Through finding this site I'm getting into building my own stereo kit, first project is to rebuild the crossovers for my first speakers, 1977 Monitor Audio MA5s, currently gone all mushy, due , I believe from research here, to worn out capacitors. Removing the crossovers I can see that there is heat damage on the board from the resistors so I intend to replace all caps and resitors. MA do not have a circuit diagram available so I've looed up the values and would need your help to conform that these look correct and also where to source. The inductors look fine. I have no expereice in these matters and am relying on O level physics from 40 years ago!!!!!
High pass filter,
in series, one resistor, marked ERG 74ER 3R3m is this 3.3 Ohms.
two identical capacitors, spilt with the inductor to ground, orange, orange green white brown, are these 3,300,000pF +-10%, 100v?
Low pass filter
resistor to ground, 56R ( I think, badly faded) 56ohms?
cap to ground -yellow,violet green.white, brown , 4,700,000pF, + 10%, 100v?
resistor in parallel to inductor, 33R, 33Ohms?
three further questions
what should be the wattage for the resistors?
where can I buy this stuff?
what do the pF numbers traslate to in today's capacitors
Any help very gratefully received. Thanks in anticipation
High pass filter,
in series, one resistor, marked ERG 74ER 3R3m is this 3.3 Ohms.
two identical capacitors, spilt with the inductor to ground, orange, orange green white brown, are these 3,300,000pF +-10%, 100v?
Low pass filter
resistor to ground, 56R ( I think, badly faded) 56ohms?
cap to ground -yellow,violet green.white, brown , 4,700,000pF, + 10%, 100v?
resistor in parallel to inductor, 33R, 33Ohms?
three further questions
what should be the wattage for the resistors?
where can I buy this stuff?
what do the pF numbers traslate to in today's capacitors
Any help very gratefully received. Thanks in anticipation
I PMed you.
Lift the white wire from the network board to get the tweeter out of the circuit, then check the tweeter's voice coil's DCR
I think you will find that it is nearly a dead short; mine read .5 ohms
Sorry to say it, but you (and me) need new tweeters
Marshall
Lift the white wire from the network board to get the tweeter out of the circuit, then check the tweeter's voice coil's DCR
I think you will find that it is nearly a dead short; mine read .5 ohms
Sorry to say it, but you (and me) need new tweeters
Marshall
Welcome to the Monitor Audio restoration club! 😀
These MA5's look fun:
I'd say those are a typical 70's KEF/Chartwell/BBC clone with a 8" bextrene or polypropylene bass and what looks like an Isofon (KK8?) plastic tweeter possibly with a modified metal front plate. All quite fixable. Morel do a CAT-298 94mm or a 104mm that might do it at wilmslow audio. But you might just have corroded connectors.
Morel Classic CAT298
Worth fixing, for sure. You'll doubtless have a 3rd. order tweeter crossover, simpler than this KEF 104 design. Maybe second order bass crossover. The bass resistor values aren't making much sense to me. Try and draw the circuit and they fall into place. 5-10W Wirewound resistors available at Maplin along with pricey (but worth it) audio grade polypropylene capacitors. Laters...😎
Oh wait...old post...LOL
These MA5's look fun:
An externally hosted image should be here but it was not working when we last tested it.
I'd say those are a typical 70's KEF/Chartwell/BBC clone with a 8" bextrene or polypropylene bass and what looks like an Isofon (KK8?) plastic tweeter possibly with a modified metal front plate. All quite fixable. Morel do a CAT-298 94mm or a 104mm that might do it at wilmslow audio. But you might just have corroded connectors.
Morel Classic CAT298
Worth fixing, for sure. You'll doubtless have a 3rd. order tweeter crossover, simpler than this KEF 104 design. Maybe second order bass crossover. The bass resistor values aren't making much sense to me. Try and draw the circuit and they fall into place. 5-10W Wirewound resistors available at Maplin along with pricey (but worth it) audio grade polypropylene capacitors. Laters...😎
Oh wait...old post...LOL
Attachments
Last edited:
Get 10 Watt resistors. You can keep the set up for the woofer but switch out the stuff for the tweeter. Go with 5.6 uf, inductor, 22 uf, 3.3 ohm, tweeter. Try switching the plus and minus on the tweeter and see what sounds the best. Another fix for the tweeter is to skip the 22 uf, replace with wire. See what sounds the best. This is a crossover at 3 kHz for both drivers. Look at ebay for new parts.
I used to have those Isophon tweeters in my Chartwell PM400 units, strawberry. Fried them at parties too! 😀
Turns out Monitor Audio used genuine KEF B200 units in the early days. Not sure which SP numbers though.
An externally hosted image should be here but it was not working when we last tested it.
Turns out Monitor Audio used genuine KEF B200 units in the early days. Not sure which SP numbers though.
GUys
thaks forthe interst. Thanks to marshalls sugestions I have now confirmed that the tweeters are blown. My sopeakers are as pictured above but with w non-MA badged tweter and the B200 SP1014 mid/bass. I've also found a look up table for crossover design and can only deduce that i worgly read the coulor coding on the original speakers.
thaks forthe interst. Thanks to marshalls sugestions I have now confirmed that the tweeters are blown. My sopeakers are as pictured above but with w non-MA badged tweter and the B200 SP1014 mid/bass. I've also found a look up table for crossover design and can only deduce that i worgly read the coulor coding on the original speakers.
Somebody asked me how you go about replacing these blown Isofon units with something similar. 🙂
I also quite like the look of the SEAS Prestige 27TFFC in this case, being quite similar to a 104mm KEF T27 overall which was the weapon of choice in the old days with KEF B200 units.
Seas Prestige 27TFFC H881
It's 91dB efficient at an Re of 4.8 ohms. I'd think you could just slot it right in. But if it's too loud, or if the crossover is misbehaving due to the different impedance, you would simply pad it with a couple of 10W wirewound resistors.
Suppose the isofon was actually Re 7 ohms on a multimeter. You'd simply add 2.2 ohm wirewound in series. With an efficiently loud replacement you can also do a proper L-pad with a shunt resistance of say 10 ohms and a series resistance of say, 3 ohms. This gets close to the old Acoustic Research attenuator we know and love. It seems to have some nice sonic qualities.
The math isn't too hard and explored here:
L-Pad (Driver Attenuation Circuit) Designer / Calculator
It's worth knowing that a crossover with a missing or fried tweeter presents a very nasty resonant load to an amplifier, so don't test at high levels till it's all working right. 🙂
I also quite like the look of the SEAS Prestige 27TFFC in this case, being quite similar to a 104mm KEF T27 overall which was the weapon of choice in the old days with KEF B200 units.
Seas Prestige 27TFFC H881
It's 91dB efficient at an Re of 4.8 ohms. I'd think you could just slot it right in. But if it's too loud, or if the crossover is misbehaving due to the different impedance, you would simply pad it with a couple of 10W wirewound resistors.
Suppose the isofon was actually Re 7 ohms on a multimeter. You'd simply add 2.2 ohm wirewound in series. With an efficiently loud replacement you can also do a proper L-pad with a shunt resistance of say 10 ohms and a series resistance of say, 3 ohms. This gets close to the old Acoustic Research attenuator we know and love. It seems to have some nice sonic qualities.
The math isn't too hard and explored here:
L-Pad (Driver Attenuation Circuit) Designer / Calculator
It's worth knowing that a crossover with a missing or fried tweeter presents a very nasty resonant load to an amplifier, so don't test at high levels till it's all working right. 🙂
Last edited:
I have never done this sort of substitution. I understand the addition of the resistor to bring the DCR in line with the old DCR of the Isophon being replaced.
But I have to ask to be absolutely certain I am doing this correctly, the 2.2 ohm resistor is simply inserted, in series with the wire from the cross over "feeding" the tweeter's terminal?
Is this correct?
I understand that we want to bring the new tweeter's DCR in line with that of the old, but what I don't understand is the other comment regarding "padding" with 10 watt resistors.
Are we talking about the same thing here as well, or something else; "you would simply pad it with a couple of 10W wirewound resistors"
Is this statement simply going on the assumption that we are doing this change over in both boxes? Again, simply bringing the new replacement's DCR in line with that of the old Isophon?
(which we will by the way) or something else entirely?
Pardon my sluggishness, and thank you!
Marshall
Last edited:
You are doing two separate things really. The series resistor of (maybe) 2.2 ohms is just to present the designed isofon load to the crossover. This will keep the frequency response flattish. And yes, you just put it between the crossover output and the input terminal of the tweeter. The SEAS is a 6 ohm unit in general parlance, but 4.8 ohms voice coil resistance is your starting point in L-pad design.
The attenuator L-pad consisting of two resistors can incorporate the 2.2 ohm load correction, but with different values can also attenuate the SEAS tweeter. The 91dB SEAS is likely more efficient or louder than the isofon which was possibly 88dB. It's a bit suck it and see. 3dB difference is twice as loud.
Most tweeters are set far too loud IMO. You have plenty of scope to reduce their level to something you can live with. FWIW, I would always replace BOTH tweeters together. 🙂
You're not going to blow up anything just slotting the replacement in with no modifications, but we all like to tweak things, eh?
The attenuator L-pad consisting of two resistors can incorporate the 2.2 ohm load correction, but with different values can also attenuate the SEAS tweeter. The 91dB SEAS is likely more efficient or louder than the isofon which was possibly 88dB. It's a bit suck it and see. 3dB difference is twice as loud.
Most tweeters are set far too loud IMO. You have plenty of scope to reduce their level to something you can live with. FWIW, I would always replace BOTH tweeters together. 🙂
You're not going to blow up anything just slotting the replacement in with no modifications, but we all like to tweak things, eh?
Last edited:
Again, thank you much!
I do understand the common practice of marking or rating transducers with "nominal" values which seldom correspond to the actual DCR of the voice coil when measured directly.
Our primary interest with this repair/project is to, as simply as possible, get some tweeters going as close to the originals as simply and purely as possible. For the moment, I'd like to avoid the inclusion of an L-pad.
You have already graciously answered this question already, but I am asking again now that I understand the in series resistor adjustment/installation; will we be fine simply matching the new tweeter's DCR with the old and going with that for now? The additional resistors in series don't reduce the traducer's efficiency?
I am well aware that I am now publicly revealing to all how little I really understand, but as I told you, I am no more than a skilled model builder or hang on artist; I truly appreciate your assistance.
If it does prove "too loud" subjectively, I think I would be inclined to simply insert and L-pad a la JBL fashion. That I can handle; a pot with a wide enough range would eliminate the need for me to deal with any math!
What sat ye?
And thank you again for your wonderful patience tutoring the blind!
Best regards,
Marshall
Last edited:
In that case, I think the original suggestion of the Morel Cat298 would be safer.
Morel Classic CAT298
It's an 89db unit @ Re 5.2 ohms available in 94mm and 104mm to suit with a nice metal frontplate. It's gonna be about right out of the box.
In fact, EXACT advice would really depend on an exact crossover schematic and a measurement of the Re of the working isofon unit. But generally speaking this stuff is not requiring of great accuracy. You get used to a particular sound fairly quickly. 😀
Oh, FWIW, you MAY be able to use the original faceplate of the Isofon. Can't say for sure.
Morel Classic CAT298
It's an 89db unit @ Re 5.2 ohms available in 94mm and 104mm to suit with a nice metal frontplate. It's gonna be about right out of the box.
In fact, EXACT advice would really depend on an exact crossover schematic and a measurement of the Re of the working isofon unit. But generally speaking this stuff is not requiring of great accuracy. You get used to a particular sound fairly quickly. 😀
Oh, FWIW, you MAY be able to use the original faceplate of the Isofon. Can't say for sure.
Last edited:
Don't have it here with me at the moment, but the working Isophon measures at 7+ ohms DCR, marked in RED of course "8 Ohms"
I surprise myself with my guesses sometimes...😀
Got that about right then. The new tweeter will slightly overdamp the crossover and lower the load to the amp a tidge, but nothing you'd lose sleep over. You usually wire tweeters on 3rd. order crossovers out of phase, but that's a bit suck it and see too.
What you may find is the old capacitors are past their best after 20 years if they are little blue non-polar electrolytics. Yellow polyester and polypropylenes go on forever. 🙂
They are the old British Standard color coded types resembling the Mexican flag; they will be replaced as well as the too small resistors, once we've established a base line and have two working boxes.
That was one of the reasons for my redundancy; you seemed to already know that the old Isophon was coming in at 7!
You've been wonderfully helpful.
Marshall
AND, last but not least; in your OPINION do you personally think that this modest model of Monitor Audio merits (sonically) this expenditure? Even though nothing esoteric or exceptional visually, it is a well made honest box with obvious and deliberate forethought.
Bottom line, with new tweeters and corresponding cap and resistor improvements, the total costs will come in at around $100 -$125 U.S. Dollars
Worth a shot?
Marshall
The Morel for an easy life! Both modern ferrofluid designs with flattish impedance. But let us know how you get on. That KEF bass unit is pretty good. I'd like to try it with a much lower crossover point, which is doable with modern tweeters.
But that would be a digression. 🙂
But DEFINITELY worth doing up. They made better boxes in the old days, and you'll have a very fine speaker.
Oh, and if you're wondering. Closed box is not fashionable these days. People are impressed with reflex, but actually you get closed box up off the floor and it sounds flipping excellent, as well as being more accurate and uncoloured and deep.
But that would be a digression. 🙂
But DEFINITELY worth doing up. They made better boxes in the old days, and you'll have a very fine speaker.
Oh, and if you're wondering. Closed box is not fashionable these days. People are impressed with reflex, but actually you get closed box up off the floor and it sounds flipping excellent, as well as being more accurate and uncoloured and deep.
Last edited:
AND, last but not least; in your OPINION do you personally think that this modest model of Monitor Audio merits (sonically) this expenditure? Even though nothing esoteric or exceptional visually, it is a well made honest box with obvious and deliberate forethought.
Bottom line, with new tweeters and corresponding cap and resistor improvements, the total costs will come in at around $100 -$125 U.S. Dollars
Worth a shot?
Marshall
- Status
- Not open for further replies.