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Old 21st December 2008, 09:59 AM   #2291
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by rhysh
Also, i see this amplifier requires 24v rails. Do you think i would get away with 25v capacitors, as they are half the cost of 63v, and a lot smaller in size...
NO!

what voltage appears on the output of the rectifiers if the mains voltage varies?
Here in the UK the tolerance is 216Vac to 254Vac. If you use a 230:18Vac transformer on the standard UK voltage it will measure ~18.8Vac when fully loaded.
On 254Vac it will measure ~19.9Vac when fully loaded. when unloaded these voltages will increase by the transformer regulation percentage. Big toroids can be as low as 3%, small EI can be as high as 30%. Find out the regulation of your transformer and adjust the output voltage assuming the PSU output fuses have blown and the caps are now charging up to the peak AC voltage going through the rectifiers.

Mains voltage variations can be surprisingly large and can overvoltage components if ignored. Measure and check, or easier, allow a margin on ratings for the variations that will occur.

BTW, electrolytics are available at many voltages between 25Vdc and 63Vdc, common available are 35V, 50V and less common 40V.

Finally, expect your PSU cost to be more than the cost of the amplifier and heatsinks.
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Old 21st December 2008, 10:05 AM   #2292
juma is offline juma  Serbia
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Quote:
Originally posted by rhysh
...Do you think i would get away with 25v capacitors...
You might, but only if you use well regulated power supply - but OTOH the voltage reg will cost you more than price difference between 25V and 35V or 40V caps. That's one of those "expensive savings" ...

Also, using components in "close to limit" area compromises reliability of the amp and makes the whole thing dangerous, so please think again
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Old 21st December 2008, 10:07 AM   #2293
AndrewT is offline AndrewT  Scotland
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and what if the voltage regulator fails or has a tolerance on output voltage?
What about the smoothing before the regulator?
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Old 21st December 2008, 10:13 AM   #2294
rhysh is offline rhysh  United Kingdom
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Quote:
Originally posted by AndrewT

NO!

what voltage appears on the output of the rectifiers if the mains voltage varies?
Here in the UK the tolerance is 216Vac to 254Vac. If you use a 230:18Vac transformer on the standard UK voltage it will measure ~18.8Vac when fully loaded.
On 254Vac it will measure ~19.9Vac when fully loaded. when unloaded these voltages will increase by the transformer regulation percentage. Big toroids can be as low as 3%, small EI can be as high as 30%. Find out the regulation of your transformer and adjust the output voltage assuming the PSU output fuses have blown and the caps are now charging up to the peak AC voltage going through the rectifiers.

Mains voltage variations can be surprisingly large and can overvoltage components if ignored. Measure and check, or easier, allow a margin on ratings for the variations that will occur.

BTW, electrolytics are available at many voltages between 25Vdc and 63Vdc, common available are 35V, 50V and less common 40V.

Finally, expect your PSU cost to be more than the cost of the amplifier and heatsinks.

Thankyou for the info, i have realised i can get the same part in 35V for not much more. I have decided on 2 x 100,000uf Rifa capacitors per rail, do you think this should be okay?
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Old 21st December 2008, 10:16 AM   #2295
juma is offline juma  Serbia
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Quote:
Originally posted by AndrewT
and what if the voltage regulator fails or has a tolerance on output voltage?
What about the smoothing before the regulator?

Right ! That's exactly why even that solution won't be very feasible in this case ...
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Old 21st December 2008, 11:23 AM   #2296
cviller is offline cviller  Denmark
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Quote:
Originally posted by rhysh



Thankyou for the info, i have realised i can get the same part in 35V for not much more. I have decided on 2 x 100,000uf Rifa capacitors per rail, do you think this should be okay?

Sounds very good.
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Old 21st December 2008, 12:25 PM   #2297
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Hi all.
I am a bit confused regarding the best voltage for rails, I seem to remember Mr Pass saying 25v rails are minimum and any lower every volt makes a difference, but then everybody uses 18v secondaries which after rectifiers, voltage drop etc gives approx 23.5v rails. Why not use 20v secondaries or even 22v ?
What have I missed ?
Thanks.
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Old 21st December 2008, 03:10 PM   #2298
labjr is offline labjr  United States
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Quote:
Originally posted by jotom750
Hi all.
I am a bit confused regarding the best voltage for rails, I seem to remember Mr Pass saying 25v rails are minimum and any lower every volt makes a difference, but then everybody uses 18v secondaries which after rectifiers, voltage drop etc gives approx 23.5v rails. Why not use 20v secondaries or even 22v ?
What have I missed ?
Thanks.
I don't think he said minimum of 25V. The amplifier works best with 24 volt rails. Try to get as close to 24V as possble. If the voltage is too high it will probably have to dissipate more heat, and may not sound the same.
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Old 21st December 2008, 08:51 PM   #2299
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Default low sound

hi..today i built f5 amp with huge heatsinks..i used 2sk1530 and sj201. bias is 1,5a and i see 2,83volts on r3 and r4..but the sound is very low.my soundcard is emu1212m and has enough output.what is the problem? i couldnt solve it
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Old 21st December 2008, 09:08 PM   #2300
labjr is offline labjr  United States
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Default Re: low sound

Quote:
Originally posted by umut1001
hi..today i built f5 amp with huge heatsinks..i used 2sk1530 and sj201. bias is 1,5a and i see 2,83volts on r3 and r4..but the sound is very low.my soundcard is emu1212m and has enough output.what is the problem? i couldnt solve it

I would check it with a function generator and a scope before hooking it to a source and speakers.
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