Project files: The RJM Emerald RIAA

Last week I showed my version of the “Emerald” RIAA design by Richard J. Murdey. The Emerald is a neat little design: It has switchable gain and load for MM/MC, if you use good components it’s got a very accurate RIAA-curve, and of course with just two opamps per channel as the active devices, it’s very easy to build.

Richard has graciously shared the Eagle-files for his version and so it seems only right that I do the same here. Richard is also selling boards from his website, so if you want something that is proven to be working and comes with support then I suggest you buy your boards from him instead.

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Project files: The Kuartlotron Buffer

Sometimes projects that have been on hold for a long time can restart with just a tiny nudge. A while ago I built (and showed) a clone of the Kuartlotron buffers. My original prototypes had one obvious mistake (an incorrectly connected Q3) which I fixed, but I still couldn’t properly zero the offset as described. I left the project, did nothing about it and then a few days ago by accident went back into the discussion thread on diyaudio. Here there was a single post discussing exactly that issue and a very short response from Keantoken (the “inventor”) that offset had to be zeroed with the input open. This is not what you normally do so I didn’t think about it after building my boards, but that small clue was enough for me to go back to the prototype boards and confirm they were OK. With the problem fixed I can finally share the project files here 🙂

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A real gem…

I’ve already had my hands on a couple of designs by Richard J. Murdey, mainly here and here. However, Richard has quite a few other interesting designs on his site and one of them I’ve had my eyes on for a while, namely the “Sapphire” which is a line/headphone amplifier. Unfortunately my first order of Sapphire boards never showed up and while I was working on the Sapphire I also had a look at the Emerald MM/MC RIAA. Those boards did show up and so while I wait for the reordered Sapphire boards to (hopefully) show up soon, let’s look at the Emerald instead 🙂

The Emerald is very simple RIAA-design with two opamps per channel and selectable gain for both MM and MC cartridges. The RIAA correction is a bit special as described in Richards write-up and the design also includes an on-board discrete voltage regulator.

Richard has graciously shared the Eagle files for his designs so that was what I started from, but what was originally meant to be minor touch-ups ended up taking me a bit further away from the original than I expected. Although the schematic is still (more or less) the same, I’ve made quite a few changes to the board so that it now looks more like one of mine and uses the parts footprints that I normally use – the latter is absolutely intentional, the former a bit less so. I’ve also done quite a bit of rerouting – which I think of as optimizing the layout, but others might disagree – and managed to shrink the overall board dimensions a little in the process as well.

I haven’t really sacrificed anything from the original, except maybe by downsizing the footprint for the output cap a little bit, but that’s hopefully an acceptable compromise for most people. Project files (probably) coming shortly 😀

Building an(other) F5…

Although I recently built a new type of F5 amplifier, I haven’t completely abandoned the original F5 design 🙂 Hiding in one of my many boxes were a pair of half-finished F5 boards and some matching matching fan heatsinks that only needed the last bits of assembly and calibration. That honestly didn’t take long to do once the right parts showed up and I then managed to confirm the boards were indeed working.

The boards were originally bought from ebay and are more or less the same as my original F5 build – nothing special there. I have some matching PSU boards as well, only missing the last few parts which are now in the queue for my next order and that’s going to be a standard C-R-C type thing as well.

The mechanical design is from the same time as my JLH mono blocks, so the idea is also more or less the same. This heat sink profile is too large to fit in most enclosures though, so cracking what to do took some time but I think I have it figured out now. It’s also going to be monoblocks, but much larger ones than the JLHs. From my first tests during calibration of the boards I think a slow-speed fan should be enough to keep the heat under control, so hopefully they will be living-room friendly when they are done 🙂

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).

Downloads:
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)

Downloads:
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 😀