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Old 22nd March 2010, 02:57 PM   #31
AndrewT is offline AndrewT  Scotland
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it is normal to expect an 18Vac transformer to provide sufficient voltage for an 18Vdc regulated supply.

Let's start with some assumptions.
50VA 18Vac, 15% regulation transformer. Mains voltage tolerance +-6%

At maximum mains voltage the no load output voltage will be
18 * 1.06 * 1.15 * 1.414 - rectifier voltage drop ~=30Vdc at the smoothing capacitor.

At minimum mains voltage the no load output voltage will be
18 * 0.94 * 1.15 * 1.414 - rectifier voltage drop ~=26Vdc at the smoothing capacitor.
The ripple on the cap will be zero since there is zero current demand.
The regulator has 8V to 12V to use and stay in regulation.

Now draw ~0.3Amps from the regulator. The ripple will increase from zero Vpp to ~1Vpp (depends on cap value). In addition the caps will not quite reach the peak voltage from the transformer. The maximum voltage at lowest mains supply voltage will be ~ 26-0.1 at ripple max and ~26-0.1-1.0 at ripple minimum.
We now have 24.9Vdc at worst case condition at the input of the regulator.
That is still 6.9V for the regulator to work with when drawing 300mA.
At the maximum draw that this transformer can supply, about 700mA, the minimum input voltage will be about 23.5Vdc. Any decent regulator should be able to maintain regulation with 5.5V of overhead.
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Last edited by AndrewT; 22nd March 2010 at 03:02 PM.
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Old 22nd March 2010, 03:11 PM   #32
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andrew, thanks for your advice on the unregulated section. how did you figure 700mA as a maximum current through a 50VA 18VAC trafo? wouldn't it be more like 2.8A? i'm asking because the design here draws a constant 1.6A from the supply...

also, i think the salas shunt classifies as a high-dropout regulator by most standards. typical headroom given is around 6V.

~ brad.
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Old 22nd March 2010, 03:16 PM   #33
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Quote:
Originally Posted by miket6000 View Post
... I recently designed one almost identical to it for one of my amplifiers...
mike, what are your opinions on sound quality of the amp using the shunt regs? what was the application?

~ brad.
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Old 22nd March 2010, 03:20 PM   #34
AndrewT is offline AndrewT  Scotland
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50VA 18Vac is 2.77Aac.
de-rate the transformer to ~70% for a capacitor input filter.
Now read of the maximum power available at the capacitor input filter.
Power = Voltage * current.
Max current = Max Power / voltage.
Max power ~= 50 * 0.70 ~ 35W
Max current ~= 35/18/1.414 ~1.375Adc

at this continuous output current the transformer will be running at the maximum rating that the manufacturer recommends. Most manufacturers specify a maximum VA based on maximum tolerable/reliable temperatures in the core.

Many builders and designers recommend an operating continuous current of ~ half the maximum to keep the transformer temperature much less than maximum, i.e. our 50VA 18Vac can supply a continuous 690mAdc

This is what I have adopted for a continuous load like a ClassA amplifier.
Put the other way, if you want a continuous current of 1.6Adc then select a transformer that can supply a continuous 6.4Aac, i.e. 115VA for each 18Vac secondary.
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Last edited by AndrewT; 22nd March 2010 at 03:23 PM.
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Old 22nd March 2010, 03:28 PM   #35
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by geekysuavo View Post
the salas shunt classifies as a high-dropout regulator by most standards. typical headroom given is around 6V.
the typical CCS+Shunt regulator is a normal drop out regulator. The Salas is no different in this respect.
The Salas will regulate with a lowish drop out, although not quite into the low drop out range.
The 6V to 10V of overhead is recommended for best audio quality.
I have the DCB1 operating at ~10.3Vdc.
I chose a 15Vac transformer to achieve a 10V overhead. Not because a 12Vac would not allow regulation in worst case conditions.
My first DCB1 build would regulate with 9Vac on the secondary of the transformer or about 2Vdc above supply ripple + regulated output.
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Last edited by AndrewT; 22nd March 2010 at 03:32 PM.
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Old 22nd March 2010, 06:12 PM   #36
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Question hmm... dual mono then?

well, in that case i suppose i need to search for a different power transformer, or completely separate the unregulated supply (true dual mono) for each channel... any reason why door number two here would prove unsatisfactory?

~ brad.
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Old 22nd March 2010, 09:23 PM   #37
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considering the cost of trafos, door number one will yield a cheaper BOM cost. i'm thinking about a 100VA trafo. (digikey 237-1341-ND) while it's still not quite at the 'halfway margin' mark, i think it should be more suited.

(i guess i was calling 'regular-dropout' regulators high-dropout regulators... lol)

~ brad.
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Old 22nd March 2010, 10:50 PM   #38
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Wink yet another revision!

ok, i've attached the changes to the power supply and regulators. a 2sk170 jfet is used in place of the current regulating diode, the new filter caps are 2200uF/50V FC types (x5), and the trafo is a 100VA type. crude simulations show ripple to be 1.0Vpp @ ~23Vout from the unregulated section.

~ brad.
Attached Files
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File Type: txt bom.txt (1.4 KB, 9 views)

Last edited by geekysuavo; 22nd March 2010 at 10:51 PM. Reason: added bom
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Old 23rd March 2010, 01:24 AM   #39
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On what AndrewT has said, there will always be a voltage ripple on the cap because the power supplys are drawing a constant 1.6A giving a voltage ripple of approximately dv = 1/(120*C) (Assuming 60Hz). This is a slightly pessimistic approximation, but it never hurts to make things a little better.

As for the transformer current rating, I think you've performed the same derating twice. Derating a transformer with a capacitive load is a simplification based on the fact that the output voltage will be sqrt(2) higher than the transformers rated voltage. To look at it another way, if you have a 50VA 18V transformer, and rectify the output with a large bulk cap the output voltage will be 25.4V if you draw 1.97A from this you're drawing 50VA. This will be aproximately 70% of the rating on the transformer because they rate transformers on their RMS voltage so 50VA/18V = 2.78A.

Short answer is it's not necessary to derate the transformer as much as AndrewT has and the one you had previously will be enough.

Mike
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Old 23rd March 2010, 03:55 AM   #40
Jen-B is offline Jen-B  United Kingdom
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Just thinking out loud... in theory would there be any advantage in using the same fet for follower and ccs? Both IRF610 or both IRF640? Maybe temp stability of similar parts? (I'm not knowledgeable on fets, so please forgive me if this is a silly question).

Thank you,
J.
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