First Neurochrome Preamp Build

birkbott

Member
Hi All,
I'm looking to start my first neurochrome preamp build. I don't have a lot of experience, I tried and failed to build a B1 buffer years ago but I'm ready to jump back in. Here is what I would like to have:

- Active
- 2x RCA inputs (on DPDT switch)
- XLR and RCA outs
- Volume control with remote
- 12v trigger out for amp
- Plexiglass/Acrylic case 10" wide

Based on what I've read on the site I am planning on buying the following:
- Neurochrome Preamp power supply (12v for the buffer and 5v for the volume control, 12v trigger)
- Neurochrome Universal buffer
- Khozmo 64 step ladder attenuator with remote
- everything else (RCA jacks, chassis, etc)

Am I missing anything? Is 10"x10"x2" big enough to fit everything? Is there anything wrong with using plexi for the case? I'm assuming I'll need to drill some vent holes.
 
I started yesterday!
link
Not sure where you are in the world, but modushop case (suggested by tom is pretty nice), needs some work to ground it as I'm just learning. They drilled and tapped all the holes which I think would be a pain otherwise. I also looked at PowerCon connectors to avoid making sqaure holes, but easier in plexiglass.

You need 10mm standoffs according to the instructions so add that to the height of the power supply when you consider the case.
You might also consider XLR inputs and then make an RCA to XLR cable so its upgradable...
 
It will be interesting to see the suggestions for volume control with a remote. Without a remote, the easiest way to get decent volume control is the standard Alps blue pot. I'm thinking the easiest way to add remote control is to get the motorized version of that. You'll still need to somehow drive the motor and control it with a remote though.
UK-based Hificollective sells boards that do it:
You can even order them with the pot you want, they have Alps and TKD.

If you only have RCA in, you can put the volume control in the input path and use a single ended option to make life easier.

I got interested in the different digital volume control options. There are a bunch of chips that offer low noise performance. I found some DIY projects but the nicest ones are SMD based and I'm not up for it. Also not convinced that I can achieve really good SMD performance with anything I can make at home. I have one of these laying around that I will probably test on the universal buffer I haven't tried yet:
They have a single ended version too.

About the motorized pots: I wondered once how easy it is to drive them with a DIY Arduino solution. To do it right, you need an additional board next to an arduino that can provide enough current to drive such a motor. On Alps it's about 100 mA. You need this because a single output pin on the Arduino can only provide 20 mA. I thought what the hell, 5 of them together are 100 mA, and it works. At the risk of destroying the Arduino probably. On the other hand you're not adjusting volume for a long time ever, so you might get away with it. I'm an Arduino n00b, but my working code is here: https://github.com/jwdevos/arduino-remote-volume-control
 
It will be interesting to see the suggestions for volume control with a remote. Without a remote, the easiest way to get decent volume control is the standard Alps blue pot. I'm thinking the easiest way to add remote control is to get the motorized version of that. You'll still need to somehow drive the motor and control it with a remote though.
UK-based Hificollective sells boards that do it:
You can even order them with the pot you want, they have Alps and TKD.

If you only have RCA in, you can put the volume control in the input path and use a single ended option to make life easier.

I got interested in the different digital volume control options. There are a bunch of chips that offer low noise performance. I found some DIY projects but the nicest ones are SMD based and I'm not up for it. Also not convinced that I can achieve really good SMD performance with anything I can make at home. I have one of these laying around that I will probably test on the universal buffer I haven't tried yet:
They have a single ended version too.

About the motorized pots: I wondered once how easy it is to drive them with a DIY Arduino solution. To do it right, you need an additional board next to an arduino that can provide enough current to drive such a motor. On Alps it's about 100 mA. You need this because a single output pin on the Arduino can only provide 20 mA. I thought what the hell, 5 of them together are 100 mA, and it works. At the risk of destroying the Arduino probably. On the other hand you're not adjusting volume for a long time ever, so you might get away with it. I'm an Arduino n00b, but my working code is here: https://github.com/jwdevos/arduino-remote-volume-control
Yeah I’m actually now wondering if the 5v supply from the preamp power supply is enough for the Khozmo because it says it’s rated for 80mA and the spec on the Khozmo site is for 1A. They do sell their own power supply though so I can just add that.

A motorized Alps would be easy, but I do prefer a stepped attenuator or digital control for channel matching.
 
I doubt the 5 V output of the Preamp Power Supply provides enough current for a relay-based attenuator. With all the relays engaged you could be looking at a few hundred mA, at least. Less if the designer bothered to use the more expensive low-current relays. You can always look up the data sheet for the relay and see what the typical coil current is.

You could consider something like a Mean Well IRM-05-5 (5 V, 5 W) or IRM-10-5 (5 V, 10 W) power supply on a piece of prototype board for powering the Khozmo.

Tom
 
I doubt the 5 V output of the Preamp Power Supply provides enough current for a relay-based attenuator. With all the relays engaged you could be looking at a few hundred mA, at least. Less if the designer bothered to use the more expensive low-current relays. You can always look up the data sheet for the relay and see what the typical coil current is.

You could consider something like a Mean Well IRM-05-5 (5 V, 5 W) or IRM-10-5 (5 V, 10 W) power supply on a piece of prototype board for powering the Khozmo.

Tom
Thanks, I threw in the Khozmo power supply with my order I’m assuming I can use that along with the preamp power supply. I guess in that scenario I’d need to use a mains power switch and not the power switch slot on the preamp power supply.

Speaking of power the “trigger” on the preamp power supply that’s for the 12v trigger out?
 
The trigger output on the Preamp Power Supply is intended as a trigger signal for a power amp. That way the power amp can be turned on from the preamp. It's a pretty common feature on high-end preamps.

You could use the trigger output to control a relay that then controls the power to the Khozmo power supply (or the output of it).

Tom
 
The trigger output on the Preamp Power Supply is intended as a trigger signal for a power amp. That way the power amp can be turned on from the preamp. It's a pretty common feature on high-end preamps.

You could use the trigger output to control a relay that then controls the power to the Khozmo power supply (or the output of it).

Tom
Ok yeah my plan was to trigger a power amp, but that’s a good alternative use, thanks
 
Actually... No properly caffeinated the neurons in my brain came up with this: Use the 12 V output of the Preamp Power Supply to drive a 12 V relay. Then use the contacts of the relay to turn the power on/off to the Khozmo power supply. You can do this on the mains side of the Khozmo supply or, if you're not comfortable with that, you can switch the power on the low-voltage side.

Just remember to put a diode across the relay coil to catch the flyback voltage when the current in the coil is interrupted. 1N4007 will be fine here.

Tom
 
Actually... No properly caffeinated the neurons in my brain came up with this: Use the 12 V output of the Preamp Power Supply to drive a 12 V relay. Then use the contacts of the relay to turn the power on/off to the Khozmo power supply. You can do this on the mains side of the Khozmo supply or, if you're not comfortable with that, you can switch the power on the low-voltage side.

Just remember to put a diode across the relay coil to catch the flyback voltage when the current in the coil is interrupted. 1N4007 will be fine here.

Tom
Now there’s an idea. A little beyond my skill set at the moment but I’ll do some research on it.
 
Actually... No properly caffeinated the neurons in my brain came up with this: Use the 12 V output of the Preamp Power Supply to drive a 12 V relay. Then use the contacts of the relay to turn the power on/off to the Khozmo power supply. You can do this on the mains side of the Khozmo supply or, if you're not comfortable with that, you can switch the power on the low-voltage side.

Just remember to put a diode across the relay coil to catch the flyback voltage when the current in the coil is interrupted. 1N4007 will be fine here.

Tom
If this is using the same 12V supply as the Universal Buffer is using, is there a chance that the load from the relay could impact the UB's performance?
 
Speaking of relays, since I only need 2 inputs would it be possible to put that 5v from the power supply to a spdt switch to select between 2 TXS2-4.5v relays? Would diodes be necessary in that context? And if so the same value as above?
 
@sledwards and I give some guidelines for preamp chassis layout here: https://ncforo.com/index.php?threads/universal-buffer-single-input-w-volume-control.9/

The sensitive nodes on the Universal Buffer are those on the input so keep those connections short and away from interference sources, such as the mains connections.

The 5 V output can drive 100 mA, so yeah, it'll power two TXS2 relays. I seem to recall that they draw 16 mA, each, but check the data sheet to make sure. Note that the 5 V output of the Preamp Power Supply is always on.

Tom
 
12v relay for volume control, I believe your drawing was for the bottom view of the relay please let me know if I have the diode in the correct orientation. Black is 12v pos green is ground
IMG_7321.jpegIMG_7323.jpeg
 
I dunno. You'd have to check the relay data sheet. Does the relay click and make contact when you apply the correct coil voltage?

The relay coil generally doesn't care about polarity. Some relays have a magnet inside to make the relay action a bit faster. Those relays do care about polarity. The only way to find out is to check the data sheet.

... and the easiest way to find the data sheet is to type the relay part number into Mouser/Digikey and click on the data sheet link.

Tom
 
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