Headscratching with voltage & low leakage

[Oceanw]

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/2013/01/16/nad-214-poweramp-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.

An externally hosted image should be here but it was not working when we last tested it.

imagen

 

[DF96]

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.

 

[Mark Johnson]

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.

 

[jitter]

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).

 

[Mark Johnson]

... 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.

 

[jitter]

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.

 

[Oceanw]

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!

 

[jitter]

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.

 

[Oceanw]

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.

 

[jitter]

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...

 

[Oceanw]

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...

Great, ill keep them as they are (220uF/160V)

If you can tolerate some more questions, here they are:

1) As you can see at the schematic of first post, there is last cap pair in psu/regulator and they are C220 and C221. Their values are 100uF/63V.
Would it be beneficial to try MKP caps same value at this position? My sense says that if i use better cap somewhere in psu they should be those last ones? Am i right?

2) Any basic guideline about bypassing psu caps and how would you do that in this particular case?

 

[DF96]

The basic guideline on bypassing PSU caps is "don't do it!". This assumes you are asking about circuit theory and not fashion.

 

[Oceanw]

The basic guideline on bypassing PSU caps is "don't do it!". This assumes you are asking about circuit theory and not fashion.

This is great info also, makes my job easier & saves money.

However, im still puzzling with my original question which concerned those Nichicon VX caps. They were used in this amp originally and have lower "low leakage" parameter than my planned replacement: Nichicon PW.

This is quote from other thread and it haunts me:
"I just wanted to share some opinions and experience on electrolytics that I have gained through several recapping projects over the last couple years.
Project 1: KPS20i CD player- I originally intended to recap this with original caps only to find they weren't available in original format. Nichicon claimed to replace the VX with VR & that they were equivalent. I recapped main supply with the VR series only to find the sound to be totally different. Bass was much thinner & the sound was not nearly as smooth. I did some research to find that the VR series & VX series have totally different specs. The VX have a much lower DC leakage current which I now beleive to be one of the secrets to the smoothness & powerful bass on this unit. I did more research to find that the Nichicon fine gold series has almost the exact same specs as the older VX series. I replaced all possible with fine gold series & the higher values with VX axial caps(Had to jumper some of these to fit).Now sound is smooth & bass is deep just like factory."

Source: http://www.diyaudio.com/forums/parts/151392-best-electrolytic-capacitors-36.html


So, are anybody aware caps which would not upset this circuit or am i doing this too difficult?

 

[Oceanw]

And those original caps which were Nicihicon VX series are 220uF / 160V.
No Fine golds available at that series. This is leading to birdnest design & axial caps, im afraid..

 

[DF96]

Quoting someone else:

The VX have a much lower DC leakage current which I now beleive to be one of the secrets to the smoothness & powerful bass on this unit.

I think you can probably ignore any advice from someone who thinks that PSU cap leakage current has any effect on amplifier bass response. Cap ESR may have a small effect, but not leakage current.

 

[Oceanw]

Quoting someone else:

I think you can probably ignore any advice from someone who thinks that PSU cap leakage current has any effect on amplifier bass response. Cap ESR may have a small effect, but not leakage current.

Relieved to hear that, thanks again!

How about this:
1) As you can see at the schematic of first post, there is last cap pair in psu/regulator and they are C220 and C221. Their values are 100uF/63V.
Would it be beneficial to try MKP caps same value at this position? My sense says that if i use better cap somewhere in psu they should be those last ones? Am i right?

 

[jitter]

When it comes to caps in the signal path, I would agree that in some circuits an electrolytic might not be the best choice.

When it comes to PSUs, I have learned to be very careful. Some regulators really don't like it if you put low ESR caps (like MKP) on the power rails.
Read e.g. this and the next post in that thread.

My advice would be to stick to the 100 uF/ 63 V caps.

 

[Oceanw]

When it comes to caps in the signal path, I would agree that in some circuits an electrolytic might not be the best choice.

When it comes to PSUs, I have learned to be very careful. Some regulators really don't like it if you put low ESR caps (like MKP) on the power rails.
Read e.g. this and the next post in that thread.

My advice would be to stick to the 100 uF/ 63 V caps.

Ill stick with Nichicon PW, thanks!

 

[DF96]

As you can see at the schematic of first post, there is last cap pair in psu/regulator and they are C220 and C221. Their values are 100uF/63V.
Would it be beneficial to try MKP caps same value at this position? My sense says that if i use better cap somewhere in psu they should be those last ones? Am i right?

What does "better cap" mean? It has to be taken in context: what is a 'better cap' at this point in this circuit? In some cases the designer may be relying (deliberately or accidentally) on the behaviour of a 'worse' cap, as jitter says.

As I often say, in order to improve a circuit you first have to understand it better than its original designer. Few tweakers bother to do this. Of course, it is easy to make a circuit sound different and if enough time and money has been spent on this then the tweaker will firmly believe that he has improved things.

 

[Oceanw]

What does "better cap" mean? It has to be taken in context: what is a 'better cap' at this point in this circuit? In some cases the designer may be relying (deliberately or accidentally) on the behaviour of a 'worse' cap, as jitter says.

As I often say, in order to improve a circuit you first have to understand it better than its original designer. Few tweakers bother to do this. Of course, it is easy to make a circuit sound different and if enough time and money has been spent on this then the tweaker will firmly believe that he has improved things.

Ok, i was in naive assumption that mkp would beat electrolytic almost in any position, and that at this pricelevel there has been taken many shortcuts in component quality.

I ordered components last night.
Here they are (might do new thread about whole project)

Main big psu caps: Chemi-Con KMH
Other psu caps: Nichicon HE and PW, according voltage.
Signal path: 80% Panasonic ECQ-E polyester film and rest 20% Elna Silmic II.
Op-amp circuit: Nichicon PW & FG.
Input caps: MKT1822 1uF /63V (same size as original)

I keeped original signal path bypass 0.1uF polyesters and polystryrenes.
Wonder how same size Panasonic ECQ-E polyesterfilm will suit for replacing 10uF / 63V electrolytics.