Mini-Dsp - Let's break it down - Home Theater Forum and Systems - HomeTheaterShack.com

Old 03-19-14, 09:44 PM Thread Starter
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Matt

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Mini-Dsp - Let's break it down

I am looking, and have been looking, to purchase a 2x8 or 4x10. Going for sub use only, and only going for the 2x8/4x10 units for their higher input voltage capabilities.

I'm leaning towards the 4x10 for it's 4 selectable settings that can be changed via remote control.

I would be running one sub line (unbalanced) out from my Pioneer AVR into the mini. Then using the appropriate (4 out) 48 KHz plugin for the best <10 Hz optimization.

What concerns me is the fact that the MiniDsp specs are wonky at best. In reality all I need is the 2x4 (1 in 4 out) but the input voltage is limiting. Very close to clipping once a person adds 15+DB of boost down low.

I was lucky and got the help of a knowledgeable person. Here is his assessment after reading the spec sheets on the 2x8 / 4x10 and their voltage ratings.

The conclusion was it can be either 4 volts or 8 volts, but not both.

I've gone round and round with MiniDsp and they can't answer the question with clarity.

Does anyone know if the Mini-Dsp 2x8/4x10 has an input limit of 4 or 8 volts?

_______________________________________________________________________

I just looked at the input voltage spec of the 2x8 and it is contradictory. I'll explain why below.

A balanced line-level input uses a circuit called a differential amplifier (diff amp), usually implemented with an IC op-amp and four resistors. I'm going to do a little math on this thing, because it's the only way I know to explain it without being ambiguous myself. In typical operation, the diff amp takes two input voltages that vary with time, neither of which is zero. Let''s name these voltages as follows:

Let input voltage #1 be called v1(t)
Let input voltage #2 be called v2(t)

The output voltage of the diff amp is a single ground-referenced voltage that is applied to the input of the A/D converter chip. Let this voltage be named as follows:

Let the diff amp output voltage (= input voltage to the A/D) be called vO(t)

Now let the gain from input 1 of the diff amp to the diff amp output, with input 2 grounded be called A1. Also, let the gain from input 2 of the diff amp to the diff amp output, with input 1 grounded be called A2. In linear operation, superpositon applies, which means vO(t) is determined by a simple formula:

vO(t) = A1 * v1(t) + A2 * v2(t)

We're almost there. To get a formula that has a better relationship to reality, we must make a change of variables. We define the difference-mode voltage vDM(t) as the difference between v1(t) and v2(t), and we define the common-mode voltage vCM(t) as the average of v1(t) and v2(t). In formulas, it looks like this:

vDM(t) = v1(t) - v2(t)
vCM(t) = 0.5 * (v1(t) + v2(t))

Now we can calculate vO(t) in terms of the difference-mode and common-mode input voltages. I'll skip the math in between and go straight to the result:

vO(t) = (A1 + A2) * vCM(t) + 0.5 * (A1 - A2) * vDM(t)

Now, here is the trick: If we design A2 so it's equal to the negative of A1, then by the formula above, the signal vO(t) applied to the A/D depends only on the difference between the two input voltages (the difference-mode voltage vDM(t)) and all dependency on the average of the input voltages (the common-mode voltage vCM(t)) disappears! This is called common-mode rejection. You may have heard of that term.

In a typical input diff amp of a balanced system, A1 = 1 and A2 = -1, giving:

vO(t) = vDM(t)

This follows from the longer formula above, with A1 = 1 and A2 = -1.

So, how does this relate to the miniDSP input voltage spec? (finally!). Well, the 2x8 spec says the maximum balanced input voltage is 8 Volts RMS (= 20 dBu). But remember, a properly-designed diff amp responds only to the difference-mode voltage at the inputs, and rejects the common-mode voltage. So this 8 Volt RMS balanced input voltage in a typical balanced system is achieved by having two equal-amplitude 4 Volt RMS signals at the input that are 180 degrees out of phase with each other. But there's another way, and that is to make one of the input voltages zero, and the other 8 Volts RMS. That also gives the same 8 Volt RMS difference-mode input voltage.

The conclusion? If the 8 Volt RMS balanced maximum input voltage is correct, then that means the maximum unbalanced input voltage is also 8 Volts RMS (because the diff amp only responds to the difference of the two input voltages, and within limits, it doesn't matter how that difference is achieved). But the spec says 4 Volts RMS!

Now there is one caveat, which deals with an unspecified parameter that relates to nonlinear operation: the maximum common-mode input voltage. From the math above, the common-mode input voltage is the average of the voltages at the two inputs, For a fully balanced system, the average of the input voltages is always zero for all time, because the two input voltages are negatives of each other. For a single-ended input signal of 8 Volts RMS, the common-mode input voltage is 4 Volts RMS (the average of 8 and zero). In a proper design, with a typical op-amp, the circuit will be safe with this common-mode input voltage.

So if all is well, it looks like the 2x8 has 4x the input voltage capability of the 2x4 (8 Volts RMS vs. 2 Volts RMS), assuming the 8 Volt RMS balanced maximum input voltage spec is correct.

Sorry for all the math, but if you take the time to understand the above, you will know more than miniDSP tech support and the people who wrote the miniDSP specs.

Last edited by pdxrealtor; 03-20-14 at 12:53 PM.
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Old 03-22-14, 11:43 AM
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Re: Mini-Dsp - Let's break it down

Are you sure you need that much boost? I would think that if you need that much boost you might be better off getting a different sub. Don't forget you may be exceeding the capability of the sub at the frequency you are trying to boost by that much. Have you tried moving the sub to a different position... As maybe you have a room null causing that much of a loss at that frequency.

tia,
Ron

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Old 03-22-14, 01:59 PM Thread Starter
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Matt

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Re: Mini-Dsp - Let's break it down

Quote:
ellisr63 wrote: View Post
Are you sure you need that much boost? I would think that if you need that much boost you might be better off getting a different sub. Don't forget you may be exceeding the capability of the sub at the frequency you are trying to boost by that much. Have you tried moving the sub to a different position... As maybe you have a room null causing that much of a loss at that frequency.
No it has nothing to do with subs or location.

In the past I've only used the tool for response shape fine tuning. Playing around with house curves, hard knee curves, low shelf filters, etc. I try not to pass 9 db of total boost (seems to work good in 3db increments at different frequencies), but also add in some cuts. If I add 9 and take 9 out the net is zero, and I'm still very close to clipping on the 2x4 balanced board w/ jumper @ 2volts.

Add in 9 and take out 0 and you've lost 9db of headroom.

I've been reading a little bit about L/T circuits and a boost of 12 db doesn't seem like much. Add in any pre L/T frequency shaping and it could add up quick.

I won't boost a true null, as it doesn't seem to work. With my last six subs I could get acceptable response with just delay adjustments, leaving me with a good base to fine tune the curve to my liking.

I moved on to four subs, four cabinets, and four channels for ultimate flexibility in tuning. Haven't got them fired up yet, but, I am expecting to be in the same situation of using level and delay tweaks to get a fairly decent base.

I mainly just want to understand the tool I'm using. Is it 4 volts of input or 8 volts of input. The guy who has helped me out with the above analysis says it can't be both.

I finally got some traction on the mini-dsp forum and I'm pretty sure I have the correct answer and I understand it. But would still like further discussion on the matter to better understand.
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Old 03-22-14, 04:51 PM
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Re: Mini-Dsp - Let's break it down

Either the unbalanced or the balanced DSP units should work just fine.

Most all SW amps have a level control that changes the sensitivity to the input signal.

If you are clipping the DSP with the initial setting then you can just turn up the level control to increase the sensitivity of the SW amp. It will then require less voltage to reach a higher SPL. The DSP will pass the lower voltage without clipping.

If you make that change then the SW level control in the AVR or Pre-Pro will need to be turned down the same amount to compensate for the change. That way the relative level between the Main speakers and SW will remain unchanged.
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Old 03-22-14, 05:04 PM Thread Starter
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Matt

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Re: Mini-Dsp - Let's break it down

Quote:
jtalden wrote: View Post
Either the unbalanced or the balanced DSP units should work just fine.

Most all SW amps have a level control that changes the sensitivity to the input signal.

If you are clipping the DSP with the initial setting then you can just turn up the level control to increase the sensitivity of the SW amp. It will then require less voltage to reach a higher SPL. The DSP will pass the lower voltage without clipping.

If you make that change then the SW level control in the AVR or Pre-Pro will need to be turned down the same amount to compensate for the change. That way the relative level between the Main speakers and SW will remain unchanged.
Right right. I agree 100%. What you describe is gain structure.

What I am after is what the actual specs of the mini-dsp units are. If not for anything more than to understand exactly what the specs are of the piece of equipment I just spent 500.00 on.

Better put is I am trying to understand how the actual stated specs work. What configuration options do I have? Can I take a 2x8 board and use a line converter box on my sub out (convert from unbalanced to balanced) to gain 4 volts of input headroom?

No one on the MDSP forum or the devteam via email could explain it so I could understand it. I'm in no way saying I was not a hard one to explain things to. Because of this fact I sought help from someone who is knowledgeable in electronics. That is his 'help' to me above in my first post. He's saying it doesn't make sense. There can't be a 4 volt and 8 volt spec based on whether the connection at the same location (phoenix terminals) is balanced or unbalanced.

Since I posted this here, I also posted on the mini-dsp forum in another attempt to get some information. That paid off as a very kind, and patient, forumite explained it so I think I understand it. He also said things that none of the other mini-dsp 'team members' have said it in the past. They are so quick to point to spec sheets in an attempt to answer questions but their spec sheets are confusing to even some of the more educated so how is a guy like me supposed to make sense of it?

I'm still trying to get in touch with the writer of the above material to get his take. And I'd also like to get a bit more info from the 'forumite' , but I think he was scared away by a problem poster over there who likes to answer people's questions by pointing them to spec sheets. Too bad he can't convey his words in constructive manners to help the truly 'un-educated', because he seems to know a lot about the mini-dsp.
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Old 03-22-14, 05:34 PM
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Re: Mini-Dsp - Let's break it down

Well, be sure and get back with us here and let us know what you find out.

Regards,
Wayne

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Old 03-23-14, 10:37 AM
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Re: Mini-Dsp - Let's break it down

Your questions are not clear to me and maybe that is the problem with getting clear answers? I will offer some comments to your questions in a very general way. They well may not help as I am unclear if there is any real problem you are concerned with. I do not own the MiniDSP so my comments are based on general knowledge and my experience with the Behringer DCX.

Quote:
pdxrealtor wrote: View Post
... Better put is I am trying to understand how the actual stated specs work.
I would expect the MiniDSP to be designed as unity gain; x Volts in = x Volts out. If filters are applied then it applies their effect on the Voltage at those freqs. There will also probably be an input and output gain control so some modest gain structure adjustment could be applied. It is not intended to convert from consumer level to pro level as that is ~+12dB. Adding that much gain on on top of any significant boost filters is likely to overload it. MiniDSP's expectation is that the control of the gain structure is handled in the other units such that there is at most a modest gain or cut in the MiniDSP.

Quote:
What configuration options do I have?
?? To solve what problem; Noise, Overload of the MiniDSP, or ?? Are MiniDSP's connection instructions not clear?

Quote:
Can I take a 2x8 board and use a line converter box on my sub out (convert from unbalanced to balanced) to gain 4 volts of input headroom?
A Line converter box does not gain 4V of headroom for a MiniDSP (or anything else). It will just boost an unbalanced input V ~12dB and convert it to a balanced signal. It is not clear to me why you would think about adding another box.

If you have an unbalanced SW output and select a balanced MiniDSP then that does add 6dB of headroom within the miniDSP, the cost of which is probably 6dB of additional noise. The additional noise shouldn't be an issue for a SW application however so that is an option. There is no obvious need for the additional headroom in my mind as the unbalanced MiniDSP should be designed to work well with the unbalanced source including a SW.

If you have an unbalanced source unit and are concerned about overload of the unbalanced MiniDSP then that is the question you could ask. Are there users that have run into this problem? Under what conditions? I would think most all SWs have enough gain control to handle this. That is the purpose of that control and that is where the adjustment must be made. Those that have a problem in this regard should be able to solve it that way.

There are always special cases, but you have not offered anything to suggest you have a special case so it is difficult to help.
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Old 03-23-14, 02:05 PM Thread Starter
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Matt

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Quote:
jtalden wrote: View Post
This is all regarding input. I have no questions on output voltage, as it is plenty to drive my amps. But feel free to discuss that too.

It's also no secret that their documentation is poor and even some of the smartest guys cant make sense of it. So, half of my post was really not a question but more an attempt to start a discussion about this box that has so much potential.

One electronic engineer says input volts cant be both 4 and 8 volts based on connection type, it must be one or the other. (See post 1) Another says it can be both. Mini dsp cant explain it to me, and it vey well could be because I don't understand enough to ask the question in the right way. But! Look around..... Half the users of the mini dsp are as uneducated as me in this area.

SO -- Assuming it can be both.......

2x8 / 4x10 mini dsp spec is 4 volts unbalanced or 8 volts balanced, at the phoenix INPUT terminals.

Question - if I convert my unbalanced connection to a balanced one, using a converter box, will I now have 8 volts of input headroom.

If not, why? If so, how? The spec clearly states 4 volts unbalanced, 8 volts balanced.

Question- what do you think? Can the INPUT voltage LIMIT double if using a balanced line? How can it be halved if using an unbalanced line.

Info-
My AVR puts sub output is over 10 volts at 80% max volume and sub trim maxed using a 60 hz tone -3dbfs. Dialed back to 'normal' (sub trim -7 MV 0) listening levels still clips the inputs on the 2x4 mini-dsp in rare instances, which has a spec of 2 volts input with proper jumper configuration. How rare Im not sure since their level meters are hard to read. I have a feeling the clip indicators were going off on my amp because the mini was being clipped on the inputs, sending that signal right down the line. Thats kind of irrelevant and I wont go into what makes me think that, but.none the less.

The mini dsp level controls do nothing to lower the input voltage until after the ADC, by that time its to late. The signal is clipped and goes right down the line through the dac and onto the amp.
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Old 03-23-14, 05:51 PM
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Re: Mini-Dsp - Let's break it down

RE max input V for a MiniDSP:
All the "consumer" audio equipment that provides both balanced and unbalanced inputs are designed to accept a standard consumer level signal on the unbalanced (RCA) input and 2x that (+6dB) on the balanced input.

You stated that the MiniDSP specs an input max of 4 V (rms or peak or p-p?) and the unbalanced input is 8 V so that is consistent with all others I am aware of.

If you are inputting an unbalanced signal into the balanced block then 8 V is still the max input rating assuming the connection is across the ± signal terminals. Some units allow it to be applied across the positive and the signal ground in which case the max input is again 4 V. So it depends how it is connected.

I'm sorry to continue on with this, but your problem would seem to be unusual.

Your AVR output capacity comments would not seem to be relevant. The AVR output is dependent on the input level of the SW amp needed to provide the SPL output. The higher the gain of the SW amp the lower the input voltage requirement.

It is possible that you have some collection of equipment that either; does not have enough SW amp gain available to drop the required voltage in the AVR/MiniDSP to an acceptable low level, or when the gain is raised that much then the AVR SW offset level cannot be reduced far enough to balance with the level with the main speakers. I think this should be pretty rare, but maybe not and it may in fact be your case. Did you really turn the SW gain to maximum and adjust the AVR SW offset level down as needed? If so, which of the two problems did you run into? What was the other equipment (AVR, SW and mains)? The information may be useful to others who are looking for help with this problem.
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Old 03-23-14, 08:28 PM Thread Starter
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Matt

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Re: Mini-Dsp - Let's break it down

Quote:
RE max input V for a MiniDSP:
All the "consumer" audio equipment that provides both balanced and unbalanced inputs are designed to accept a standard consumer level signal on the unbalanced (RCA) input and 2x that (+6dB) on the balanced input.
You lost me. Let's use the DCX for example, or any device that only accepts balanced inputs. Let's call the rating 10 volts RMS max in. Now let's say I have take an unbalanced line and jump - and S, as is typically done.

Question: does that change the input volt rating?

Quote:
You stated that the MiniDSP specs an input max of 4 V (rms or peak or p-p?) and the unbalanced input is 8 V so that is consistent with all others I am aware of.
If I did state that it was a typo- apologies. The mini-dsp specs a phoenix connector MAX INPUT VOLT RMS as 4 volts unbalanced, and 8 volts balanced. This is VOLTS RMS. If using unbalanced the instructions state to jump - and S.

That would seem to contradict what is consistent with all the others you are aware of.

Quote:
If you are inputting an unbalanced signal into the balanced block then 8 V is still the max input rating assuming the connection is across the ± signal terminals. Some units allow it to be applied across the positive and the signal ground in which case the max input is again 4 V. So it depends how it is connected.
Again they instruct an unbalanced connection to be connected as + and S and the S and - be jumped. I don't know what would happen on the mini-dsp board (2x8 or 4x10) if you connect unbalanced to +/-.

I know that on the OUTPUT of the mini-dsp I myself have to float the S (float pin 1) because as far as I can tell their boards are grounded to the PCB, not the chassis. This creates, in my system, a hum. So, I have no idea what affect floating pin 1 would do on the INPUT RE: voltage limits. These are the kind of things I'd like to know.

Quote:
I'm sorry to continue on with this, but your problem would seem to be unusual.

Your AVR output capacity comments would not seem to be relevant. The AVR output is dependent on the input level of the SW amp needed to provide the SPL output. The higher the gain of the SW amp the lower the input voltage requirement.
No worries- that's why we are here. To discuss and help everyone understand.

When one has to lower the output of the AVR to max to barely clear input voltage limitations of the mini-dsp it does not matter what the sub amplifier does. As long as it can still reach max voltage needed to fully power the subs.

I have a Pioneer AVR. SC-1522k. Onkyo's are known to have high output voltage too. Many are clipping and have no clue at all. How many times have you read of guys running 10DB hot? Even without boost, dialed back to minimum sub out to get the proper INPUT voltage to the mini dsp doesn't matter much if you like to turn it up 10db. Add some boost into that scenario and you've got problems with an AVR that puts out high voltage.

Mini-dsp suggested I put a potentiometer on the sub line in to the mini-dsp. That's great. How about they offer the option to purchase one. Or put out an app note on their website. I don't know, it just seems to me at least, that's a hokey suggestion for a consumer product.

Quote:
It is possible that you have some collection of equipment that either; does not have enough SW amp gain available to drop the required voltage in the AVR/MiniDSP to an acceptable low level, or when the gain is raised that much then the AVR SW offset level cannot be reduced far enough to balance with the level with the main speakers. I think this should be pretty rare, but maybe not and it may in fact be your case. Did you really turn the SW gain to maximum and adjust the AVR SW offset level down as needed? If so, which of the two problems did you run into? What was the other equipment (AVR, SW and mains)? The information may be useful to others who are looking for help with this problem.
Most I answered just above this quote. I will add though, you cannot lower the voltage with the mini-dsp, any of them. The input levels attenuate AFTER the ADC, so the signal is already clipped by the time the voltage is lowered. This can be referenced here - http://www.diyaudio.com/forums/minid...sp-x-over.html

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