Project files: Little helpers – Alps PCBs

These “little helpers” are small supporting boards I have developed for my various audio projects. I have a few different ones and I don’t think they are really “big” and sophisticated enough to warrant a post for each one so I’ve decided to lump them together in a few groups instead.

What is it?
The first collection is PCBs for mounting Alps (and a few other types, including the Noble I have used in some of the pictures :) ). Included are boards for different types of potentiometers namely stereo without motor and quad with and without motor (i.e. Alps types RK27112 and RK27114/RK27114MC).

For the unmotorised version there are two variants: “Preamp” groups input and output terminals together and “Breakout” groups the individual decks of the pot together. The “preamp” version is used when you want the signal to flow through the pot (such as in a pre-amp), whereas the “breakout” version is used when you want to insert the pot into an existing circuit, i.e. for wiring an off board pot to an amplifier PCB that has onboard space for it. This distinction is mainly to help simplify wiring – unless the circuit has a really strange design with respect to grounding, you can of course use the boards interchangeably.

How big are the boards?
Small… I don’t want to list them all here. The biggest board is the quad motorised which measures 3.5″ x 1.75″ (app. 89 x 45 mm).

What is the status of the boards?
The boards have been tested in v1.0 and for a couple I made some cosmetic changes, including changes to the footprint of the motorised quad pot, to upgrade them to v1.1.

Does it use any special/expensive/hard-to-find parts?
Mostly there’s only one real part on the board and that is the pot itself which can be a bit expensive, so yes, I guess so :D

Anything else I need to know?

  • The stereo boards have a four-hole footprint to fit various pots that use that configuration, but the last set of pads isn’t connected to anything so will not work directly with a loudness tap.
  • The various quad boards have all holes in the same places so the motorised and unmotoriseed boards can use the same chassis footprint (even if of course the unmotorised board is smaller)
  • The “preamp” style boards have a ground plane and a ground pad that can be used if you grounding scheme requires the shaft of the pot to be grounded. Use a piece of wire connected from the ground pad to either one of the screws on the back of the pot or soldered to a ring terminal wedged between the pot and the chassis.
  • On the motorised quad pot the shaft is connected electrically to the solder pins for the motor housing, so if you want to ground one of these you only need to connect a small jumper on the underside of the board from one of the housing pins to the ground pad – no messy wiring needed.
  • The eagle footprints for the quad pots are not “official” but ones I made myself by chopping up the stereo footprint. They work fine, but they probably aren’t the last word in accuracy and the holes for the housing pins could have been done more elegantly…

Downloads:
Download design files here

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

As usual, please remember to consult the manufacturer’s datasheet as well.

 

EDIT July 28th 2014: Added pictures (which I stupidly forgot when I posted this…)

DC-DC converter experiments…

A while ago I started looking at DC-DC converters for powering some of my audio circuits. I am not quite sure what triggered this to be honest, but mainly it was because I wanted to be able to make some more compact headphone amps and preamps than what the usual linear supplies can provide. At least around here there are very few external dual supplies available and so building a preamp or a headphone amp with an external PSU can be a bit tricky. Another option is a single-rail amplifier design, but they are often compromised because they need big input and output caps, so what do you do then?

Enter the DC-DC converter, a switching device that chops up a DC voltage and then transforms it up or down as needed. Most types include regulation of the output and galvanic isolation as well. The DC-DC converter gives tremendous flexibility because instead of needing a 2×12-15V PSU with a transformer, rectifier/caps and a wasteful linear regulator that must be heat sinked, you get a very small PCB that only needs a single DC input from a wall adapter or other DC source to work.  Some, like the Traco THN-series I have used offer a 4:1 input range and up to 20W output from a 1″x1″ brick, meaning you can get a fairly powerful regulated +/- 15V PSU in a tiny form factor and with just a single 9-36VDC input  – that’s pretty impressive I think!.

Another useful feature is that many of the converters have a remote on/off feature to put the converter into standby-mode. This means that if you are using a circuit that already has a micro-controller built in you can turn the converter (and the circuit it powers) on and off using the uC or even by a simple low-voltage mechanical switch.

The downsides of this power and versatility are mainly that DC-DC converters can be quite expensive and then that they (like most switching supplies) have rather a poor reputation for audio use. In some cases that is probably well-deserved, but I don’t like ruling out a whole technology just because of a few poor examples – in my world there are very few absolutes like that in audio. However, using switching technologies of any kind doesn’t really eliminate problems, it just replaces one set of well-known problems (heat, space, weight etc.) with another, perhaps less common set (EMI, high-frequency noise, inrush currents etc.). Depending on the application these “new” problems may be significantly harder or easier to deal with than the “traditional” ones.

So, after a bit more digging, reading datasheets and application notes etc. I have built some prototype boards for various DC-DC converters. Below are some pictures of my prototype boards, using industry standard footprints for 1”x1” and 1”x2” converters (more versions in the works). I’ve followed the application information from a few different Traco and Recom application notes but the boards will generally fit converters from a variety of manufacturers – one of those pictured are some old Tyco ones I bought a few years ago without really being sure what I wanted to use them for.

As for audio quality I can’t really say anything just yet, but as one of the pictures below show I have tried to power my universal preamp/headamp from a +/- 12V converter and I think it sounds pretty good. There are no audible artifacts such as hum or noise from the converter and the remote on/off feature works great as well. So while more testing is definitely needed, I am already starting to look at look at some of my current projects in a new way :D If anyone has used standard DC-DC converters to power audio circuits I’d love to see some examples :)

A comment on audio reviews

No, it’s not me making it….

I normally don’t just link to contents like this, but if you’ve ever read an audio review and been annoyed/frustrated/puzzled about how reviews are written then this post by Srajan Ebaen (the owner and founder of 6moons.com) is well worth a read. You may not agree with the man, but he does make some important points I think.

Incidentally, both 6moons and digitalaudioreview.net where the post is published are well worth following if you aren’t already :)

Project files: High current regulators

What is it?
These are high-current regulators designed for LM/LT108x-type regulators with current limits up to 5A or 7.5A dependent on the package. There are two versions, one with an on-board heat sink (as used to power the JLH1969 here) and the other without an onboard heat sink. The PCB with heat sink is intended for regulators in TO-220 packages. This will give you up to 5 amp current capability with an LM338 or a LM/LT1084 reglator IC. The PCB without heat sink is intended to use a TO-247 packaged regulator (LM/LT108xCP – up to 7.5A output current) and should be mounted on a suitable heat sink instead. Whether you use one or the other board version, remember to always calculate the heat dissipated in the regulator – if you are expecting to draw a couple of amps or more, the heat dissipation in the regulator quickly becomes quite large.
Also included is a small DC-DC regulator that fits on top of both regulator PCBs and can be used to generate an additional DC voltage from the main rail to power auxillary circuitry. Depending on the voltage differential and current draw, the AUXreg can use either a standard 78xx regulator or a switching type like the Traco TSR-1 or the Recom R78xx. The compact size means it can also be used as a “voltage thief” in many other places where you have a main DC supply but need a small extra DC voltage for a fan, a microcontroller or similar.

How big are the boards?

  • Regulator with heatsink (onboard-HS): 3.925″ x 20″ (app. 100×51 mm)
  • Regulator without heatsink (non-HS): 2.0″ x 2.0″ (app. 51×51 mm)
  • AUXreg: 0.4375″x2.0″ (app. 11x51mm)

What is the status of the boards?
The onboard-HS board is in v1.1. I built v1.0 and made some small adjustments subsequently, including moving the regulator footprint a little forward because it was too close to the heat sink and also tweaked the silkscreen a bit.
The AUXreg and the non-HS regulator are both in v1.0. I have built the AUXreg and it seems to work well as-is. I haven’t built the non-HS regulator but it is electrically identical to the onboard version it should be fine.

Does it use any special/expensive/hard-to-find parts?
Not really. The only potential exception is if your application requires a switching regulator for the AUXreg board – they can be a bit expensive.

Anything else I need to know?

  • The output voltage on the main regulator can be variable within a certain interval. If R1 is 121R, R2 will set the minimum output voltage (use my spreadsheet to calculate) and using a 500R/1k trimpot for P1 will give app. a 5V/10V adjustment range on the output from the minimum voltage. Note that with R1 = 121R, then C3 should be at least 22uF.
  • For the small caps on the main regulator boards, I recommend types that are rated for a 105C operating temperature as these sit very close to the heat sink. A 105C rating will help improve the reliability and overall lifespan of the caps.
  • The on-board heat sink is a Fischer type SK68/50. It is possible to fit a 40mm fan to the slots on top for improved heat dissipation.
  • Mounting considerations: Mounting the regulator and the PCB to the SK68 heat sink requires a bit of manual support because the screws tend to “slip” sideways in the slots. Also, you’ll want to mount the C1 capacitor last as it obscures the access to the mounting screw for the regulator.
  • For the AUXreg: The cap values C1 and C2 should be 330nF and 100nF respectively (ceramic or film types) for an 78xx regulator. For a switching regulator normally only a small electrolytic is required at the C1 position, but please consult the datasheet for the specific regulator you are using to be sure.

Downloads:
Download design files here

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

Always read the manufacturer’s datasheets for regulators etc. to confirm component values are correct. Even for “generic” types there may be slight differences between each manufacturer’s recommendations.

Project files: Mini JLH1969

What is it?
This is the project files for my version of the JLH1969 amplifier as shown here. The files for the matching PSU will follow shortly.

How big are the boards?
The board measures 2.25″x2.3″ (app. 57x58mm).

What is the status of the boards?
The board is in version 1.0. I have built and tested a couple of prototypes (as described in the original post) and although they play music just fine, you should still consider this a “work-in-progress”. I would be especially concerned with confirming the thermal stability of the amp because of the compact dimensions, so if anyone decides to build this please share your results :)

Does it use any special/expensive/hard-to-find parts?
No. I have used MJL21196 output transistors because I had them, but versions of the MJE/TIP3055 should be OK as well as long as they are in suitable (meaning TO-247 or TO-264) packages. Do not be tempted to use faster transistors such as the MJL/NJL3281-types as the amp will most likely not be stable with these. Have a look at the TCAAS page on transistor substitutes for more info.

Anything else I need to know?

  • The only change I have done from the original schematic is to replace R5 with a 200k trimpot to make it easier to adjust the operating point. Oh, and I’ve made up designators for the parts that don’t have a number on the original schematic ;)
  • In my opinion the output cap should be 4700uF or larger, even with an 8 ohm load. Even if the supply voltage is 27V, I’d be comfortable with using a 25V cap in this position. All the other electrolytics should be 35V or higher.
  • Heat sink T3 as it might get hot. You’ll probably not be able to find a standard heat sink that fits. so my recommendation would be a small piece of bent aluminium or copper with a hole tapped in it for easy mounting.
  • Raise the R2 resistor 5-10 mm above the board for cooling (I haven’t done this in the prototype version but it gets quite hot).
  • The footprint for the input transistor is for a 2N3906 as described in the original BoM, but other types can be used as well. An easy to get choice is a BC560C which needs to be turned 180 deg. in order for the footprints to match.

Downloads:
Download design files here

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

As usual, both TCAAS and this diyaudio.com thread contains lots of worthwhile information.

minijlh1969pcb-1

JLH 1969 PSU

As promised, here is the matching PSU for the mini JLH1969 amp. Based (roughly) on the PSU designed for the JLH1996 amp, this is a single-rail regulated PSU based on an LM1084 (or LM338) regulator giving a 5A current limit. The board measures 5×10 cm and is intended to be used for one amplifier channel. The onboard heat sink is a Fischer SK68/50 which should be enough for an amp that draws app. 1.3A, especially if the low-drop LM1084 regulator and a suitable transformer is used.

The PSU works, but I haven’t had time to test it properly with the amp just yet (hopefully in the coming weekend). More pictures and information when that is done and I can post the project files :)

jlh1969psu-1

WCF tube amp…

Another installment in my “take a cheap ebay-kit and tweak it” series – a tube headphone amplifier :D. I do what I normally do with these – I prefer to buy a kit instead of a finished board (even if the price difference is very small) and then replace any parts that look “questionable” or should be upgraded.

The circuit in question is a White Cathode Follower (WCF) and the tubes needed are (equivalents of) ECC88 (input) and ECC99 (output). I haven’t actually checked if the circuit is properly calculated as a WCF, but it’s probably close enough for me… :) The main attraction of this kit was that that it was simple, cheap (25 USD shipped excl. the Alps pot and the tubes) and that it needed voltages that I already had a suitable spare transformer for (200VAC and 6.3VAC). I have reused most of the components that came with the kit, diodes, resistors, screw terminals etc. The tube sockets I replaced, but the ones that came with the kit were quite good as well. For 25 USD including postage anything other than the PCB I almost consider a bonus, really :)

One thing I have replaced though is all the electrolytic capacitors. They looked fine on the outside, but when I checked them against the manufacturers datasheet I found problems on a couple of them –  the case size wasn’t listed for that particular capacitance/voltage combination!. This is a major warning sign that either means that they are a special-order item bought cheaply as surplus by the kit manufacturer (possible) or that they are of questionable origin, i.e. counterfeit, relabeled used caps or similar (which is sadly also definitely possible).

Since we’re dealing is a high-voltage amp I did not want to take any chances and so I purchased new electrolytics from Mouser which should be completely safe. I also chose different values (for better bass with low-impedance phones) and higher temp. rating (for increased reliability).

Soundwise this definitely sounds like a tube amp :) The bigger output caps gives decent bass even with mid-impedance (60-150 ohms) phones and moving to higher impedance phones seems to improve the sound further. I have no imminent plans to case this yet, but as a cheap and cheerful build I am quite happy :D

JLH 1969 revisited…

Finally a bit of progress around here :D

Even though I have already built a Linsley-Hood JLH1969 clone and another is in the works, I still wanted to make my own layout for this classic amplifier – and here it is!.

The board measures measurements are 2.3″ by 2.25″ (app 57×58 mm.) and as usual the layout is as tight (some would say “cramped” as I could make it. The transistors used are based on Geoff Moss’ recommendations. I actually have some NOS 2N1711, but the BD139-16 actually measures better (higher hfe). The output devices are MJL21196 because I had them available and because the larger TO-264 package is nice.

The board will be powered by a regulated PSU (hopefully ready shortly – the last parts are in the mail) based on a normal 3-pin regulator as shown in the 1996 JLH article (if all these references seem a bit like you have to be “in-the-know” to keep up, I suggest you start reading the pdf-articles here instead :) ).

Now, I haven’t really done much with this beyond testing that it plays music, but when the PSUs are up and running I’ll look into more testing. In terms of value-for-money this isn’t likely to compete with the cheap ebay-kits, but hey – if I wanted to save money I wouldn’t be building amps I would just buy one that worked! :D

Thirty thousand and counting…

Another milestone of sorts: Yesterday this blog reached 30k pageviews, which means twice as many views as in the beginning of the year :) The views come from all over the world, more precisely 90(!) different countries according to the WP stats.

I am very grateful that so many people from all over the world look at my projects and I will do my very best to keep posting updates. My job still keeps me very busy (I am in fact writing this from a hotel room in a different country and not from home :) ), but even if the frequency of posts is a bit reduced compared to when I started there is still plenty of stuff in the works :)

Until those things are ready to show – all the best to all of you out there! :D

Project files: Audio meters

What is it?
Two designs for audio level (VU) meters. One is a driver for analogue VU meters based on this design.
The other is a small VU meter based on the National/TI LM3916 VU-meter IC. This design comes from here and incorporates a peak-detector with (adjustable) hold.

How big are the boards?

  • Analogue driver: 1.95″x1.625″ (app. 50×41 mm.)
  • LM3916 meter: 2.175″x1.275″ (app. 55×32 mm.)

What is the status of the boards?
The analogue driver board is in version 1.1. It has been built and tested and looks to be working (only a quick functional test, nothing extensive). I have made some cosmetic fixes (silkscreen etc.) for v1.1.
The LM3916 layout is in version 2.1. It is a revision of an old design of mine, so was upgraded to v2.x. I built and tested v2.0 and then fixed a couple of mistakes (including an unconnected power rail – doh!) to get to v2.1.

Does it use any special/expensive/hard-to-find parts?
Not really. The 10-segment LED matrix for the LM3916 circuit is available in both bicolor (red/green) and tricolor (red/yellow/green) versions that match the audio indication levels of the LM3916, but they can be a bit hard to find. I believe they are made by Kingbright and they typically show up on ebay – i honestly can’t remember where I bought mine, but it was probably there :D

Anything else I need to know?

  • Due to the peak detection circuit, the LM3916 circuit isn’t likely to work with the LM3914 (linear scale) or LM3915 (“standard” log scale) ICs. If this is what you want, have a look at the respective data sheets and google – plenty of other designs out there then :-)
  • The LED matrix should be mounted on the back of the PCB, but then the board should be reversed for mounting on a panel. The gives the correct orientation for a multicolor matrix as well.
  • You should note that the analogue driver is designed with a balanced input.

Downloads:
Download design files here

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

Please refer to the respective design articles and to the data sheet for the LM3916 IC for additional information.

univumeter-1

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