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Old 16th March 2013, 01:26 PM   #1
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Default Transformers and current output

Hi !
I have seen low to medium power amps with different choices of voltage rails
My amp has a transformer with output 20+20V and it is a 40W/8ohm
I have seen other ones with same power (i.e. 40W/8ohm ) but higher voltage rails, even 35+35
My question is :
what would you prefer (once fixed the transformer power) lower voltage output and more current or the viceversa ?
Is not more important current than voltage ?
Of course I am referring only to power amps up to 40W that do not need high voltage rails
Thanks a lot
Regards,
bg

Last edited by ginetto61; 16th March 2013 at 01:30 PM.
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Old 16th March 2013, 01:45 PM   #2
JMFahey is offline JMFahey  Argentina
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Quote:
My amp has a transformer with output 20+20V and it is a 40W/8ohm
I have seen other ones with same power (i.e. 40W/8ohm ) but higher voltage rails, even 35+35
Not possible .
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Old 16th March 2013, 01:46 PM   #3
jcx is offline jcx  United States
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40 Wrms @ 8 Ohms requires ~25 V peak amplitude - so maybe with some hokey PEP rating relying on 28 V peak from the 20+20 Vac xfmr, fully charged resevoir C


but current reserve also depends on loading assumptions - many respected audiophile amps brag about doubling power as load is halved - some suppy 4x current, power to 2 Ohms vs their 8 Ohm load rating

will you ever hook up 4 Ohm speakers, or lumpy impedance speakers that may dip below |4 Ohms| at certain frequencies even with "8 Ohm" on the nameplate

if you assume evil "matched" signal to a lumpy impedance speaker it is possibe to see large multiples of the nominal current requirement - at least up to 6x

Last edited by jcx; 16th March 2013 at 01:54 PM.
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Old 16th March 2013, 01:57 PM   #4
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Quote:
Originally Posted by JMFahey View Post
Not possible .
the voltage measured on the pc caps is about 28-29V
20-0-20 V is the transfo output not the voltage rails
I do not have the original schematic but i found this in the web

Click the image to open in full size.

Regards,
gino
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Old 16th March 2013, 02:02 PM   #5
JMFahey is offline JMFahey  Argentina
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Fine, but please make it clear.
I originally thought you were referring to VAC (which is proper) but the immediate next line you wrote spoke of "but higher voltage rails, even 35+35".
The "but" particle and the "higher" reference makes it a continuation of the earlier phrase.
And here you qualify the voltage mentioned as "voltage rails".
But, of course, thanks for the clarification
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Old 16th March 2013, 03:30 PM   #6
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Quote:
Originally Posted by JMFahey View Post
Fine, but please make it clear.
I originally thought you were referring to VAC (which is proper) but the immediate next line you wrote spoke of "but higher voltage rails, even 35+35".
The "but" particle and the "higher" reference makes it a continuation of the earlier phrase.
And here you qualify the voltage mentioned as "voltage rails".
But, of course, thanks for the clarification
Yes you are right i am confusing sorry.
I presume that the schema is correct but i do not have the real original one
The voltage at the secondaries is about 20-21 VAC that after rectification become actually 28-29 VDC
And actually i have seen amps with similar wattage but employing transformers with 35 VAC at the secondaries
I try to explain why i am asking
Listening even at low volume some amps with less watt gives a more full range sound than amps with higher wattage.
When you raise the volume on these latter amps you never get the low end, just mid and high range.
Like as they were with a low filter engaged
This is not realistic ... you cannot listen to music this way. You will never get a realistic feeling
It is extremely frustrating
To an extreme a krell ksa 50 can have hugely stronger bass than another and cheaply built 100 W/8 ohm
So my real question is how i can read the specifications to understand if the amp is really delivering power or just starts screaming when i raise the volume ?
I think that they should specify the maximum current output without distortion or something like this
Thanks again
regards,
gino

Last edited by ginetto61; 16th March 2013 at 03:50 PM.
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Old 16th March 2013, 03:47 PM   #7
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Quote:
Originally Posted by jcx View Post
40 Wrms @ 8 Ohms requires ~25 V peak amplitude - so maybe with some hokey PEP rating relying on 28 V peak from the 20+20 Vac xfmr, fully charged resevoir C
but current reserve also depends on loading assumptions - many respected audiophile amps brag about doubling power as load is halved - some suppy 4x current, power to 2 Ohms vs their 8 Ohm load rating
Thank you very much indeed for your kind and valuable explanation
Is it possible from the am specifications to predict if the amp is able to deliver current ?
I remember an experience
I went to listen to a pair of huge tannoy pro monitors ... a very tough load indeed i guess
They were connected to a 250W/channel pro amp
The audition started at low volume
Then raising the volume the bass did not come out at all .... it was only mid and high frequencies ... and also not very nice
I was shocked thinking at the power ratings of the amp
Then I saw some measurements on commercial amps
There are amps that cannot deliver more than 3-4 ampere without distortion
This is very little
The output transistors are usually 150W types even in cheap amps
The main suspect for me is the transformer
It is the first and main bottleneck for current
I read of famous little low wattage amp with a very string sound
I usually listen a low volume but i would like to have a complete sound not just mids and highs, like a head without body.

Quote:
will you ever hook up 4 Ohm speakers, or lumpy impedance speakers that may dip below |4 Ohms| at certain frequencies even with "8 Ohm" on the nameplate
if you assume evil "matched" signal to a lumpy impedance speaker it is possible to see large multiples of the nominal current requirement - at least up to 6x
I think this is the very point
so many real speakers are very far from an ideal resistive load
The low impedances are wells for the current
To find an amp that is not undersized in the ps means to go with very high price units I am afraid
Still i think that some measurements can show this behavior clearly
But i never see them
Thanks again and kind regards,
gino

Last edited by ginetto61; 16th March 2013 at 03:52 PM.
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Old 16th March 2013, 04:46 PM   #8
gootee is offline gootee  United States
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Maybe the amps that can't provide more bass when louder simply don't have enough reservoir capacitance. Remember that most of the speaker current comes from the caps. So there must be enough capacitance to supply full max output current for the entire time between charging pulses, without letting the rail voltage sag too much. And also, the caps must then be able to be recharged fully by one charging pulse. It helps, for that, to have a large-enough transformer, with low leakage inductance and resistance, and capacitors with low ESR (series resistance), and thick low-inductance conductors everywhere.

Last edited by gootee; 16th March 2013 at 04:48 PM.
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Old 16th March 2013, 04:53 PM   #9
DF96 is offline DF96  England
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I suggest the OP makes himself a cup of coffee (or whatever beverage suits him) and then sits down to read the 'reservoir cap size' thread. All his questions will be answered, in more detail than he probably wants at this stage. The length of that thread, and its occasional meanderings, shows that the answers are not quite as simple as some people think but not as esoteric as others like to imagine. It all boils down to a ripple calculation. No magic needed, just a calculator or one of gootee's spreadsheets.
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Old 16th March 2013, 05:05 PM   #10
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Quote:
Originally Posted by gootee View Post
Maybe the amps that can't provide more bass when louder simply don't have enough reservoir capacitance. Remember that most of the speaker current comes from the caps. So there must be enough capacitance to supply full max output current for the entire time between charging pulses, without letting the rail voltage sag too much. And also, the caps must then be able to be recharged fully by one charging pulse. It helps, for that, to have a large-enough transformer, with low leakage inductance and resistance, and capacitors with low ESR (series resistance), and thick low-inductance conductors everywhere.
Thank you very much and very interesting indeed
Speaking of ps caps do a rule of thumb exists ?
I mean, for one channel 2 x 10.000 uF could be an acceptable solution ? will they be enough ?
I will read the relevant 3d by the way. i am very interested in this issue
Thanks and regards,
gino
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