Project files: PA100 parallel gainclone

What is it?
Board files for my “PA100” parallel chip amp with the LM3886 first presented here.

I’ve used the app. note version of the circuit which is non-inverting and uses low-tolerance components to minimise offset between the two ICs. There is also the Jeff Rowland-derived inverting circuit that is normally employed as a PA150/BPA300 configuration with three ICs per board.

I’ve mosty stuck to the datasheet circuit, but in some areas I have drawn inspiration from Tom Christensens article on the LM3886 IC. I’ve used SMT-components where I believe it makes sense to get a tight layout, but mostly its nice and diy-friendly leaded parts 🙂

How big are the boards?
The board measures 3.9” x 2.4” (app. 99 x 61 mm).

What is the status of the boards?
The files are for board version 1.1. I’ve made the following changes compared to the v1.0 prototype.

  • Mute capacitor footprint enlarged.
  • Mute resistor moved to the center of the board to make space for the larger capacitor.
  • Footprint for the LM3886 changed as the holes were very too small.
  • Made a small space between the large reservoir capacitors so they don’t touch each other.

Note that I haven’t tested the v1.1 (yet – will include them with my next PCB order) but I don’t expect any adverse effects of these changes.

Does it use any special/expensive/hard-to-find parts?
Not really, but the recommended resistors are lower tolerance than what is common (the 0805 resistors are 0.1% and the 0R1/3W output resistors are 1%). Mouser has them all and there should be plenty of other sources. The amp will work with standard tolerances (1% for the SMTs, 5% for the outputs) but if you’re unlucky with the tolerances then performance will suffer a bit (higher DC-offset on the output and higher idle dissipation in the ICs). The recommended parts are not much more expensive so I definitely recommend you stick to them.

Anything else I need to know?

  • The gain setting resistors (the SMD-ones) should be 0.1% tolerance for best performance (see above).
  • Similarly, the load-sharing resistors on the output should be 1% tolerance for best performance (see above).
  • The power LED on the board is only between the negative supply and ground, so it is not a 100% indication that everything is OK.
  • The board obviously works with both versions of the LM3886, but I recommend the isolated (TF) version because it’s easier to mount.
  • Decoupling: My decoupling scheme is somewhere between the datasheet recommendation and TomChrs decoupling scheme. The topside parts are intended to be 100nF MKT or X7R MLCCs which is more or less what the data sheet specifies, but on the bottom there are pads for 1206/1210 SMD caps which you can fill with 4u7-10uF X7R MLCCs. You can also use the SMD pads for 100nF MLCCs and then mount electrolytic on top, but there isn’t much space so be a bit careful.
  • The board should be fed from a DC power supply, linear or switching. The large reservoir caps can be as big as you like, but as my prototype boards are intended to be powered by an SMPS (which is sensitive to capacitive loading) I’ve used fairly small capacitors. If you use a linear supply by all means use bigger capacitors.
  • Bridging: You can bridge two boards to create a BPA200 amplifier, but remember a) to lower the supply voltage to around +/-28VDC and b) that you need either a fully-balanced source/preamp or you need to invert the phase using a balanced line driver such as a DRV134/THAT1646 or or fully-differential amplifier of some sort.
  • Mechanics: The C-to-C spacing between the ICs is 1.5” (38 mm).

Download design files here

Related information:
Note: Always read the “intro post” for additional important information about my designs.

You can find additional information about the LM3886 amplifiers in the data sheet, the AN-1192 appnote linked above and several other resources – check them all out 🙂


The “Whammy” headphone amplifier…

Although I did my own version of the Pass/Colburn “Whammy” headphone amplifier before there were boards available for sale in the diyaudio store (and before it was officially called the Whammy), I have still considered getting an original all-in-one board as well.

The cost of shipping from the US originally deterred me enough to do my own version, but a couple of weeks ago a board popped up on diyaudio from a fellow hobbyist in Europe, so I was able to get one at a reasonable cost. Unlike the diyaudio board this one is green (which I massively approve of) and also 2mm thick and plated with gold (ENIG) so it looks and feels really great. Because the board was thicker than usual and I knew I had to mount it in a big case I decided to go “all out”, use tall caps and heatsinks and maybe experiment with turning up the current compared to normal (haven’t do that yet though).

The power supply is running at 20V courtesy of some 7×18-regulators and a pair of green LEDs. This limits my choice of opamps, more or less to either the original OPA2604 or the (now-discontinued) LME49860 which is supposed to be a 22V-tolerant LME49720. Not sure if that is true, but I did chose the latter and I have no complaints about the sound. I might try the OPA2604 at some point instead since I haven’t listened to that since the comparison was an OPA2134 – that’s been a while. The output FETs are the recommended 2SK2013/2SJ313 which I already had matched pairs of, but obviously plenty of other options available that are easier to source.

Just like my clone version this one worked immediately after being powered up, but that is probably more to Wayne’s credit than mine 🙂 I don’t have a case idea just yet, so for the moment it’s going in a box until I come up with a plan for what to do next – still sounds great though 😀

A parallel amplifier with the LM3886…

Gainclones or chipamps are a popular DIY-topic and I’ve done a couple of designs myself and assembled a few others as well. The only one of the “original” National semi amplifier IC’s that I haven’t really done anything with – and coincidentally the only one that’s still in production – is the LM3886.

But not any more, because I just finished a simple design with two LM3886s in parallel configuration. The circuit is built (mostly) according to the “PA100” design from the original National application note (AN-1192) and not the Jeff Rowland-derived PA150/BPA300 that has different configuration and of course a third IC per board.

The configuration with two parallel ICs gives full current output at +/-35V into 4 ohms where a single IC would otherwise be thermally limited, but of course the power is still modest. As I recently swapped my faithful Sonus Faber speakers for a set of Scansonic MB towers which have a fairly low impedance, that’s exactly what I needed though (not to mention that I had a 35V supply left over from another project 🙂 ). The two-chip configuration also means boards can be kept small (and cheap), and there’s still the option of using two boards per channel in bridge-mode to make a BPA200, although the supply voltage would have to be reduced – only the BPA300 will run at 35V rails in BTL-mode as well.

The boards worked first time on power-up and seem to be well-behaved (quick tests only though). I need to do a bit more testing and make some minor (mechanical) changes to the layout and then I’ll publish the project files 😀

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? 😀

Project files: Line Attenuators

What is it?
If you are using a preamp with gain you may have problems with only using a fraction of the available range on your volume control which is very annoying. The problem is usually caused by too much gain and/or an incorrect gain structure. If it is not possible to reduce the gain of one or more of the amplifiers in the chain, a solution can be to use inline attenuators from e.g. Rothwell Audio instead. These are quite expensive though, and they only come in predefined attenuation levels so for testing purposes a DIY-option such as I am presenting here might be better.

The attenuator is built on a small board with RCA sockets for input and output, as well as an option for fitting two parallel resistors on the output side. The gives two (or even three) selectable attenuation values. The selection can be either by jumpers or even via a switch to make the boards suitable for testing etc.

How big are the boards?
The board measures 1.75″ by 0.9″ (app. 44 x 23 mm) – plus of course the off-board part of the connectors.

What is the status of the boards?
The board is in v1.0, meaning it has been tested and confirmed working.

Does it use any special/expensive/hard-to-find parts?
The RCA sockets are clones of Vampire RCAs. They are normally the best board-mounted RCAs I know of and available on ebay. If you don’t want to use connectors or can’t find them, just connect the signal via a 0.1” header (or a JST XH/Molex KK connector) instead.

Anything else I need to know?

  • Important: The reason that Rothwells are built into the RCA-plug is to keep the signal path short and especially the load capacitance on the output side as low as possible. Use the shortest possible cables on the output of these to avoid the cables inducing an RF-rolloff.
  • The resistor values are quite important and should ideally be matched to the source and load impedance. I’ve used this thread (post #6) as a starting point but it’s worth reading up on the theory behind the operation as there are a few trade-offs involved.
  • The center-to-center spacing of the RCAs is 1.1″ (28mm)

Download design files here

Related information:
Note: Always read the “intro post” for additional important information about my designs.

Inching forward…

Another long(ish) break from posting – this time mostly courtesy of some extremely nice late-spring weather and a couple of house-related DIY-projects. Just about the only thing that has moved forward (at least enough to notice) are my ICEpower 700ASC-based mono blocks (which I discussed here). A couple of weeks ago I got the mounting plates I designed for the modules + supporting circuitry which meant I could drill the chassis and start putting some mechanicals together at last.

Some of you may have guessed that this is where my BalBUF design is supposed to end up, but there was a piece missing. A matching power supply to drop the 700ASC’s 15V aux power supply to something more manageable for the OPA1632 (which gets very hot in operation). Because I was running out of space in the enclosure I wanted to use, a key design criteria was that the PSU should be “stackable” with the BalBUF board.

I quickly found what looks like the perfect device for this use – the TPS7A39 from TI – which is a dual pos/neg low-noise regulator with the right specs. Unfortunately, it is also only available in a 3×3 mm leadless package and as my odds of hand-soldering that are pretty much = 0 I dropped that pretty quickly. Instead I went for a bog-standard LM3x7-based design, but managed to squeeze it down to size because of the modest heat sinking requirements.

In a nod to “reusability”, which is something I always aim for where possible, the PSU board includes SMD resistors on the bottom in front of the caps, which means it can also be used with the unregulated supplies on the other ASX-boards such as the 50ASX and 125ASX. This means that you can use the BalBUF with any ASX-module without a separate offboard supply for the low-voltage circuitry, and because the BalBUF and the PSU stack on top of each other it should be very compact. Assuming everything works as expected with the 700ASC when I test it, I’m pretty sure that means I’ve just figured out what to do with my last remaining pair of 50ASX’es 😀

The sketch for the rear panels is also pretty much done, but given that Schaeffer/FPX panel work is getting more and more expensive I have decided not to order the rear panels “blind”, i.e. before I have tested that the monos work electrically. If this weather continues, that might be a while though 😀

Meet the BalBUF…

For a while I have been looking for a simple buffer/preamp circuit that could be used with balanced inputs. In general it would be nice to have, but I have a specific project in mind that would need it (no, I am not going to tell you just yet 😀 ). Also, it would have to be compact and would have to operate on existing supplies. An obvious candidate that I have been interested in for a (long) while is a Fully-differential Amplifier (FDA) in the form of the TI OPA1632.

The OPA1632 includes a Nelson Pass patent called SuperSymmetry (SuSy for short) that gives an inherently balanced topology and therefore allows for all combinations of Bal and SE to be used on both inputs and outputs. To supplement the FDA is what’s called an instrumentation amp front end using a dual opamp. This performs input buffering to keep the FDA happy and can add gain if needed.

The OPA1632 isn’t a new IC by any means, but it is still interesting and something I have been fiddling with for a while (actually for years). However, it had remained on the drawing board and as some prototype boards that I for a long time didn’t really dare assemble and test – I didn’t fully understand the concept of an FDA and so I could not really be sure I had the schematic figured out correctly as I was starting from bits and pieces collated from other designs.

When AMB picked up the OPA1632 again for his Alpha24 (A24) and KappaDCX designs then I finally had a very clear schematic to work from and so I decided to dust off the old board designs and see if I could get it to work this time. In comparison to the A24 I have omitted some of the configurable options of the A24 and deleted the last stage that sums the balanced signal back to SE – that would be rather pointless here 🙂 I was also going to break out and use the enable-pin as well, but as AMB reported (here) that it doesn’t really work well as a mute circuit I decided not to bother.

Technically I haven’t actually used the OPA1632 yet, but instead its “industrial” cousin, the THS413x. There were speculation when the ICs were released that these two are actually the same die but just tested and marked as two different parts depending on achieved specs. Even if that isn’t the case (I don’t think it was ever actually confirmed) they chips are pin compatible and close enough in specs that the differences should not matter.

For the front end I used an OPA1642 which is TIs current highest-spec FET input opamp. It sounds great, but just about any dual SOIC opamp should be fine as a substitute – I just happened to have 3 left over from something else.

As the pics show I’ve just built a single prototype for now which I will keep for testing, but I need to build a new pair as well. Now, as mentioned I am not going to reveal exactly what these are going to be used for because there is a still a piece missing, but if everything works then I think this is actually a very important design (or designs I should say because there is a second PCB on the way as well…)

Project files: Simple power-on delay (with 555 IC)

As mentioned in a couple of previous post I have been looking for a simple delay circuit for headphone amps for a while. The original trigger was the Borbely amp project, but many other circuits benefit from a delay on the output to protect speakers and headphones against turn-on and turn-off transients. My (renewed) search led me to this page which has a great circuit. However, the board also has an onboard headphone jack which I don’t want, so roll out Eagle to do another layout 🙂

I already have made an ATtiny-based delay board that could be used but sometimes you want the bulletproof reliability of a design that doesn’t contain any software 😀 And honestly, using a microcontroller for a simple delay circuit is a bit unnecessary – a standard 555 is just fine.

What is it?
A simple power-on delay circuit that can be used to mute headphone outs, speaker outs or similar to protect against DC transients and also potentially e.g as a B+ delay for tube amps and so on. The board is based on the 555 timer IC in monostable mode.

There are two board versions, one with an onboard relay for headphones and line level signals, and one without a relay for use in other applications and for speakers etc. that require high-power relays. The two boards are identical apart from the size (of course) and the fact that the high-power version has bigger voltage regulator and a bigger protection diode because the relay current may exceed the 100mA that the 78Lxx regulator on the low-power board can supply.

The design has two intentional limitations: One is that the regulator powers the 555 directly, meaning you are in practice limited to using 5V-12V relays. The 555 can handle from 3-18V, but relays are mostly 5V and 12V so normally that’s your choice. However, for most of the intended applications this is just fine and the onboard voltage regulator increases the flexibility a bit (and it can be omitted). The other limitation is that there is only one fixed resistor to set the delay time, so no way to shorten it for testing. However, given the intended application I think that should be OK.

How big are the boards?
The no-relay board measures 1.25″ x 1.75″ (app. 32 x 44.5 mm) and the relay-version is a little longer at 1.25″ x 2.45″ (app. 32 x 62 mm)

What is the status of the boards?
Both boards are v1.0. I haven’t tried the no-relay version yet (prototype boards are in the mail), but the circuits are so close to each other that I am fully confident it will work.

Does it use any special/expensive/hard-to-find parts?

Anything else I need to know?

  • The equation for the delay time is 1.1*Rt*Ct, meaning that a capacitor of 22uF and a resistor of 470k gives a nominal delay of app. 11 seconds (not accounting for component tolerances). If you are unsure about the exact times you need/want, size up the capacitor to the next larger size. Partly because tolerances and leakages in the capacitor may reduce the time and partly because it’s always easier to parallel a second resistor on the back of the board to get a lower value 🙂
  • The header marked “MT” forces the output into mute by simply disconnecting power to the relay. You can skip this feature by simply soldering a bridge across the pads or you can use it for a mute switch. The intention is to have a physical mute switch here, but it can actually also be an electrical switch (transistor) from another circuit. This makes it possible to keep the delay function separated, but still disconnect the output in case of a fault.
  • If you are building the no-relay board an isolated 78xx regulator is recommended to protect against unintentional shorts. If you draw a lot of power (with big speaker relays) or if you use the regulator to drop a lot of voltage, a small piece of aluminium as a heat sink would be required. If you don’t need the regulator because you already have a suitable regulated voltage available, just bridge the input and output pins.
  • If for whatever reason, you need the opposite function of this board, namely that the relay is on during the delay period and then it turns off, then you can simply replace the PNP transistor with an NPN-type with the same pinout (such as a BC54x). Don’t bother asking how I found out that this actually works quite well… 😉

Download design files here (EDIT 11th May 2018: File updated to v1.0a to include a BoM-file as well)

Related information:
Note: Always read the “intro post” for additional important information about my designs.

Building a Kuartlotron… (part 1)

No, I don’t know where the name “Kuartlotron” comes from either, but I can assure you it sounds scarier than it really is 😉

I wouldn’t say simple buffer circuits are a mainstay of this site, but they are definitely both useful and enticing and so when a reader pointed me to the Kuartlotron and its accompanying (and quite massive) diyaudio-thread a while ago, it did pique my interest. A bit of reading later and I was a) still interested and b) deciding to have a go at my own PCB-layout.

The Kuartlotron is the work of diyuser Keantoken and unlike most other simple discrete buffer circuits, it doesn’t employ traditional feedback but instead a type of error correction invented by Professor Malcolm Hawksford. I’ll be honest – I haven’t bothered too much with reading the theory and the technical details, because it’s a simple circuit so it was easier to just have a go at building it.

What I have done in comparison to the original circuit is to lose the thermal coupling between the transistors (which may or may not be a good idea), lose one of the trimpots (because I don’t have the ability to measure THD anyway) and try to minimise the board layout as much as I can.

Now the status of the project so far is that it more or less works. First off I had some major offset-problems and some weird noise issues. Making sure all 4 the transistors were hooked up correctly mostly cured that (…), and so now the noise performance is definitely where I would expected it to be (i.e. there isn’t really any noise 😀 ). The DC-offset is lower than before, but still higher than I would prefer at 25-30mV and the offset adjustment pot doesn’t seem to work as I expected. A few more tests to go then and potentially a rerun of the PCB and then I might publish my final work here, but if I can get it to work then it’s potentially a very interesting circuit either for class D modules in SE-mode or as a simple “no-gain” preamp.

As usual, if you can’t wait for my experiments or if you want something that is tried, tested and supported then I highly recommend buying a PCB from Keantoken instead.

Naim the clone…

Yes, sorry for that terrible headline 😉 One of the ebay-kits I mentioned I my last post is this one.

It’s (supposedly) a clone of the Naim NAC152XS preamp. Now, I’ve not spent a lot of time working out the circuit details (a bit of information available here), but apparently it consists of a simple gain stage and an active filter which also serves as a buffer (and a volume pot in between). It could well be that this is a somewhat bastardised version of the original Naim circuits, but that’s not terrible important for me.

Although part of what made this kit interesting to try was that Naim is one of those hifi-brands that have a distinct sound signature – and a loyal following because of that (some would undoubtedly claim that you definitely don’t buy Naim gear for the looks so it has to sound good… 😀 ). What really caught my eye was that it is a very simple discrete circuit and it is single-supply. Discrete is always fun and while single supply circuits do have some drawbacks (additional capacitors in the signal path etc.), they also have some advantages for DIY’ers. One large advantage is that the single supply rail is normally easier to make and certainly easier to transfer between boxes, so an external PSU suddenly becomes a more viable/desirable option – and that’s where this will go as well when I get that far.

My contribution to this project – apart from soldering all the parts in the right places – consisted of replacing the capacitors in the signal path with Nichicon ES bipolars which are a bit better suited to the job (and nice and green!), and then just matching the supplied transistors as good as I could to make two identical channels. I also supplied the four board-mounted RCA connectors which I had left over – and then immediately after soldering them in place I remembered I had actually put them aside for another project 😉

Now, as usual I don’t post detailed impressions of the sound quality (mainly because I don’t have any right now), but my initial impressions are definitely positive enough that I’ll go forward with finding a suitable enclosure for these boards because I think they deserve that 🙂