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Old 21st April 2014, 02:36 PM   #1
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Default SystemD LiteAmp

In March a friend asked me if I would like to design something simple
with the IRS2092. First I was not enthusiastic, because I had doubts that
it would work to achieve something better than the IRAUD7 and its clones.
Better means: Improved speaker control (==> post filter feedback), less noise and at least not worse distortion.
Nevertheless I had look into the topic.
My first attempts were fitting more or less to my expectations.
Good control of the filter and speaker, blameless step response, but harmonics in the -70db category all over the place. Ugh..
Also the gap between simulation and reality was to big for my taste.
Operating frequency and distortion figures did not fit sufficiently.
I digged deeper into the non documented weaknesses of the IRS2092.
Key points:
Transfer function of the OTA
Noise of the OTA
Jittery / ambitious flipping of the comparator
Internal delay times

After all this I updated my simulation model of the IRS2092 and soon came to
a changed structure, which behaves fine in simulation and reality.
Can't tell an official name for the structure.
With a good portion of phantasy we could call it a post filter feedback sigma delta modulator.
Above 60kHz F1(s) and F2(s) together are acting like an integrator for the 1Bit data stream coming from the half bridge...

In the mean time the results by far surpass the IRAUD7, but the complexity is very similar.
Step response is blameless with rise times below 5us, output noise is below 100uVrms, very low THD and nice dual tone behavior.
It is promising enough that I intend to go for a proper PCB.
Attached a scheme of the control structure and some measurements at 10W into 4R.
Attached Images
File Type: jpg Breadboard.jpg (215.5 KB, 1289 views)
File Type: jpg LiteAmp_ControlStructure.JPG (234.7 KB, 1274 views)
File Type: jpg 10W_4R_1kHz.JPG (60.9 KB, 1207 views)
File Type: jpg 10W_4R_DualTone.JPG (62.3 KB, 1132 views)
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Old 21st April 2014, 05:37 PM   #2
astx is offline astx  Austria
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Looks very interesting! Especially post filter feed back idea! How is cross channel intermodulation avoided - or is there a master clock to drive e.g. 4 channels from same clock source?

I have build some time ago a simple 4 channel class D amplifier based on TDA8950j, but this chip has some really noticeable THD problems at about 5 - 7 kHz. So I decided to give IRS2092 a trial and bought some IRS2092 (DIP case) and couple of IRFI4019. But haven't had time to finish the project based on IRAUDAMP examples.
As a silent reader of your Class D projects I think you have the skills to get the best out of IRS2092, maybe more as the IRF test samples.
Keep on going.

BR Toni

P.S.: this was my project with central timing clock to avoid cross channel intermodulation products: 4 Channel BTL 300W 8R TDA8950j power Amplifier
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Old 21st April 2014, 08:58 PM   #3
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Quote:
Originally Posted by astx View Post
How is cross channel intermodulation avoided - ...?
It is not avoided.
But note: The intermodulation results in a product which is an amplitude modulated HF. Without amplitude demodulator it remains inaudible like the carriers themselves.
The only realistic way to make it audible would be to operate in bridged mode and use a very non linear tweeter acting as demodulator. ...not sure where, but somewhere here on DIYaudio I had a seperate thread for this - before I decided to give self oscilating topologies a chance.
Besides the AM products, poor implementations of self oscilating amplifiers may suffer from direct interference, typically caused by short comings in layout and/or wiring.

In real life I never had trouble with any of the above mechanisms.
Furtheron I am listening since half a year with pleasure to a self oscilating design without any issues.
So in most cases the simplicity of self oscilating designs is winning the race.
Especially for this project - which is intended to be simple.


Quote:
Originally Posted by astx View Post
P.S.: this was my project with central timing clock to avoid cross channel intermodulation products: 4 Channel BTL 300W 8R TDA8950j power Amplifier
...no matter if you are dissatisfied with the THD, but the build is pretty cool !
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Old 22nd April 2014, 02:38 PM   #4
ungie is offline ungie  Canada
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Very interesting ChocoHolic! I firmly believe that there is much untapped potential in the IRS2092...it has many excellent characteristics but is somewhat of a letdown when we start looking at the basic measurements. I am hoping you will detail this thread in the same manner as your other excellent threads...I'm very interested to know how the feedback characteristic is determined in order to adjust the parameters for a given application, even if this requires some first-principals discussion of feedback as applicable to self-oscillating Class D.
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Old 22nd April 2014, 06:41 PM   #5
astx is offline astx  Austria
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Do you mean it would be no problem using bridging with your new design - or should bridging strictly be avoided?

Would it be possible for you to share the ltspice files during your design process so we can learn more in depth?


Quote:
Originally Posted by ChocoHolic View Post
... but the build is pretty cool !
THX!

BR, Toni
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Old 22nd April 2014, 07:00 PM   #6
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Quote:
Originally Posted by ChocoHolic View Post
In real life I never had trouble with any of the above mechanisms.
In fact what I see in reality more is that multiple self oscillating systems in close proximity have the strong tendency to auto sync. No matter if we are talking about SMPS or class D amps any other self oscillating power electronics.


Quote:
Originally Posted by ungie View Post
...much untapped potential in the IRS2092...it has many excellent characteristics but is somewhat of a letdown when we start looking at the basic measurements.
Which specific properties do you see as main 'letdown' of the IRS2092?

Please be aware that in this particular thread I will be more reluctant in disclosing every detail. The combination of low complexity and high grade results is something which I would enjoy to see first on many benches of passionated DIY enthusiast before seeing it stolen and sold on ebay by the commercial copy cats.
So I am considering to provide kits of key components + BOM to hobbyists on a non profit basis. Depends on forum resonance.
The shown breadboard is a low power version using two paralleled IRFI4212, running from +/-40V and providing 250Wrms into 2R or bridged 500Wrms into 4R.
Originally I intended to stick to this, because the 100V MosFets have great body diodes, while higher voltage types with poor body diodes for sure will degrade the achievable distortion level to some extend.
But seeing the nice results it would be worth to dig into a version with two paralleled IRFI4020, running from +/-70V and delivering 400Wrms into 4R or bridged 800Wrms into 8R. Should be possible with still reasonably good results, even if not as extremely low THD like the small one.

Which version would be of higher interest?
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Old 22nd April 2014, 07:07 PM   #7
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Quote:
Originally Posted by astx View Post
Do you mean it would be no problem using bridging with your new design - or should bridging strictly be avoided?
You can expect trouble free operation also in bridged operation.
My last design, the 2kW one, also is fully self oscilating and runs fine in bridged mode. So I am not afraid of this.
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Old 22nd April 2014, 07:30 PM   #8
astx is offline astx  Austria
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Quote:
Originally Posted by ChocoHolic View Post
...
Which version would be of higher interest?
Starting with the lower power version (bridged 500W@4R or 250W@8R) would be fine. Think I could reuse my IRFI4019 also for this version ...
Hopefully you are doing a mostly through hole design?

BR, Toni
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Old 22nd April 2014, 09:07 PM   #9
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Being a great fan of free oscillating, post-filter-fb class-d, I am looking forward to your design. Hau rein, Choco!
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Old 24th April 2014, 07:39 PM   #10
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Using IRFI4019 instead of 4012:
In order to avoid increased idle losses and worsened idle EMI you will need to readjust DT and live with slightly higher distortion (some more advanced options for additional adjustment of the gate drive in order to reoptimize distortion...)
Also you will have to accept the fact that the current capability of the IRFI4019 is not sufficient for reliable operation into 2R (or bridged into 4R) at high levels ==> shut down expected when pushing hard.

Mostly pin through hole components:
Agreed. Some few SMD in key positions, but just few of them and nothing smaller than 0805.
IRS2092 is clearly planned to be DIP.

Noise:
The IRS2092 has a noisy OTA and a jittery flippy comparator.
Data sheet noise values translate to an equivalent input voltage noise density of 30nV/sqrt(Hz),
and my personal OTA measurements are pointing even to 50nV/sqrt(Hz).
Output noise measurements on the CDA-224 (somewhere between 200uVrms – 250uVrms) show that it is close to the theoretical limit , but still by factor 2-3 above my personal limit for amps, which are intended to be used in small living rooms with normal speakers (say 90..95db/W @ 1m).
Amps like the LD25 with even more gain and a single layer layout are no surprise, when showing 400uVrms-500uVrms output noise.
The only way to achieve low output noise (Max: 100uVrms. Goal: 70uVrms.) while using such a noisy input stage is to drive the input with large signals.
Already the OTA of the IRS2092 is generally too noisy to achieve good results directly from a 0db line level.
So the power amp of the LiteAmp is designed for input levels above 2Vrms for full output power. We could consider even higher input levels, until the point where the noise does not decrease anymore because of the flippy comparator....
An optional daughter board with a high end instrumentation amplifier gain stage (noninverting/inverting/symmetric) is part of the project. This daughter board also will give freedom to all DIYers for custom experiments with different OP amps or input caps or DC coupling.

Last edited by ChocoHolic; 24th April 2014 at 07:42 PM. Reason: typo: changed noise into not
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