ePot.V4 LDR Attenuator/Controller, Buffer & Power Supply

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tortugaranger

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Hi All,

Development work is progressing on the new ePot.V4 series attenuator/controller. The V4 builds on the V3 keeping the core LDR control scheme the same (closed loop LDR current control), and same physical size, but with interesting changes & options that will hopefully keep costs down while delivering overall improved performance.

This is plenty to keep me busy and since I'm no longer designing/building finished preamps, I now have more time to focus on core components which is what I enjoy the most.

Here's an outline of coming attractions..

Input Switching - The V4 moves on from using analog switches (integrated circuit chips) and instead employs conventional miniature electro-mechanical relays. The analog switches worked well for the V3 but under certain circumstances would exhibit leak-thru behavior which oddly enough I could never duplicate here in my development rig no matter what I tried. Since the analog switch I was using is yet another semiconductor chip available only from one manufacturer, I decided that the V4 would use more conventional switching hardware while eliminating another often hard to find chip whose price has doubled. The downside to this decision is I could only squeeze 4 relays on the same real estate as the 6 analog switches. Sure, I could have made the board a little longer to accommodate 6 but few people really need/want 6 input channels and I wanted to keep the board dimensions identical to the V3 so retrofitting/upgrading would be easy.

Firmware Updating Port - The V3 uses a UART serial port for firmware updating. This worked fine, but to interface the V3 with a PC or MAC required an extra USB-to-UART conversion board or special cable which is frankly a PITA for both designers and users. The earlier V25 used a simpler direct USB port but when I designed the V3, I needed to eliminate the external crystal clock to free up some real estate for other things. But without the accuracy of a crystal clock, USB was no longer an option. With the V4, the crystal clock is back and so is the USB port.

PWM vs DAC - The V3 uses a 4-channel 16 bit DAC chip to generate the voltage setpoint to the 4 LDR (light dependent resistors) attenuator closed-loop current controllers. Over the past 2 years the cost of this chip has almost tripled and has  become difficult to source reliably. Time to jettison yet another chip. Instead, the V4 employs 16 bit PWM (pulse width modulation) which is generated directly by the microcontroller together with external first order filters to smooth out the pulses. This change is arguably the biggest departure from earlier designs but should prove to be highly effective. I've yet to fully test this out but expect to be able to within the next month or so.

Plug-in Buffer Module - I've redesigned the stand alone SSPB.V2 Solid State Buffer board into a plug-in buffer module (board). It plugs right into the LDR module socket of the new V4 board. The LDR module then plugs into the buffer board. The buffer board essentially converts the V4 from a passive attenuator into a very compact solid state active attenuator that sounds just as great as the passive but with a touch more authority and punch. The new buffer module has its own built-in split voltage switch mode power supply that needs an external source of nominally 12 VDC power. When plugged into the new ePot.V4 board, the buffer module also powers the V4. But there are alternative power supply options as well. More on that in the next section. 

Power Supply Options - There are several power options. First, like the V3, the V4 can be powered by a small split-voltage switching power board located on the underside of the main board that needs an external source of 12 VDC. Secondly, when a buffer module is plugged into the V4, the V4 is powered directly from the buffer board's split-voltage switching power supply which is a higher quality power source. A third option is a new stand-alone linear power supply board that is the same size as the V4 board and is powered from AC mains (120VAC or 240VAC - set by jumpers). Typically, the linear power supply board will sit next to the V4 board and there's a short 5 wire power cable that connects from the linear power supply to either the V4 by itself, or to the buffer board when present. When a V4 or buffer board is powered by the linear power supply, the V4/buffer board's own power supply components are excluded from the build. The advantage of the new linear power supply is the complete elimination of any high frequency noise/artifacts associated with switching power supplies resulting in the cleanest possible source of power for the V4 and buffer module. 

Cheers,
Morten

tortugaranger

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Re: ePot.V4 LDR Attenuator/Controller, Buffer & Power Supply
« Reply #1 on: 4 Jul 2022, 01:58 pm »
The first pic shows the layout of the ePot.V4 attenuator/controller prototype currently in development. The V4 is the V3 evolved.

The second pic shows the layout of the SSBM.V4 Solid State Buffer Module prototype also currently in development. The Solid State Buffer Module is a piggy-back board that plugs directly into the ePot.V4 converting the V4 from a passive to a solid state active attenuator built around the same op-amp buffer input stage and a discrete, direct coupled, JFET buffer output stage used in our earlier SSPB.V2 stand-alone buffer board.

The third pic attempts to show the solid state buffer module superimposed on top of the V4 attenuator/controller as it would appear when plugged into the V4. The LDR attenuation module would then plug into the buffer module resulting in a compact triple layer V4->Buffer->LDR sandwich. This is awkward to show 2 dimensionally but will be quite clear once I have actual boards/hardware.

Prototypes of these boards are on order. I hope to have working prototypes of both by the end of July.






Chilkoot

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Re: ePot.V4 LDR Attenuator/Controller, Buffer & Power Supply
« Reply #2 on: 3 Aug 2022, 05:58 am »
Looks good. I am hoping to roll one of these into my pre!

tortugaranger

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Re: ePot.V4 LDR Attenuator/Controller, Buffer & Power Supply
« Reply #3 on: 3 Aug 2022, 01:36 pm »
Looks good. I am hoping to roll one of these into my pre!


Prototype boards are in. Now I have to set aside some time to populate them and run through initial testing to assess operation and performance. Probably another week before I'm there.