bridge rectifier error?!
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Re: bridge rectifier error?!

by gompie on Tue May 29, 2012 2:35 am

evostars wrote:I understand the opamp that makes the 5.333 V &12 V needs 2v headroom. So 14V should be enough. Actually the tr303 had 12 V feeding the opamp i believe. Correct me if I'm wrong....
So with the modded bridge rectifier, the opamp gets powered like the TR303. That's another plus. Anybody like warble?
:-P


It's not the opamp that makes the 5.333v. the 5.333v is put onto the input of the left opamp. the left opamp is set to follow the input voltage. IC22 is responcible for the 5.333v. IC22 is an adjustible zener-diode with a 0.5v range. the resistors and potmeter on the right is the circuit responcible to tune to the 5.333v.

With the 12V you are correct, that one is generated by the op-amp, due to the way the output of the right op-amp is fed back to it's input via the resistors on the right of the right op-amp (still follow me? :) )

To be able to get to the 12V output, the op-amp needs to be fed with a voltage that is at least 14V, due to the power that is lost in the op-amp circuitry (an op-amp is made out of many transistors as you might know) I got the 2V power drop from the specs of the op-amp.
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Re: bridge rectifier error?!

by gompie on Tue May 29, 2012 2:42 am

evostars wrote:I understand the opamp that makes the 5.333 V &12 V needs 2v headroom. So 14V should be enough. Actually the tr303 had 12 V feeding the opamp i believe. Correct me if I'm wrong....
So with the modded bridge rectifier, the opamp gets powered like the TR303. That's another plus. Anybody like warble?
:-P


It would be a very interesting theory to test if a low voltage on the op-amp would create a desired warble.
One thing is for sure, below 14V on C5, the 12V output will be affected first (and that one is not respocible for the tuning).
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Re: bridge rectifier error?!

by gompie on Tue May 29, 2012 3:08 am

simplified the powersuply looks like this:
Attachments
diode%20bridge%20x0xb0x.png
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Last edited by gompie on Tue May 29, 2012 7:41 am, edited 1 time in total.
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Re: bridge rectifier error?!

by evostars on Tue May 29, 2012 5:17 am

Hmm no Gompie, c3 is not half rectified. Its not rectified at all. It gets ac straight from tthe adaptor.... Not trough a diode at all...
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Re: bridge rectifier error?!

by evostars on Tue May 29, 2012 5:50 am

evostars wrote:Hmm no Gompie, c3 is not half rectified. Its not rectified at all. It gets ac straight from tthe adaptor.... Not trough a diode at all...

hmm, but there is a diode to earth, so the negative side is connected to earth. so it is rectified...
Still its weird, because its also connected to C5!
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Re: bridge rectifier error?!

by gompie on Tue May 29, 2012 6:42 am

evostars wrote:
evostars wrote:Hmm no Gompie, c3 is not half rectified. Its not rectified at all. It gets ac straight from tthe adaptor.... Not trough a diode at all...

hmm, but there is a diode to earth, so the negative side is connected to earth. so it is rectified...
Still its weird, because its also connected to C5!


I agree that it would not be something I would design...

to get back to your comment: it's also connected to de - side of C5 to raise the base voltage at the - side of C5 to the level of C3 (otherwise the voltage doubler would not work)
Take into acount that to C5 it does not matter if the basis is 0V or 12V to earth (since it is not connected to the boards earth) the 12V (let's assume it's 12V) is now the new 0V for C5.

Measure the plus side of C5 to earth, it's 25V or higher. If you build your x0xb0x build is finished and the powersupply is still above 25V to the op-amp, I would change the AC adapter for a lower voltage one, or apply your suggested modification.

I agree that these voltages are a bit overkill in this stage of building, but it might prevent some unwanted troubles later on.
I will let you know when I finished my x0xb0x what the measured voltages are and still would like to go for a powersupply mod.
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Re: bridge rectifier error?!

by gompie on Tue May 29, 2012 7:27 am

evostars wrote:
evostars wrote:Hmm no Gompie, c3 is not half rectified. Its not rectified at all. It gets ac straight from tthe adaptor.... Not trough a diode at all...

hmm, but there is a diode to earth, so the negative side is connected to earth. so it is rectified...
Still its weird, because its also connected to C5!


I have been looking at the drawing for a while now, trying to figure how c3 can be charged in the first place...
The diode is indeed the explanation. It's not the + side of C3 that gets filled because of this diode, but the - side that gets drained because of this diode! Imagine the place where C3 and C5 connect to be te common, and the GND gets charged to -12V because of C3 and the one diode. C5 gets charged to +12V. The difference is still 24V between the +side of C5 and the -side of C3, except now the concept works :)

I think we're too hard looking at the GND to be Earth and figure it should therefor be 0V, but GND is not defined to an absolute voltage.

Could this be why it works?? haha
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Re: bridge rectifier error?!

by machine.cuisine on Tue May 29, 2012 10:48 am

gompie, you are right - it is a voltage doubler. I'm sorry about that confusion!

the ~24V from the doubler powers the dual-opamp, but also the 1st opamp "creates" and buffers (non-inverting) the 5.33 V, also using the "doubled voltage";
this 5.33V then goes to the 2nd opamp (non-inverting) who multiplies the 5.33V by (1 + 2.2/1.8 ), which is 11.8V.

so it looks like the capacitor C5 just needs a higher voltage rating, right? and probably r178 or179 should be a trimmer to get exactly 12V?
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Re: bridge rectifier error?!

by evostars on Tue May 29, 2012 1:32 pm

machine.cuisine wrote:gompie, you are right - it is a voltage doubler. I'm sorry about that confusion!

the ~24V from the doubler powers the dual-opamp, but also the 1st opamp "creates" and buffers (non-inverting) the 5.33 V, also using the "doubled voltage";
this 5.33V then goes to the 2nd opamp (non-inverting) who multiplies the 5.33V by (1 + 2.2/1.8 ), which is 11.8V.

so it looks like the capacitor C5 just needs a higher voltage rating, right? and probably r178 or179 should be a trimmer to get exactly 12V?

Agreed!
But c5 rides in top of the voltage of c3 so it should be good.
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Re: bridge rectifier error?!

by evostars on Tue May 29, 2012 1:37 pm

Great thinking everybody! Learned a lot.
Thanks :-D
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Re: bridge rectifier error?!

by gompie on Tue May 29, 2012 4:31 pm

machine.cuisine wrote:gompie, you are right - it is a voltage doubler. I'm sorry about that confusion!

the ~24V from the doubler powers the dual-opamp, but also the 1st opamp "creates" and buffers (non-inverting) the 5.33 V, also using the "doubled voltage";
this 5.33V then goes to the 2nd opamp (non-inverting) who multiplies the 5.33V by (1 + 2.2/1.8 ), which is 11.8V.

so it looks like the capacitor C5 just needs a higher voltage rating, right? and probably r178 or179 should be a trimmer to get exactly 12V?


C5 is just fine because its rated 16v. As long as the power measured over (between + and - of C5) does not exceed 16v.
C7 and C8 are rated 25v. Again this is good as long as the voltage over these C's stays under the 25v.
If this is not the case and the measured power is higher, the quickest and easiest way to do something about that is to look at the AC adapter and change it for a proper/lower rated one.

The op-amps do not use the doubled voltage other than as a source of power to be able to regulate towards on their outputs. the higher to supplied power, the higher the output of the op-amp can be. However in this case a higher power than 12V is not needed. As long as the supplied power to the op-amps (from the voltage doubler) stays above 14V (and not higher than 30v), all is good.

Now, the input of the first op-amp should be 5.333v. the output of that op-amp follows the input.
You are right, the output of the second op-amp depends on the 5.333v.
The two resistors r178 and r179 are responsible for output voltage, thats correct.

Because of the loopback on the second op-amp, the op-amp tries to minimize the difference between the + and - of the two inputs.
This ideally means that 5.333V over 1.8K means there is a current flowing of 2.96 mA.
Because of the high impedance of the op-amp inputs, the same current is flowing through the 2.2K resistor.
the output voltage should therefore ideally be (the resistors still have a 1% tolerance if I'm correct.) 2.96 x 4 = 11.85V

If you want to increase the output voltage of the op-amp, R178 should be increased in value.
I see no direct reason for this, but it can be done of course :)
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Re: bridge rectifier error?!

by antto on Tue May 29, 2012 5:45 pm

yes, my measurement of the "12V" usually reads 11.87V

this would make a slight difference for the square wave
i'm very curious what is the actual reading on the "12V" in a 303 :?:
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Re: bridge rectifier error?!

by aminoacid on Tue May 29, 2012 5:52 pm

there also seems to be another zener (d42) in the 303 that prevents the voltage, feeding the opamp, to exceed 15V.
it also looks like its part of the dc-ac convertation to regulate oscilations or something...

maybe one should try adding a 15V zener..?
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Re: bridge rectifier error?!

by evostars on Tue May 29, 2012 6:22 pm

aminoacid wrote:there also seems to be another zener (d42) in the 303 that prevents the voltage, feeding the opamp, to exceed 15V.
it also looks like its part of the dc-ac convertation to regulate oscilations or something...

maybe one should try adding a 15V zener..?

and d43, a 6.4V schottky
I'm turning that one into a mod:
viewtopic.php?f=13&t=28561
I'm trying to get closer to the original tr303 PS for the 5.333 and 12 V
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Re: bridge rectifier error?!

by gompie on Wed May 30, 2012 2:21 am

aminoacid wrote:there also seems to be another zener (d42) in the 303 that prevents the voltage, feeding the opamp, to exceed 15V.
it also looks like its part of the dc-ac convertation to regulate oscilations or something...

maybe one should try adding a 15V zener..?


It's worth the try! See if its affecting the sound in any way :)
You would take away possible modulation/variation on the op-amp power supply, because of changing load on the x0xb0x power supply (due to turning knobs, leds that go on and off, etc.)
Now op-amps are very quick creatures, but it's definately worth the try if it would fix something in the sound. I figure that other people would disagree right away, because the unwanted extra modulations might just define the caracter of the sound in a pleasant uncontrollable way...

Cheers!
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