Project files: The ACP+ clone…

Well, both my ACP+ clone boards are now fully populated with relays and working as expected so I guess it is appropriate to share the design files 🙂

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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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Reworking the ACP+…

Last weekend was this year’s “Burning Amp” festival in San Francisco. I wasn’t there (it’s a bit far from Denmark for a weekend trip…), but as usual there was a thread on diyaudio.

Burning Amp has frquently been a “launchpad” for new Nelson Pass designs and this year was no exception – the Amp Camp Pre (ACP+) was shown and the article is now on the FirstWatt website. As usual when Nelson releases a new design you sit up and take notice, but this one was just what I wanted to see (because there is only so many 25W class A amps you can use 😉 ). The ACP+ is a discrete preamp/headphone amp with the same basic architecture as a Pass J2 power amplifier. It’s discrete, doesn’t use a lot of components and runs from a single supply. The only fly in the proverbial ointment is that the amp uses P-channel JFETs for the input (either 2SJ74 or LSJ74), which are either impossible to get (2SJ) or just plain expensive (LSJ). However, I’m certainly not going to let that minor inconvenience stop me.

Nelson has of course done a board for the ACP+ already which will eventually find its way to the diyaudio store I’m sure. However, the original board breaks one of my rules because it has connectors on two edges. It also doesn’t look like the onboard RCAs are particularly good quality. As usual (I am tempted to say) I prefer a more modular approach, with the power supply, the amplifier, the volume pot etc. separated and so as I’ve done in the past I am going to have a go at redoing the ACP+ in modules instead. When I dig into the design I am sure i will be tempted to add a few changes, but let’s see. I expect I am going to build the original proposed linear supply, but an obvious candidate (in my mind) is a filtered IRM-module.

PCB order (hopefully) going out shortly, so with the usual shipping lead time this is going to be my X-mas present for myself this year 🙂

Picture of the prototype amp from the diyaudio-thread.

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 BBA3FE

Haven’t really had time to fully complete my BBA3FE project yet, but as I am otherwise happy with the design I might as well release it in case anyone wants to have a play with it 🙂

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Project files: The BalBUF and PSU

I’ve made a little bit of progress on my balanced mono block 700ASC-amplifiers lately, so now is probably a good time to release the project files for the balanced buffer input board and the accompanying PSU that I used in that design. It’s a pretty obvious clone of AMBs Alpha24, but since I did my own board and ditched some of the configurability I figured it’d be OK. Big thanks to Ti Kan though for actually showing how to build this circuit which I previously attempted a couple of times without getting it right.

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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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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: LED tester

What is it?
The PCB files for my version of Håvard Skrodahls LED-tester as described here.

How big are the boards?
The board measures 2.0” x 1.6” (app. 51 x 41 mm.).

What is the status of the boards?
This is version 1.0 as everything (for once) worked the first time 🙂

Does it use any special/expensive/hard-to-find parts?
None, really. The 16mm pots can be bought from ebay and everything else you should be able to get from multiple different sources. If possible, I would suggest using a stereo 5k-10k pot and the fully-isolated version of the LM317. The former gives the best adjustment range and the latter helps protect against mishaps with flying test leads 😀

Anything else I need to know?

  • For information about how the circuit works, read the hackaday-post linked above.
  • Output current can be calculated as 1.25V/Rtot. For max. current Rtot = R1 and for min. current the value is Rtot = R1 +  the pot value (with the decks in parallel if you are using a stereo pot obviously)
  • There is a difference between Lin/Log pot as described in the build article, so you’ll have to decide up front which adjustment profile fits you best (or keep the pot offboard so you can change – or just build two boards 🙂 ).
  • If you want to use the “high-current” mode, populate R2 as well and short the jumper. Remember that power dissipation in both the resistor and the LM317 regulator increases with higher current. The calculations for min and max current above have to be adjusted to reflect the fact that R1 and R2 are in parallel.
  • The connection for the ammeter is required as it is in series with the LED being tested. If you don’t want the ammeter, bridge If+ and If- connections as shown in the picture. The connection for the voltmeter is optional.  Note that I have tried using a cheap LED meter from ebay for the ammeter and I had some problems with it, whereas if i connect my normal multimeters everything works fine – YMMV.

Downloads:
Download design files here

Related information:
Be sure to read the original post for the exact circuit description, information and tips.

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