Rane MP24 -- Performance Modification Question...

[rs_]

Hi, thanks to those who helped me get this mixer going 2 weeks ago. I have a lot of respect for the Rane designers; I feel that the maintainability and quality of the MP24 is up there with the UREI 1620 -- and in some ways it's better. A criminally-underrated mixer in the year 2013.

While I await some other parts, I was taking a look at the schematic:
http://www.rane.com/pdf/old/mp24lcsch.pdf

I noticed an 18dB/oct highpass filter at 15Hz just after the Program summing stage. (Page 6 of the PDF.) Based on discussions with some people who make a living modding equipment for studios, these are the kinds of things they often remove to improve sound quality. This mixer is no longer being used with turntables, instead being used exclusively with a Serato line-level box. If the purpose of the highpass filter was to protect from turntable rumble, it is no longer needed. Furthermore, the mixer is being used with systems that have numerous other protection measures built in anyway.

I'd like to bypass this filter entirely. If I'm correct, this HPF can be seen at the top-right corner of Page 7 of the PDF. If I understand correctly, I should be able to bypass it by jumping from the output of R144 to the output of Z7, and removing Z7 entirely. Does this seem correct?

Thanks,
rs

[Enzo]

I would retain C127 or some other large-ish cap to block any DC coming or going at that jack. Look at all other output jacks in th system, they seem to agree not to ride bareback.

HP filters do protect against turntable rumble as well as some TT feedback, but also things like mic stand rumble and stage floor noise. A HP filter is not there to protect anything, it is there to maintain the quality of your sound. Think of it as an electrical equivalent of putting a foam ball over a microphone to reduce blast noise.

Just my opinion, but for me this falls into the category of "just what is improvement?" Like someome arguing that some op amp is better because it has bandwidth to 5MHz instead of the 3.5MHz of the original. Um, this is audio, who cares about "better" high end respsonse when we are talking Megahertz? So you intend to flatten out the response under 15Hz? Subwoofers have response down to about 40Hz, maybe even 30Hz. 15Hz is an octave below even that. I found one behringer sub that claimed a 20Hz bottom. I will believe that when I see it, and how many db down was that anyway? But even 20Hz is below your filter freq. SO what would this accomplish, other than to have some personal touch to the innards?

SInce this is a DJ mixer, I am assuming it is being used in live DJ work, not recording studio work. SO I have to consider it is entertaining dancers and drinkers and drinking dancers. Studio guys are clever, and they want to squeeze as much as they can out of gear. Frequencies are something you can't get back when lost. SO if they can get response out either end of the scale and octave farther, they will do it, just so it will be there later. But in live sound, I'd put up this experiment any day. Play a set of music to get people acclimated. Now don;t tell anyone, but drop all the EQ sliders over 10kHz to -12db or whatever you graphic has as a cut limit, and see if anyone notices. Those people are not there to sit in the sweet spot and critically listen to a symphony. They want a thumping beat and a familiar tune.

SO I have to wonder how doing this helps anything.


You want to do a useful modification? Since you no longer use turntables, ditch the three RIAA preamps and convert them to more line ins. That will add three more useful pairs of inputs. And should be no more complex than the mod you proposed above.

[rs_]

Even though I didn't write it in the original question, the "true mod benefit" was one "conceptual" question I was wondering about, so thanks Enzo for addressing it.

I hear the "people won't notice in a noisy club" notion expressed a lot, and for a long time I subscribed fully to it. But as DJing has "evolved" (or at least "changed") to the point where anybody with an iPod and a set of Behringer speakers calls themselves a DJ, I've gone more and more in the "audiophile" direction as one way of offering something that most DJ's do not give serious thought to. Even if it's a sonic change that "shouldn't" be noticeable, I've started laboring under the assumption that people are extremely perceptive, and will notice it. I have gotten a lot of positive comments about the sound from ordinary people, saying it sounds much better/"warmer"/etc... than the average Chain-Store DJ setup. Maybe I'm just happier playing when the sound quality is slightly better, or maybe it's a case of people "hearing with their eyes" like Les Paul explained, but whatever it is I've found some sonic "mods" to be worthwhile.

***

Back on a technical level, it does bear mentioning that no microphone signal passes through this summing amp and HPF, according to the signal flow diagram. The microphones are summed in later, after the FX loops and the Program EQ. So, other than the potential for turntable rumble, I guess that some DC could potentially find its way into this portion of the signal path?

Regardless, this mod is on the "back burner" I think. Once I've got the rotary conversion done I may revisit it -- keeping in mind what Enzo said about the DC blocking capacitor. Thanks folks as always!

rs

[Enzo]

I think people notice if it sounds good or not. But I also think they won't hear the 15k. I also think the ability to run the system counts. Some guy rents a DJ rig from a chain store, puts his smiley face EQ curve on the graphic and is gratified to hear thumping bass out the subs. You come along and properly EQ the speakers and with the same gear sound 100% better.

[rs_]

It's hard for me to know where the benefits of "lots of practice" are ending, and where the benefits of "souping up electronics" are starting, probably since I'm so tied up in it on an artistic level.

Either way, thanks for sharing your knowledge so generously here in general.

rs

[Jazz P Bass]

Do you want to be the 'best' sound guy?
Wear hearing protection.
Take it off to adjust & then put it back on.
Once those little cilia hairs are gone, they are gone.
My 2 pence.

[km6xz]

As a studio guy and a club guy, and an electronics engineer, I think it is unwise to mess with a design that works very well in that amateurs seldom consider the apt notion of "there is no free lunch" in design. What specific and general design or performance criteria are you willing to give up to gain in some specific criteria? You have posed the question as in general terms of expecting it to sound better. That makes no sense. WHAT sound better and how to you design for it and then measure it to confirm that your specific goal was reached? What are the benefits of <15hz signals passing through to later stages? Why was it in there in the first place, not rumble, there is already bandpass tailoring being done earlier. This is where the "no free lunch" comes in. What about summing amp settling time when integrating multiple channels or stability? If anything excessive uncontrolled LF content does more to ruin sound than it could possibly benefit. It lowers headroom in the system, causes intermodulation, stresses speakers without generating sound, and lowers reliability. That is the "lunch" part.
Any mod needs to be for a specific reason, technically, not some ethereal belief that a simpler path is more "pure goodness". If the specific technical narrowly defined goal outweighs the cost of the "lunch", have at it. But in this case there is no definite goal that corrects a problem, just a hope.

[Steve Conner]

I'm pretty sure that filter is there to protect against subsonics from turntables. This was a serious problem in the vinyl era. Vibrations from the bass bins could travel back through the stage to the turntables and start low-frequency feedback. This was one reason for the Technics 1200's heavy chassis and squishy anti-vibration feet, but even so it needed all the electronic help it could get.

I guess you could bypass the filter if you were sure that you wouldn't use vinyl any more and your digital sources wouldn't produce any subsonics. However, I don't expect bypassing the filter would make an audible difference, unless you were overdriving things further down the signal chain, in which case you would get the intermodulation that Stan refers to. But what audiophile DJ would drive things into the red?