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Old 27th October 2003, 09:46 AM   #181
Electrons are yellow and more is better!
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Default Re: LM317s and level shifting

Quote:
Originally posted by Dave S
ALW in his application note states that decoupling the ref pin on the LM317 pre-reg is a must do (BTS!!).
It's very normal in general to have a cap in the feedback network of a LM317. One of the first thing mentioned in the datasheets.
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Old 27th October 2003, 10:16 AM   #182
Dave S is offline Dave S  United Kingdom
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Default Bts

I agree it's normal to have the LM317 ref decoupled, my point was that if this makes such a big difference to the sound quality for the whole regulator then something downstream is sensitive to this change - and I think this is where Fred is coming from.

Dave

BTS - Bye, Tata, Seeyou.
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Old 27th October 2003, 12:19 PM   #183
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Since we are sliding into the extreme department, has anyone tested LTZ1000 as a reference?
http://www.linear.com/prod/datasheet.html?datasheet=141

But then I'll guess it's time to change the opamp to something like AD797+MAX420 combo or LT1028+MAX420
Page 16 here
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Old 27th October 2003, 04:13 PM   #184
WaltJ is offline WaltJ  United States
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Default Isource improvement(s)

Quote:
I replaced the 10K resistor bias for the current source LED with a jfet current source. This should result in about a 45dB improvement in LED voltage change with respect to input voltage change. Replacing the 10K with two 5K resistors with a cap to ground might be an even better approach.
These comments are most germane to the Audio Electronics 4/2000 article, and also indirectly to Andy Weekes' (ALW) PCB implementation upon which Fred D. is commenting above.

Shortly after the AE 4/2000 article was published, I received an email from Russell Twining, who suggested an alternative of essentially what Fred D. tried to say above (but Fred got it got wrong). Russell suggested that instead of an LED a 2.5V LM336 could be used. I agree with this point entirely, it would be a better solution that making the 10k feed resistor a JFET Isource (expensive!). Note that the ED 6/23/1997 regulator design referenced in post #350 uses this approach, with a 1.2V diode.

Yes, one could say that the current source circuits as published in AE 4/2000 have a weakness. I did in fact allude to this in the article, saying it is difficult to improve with a simple fix which does *not* mod the PCB. The alternate step of the 2.5V or 1.25V reference diode is quite worthwhile, and can be implemented without any PCB surgery to the Didden PCB, or the ALW PCB. It does leave the Early effect of the current source transistor(s) as a remaining error.

Russell also suggested that bypassing of a split 10k feed resistor (into two equal values), with a capacitive bypass from the split node to the Vin rail (not ground) would be another improvement. I agree with this as another possibility, unfortunately one not compatible with the Didden PCB which was the environment of the AE 4/2000 article.

Because of all of this, and the fact that I was attempting to make the new electrical design as close to a "zero PCB change" as would be possible, I took the approach of the prereg as a next-best system solution. Use of the prereg renders any imperfections in the Isource such as Fred D. has targeted rather moot.

If anyone implements the prereg, I think they'll agree it sidesteps all of the above issues. Plus, it also fixes the Vcb of the pass transistors, which is also worthwhile.

wj
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Old 27th October 2003, 05:19 PM   #185
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Default Re: Isource improvement(s)

Quote:
Originally posted by WaltJ


These comments are most germane to the Audio Electronics 4/2000 article, and also indirectly to Andy Weekes' (ALW) PCB implementation upon which Fred D. is commenting above.

Shortly after the AE 4/2000 article was published, I received an email from Russell Twining, who suggested an alternative of essentially what Fred D. tried to say above (but Fred got it got wrong). Russell suggested that instead of an LED a 2.5V LM336 could be used. I agree with this point entirely, it would be a better solution that making the 10k feed resistor a JFET Isource (expensive!). Note that the ED 6/23/1997 regulator design referenced in post #350 uses this approach, with a 1.2V diode.

Yes, one could say that the current source circuits as published in AE 4/2000 have a weakness. I did in fact allude to this in the article, saying it is difficult to improve with a simple fix which does *not* mod the PCB. The alternate step of the 2.5V or 1.25V reference diode is quite worthwhile, and can be implemented without any PCB surgery to the Didden PCB, or the ALW PCB. It does leave the Early effect of the current source transistor(s) as a remaining error.

Russell also suggested that bypassing of a split 10k feed resistor (into two equal values), with a capacitive bypass from the split node to the Vin rail (not ground) would be another improvement. I agree with this as another possibility, unfortunately one not compatible with the Didden PCB which was the environment of the AE 4/2000 article.

Because of all of this, and the fact that I was attempting to make the new electrical design as close to a "zero PCB change" as would be possible, I took the approach of the prereg as a next-best system solution. Use of the prereg renders any imperfections in the Isource such as Fred D. has targeted rather moot.

If anyone implements the prereg, I think they'll agree it sidesteps all of the above issues. Plus, it also fixes the Vcb of the pass transistors, which is also worthwhile.

wj
Walt, hi,

One change I have tried and also described in a post somewhere up north was the replacement of the entire input current source by a JFET current source. In other words, get rid of the LED, the PNP and related resistors and connect a FET CS directly from raw input (or after the prereg if implemented) to the pass transistor base. This improves the input ripple/noise rejection manyfold. But I found that a 1 or 2 mA CS was not enough to obtain good regulation above 100mA load or so. I had to look for a CS spec'ed at 5.6mA, that worked fine. As I noted earlier, these CS'es need some 5+ V across them so they ruin your dropout voltage. Still, it is an elegant and worthwhile mod if you have the extra input voltage. And it wouldn't need a PCB change I think.

I thought that this was also what Fred meant, but I now see that this was no so. Anyway, any comments from your side?

Jan Didden
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Old 27th October 2003, 06:58 PM   #186
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Default Ain't no mountain high enough

A cap to Vcc for a voltage for an RC filter for the current through The LED is of course correct, which will be quickly apparent when one draws the schematic. I do think the different approaches with the jfet current source and LM336-2.5 are worth examining. A green LED biased at 0.4 mA has an AC impedance or about 100 ohms and a voltage drop of about 1.8 volts. The J502 has a typical impedance of 7 megohms. the ratio 7e6 / 100 is about 97 dB of rejection.

The 0.4 ohms impedance of the LM336 with a 10 K resistor gives about 88 dB of rejection. This is further compromised by fact the impedance of the LM336 starts to rise above 1 KHz as it's feedback falls off. The rejection is then decreasing by 20 dB per decade. I believe the voltage noise is also a little greater with the LM336 than with the LED. And of course the drop out voltage becomes slightly higher. These are still outstanding numbers and certainly a very valid approach that may be cheaper and easier to find parts for. No PCB surgery should be necessary for either approach. Differences in approach mainly those of preference for design with ICs or with discrete transistors, and there is certainly room for both in audio design.

There are many paths up the mountain. Walt has led 95 per cent of the way for the development of this very sophisticated regulator design. We are all extremely thankful for his fine work. I truly appreciate his input. Post like this indicate why he is such a welcome addition to the forum. We eagerly look forward to his further insight into circuit design. I feel sure he would not begrudge anyone the satisfaction of choosing his own path for the last few hundred yards to the summit.

Respectfully,

Fred
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Old 27th October 2003, 07:24 PM   #187
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Default Re: Ain't no mountain high enough

Quote:
Originally posted by Fred Dieckmann
A cap to Vcc for a voltage for an RC filter for the current through The LED is of course correct, which will be quickly apparent when one draws the schematic. I do think the different approaches with the jfet current source and LM336-2.5 are worth examining. A green LED biased at 0.4 mA has an AC impedance or about 100 ohms and a voltage drop of about 1.8 volts. The J502 has a typical impedance of 7 megohms. the ratio 7e6 / 100 is about 97 dB of rejection.

The 0.4 ohms impedance of the LM336 with a 10 K resistor gives about 88 dB of rejection. This is further compromised by fact the impedance of the LM336 starts to rise above 1 KHz as it's feedback falls off. The rejection is then decreasing by 20 dB per decade. I believe the voltage noise is also a little greater with the LM336 than with the LED. And of course the drop out voltage becomes slightly higher. These are still outstanding numbers and certainly a very valid approach that may be cheaper and easier to find parts for. No PCB surgery should be necessary for either approach. Differences in approach mainly those of preference for design with ICs or with discrete transistors, and there is certainly room for both in audio design.

There are many paths up the mountain. Walt has led 95 per cent of the way for the development of this very sophisticated regulator design. We are all extremely thankful for his fine work. I truly appreciate his input. Post like this indicate why he is such a welcome addition to the forum. We eagerly look forward to his further insight into circuit design. I feel sure he would not begrudge anyone the satisfaction of choosing his own path for the last few hundred yards to the summit.

Respectfully,

Fred

Fred, what about a LM336 and the JFet CCS?
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Old 27th October 2003, 10:16 PM   #188
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Folks, sorry to break in, without checking up on all previous inputs. First of all, what Fred just put here is a typical current source used by designers for at least the last 25 years. Yes, it works. It is also very good.
When 'optimizing' a regulator, several factors must be thought through.
1. What do you want it to do?
2. How noisy is it?
3. What is its practical output impedance? Over frequency?
4.What is its transient response, both to load changes, and source changes?
There may be many other factors that I cannot think of just now.
When you change op amps, you change many of these factors.
Please keep this in mind.
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Old 27th October 2003, 10:36 PM   #189
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JC> What is its transient response... to load changes

One thing that "bothers" me about many published regulators:

Where is the pull-down?

If you hang a resistor or light-bulb on them, they only have to source current. If the load turns off, they just have to stop sourcing very quick.

But if you hang a choke-loaded stage on a source-only regulator, and clip it, the load "kicks-up" and the regulator can't do a thing about it.

That's rare (though on another forum everybody is hanging transformers on mike-amps). But very short "kick-ups" happen in high-speed circuits. The kick alone may not be a big deal, but if the regulator freaks-out and does not recover quickly, it turns a short kick into a longer bobble.

Sure, we can suppress such kicks with a capacitor. But then why not suppress everything with a cap? Call me obsolete: I still like 4,700µFd caps. I use regulators only to work close to chip limits; but it is amazing how many of my chips have run on UNregulated +/-17V for years.
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Old 27th October 2003, 10:44 PM   #190
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Hi Paul,

Quote:
The kick alone may not be a big deal, but if the regulator freaks-out and does not recover quickly, it turns a short kick into a longer bobble.
That kind of behaviour is to be expected but:

Do you think this would actually happen in a real life condition?

Not too often IME...

Quote:
Call me obsolete: I still like 4,700µFd caps.
Believe me, you're not the only one.


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