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MMG MiniDSP and bi-amp Questions

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Posted on February 23, 2012 at 06:23:49
acroy
Audiophile

Posts: 136
Location: Dallas-area
Joined: December 5, 2011
 I’ve been running the MMG’s bi-amped for a few weeks with an inexpensive Pyle active crossover. It works as advertised but has limited x-over point selection. The sound is so very much improved. The treble and especially the midrange/vocal clarity are wonderful. Given these results I decided to ‘do it right’ using a 2x4 Mini-DSP.

I have 2 questions on setting up the MMG’s for bi-amp

1)Recommended crossover points and slopes for the Mini-DSP
2)How to ensure the mid/bass and tweeter panels are being driven in-phase? The wiring strip with 4 connections: I assume it is +-+- The sound is good as-is though I have not tried reversing it…

I’m sure both have been covered before, but could not find them easily. Any other tips & tricks would be appreciated!

Bass duties are being handled by an active subwoofer, currently crossed over at 50hz.

Once the MMG’s are set up I will do some serious a/b comparisons with the 1.7’s.

For those who are interested, a low-end Onkyo receiver is handling all 4 channels of amplification. The receiver is not rated for the load, and initially ran quite warm. As I have little investment in this piece of gear, I added a small 120v industrial fan to the heat sink section. The amp section now stays quite cool. Long-term plans include dedicated stereo amps for this setup.

 

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RE: MMG MiniDSP and bi-amp Questions, posted on February 23, 2012 at 06:49:56
Davey
 IMHO, the recommended electrical crossover slopes (to start) should be the same as created by the stock crossover. You can view these on my webpage or some of the other pages from users here.

You are correct on the polarity configuration. Make sure to keep close track of that because different crossover combinations might require a relative polarity adjustment between woofer and tweeter. You can do that easily with the miniDSP.

The miniDSP has some pitfalls that might catch you. If you need help send me a PM.

Cheers,

Dave.

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 23, 2012 at 09:29:12
acroy
Audiophile

Posts: 136
Location: Dallas-area
Joined: December 5, 2011
 Excellent, thanks
Dumb question, but where is your webpage?

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 23, 2012 at 15:32:28
neolith
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Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 You didn't mention whether you are using the "newer" MMG's or the original model. For the newer version the filters are 1st order - LP at 290 and HP at 3300. For the original the LP filter is 2nd order at 680 and the HP is 1st order at 1660 2550.
If the phase of the tweeter and woofer is incorrect you will hear a drop in the output around 800-1000 hz.



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 23, 2012 at 21:33:45
JBen
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Posts: 3082
Location: South FL
Joined: May 18, 2008
Contributor
  Since: July 26, 2010
 Huh? Neo, I must have been smoking some very bad crap for far too long but could you recheck the LP spec for the original MMG?

Mine is the old design and came with the same 2.2mH coil as the newer ones. The only difference was the 2nd order caps (25uF). So, the x point would nominally be the same but with a faster slope.

An odd thing about the older tweeter is probably not widely known. The old tweeters in mine are 2.6 ohms (measured). The new tweeter is "probably" 3.5 ohms (extrapolated, not measured). With the caps that came with them, it would seem that:
- Old MMGs tweeter with 24uf@2.6ohm = 2550hz
- New MMGs tweeter with 12uF@3.5ohm = 3790hz

That 2.6ohm in mine caused me quite some anxiety when I tried Tazmon's capless HP mod and adjusted it for "flat" rsponse, years ago.

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 07:03:21
neolith
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Posts: 4842
Location: Virginia
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Contributor
  Since: December 2, 2004
 For a first order LP filter fc= R/(2*pi*L) which works out to 290 hz for a 4 ohm load with a 2.2 mH inductor. For a 2nd order filter the fc= 1/[2*pi*sqrt(L*C)], so for the original MMG this is 1/[(2*pi*sqrt(2.2*24*10^-9)] or 680 Hz.
I was unaware that the original MMG tweeter had an impedance of 2.6 ohm, in which case the fc is 2550 Hz as you stated. I will need to update my spreadsheet.
BTW, Tasmon reported the woofer inductor as 0.75 mH while the official schematic (both on the Tweaks page) is 2.2. I never understood why one would want to go to a capless crossover as the inductor is the least ideal of the passive devices -relatively expensive, poor tolerances, very real capacitance and resistance, limited values, etc.



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 07:13:29
acroy
Audiophile

Posts: 136
Location: Dallas-area
Joined: December 5, 2011
 I have the 'newer' crossover with 1st order slopes.

You state LP 290 and HP 3300

Maybe I am being dense - but where is the sound coming from in that huge hole from 290 to 3300? Is the rolloff so gentle, and the panel response curves so, um, 'rising-rate' (for lack of a better term) that the 290-3300hz range is covered even with the 1st order slopes in the way?

Many thanks as always. I am sure these questions ahve been answered a million times. I boy-scount-honor swear I'll help educate the next batch of newbies once I'm educated myself ;)

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 08:58:51
Davey
 He either mis-measured the inductor at 0.75mH or made a decision to change (lower) the value for his "cap-less" crossover.

Everything he did is highly suspect, IMHO. It really should probably be removed from the tweaks page.....or at least explained. Many folks have raised questions regarding this "mod."

I don't think I'd analyze these MMG crossover in terms of crossover frequency. Look at them as two separate filter sections and model/analyze based on a defined -3db (or -6db) point. It's less confusing that way.

Cheers,

Dave.

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 09:08:35
Davey
 See my post below regarding filter frequencies.

There would be "huge hole" IF the transducers responses were flat and wide ranging. But they're really not. This is definitely not a "textbook" crossover design. It's the acoustic responses that are ultimately important. You adjust the electrical responses to achieve whatever the target acoustic response is......regardless of how funny the ultimate electrical responses look.

Cheers,

Dave.

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 09:21:10
neolith
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Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 Ideally (perhaps) the filter's should look like step-functions or "cliffs" with sudden drop off at the cutoff frequency, but this is not possible.
Here's a diagram of what's going on,electrically. As Davey points out the actual acoustic response is different and that -6db dip @ 1000 Hz may not exist when you listen to the speakers.





"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 09:34:42
acroy
Audiophile

Posts: 136
Location: Dallas-area
Joined: December 5, 2011
 excellent- thanks much. Graph helps a lot. What tool is that - a freebie simulator would be fun to play with. I understand the basics, i.e.crossover orders, but would have to pencil-whip the numbers.

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 11:04:32
Dawnrazor
Audiophile

Posts: 12589
Location: N. California
Joined: April 9, 2004
 Hey Neo,

If the phase of the tweeter and woofer is incorrect you will hear a drop in the output around 800-1000 hz.

I had assumed that the 1st order slopes have the tweeter and woofer in phase. When I was messing with the allocator software to mimic the stock slopes I never could get the right graph. Davey showed me that even in this config the tweeter and woofer are wired out of phase so any crossover mod should keep that same orientation.

So "incorrect" in this case is in phase with each other???

For instance, here is the curve with the drivers out of phase:





and here it is with them in phase:





I guess I thought the drivers were supposed to be in phase on 1st order slopes. Where can I read more about that stuff??

Cut to razor sounding violins

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 12:04:42
JBen
Audiophile

Posts: 3082
Location: South FL
Joined: May 18, 2008
Contributor
  Since: July 26, 2010
 100% in agreement on both counts.

The Tazmon tweak worked for me because I had the feedback from mic measurements. It didn't take long for me to see that the low pass was not right. I used the 2.2mH instead and things began to fall in place. Pity that the HP has to waste so much energy as heat. Shy of a line-level xover, it sounds delightfully clean and extended. We should insert clarifications for this mod.

Later, as I went on to line-level filtering, once again the feedback from the measurements showed what you just said with regard to the MMG modeling.

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 12:11:51
JBen
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Posts: 3082
Location: South FL
Joined: May 18, 2008
Contributor
  Since: July 26, 2010
 Yes, also seen at my end, Davey.

When one measures the actual non-filtered output of the drivers, more so the mid/bass, it becomes very clear why Magnepan chose to do things in not-quite conventional ways.

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 12:45:15
neolith
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Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 If you have a symmetric first order crossover than the phase difference of the LP filter and and HP filter is a constant 90 degrees at all frequencies. It doesn't matter if the polarity is reversed since the difference is still 90 degrees.
However this does hold up with an asymmetric crossover and in that case changing the polarity may very well result in a different phase relation and a "dip".



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 13:10:34
JBen
Audiophile

Posts: 3082
Location: South FL
Joined: May 18, 2008
Contributor
  Since: July 26, 2010
 Neo mentions: "If you have a symmetric first order crossover than the phase difference of the LP filter and and HP filter is a constant 90 degrees at all frequencies. It doesn't matter if the polarity is reversed since the difference is still 90 degrees."

Neo, that's precisely what I thought at first. It didn't work quite that way in practice but I'd have to look up the charts to refresh my mind. (It's Friday at work and my mind is having a major dip at the xover point :-))

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 14:18:41
Davey
 Hint. You should think polar response here. Horizontal polar response, to be more specific.

Cheers,

Dave.

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 17:21:45
neolith
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Posts: 4842
Location: Virginia
Joined: February 21, 2002
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  Since: December 2, 2004
 I use a spreadsheet that I developed in Open Office. Open Office does everything that MS Office does and is free.



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 18:32:30
neolith
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Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 The phase relations are determined by the math. Here's two graphs that show the difference between a symmetric 1st order crossing at 800 hz and an asymmetric 1st order also crossing at 800 hz.
With the symmetric the phase difference is a constant 90 and the output of normal polarity and inverted polarity are superimposed on each other.

With the asymmetric, the phase difference varies with frequency and the normal polarity creates a dip while the inverted polarity has a much shallower dip.




"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 24, 2012 at 21:39:34
JBen
Audiophile

Posts: 3082
Location: South FL
Joined: May 18, 2008
Contributor
  Since: July 26, 2010
 Neo, I was wondering if we were talking about 2 different things. By asymmetric, I meant the original MMG xover with the mix of 1st/2nd order slopes. In this case, the current design would be symmetric for having 1st/1st.

Perhaps the symmetry you refer to is with regard to another set of parameters? I believe you mean that the points for each driver are at the same frequency. This latter case is when one should expect to see no difference? I guess this is it. Which would explain why, by being asymmetric in this sense, I didn't see what I expected from a current design. Of course, MMGs don't want to use the same points because, hey, that would be boring and conventional. LOL!

So, both MMG xover designs share this kind of approach to frequency point asymmetry. In addition, the original design is further asymmetric in the slopes used.

All of which helps us have so much fun. :-))

 

RE: MMG MiniDSP and bi-amp Questions, posted on February 25, 2012 at 05:05:00
neolith
Audiophile

Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 It's my understanding that a symmetric crossover is like the typical L-R in that the fc of the LP and HP are the same as are the orders of the two filters. Anything else is asymmetric. Magnepan almost exclusivly uses asymmetric crossovers with the exception of the SMG(a)(series 1st order) and IIa (parallel 3rd order).



"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: MMG MiniDSP and bi-amp Questions, posted on March 5, 2012 at 10:42:40
acroy
Audiophile

Posts: 136
Location: Dallas-area
Joined: December 5, 2011
 I finally played with the MiniDSP over the weekend, it is a fun little toy. I've tried both the 1st order and 2nd order setups with x-over points as above.

Sound quality seems very good. I do not detect any high-frequency harshness or noise at all. Noise floor is outstanding. With computer, miniDSP, and amp gains all turned up to the max, there is zero noise from the panel even with my ear against it.

So far my preference is for the 'new' 1st order corssover setup, which is what my MMG's originally came with. It is entertaining changing the slopes and frequencies on the fly, and hearing the immediate change.

Another good function is being able to invert driver phase on the fly to ensure correct polarity. Sure enough, when out of phase, there is a midrange dip. Not as severe as I'd thought it would be, but pronounced, sepecially when you can a/b instantly.

More experimentation to follow. Next up with be playing with REW and feeding the results into the miniDSP.

 

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