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Old 19th January 2010, 04:06 PM   #1
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Default Reinventing IcePower topology

Hi all

Lastly it stroke me that B&O IcePower do some tricks on their amps and I think I start to understand why, need some sanity check here.

First they advertise the tricks which are to my understanding:

1. Class BD full bridge - with some well known advantages and disadvatages compared to AD or half bridge

2. Nested feedback with local differentialling self-oscillating pre-filter loop and global post-filter voltage feedback. The differential characteristics of self oscillator together with output LC give a single dominant pole global loop (hopefully), which is something feedback loops do like.

BUT, assume one uses such a nested feedback arrangenent with half-bridge.
It won't work!!!

The global loop can and will take over the self-oscillation and switch into UcD mode at somewhere close to filter resonance frequency. The inner loop will get permanently overdriven in either direction so that it does not self-oscillate any more.
My point is that the nested feedback arrangement like this can ONLY work with class BD and balanced feedback (or withvBCA, which is patented by someone else), due to the fact, that the output does not see the switching ripple of the first harmonic, same for global feedback loop.
Any base-frequency switching ripple would immediately turn the whole effort in a global voltage ripple-based modulator (like UcD).

Need opinions on that, thanks in advance.
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Old 19th January 2010, 05:11 PM   #2
Eva is offline Eva  Spain
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Icepower is plain class AD, I have just removed a 1000ASP PCB from the heatsink and checked to be sure.

The inner loop produces the phase shifted self oscillation, but there are two op-amps on the way of the pseudo "triangle wave", which is not a good thing at all because it contains frequencies that two MC33078 can't handle properly. The outer loop drives the inner loop with gentle negative feedback for further linearization.

I think they use two feedback loops (one pre-filter and other post-filter) because they feared using the carrier residual on the output directly as a "triangle wave" as done in UcD. This requires a high quality carrier residual, which implies an high quality output filter with negligible parasitics, and ICEpower output filters exhibit high parasitic inductance on the (big) capacitors, not to mention the complete absence of ground planes and the compromised layout to reach such a small size.

Then Bruno demonstrated that it was possible to use the carrier residual on the output directly as a "triangle wave", and he did it routing the carrier residual directly to the comparator without any op-amps on the way, and using a differential approach. Bruno layouts use ground planes, and the modulator is in a spatial plane perpendicular to the power plane, so they are much better.

ICEpower comparator (LM319) is not wired in differential mode like in UcD, they rely on the MC33078 to add up both carrier residuals (pseudo "triangle waves") and then the signal goes to both sections of the LM319, one wired as inverting and the other as non inverting, to generate complimentary logic outputs and drive the two IR2010. This is not very clever.

I derived my own self oscillating modulator scheme from these two, which also uses two feedback loops, but both are post-filter...
My output filter has high C, low L and negligible series inductance on the capacitor. My carrier residual is sensed fully differential and goes to a TL3016 high quality comparator with only passives on its way. Only audio frequencies go through op-amps in my modulator, and all their inputs (from "outer world") have capacitors to ground, so RF rejection is very high. I use two poles, one passive and one active. ICEpower also seems to use two poles.

The resulting amplifier was just presented at NAMM last weekend... It's about to be released.

In general, you can safely and completely forget about ICEpower (and all their marketing hype) if you understand UcD, because UcD is ICEpower and a lot more, but made simpler (although it's useful to know the weaknesses of ICEpower to avoid repeating them)

EDIT: Another major difference from ICEpower to UcD is that ICEpower modulator does not damp output filter properly and relies on a 5 ohm resistor mounted on the heatsink for some damping (which imposes limitations on sustained trebble output), while UcD (and my approach) result in load-independent frequency response.
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Last edited by Eva; 19th January 2010 at 05:29 PM.
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Old 19th January 2010, 05:51 PM   #3
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Thanks a lot for clarifying that.
Do you know the number of how much global feedback they use ("gentle"<<30dB?)
Congrats for your amp!
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Old 19th January 2010, 08:03 PM   #4
luka is offline luka  Slovenia
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what was the design name, that was presented on namm
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Old 19th January 2010, 08:16 PM   #5
Eva is offline Eva  Spain
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Sorry, I don't have answers for pseudo-science questions
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Old 19th January 2010, 08:21 PM   #6
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Quote:
Originally Posted by Eva View Post
Sorry, I don't have answers for pseudo-science questions
Dear Eva,

You mean ME with the "pseudo science"??
The question was more opinion based than scientific....

Anyhow, back to Q1, was thet your Amp? what is the Brand???

cheers,

Tamas
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Old 19th January 2010, 08:45 PM   #7
Eva is offline Eva  Spain
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Yes, I meant you with pseudo-science. Sound is a loudspeaker thing

Concening the other question I can't tell much for the moment. I was just hired to design the amp, and my customer told me that they were at NAMM and they presented the product (preliminary prototypes) and many customers got interested due to the high power and compact size. I wasn't there, but I think there were another thousand companies presenting several thousand other products, and my customer only sells plate amps to high volume speaker manufacturers, so even if I told more, you have little chances to find more information or references on the web (or get the product once it's on sale). Sorry.


To darkfenriz:

Concerning ICEpower open loop gain, I don't know, but frequency response plots from datasheet tell that the outer loop is probably running out of OL gain around 20khz, and it's likely to increase at 12dB/oct below that frequency. The inner loop probably has some gain up to a higher frequency, but it's not very useful because it does not provide linearization or damping to output filter.

Have you seen this?
Sanyo/B&O Icepower 200AC Rev.D, schematic about rev. engin., not complete finished

This schematic seems to have several mistakes but it's still useful for learning.
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Last edited by Eva; 19th January 2010 at 08:57 PM.
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Old 20th January 2010, 06:40 PM   #8
Baldin is offline Baldin  Denmark
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Congratulations with the amp Eva (a picture wouldn't hurt )

Hadn't seen that (B&O schematic).
Seems thay are using 4. order output filtering ... didn't know that.
I have a hard time understanding why they use LM319 .... it's just so damn slow ....
I use TL1711 for my latest design. Rail to rail, damn fast and you can run the output as +-5V if desired. Only draw back is the size when hand soldering .... but it's doable

Wonder why thay want to be able to switch between different resistors in the feedback path ... does it have to be set according to PSU voltage or something??
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Old 20th January 2010, 06:53 PM   #9
Vibeguy is offline Vibeguy  United States
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I was at NAMM, saw the module.WOW.Stunning!

Superb design effort,Eva!!! congrats!!
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Old 20th January 2010, 07:02 PM   #10
Eva is offline Eva  Spain
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The 1000ASP that I have does not use 4th order output filtering, the output inductor has 4 sections but they are just connected as two series pairs. I think the schematic has some mistakes.

If you don't understand why they use LM319 (which is just one step ahead of LM393 and LM311, and is not that bad) compare prices, particularly for high volumes. They did a good cost reduction job. LT1711 seems crazy expensive.

btw: I use LM319 for current limiting for the same reason, it's dirt cheap and propagation delay with low overdrive voltages is not that bad.
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