Re: Estimating transformer voltage for B&K CS117 preamp

On 15 Mar, 11:45, "Arfa Daily" <arfa.da...@xxxxxxxxxxxx> wrote:
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On 14 Mar, 17:22, "Arfa Daily" <arfa.da...@xxxxxxxxxxxx> wrote:
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On 14 Mar, 12:41, "powerdoc" <dkingl...@xxxxxxxxxxxxxx> wrote:
On Mar 14, 4:05 am, "N Cook" <diver...@xxxxxxxxx> wrote:
powerdoc<dkingl...@xxxxxxxxxxxxxx> wrote in message
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I have a CS-117 with a defunct transformer. The

Lets guess at 4v drop, which is on the generous / safe side, so that
means you need minimum 19v dc input.

Its a preamp so low current supply, so say 2v BR drop and perhaps a
25% regulation transformer. Allow for 15% mains drop. So your
transformer ac voltage will be

(19 + 2 ) /.85 x .707= 17.5v.

You'll need to allow for copper loss too, so 18+18 wont be enough.

Top voltage limit will be dictated by margins & cap rating.
If 25% transformer regualtion and 10% mains overvoltage get us to 40v,
running loaded v = 40/1.375 = 29v.

25v ac would be a good guesstimate. If its a split secondary with 2
diode rectifier, 25-0-25.

A tranny of nominal output 25v RMS is going to push those caps VERY near
their limits.

no, near their ratings.

I replace many electrolytic caps a week, and oftentimes, cheapo hifi's will
have caps rated at only 10v across rails that have 8v or so on them. Trust
me, they don't last long. For long term reliability, that rating figure
should be taken as an absolute limit. It is common design practice to derate
electros by working voltage, by a factor of 30 to 50%

If you see high failure rates at 80% rated voltage, something's wrong.
Perhaps the circuits are such that voltage fluctuates. Or maybe theyre
low grade caps.


If it's just a preamp, the current demand on it is not going
to be any more than a couple of hundred mA tops,

yes, probably less.

which will not load any
reasonably rated transformer anywhere near into copper losses IMO.

Eh? A small lower power transformer is going to have poor regulation,
ie relatively high winding R, and you've got a peaky current waveform
being drawn. Vdrop in the copper will thus be significant.

Why a " peaky " current waveform ?

The load is a bridge rec + reservoir, so it only charges the reservoir
caps at the peaks. Most of the time i=0, and at peak i= several times
average. Copper losses have a bigger effect with peaky waveform on a
low power and thus poor regulation transformer.


With a low current demand circuit such as
a preamp, the current drain from the transformer should be pretty constant,

averaged over each cycle yes, but instantaneously its the other way
round.

the main rectifier resevoir caps taking care of supplying any transient
requirements. Even a 'small' transformer at 18-0-18 is likely to have a
current rating of at least 500mA per limb,

no... thats a 9w transformer. Why would one fit a 9w tf to a 3w app?

and with the low demand of this
type of circuit, I would not expect to see barely a small drop due to copper
losses. The poor regulation will ensure that the output voltage is high on
the nominal design figure, and will likely remain so.

an old fashioned inefficient way to do things. Cheap volt regs make
such practices unnecessary today.


In fact,
a transformer rated at 25v nominal will likely have a low / off load
output
of up to perhaps 28v RMS.

12% regulation, maybe, or maybe higher. We really dont know.

Which potentially makes the situation even worse ...

12% regulation is a rephrasing of what you stated there. Unless you
mean 28v due to some other cause.


Multiply that up by 1.4 to get to the peak voltage
and you will be just about at 40v across the caps.

You're ignoring the effect of loading plus winding resistance. R has
more effect on a peaky waveform.

I don't think I am. I am employing years of practical experience with this
sort of thing.

exactly. If you work through the theory + numbers you'll see what
youre doing creates results that work fine until mains sags, then they
go wrong. A designer has to make circuits that tolerate the usual
overvoltage and undervoltage limits, whereas when repairing this is
optional in practice.


If we were talking about a power amp, then yes, factors such
as transformer regulation and copper losses have to be taken into account
for voltage sag calculations, but in low demand power supplies, it's more
relevant to look at it from the opposite angle, and work out how much higher
the output voltage will be than expected.

Surely it should be as expected, else you've miscalculated.


Those
regulators should have 20 to 25v going into them,

If they see 20v then 15% mains sag and the regs drop out of
regulation. 78 series require a 4v overhead.

So as I said, 20 ( a 5v overhead ) to 25 ( a 10v overhead ) is correct. If

ok

you were unfortunate enough to live somewhere with 15% sags on your incoming
line voltage, I would suggest that there would be a lot of equipment in the
house suffering odd problems. Taking the case of poor mains regulation, if
it can go down by 15%, you'd have to reckon on it being able to go up as
well. That's going to take those 40v caps over their limit, or 'rating' if
you prefer.

Arfa

Here (EU) all new goods can be expected to function correctly with
real world mains over- and under- voltage.


NT

.

Relevant Pages

  • Re: Estimating transformer voltage for B&K CS117 preamp
    ... 25% regulation transformer. ... I replace many electrolytic caps a week, and oftentimes, cheapo hifi's ... electros by working voltage, by a factor of 30 to 50% ... low power and thus poor regulation transformer. ...
    (sci.electronics.repair)
  • Re: Estimating transformer voltage for B&K CS117 preamp
    ... 25% regulation transformer. ... electros by working voltage, by a factor of 30 to 50% ... If there's some there, but not at the reg, then ...
    (sci.electronics.repair)
  • Re: Estimating transformer voltage for B&K CS117 preamp
    ... 25% regulation transformer. ... I replace many electrolytic caps a week, and oftentimes, cheapo hifi's will ... electros by working voltage, by a factor of 30 to 50% ... A small lower power transformer is going to have poor regulation, ...
    (sci.electronics.repair)
  • Re: Estimating transformer voltage for B&K CS117 preamp
    ... 25% regulation transformer. ... I replace many electrolytic caps a week, and oftentimes, cheapo hifi's ... electros by working voltage, by a factor of 30 to 50% ... The poor regulation will ensure that the output voltage is high ...
    (sci.electronics.repair)
  • Re: Estimating transformer voltage for B&K CS117 preamp
    ... 25% regulation transformer. ... electros by working voltage, by a factor of 30 to 50% ... The poor regulation will ensure that the output voltage is high ... it will be a bridge across the outer limbs of the winding, ...
    (sci.electronics.repair)