[Gary Blake]

Hello, new to this circle.

I saw an older post here regarding upgrading a speaker crossover with newer higher quality parts while keeping the same values of the components without testing and re-designing.

I have a circa 1983 pair of Merak Acoustics model M4 speakers that I want to upgrade, specifically the crossover, internal wiring and binding posts.
These were a high end speaker back in the day (Think I paid about $12-1500.00 for them in 1983) They still sound pretty darn good. I have them hooked up to a similarly high end Harman Kardon system of the same vintage and I have restored the PM 660 amp and T60 Turntable.

These were a very limited production item and as such there is very little information available on the internet/google.

So far on the speakers I have re foamed the dual passive radiators and the polypropylene woofers and put a fresh coat of finish on the oak veneer cabinets.

I have been watching a lot of Danny Richie’s videos and have caught the upgrade bug again.

These speakers have very good quality drivers and a solidly built well braced and damped cabinet (Weigh in at 40 pounds each). I like them a lot and am prepared to go down the “rabbit hole” with the upgrades.

When I opened them the crossover parts are obviously on the cheaper side along with the wiring and binding posts and driver connections.

Plan is to replace the crossover parts with parts of equal value but higher quality as per the older posts question and upgrade the wiring and probably add tube connectors to replace the cheap binding posts.
I have run in to a bit of a problem identifying the parts on the crossover. It is a very simple crossover with only 6 parts per speaker. These are two way speakers with one woofer and one tweeter.

The capacitor is not a problem it is an electrolytic 10 uf 50 volt generic, I plan on replacing this with a Gen I Sonicap 10 uF 200VDC Part# 506741  or Audiocap Theta 10uF 200VDC Part # 426650  (Sonic Craft part numbers)

There only two resister values involved, presently marked “TRW PW5 5W 10%, 1R0 8144”, (5 Watt 2 Ohm?, from what I can gather, could use some help here is anyone knows how to read these markings on the older TRW resisters)

The second one is marked “TRW PW10 10W 10% 39R 7949” (10 Watt 39 Ohm? Same request here if I am reading these marking right)

If I have the right values I plan on replacing these with 1.8 Ohm Mills 5 watt MRA-5, Part # 626130 or 2.5 Ohm Mills 5 watt MRA-5 Part # 626145 (Sonic Craft part numbers again and they don’t have a 2ohm version) and 39 Ohm Mills 10 watt MRA-10, Part# 630369

The inductors I cannot find any markings on so am at a complete loss here. I have contacted the manufacturer (Now called Axiom) but they have not kept any specification records from that far back.

I do have the original brochure for the speakers but all this regarding the crossover is “2500 Hz and 70 Hz with accommodating Gaussian configuration”.

Not sure what this means or if it helps to identify inductor values.

Other technical spec that may help –Frequency response 30-20,000 Hz +/- 3 db, 8 ohms, Sensitivity – 92 dB SPL for 2.8 V RMS pink noise input measured at 1 meter in average listening environment, Power requirement  - 15 watts minimum, Power Handling – 50 watts nominal, 200 watts maximum.

Hoping someone can help me out with the unknowns and question marks.

I want to try this first and will probably add one of GR Research kits to the list of thing to try, the new NX Studio model looks like it may suit my listening environment quite well

Can add a photo of the current crossover board if that helps.

Gary

[77SunsetStrip]

Replacing crossover parts with better quality of the same value can breathe new life into your speakers.  Getting rid of electrolytic capacitors in vintage speakers is a must.

1R0 indicates a 1 ohm resistance.  39R0 is 39 ohm.  The only way to identify the inductors is find a value marked somewhere or measure them.  There is a slim chance posting pictures might reveal something.

[Doublej]

Pictures help as does a crossover schematic if you can draw one. Since R, C, and the crossover frequencies are known, I think one could determine the L values with a schematic.

The marketing brochure says they are a three way speaker but they have only two drivers plus a passive radiator so I don't know what the 70 Hz crossover is doing.

[Gary Blake]

Thanks for the replies. I think the "3 way" speaker description on the brochure may just be marketing people thinking the passive radiators qualify as 3 way.

Added a photo and visual drawing of the circuit (Electronically and software handicapped, appreciate if some could draw a proper circuit diagram for me)

I am also wondering why there are two inductors and only one capacitor on this crossover?

[Danny Richie]

You can send the inductors in and we can measure them for you.

We can also provide you with all of those parts.

We even send out the Sonicaps in matched pairs.

[Gary Blake]

Danny, thanks for the response. I have found a local source that will test and measure the inductor values for me, that will be happening later today. (That will save shipping them all the way to Texas and speed things up a bit)

I will definitely be in touch with you regarding parts, I already have most of the items picked out, just waiting for the inductor values to finish my shopping list for this project.

[Gary Blake]

Now I have had the inductor values measured and the results are as follows:

Woofer Inductor: 1.1 Mh, 19 AWG wire, 0.7 Ohm resistance
Tweeter Inductor: 0.46 Mh, 22 AWG wire, 0.7 Ohm resistance

I have also found the spec sheet for the tweeter which is an SEAS H 253 1” dome and will attach below.

The person that did the measurements was confused as to the overall design of the crossover (He has 45 years of experience in speaker/crossover design and has designed and manufactured his own line of speakers and seemed quite knowledgeable)
 
Just by looking at the crossover design he seemed to think the original design would cause the drivers to be out of phase by quite a bit and that the full efficiency of the drivers was not being utilized.

He suggested an alternate design that would eliminate the three resistors entirely, replace the 10 uF capacitor with a 4.7uF, eliminate the tweeter inductor and just retain the woofer inductor but change the value to 0.8 Mh.

He was also confused with brochure’s description of this being a 3 way speaker and the fact that the brochure lists 2 crossover points (2500 Hz and 70 Hz)

So now I am a bit confused and have some questions that hopefully someone can answer for me.

As to the crossover appearing to generate out of phase drivers – could this be a result of the original crossover design trying to compensate for the sloped front of the cabinet design which places the tweeter cone approximately 2 inches horizontally behind the woofer cone?

(The top 2/3 face of the cabinet housing the woofer and tweeter is sloped about 15 degrees backwards of 90 degrees, the passive radiators are housed in the bottom of the cabinet which is not sloped remaining at 90 degrees)

Could the 70 Hz crossover point be related to the passive radiators?

Do the passive radiators need to be accounted for in the crossover design and is this what the brochure is referring to in the description of the crossover ie “2500 Hz and 70 Hz with accommodating Gaussian configuration”?

Although the brochure states impedance at 8 Ohms (nominal?) for these speakers, the spec sheet for the tweeter states it is rated at 6 Ohms. I measured it to be 5.56 Ohms and the woofer at just under 5 Ohms.

Is this being accounted for in the original crossover design?





I am halfway thinking of rebuilding the crossover in an external enclosure and doing a/b listening tests with the original crossover design and the modification suggested above.

[Doublej]

Here's what I would do.

1. Watch Danny's video on how to measure speakers. His and most measurements include amplitude, phase, and impedance sweep graphs.
2. Obtain the hardware and software required to measure the speakers.
3. Measure your speakers.
4. Determine if you want to redesign the crossover after you have the measurements.

The brochure's claim of the speaker being a 3 way is not consistent with what is considered a 3 way speaker today, so I would ignore this. It's a 2 way speaker with a passive radiator.

From your diagram it appears that the electrical crossover for the woofer is a single inductor. This is a first order crossover. The inductor is a low pass filter. According to crossover calculator I found, the formula calculates the woofer impedance to be 3.4 ohms if the crossover frequency is 70 Hz.

https://www.omnicalculator.com/physics/crossover

It is highly likely that the crossover was designed to take into consideration the physical alignment of the drivers. What one cares about is the response of the speaker at the listener's ears. This is a specific distance from the listener and with the tweeter height often aligned with the listener's ears.

Once I have decided what to do, I would buy what I need from GR Research. Danny sources quality parts, support this site, and is very generous with his time to help people like yourself.

[Danny Richie]

You are making enough changes now that speculation as to what values are needed or what crossover will or will not work really all goes out the window. The actual response could be anywhere.

You are welcome to send one in. The measuring and testing service is free of charge.

[Gary Blake]

Thank you for the replies, all very helpful in this learning process.

Now that I have the inductor values, this further input and more research (Mostly Danny's videos - great way to learn) I will be going back to my original plan - rebuild the crossover with new higher quality parts of the same values and in the same configuration.

There was no problem with how the speakers sounded with the original crossover design.

I either missed or glossed over this part of specifications in the Merak brochure " Driver Unit Configuration: Time compensated vertical inline units."

From what I now know this tells me that the crossover was designed to compensate for the horizontal offset between the tweeter and the woofer. I did not communicate this information properly to the technician that measured the inductor values which caused him to question the crossover's original design and suggest an alternate design. My bad.

With upgrade parts of equal value and higher quality wiring the speakers should end up being pretty close to the original design intent.

I will also be upgrading the connectors on the speaker and amp and will be ordering one of the GR Research speaker cable kits and cable. All this should give me the improvement I am hoping for.

I will also probably be trying out Doublej's suggestion of picking up the hardware and software necessary to measure the results - the hardware is not that expensive for basic stuff and there are some open source software available.

I like tinkering with stuff like this. Right now I know just enough to be dangerous

Danny: I like your suggestion but shipping anything from Canada is ridiculously expensive, these speakers are 39 1/2 inches tall and weigh in at 40 pound apiece. Canada Customs can be a real headache with shipping and returning items as well (Been down that road before). To make things even worse the Canadian dollar is in the toilet right now.

[Gary Blake]

With the help from responses received on this forum, GR Research and accurate measurements of the inductors I have started ordering parts to go ahead with this project.

Even though these M4 speakers are pretty few and far between the good thing about these forums is that the information contained gets searched by Google. That means if there does happen to be anyone else out there looking to refurbish these speakers they may find some useful information in this thread so I will keep it going and update until the project is finished. (This is how I found a lot of useful information when doing my amplifier and turntable upgrades)

The inductors measured out as follows:
Woofer Inductor: 1.1 Mh, 19 AWG wire, 0.7 Ohm resistance
Tweeter Inductor: 0.46 Mh, 22 AWG wire, 0.7 Ohm resistance

I have decided to go with the following crossover upgrades:

Capacitor: Mundorf Capacitor 10uF 450Vdc MCap® EVO SilverGoldOil x 2
Bypassed with: Duelund 0.01uF 600Vdc JDM Series Silver Foil & Wax/Oil Precision Capacitor x 2
Resistors: Mills Resistor 39R Ohm 12W MRA-12 Series x 2 ( 39 ohm)
                   Mills Resistor 1R Ohm 5W MRA-5 Series x 4 (1 ohm)
Inductors: Mundorf CFC (Foil) Inductor 1.20mH 14awg x 2
                   Mundorf CFC (Foil) Inductor 0.47mH 14awg x 2

For this project I will also be upgrading wires and connectors and will be going with these items from GR Research. I will be adding the Electra Tube connectors on the amplifier side as well.

24 Strand DIY Speaker Cable - 23 feet
DIY Audiophile Speaker Cable Kit – electra cable tube connectors both ends for a pair of cables
Electra Cable Tube Connectors – 2 pair for amplifier side connections.
B24 Power Cable 4 Feet

Received the parts from GR Research, (Very high quality components)  still waiting on the crossover parts.