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Old 24th February 2006, 12:11 PM   #1
SQFan is offline SQFan  South Africa
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Default A different DIY project

Hi everyone. I am relatively new to this forum and would like to take this opp. to congratulate all who have contributed to the forum. I have undertaken a DIY ribbon project. I am in the testing stages and have a few questions. Please note that i am not all that technically clued up (like you boffins) but would like to know:
a) if for example, i use a standard car audio amp. ( i have a few lying around and also have a decent 12V PSU) to drive a ribbon, it is running fine for about an hour, then starts to heat up. PS. there are only two paralleled caps of 6,8mfd 400V wired in series to the positive terminal of the ribbon. Can I possibly wire in a 4 ohm 5 W resistor in series with the output to the ribbon to prevent the amplifier fronm seeing an almost zero ohm load ? Any suggestions will be appreciated.

b) is there a way of impedance matching a ribbon to the amp as mentioned above. I have not got the expertise to build a transformer right now, but if there is a simple method to complete my testing, thereafter i can get to the more serious technical aspects.
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Old 24th February 2006, 02:15 PM   #2
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Hi SQFan,

A: Using a resistor in series will not cause any problems and will make life a lot easier for your amp. Although it will also lower the output from your ribbon.

B: In the long run building a transformer is a much better solution than wasting a lot of power in a series resistor.

Of course if you end up with a ribbon much more efficient than the mid/low units you are going to pair it with, then you could just stick to the resistor.

Are you building a single conductor ribbon?

Cheers,
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Old 5th March 2006, 02:03 PM   #3
SQFan is offline SQFan  South Africa
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Hi JVJessen, i really appreciate your assistance. My testing stages have progressed quite well. I have run 3 paralleled 6,8uF 150V polyprop. caps on the positive line of the amplifier output terminal and then to the positive of the ribbon. This seemed to be a better solution than that of using the resistor idea, as the resistor i tested with was 3.9ohm 10W, and it heated up tremendously. Also, what i found was that the quality of the sound from the ribbon increased using 3 caps (previously tested using two), and also found that the volume from the head unit (mini hi-fi) could be increased much more without the ribbon distorting. I can safely say that this is what i wanted to achieve from the ribbon project. Frequency wise, i think that the ribbon is running between late 400Hz up to mid 5kHz. Ultimately, i would like to cut off the higher freq. range and bring it down to about 2,5 kHz. This I will do via a parametric crossover once installed.

Now to answer your question, the ribbon i built is similar to the one by Michael. I was already complete with the first one, when i came across this website. This website just made everything much more easier for me to get the results i needed.

Regards,
SQFan
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Old 5th March 2006, 09:32 PM   #4
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SQfan,

I think to save your amp from destruction it would be best to keep the 3-4 ohm series resistor. The capacitors you are using become a low impedance at high frequencies and then the amp sees the ribbon as a short circuit. To filter out the low frequencies from the ribbon you will still need a series capacitor to create a RC filter. Ribbons are fragile, and most designs use a steep 4-8 order crossover to keep the low frequencies away. You can absorb the 3-4 ohm padding resistance into a high order filter's inductors.

There are a few very detailed DIY transformer threads on this site to drive very low impedance ribbons.
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Old 10th March 2006, 08:02 AM   #5
SQFan is offline SQFan  South Africa
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Hi Linesource,

Thanks for the info. I have tested the ribbons with & without the series resistor, and there seems to be very little change in the heating up of the amplifier, however, i would still like to build the transformer. Have you got any links to the 'DIY transformers' ? If so, kindly advise.

Also, the following is a bit too complicated for me "To filter out the low frequencies from the ribbon you will still need a series capacitor to create a RC filter. Ribbons are fragile, and most designs use a steep 4-8 order crossover to keep the low frequencies away. You can absorb the 3-4 ohm padding resistance into a high order filter's inductors. As i mentioned initially on this topic, i am not that well clued up on the technical aspects.

I am not getting the low frequencies coming through, and if any, very minimal for some reason.

Regards,

SQFan
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