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JohnA

Raising P.S. Capacitance

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I still have the Technics SA-200 receiver I bought in college, and it still works, but with a little static in the knobs.  I intend to re-cap it.  It's 8 ohm rating is 25 wpc and its 4 ohm rating is 30 wpc.  Typical of these amps, the heat sinks are small as are the P.S. caps.  Recovery from clipping is audibly slow.  The original caps are 35V, 6800 uF.  If I have the power supply cap size increased, what will I do to the amp?  There is plenty of room to add more Al plate for heat sink and looks like room for a pair of 25 x 80mm+ caps.  

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Deoxit for the pots and no problem with increasing capacitance or voltage rating of caps. 

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2 minutes ago, wdecho said:

Deoxit for the pots and no problem with increasing capacitance or voltage rating of caps. 

Headroom!

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I am currently refurb'ing some Crown D75 amps.The stock caps were 10,000uF. I increased them to the biggest I could find that would still fit 22,000uF. The at rest(no signal voltage) has increased from + and -30 volts to about 36 and at full load just before clip I am still getting about 30. I did not measure what the original caps dipped to at full load. Also have not or did not check the ripple yet. But @John Warren did some of this with D45s and documented it quite well.  See: Well I can't get to his forum right now, don't know why:( But perhaps you have seen some of his posts here. I think this extra capacitance will give a reserve of power for low freq transients and I'm sure the added voltage probably makes the outputs run a little hotter. But with our efficient Klipsch speakers I will never run these things up high anyway. In short i don't think you can hurt your receiver by upping the capacitance. Of course make sure they're still rated to work at the correct voltage.

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28 minutes ago, CECAA850 said:

Headroom!

Not sure exactly what you mean. Are you talking about the physical size of the cap? Knowing if a larger value cap will fit should be a no brainer for anyone working on electronic equipment. 

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Just now, wdecho said:

Not sure exactly what you mean. Are you talking about the physical size of the cap? Knowing if a larger value cap will fit should be a no brainer for anyone working on electronic equipment. 

Headroom = the minimum you need + extra.

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I was hoping/thinking I could get more headroom, of sorts.  I want to try to reduce the recovery time from clipping and maybe increase the millisecond output into 4 ohms.  These old low end receivers were built to pass the ratings test, but no more.  It sounds fine, until it clips a little. 

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35 minutes ago, CECAA850 said:

Headroom = the minimum you need + extra.

Correct. One could too far in the extreme with a diy but hardly with a production box due to physical restraints. Typical capacitance with a Class A is 60,000uf per bank of caps. Some builders go much higher. When you get too far in the extreme other factors do come into play but hardly worth mentioning in a production amp. A higher voltage rating is desirable in a cap along with a higher temp rating as well. Typically the temp ranges available are 85C and 105C with 105C preferable. Probably overkill for most circumstances. 

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Keep an eye out for the dissipation factor (ESR) on the larger value caps. You can actually hit a point where the larger value capacitor is worse. To put it another way, if you're increasing the capacitance, then make sure the physical size is increasing too.

 

These are horrible rules of thumb because this should be calculated, but that's beyond the scope of the forum. Most times it's probably better, but not always...

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2 hours ago, DrWho said:

Keep an eye out for the dissipation factor (ESR) on the larger value caps. You can actually hit a point where the larger value capacitor is worse. To put it another way, if you're increasing the capacitance, then make sure the physical size is increasing too.

 

These are horrible rules of thumb because this should be calculated, but that's beyond the scope of the forum. Most times it's probably better, but not always...

 

The story of all engineering.  My concern would be the in-rush current damaging a diode or causing a destructive thump.  I quickly found a 10k uF cap on Mouser that was 3mm larger in diameter and 10 to 15mm taller.  I have plenty of room for that. 

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8 hours ago, JohnA said:

 

  My concern would be the in-rush current damaging a diode or causing a destructive thump.  

CL-60 in series on mains if concerned. With just the value increase you are talking about I would consider it not necessary though if you had no problem before. I would not consider ESR a problem on a new cap for power supply. Even the best pill has side effects. One has to weigh the value vs consequence. 

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15 hours ago, JohnA said:

I was hoping/thinking I could get more headroom, of sorts.  I want to try to reduce the recovery time from clipping and maybe increase the millisecond output into 4 ohms.  These old low end receivers were built to pass the ratings test, but no more.  It sounds fine, until it clips a little. 

You will have greater turn on surge as the caps charge. It may sag your line voltage for a very short time, but no worries.

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