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miniDSP Low cost, modular Digital Signal Processor (DSP) kits for the DIYer from miniDSP.

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Old 2nd November 2016, 06:41 PM   #1
tcpip is offline tcpip  India
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Default MiniDSP preamp

Most of you who have built active speakers with MiniDSP based xo will know what I'm talking about here. You need a preamp around the MiniDSP, and there's no product from the manufacturer which meets the bill. (There's a product from the Hypex active xo which meets these needs for their products.)

The requirements
  • Everyone needs something to do input selection, support a reasonable number of inputs, and do volume control.
  • If you're using the MiniDSP, you can't use a conventional preamp upstream of the MiniDSP, because the volume-attenuated signal level will be too low when it enters the MiniDSP ADC, leading to sharp worsening of SNR. So you either do multi-channel analog volume control downstream of the MiniDSP, or you use the MiniDSP for digital volume control. And a conventional stereo preamp won't do at the output for volume control -- you need multi-channel.
  • If you're using the MiniDSP, your input selection must be upstream of the MiniDSP. And we've already seen that a conventional preamp upstream of the MiniDSP causes other problems.
  • Any source components which can generate digital output must be connected straight (i.e. digital) to the MiniDSP's digital input, to avoid a signal-degrading hop of D-to-A-to-D. Therefore, you will never want to feed the analog output from a CD player to the MiniDSP's analog inputs -- you'll want to keep the link digital.

So, you need a special preamp if you're using MiniDSP active xo. This special preamp must have input selection at the input of the MiniDSP module, and volume control inside or after the MiniDSP. Also, this special preamp must be able to let you select one input from a collection of analog and digital inputs, and direct the right signal to the right MiniDSP socket.

The closest that you get to this is by using the MiniDSP 4x10HD box. This box has (for most of us at least),
  • one SPDIF coax digital input
  • one Toslink optical digital input
  • one analog input
and lousy ergonomics. It's not a good solution.

Ergonomics issues with the 4x10HD
  • The volume control, input selection, and config selection are all done with one knob, leaving careless or non-geek users with the risk of accidentally changing one while changing the other.
  • There's no visual feedback about the current volume setting
Plus, there's the constraint of just one input of each type. Very few of us have such a small setup that just one input of each type is adequate. I'd like four digital audio inputs, and at least two analog inputs.

The natural instinct will be to think: we're DIYers, we'll fix it.

But we then realise that in many small ways, the MiniDSP products and policies are not as DIY-friendly as we'd like.

DIY challenges with MiniDSP products
  • Hard to panel-mount the sockets: Input sockets are always soldered directly on the MiniDSP motherboard. And with the 2x4 boards, the input and output sockets are on either side of a small board. If you fix the board right next to the rear panel of your chassis, you will either get only the input sockets or the output sockets to project out of the rear panel. Therefore, you have to carefully desolder the sockets from the board, voiding its warranty, before you can integrate the module with a "normal", usable chassis. A more DIY-friendly design would have left the RCA, SPDIF and Toslink sockets off-board.
  • Ditto, IR sensor: The IR sensor for remote control is also hard-soldered on the board. You can't use the IR sensor there if you want to fit the board into a chassis. The only exception is if you use their Vol-FP module and choose to mount the entire module, with its UI components, all on the front panel. I'd have just preferred the flexibility of fixing my IR receiver on the front panel wherever I wanted. A more DIY-friendly design would have just put pin-headers for the IR receiver, and provided a cable + IR receiver separately.
  • Opaque control protocols: The way in which a microcontroller can control the 2x8 board is not documented. Intrepid DIYers have actually reverse-engineered the I2C protocol between the Vol-FP and the 2x8 board (the whole scoop here), and have later discovered that new versions of the board have changed the protocol. Therefore, only the toughest DIYers can easily integrate their own DIY control units with the 2x8 back-end module. Not everyone's cup of tea.
  • Hard to use rotary switch for input selection: The average DIYer would like to use a rotary switch to build an input selector. It's inexpensive, easy to use and reliable. With the MinIDIGI, it's possible to select one out of four digital inputs using a rotary switch, but not from a combination of analog and digital inputs. With the Digi-FP, it's not possible to use a rotary switch at all.

Proposed solution

I am embarking on a project to design and build a preamp around the MiniDSP, which I will then make available as open source. I am hoping to solve these DIY challenges. My design (if it works!) will work with
  • MiniDSP 2x8 + MiniDIGI, at 96 Ks/sec, giving you an 8-channel active xo
  • MiniDSP 2x4 (balanced or single-ended models, but not the new HD variant) + MiniDIGI, at 48 Ks/sec, giving you 4 output channels

My aim is to support:
  • Input selection using a rotary switch + a relay board. A 6-way rotary switch will allow you to select from among 2 digital coax inputs, 2 digital optical inputs and 2 analog inputs. An 8-way rotary switch will allow you to select from 2 coax, 2 optical + 4 analog inputs. Balanced or unbalanced, depending on the MiniDSP board you're using
  • Volume control using a motor pot and a separate remote-controlled volume control module (pick up any from eBay or elsewhere, which can drive a linear motor pot of your choice)
  • If you select a digital input, the digital signal will go into the digital input and ASRC of the MiniDSP. If you select an analog input, the analog signal will go into the ADC input of the MiniDSP. No unnecessary conversions anywhere.
  • everything inside a single chassis, to avoid box-count bloat which so many of us suffer from
  • no remote control of input selection
  • no soldering or desoldering on any MiniDSP module, thus avoiding such surgery

My proposed solution will contain, in its chassis:
  • a toroidal xfmr plus linear PSU board to generate the various DC supply rails, or a set of enclosed SMPS modules to do this
  • a remote volume control module from some third-party to drive a motor pot
  • a MiniDSP module: the 2x8 or one of the two 2x4 boards supported, with their analog outputs connected to the rear of the chassis
  • a MiniDIGI module with all its four digital inputs peeping out of the rear panel of the chassis
  • a relay board to select from one of the analog+digital inputs supported
  • an 8-way rotary switch, a motor pot, an IR receiver and a power switch mounted on the front panel

Why the MiniDIGI instead of the DIGI-FP? Well, the MiniDIGI gives me four useful digital inputs to choose from, plus allows input selection using an external switch. The DIGI-FP gives me a choice of just two useful digital inputs, as far as I am concerned, and also does not support external mechanical switches/relays for input selection.

Why a volume control using a pot? It allows me to use a remote volume control kit separately (and this can be from among any relay-based or motor-pot-based kits which implement a linear potentiometer), without being forced to use the flawed ergonomics of the Vol-FP. Using a volume pot, I get visual feedback of the current setting of volume. The Vol-FP does not give us this.

Why only these three MiniDSP boards supported but none of the others? Well, these are the only three boards which work with the MiniDIGI and support an external potentiometer for volume control.

I feel this sort of preamp is sorely lacking in the MiniDSP eco-system.

Why am I fussing so much to fill these gaps with the MiniDSP system, instead of shifting to Hypex? I would have, believe me. But AFAIK, Hypex digital active xo does not support entering of arbitrary biquads to define IIR filters of my shape and size. It forces me to select from a drop-down menu of canned choices. This is too restrictive.

What do you think? Will this be useful? Is this a sensible direction to proceed in?

Last edited by tcpip; 2nd November 2016 at 06:53 PM.
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Old 3rd November 2016, 01:21 AM   #2
seanp2k is offline seanp2k  United States
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I think this is a good idea. I was just planning on using all TOSLINK and an external remote-control switched ( e.g. ViewHD SPDIF / TOSLINK Optical Digital Audio 4X2 True Matrix with Remote Control | VHD-SM4X2 ) plus the integrated digital volume control, ran by a KlikR Universal Remote.

I almost got the 10x10HD but after looking through the XML configs for the LXMini +2, adapting them for the 10x10 seemed not as easy as I thought. Maybe it is easy, but this is my first minidsp product and though I work in software, I just want it to work right at first. I was under the impression that the 10x10 can do analog input switching. It's still just 2 digital channels regardless of interface, which quashed my dream of running 6-channel room-corrected Dirac Live on a SFF PC through an MOTU UltraLite-mk4 over ADAT, but the Minidsp products all only support 2 channels of digital input.

The digital volume control still bothers me, since as I understand it, I'll be losing bit depth. Effective number of bits---or why "you have to keep software at full volume" is nonsense Seems to have a deeper explanation / debate, but that was my reasoning behind looking into 6/8ch analog stepped attenuators. I was also considering making my own 6ch Light-Dependent Resistor-based passive preamp using the Tortuga Audio components, but I figured I'd play with just the 4x10HD first before making a decision.
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Old 5th November 2016, 12:28 PM   #3
Henkjan is offline Henkjan  Netherlands
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great project.

have you seen knuisje's work?
MiniDSP-based system GALLERY
he built a preamp around the 2x4 and minidigi

just an idea: how about multi minidigi's to enable digital output for all outputs, so one can use another DAC

Last edited by Henkjan; 5th November 2016 at 12:32 PM.
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Old 5th November 2016, 04:17 PM   #4
Davey is offline Davey  United States
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To me, this seems a solution in search of a problem.

I do agree a visual volume indicator (of some sort) is needed for the miniDSP products, but the rest of your concerns I think can be solved/understood by changing your operational mindset.

I don't agree with three of your four bullet points in the "The requirements" portion.
The number of digital inputs 'might' be an issue for some users, but I suspect most of us have a single digital input we normally use. However, you can still select three different ones via either the front panel encoder or the IR remote.
A conventional preamp used upstream of the miniDSP unit does not change the internal signal levels within the DSP/DAC portion of the unit. Yes, you would be using the analog inputs and ADC portion of the unit, but I see no reason to do this with line-level gear that most likely has a digital output anyway. Only those users who spin vinyl will need to interface in the analog domain with their phono-preamp unit. Nominal output from those should easily place the signal level in a good range. And there is also a selectable input attenuator to help further.
I wouldn't normally feed analog line-level equipment to the miniDSP analog inputs, but you certainly can do it and still maintain good SNR from the onboard ADC.
Concern about the digital volume control within the miniDSP is unfounded. Even with 16bit source material you will not have resolution loss at settings above -48db. If your gain structure is properly implemented you will never be anywhere close to that setting. Thankfully, the days of adding multi-channel post DAC volume-controls are well behind us.

Sean, if you need support translating settings, then a simple query on the OPLUG forum would have attracted my attention.

Cheers,

Dave.

Last edited by Davey; 5th November 2016 at 04:20 PM.
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Old 5th November 2016, 09:51 PM   #5
tcpip is offline tcpip  India
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Quote:
Originally Posted by Henkjan View Post
have you seen knuisje's work?
MiniDSP-based system GALLERY
he built a preamp around the 2x4 and minidigi
Thanks for pointing this out -- I went through the details. The last post about his project is a query from another forum member asking whether the design is available for others to build. There's no answer to that. So, I guess the next guy will have to re-invent all this anyway.

In any case, the approach he's taken involves programming a microcontroller -- my approach hopes to avoid that. I'm lazy.

Quote:
just an idea: how about multi minidigi's to enable digital output for all outputs, so one can use another DAC
Am a little confused here. If you want more digital outputs, isn't it easier to move to the nanoDIGI 2x8 and add your own DACs all around?
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Old 5th November 2016, 09:58 PM   #6
tcpip is offline tcpip  India
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Quote:
Originally Posted by Davey View Post
I do agree a visual volume indicator (of some sort) is needed for the miniDSP products, but the rest of your concerns I think can be solved/understood by changing your operational mindset.
Am keen to see how.

Quote:
The number of digital inputs 'might' be an issue for some users, but I suspect most of us have a single digital input we normally use. However, you can still select three different ones via either the front panel encoder or the IR remote.
I have already explained my problem with this. The Digi-FP gives me too few usable digital inputs, and the basic board gives me just a single analog input, no switching at all. And out of the three digital inputs supported, I've never had any equipment, even at a friend's place, which generates the AES/EBU digital stream. So, as far as I'm concerned, I get two usable digital inputs with the Digi-FP.

It's possible that these are adequate for your needs. I can't see how it makes my requirements invalid.

Quote:
A conventional preamp used upstream of the miniDSP unit does not change the internal signal levels within the DSP/DAC portion of the unit. Yes, you would be using the analog inputs and ADC portion of the unit, but I see no reason to do this with line-level gear that most likely has a digital output anyway.
It seems here you're making assumptions about the gear others use. Maybe we can just say that some of us still like to use analog outputs of some devices, or have devices which generate only analog outputs? This is particularly true of people who keep trying out various source components.

Quote:
Only those users who spin vinyl will need to interface in the analog domain with their phono-preamp unit. Nominal output from those should easily place the signal level in a good range. And there is also a selectable input attenuator to help further.
I am confused here. Is there an "input attenuator" which can be controlled directly or indirectly from the front panel of a chassis?

Quote:
I wouldn't normally feed analog line-level equipment to the miniDSP analog inputs, but you certainly can do it and still maintain good SNR from the onboard ADC.
Thank you. We agree here. I'm okay feeding analog line-level when analog line-level is all I get.

Quote:
Concern about the digital volume control within the miniDSP is unfounded. Even with 16bit source material you will not have resolution loss at settings above -48db.
Thanks again. We agree here.

As you can see, I don't understand what you're saying, other than guessing that you may be disagreeing with the basic premise that someone may need more inputs than what is available from a Digi-FP plus base DSP board.

Last edited by tcpip; 5th November 2016 at 10:28 PM.
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Old 6th November 2016, 01:29 AM   #7
Davey is offline Davey  United States
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Well, it seems you're keen to build something, and since this is indeed a DIY audio forum I will not discourage you further.
There's always something to learn and I will be interested to see what you come up with.
Let me know if I can help you in any way.

Cheers,

Dave.
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Old 6th November 2016, 01:44 AM   #8
tcpip is offline tcpip  India
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Quote:
Originally Posted by Davey View Post
Well, it seems you're keen to build something, and since this is indeed a DIY audio forum I will not discourage you further.
There's always something to learn and I will be interested to see what you come up with.
Let me know if I can help you in any way.

Cheers,

Dave.
Forget about discouraging me Can you please help me understand what I may be missing in your earlier post?
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Old 6th November 2016, 07:14 AM   #9
tcpip is offline tcpip  India
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As far as I can see, my thoughts are pretty straightforward:
  • You need more inputs than what MiniDSP modules give you. In particular, you need 2 or more analog inputs, 4 or more digital inputs, and all of these (analog and digital combined) should be selectable from a single mechanical rotary switch. Their existing multi-function knob is ergonomically unsound even if they gave you 6+ inputs hooked to it, and I also don't like input selection purely from a remote. I like to have a front-panel option too. (In fact, I don't use remotes for input switching on audio systems -- never felt the need.)
  • There should be an indication of what's the current volume.

These are my basic premises.

To implement these, the following attributes of the solution would be nice to have:
  • Avoid programming a microcontroller, or worse still, avoid actually designing microcontroller hardware and programming it. Use pre-programmed microcontrollers if available and necessary. Keep the custom design and build to a minimum.
  • Avoid voiding warranty of the MiniDSP modules -- i.e. don't desolder things from the boards if you can get away without doing it
  • Trust the MiniDSP ADC, DAC, digital volume control, etc, for simplicity -- don't try to bypass or reinvent these things.
  • Avoid having to reverse-engineer undocumented features of the MiniDSP boards, because the designers are known to leave these undocumented and change them from version to version without announcing them. If you take this precaution, your own design will be less likely to be affected by versions of their boards.

These are my ideas about the solution.

Now please tell me what I'm missing and how my project is a solution in search of a problem?

Last edited by tcpip; 6th November 2016 at 07:17 AM.
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Old 6th November 2016, 10:33 AM   #10
Henkjan is offline Henkjan  Netherlands
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Quote:
Originally Posted by tcpip View Post
Thanks for pointing this out -- I went through the details. The last post about his project is a query from another forum member asking whether the design is available for others to build. There's no answer to that. So, I guess the next guy will have to re-invent all this anyway.

In any case, the approach he's taken involves programming a microcontroller -- my approach hopes to avoid that. I'm lazy.
his design is available (on our Dutch DIY forum it was rather popular, incl a GB for the prints; I have one myself), I guess he just didn't see the question in that topic. I'll point him to this topic.


Quote:
Originally Posted by tcpip View Post
Am a little confused here. If you want more digital outputs, isn't it easier to move to the nanoDIGI 2x8 and add your own DACs all around?
yes, but then there is only a single input again; so let's see if your project is also suitable for the nanodigi
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