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Old 1st October 2012, 08:07 AM   #21
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Originally Posted by rcw666 View Post
The diode only lets positive going signals in, when this happens there is feedback the negative going signals go via the input and feedback resistors, in this case there is no feedback until the op amp output goes positive again.

This is about the simplest precision full wave rectifier you can make.
Very cool. Got some ballpark values?
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Old 1st October 2012, 08:31 AM   #22
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Originally Posted by phase_accurate View Post
The one function that showed the best result according to the AES articel was some exponential with an offset. From the complexity point-of-view this shouldn't be difficult to be done in analog circuitry. The enemies (as always when doing tricks with nonlinear analog circuits) are component tolarances and thermal runaway.

I will have a look at the paper again and will show a circuit that could be used but I will not do any fine-tuning etc.

The full-wave rectifier wasn't perceived as being too good in the JAES article.


Oh wow, thank you very much!
Thermal runaway shouldn't be too bad on a little battery powered device.
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Old 1st October 2012, 09:36 AM   #23
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I can post the design formula for the gadget, I will assemble them and post them soon.
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Old 1st October 2012, 11:36 AM   #24
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No circuits, but a lot of information on the techniques and the science behind psychoacoustic processing for simulating bass from tiny speakers:

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Old 1st October 2012, 04:40 PM   #25
Elvee is offline Elvee  Belgium
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Here is the tube circuit converted to jFET.
First pic shows the output at 30Hz (distorted)
Second pic at 1KHz (undistorted)

All the pots are centered. Other values can be tweaked too (corner frequency of lowpass, etc).

If you want to make a virtual test, LTspice can accept a wav file as input voltage, process it and write it to an output file.

The operation will be similar to the original tube circuit (ie. pretty crude)

The nominal voltage is around 0.5Vrms
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Old 2nd October 2012, 02:24 AM   #26
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If you put the three resistors in the rectifier, i.e. Ri, Rf, Rl, in the ratio 2:1:3, the result is equal gain for both directions, there might be some sonic advantage in asymmetry but I am not sure.

The Philips references do report that people seemed to prefer the second harmonic only scheme such as this although in theory the third should also be present.

There might be something gained by using a simple diode limiter to give some third harmonic, it is certainly worth the experiment.
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Old 2nd October 2012, 06:47 AM   #27
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I want to thank Don for the links. Interesting stuff, including a nice little summary of psychoacoustic concepts and research for us less-than-informed types.
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Old 2nd October 2012, 12:39 PM   #28
sreten is offline sreten  United Kingdom
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Probably the easiest way of doing this is :

Instead of a cheap, fairly nasty sounding crude analogue implementation,
create some MP3 files with the effect included with a good PC plugin.

Use the intended rock box as the PC sound monitor whilst doing this.
Whilst your at it also EQ the files and compress the dynamic range of
the bass and generally fiddle with the files to your hearts content.

Or :

You can get a chip described as a portable tone processor that boosts
and controls bass levels with a fixed 4dB treble boost, otimised it says
for portable drivers around 3". I'm pretty sure such a chip is built into
my computer speakers :

see Computer Speakers - what you can get for 25 USD or 15GBP

No real shenanigans here, low bass is cut-off and what harmonics
are already in the signal are boosted, driver excursion is controlled.

I don't know the exact chip used, but a search should find something.


If you further search on SRS or WOW chips, something like :

NJM2195L datasheet - SRS Wow Audio Processor

can be added, but I think you'll still need the first chip.

rds, sreten.

Last edited by sreten; 2nd October 2012 at 01:01 PM.
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Old 2nd October 2012, 01:03 PM   #29
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This circuit is a dynamic level control but no synthesis circuit. It might work quite wll though but it doesn't do what the OP intended.


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Old 2nd October 2012, 01:26 PM   #30
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I have a portable stereo that has two 3" drivers and uses a "speaten filter" IC to boost the bass and treble. I really haven't bothered to read the datasheet for the IC, but I don't think it is doing any sort of psychoacoutstics.

I don't know whether it's just the implementation or the drivers used, but to be honest this thing sounds pretty bad. Annoyingly "boomy" and the drivers reach xmax at even moderate levels because of the bass boost.

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