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You may have noticed an article in the November 2006 Absolute Sound on switching (Class D/T) amps that concluded that they remain inferior to their Class A/B counterparts and that they are perhaps inherently flawed. Well – you can do something about this! Class D amps offer many modding opportunities – perhaps just as many as Class A/B amps.First – let’s start with a list of issues with currently available class D/T amps:
- Most use one or more op amps ahead of the amp chip to deal with balanced/single ended conversions. The op amps are typically mid-pack performers and their implementation often leaves much to be desired. Interestingly enough – Class D amps don’t need amplification ahead of the amp chip – the amp chip can do all of the heavy lifting.
- To deal with DC offset, the majority of Class D amps use coupling caps – and not especially good ones. If a servo is used, the servo cap is typically crap.
- The amp chip itself typically contains one or more op amps. The manufacturers should at least offer the option of bypassing the internal op amps so the designer has a choice of op amps or can design an input buffer using discrete components.
- The gate resistors used between the amp chip and the output MOSFETs are typically of the metal oxide variety. Sturdy and cheap – but nothing that should be in the signal path of a good amplifier.
- The output LC filters and Zobels are typically implemented with parts costing $2 to $3 and they are every bit as good as the cost would suggest. The inductors are wound with ordinary heavy gauge, solid-core wire and even the best caps used are just so so.
- The power supplies to the amp chips (typically +5V) are completely unremarkable – even for analog. But these are high speed chips that benefit greatly from ultra low ripple, ultra low impedance, ultra stable power supplies.
- If the amps use linear power supplies, they are typically not designed for switching applications. The electrolytic caps are often low-cost general purpose caps – not the low inductance caps one would hope to find (or 4-pole caps for superior noise reduction). I haven’t had a chance to evaluate the switching power supplies used in ICEpower-based amps and Nuforce amps – and they may be better – or not.
- Class D/T amps require filtering on the main AC supply to meet noise emission requirements (and to avoid feeding noise to your front end equipment). The design of these filters often limits dynamics. And this problem is typically compounded by using a fuse to protect the amp rather than a magnetic breaker.
- And then there is the usual lack of care in selecting the RCA/XLR connectors, speaker posts and internal wiring.
What’s remarkable is that with all of these shortcomings in the implementation of Class D/T amps – they sound quite good. Most reviewers and owners have noticed remarkable clarity, deep clean bass and at least decent dynamics. But to get what’s missing – you’re going to have to do some soldering.
Over the next few weeks, I’ll be posting my experiences with modding a Bel Canto Evo 2, Gen II. Why the Bel Canto? Well – the basic design is solid, there’s plenty of room inside the case, the main circuit board isn’t so densely packed that it’s impossible to mod, it has a large power supply and you can pick one up on Audiogon for about $1,000.
I posted the first installment on modding the amp way back in 2005 – and but I’ve changed a couple things so I’m reissuing a revised version below. This first installment is just about some of the obvious basics. The good stuff will appear in Parts II and III to be posted over the next two weeks.
Connector Mods
This Bel Canto Evo 2, Gen II has received rave reviews from Stereophile, Stereo Times, 6moons Audio and SoundStage. And there are certainly aspects of its performance that are class A right out of the box. The midrange is startlingly clear and natural sounding and the bass is ample. But I found the treble to be somewhat recessed, the dynamics to be overly relaxed and the overall presentation to be a bit thin. This first set of mods does much to improve on these weaknesses – all for relatively little money and trouble.
The mods in this post are almost “bolt on” mods. And if there is a theme to the mods, it’s about getting power and low level signal in and high level signal out. Amplifier designers tend to be focused on their circuit boards and as a result -- most amps can be improved by upgrading the pathways to and from the boards.
I started with the IEC power filter module. The stock model is a Curtis F2402CA06 rated at 6 amps. It works in conjunction with a second filter on the Power Board implemented with discrete parts. Double filtering is a common strategy to control outbound power supply noise – and switching amps are definitely noisy.
I replaced the 6 amp Curtis filter with a Corcom 10EC1 “high performance” filter rated at 10 amps. This would be just a bolt-on upgrade, except that Bel Canto sized the opening in the case exactly for the Curtis part. The Corcom module has a slightly larger lip around the IEC connector opening – so you’ll have to file either the Corcom filter or the case a bit. I prefer filing plastic to steel – but the choice is yours.
The first things you will notice with this mod are much better bass response and a fuller, more robust sound overall. Dynamics are better – but still too smooth. And there is some improvement in apparent detail retrieval – even with the fuller sound.
I also replaced the 10 amp slow-blow fuse with a 10 amp Carling Technologies M Series magnetic circuit breaker. Since fuses are inductive, magnetic circuit breakers are a much better choice for amps – particularly switching amps that modulate the power supply at relatively high frequencies.
This mod is even more effective than replacing the IEC filter. Now there is decent rhythm and pace! And curiously enough – you will also notice an improvement in high frequency extension.
There is room in the case to leave the breaker inside – although you will have to open the cabinet if the breaker ever needs resetting. There is also room in the back panel to mount the breaker if you feel so inclined. And I just abandoned the fuse holder in place, wanting to get on with upgrades that improve the sound.
This would be a 5 minute mod except for one little problem. If you check the 10 ohm resistor in the Power Board that is supposed to limit inrush current on start up (R7) – you’ll find that it’s toast. With a low value and a relatively low wattage rating (5W), it can’t withstand the momentary inrush current. Hence, your new breaker will blow on start-up. Since the fuse is of the slow-blow variety, it survives the inrush current.
There’s no way of fitting a properly sized power resistor on the Power Board – but there is plenty of space to mount the resistor inside the cabinet. I used a Dale 100 ohm, 50 watt resistor and ran new lead wires (20-gauge fits the board) between the Power Board and the resistor. I’m happy to report that this resistor has worked perfectly for two years.
While replacing the IEC connector and fuse, you will also notice that there is 20-gauge wiring between the IEC connector, fuse and Power Board. Replacing this with 16-gauge wire (the biggest that fits the board) makes another nice improvement not unlike swapping a circuit breaker for the fuse.
And while you’re thinking about power supply issues, you’ll definitely want to replace the stock 16-gauge power cord. I’d recommend one with high quality gold or silver plated connectors and wire that is more on the order of 10-gauge.
Moving from power to signal, here’s a mod for getting the signal to the Amplifier Board. Bel Canto has mounted a pair of RCA jacks and a pair of XLR jacks on a small Input Board. The RCA’s appear to be Vampire PCB2F/A’s and XLRs look like Neutrik NC3FMD-H’s. The Input Board has switches for choosing between the RCAs and XLRs and for choosing between stereo and monoblock operation. The Input Board is then connected to the Amplifier Board by a ribbon cable that appears to be of non-audiophile origin.
In figuring out what to do with the Input Board – it’s helpful to know a little about the Tripath chip. It’s designed to receive unbalanced inputs for stereo operation or balanced inputs for mono operation. So if you’re going to use the amp as a stereo amp, it’s probably best to use the RCA inputs. If you use the balanced inputs in stereo mode – the positive and negative signals will be summed by op amps on the Amplifier Board. Conversely, if you are using the amp as a monoblock, you should use a balanced input. Otherwise the unbalanced input signal will be parsed by op amps on the Amplifier Board.
Since I’m using the amp as a stereo amp, I focused on modding the RCA inputs. I replaced the stock RCA’s with WBT Nextgen silver RCAs. The Nextgen connectors in either silver or copper are wonderful – and they fit the stock openings in the cabinet. When soldering, be sure to stick an old male connector in the jack to act as a heat sink. I did away with the Input Board entirely and ran lengths of Kimber AGSS directly from the RCA jacks to the Amplifier Board -- but feel free to use your favorite wire. Be sure to keep the wire a bit away from the Power Board or you will acquire a 60 cycle hum.
The input connectors and wire help to address the recessed, somewhat muffled treble presentation. Along with the improved treble performance comes a better sense of the recorded soundstage. And keep in mind that since this mod involves wire – there will be a break-in period.
And then there is the matter of getting the amplified signal off the Amplifier Board. It appears that the speaker posts are also made by Vampire – but they are not Vampire’s best. I replaced the speaker posts with OFC Cardas posts -- my personal favorites – and I like them unplated.
Unfortunately, this is not quite a slip-in-place mod. The chassis holes need to be enlarged and you’ll need a Unibit to do this accurately. In addition, you’ll need a soldering gun with some cahones to solder the Cardas posts – they’re terrific heat sinks. I use a 260 watt Weller gun – a product that can be found at industrial supply companies.
Bel Canto uses an Output Board that performs a filtering function and probably facilitates assembly. The board contains three small ceramic caps to filter high frequency hash from the outputs. I soldered the wire directly to the speaker posts – but I also reinstalled the Output Board to retain the filtering function. The caps are still wired correctly when the output wires are soldered directly to the posts.
You will also notice that Bel Canto ties the left channel output ground to the chassis --apparently their way of grounding the power supply. When you put the Cardas posts in place with the supplied insulated washers -- you’ll lose this ground connection – and that’s a good thing. The correct place to ground the power supply is on the Power Board before the connection to the Amp Board. I soldered a separate ground wire to the ground output pin on the Power Board and ran it back to the unused IEC connector ground pin. Revising the grounding removes some bass bloat, and overtones are more apparent while imaging and detail is improved.
I did not replace the wire and connectors between the Amplifier Board and the speaker posts – at least not yet. I’m doing more mods – and the connectors make it easy to remove the Amplifier Board. The wire is rather light gage and stranded – but the insulation appears to be Teflon.
This mod is less effective than input mod, but is definitely worth doing. The most noticeable improvements are increased body and more powerful bass. Some improvement in overall articulation will also be noticed.
These rather straightforward mods have made more difference than I ever expected. They do a lot to improve the bass and midrange– but there’s still a ways to go, especially with the treble and dynamics. More mods will be posted in a week.
Here’s the parts list for this set of mods:
1 ea Corcom 10EC1 10 amp Power Entry Module $23.65 Digikey
1 ea Carling MA1-B-34-610-1A26-B-C 10 amp Circuit Breaker $23.37 Digikey
1 ea Dale 100 ohm, 50 watt Resistor $5.38 Digikey
1 set Cardas CCBP-S Unplated $49.00 Parts Connexion
1 pr WBT 0210-AG Nextgen RCA $86.80 Parts Connexion
2 ft Kimber AGSS Wire $120.00 Parts Connexion
Total $308.20
Follow Ups:
I especially like the way you detailed the the diameter of the board receptacles. I got into a little trouble replacing the op amps in my PS Audio HCA-2. Nice.I realize that the size of NuForce's case work makes them a questionable candidate for modification but it would be nice to hear your evaluation of the potential improvement. I had fun doing the mods to my HCA-2 and the sonic improvements were very worthwhile.
I'm a fan of class D yet I still found the TAS article questionable considering that publication's selection for Amplifier Of The Year 2005.
Thanks again for your post.
I don’t know much about the NuForce amps. I’ve never had the opportunity to examine or listen to one and there is virtually no technical information available since the amp uses proprietary technology. But NuForce offers a tweaked-out Special Edition model of their own and NuForce amps are being modded by Reference Audio Mods. Rick Schultz (EVS) used to mod them but found them to be problematic and switched to ICEpower amps. (Check out the websites for more information.)One of the reasons I tweaked a Tripath-based amp is that good technical information is available on the Tripath website. There’s also a fair amount of information out there on UCD-based amps, but the NuForce and ICEpower amps are more uncharted territory. And the “try and see” approach to modding Class D/T amps can result in spectacular failures.
I’ll be posting more mods next week and the following week that may provide some ideas for modding other amps.
I believe you may be able to bypass the OpAmp's in some chips.
Actually it’s easier to mod a class D/T amp than to build one from scratch – even if the mods are extensive. Designing a good Class D/T circuit board is a challenge – which is one of the reasons many manufacturers just buy complete amp modules.
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Then you will magically lose the desire/interest to even think about doing silly useless stuff like this.
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