PSU question for A30

[Buhl]

In the phase of collectiong the pieces for my A30 amp, im a bit confused about what PSU design I should go for - a resistor in the area of 0,47 ohm in a CRC filter, or just a simple big old C ?
My rough calculations gives me that a 0,47 ohm resistor will drop the voltage about 1,88 volts, and at approx. 2 amps pr. channel dispatch some 7-8 watts - not that much - and thus i will incorporate the secondary drop in the design, and use a 20 volt tranny, and thus end up around 24 volts - that is if I use a 1,3 instead of square root of 2 (witch I recall nelson suggested to use, for more life like calculations)

In the haze of the post returned to me by the schearch machine, when entering CLC or CRC i found a thread that stated that CLC or LC was the only way to go....

Is the effort and heat of the R worth it ? I saw that Nelson uses it in the FW amp - or should i just go with with the C of about 220.000uF / channel, like the original alephs ?

If I should use a resistor, is 0,1 ohm enough, or is 0,47 better ?

Any other tips, besides the use of small polypropylenes acros the big C's ? Im not a fan of CAD-software, more the "better big than sorry" type of apporach.......

And just a short 2. Q: the voltage ratings of the 220uF caps in the A30 - is 35 volts ok, or should I use 63 volts ?


Cheers !
Hans

 

[wuffwaff]

Hi,

since your bias won´t be that high I would go for a CLC approach. It wastes less energy and is much more effective. For example something like 44000-2.2mH-66000.........
You can have a look at the effects of resistors and chokes with PSU Designer II (no CAD!)

The 220uF caps can even be 25V without a problem (the coupling caps in the input and current source could even be 6,3V BG NX)

William

 

[Nelson Pass]

I have built them successfully using a .47 ohm value per
channel, which isolates the two channels and give a 1 volt
drop across the resistor. This gives a reduction in the supply
ripple by a factor of about 10.

 

[Buhl]

PSU for A30

Wuffwaff and Nelson - thnx for the answers - I would like to use R's instead of L's - as they are somewhat harder to get for me, and to be frank - i dont mind the extra heat.... ;-)
I suspect that the the cap at the current source is C9 close to Q5 - or is it C10 ?? Wuff - you mention the use of BG caps here - is this location and the input coupling cap a good place to spend some extra $$$ in trade of quality ? I plan on using Panasonic FC, as I have them lying in my "box O'goodies".

Nelson, the use of Resistors that you mention to give isolation, as well as a factor 10 in reduced ripple (sounds as it is worth to include ??) - is that the "normal" use whare you place the resistor in the positive and negative rail between two banks of Caps ? How could this isolate the channels - is that because that you use a seperate cap bank for each channel ? and finally ( as if I didnt ask enough questions...) are there any pros by using a cap bank for each channel, with its own bridge ?(or two bridges, but I would like to thy and use one, as this i whats used in the A30 schematics)

Thank you for taking the time to answer and contribute to my knowledge increase ;-)

Cheers !
Hans

 

[wuffwaff]

Hi,

the cap at the current source is the one connected to the base of Q5.
BG NX 220uF 6,3V are not very expensive and I used them in my Aleph5. There is another thread "coupling cap in Aleph" were these caps are discussed at lenght ending with a bunch of BG´s used for the input cap and a panasonic FC + 10nF at the other position.

William

 

[Buhl]

A30 psu

Hi again
as I read the schematic, the only cap attached to the base of Q5 is C7, witch is 0.001uF ?
C9 is connected acros the collector and emitter, and C10 to emitter, and to base via the R21@820 ohms - R9&R10 is 220uF ?
I a bit confused now - ??

Cheers!
Hans

 

[wuffwaff]

it´s the one connected to R21. Insome Alephs the cap comes first then the resistor. Here it seems to be the other way round.......

sorry,

William

 

[jh6you]

I think that 220uF of 35V grade is ok for C9 and C10.
If you use 63V grade, the life of the caps could be vertually permanent.

Regards

 

[Buhl]

PSU

OK, good to know - I'll stick with the 35 volt - permanent is almost to good for me

But still Im a bit confused about the use of resistors (or inductors) in the PSU - I suspect that nelson uses it in the FW amps because they have to drive very sensetive speakers, and/or the circuit itself is more prone to pick up noise from the supply ? I recall that some have said that they did not think that further regulation beyond a simple C of the aleph supply gave any improvement ?

Ups, and another thing - I want to be able to play with BIAS (dont we all ??) and thus want to calculate for the greater current draw when ordering my toroid - how much does the A30 draw when R19 is removed - its about 2 amps/channel when not removed ?

Cheers !
Hans

 

[wuffwaff]

Hi Hans,

with 0,47 Ohm source resistors this will probably be over 3A / Channel. Instead of removing it you could also change it to a higher value. You will need a pot in this position anyway to set the amp up in the first place. Just change the setting and measure the pot so you´ll know what resistor you need for a given bias.

All the Aleph´s were without C-R-C or C-L-C so this will work just fine.
The nice thing about DIY is that you can try other things too without having to worry too much about cost or customers.
A nice C-L-C supply will give you far less ripple. Why don´t you just try with and without. Measure the ripple voltage and listen to the changes in sound.

William

 

[jh6you]

In addition to William's.

I estimate that the current draw after removing R19 is about 4.5A per channel. Therefore, for us to be able to to play the bias, we need a toroidal transformer of 2 x 18~20V min. 800VA. I estimated the value with plus tolerance, considering potential error between my calculation and the real measurement.

Have fun.

Regards

 

[Buhl]

A30 PSU

Yes, Wuffwaff, youre rigth, its very easy to try with or without - but, the question for me is, if the hazzle of getting the chokes, witch is quite hard for me, is worth the difference between resistors and chokes ? If I use resistors, I just buy a toroid with 2 volts more on the secondary. How much would the DC resistance of a 2mH coil typical be ? should I calculate for the same voltage drop as with the resistor?

Is was planning on a 1000VA 2x18 volt, or two 500VA 2x20 volt - the big one is cheaper than two small ones, so the price of chokes would even out the total price i guess.
4,5 amps - thats ok !!! is it correct that its only about 2 amps with R19 included ?? This would also bee a problem if I use CRC, thus the voltage drop over the R would be a lot greater (double??)

Cheers !

Hans

 

[wuffwaff]

Hi,

the voltage drop over a choke is much lower for the same filtering effect. Normally dc-resistance is between 0,12-0,3 Ohms for a 2.2mH. There are easy to get Intertechnik air chokes 1.8mH I used in my Aleph 5 or for bigger currents the torobars (I tested them at 10A) with 2.2mH
The voltage drop will double going from 2 to 4 A. How much you loose depends on the resistor/choke, total capacity and transformer.
Like I said before you´ll have to use a pot for R19 to set a precise bias!

William

 

[Buhl]

A30 PSU

HI Wuffwaff
That was just what i thougth, but, the Intertechnic air coil you are talkin about, arent they for loudspeaker crossovers ?? I thougth that this kind of coils were a bad choice for DC applications ?? Should it not be a "real" DC coil ??


Cheers !
Hans

 

[wuffwaff]

Hi Hans,

yes they are for Loudspeaker filters.
The air chokes so they can´t be saturated so they will work fine but have a bit higher resistance.
The Torobars can be saturated but like I said worked fin up to 10A. (didn´t test them any higher cause my load started to cook )

William

 

[Buhl]

Well, I have run into a small issue constructing this PSU - I ended up using 4 pcs. of 20-0 volt, 150VA transformer - using two for each channel, giving 20-0-20 volt. I have hooked up the transformers, and get 20 volt on the secondary - but I have a h*** of a buzz - mechanical, 50Hz buzz. I have connected the transformers as shown in the attached picture - they have twin 110 volt primarys, which I have wired in series for the 230 volts in my contry - but i am in doubt if I should reverse one of the primarys? Or should I go for a DC filter on the primary?

Kind regards
Hans

 

[Bill Fuss]

It does sound like your primaries are out of phase if you have no load on the secondaries. Try it with a 1/2 A fuse on the primaries,if it's out of phase it should blow the fuse.
300va is enough for one channel, it should work.

5 years, I thought I was slow.

 

[Buhl]

Hi Bill - thanks for the answer. And yes - when we are talking about slow projects, this one takes the prize!! Do you mean that without load on the secondarys, the transformer is likely to buzz?

In Denmark, and in fact all of EU, we are not allowed to have phase indicators on the power outlet, so guessing which is phase is quite hard - however it can be measured I have tried to switch phase, without audible difference, but maybe that will change with load.

In the meanwhile, I have played around with different configuration - putting two of the four transformers in reverse to the other two, rubber insulation between transformer and cabinet etc. - and I think I will go with the rubberfeet, which definetely helps, and all transformers in same phase - that way I can switch the phase by rotating the mains plug 180 degreees. Will also do an external DC blocker with a couple of caps and diodes.

If the buzz is still to much after the amp is finished (knocks under the table for good luck) - I will have to get another and rebuild the amp

/Hans

 

[Bill Fuss]

Hi Hans,
I was speaking of each individual transformer. Are you sure you have the 2 primaries series connected in phase with each other. If they are not the transformer should blow a 1/2amp fuse without any load applied. But if you have the amp operating I dont think that's the problem.

 

[Buhl]

Hi Bill
No, I do not have the amp working...yet - I am in the process of assembling it, and was doing tests on the PSU when I discovered the buzz. I have tried the phase reverse the primarys, without any noticeable difference in buzz, so unless the DC filter helps out, I think it is just a case of bad transformers

Thanks for taking your time helping out!

/Hans