Bridging amplifiers…

In response to one specific (and several non-specific) requests and questions, here is a little overview of what to be aware of when bridging single-ended amplifiers. This could be either two channels on certain ICEpower-modules (with some particular considerations mentioned at the end) or e.g. two PA100/PA150 chipamps. Note: The following assumes you are bridging two existing amplifiers, either at board-level or two channels on a finished amplifier. If you are designing a BTL amplifier from scratch or connecting an amplifier IC that supports BTL mode, I suggest you look elsewhere (such as the data sheet for the IC) or this Infineon application note if you are designing from scratch.

On a normal SE-amplifier, both the input and output signals are referenced to a common “ground” (“0V” is probably more accurate, but never mind that for the moment). When you connect two amplifiers together for BTL you couple their grounds together and drive the signal inputs separately. That way, one amplifier “pushes” when another “pulls” the loudspeaker and in theory you can get four times the power (= twice the voltage swing) this way, but there are a few caveats to note.

Firstly, in BTL-mode each amplifier sees half the effective load impedance. Since most normal amplifiers don’t double their voltage swing into a half-impedance load, the max. power you really get is frequently less than four times but still higher than the original.

Secondly, the lower load impedance also means the amplifier has to be able to deliver more current, so the “price” of more power is typically increased heat and a higher risk of tripping an amplifier’s protection circuitry. This is also why many commercial amplifiers that can be bridged state the they should only be connected to loads of 8 ohms or more in BTL-mode, because the amplifiers are not able to handle the 2-3 ohm loads that they would otherwise see.

Lastly, on the signal side a prerequisite for more power in BTL-mode is that you feed one amplifier channel an input signal that is out of phase to the other. Only this way will one amplifier “pull” when then other “pushes”, which is how you double the output voltage swing. That means you need a “balanced” signal, meaning a signal with separate hot and cold pins with respect to a mid-point (ground). You can’t cheat and connect the normal SE input signal as hot and cold because that is also referenced to ground and you would have grounding issues (hum/noise).

To get a balanced signal you will have to either use a balanced source or include some sort of SE-to-BAL converter in the signal chain. This converter can be passive (= a balancing transformer) or it can be an active circuit either discrete or simply built from two opamps. Last but not least it can be a purpose-built integrated circuit in the form of something like a DRV134/THAT1646 line driver or a fully-differential opamp like the OPA1632 (examples of both of which are on this site 😉 ).

I honestly don’t know the textbook definition of “fully-differential”, but whereas a BTL amplifier is two separated channels connected together, a fully-differential amplifier at least has a feedback loop which is shared between the two sides. A fully-differential amplifier will usually perform any combination of SE<->BAL conversion you can think of. If you want an unbalanced input you simply ground the “cold” side of the signal, and if you want an unbalanced output you can take it from the “hot” output and ground.

Specifically for the ICEpower amps that allow bridging such as the 125ASX2 and 250ASX2: The principles are exactly the same as decribed above and as showed in the datasheet, but there is a “BTL sync” pin that should be pulled low as well. The reason is that the ICE-amps have load-dependent switching frequencies and the BTL sync pin ensures that the switcing frequencies track each other (with a fixed offset between them), no matter how you load the two channels. This prevents any risk of the two channels interfering with each other, even under adverse load conditions. Would it work without this connection – yes it should, but why risk problems that are so easily mitigated 🙂

I know this post should have come with a drawing or two, but I can’t be bothered to do that now – plenty of “googlable” examples anyway 😀

Idling…

Another long break since the last post, but to be honest nothing much has happened over the last few weeks. I have been waiting for some PCBs to turn up and have been busy with non-audio related DIY as well. I’ve also added a few new empty boards from eBay to the project pile but that’s about it.

The PCBs I have been waiting for from manufacturing were a simple dual-PSU based on IRM-modules and another Borbely design – hopefully I’ll have those populated in a few weeks so I can showcase them.

I’ve also previously promised myself to resist the urge to take up more loudspeaker projects, but a few weeks ago a MiniDSP PWR-ICE125 amplifier popped op on my local classifieds page at a reasonable price so now that promise has been broken as well.. No concrete plans yet, but one of the things I’ve investigated before is how to do a “soundbar/soundbase”-style active TV-speaker and some DSP-capabilities would come in handy there. I’ll start by looking at the drivers I have in stock and see if there is anything that be used for that purpose 🙂

Oh, and a final moan about parts availability. I was browsing through Mouser the other day trying to find an 7905 regulator in an isolated package. It looks like JRC are discontinuing some of these negative variants, which is a bit alarming. I genuinely never thought I’d see the day where I’d be concerned about availability of things like three-terminal regulators and BC54x-transistors, but here we are. On the plus side, I’ve noticed that some of the newer audio-grade opamps from TI (OPA16xx) are surprisingly cheap (but of course SMD only) so it’s not all bad but in general parts availability for “normal” diy’ers is declining so rapidly that you can either worry, stockpile or both! 🙂

Trial and errors….

Like most blogs, social media showcases etc. this page is to some extent a massive display of selection bias – you only see the stuff that works, and only when it works. You never (or at least rarely) see the things that don’t work. Because of that, I just thought it would be funny to at least give you a few examples of the memorable mistakes I’ve made during the life time of this blog – along with the lessons I’ve (hopefully) learned from them.

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In search of synergy…

Slightly off-topic post, but I have written a few times about how I think that system-matching is much more important than any “absolute” sound quality, at least as far as enjoying the music is concerned. Well, today was another reminder that I still think this is the case 🙂

A couple of months ago I got new speakers, trading my old (and much-loved) standmount Sonus Fabers for some floor standing Scansonics that offered a bit more low-end slam. I was quite happy with the trade from the beginning and I have absolutely no regrets, but after a time the inevitable restlessness sets in and you start thinking about change (at least I do…). I’ve been running the Scansonics with a simple 125ASX amp on my Harman/Kardon preamp, but just to try it I dug out another ICEpower-amp from my collection, this time based on the older 200ASC-modules.

Although I would definitely still class the 125ASX as the better amp overall, the Scansonics (which are just a little bit bright) immediately benefitted from the more “closed-in” presentation style of the 200ASC, so as usual after initially listening to half a track I started to go through my normal playlist of tracks I know well and just enjoyed listening to some music that I would normally say I know back-to-front already.

To be fair I am honestly not surprised at this, because I saw the same change when I switched from the even older Elac speakers that much preferred the warmer sound of a 50ASX amp whereas the Sonus Fabers really came to life with the more lively presentation of the 125ASX. However, I still think that it’s nice to be reminded once again what really matters when putting a well-rounded system together and of course experimentation is always fun (although it can sometimes be very expensive as well…)

Waiting for parts…

The summer weather still doesn’t show any signs of slowing down here – at least not significantly – and so building is a little on the backburner. However, I have been keeping up a steady flow of PCB-orders over the last weeks (partly my own designs, partly not) so that when I go on holiday in a couple of weeks the finished boards should be waiting for me. Assuming the weather is more suitable for indoor activities at that point, there should be a few interesting things coming up in the not-too-distant future then 😀

Already now though, I have started putting together a few things including another line-level buffer, an ebay tube-kit and a couple of headphone amplifiers but it’s stop-start traffic most of the way. A constant interruption to these builds are a lack of parts – not massively so, but a resistor here and a capacitor there is enough to slow everything down. Case in point is a buffer by Kevin Gilmore where I have the boards (and have had them for a while) and most of the assembly is done, except that I am missing four ceramic caps (odd value and specific form factor) and four RN60 resistors (a standard value that I simply ran out of).

For some odd reason this actually tends to delay overall progress by quite a lot because by the time I’ve accumulated enough volume for an order from a specific vendor and the missing parts show up, usually something else has caught my eye…  😀

Anyway, Mouser order just completed so the last parts for the buffer and a few other half-finished projects should be here by the end of the week. Maybe I should spend my holidays working out a queuing system for new builds of some sort instead? 😀

Spring cleaning…

Although spring is still some way off weather-wise around here, the Easter break certainly gives time for some (much-needed) spring cleaning. We’ve got quite a few official holidays in Denmark already, so with just three extra holidays from your allowance you can get a full 10-day break – nice!

As the weather most of the week turned out to be well-suited for indoor activities (…) this meant I had time to work on getting my “lab” in order and actually start work on some of the various “backlog” projects that my moving uncovered. Highlights worth mentioning 🙂

I mentioned a while ago that I was building an F4 and that is a backlog design that I would really like to finish and try out. I managed to confirm that the boards are working correctly so more or less all that’s left now is to fix a stupid mistake that I made in the (otherwise nearly completed) chassis work.

Another “no-gain” amplifier design is the “MoFo”. As I alluded to in an earlier post I have made my own board which I would also like to finish and test. I’ll be using the smaller 193T chokes as per the build article, so it’s not much power but it should still be an interesting listen. As you can see from the pictures I have the heat sinks sorted, but a few parts are still missing (incl. the chokes). Of course my plan to get the last remaining board parts here in time for Easter did not work when Mouser shipped late, so they should turn up next week instead. But hey, then there’s something to do for next weekend as well 🙂

I also managed to half-finish a couple of new ebay-kit designs, so another Mouser-order is called for to secure the remaining parts. No ETA on either of these, it’s just stuff I bought because I thought they looked like interesting designs.

Last but not least, since I’m not the only one spring cleaning I’ve been keeping my eyes on the local classified sites and picked up a couple of interesting (for me) products that I have been testing. That experience has (once again) convinced me that I am not looking for absolute sound quality, but more an optimal match between the various parts of the system to get the most pleasing result. Maybe a bit contentious for some, but the difference in musical enjoyment is enough to convince me that I am right 😀

 

Slow-turning wheels….

I’ve been complaining about my pile of unfinished projects for the last few posts, but now I am at least at a stage where I can start to do something about it. The first step was of course to add cupboards where I can hide the mess ( 😃), but obviously the only really viable solution is start finishing up some of these projects, so I will try to get started on that during the Easter break – fingers crossed!.

Meanwhile, there are also other stuff to be worked on. One of the projects I did start warming up to has been my ICEpower700ASC-based amplifiers where I have managed to decide on an overall architecture. It’s going to be monoblock chassis with two switched inputs, provision for some form of buffering and an external trigger option. The ASC-board has a whole host of useful features and connections which it would be a shame not to exploit, but in order to avoid too many air wires I decided that a small breakout-board was in order. This will put the control signals and the aux power on more easily-accessible headers. Prototypes have been ordered! 🙂

Buffering isn’t 100% decided yet but since the 700ASC-module has a balanced input, the requirement was really for a fully-balanced buffer – ideally with Bal/SE conversion built-in. The obvious choice for that (and one which I haven’t really used before) is a fully-differential amplifier (FDA) such as the OPA1632. I’ve previously looked at this IC and done some sample board layouts, but nothing ever really came of it. This time, I’ve started from the schematic of AMBs excellent “Alpha24” design and started hacking it to suit my usage. The starting point for the board layout is one of my old ones, but significantly cleaned up compared to those previous experiments so hopefully everything works as it should (the OPA1632 is fairly high-speed and so board layout is a bit critical to get good performance and low noise).

I’ve also made rough mechanical sketches which are only really waiting for the boards etc. to become available so that the dimensions and placement of the various holes can be 100% finalised – paper mockups are a great way to do the initial prototyping though. The turnaround time for board deliveries from China seem to have slowed down a bit lately, but since all the Danish public holidays are basically in April and May there should be a chance to make some more progress when the boards do show 😀

Small thing – big difference

I don’t normally write (much) about the commercial gear that I buy, but for once I’ll make an exception.

I’ve actually owned both the previous versions of the Audioquest Dragonfly (DF) USB DACs but sold them after a relatively short time because I did not really need them anyway. Thanks to an ad on a local classifieds page I now find myself as the owner of the third generation DF as well – the Dragonfly Red. Apart from the new looks – which I really like – the new series of DFs also have the benefit of much lower power consumption, meaning they can be used with mobile devices.

I therefore tried hooking up the DF to both my iPhone and iPad. To be honest I wasn’t expecting that much, but it really does make a significant difference to the sound quality. The downside is of course that your phone becomes quite a bit less portable with a couple of extra dongles and adapters hanging off it, but the benefit in sound quality seems worth it. I wouldn’t use it every day, but if I really had to travel light then using the DF would save me packing and carrying a separate music player which could be very handy.

Photo of the device and a “real” red Dragonfly as well for comparison 😉 (that picture was taken by yours truly in Hong Kong – coincidentally exactly two years ago today)

Shopping in Japan (again…)

Yes, I’ve just returned from a two-week trip to Japan – my third in as many years. Apart from a load of sightseeing and general holiday’ing, just as the two previous trips (see here and here) I had a chance to do some shopping. Not the only reason for going, shopping in Japan is in my opinion an opportunity that shouldn’t be missed for any audio and electronics enthusiast 🙂

Although it is no doubt just a shadow of its former self in this respect, Tokyo’s Akihabara district (and it’s less well-known counterpart in Osaka, Nipponbashi or “Den-den”-town) are still interesting places for DIY’ers to walk around and browse. The pictures below are from a couple of the shops I’ve passed on my way and I’m sure you’ll agree it looks interesting 🙂 Finding adresses can be a bit tricky – and not everything in Japan is on the ground floor for all to see – but there are a few good resources available online on where to go, such as Pete Millet’s “Parts in Asia” page that covers Tokyo and various blog posts.

Is it cheaper than buying online? Not always to be honest, but it’s definitely much more fun! 😀

So, what can (or should) you buy in Japan then?

Well, if you are from Europe like me, most Japan-made items will be cheaper there. If you are in the US, the prices might not be all that competitive for everything but it’s still worth having a look around.

Apart from finished electronics that aren’t wall-powered (anything wall-powered is often 100V-only for the Japanese market and so not useable anywhere else), that means headphones and other gear from the likes of Stax, Audio-Technica and all the usual big-name brands like Sony, Pioneer, Denon and Onkyo. Smaller electrical items which use outboard power supplies may also work, provided you factor in the cost of replacing the PSU and of course accept that the warranty on Japanese items usually isn’t valid outside of Japan.

It also means cables and connectors from the likes of Canare, Mogami and Oyaide as well as a heap of excellent-quality tools. I’d especially recommend the Japanese “Engineer” brand where everything I’ve seen and tried seems to be excellent quality. There are several other interesting tool brands as well, but the stuff from Engineer seems to be consistently good and prices in Japan can be 30-50% lower than the EU prices I’ve seen (although the yen has climbed a fair bit against the Euro over the last year).

I also saw several places selling loose connectors of the most well-known series from Molex and JST. These can be hard to get as well, so being able to get singles just off the street might be helpful. There were a few shops with audio-grade parts like ICs, pots and capacitors and again, Japanese brands like Muse capacitors and Alps pots were generally cheaper. A bonus should be that these parts are probably less likely to be fakes than if you shop on ebay etc.

If you are into tubes, there are a few good places for both tubes and accessories such as transformers (see Pete Millets page for details). Don’t expect to find screaming bargains (although you might) and ignore at your own peril that tubes don’t necessarily travel well and transformers will tend to take a big chunk out of your airline luggage allowance 🙂

Oh, and of course regardless of whether your shopping allowance is more limited (or far greater) than mine, Japan is still a phenomenally interesting place that I highly recommend visiting if you get the chance 🙂

Scope creep…

For those not already familiar with the term it is when projects suddenly grow in size and complexity – often without anyone actually realising why. This little teaser is exactly one of those projects 🙂

I recently (and very innocently) found two PCBs that someone was trying to offload cheaply on diyaudio. I decided to buy them because I like the design and because I knew I had most of the parts to build them (no, I’m not telling you what it is just now 😀 ).

However, once I starting thinking about how to actually use the PCBs, what was originally intended as a simple job for a rainy Sunday afternoon quickly grew a bit more complicated. As a result, I’ve already spent more or less the first Sunday afternoon fabricating two slabs of aluminium to mount the boards on and designing the rest of the chassis components. I still need to go through the BoM to validate that I do have all the components and no – I haven’t even started on populating the PCBs yet 🙂

On one hand it’s part of the fun to just take a project and see where it goes – on the other hand I do sometimes wish that I was a little more realistic with my up-front planning 😉

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