LDR3x DIY Passive Preamp Controller Board w/ Remote

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robertopisa

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #140 on: 10 Mar 2016, 05:54 am »
Thanks Morten for letting us discuss your design, I think this should be listed as one the features of your preamps :)

Arduino was an example, I think once you put a more powerful system this is good as long as it is Linux. But it has drawbacks, as you say, it adds complexity. Our preamp should boot and periodically be updated when a new version of the OS or when a new driver is available, as it happens for our NAS. Turning off becomes a shutdown, which takes some time. At the end, the LDR preamp is going to become a sophisticated machine on the SW side, which is good but I am afraid of the complexity. I prefer Occam's razor principle, and go for the simplest way to get the job done (and I have to manage a lot of complexity in my daily work). Everyone has experienced the computer getting stuck with a blue screen, very often because of drivers: what if the preamp got stuck with a blue screen and the volume jumps to maximum? With a micro controller, things are more under control, I would use it in an elevator, not a Raspberry. So it seems to me an overkill to use a Linux system to control the preamp, unless we want Spotify directly on your preamp, so it becomes a network player, another beast :)

As for the noise, I would not underestimate its impact as people spend good money on isolating that noise in computer-based high-end audio. Sonically speaking, it is not a hum but its effect is that it flattens soundstage and makes mid-range dull.

I see that there many requests for many inputs, it makes sense. I prefer the shortest path, and maybe it suffices having extra solder pads close to the LDRs.

With the removable box, one could have connectors already soldered on it, so the signal path would be all isolated from the rest, I like this idea of yours :) Note that the box is still plugged in as you say, but in the control signal path, not in the analog audio signal path, which is different to my eyes:

- current analog audio signal path (my guess, assuming no input switch): connector (XLR/RCA) -> soldering -> wire -> soldering -> trace on the board -> soldering -> socket -> plug -> soldering -> LDR -> soldering -> plug -> socket -> soldering -> trace on board -> soldering -> wire -> soldering -> connector (XLR/RCA)

- analog audio signal path in the box with connectors: connector (XLR/RCA) -> soldering ->  trace on the board -> soldering -> LDR -> soldering -> trace on board -> soldering -> connector (XLR/RCA)

As you can see, the number of steps is half in the signal path. Increasing the steps in the control path is less detrimental to my eyes.

Maybe I am too extreme, but you know, this is our hobby: a preamp is a preamp, a computer is a computer :)

Cheers
-Roberto

Hi Roberto. Good to hear from you and thank you for your thoughtful input.

There's little to distinguish an Arduino from our controller board aside from the fact that the Arduino's are mass produced. Both use standard available microcontrollers and are programmed at the basic hardware level. We would have to mate any Arduino with our own custom interface board same as we would for the Raspberry. I prefer the Microchip family of microcontrollers whereas the Arduino uses Atmel although I've worked with both. Our earliest prototype LDR board used an Arduino. Personally, I'm not very keen on the Arduino programming language and prefer working in either C or Pascal. I currently use MirkroElectronika's development hardware and compilers.


The Raspberry Pi is totally different animal wherein we'd be programming on top of a full Linux operating system. A lot more complexity under the hood and we'd be insulated from the microprocessor at the hardware level. Fortunately there are sufficient drivers available for everything we need - and then some - to handle IR receiver and SPI communications, USB, wireless etc. not to mention high level display functionality. Concerns over noise and shielding may be valid but then again our current boards are already running 16 MHZ microcontrollers with shared power supply driving the LDRs and no shielding all without discernible downside.

I'd say it's around 50/50 in terms of comments I receive on the topic of number of inputs. Half are asking for more than 3 and the other half can't fathom why more than 2, perhaps 3 at the most...and some say why not just 1. There's no perfect one size fits all solution here. The current prototype board design comes close  insofar as it has the flexibility to only have 1 input or have up to 6. It's only a matter of adding the LDR modules which are plug in.

It would be a simple matter to minimize trace lengths by eliminating J1L and J1R as such and simply having individual solder pads as close as possible to each input LDR switch. This would have the added benefit of shortening the board up by perhaps another 0.5 inch. However, as a practical matter this doesn't really shorten the signal path. You still have to run wires from each input RCA/XLR jack to the board. The traces may be shorter but the wires to each trace/pad would have to be longer. Pick your poison.

One of the more interesting philosophical issues around design is whether one is designing around a real or imaginary problem. Is noise really a dominant problem here that needs a solution? In my view that remains an interesting question as it relates to the bigger topic of perceived benefit of balanced audio within short run stereo audio...to which I say... I don't know. I can say that with some conviction that noise has not been an issue with any of our LDR preamps aside from the your run-of-the-mill ground loop hum issues which are really a separate noise category best solved through good grounding practices. RF/EM noise has not been a problem as far as I'm aware. Therefore taking special precautions such as shielded boxes and/or cables internally within the preamp do not seem warranted.

Also I doubt anything is gained by having to replace a whole set of LDRs within a removeable shielded box vs. individual LDRs modules. If it's removeable it's still plugged in and not soldered in. Tubes live quite effectively within a plugged in reality, so my thought is LDRs should be able to as well. We use gold plated pins and sockets so corrosion shouldn't be an issue.

One design architecture that we've explored quite a lot is to move the input switching out to the individual RCA/XLR input jack. While this would indeed minimize the signal path of each individual input, you still end up with common signal(s) that have to run back to the LDR board. Plus you'd end up with numerous switching control signal cables running from the LDR board to each input jack which frankly would end up looking really awful.

After going around the track numerous times with various alternatives we arrived at the design you see below in the recent post. By locating the LDR board as close to the rear panel as practical you minimize the signal wire runs to/from the LDR board and ultimately the total signal run distance between the rear panel inputs/outputs and the LDRs. At least that's the thought behind it.   :thumb:

There's one additional step we could take but have resisted. The LDR board could be custom designed to mate up directly with the RCA or XLR jacks. This eliminates any additional wiring between the jacks and the board. The issue  with that is you end up with a different custom board for each and every model of preamp. Many designs use this approach since it has the added benefit of reducing the manhours needed to build a preamp.

tortugaranger

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #141 on: 10 Mar 2016, 03:18 pm »
Thanks Morten for letting us discuss your design, I think this should be listed as one the features of your preamps :)

Arduino was an example, I think once you put a more powerful system this is good as long as it is Linux. But it has drawbacks, as you say, it adds complexity. Our preamp should boot and periodically be updated when a new version of the OS or when a new driver is available, as it happens for our NAS. Turning off becomes a shutdown, which takes some time. At the end, the LDR preamp is going to become a sophisticated machine on the SW side, which is good but I am afraid of the complexity. I prefer Occam's razor principle, and go for the simplest way to get the job done (and I have to manage a lot of complexity in my daily work). Everyone has experienced the computer getting stuck with a blue screen, very often because of drivers: what if the preamp got stuck with a blue screen and the volume jumps to maximum? With a micro controller, things are more under control, I would use it in an elevator, not a Raspberry. So it seems to me an overkill to use a Linux system to control the preamp, unless we want Spotify directly on your preamp, so it becomes a network player, another beast :)

After further research into the Raspberry Pi we've decided to stay with a microcontroller. There's just too much operating system overhead to deal with. Just turning it on and waiting for it to boot each time would be too much. It was tempting but just saying no at this time. Will continue to keep an eye on the RPi as it evolves because I see it eventually becoming a widely used embedded controller for Internet of Things applications.

Quote
As for the noise, I would not underestimate its impact as people spend good money on isolating that noise in computer-based high-end audio. Sonically speaking, it is not a hum but its effect is that it flattens soundstage and makes mid-range dull.

One thing that will be different with V3 is physically/electronically separating the microcontroller and it's power regulator from the power regulator feeding the LDR board. And being that it's still a resistive/passive attenuator, there is still no active contact between the audio signal and the power unless you count the transmission of light as active contact.

Quote
- analog audio signal path in the box with connectors: connector (XLR/RCA) -> soldering ->  trace on the board -> soldering -> LDR -> soldering -> trace on board -> soldering -> connector (XLR/RCA)
As you can see, the number of steps is half in the signal path. Increasing the steps in the control path is less detrimental to my eyes.

This would require each input to have its own series/shunt LDR attenuator circuit plus input/output shut off/isolation LDRs. The complexity and cost would increase drastically. I don't see it as a practical solution but I agree that the cleanest, shortest signal path is the best.

Meanwhile, we are continuing to tweak the design. The latest version is shown below. Here we've minimized the audio signal PCB trace lengths with the resulting trade-off being somewhat longer hookup wires from the jacks to the board. We've also added a "Mono" switch feature. There are other minor changes to the LDR control circuit including a more robust low-pass filter in the LDR control signal to suppress noise. We are also going to add another voltage regulator to this board such that the controller board only provides nominal 5V DC to the LDR board and the LDR board has its own regulators for the DACs, ADCs, and op amps. All communications back to the controller board microcontroller is via 4 wire SPI serial data. We'll soon have prototype boards built so we can continue development work and further shake down the design.


craig sawyers

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #142 on: 10 Mar 2016, 07:28 pm »
Although the SPI links are likely to be short if the controller is in the same case as the analog stuff, might it be an idea to put the controller in a separate enclosure and galvanically isolate the SPI?  That could be arranged to put the kibosh on any potential for digital breakthrough onto the audio, particularly common mode or coupled ground based.

This sort of thing http://www.linear.com/product/LTM2892 and its ilk.

robertopisa

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #143 on: 11 Mar 2016, 03:05 pm »
I like the new layout.

I agree that there is no perfect solution for everybody :) I am a bit extreme: the ideal for me is without input selector, a box (containing just the XLR connectors and the soldered LDRs) that is directly fastened on the enclosure and connected with a ribbon cable (for the control path and grounding) to the rest of the board.

-R

tortugaranger

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #144 on: 11 Mar 2016, 03:59 pm »
Although the SPI links are likely to be short if the controller is in the same case as the analog stuff, might it be an idea to put the controller in a separate enclosure and galvanically isolate the SPI?  That could be arranged to put the kibosh on any potential for digital breakthrough onto the audio, particularly common mode or coupled ground based.

This sort of thing http://www.linear.com/product/LTM2892 and its ilk.


That's a possibility I hadn't considered. The argument for it not being necessary or advantageous is the fact that serial data is only active (clock and data pulses) when volume is being changed and not during normal listening. The argument for using this is to eliminate any microcontroller MHz clock bleed through from the controller board to the LDR board along the 4 SPI lines.


DaveC113

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #145 on: 11 Mar 2016, 04:56 pm »

That's a possibility I hadn't considered. The argument for it not being necessary or advantageous is the fact that serial data is only active (clock and data pulses) when volume is being changed and not during normal listening. The argument for using this is to eliminate any microcontroller MHz clock bleed through from the controller board to the LDR board along the 4 SPI lines.

I think it would be a good idea.... When I was building the preamp there was a bit of noise evident on the signal when touching the encoder before the display panel/encoder assembly was grounded to the chassis. I don't understand how this is possible but it was on the scope... The noise did not show up when touching a grounded conductor at the same time, and grounding the display/encoder assembly eliminated it.

tortugaranger

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #146 on: 13 Mar 2016, 06:07 pm »
I think it would be a good idea.... When I was building the preamp there was a bit of noise evident on the signal when touching the encoder before the display panel/encoder assembly was grounded to the chassis. I don't understand how this is possible but it was on the scope... The noise did not show up when touching a grounded conductor at the same time, and grounding the display/encoder assembly eliminated it.


The way the ALPS encoder is designed, the control shaft (and metal knob) is connected to the common leg (ground) of the encoder. Unless you have everything else properly grounded, touching the encoder may show up as noise. Really no different than touching the metal chassis of most any audio device where the chassis is tied to earth/ground.

craig sawyers

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #147 on: 14 Mar 2016, 11:02 am »
I get a LOT of inquiries asking for more than 3 inputs.

Hi Morten

The LDRx has the option of 3, 4 or 5 inputs.  I guess a fifth input on the LDRxB is not possible because of rear panel real estate.

But on the LDRx, what is the take up of the 4 and 5 input options?

tortugaranger

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #148 on: 15 Mar 2016, 01:36 am »
But on the LDRx, what is the take up of the 4 and 5 input options?

Not exactly sure what you mean by "take up" but if you're asking how we handle the additional inputs our current LDR input switching board design handles 3 inputs. To accommodate additional inputs we have to add a second input switching board which piggy backs on the first one.

craig sawyers

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #149 on: 15 Mar 2016, 07:22 am »
Not exactly sure what you mean by "take up" but if you're asking how we handle the additional inputs our current LDR input switching board design handles 3 inputs. To accommodate additional inputs we have to add a second input switching board which piggy backs on the first one.

I was just curious, in the context of the recent discussion in which some are completely happy with one single fixed input, some with three, and others who would like six - how much demand there was for the five input version of the LRDx.  I totally understand that this might be commercially sensitive and you'd prefer not to say, but the demand statistics could inform the details of the new design.

tortugaranger

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #150 on: 15 Mar 2016, 04:58 pm »
I was just curious, in the context of the recent discussion in which some are completely happy with one single fixed input, some with three, and others who would like six - how much demand there was for the five input version of the LRDx.  I totally understand that this might be commercially sensitive and you'd prefer not to say, but the demand statistics could inform the details of the new design.

In reality the majority LDRx units sold have the default 3 inputs yet there are still requests for 4-6 inputs often enough that we continue to try to find the most flexible approach. The LDR3.V2 has 3 inputs & 2 outputs and I'd say that is the sweet spot for most.


jquark

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #151 on: 26 Jul 2016, 07:03 am »
Hello Morten,

Currently auditioning a v1 board and am VERY impressed. Any updates on the v3 board? Will it be released in 2016? 

Thank you for being so interactive with customers and for continuing to improve your already wonderful product.

tortugaranger

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #152 on: 26 Jul 2016, 12:31 pm »
Hello Morten,

Currently auditioning a v1 board and am VERY impressed. Any updates on the v3 board? Will it be released in 2016? 

Thank you for being so interactive with customers and for continuing to improve your already wonderful product.

Thanks and glad you like what you're hearing. Yours is a familiar reaction of folks who give our preamp/attenuator a real world test drive with their ears.

The V3 is under active development and our goal is to release it in 2016. At this point that remains a goal only. Still too early to project release dates. The V3 is our most ambitious project to date and we are very excited about its potential.

Cheers,  :thumb:
Morten

tortugaranger

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Update on the V3 Preamp Controller Board in Development
« Reply #153 on: 4 Sep 2016, 07:48 pm »
We just published some descriptive info on the LDR.V3 (the "V3") board which continues to be under active development.
It is still WAY too early to suggest a release date. While the hardware design is settling down there's still a ton of software development work to be done.

Here's a link to the V3 article:  http://www.tortugaaudio.com/ldr-v3-preamp-controller-system/


glynnw

Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #154 on: 5 Sep 2016, 01:21 am »
You have been a busy boy.  Not sure if I read the new info right, but please make the display available in red.  I recently built (with your help) a new preamp because I could no longer read the blue from about 10 feet.  According to the all wise internet, this is a common condition.

tortugaranger

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #155 on: 5 Sep 2016, 02:07 pm »
You have been a busy boy.  Not sure if I read the new info right, but please make the display available in red.  I recently built (with your help) a new preamp because I could no longer read the blue from about 10 feet.  According to the all wise internet, this is a common condition.

As much as I like the cool blue of our current display modules I'm aware that there's something about human vision that can cause blue to be blurry and hard to focus on. The blue of our V3 OLED display has a different appearance compared to the 7 segment display modules which I think make the OLED much easier to look at. Probably something to do with a different wavelength of blue and the fact that the light comes from an organic LED source.  That said, the OLED display model we chose also comes in yellow but not red. I will definitely make sure we try both blue and yellow and keep this issue in mind.

kernelbob

Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #156 on: 5 Sep 2016, 03:32 pm »
I'm glad to hear that I'm not the only person that finds blue displays to be more difficult to read, particularly at a distance.  Morten was able to use orange (yellowish orange?) displays in my Tortugas.  The only downside is that these displays, even at their maximum level, are not as bright as the blue ones.

Best,
Robert

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #157 on: 5 Sep 2016, 04:47 pm »

I found the following explanation about difficulty focusing on blue light which seems to cover it quite well.  Included the link & text.

http://www.candlepowerforums.com/vb/showthread.php?343259-Why-is-it-so-difficult-to-focus-on-blue-LED-lights&s=ca83c861ab440bc750f26569681e7cb4&p=4016046&viewfull=1#post4016046
Quote
Red and green (which are, from the evolutionary standpoint, the same frequency, as the red and green cones are recent variants of one another while the blue cones are distinct, they use a different photoreactive enzyme) are the dominant *resolution source* for human vision, while blue cones are much more important for expanding our ability to detect color (chrominance). It's why we have more red and green cones than blue ones (IIRC the ration is 10:1, though that may be way off).Since focus is much more important to spatial resolution (details) rather than color resolution, the human "autofocus" is biased to favor the red-green band. Blue gets short shrift because of its relatively minimal importance to resolution. An experiment: if you mess around with the three color channels of an image in a photo program like Photoshop, messing up the blue channel affects the final color image the least, given the same amount of "messing" (like noise or deresolution).Last but not least: as noted, chromatic aberration means that the different colors won't quite match up. It's not normally visible, likely because our brains auto-compensate when combining the different cone inputs into the final image (Canon digital cameras recently acquired a similar ability.) Not everyone is going to notice the effect: this is probably because blue focusses "shorter" in terms of lens focal length than red. So, you are much more likely to notice "blue fringing" if your unaided vision tends towards the nearsighted side (like mine does), given the focus center on red/green -- and you'll see it worst with distant blue sources than near ones. If you know someone who has worse nearsighted vision than yours and wears glasses (and neither of you have astigmatism) try using their glasses and see what happens.

craig sawyers

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Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #158 on: 7 Sep 2016, 08:25 am »
Colour vision is a strange and wonderful thing.  Basically even with significantly skewed lighting, they eye/brain decodes the true colours of objects. Edwin Land, the founder of Polaroid, introduces the Retinex theory of colour vision to explain this, and set up carefully designed experiments to demonstrate colour constancy.

In fact the initial films that Kodak introduced did not include blue at all (eg http://zauberklang.ch/filmcolors/timeline-entry/1230/ ), because of chemical problems with fixable blue.  But although these early films were only two-colour red/green, the images look perfectly acceptable. I used this effect in the early-ish 1980's to demonstrate the feasibility of a video endoscope. Because this was the very early days of full colour CCD's, I used blooming-free Charge Injection Devices (CID) from GE, and a two colour LCD stack, so the instrument did frame sequential green/red switching.

In fact in early experiments using different colour highlighting pens, illuminated with two slightly different red filters, viewers could identify most of the pen colours.

Worked a treat.

konut

Re: LDR3x DIY Passive Preamp Controller Board w/ Remote
« Reply #159 on: 7 Sep 2016, 03:39 pm »
Appropriate to nothing, it appears that humans perception of blue is a, relatively, recent adaptation.

http://www.radiolab.org/story/211213-sky-isnt-blue/