Mission 753 speaker modding

[Roppu]

Hi Richard.
Yes we do not know where those wires go but if the colouring criteria is the same of mine, red and black wires are for woofers. And two equal colours starting from the same pin can't be series connection.
For the low DCR I can only believe you...
I'm working on rewiring one of the speakers last days. I want to compare them when this one will be ready. Today new caps arrived. Worse than cheap old ones they won't be, I think.
I'm driving them with a Arcam Delta 290 integrated connected to HF and a power Arcam Delta 290 connected to LF but they were lows poor with other amplifiers too.
Still waiting for an answer from IAG!

[Richard]

Hi Roppu,

That's good let us know how the re-wired one sounds and what its LF- LF+ DCR is.

If you have the chance you may find it interesting to meter any passive speaker and you most likely will find it checks very similar to its nominal impedance. (A nominal 8 ohm impedance speaker will meter DRC around 6-8R etc). It doesn't need to be bi-wire type to do this as the tweeter series cap blocks the meter so the tweeter won't be in circuit and the meter will just see (usually) the bass inductor and speaker voice coil in series.

If you have accesss to an inductance meter you could check your inductors and order the missing ones from a supplier such as Falcon in the UK https://www.falconacoustics.co.uk/au...iron-dust.html or locally. They will be high power ferrite core so not expensive and not a large enough DCR to make much difference to the exact specification, the inductance will be more important as it will affect the filter freqency/slope.

The Arcam amps are very good and are probably doing a good job but less than 2 ohms is very low even for them.

[Roppu]

Originally Posted by Richard

Hi Roppu,

That's good let us know how the re-wired one sounds and what its LF- LF+ DCR is.

If you have the chance you may find it interesting to meter any passive speaker and you most likely will find it checks very similar to its nominal impedance. (A nominal 8 ohm impedance speaker will meter DRC around 6-8R etc). It doesn't need to be bi-wire type to do this as the tweeter series cap blocks the meter so the tweeter won't be in circuit and the meter will just see (usually) the bass inductor and speaker voice coil in series.

This is very interesting. I will do so

Originally Posted by Richard

If you have accesss to an inductance meter you could check your inductors and order the missing ones from a supplier such as Falcon in the UK https://www.falconacoustics.co.uk/au...iron-dust.html or locally. They will be high power ferrite core so not expensive and not a large enough DCR to make much difference to the exact specification, the inductance will be more important as it will affect the filter freqency/slope.

I was just thinking to buy an inductance meter. I saw one which measures caps ESR too.
But right now I don't have one. Actually you're right. Doing all this without checking the inductors has no sense in case they were the culprit. Thanks for the link. I will check it out.

Originally Posted by Richard

The Arcam amps are very good and are probably doing a good job but less than 2 ohms is very low even for them.

Thanks again for your help Richard. It's much appreciated. I will dig more into all this

[Roppu]

By the way, I noticed the multimeter needs time to stabilise on a measure when I check the DCR. Should I wait for it to be fixed? Is it safe for the drivers?

[Richard]

Hi, yes I checked 4 pairs of speakers around the house and they were all similar, a little lower DCR to their rated impedance.

The meter didn't have a problem but I have had what you described when trying to check resistors in situe in amps etc when there is a large value electrolytic cap also in circuit. Possibly your large electros are causing the slow meter response. Check the meter against some marked values of single resistors out of circuit to see if it works ok and yes sometimes components need to be removed for testing.

The only speakers I checked which are a bit strange are my Tannoy XT8F which are specced at nominal 8 ohms but where the bass section measures 2.9 DCR. I remembered the HFW review (I bought them several years ago) and upon checking it it says this,

from HiFi World Tannoy XT8F test measurements July 2015,

“But the bass unit has a d c r of just 2.7 Ohms – the lowest we have ever measured – and our impedance curve shows the Tannoy is a very low impedance loudspeaker – down to 3 Ohms at low frequencies.”

So clearly Tannoy are stretching the specs in that instance. That's one speaker that bucks the rule and your Missions may be another so keep an open mind.

[Roppu]

Originally Posted by Richard

Hi, yes I checked 4 pairs of speakers around the house and they were all similar, a little lower DCR to their rated impedance.

The meter didn't have a problem but I have had what you described when trying to check resistors in situe in amps etc when there is a large value electrolytic cap also in circuit. Possibly your large electros are causing the slow meter response. Check the meter against some marked values of single resistors out of circuit to see if it works ok and yes sometimes components need to be removed for testing.

The only speakers I checked which are a bit strange are my Tannoy XT8F which are specced at nominal 8 ohms but where the bass section measures 2.9 DCR. I remembered the HFW review (I bought them several years ago) and upon checking it it says this,

from HiFi World Tannoy XT8F test measurements July 2015,

“But the bass unit has a d c r of just 2.7 Ohms – the lowest we have ever measured – and our impedance curve shows the Tannoy is a very low impedance loudspeaker – down to 3 Ohms at low frequencies.”

So clearly Tannoy are stretching the specs in that instance. That's one speaker that bucks the rule and your Missions may be another so keep an open mind.

Sorry for my late reply Richard. I started a new job and it's taking all my time.
I will read carefully what you're telling me... In the meantime somebody answered me here:
https://www.diyaudio.com/community/t...3#post-7018484
As you can see the schematics is just the same.
Btw: i never got an answer for the schematics from the producer

[Richard]

Hi Roppu,

Looking over these threads again it seems Justblair has Mission 753 and you and Threndor have 753 Freedoms. 753 came out around 1992 and 753 Freedoms around 1995.

That model change may explain the crossover differences.

The top user review on this thread from The Rhodian who’s had both versions for many years says there are big differences between the two speakers and much of that is in the bass which may be explained by a large difference in crossovers,

http://www.audioreview.com/product/s...3-freedom.html

Did you check LF- to LF+ for DCR? If Threndor would check his DCR too and it agrees with yours it would be a useful confirmation that your drawings are correct for the Freedom model.