Project files: The NE class A headphone amp…

I’ve now finished my testing on the NE class A headamp I described earlier, so here are the design files if anyone else thinks it looks like an interesting design and wants to give it a go 🙂

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Project files: The Borbely non-hybrid headamp

To supplement the original Borbely tube hybrid headphone amplifier are here the files for the solid-state version as described previously here. Have fun!

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Project files: Filtered IRM power supplies (part 1)

As promised a while ago, here are my designs for the “filtered” power supplies based on IRM AC/DC modules. These are excellent for adding compact and powerful single and dual supplies which still have a reasonably good performance to any small preamp/headamp amplifier.

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Project files: Electronic Load files (untested)

My post on Kerry Wong’s electronic load continues to be one of the posts that consistently drives referrals and traffic to the site, even four years on from publishing it. I guess this is more of a testatement to Kerry Wongs’s original idea and design than to my efforts, but it also means that regularly someone will email me and ask what happened to the project. For the last few years I’ve only really be able to answer “not much more than what you can see” and “no, I am not expecting to do anymore about it in the foreseeable future”.

However, I have been thinking that I should get a grip on myself and at least package up my design files so that someone else might be able to carry the project forward forward. Last week another email from a reader managed to prod me just enough that I finally relented and started doing just that, so here I am publishing something that I generally do not like – a completely untested design. However, i do hope that someone can continue with the design and get it tested and running because I’m sure it is worth it (and if at some point I need a load myself, I will be grateful that someone else has done the work 😉 )

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Project files: Hypex UcD400OEM adapters

Well, since I shared these on diyaudio already I supposed they should be here as well 🙂

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Project files: The “MoFo” power follower

I did this version of the “MoFo”-design a while ago and also mentioned it briefly (here) but didn’t manage to complete it or even test the boards. In the mean time the “official” boards have become available from the diyaudio store, but since I now finally got round to testing my boards I still thought I’d share my version as well.

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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: 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 🙂

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.

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?
No.

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… 😉

Downloads:
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.