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Old 13th April 2012, 09:26 AM   #101
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Default Noise

Hi David,

I think it's feasible to get as low as 2nV/rt Hz if you lower the source resistors to 50 Ohms. But keep in mind that it also has an effect on ULGF. So you have to adjust the compensation stuff as well.

Regarding the noise of the current mirror degen. resistors, not much is gained if you make them much larger than 22 Ohms. In the extreme case of 1kOhms, the noise will drop by about 0.17 nV/ rt Hz (depending on the amount of noise from other sources, of course).

BTW, what about the noise of your pre-amp and the volume pot?

Cheers,
E.
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Old 13th April 2012, 12:18 PM   #102
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Edmond Stuart View Post
what about the noise of your pre-amp and the volume pot?
One ensures adequate, but not excessive, system gain for each of the alternative (frequency) channels.
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Old 13th April 2012, 01:37 PM   #103
PHEONIX is offline PHEONIX  Australia
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Default TIS with simple BJT output THD Sim

Hello Edmond

I did some some THD simulations with circuit attached and into 8R 20Khz I achieved 0.002% THD at 80V PP.

I must confess this seems to me a little bit high with the TIS input stage. Have you done sims with a simple output stage what sort of results do you get .

Regards
Arthur
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File Type: png TIS input _BJT ouput.png (59.3 KB, 417 views)
File Type: png TIS input _BJT ouput THD.png (55.3 KB, 410 views)
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Old 13th April 2012, 02:32 PM   #104
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Default THD20k = 20ppm

Hi Arthur,

I'm not surprised, as I got similar results with a BJT OPS.
This TIS is especially designed for the AB2 OPS with MOSFETs.

edit: Remember, compared to more conventional amps, your circuit has one gain stage less, i.e the VAS.

Cheers,
E.
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Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)

Last edited by Edmond Stuart; 13th April 2012 at 02:45 PM.
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Old 14th April 2012, 04:33 AM   #105
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Quote:
Originally Posted by Edmond Stuart View Post
Hi David,

I think it's feasible to get as low as 2nV/rt Hz if you lower the source resistors to 50 Ohms. But keep in mind that it also has an effect on ULGF. So you have to adjust the compensation stuff as well.

Regarding the noise of the current mirror degen. resistors, not much is gained if you make them much larger than 22 Ohms. In the extreme case of 1kOhms, the noise will drop by about 0.17 nV/ rt Hz (depending on the amount of noise from other sources, of course).

BTW, what about the noise of your pre-amp and the volume pot?

Cheers,
E.
2 nV/rt Hz is acceptable - just! If we have the distortion down to -160dB we may as well make the noise at least inaudible!
If I worked it correctly, the best way to analyse the CM resistor noise is not to consider the resistance itself but the voltage across it. Sort of like "quiescent current" in the OPS is better is expressed as the Oliver criterium of so many millivolts on the emitter resistors. Ideally one wants a couple of volts across the CM resistors to make any further improvements of CM noise trivial.
So we know what impact on output volts.
But then, as you point out, there's the other stuff. The pre-amp is the _next_ project! Volume control by manipulation of the feedback loop perhaps. Need to study this stuff too.

Thanks
David

Last edited by Dave Zan; 14th April 2012 at 04:53 AM.
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Old 14th April 2012, 05:11 AM   #106
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Originally Posted by Edmond Stuart View Post
Thanks, David.


I'm glad we share the same 'instinct'


BTW, 20mA is a lot of current for an IPS; trannies will get pretty hot.


Cheers,
E.
20 mA would be a bit hot without an IPS cascode, but since your circuit has one I may as well use the lower drop across the diff. pair. The extra current in the Baxandall pairs is about optimum to maximize their Ft and that also seems the correct direction.

In answer to your actual question- No one has mentioned the AD797. Is this not a bit similar? I have only ever seen simplified circuits of that op-amp and they had no Baxandall pairs but it has plenty of open loop so some of the schematic "transistors" are perhaps physical pairs.

This is a repost of an earlier attempt that seems to have failed. Sorry if a bit late.
David

Last edited by Dave Zan; 14th April 2012 at 05:26 AM.
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Old 14th April 2012, 05:25 AM   #107
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Quote:
Originally Posted by PHEONIX View Post
Hello Edmond

I did some some THD simulations with circuit attached and into 8R 20Khz I achieved 0.002% THD at 80V PP.

I must confess this seems to me a little bit high with the TIS input stage. Have you done sims with a simple output stage what sort of results do you get .

Regards
Arthur
Quite apart from possible benefits to the diff. pair, would it help to increase the Baxandall pair current? Will sim this when I can but it seems this may increase its ability to drive the OPS?

Best Wishes
David
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Old 14th April 2012, 08:54 AM   #108
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Quote:
Originally Posted by PHEONIX View Post
Hello Edmond

I did some some THD simulations with circuit attached and into 8R 20Khz I achieved 0.002% THD at 80V PP.
[...]

Regards
Arthur
Hi Arthur,

Perhaps you get better results if you connect the compensation cap(s) to the emitters of the pre-drivers. See fig.2 on my website.

Cheers,
E.
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Old 14th April 2012, 09:38 AM   #109
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Default AD797

Quote:
Originally Posted by Dave Zan View Post
[...]
In answer to your actual question- No one has mentioned the AD797. Is this not a bit similar? I have only ever seen simplified circuits of that op-amp and they had no Baxandall pairs but it has plenty of open loop so some of the schematic "transistors" are perhaps physical pairs.
[...]
David
Hi David,

Although I wasn't inspired by the AD797, it just happened that basically both circuits share indeed the same philosophy: only two stages with high gain and folded cascodes. On the other hand, there are also big differences: apart from Baxandall super pairs, no floating current mirror and a totally different frequency compensation.

>I have only ever seen simplified circuits of that op-amp
See below for more details of the AD797. Regrettably I don't have accurate models of the transistors.

Cheers,
E.
Attached Images
File Type: png AD797SC.png (26.3 KB, 351 views)
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Old 21st April 2012, 12:18 PM   #110
miralin is offline miralin  Russian Federation
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Hi Edmond
Congratulate you with inventing and introducing relatively simple circuit with superb linearity.
I think it can become a standard for the years.
But why do you call it “transimpedance”? Common base stage is not transimpedance, even if you use a super pair instead of a single transistor, and the whole circuit is definitely transconductance (98 mA/V).
IMHO, it can be called something like “an LTP loaded with current mirror fully cascoded with super pair in a complementary configuration”, where “fully cascoded” means your combination of conventional and folded cascoding, that I’ve never seen before.
This long name is inconvenient, but shows the way to define what each component of the whole circuit gives to excellent performance.
1.We can exclude “complementary configuration” by connecting bases of Q1 and Q6 to the ground (I mean Fig.1 on http://home.tiscali.nl/data.odyssey/Super_TIS.html).
2.We can exclude “fully cascoded” by connecting collectors of Q5 and Q6 to +50v and -50v.
3.We can exclude “super pair” by connecting bases of Q10 and Q11 to V4 and V5 (1.5v).
4.We can make combinations of 1,2,3, each time measuring THD20k.
Would you like to do this?
With best wishes – Mir
PS. A short name may be “Stewart Audio Front End” (SAFE) – like “Baxandall Super Pair”. S may also mean “super”.
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