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Old 12th December 2010, 04:38 AM   #1
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Default 7815 and 7915, which type A or C?

Hello,

I am not sure about the differences between the A type vs the C type.
On spec sheets the current voltages look a bit different, but nothing I can see that makes a huge difference.

What are the differences?

Thank You,
robert
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Old 12th December 2010, 05:33 AM   #2
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The A version has a tighter tolerance on its voltage reference, so will have a narrower window of predicted output voltage. Other than that there is little significant difference.
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Old 12th December 2010, 06:16 AM   #3
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Thank You!

I used a new C type to replace an older A type, because I thought the current noise spec was better 90uV vs 320uV.

Maybe I should go back to the old A type to get a tighter voltage output.

any thoughts?

robert
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Old 12th December 2010, 07:16 AM   #4
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Keep the less noisy regulators in. IMHO less noise is more critical than voltage precision in audio applications.
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Old 12th December 2010, 07:59 AM   #5
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From what I remember ON Semiconductor has the lowest noise figures for regulators, at least in the datasheet.
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Old 12th December 2010, 04:12 PM   #6
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Thank You guys.

I will keep the lower noise one in, it is an On-Semi brand 7915 C type.

What do you guys think the more important the ripple reject number or the output noise voltage number?

I am thinking of changing the 7815 for a better part. The one I have in there is a LM7815A.

Thank you again. Reading the specs sheets after a while makes one lose perspective.

Robert
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Old 12th December 2010, 05:55 PM   #7
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Quote:
Originally Posted by rfenergy View Post
Hello,

I am not sure about the differences between the A type vs the C type.
On spec sheets the current voltages look a bit different, but nothing I can see that makes a huge difference.

What are the differences?

Thank You,
robert
I have found a problem using these regulators.
I found I had to decouple the output of the regulator with an electrolytic very close to the regulator or I got instability on the reg output.
Perhaps I just bought cheap versions.
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Old 12th December 2010, 08:29 PM   #8
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Quote:
Originally Posted by nigelwright7557 View Post
I have found a problem using these regulators.
I found I had to decouple the output of the regulator with an electrolytic very close to the regulator or I got instability on the reg output.
Perhaps I just bought cheap versions.
It used to be standard practice to put a 1uF tantalum capacitor on both input and output of the 78xx and 79xx regulators.
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Old 14th December 2010, 12:31 AM   #9
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The design I am upgrading uses 10uF on the in and outs of both the 7815 and the 7915.

Anyone have any thoughts on specs of what is more important. Either the ripple rejection number or the output noise voltage number?

I can use the old 7815 in the circuit that has a 80db rejection and 90uV/VO noise or a new 7815 with 70db and 90uV noise total. The new one is made by ON-Semi, (I hope I read the specs right) it is the On-Semi NCP7815TG and is says its good for appliances, but no mention of audio use. I hope I am not chasing my tail with all the specs.

I am thinking the lower noise would be better (per Jitter's comment), but I am uncertain on the rejection of ripple. (side note, I think the new 7915 with 90uV noise and 60db rejection sounds cleaner than the old one with 320uV of noise, could be my imagination...I do not have proper measuring equipment to prove this)

Thank You,
Robert

Last edited by rfenergy; 14th December 2010 at 12:37 AM.
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Old 14th December 2010, 12:36 AM   #10
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Quote:
Originally Posted by metalsculptor View Post
It used to be standard practice to put a 1uF tantalum capacitor on both input and output of the 78xx and 79xx regulators.
The datasheet says you dont need a decoupling capacitor on the output.

I found I did need a capacitor on the output, my class D amp was making very high frequency noise on the 12 volt rail and causing the amp to howl.
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