute the audio output when changing inputs for example. There is some perception that removing these from CD players in the analog path yields noticeable improvement. Some evidence suggest the BJT junction has parasitic capacitance that is frequency dependent. This effect supposedly impairs stereo imaging.I am not sure how much truth there is to this claim. There is a real risk that "thumping" may be heard from the speaker outputs if these are removed, especially when changing input sources for example.
         Higher-end equipment tend to use a signal relay instead of a BJT.
  • Removing all possible AC audio coupling capacitors in the audio path of the amplifier. In theory the design of the circuitry by using split dual power supply rails (-ve and +ve) should help accomplish this. The nature of dual supply rails is that the circuit is naturally biased with no DC offset. On the look for DC offset at the op-amp outputs. Such DC offsets can be found reasonably easy by looking at the incoming point of an AC coupling capacitor. Depending on the amplitude of the DC offset will determine if the capacitor stays in or gets replaced by a piece of link wire. Target candidates are unity gain / buffer op-amp stages.



Update 24/0712 Pre-Amplifier Modifications

Removal of AC coupling (DC blocking) capacitors from the audio signal path. The Marantz SR7500 receiver has plenty of scope for this to happen.

1. The circuit schematic of the INPUT board, has many op-amps in buffer configuration. All of these have AC coupling capacitors on both the input and the output.
These can be safely removed. Replace the capacitor with a tinned copper wire link. A biasing resistor to ground will also have to be removed.

2. Volume board, has a set of AC coupling capacitors and bias resistors that can safely be removed.

3. The muting transistors can be removed. I have found no thumping to occur at the speaker outputs when changing input sources.

4. DSP board. AC coupling capacitors and bias resistors can be removed from differential op-amp interfacing to the CS4392 DAC converter.

Note: Care needs to be taken with op-amps with voltage gains  >1.This is because if any DC offset is present to the op-amp inputs, the output will  be amplified by the gain. (both AC and DC amplitudes) it is possible to run out of headroom into saturation (clipping) of the supply rails, resulting in severe distortion.


The result is quite a different sounding amplifier. In contrast, low fidelity audio could be best described as a monotonic bassy boom. This is far from that, much more dynamic and clear. The low frequency bass response is much more tighter.
In a well designed pre-amplifier there should be no AC coupling capacitors in the audio path.

AC coupling capacitors in the audio path are undesirable as it causes distortion and changes the characteristics of the sound.

The end result is I had removed a staggering 58 AC coupling capacitors and biasing resistors! This could have shaved a few dollars off the manufactured cost of the unit. 
The Marantz SR7500/SR8500 are very good receivers if you have some knowledge of electronics and wish to improve the units sound reproduction.

As always - For new players the author does not recommend attempting to repair  mains powered equipment. Mains equipment requires care, intention and are at lethal voltages.

Disclaimer: The information contained in this post is for educational purposes only. The author accepts no responsibility for any damages or harm that may arise.

Wednesday, 29 February 2012

Marantz SR7500 repair


This receiver had 3 different faults, which lead me to think it had been used as spare parts! A quick resistance check of the primary & secondary windings of the mains transformer and no open or short circuits (thank goodness).




http://www.hometheaterhifi.com/volume_12_4/marantz-sr7500-receiver-10-2005.html




Upon opening it up and removing sub-boards - there were the tell tale touch up solder on certain points  - quite obvious it had been looked it.

This unit had been manufactured late 2005, as stamped inside the case. It mostly consisted of through hole (leaded) components on single sided PCBs.

The DSP/CPU board however was a double sided PCB with mostly surface mount components.

image taken from http://www.nubert-forum.de/nuforum/viewtopic.php?f=10&t=16013

What really shocked me was that the manufactured build quality of this unit was far from the best I have seen. Solder joints looked dull (cold), especially on the jumper links of the single sided boards. Spent some time re-soldering these joints.

Power on issue


The power on issue was caused by a low value resistor (essentially used as a fuse - in red). I initially thought it was a faulty bridge rectifier (in green) since the resistor measured OK when not powered  up. There was also no output from the bridge rectifier. After replacing the bridge rectifier I knew straight away it was the resistor. It must have been  going high impedance when current was being drawn from it. This supplies the remainder of the low voltage rails necessary to power up the CPU/DSP section and display.



After addressing this - I was relieved to see the display and unit powered up.However it was now going into Protection mode.

Standby board

This unit uses a standby board. How this works.
1. When the main ON/OFF switch is initially turned on, this powers on a standby transformer. This is a step down transformer that provides a +5V rail to power up the main CPU.
2. A mains relay that is controlled by the CPU. This switches in the primary side of the mains transformer.
3. The CPU performs, some checks namely low voltage line checks and amplifier checks namely:
  • Over current
  • Offset voltage
  • Over temperature (high heat sink temperature >80C, amplifier running at to much load continuously)
Any one of these three conditions will cause the unit to go into protection mode.

Repairing an amplifier channel

The unit also had a blown amplifier channel. This was easy to determine by checking for short circuits across the output driving transistors. The worry with a blown channel is that it is likely to have also taken out a number of other components. This proved to be the case.
Firstly I removed both transistors from the blown channel, as it was short circuiting the audio bus voltage. I did this so I could power up the amplifier to ascertain which channels worked and which did not.

After powering the unit up and driving output to speakers, I found that the front left, front right channels and surround left and right channels were operating. I had no center, sub woofer and rear surround left and right. My initial thought was it had to be something common that affected these channels.
AV receivers have a test tone mode that you can put through each channel. I did this while probing an oscilloscope onto the channel outputs of the Electronic Volume Control IC's (Toshiba TC9482). Sure enough had the test signal coming into the Volume control IC inputs, but no output! Faulty TC9482 (circled in red below).

 image taken from http://www.nubert-forum.de/nuforum/viewtopic.php?f=10&t=16013

These seem difficult to get as they are typically only used in audio equipment.

Going back to the blown amplifier channel - rear surround left.

Using a multimeter set to measure resistance I checked a number of components and found along with the 2 output transistors (big red circles) diodes, and zener diodes had also gone short circuit, the resistor had gone high impedance.


The audio output transistors are difficult to get. Sanken SAP17N , and SAP17P, 5 terminal darlington audio output transistors.

Protection Mode


After replacing these components in the amplifier the unit was still going into protection mode.


As soon as the CPU detects one of the three conditions for protection mode the CPU switches off the main relay and thus the primary power side of the main power transformer. 
There were still problems in the rear surround channel circuit, as I had the amplifier running when I had removed the output transistors.

The next problem was how to run the unit without it switching OFF? Solder a wire across the main relay contacts on the standby board, so the main transformer stays ON!
Now I could measure some voltages without the entire unit powering OFF into protect mode.

I found a -47V dc offset at the output, and virtually -47Vdc on both ends of the AC coupling capacitor (circled in red).The output of the NJM2068 op-amp (in blue) was -11Vdc. This is what was clearly driving the output negative. It looks like when the output transistor pair blew it also took out the capacitor and op-amp input (due to over voltage). 


I'm not sure where to buy NJM2068 operational amplifiers (op-amp) from. Element14 (formerly Farnell) my local go to does not carry them.
I decided on replacing the '2068 with a On-Semi MC33078. Similar specifications on paper, and cheap enough in 1 off quantities under $2 for an audio op-amp.


Replaced both capacitor and op-amp, and removed the wire link from the mains relay on standby board. Presto - unit does not go into protection mode.
Tested each individual channel out driving it with sound - and all appears well and good. I will be keeping it even though it does not have HDMI.


Note: For new players the author does not recommend attempting to repair  mains powered equipment.Mains equipment requires care, attention and are at lethal voltages.

Disclaimer: The information contained in this post is for educational purposes only. The author accepts no responsibility for any damages or harm that may arise.