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Old 14th March 2015, 09:06 AM   #1
Oceanw is offline Oceanw  Finland
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Default Headscratching with voltage & low leakage

Hello,

I bought two Nad 214 poweramps from fleamarket. They work, but are humming and scratching occasionally. Found blog about servicing them to order: Omron relay, main big caps replace etc. Instructions can be found here:
https://fittingmedia.wordpress.com/2...refurbishment/
Big thanks to thijsdebont!

Now im proceeding and doing some shopping lists at Mouser. Plan is to recap all electrolytics. I got enjoyment in my mind when reading info about different caps from here and audiokarma. Very time consuming and you can stuck at forums for hours..

Some info about psu: (schematic in attachment)
NAD 214 consist 2pcs of 4700uF /63V for one channel (originals are Nichicon LQ(M). There is place for third cap per channel, it is for bigger 216 model. OK, ill replace these with total of 6pcs of CDE SLPX 6800uF / 63V.
They will fit nicely, diam:25mm. No need to make custom tweaks. This is fine thisfar.

Next there is 2 x 220uF/63V (brand: EIC). Ill put same voltage & value Nichicon PW. Now were going to my problem:

Then there is large pattern of 4pcs of 220uF/160V(?!). I checked that these are special ones: Nichicon VX. After those caps EIC 47uF/100V and later only 63V cheap EIC.

Question: Why those C210,211,212,213 are 160V? Could i replace them with 63V ones?
I read that those VX are low leakage types and not so bad caps overall. Some say that their sonics are great. I cannot get them anymore. I can replace them with 220uf/160V Nichicon PW but is PW right cap for low leakage position? I heard that you can get hum & reduced bass if putting wrong caps to vintage equipment positions when there is called for low leak caps.

So, was Nad using 160V Nichicon series because of 160V rating is better in sonics than 63V version and also getting low leak benefits of VX series.
If i can put 63V versions like all other caps nearby, i could use low leak Nichicon KL 220uf / 63V and everything would be fine.

Comments are well noted.. Schematic is in attachment.

Click the image to open in full size.
imagen
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Old 14th March 2015, 12:10 PM   #2
DF96 is offline DF96  England
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Doubling the amount of reservoir capacitance may do no harm, or may increase buzz. It depends on the details of the wiring, impedance of the transformer etc.
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Old 14th March 2015, 01:09 PM   #3
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It seems to me that C206-C207, plus D202-D207, work together to build a pair of voltage doublers. I expect that C208 has got about +100V across it, and C209 has got about -100V across it.

The two 27V zener diodes and the Vbe of transistor Q203, set the negative output voltage (across C221) to -55V. The voltage divider made of 1% resistors R212-R217, sets the positive output voltage to be the inverse of the negative output voltage, namely, +55V.

I imagine the reason why C210-C213 are rated for 160V, is because in normal operation they see about 110 volts, and the designer wants some safety margin on top of that.
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Old 14th March 2015, 01:18 PM   #4
jitter is offline jitter  Netherlands
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I didn't really recognize the input of the +/- 55 V regulator until a realized that maybe this is some kind of a voltage doubler. Edit: Mark beat me to it and confirmed my presumption of the voltage doubler. But my interpretation differs.
Based on the voltage doubler on this site, I would have thought that D202 and D207 are not part of the doubler circuit, but the other diodes and caps are.
If I think about it, D202 and D207 might actually limit the doubling to a lesser amount by limiting the AC voltage to half a wave. You don't want to double the voltage and then have to dissipate most of it again in the next stage. But I might be totally wrong, if so, feel free to correct me.

Both the +/- 52 V supply and the +/- 55 V supply operate off the same windings of the transformer (AC1 and AC2). The +/- 52 V section is full-wave rectified with D201 and smoothed by C202..C204. So AC1 and AC2 yield a rectified voltage that is no more than +/- 52 V, and yet you're going to also use the same windings to create a regulated +/- 55V?

If you're going to need a regulated +/- 55 V you will need more volts on the input to account for the voltage drop in the regulator part of the circuit. What NAD seems to do is double (well, maybe not actually double, but at least up) the voltage, so the 160 V capacitors are there for a very good reason. Don't replace them with lower voltage caps.

Since the amp is working, you might measure the voltage across C210 or C211 and see what it is (careful, you're measuring dangerous voltage levels).

Last edited by jitter; 14th March 2015 at 01:43 PM.
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Old 14th March 2015, 02:19 PM   #5
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Quote:
Originally Posted by jitter View Post
... If I think about it, D202 and D207 might actually limit the doubling to a lesser amount by limiting the AC voltage to half a wave.
I predict that when you simulate the circuit with LTSPICE or TINA, you'll discover that the purpose of D202 and D203 is to accelerate start-up. Those two diodes guarantee that (Vin - Vdiode) is pumped to C210-C211, on the very first cycle of the mains AC waveform. This cuts the start-up time in half, compared to a circuit which omits D202 and D203.
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Old 14th March 2015, 07:06 PM   #6
jitter is offline jitter  Netherlands
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Thanks for the clarification.
The proof of the pudding is in the eating, so I breadboarded the circuit with Schottky diodes and 10 uF caps and hooked it up to a scope.

A 12 V transformer gave an (unloaded) voltage of about 35 VDC from the voltage doubler. The output voltage did not change by putting the diode across the input cap, my assupmption that it would was wrong.
With a DSO I might have been able to measure the difference in start-up time, but I have to make do with an analogue scope. I'll take your word for it that SPICE would show the difference.
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Old 15th March 2015, 07:14 AM   #7
Oceanw is offline Oceanw  Finland
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Hey guys! Thank you very much!!
It seems that i have to keep 160V voltage for caps.
I think i go with Nichicon PW series, not so many choices for 160V and PW series seems to have proven in psu.

Hmm.. If im changing big caps capacitance from 4700uF /pcs to 6800uF/pcs is there reason to add littlebit more at lower of chain? So, 220uf --> 330uF etc.
Any other ideas which would gain some improvement. Have to get all elkos away so now its change to tweak things..

Thanks again for all about comments regarding this 160V mystery!
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Old 15th March 2015, 07:50 AM   #8
jitter is offline jitter  Netherlands
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Hold on, I do agree with DF96 that upping the capacity too much might actually lead to disappointments.

A much higher capacitance has to have an effect on the current pulses charging the smoothing caps (higher capacity => lower impedance => higher peak charging current => higher EMI). This might cause unwanted noise as DF96 pointed out.

You may go a little up on the capacity, but doubling?? I'm not so sure about that.

Last edited by jitter; 15th March 2015 at 07:56 AM.
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Old 15th March 2015, 08:02 AM   #9
Oceanw is offline Oceanw  Finland
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Quote:
Originally Posted by jitter View Post
Hold on, I do agree with DF96 that upping the capacity too much might actually lead to disappointments.

A much higher capacitance has to have an effect on the current pulses charging the smoothing caps (higher capacity => lower impedance => higher peak charging current => higher EMI). This might cause unwanted noise as DF96 pointed out.

You may go a little up on the capacity, but doubling?? I'm not so sure about that.
Do you mean that doubling about lower valued elcos may bring me more EMI or are you regarding to those big main caps which i planned to change from 4700uF to 6800uF (and one pcs more for channel) ? I suppose you and DF96 meant those big ones. Then ill cut the risk and get better quality 4700uF on just add one pcs more per channel like in 216 model.
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Old 15th March 2015, 10:10 AM   #10
jitter is offline jitter  Netherlands
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Yes, I meant the big ones.

Enlarging the 220 uF caps might give some benefits. Voltage doubler circuits don't need much load to lead to large amounts of ripple. A bigger cap reduces ripple, but I expect that NAD didn't undersize these to begin with, and the regulator part of the circuit takes care of the rest.
But here too, don't take it too far...

Last edited by jitter; 15th March 2015 at 10:17 AM.
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