Posts Tagged transfer function

blueprinting and characterizing studio monitors — Phase Technology PC2

This seems to be the quarter for speaker matching.

Last week, I trundled off to Nova-K studios with an audio analyzer and a truck full of surplus acoustical treatments. My mission was to figure out how much improvement in the monitoring environment could be made ‘on the cheap’.

I had already recorded several projects at this particular studio. I knew that it was a challenge to create mixes there that translated well to other systems. After living with the situation, we finally reached our breaking point one day.

The smoking gun

One one tune, we wanted to add some room sound to glue the electronic sampled drums together as a cohesive entity. In a stroke of madness, we decided to reamp the drum tracks through the control room monitors, and record the resultant sound using a matched pair of small diaphragm condenser mics. Upon playback, the sound was unusable due to a nasty resonance. In fact, the resonance was so pronounced that a guitar tuner program built into my phone had absolutely no problem in identifying the resonance as 67.some-odd Hz.

We tried reorienting the mics & monitors, trying different mics, etc. We could not eliminate the resonance. Further, now aware of its presence, we could clearly hear its effect in the mere playback of almost any program material.

A quick back-of-the-envelope analysis of wavelengths, room dimensions, and modal theory revealed that this resonance was unsurprising. We will return to this theme in the future. For the purposes of today’s post, we need to move forward a bit.

Matched pair? Riiight.

So I arrived at Nova-K with my analysis rig, and set it up. I figured I’d get some raw shoots of the current situation to establish a baseline before tweaking anything.

So I set my measurement mic up in the mix position, and proceeded to measure the output of the left channel. There were significant deviations from flat response, including the expected huge peak in the 68 Hz region. We measured a similarly dismal response in the right channel. Further, the left side was weaker by 4 to 8 dB through the range from about 800 Hz – 8 kHz. This seemed extreme, but plausible given the lath-and-plaster nature of the room, and its asymmetries.

We moved the mic in turn to approximately 1 foot in front of each monitor in turn, expecting the left-to-right deviation to all but disappear. We were surprised, but not completely astonished to learn that the discrepancy from side to side in the upper mids to remain about the same.

Starting to question the speakers (as opposed to the room), we swapped the left speaker to the right mount and the right speaker to the left mount. Measuring each again showed that the upper midrange difference moved with the speaker, rather than staying with the location in the room.

Now pretty convinced that the difference was in the speakers, we wanted to prove this. We placed them at chair height, right next to each other, and baffled them off front, sides, and top from the room with heavy gobos, and in the rear with blankets. Placing the mic about one foot in front of the seam between them, we measured again. The following graph clearly shows the discrepancy in response — the ‘left’ speaker is in orange, and the ‘right’ speaker is in blue.

nova-k studio monitors response before

nova-k studio monitors response before

One can readily see that, save for a strong peak at about 1300 Hz, the left speaker has a significantly weaker response throughout the upper mids as compared to the right speaker.

The Speaker Scrutinizer

It was evident that, given the speaker to speaker inconsistency, this was a problem that needed addressing before the room treatments. Accordingly, we changed tack.

The speakers were model PC2 from Phase Technology. Other then hearing them at Nova-K, I had no knowledge of this manufacturer. While not marketed as ‘studio monitors’, per se, further investigation revealed that they have a decent reputation in audiophile communities. These are a two-way, bass reflex design, with ~ 6.5″ kevlar woofer with a flat plate, and a 1″ soft dome tweeter. These had been in service at Nova-K for approximately 13 years.

So we started disassembling one. This disassembly revealed a rather solidly built unit, with a hefty woofer, and a surprisingly complex crossover network for a two-way bookshelf system:

Phase Technologies PC2 crossover network

Phase Technologies PC2 crossover network

A visual inspection tuned up nothing amiss. So we tore into the other unit. We were surprised to find that the rather substantial internal bracing from the first unit (s/n 01290A, previously ‘left’) was missing from the second unit (s/n 01021B, previously ‘right’), and that the second unit’s XO was mounted at a rakish angle.

Again, a visual inspection turned up no issues with the right speaker that would seem to explain the discrepancy in frequency response. It was obvious that we needed to measure each individual component to identify the source of the discrepancy. Lacking a full array of measurement gear, I took the speakers back to Rocket Surgery Labs (a subsidiary of my sound company q music inc.) for a more detailed analysis.

On the bench

Back at the Lab, it was time to characterize each component, looking for differences from unit to unit. The speakers each consist of essentially three components — the woofer, the tweeter, and the crossover network (XO). Access to each of these is through the woofer’s mounting hole.

Reasoning that there were no ‘strained’ sounds from any of the drivers indicating damage, and that the XO was rather complex for a two-way design, I started my investigation with the XOs. I pulled them from the cabinets. I decided to characterize them using a process similar to that described in this previous post. This process employs SMAART, as a dual channel FFT, to generate a Transfer Function for each XO output. See the aforementioned post for more info on this process.

Here is an overview of the test setup:

PC2 Crossover Test Setup

PC2 Crossover Test Setup

Before setting up the test rig, I measured each of the woofers and tweeters at a DC resistance of 2.7-3.2 ohms. Using faulty reasoning that I now discount, I assumed these to be three ohm nominal drivers. In retrospect, these are likely four ohm nominal drivers. However, as we will see, this is immaterial for the purpose at hand.

Accordingly, I loaded down the LF output of the XO with a 3 ohm wirewound load resistor, and the HF out with a 3 ohm load composed of two 2 ohm resistors in parallel, for a resistance of 1 ohm, in series with another 2 ohm resistor, for a total of 3 ohms. Here is a closer look:

Crossover test rig connections

Crossover test rig connections

In this photo, one can readily see the output of the amp on the gray-sheathed red and black wires, connected to the XO inputs (white and black), the XO LF outs (blue and black) connected to the green wirewound resistor with the alligator clip leads (red and black {offscreen}) and the XO HF out (red and black) connected to the resistor network through the alligator test leads (yellow and green at top). Also attached to the LF outs is the Interface’s DUT input, through the red and green alligator leads in the foreground.

In the process of this testing, I discovered a cold solder joint on one of the inductors. After repairing this, the XO’s Transfer Functions were remarkably similar:

L & R XO LF & HF outs with 3 ohm dummy loads

L & R XO LF & HF outs with 3 ohm dummy loads

Remarkably similar Transfer Functions. I next reassembled the units, but included leads from each XO output <> driver connection, each passed out the bass reflex port. This allowed me to measure electrically at the crossover outputs with the complex load of the drivers and the cabinets, rather than the simple dummy load. Here is how this looks physically:

assembled speakers instrumented electrically

assembled speakers instrumented electrically

While the photo is blurry, you can see the tape flags labeled LF on the red and black pair, and HF on the yellow and green pair.

With the real load of the speakers on the XO’s, they still appear well-matched:

Both XO's LF & HF TFs with the actual drivers

Both XO's LF & HF TFs with the actual drivers

Both amplitude and phase are very well matched, for both LF and HF, from unit to unit. Accordingly, we can rule out the XOs as the source of the frequency response aberration.

For the interest of completeness, the following graph compares the XO TFs in both the case of dummy load, and of actual drivers (and cabinet acoustics) as loads:

Comparison of XO outs with resistive dummy loads, and actual drivers

Comparison of XO outs with resistive dummy loads, and actual drivers

We can see that the real load of the speaker drivers, as well as the reflected acoustic impedance of the cabinets, have a non-negligible effect on the crossover transfer functions.

Woofers

I next decided to move the L woofer to the R cabinet and vice versa. If the aberration went with the woofer, then the woofer would be implicated as the differing component. If the difference stayed with the rest of the unit (cabinet, XO and tweeter), then the woofer would be absolved. This would require acoustic measurements. Further, I would need to re-baseline the measurements due to the different acoustical environment in which I would be testing.

I placed the speakers side-by-side on the corner of the workbench, and carefully positioned my measurement mic to be equidistant from the axis of each driver. Here is an overview:

acoustic test setup overview

acoustic test setup overview

You can see the similar configuration of the SMAART test computer, the Interface, and the Power Amp. The DUT input of the interface, however, is now being fed by the measurement mic visible in the foreground. The next picture shows the view down the axis of the mic:

acoustic test setup mic

acoustic test mic setup

One can see the test leads from a previously described test exiting the port one of the speakers. These are not connected in this test.

With this rig, I made baseline measurements. These are shown below. The left speaker is in orange, and the right speaker is in magenta:

Nearfield response - left vs right

Nearfield response - left vs right

Hey! In this acoustic environment, the right speaker is impressively flat – within +/- 3dB from about 50 Hz to about 18 kHz. The phase response is also very smooth.

One can clearly see that there is still a significant loss of amplitude in the upper mids in the left speaker. Interestingly, the 1.3 kHz spike is no longer present. Was this a byproduct of the cold solder joint on the XO? More on this later.

I then swapped the woofers from cabinet to cabinet and remeasured. The problem did not move with the woofer — it stayed with the rest of the speaker. Here is the curve for the right speaker with the woofer from the left in green, along with the right speaker with right woofer from the above measurement in magenta:

nearfield R vs R with L woofer

nearfield R vs R with L woofer

And similarly, replacing left’s woofer with that of the right yields similarly poor performance for the combination. Here is the earlier left measurement in orange, with the new measurement of left with right’s woofer in blue:

nearfield L vs L with R woofer

nearfield L vs L with R woofer

As the problem stayed with the system, and did not move with the woofer, we conclude that the woofers are substantially identical. I returned the woofers back to their original systems.

Tweeters

As the XOs are identical, and the woofers are identical, the difference must be either in the tweeters or the cabinets themselves. I prepared to swap the tweeters from cabinet to cabinet to make another measurement. Upon removing the diffraction foam on the face of the baffle, I was met with quite a surprise. The tweeters are of substantially different design! See the photo below:

Tweeter Comparison

Tweeter Comparison

The tweeter on the left appears to have a trim ring around it, pressed into a recess in the face of the baffle. The right tweeter has an integral mounting flange. Investigating further, I snapped some photos from the inside of the cabinet at the backs of the tweeters. Here is the left:

L tweeter inside

L tweeter inside

Note that the tweeter itself does not extend through the baffle.  It is merely screwed to the front of the baffle, and the speaker wires are passed through, and sealed with modeling clay.

On the other hand, the right tweeter does extend through the baffle:

R Tweeter from inside

R Tweeter from inside

This tweeter appears to have a back chamber, adding perhaps an inch and a half of depth behind the face of the tweeter. No wonder it sounds different.

C’est la vie

Despite being sold as a matched pair (!), they are of considerably differing design. Oddly, the presumably newer one (based upon serial number), with the additional cabinet bracing, has the poorer tweeter configuration. Ideally, we would procure one of the old style tweeters for the other speaker, thereby bringing it up to performance with the better one. However, these are thirteen year old speakers.  Accordingly, sourcing may be a problem. While attempting to locate a suitable replacement, we still have mixes to do. Accordingly, we will accept the differing performance for now, and patch around it as best we can with eq. As the phase looks rather smooth through this region, we expect to be able to largely mask this difference in performance with some tuning. At least we eliminated the nasty peak at 1.3 kHz.

The room tuning will be the subject of my next post…

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EAW KF650isfx crossover repair and characterization

In a previous post, I described how I performed some troubleshooting on a number of crossover (XO) networks for the EAW KF650 — specifically KF650isfx. At the conclusion of that post, I described what I thought were the failed components of each XO assembly.

Since then, the good news is that EAW is still stocking the required parts to effect repair. Almost is good is that I received all the necessary parts, delivered, for less than a Franklin.

So I repaired the XOs with the replacement parts.

After repair, I used SMAART to measure the transfer function (TF) of each of the XO assemblies.The set setup was the same as in the previous post.

Here is a TF of all five LF modules, superimposed:

EAW KF650isfx LF XO - all 5 superimposed

EAW KF650isfx LF XO - all 5 superimposed

The lack of correlation at ~1.25KHz is puzzling. However, all five demonstrated the same issue, and the level is already attenuated by about -60 dB at this frequency.

Here are TFs of all five MF XO modules, using the same IO Panel 3203 for a control:

TF: EAW KF650isfx - all 5 MF XO w/ 3203 IO Panel

TF: EAW KF650isfx - all 5 MF XO w/ 3203 IO Panel

Excellent! And all five IO Panels, using the same MF module 3203 as a control:

TF: EAW KF650isfx - all 5 IO Panel w/ 3203 MF XO

TF: EAW KF650isfx - all 5 IO Panel w/ 3203 MF XO

Again, excellent.

Lastly, we have TFs for all five HF XO modules:

TF: EAW KF650isfx - all 5 HF XO

TF: EAW KF650isfx - all 5 HF XO

Unit 3204 has a slight dip at 2K. Note however, that the phase at that frequency is spot-on. Other than this slight attenuation, they match almost exactly.

The upshot of all this is that the repairs have been effected, and the XO’s match extremely well with each other.

Three-way operation

In order to visualize the operation of the crossovers as a whole, the following is a graph of representative LF, MF, and HF modules superimposed:

TF: EAW KF650isfx - all bands

TF: EAW KF650isfx - all bands

Note that the mids and highs are horn loaded. Accordingly, they are acoustically more efficient than the lows. In terms of acoustic output, it would help to visualize the MF and HF curves as boosted by 7dB from their current values.

Here is a high resolution look at the magnitude curves:

TF: EAW KF650isfx - all bands - hires magnitude

TF: EAW KF650isfx - all bands - hires magnitude

And a look at the phase curves when unwrapped:

TF: EAW KF650isfx - all bands - phase unwrapped

TF: EAW KF650isfx - all bands - phase unwrapped

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EAW KF650isfx crossover operation

The EAW KF650isfx

So a while back, my sound reinforcement company Rocket Surgery Sound (a division of q music inc.) bought five used EAW KF650isfxP speakers. These are true professional-grade three-way arrayable speakers for live sound reinforcement. The KF650- is the base model name, the -P suffix indicates they were made for permanent installation, the -is- indicates the first iteration of these speakers, and the -fx- indicates that they were a special Build To Order version with an integral three way passive crossover, meant to be operated full-range only.

The standard product most like my speakers are the KF650is (without the -fx-). These standard units are meant to be either bi-amped or tri-amped. So rather than the single crossover (XO) of the standard product, the KF650isfx has three XO networks spread across four separate modules. The woofer is filtered by the LF XO module, the tweeter is filtered by the HF XO module, and the mid is filtered by the MF network, which is spread across the MF XO module and the IO panel.

Problem and Potential Solution

Upon receiving these speakers, each sounded radically different than the other. Cabinetry seems identical. The drivers all seem roughly equivalent — though I will characterize them fully in a future installment. Apparently, the difference in sound from speaker to speaker is due to variation in the XO networks.

In order to restore the proper operation of the XOs, the plan is to characterize each network’s Transfer Function. With the assumption that at least some XO networks are in good shape, I should be able to determine which XO networks have drifted from their factory values, and then drill down to component level troubleshooting to identify the failed components.

I have access to a tool called SMAART, which is a computer program that measures and compares audio signals. Using SMAART, I can compare the relative amplitude and phase, between the input to an XO and its output, across the entire audio frequency range. This generates a Transfer Function.

Accordingly, I will capture a Transfer Function of each XO in turn, and compare these for any anomalies.

Test Setup

The following is a diagram of my test setup:

XO test setup diagram

XO test setup diagram

Note that I am using an 8 ohm resistor to load the output of the XO. Each driver in the KF650is* is an 8 ohm driver. While a speaker driver is a more complex load than a resistor, the resistor will serve for this characterization. Recall we are just needing to compare Transfer Functions from XO to XO to look for differences.

The rather low impedance requires us to employ a power amp to drive the XO. The power amp is being driven by an audio interface, which is fed a signal from SMAART.

The inputs to the interface, and thereby the inputs to SMAART, are at the input of the XO (V(omega)in) as the reference signal, and at the output of the XO (V(omega)out) as the measured signal.

SMAART is able to perform an FFT (Fast Fourier Transform) on both the reference signal and the measured signal and compare them. This generates the system Transfer Function

H(omega) = V(omega)out / V(omega)in

The following is a photo of the test setup:

XO Test Setup

XO Test Setup

Here, at the left you can see he computer running SMAART atop the amplifier, the interface in the center, and one of the XO modules on the right. The load resistor is the green cylinder behind the XO. There is also a DMM in the picture which may come in handy. Last, there is an assembly drawing and a schematic for the XO.

Low Frequency XO

For no particular reason, I started with the low frequency (LF) XOs. I found that four out of the five LF XO networks matched very closely. See this following for a Transfer Function (TF) graph:

LF TF - 3203, 3204, 3206, 3207

LF TF - 3203, 3204, 3206, 3207

This graph shows all four TFs match within a fraction of a dB for all frequencies of interest.

The following is a higher resolution view of the same:

LF TF - 3203, 3204, 3206, 3207 - hi res

LF TF - 3203, 3204, 3206, 3207 - hi res

I was quite pleased that four of the five matched so closely. Indeed, even the phase trace is as tight as I could hope for. Phase only starts to deviate around 1 kHz, but the signal is already in excess of -50 dB down by this point, essentially being inaudible.

With such tight correlation, I am quite sure that these XOs have not drifted appreciably since leaving the factory. This is testament to the quality of truly pro level gear.

It is worth noting that the graphs show approximately -1.5 dB of insertion loss for the LF XO. This represents a waste of power. Converting to bi- or tri-amp operation at some future date could regain this lost amplifier power.

But not all is perfect

Unfortunately, the fifth LF XO (s/n 3205) did not fare as well. The following shows this TF superimposed upon the other four:

LF TF all

LF TF all

Note the discrepancy of the orange trace. This looks as if the main XO network 1s operating properly, but a second tuned filter at about 250 has failed.

By performing some resistance and capacitance measurements of the failing XO, compared with that of the good ones, I determined that L10 had failed on the LF XO 3205. This is a 0.9 mH inductor. The failed unit displayed a DC resistance of 0.2 ohms, while the healthy ones had a DC resistance of 0.6 ohms. Visual inspection suggested that the failing L10 may have overheated, melting insulation, and shorting the coil.

To verify that this was the only failed component I swapped L10 between the bad XO 3205 and the good XO 3203. The previously good 3203 with the bad L10 now failed:

LF TF - 3203 with bad L10

LF TF - 3203 with bad L10

The orange trace is the first measurement of the failed XO, while the blue is the previously good 3202 with L10 from the failing unit.

I next captured a TF for the previously failing XO 3203, with L10 replaced with one from a good XO:

LF TF - previously bad XO with good L10

LF TF - previously bad XO with good L10

Note how closely the ‘repaired’ XO (orange) matches one of the four matching traces from above (blue).

Conclusion – a new L10 will restore LF XO 3203 to proper operation. At that point, all five LF XOs will be effectively blueprinted. I sure hope EAW can provide these inductors.

Mid Frequency XO

The MF XOs are somewhat more involved, as they are spread across two assemblies – the IO panel and the MF XO proper.

I first used the IO Panel sn 3203 as a control, and measured the five MF XOs.

As the response of the MF XO does not extend to DC, SMAART must compensate for the concomitant delay. I started by taking an impulse response:

MF Impulse response

MF Impulse response

And set SMAART’s internal delay to the corresponding .41 ms.

I then measured all five MF XO’s, using the same IO panel. Much to my delight, all five MF XOs exhibited tight correlation:

MF TF - all five modules

MF TF - all five modules

There is a little phase difference creeping in around 100 Hz on the low end, but the amplitude is already minimal by that point.

IO Panel woes

Unfortunately, the picture was not so rosy for the IO panels. Using the MF XO 3203 as a control, I measured each IO panel in turn. As I will explain later, I determined that IO Panel 3203 was one of the ones in good shape. As I used this IO panel for the MF XO measurements, this meant that I did not have invalidate all the MF XO TFs.

The following shows all five IO panel TFs superimposed. from the picture, one can see that there is a group of two, another group of two, and a third (fifth) outlier.

TF all IO panels

TF all IO panels

There were two IO panels that exhibited proper operation — 3203 and 3206. Their TFs are displayed below:

IO Panels 3203 and 3206 -- good

IO Panels 3203 and 3206 -- good

Excellent correlation.

The other group of two are as follows:

IO Panels 3204 and 3207 -- bad

IO Panels 3204 and 3207 -- bad

Visual inspection revealed that a capacitor lead had been clipped on these two IO panels, essentially removing them from the circuit. Jumpering these leads caused the unit to whine loudly. Using a DMM and capacitance meter, I determined that these clipped capacitors were faulty. Some previous tech’s means of ‘fixing’ the units after failure was to remove them from the circuit. This removal is the cause of the radically different TF.

These caps are 15 uF / 100 V bipolar electrolytic. As two out of the five have failed, it seems likely that this is a weak area of the KF650isfx design. It would seem prudent to increase the voltage rating of this cap to 200 V or so.

The last IO Panel is worse yet:

IO Panel 3205 - dead

IO Panel 3205 - dead

Note that, at the middle of its passband, the TF is essentially dead – the level is attenuated in excess of -50 dB.

By measurement, it was discovered that the other cap was faulty. This is a 30 uF / 100 V unit. Again,would seem prudent to increase the voltage rating of this cap to 200 V or so.

High Frequency XO

I performed an Impulse response on the HF XO:

HF Impulse

HF Impulse

After setting the delay to 0.02 ms, I captured TFs for all five of the HF XOs. Much to my delight, all five exhibited excellent correlation:

HF TFs - all

HF TFs - all

In the process, I learned that the HF output wires are unconventionally colored. The black wire is hot and the red wire is ground. By hooking the colored wires to the correspondingly colored terminals on the tweeter driver, one obtains a 180 degree polarity flip. This is likely an intentional design artifact.

Unfortunately, before I realized this, I blew up my amp by connecting its output hot to the interface’s ground. Oh well.

Summary

Well, if my analysis is correct, it should be easy and cheap to restore the crossovers to factory fresh operation. Three capacitors (cheap and easy) and one inductor (moderate expense, and perhaps difficult to find) are all that is required.

I will follow up some day with a post-repair report.

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