15V for opamps from 35V ? - Page 2 - diyAudio
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Old 17th June 2005, 04:48 AM   #11
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As we said is 35 V dangerously near the limit of the 78xx/79xx but as Carlos point out a series resistor may be inserted but you must know the load in order to calculate the resistor value.

The only disadvantage as I see it is more parts around the LM317/337 but in this particular case it's only one more
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Old 24th June 2005, 11:05 AM   #12
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Dont forget to decoupling caps near the Vin and Vout near the voltage regulators.
Cheers!
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Old 27th June 2005, 02:18 PM   #13
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granted i'm still just the new guy here, i have a little insight to this problem. i have been repairing Peavey amplifiers for a little over 3 years now. i'll use the CS series as an example their rails are aprox +/- 59 VDC it's split like this, the output gets the full +/- 59 then there is a series resistor the next stage goes to the driver board for the driver output transistors. they drop the voltage a little more then there is another resistor to drop it to the final range. (looking at the schematic now) for their needs they use a 10 watt 10 ohm and then a 5 watt 1k. but case and point series voltage dividers are probably the safest and cheapest way to go
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Old 27th June 2005, 09:19 PM   #14
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You can use an 80V transistor and a zener diode to make a power-zener.
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Old 28th June 2005, 07:05 PM   #15
sbrads is offline sbrads  United Kingdom
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A few years ago I modified a Crimson Elektrik moving coil head amp's 15v power rails. I tried a zener + transistor (noisy), 7815's (extremely noisy) and ended up with LM317's fed from 50v via a dropper resistor plus filter caps as by far the quietest and simplest supply. Any hiss from the regulator got straight through the discrete mc circuit and was very audible, and decoupling caps across 7815's doesn't do a thing to reduce it either. LM317's are the way to go.
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Old 28th June 2005, 08:14 PM   #16
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Quote:
Originally posted by sbrads
LM317's are the way to go.
now correct me if i'm wrong but don't LM317's use the same PWM style as 78xx series regulators. which means they should have the same harmonic output at X voltage. it was my understanding that the only difference with the 317's was that they were variable. and that much like triacs they only varied the pulse width.


like i have said before i could be way way off but that was what i always thought

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Old 28th June 2005, 09:07 PM   #17
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The 317 and 78xx are linear regulators. Any triangular hash at their output is simply due to the feedback scheme employed. The 317 is a totally different beast to the 78xx and this is reflected in the performance.
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Old 28th June 2005, 09:16 PM   #18
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Take a look at Linear Technology's line of linear regulators, they have much lower noise than the venerable 317/337, tighter regulation and lower output impedance as well.. Try LT1086 and LT1185..

Kevin
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