Hello, I have a friend who have a a pair of the original ATC SCM12 model. It performs really good for its age, but it is a speaker on the "dark side".
The tweeter it uses is the Vifa D26NC-15-06 which is supposed to be flat and with a little spark in the 16khz area, so we don't understand why its dark sound.
I want to ask if it is possible to make some kind of mod in the crossover to make them a little bit brighter. Here are the pics he has sent me of the crossover. Thanks in advance for any help
The tweeter it uses is the Vifa D26NC-15-06 which is supposed to be flat and with a little spark in the 16khz area, so we don't understand why its dark sound.
I want to ask if it is possible to make some kind of mod in the crossover to make them a little bit brighter. Here are the pics he has sent me of the crossover. Thanks in advance for any help
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Heraldo;
Personally, I'd try going the replacement tweeter route ( for the small cost );
In Britain ; Peerless D26NC1506 replacement
or
Peerless D26NC1506 replacement in Canada
Take note that your old tweeters have ferrofluid in the VC gap / which has likely dried-up and is now overly damping the High Frequencies.
ie; I wouldn't adjust the crossover for a tweeter that needs service.
Personally, I'd try going the replacement tweeter route ( for the small cost );
In Britain ; Peerless D26NC1506 replacement
or
Peerless D26NC1506 replacement in Canada
Take note that your old tweeters have ferrofluid in the VC gap / which has likely dried-up and is now overly damping the High Frequencies.
ie; I wouldn't adjust the crossover for a tweeter that needs service.
Attachments
Hi Heraldo,
Your friend will need to remove a complete crossover from one cabinet and then draw out the schematic for the crossover. Once drawn, you'll need to post it here.
This leg-work can't be done by anyone else but him ( additionally, the pictures he's provided to you are of very little help here ).
I'm assuming there's a 2 ( to 3 ) resistive pad on the HF ( like other small ATC speakers ).
- That pad ( once identified by your friend ) can be bypassed with something like a 1.2uF cap to add back in some suppressed HF.
- Taking it further, a .08mH coil can be put in series with the 1.2uF cap to create a frequency-specific boost ) with the UHF "boost" controlled by a single resistor ( see my pic of one possible scenario ).
Here's a "What-If" mock-up based on fictional values ( ie; this is not your friends network ).
- I used the blue ( 30deg ) trace ( belonging to the Peerless version of the Vifa ) for this simulation .
- As such, on axis response might be way too hot in the UHF for some listeners.
I did some reading this morning on ATC's small monitors and it appears there's some consensus that they are somewhat "dark" sounding.
One such thread.
- That might also mean the tweeters ( in your friends SCM12's ) are working as expected ( though to be sure, they ought to be tested to confirm one way or another ).
Here's someone else's rendering of their SCM10 network ( I couldn't find any SCM12 schematics online ).
Apart from the values of the coils, your friend should find the schematic of his SCM12 is somewhat similar in layout ( give or take a few parts ).
BTW; the boost circuit pulls the UHF impedance down to about 4ohms, so one wants to use an amplifier that's stable into low impedance when using these sort of LCR circuits.
Your friend will need to remove a complete crossover from one cabinet and then draw out the schematic for the crossover. Once drawn, you'll need to post it here.
This leg-work can't be done by anyone else but him ( additionally, the pictures he's provided to you are of very little help here ).
I'm assuming there's a 2 ( to 3 ) resistive pad on the HF ( like other small ATC speakers ).
- That pad ( once identified by your friend ) can be bypassed with something like a 1.2uF cap to add back in some suppressed HF.
- Taking it further, a .08mH coil can be put in series with the 1.2uF cap to create a frequency-specific boost ) with the UHF "boost" controlled by a single resistor ( see my pic of one possible scenario ).
Here's a "What-If" mock-up based on fictional values ( ie; this is not your friends network ).
- I used the blue ( 30deg ) trace ( belonging to the Peerless version of the Vifa ) for this simulation .
- As such, on axis response might be way too hot in the UHF for some listeners.
I did some reading this morning on ATC's small monitors and it appears there's some consensus that they are somewhat "dark" sounding.
One such thread.
- That might also mean the tweeters ( in your friends SCM12's ) are working as expected ( though to be sure, they ought to be tested to confirm one way or another ).
Here's someone else's rendering of their SCM10 network ( I couldn't find any SCM12 schematics online ).
Apart from the values of the coils, your friend should find the schematic of his SCM12 is somewhat similar in layout ( give or take a few parts ).
BTW; the boost circuit pulls the UHF impedance down to about 4ohms, so one wants to use an amplifier that's stable into low impedance when using these sort of LCR circuits.
Attachments
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Based on the review of an ATC SCM 11 in Stereophile, which is not this exact loudspeaker but assuming the voicing may very well be similar, it is evident that the tweeter padding made the TW response significantly below woofer SPL, so within is the solution. It may simply suffice to lower the padding resistor and/or modify the HP network for we are not sure what kind of an effect would that make without at least simulating. Since we have original Vifa specs with graphs, we can make a good guess but I would prefer the photos to be executed again, this time with the terminal taken off to clearly see the LP of the woofer and the tweeter side again too, if it is not too much trouble. Close up shots with wire markings which goes where.
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In addition to the "darkness" in the sound there is also a difference between both tweeters he has also confirmed switching them. So to be sure he is going to replace both.
You're welcome Heraldo!
If those SCM12 continue to be dark sounding in there voicing after tweeter replacements ( then as Lojzek has mentioned ) an adjustment of the padding resistors can increase the overall response of the tweeter.
Using my fictional circuit again, one can see how easy it is to get ( across the board ) a 2db increase by simply shorting out R2 ( in the "T" pad ).
- Interesting ( to me at least ) is the new response curve still tightly tracks the old .
- I strongly suspect that an overall 2db increase is too much and that around 1db would make more sense.
- 1 db is achieved ( in my fictional circuit ) by reducing R2 to 1.1 Ohms.
If those SCM12 continue to be dark sounding in there voicing after tweeter replacements ( then as Lojzek has mentioned ) an adjustment of the padding resistors can increase the overall response of the tweeter.
Using my fictional circuit again, one can see how easy it is to get ( across the board ) a 2db increase by simply shorting out R2 ( in the "T" pad ).
- Interesting ( to me at least ) is the new response curve still tightly tracks the old .
- I strongly suspect that an overall 2db increase is too much and that around 1db would make more sense.
- 1 db is achieved ( in my fictional circuit ) by reducing R2 to 1.1 Ohms.
Attachments
You're welcome Heraldo!
If those SCM12 continue to be dark sounding in there voicing after tweeter replacements ( then as Lojzek has mentioned ) an adjustment of the padding resistors can increase the overall response of the tweeter.
Using my fictional circuit again, one can see how easy it is to get ( across the board ) a 2db increase by simply shorting out R2 ( in the "T" pad ).
- Interesting ( to me at least ) is the new response curve still tightly tracks the old .
- I strongly suspect that an overall 2db increase is too much and that around 1db would make more sense.
- 1 db is achieved ( in my fictional circuit ) by reducing R2 to 1.1 Ohms.
Looks really good, we will wait until de new tweeters come and if they do not fix the tonal issue we will try to go in deep with your recommendation
. 