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Old 28th January 2013, 11:00 PM   #1061
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Sounds familiar Edmond. Happens to me all the time!

One thing I'm concerned about is reducing the input impedance. With an input capacitance of 2.5nF, a 100k volume pot at center position will result in a 2.5KHz LP filter! This improves to 25KHz with a 10k pot, which may be acceptable depending on who you talk to. C2 can bootstrap the input capacitance but can also make the amp a source-dependent resonator/oscillator. If the LP were moved to before the pot this would be no object, but do you think EMI could be an issue then? The LTP does need a minimal input decoupling cap so stability is not source-dependent. So perhaps very careful input bootstrapping is in order.
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Old 31st January 2013, 08:51 PM   #1062
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Quote:
Originally Posted by keantoken View Post
Sounds familiar Edmond. Happens to me all the time!

One thing I'm concerned about is reducing the input impedance. With an input capacitance of 2.5nF, a 100k volume pot at center position will result in a 2.5KHz LP filter! This improves to 25KHz with a 10k pot, which may be acceptable depending on who you talk to.

C2 can bootstrap the input capacitance but can also make the amp a source-dependent resonator/oscillator. If the LP were moved to before the pot this would be no object, but do you think EMI could be an issue then? The LTP does need a minimal input decoupling cap so stability is not source-dependent. So perhaps very careful input bootstrapping is in order.

Hi Keane,

Putting a pot directly in front of the input filter is asking for troubles. Not only in this particular case, but also with any other power amp equipped with an input filter. As you know, it's well known issue.

I think the best way to circumvent these issues is to use an active volume control, i.e. the volume pot in the feedback path of an op-amp. D. Self has done this, for example. See his 'precision pre-amp', Electronics World, Sept. 1996, pp.708..716. This way you get a sufficient low output impedance (and lower noise!)

Cheers,
E.
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Old 31st January 2013, 09:29 PM   #1063
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Requiring an active volume control however increases the parts count for a build of this amp. Buffering the +- inputs would make the circuit virtually impervious to source impedance. I have not found a way of doing this that doesn't mess up the aesthetic symmetry, but it can be done.

If there is a simple way to bootstrap a 2-pole input filter to reduce input impedance, the next barrier is that off component tolerances can ruin the gains. For instance if we bootstrap an input capacitance of 2.7nF, and the 2 caps have 5% tolerance, we have +-270pF range of input capacitance. Negative capacitance would be absolutely horrifying so we'd want to swamp it with another 270pF cap giving us a final input capacitance range of 0pF-540pF.

With LTP buffers we can solve this problem by using smaller input capacitors. This way with 5% tolerance can can achieve +-27pF input capacitance with a 270pF cap, and add a 33p cap to swamp it and give some margin for error. Now we have an input impedance of say 100k and 6p-60p input capacitance; compatible with any source and tolerant of smaller input caps. No one can complain of the input impedance being too high because you can just short the input with a 10k resistor or so.

I envision this adding 2 extra BJTs and several small passive components. Might be more economical than a whole new board and circuit just for impedance fixes; unless you already planned on a preamp in front for all inputs.

Last edited by keantoken; 31st January 2013 at 09:33 PM.
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Old 1st February 2013, 09:15 AM   #1064
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Hello Keaton,

What do you mean by "aesthetic symmetry".


Regards
Arthur
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Old 5th February 2013, 07:33 PM   #1065
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Hi Keane,

> I envision this adding 2 extra BJTs and several small passive components. Might be more economical than a whole new board and circuit just for impedance fixes; unless you already planned on a preamp in front for all inputs.


Why should I fix impedance issues of a pre-amp in the next link of the audio chain, i.e. the power amp? Any decent signal generator is supposed to exhibit a sufficiently low and constant output impedance (50 Ohms or so). IMHO, the same shall apply to pre-amps.

As for volume pots in the FB path, please read this: Bruno Putzeys r4 random rants, raves and ramblings

Cheers,
E.
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Old 11th February 2013, 07:43 PM   #1066
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By symmetry I just meant the appearance of the schematic.
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Old 26th March 2013, 02:19 AM   #1067
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Default Return Ratio in the SuperTIS

Hi Edmond
Looks like you are back on the the forum so I hope your PC problems are sorted.
I want to better understand some of your comments on your SuperTIS website.
You mention a few times the need to lower the current gain to keep loop stability. Why the emphasis on current?
The Return Ratio is just a ratio, the same for current or potential.
For instance a cascode can increase the RR even with no current gain, yes?
It seems to me that power gain could be a more useful metric but I have not seen this approach used. I have seen a few references to "non ergic" feedback that seem to imply this perspective but they are a bit obscure.
Don't have a specific question, more just want to clarify my own ideas and learn your perspective.

Best wishes
David
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Old 26th March 2013, 11:53 AM   #1068
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Hi David,

I just scanned my website on 'current gain'. I found it two times, it's about the TIS itself and the current gain is just 1x. I only mentioned this, because it is much much lower than a conventional TIS. But you are right, this is only part of the story, as the power gain is much higher than 1x.

>the need to lower the current gain to keep loop stability
I couldn't find it. Could you tell me where exactly I said that?

/OT
As for my PC, it's on the air again, though not fully completed. The cooling needs a finishing touch, for example. I finally found a heat sink that really makes sense: a Silverstone NT01 pro. Why this one? Well, you can mount it in such a way that the fans are seated between the cooling ribs and the rear of the PC housing. With this arrangement the fans suck cool air from the outside, instead of just circulating warm air inside PC cabinet, as most other fans do.
Also the triple boot system (XP/W7/W8) is not ready yet. I need three OSes to test DiAna. BTW, I also tested a couple of sound cards, a Lynxstudio L22, for example. What a delight! Rather expensive, but worth every penny. At the other side, I also tested some cards from Creative Labs and ASUS (also designed by Creative). Far more distortion and spurious noise. Also the ASIO drivers don't work. The size of the software is enormous and slows down your PC. What a disgusting bloatware. Please don't touch it!

Cheers,
E.
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Last edited by Edmond Stuart; 26th March 2013 at 11:55 AM.
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Old 26th March 2013, 01:22 PM   #1069
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Quote:
Originally Posted by Edmond Stuart View Post
I just scanned my website on 'current gain'. I found it two times, it's about the TIS itself and the current gain is just 1x. I only mentioned this, because it is much much lower than a conventional TIS. But you are right, this is only part of the story, as the power gain is much higher than 1x.

>the need to lower the current gain to keep loop stability
I couldn't find it. Could you tell me where exactly I said that?
I paraphrased, but the sections on Super_TIS.HTML that I meant

To maintain sufficient stability a redistribution of gain stages is required This is done by:

1. Replacing the ubiquitous VAS (with a gain of beta or even beta^2) by a cascode (with a current gain of 1x) which is directly connected to the current mirror,


"VAS... gain" is clearly current gain. So a lower current gain to maintain loop stability.

2. Putting a pre-driver after the cascode, which restores the gain of the whole chain, see fig.3a.

Current gain implied since that is what a pre-driver does.

Although the current gain of the TIS is just 0dB, the gain-bandwidth-product of the stages enclosed by the Miller loop is still too high.

Since the VAS has been replaced by a cascode having a current gain of just 1x,

It should be noticed that the current gain and gm are still rather low.

Therefore, it's highly recommended to compensate for the lack of gain by means of a pre-driver
Current gain implied.

Best wishes
David
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Old 26th March 2013, 02:51 PM   #1070
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Hi David,

Now I understand what you mean Thx.
>You mention a few times the need to lower the current gain to keep loop stability. Why the emphasis on current?

It's a long time ago. Probably because it was easier to manipulate the current gain instead of the voltage gain. Another possibility might be the Tian gain probe showed that it was (mainly) current which determined the RR.

>It seems to me that power gain could be a more useful metric but I have not seen this approach used.

The point is that the available power gain is seldom use (at least AF amps that don't make use of transformers). It's either voltage or current that's transferred from the one stage to the next stage. So I'm not sure whether power is a more useful metric. As for HF and UHF, it's a different story, of course.

Cheers,
E.
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