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Old 25th March 2014, 03:36 AM   #1
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Default A-25 tweeter upgrade?

hi
I have a pair of vintage dynaco A-25.
I have already replced teh cap in the crossover and thought about maybe getting a better tweeter. I think the direct replacement is the seas 27tffc
The Madisound Speaker Store

anyone done this mod? Should I do it and would it be a direct replacement or I should modify the xo?
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Old 25th March 2014, 03:45 AM   #2
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Take the metal screen off the old tweeter.

And after getting levels right replace the resistors with a single quality one.

You can do stuff with the box too.

dave
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Old 25th March 2014, 03:54 AM   #3
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what resistor value do I need?
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Old 25th March 2014, 03:55 AM   #4
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Quote:
Originally Posted by planet10 View Post
Take the metal screen off the old tweeter.

And after getting levels right replace the resistors with a single quality one.

You can do stuff with the box too.

dave
the weird things Ive noticed, no matter the position on the switch, theres no difference in sound. what could be the cause? i have to see if I may have put the wrong cap value when I did the recap.
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Old 25th March 2014, 04:08 AM   #5
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Quote:
Originally Posted by murphythecat8 View Post
what resistor value do I need?
Click the image to open in full size.

dave
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Old 25th March 2014, 06:30 AM   #6
DrBoar is offline DrBoar  Sweden
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As you start with 12 ohm and go up to 20 ohm in 4 steps that is a reduction of less than 3 dB in 4 steps so less than 1 dB per step that can be hard to hear.

The bigger problem with the crossover is that the croosover is a shallow 6dB octave at a very low 950 Hz with minimal attaneuation and down to 650 Hz with all resesistorns in line. So the attenuation of the tweeter is counteracted by a lowering of the crossover frequency. With no serial resistors at all the the crossover is at 2 kHz still pretty low and shallow but usable with a dome with good power handling and a fundamental resonnance that is low in Q and Fr. I have several H-87 I might measure them.
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Old 25th March 2014, 09:33 AM   #7
DrBoar is offline DrBoar  Sweden
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The Seas I have is from two different batches or vintages so Fr is different but the pair matching is good. The Q is kind of middle value, but what happen with a singel cap in series?
Correction: 5 micro and 8 ohm is 4 kHz so the crossover with 20 ohm added in series the crossover is down to 1100 Hz and that is way to low.

Click the image to open in full size.

Here we have a Philips dome with the same total Q value but with a lower electrical A and higher mechanical Q.
Click the image to open in full size.


With a small cap in series. The tweeter is 6 dB down at 4 kHz and 12 dB down at 2 kHz from a 7-8 kHz crossover, textbook stuff so far. But then hell breaks loose and at a 700-100 Hz the attenuation is not 18-24 dB as it should but less than 5 dB.

Use an 8 ohm L attenuator and tune in the attenuation you want to have then if needed measure the serial and parallell resistances and replace with fixed resistors. The parallell resistor will flatten the impedence peak of the dome and improve the working conditions of the cap.
Click the image to open in full size.
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Old 25th March 2014, 10:41 AM   #8
system7 is offline system7  United Kingdom
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We have had a thread recently about the updated similar SEAS A26 speaker. I do suspect that there is no exactly optimum setup with this type of low crossover speaker.

EaZy used the SEAS 27TBFC/G as a replacement for the expensive modern T35C002 unit that SEAS fit. He was pleased with the results, but changed the crossover a little. I have seen DIY versions of this crossover with 2.2R in series with 8uF, and a 10R resistor shunted across the tweeter too, which might be good.

SEAS A26 Speaker Kit

He felt it had the right low frequency toleration needed. It doesn't look much different to the 27TFFC TBH, but is a metal dome. So, yes, I think your 27TFFC is a decent idea if that famous old SEAS H-087 tweeter is past it. Hope it helps anyway. Worst that happens is you refit the original I suppose.
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Last edited by system7; 25th March 2014 at 11:00 AM.
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Old 9th April 2014, 10:46 PM   #9
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the recommended tweeter replacement by SEAS is the SEAS Prestige 27TFFC , so i will go with that.

Quote:
Originally Posted by DrBoar View Post
As you start with 12 ohm and go up to 20 ohm in 4 steps that is a reduction of less than 3 dB in 4 steps so less than 1 dB per step that can be hard to hear.

The bigger problem with the crossover is that the croosover is a shallow 6dB octave at a very low 950 Hz with minimal attaneuation and down to 650 Hz with all resesistorns in line. So the attenuation of the tweeter is counteracted by a lowering of the crossover frequency. With no serial resistors at all the the crossover is at 2 kHz still pretty low and shallow but usable with a dome with good power handling and a fundamental resonnance that is low in Q and Fr. I have several H-87 I might measure them.
do you think its a good idea to go with the 27tffc?
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Old 9th April 2014, 10:50 PM   #10
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Quote:
Originally Posted by DrBoar View Post
As you start with 12 ohm and go up to 20 ohm in 4 steps that is a reduction of less than 3 dB in 4 steps so less than 1 dB per step that can be hard to hear.

The bigger problem with the crossover is that the croosover is a shallow 6dB octave at a very low 950 Hz with minimal attaneuation and down to 650 Hz with all resesistorns in line. So the attenuation of the tweeter is counteracted by a lowering of the crossover frequency. With no serial resistors at all the the crossover is at 2 kHz still pretty low and shallow but usable with a dome with good power handling and a fundamental resonnance that is low in Q and Fr. I have several H-87 I might measure them.
if I understand correctly, if I select the highest tweeter setting, this bypass the serial resistors and gives me 12 ohm so 2 khz crossover?
And if I select the lowest tweeter setting, the xo is at 1100 hz?

http://www.seas.no/index.php?option=...=81&Itemid=105

Ideally, with the replacement tweeter, it seems to be flat from around 1.5khz, so my best bet is to put the tweeter pot at the highest setting for a 2khz xo?

Last edited by murphythecat8; 9th April 2014 at 11:11 PM.
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