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Old 6th May 2012, 11:12 AM   #31
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Here is an example. The DN2540 with 1W dissipation seems adequate, with a limit current of 10mA. The added 180R in series with the LED also stabilizes the LP corner. Read trace is LED voltage for the original version, orange is for the triple-LP version.
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Old 6th May 2012, 12:05 PM   #32
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For a fast treble limiter, maybe what you're looking for is just a diode clip circuit. A CFP can act as a very abrupt turn-on diode, which will cause less distortion before the clipping point. If the clipping is done before the amplifier itself clips, the gains may be significant even if it's not "soft".

You could also try an "ideal diode" such as the LTC4411.
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Old 7th May 2012, 03:36 PM   #33
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Quote:
Originally Posted by keantoken View Post
For a fast treble limiter, maybe what you're looking for is just a diode clip circuit. A CFP can act as a very abrupt turn-on diode, which will cause less distortion before the clipping point. If the clipping is done before the amplifier itself clips, the gains may be significant even if it's not "soft". You could also try an "ideal diode" such as the LTC4411.
I had the same basic idea but didn't test it yet. A schottky based hard clipper with resistors added for a not so hard clip makes this soft clipper:
<rectifierschottky<led, resistor
>rectifierschottky>led, resistor
Nicely independent function there, good because music audio is asymmetric (except for radio broadcasts run through phase scramblers).

Even with the tiny voltage drop and near instant switch on of the 1N5819, it would still be quite hard to trim this for precision by adding more or less diodes. Well, not difficult, but the fine tuning would take some time. May also need elastics added in series to the above (elastics = schottky//resistor) to mitigate abruptness and led variances.

Well, that was the idea. Perhaps you can add some elegance?
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Old 7th May 2012, 04:00 PM   #34
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Quote:
Originally Posted by keantoken View Post
Here is an example. The DN2540 with 1W dissipation seems adequate, with a limit current of 10mA. The added 180R in series with the LED also stabilizes the LP corner. Read trace is LED voltage for the original version, orange is for the triple-LP version.
Checking to see if I understood:
Replace the original 390R, 22u with:
120R, 22u; 120R 10u; 120R, 4.7u; pretty much as shown on your schematic?

This looks like you can use more capacitance (typically makes treble better, bass worse) without making the bass worse?

Could manufacture these near miss cap values:
10u//4.7u, 4.7u//2.2u, 2.2u//1u
14.7u, 6.9u, 3.2u
Would that be better?
I have a lot of electros, but not a lot of odd values.

P.S.
BAT85 rectifier? Wait per current before switch on function of a BAT signal schottky? I wasn't sure about the reverse voltage tolerance. The 1.4a high voltage silicon bridge rectifier (looks like a little round pill, costs 20 cents) has a "flexy" behavior similar to a schottky or at least close enough, so I used it.
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Old 7th May 2012, 04:13 PM   #35
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I still seems like the best way to achieve low distortion and good clipping behavior is to just use a diode clipper and underrate the amp so that under normal listening conditions the diodes don't half-conduct.

What do you mean by "soft"? I thought you meant "not abrupt", in a timing sense, as an abrupt clip will be crunchy. It could also mean nothing to do with timing, just a non-abrupt transfer function. The problem with a non-abrupt transfer function is that it neither completely clips, nor is completely turned off. So my vote is for a abrupt transfer function, but a good LP filter, so harmonics are limited to the bass and mid-bass. Simple diode limiter for transients that occur too fast.

Problem with a diode limiter is that it's not very abrupt, so your amp will usually clip before the diode limiter turns on... And you have to overrate the amplifier output voltage by twice or more to keep the diodes completely off.
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Old 7th May 2012, 04:23 PM   #36
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I just chose the rectifiers from my SPICE list, that one came from Cordell. As I recall, Schottkeys have lower breakover, but are less abrupt and have higher impedance at high currents. So I think silicon diodes are the best option for limiters. In an input limiter it is also important not to oversize diodes because the nonlinear capacitance can cause distortion depending on your source impedance. For the limiter rectifier though it shouldn't matter much.

The values of the caps aren't very important, just keep the general relationship. Actually I'm not sure if the value stepping matters here but I'm sure there is some economy to it. My reasoning is that C6 actually has 220R source resistance at LF, so it only needs to be half value. C5 has 330R, so 1/3 etc.
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Old 7th May 2012, 04:37 PM   #37
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Maybe we should consider the way we drive the LED. Depending on whether we use voltage or current drive, the LDR transfer curve will be some sort of log thing. Looking at this datasheet, it appears we want voltage drive, for logarithmic LED current, which will give logarithmic resistance, acting as a logarithmic volume limiter, which is what we need for audio.

http://www.silonex.com/datasheets/sp...pdf/104057.pdf

In this case a BJT buffer for the LED might be desirable so a series resistor is not needed to stabilize the LP corner. But then how to add current limit?
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Old 7th May 2012, 04:44 PM   #38
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The best approach is to use a micro-controller to evaluate input/output signals and set the amount of gain reduction dynamically. Electrically it's quite simple to implement: Just a PIC sensing input signal, output signal and rail voltage, and driving the LDR with PWM. Programming-wise finding good limiter control algorithms capable to adapt themselves to the signal being played is a little challenge.
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Old 7th May 2012, 05:16 PM   #39
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That's no fun, let's do it with an analog computer...
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Old 7th May 2012, 05:54 PM   #40
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Another schematic. This illustrates the benefits of a high-order LP filter. High order harmonics are much lower. This is the result of an input voltage that is a combination of 500Hz and 490Hz, which produces a sine wave modulated at 10Hz, perfect for exploring limiter behavior.
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