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Old 16th May 2011, 12:07 PM   #11
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Quote:
Originally Posted by gpapag View Post
Hi Tony

That the 10000uF_4700uF_4700uF gives the same noise results at the regulators output (you may have a look at the difference on the input side) with the 3X 1000uF, is an indication that the LM317 is doing a good job.
Yes there was a BIG difference on the input side, that is why I originally went down the route of the big caps, but it seems that the LM317s's ability to filter out that ripple (at least with the adjust pin transistor mod) makes it less of an issue.

Quote:

Remember though that these results are valid for a fixed value load.

When the PSU will be connected to a variable load (Class AB/B amplifier), things may not be that easy for the LM317. *PS

So, please test this before you start removing caps from your final lay-out.
That's going to be a bit difficult unfortunately, as I haven't even started on the project it is going to power yet I have an idea what the load current will be, but I don't know how variable it will be. It is going to be powering my "synergy active crossover" which can be seen here --> http://www.diyaudio.com/forums/blogs...crossover.html. As I was planning on using the same breadboard that is currently hosting the PS for protyping the synergy, experimenting will be a tad diffcult

I did test with load current of 180mA and also 360mA with basically identical results, though in both cases the current is constant. I did do some tests with the scope before and this post ---> help making sense of measurements shows what happened when I used the supply to drive a square wave generator, however this was before the recent revelation as to how much of a difference scope probe placement makes to the measured result.

I'm actually thinking I'm going to drop the 10,000uF caps and change to 4700uF 3r3 4700uF 3r3 1000uF I could upgrade the supply in my chipamp with the spare 10,000uF caps (I actually have four due to a previous blonde moment). This will also mean the board is not so tight and give me a bit of extra room to improve the layout.

Quote:
*PS: Unless you plan to use this PSU to feed a pure class A amplifier (which has constant current draw = acts as a fixed value load)

>Edit: Also make sure that the measuring signal coming into your audio card is of adequate amplitude (not very low), or else the spectrum measurements will not spell the real situation (I've done this mistake myself)
Well I guess the B1 buffer parts of the circuit will be class A, but I don't think the GIC and FDNR will be.

I'm not sure how low the amplitude is.. I know the noise on the scope (just wideband noise) measured at less than 2mV peak to peak, so with 10X gain on the sound card preamp, it could be around 20mV for the noise floor and anything above the noise floor will be higher than that. Do you think that should be enough?

Tony.
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Old 16th May 2011, 03:08 PM   #12
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Hi Tony
I see you are doing progress.

Ref. the behaviour of the regulator with a variable load:

As your intended load circuit is not yet built, think of this
Low cost regulator in between Jung and Flea

It is really helpful when it comes to testing.Make the resistanse suitable to your needs.
(A very brute alternative to this circuit is a manual SPDT switch)

Quote:
I'm not sure how low the amplitude is.. I know the noise on the scope (just wideband noise) measured at less than 2mV peak to peak, so with 10X gain on the sound card preamp, it could be around 20mV for the noise floor and anything above the noise floor will be higher than that. Do you think that should be enough?
Although I really think this signal is very low , you have to check it.

You are recording the output of the voltage regulator with some software program (a wave editor) right?
Usually all these programs show - at least while recording-the input amplitude with some "VU meters".
These VU meters are the ones you have to watch for to see if you have enough signal for to (later) spectrum analyse, or you are just analysing the noise of your soundcard *PS

If your signal is really low (I wouldn’t go with anything lower than –60 to –70dB Vu meter reading), then you have to increase the gain of the preamplifier (make it X100 at least) and recheck.

[Note: If your oscilloscope has a “Y Out” you can use it to feed your soundcard.
The probe will be connected to the Y Input of the oscilloscope, so effectively you are using the scope’s preamplifier/adjustable attenuator as a good quality adjustable preamplifier. ]

Keep on good work (and report here)

Regards
George



*PS: I think, this is what was actually happening here:
Quote:
as my Audigy seems to be playing up and I couldn't get anything resembling reality (everything was -117db regardless of configuration).
LM317 experiments and measurements
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Last edited by gpapag; 16th May 2011 at 03:11 PM.
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Old 16th May 2011, 10:17 PM   #13
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Thanks George yes I am using audacity to record, Levels were around -72db so probably maginal, though I did do a test to make sure I was seeing a difference between the sound cards noise floor and the measurements of the reg. see pic three in post #3 You could be right about the audigy though! I was running it with zero db gain, the onboard card I don't know but possibly is running with some gain, I just set it to get the lowest noise floor with input shorted.

not sure if my scope has a y output, will have to check! I could take a look at the preamp circuit, not sure if it would handle a gain of 100X

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Old 16th May 2011, 10:51 PM   #14
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Quote:
not sure if my scope has a y output, will have to check!
Tony
If it has, it will be at the back (usually a BNC).
If there isn't one but you have a schematic circuit of your scope, then you can add one. It is useful.


Quote:
I could take a look at the preamp circuit, not sure if it would handle a gain of 100X
Post the schematic and it will

Regards
George
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Old 17th May 2011, 01:14 PM   #15
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Hi George, I checked the manual, and no Y out, though I do have a complete circuit diagram and description of the circuit.

I'll have to dig up the schematic but I found it uses an LM324. The opamp itself can easily manage 100X gain, what I'm not sure is whether the rest of the circuit can handle it

I thought I should also mention that I have done the final (full) circuit, ie the + and - rails and can happily report that it worked flawlessly (well at least based on the meter reading and sound card measurements)

I am going to have to be careful where I take the common ground point from though, as it is even more critical than the point I was taking the other measurements. Unfortunately I don't think I saved the really bad one, but I can easily do it again.

The measurements below were taken with the scope probes on either side of the load resistors once again, they were not taken with a common ground point, mainly because the only places I could easily attach scope probes were the resistor leads, and sharing a common load resistor lead as the ground point gave VERY bad results.

1st pic is the comparison between the +ve (left) and -ve (right) outputs I did the measurements separately as my mic preamp, sound card (and possibly the different scope probes) give different results when recorded in stereo.

2nd pic shows what I get when I measure both + and - at the same time with stereo recording.

3rd pic shows the difference between the 1000uF elna Silmic II on the output of the reg and a 100uF nichicon FG. left is the 1000uF right is the 100uF

4th pic shows difference with and without the 0r33 in series with the output cap. Both were taken with a 100uF output cap. I'm uncertain why in this instance I got a much flatter result than with the other measurements for the one with the 0r33 as all of the other measurements (unless stated) had the 0r33 in circuit, the only thing I can think of is I may have removed the 470nf that is connected directly from rail to ground for that measurement.

Anyway I guess that is enough for now

Tony.

PS, you may have noticed the variations in the 50 Hz, this seems to vary depending on the time of day, and I get the same 50Hz hump even with the scope probe shorted, so it is either the preamp picking up hum or the soundcard.
Attached Images
File Type: png +and-_measured_separately.png (27.3 KB, 783 views)
File Type: png +and-_measured_together.png (28.7 KB, 485 views)
File Type: png 1000vs100_on_output.png (28.4 KB, 486 views)
File Type: png 100uF_output_left_with_right_without_0r33.png (29.1 KB, 473 views)
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Old 17th May 2011, 02:46 PM   #16
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Quote:
I checked the manual, and no Y out, though I do have a complete circuit diagram and description of the circuit.
If you e-mail them (scans or internet link if they are posted somewhere) and I will try to sketch a solution. I have done the same on my scope (a Hameg) and works OK.

Quote:
I'll have to dig up the schematic but I found it uses an LM324. The opamp itself can easily manage 100X gain, what I'm not sure is whether the rest of the circuit can handle it
What is in the rest of the circuit that is so "fragile"?

Quote:
PS, you may have noticed the variations in the 50 Hz, this seems to vary depending on the time of day, and I get the same 50Hz hump even with the scope probe shorted, so it is either the preamp picking up hum or the soundcard.
Most probably so.

Regards
George
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Old 17th May 2011, 10:03 PM   #17
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Hi George, here is a link to the manual for my scope, quality isn't great, but I don't have a scanner at home. If this isn't good enough I'll take the manual into work and do a good quality scan of the schematic http://bama.edebris.com/download/bwd...%20824_001.pdf

what's so fragile about the preamp? probably nothing, just that I read that the 2002 silicon chip one is supposed to be superior, this is an Electronics Australia design and I'm a bit concerned it could be noise prone if the gain is increased too much. It is in a plastic jiffy box too, so no shielding.

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Old 18th May 2011, 06:10 AM   #18
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Hi Tony

The oscilloscope's documentation is readable. Don't bother with scanning it.
You'll have feedback ASAP

Try to find the schematic of your X10 preamplifier.

Regards
George
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Old 18th May 2011, 12:52 PM   #19
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Thanks George!

Here is the schematic for the sound card preamp. Note that they made a mistake (and left out the +1 in the gain calculation) The actual feedback resistors are R5 82K and R6 9.1K To get a gain of 100 I could just change R6 to 825 ohms.

Note that the original circuit also had a quite early rolloff on the low end and I replaced the 10nf input cap with a 68nf one which substantially improved it, however this is the first time I've looked at the circuit since (and I know a bit more now) and I can see that potentially some more improvement could be made by increasing the 10uF feedback cap as well, as it appears that at present the corner frequency it produces would be around 17Hz. Increasing the cap to 47uF would bring it back to around 3Hz.

Also as mentioned in another thread, the second pic is a comparison between a single 1000uF smoothing cap vs 3 X 1000uF smoothing caps separated by 3r3 resistors. As with all the other measurements this was taken with 20db (ie 10X) gain on the soundcard preamp.

Tony.
Attached Images
File Type: jpg DSC_7457.jpg (591.0 KB, 683 views)
File Type: png 3X1000uF_vs_1X1000uf.png (21.4 KB, 472 views)
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Old 18th May 2011, 09:12 PM   #20
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Tony, the amplification of this circuit is accomplished with only one op. amp. per channel.
So for an op.amp when we increase the gain from X10 to X100, we unfortunately divide the bandwidth by 10. This is the gain X bandwidth rule.

I breadboarded this non inverting gain block using TL074, C3,R5 and R6 only.

With
C3=10uF
R5=100k
R6=10k
Gain=X11
Bandwidth (-3dB): LF=1.5Hz, HF=340kHz

With
C3=10uF
R5=100k
R6=1k
Gain=X101
Bandwidth (-3dB): LF=15Hz, HF=34kHz (At 20kHz it is ~ -1.3dB)

With
C3=100uF
R5=100k
R6=1k
Gain=X101
Bandwidth (-3dB): LF=1.5Hz, HF=34kHz (At 20kHz it is ~ -1.3dB)

With
C3=10uF
R5=1M
R6=10k
Gain=X101
Bandwidth (-3dB): LF=1.5Hz, HF=34kHz (At 20kHz it is ~ -1.3dB)

The last implementation retains the low bass roll-off and you have to change only R5.

If the HF roll-off is OK for you, you are set.

If not , I think you have to go for a two stage amplifier

Regards
George
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