[sdiy] Panning with LM13700

Richie Burnett rburnett at richieburnett.co.uk
Tue Jan 10 11:48:40 CET 2017

Tom,  You might even consider doing the panning with analogue-switches as 
"VCAs" driven directly from the PWM carrier signals, depending on your 
requirements.  I normally hate the use of PWM'd analogue switches as VCAs 
because of dynamic range limitations re small pulse lengths, resolution vs 
carrier freq tradeoff etc, etc.  But for panning you don't really need much 
dynamic range.  Once you're panning something strongly to one side it really 
doesn't matter much perceptually whether the level in the opposite channel 
is -40dB or it is completely off.  You will just perceive the sound as 
coming at you from the side.  You only need a few specific attenuation 
settings for each channel, especially if you're panning based on a 7-bit 
MIDI CC value!


-----Original Message----- 
From: Matthias Herrmann
Sent: Tuesday, January 10, 2017 9:45 AM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Panning with LM13700

Same here.

We adopted the AN120 for 12V operation and used it for a 4U Stereo Mixer 
(ala Serge).

After some simulations and real life testing we were able to go with just 
the center trimmer, using 1% resistors. However, we used it with the nice 
pre-trimmed THAT2180 VCAs.

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Tom 
Sent: Tuesday, January 10, 2017 10:05 AM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] Panning with LM13700

Yes, I agree -- THAT notes often need some detailed consideration - they use 
some absolutely bizarre resistor values.

I actually used the AN120 idea basis with V2164 VCAs - took quite a lot of 
multisim investigation first and then some physical prototyping afterwards.

On 09/01/2017 21:29, microtonalsynthesis at yahoo.com wrote:

Like many app notes, the THAT AN120 is something of a DIY project in itself.

The control voltage inputs are reversed. U1 should use EC+ pin 2 and U2 
should use EC- pin 3.

The breakpoint generator is very sensitive to offsets and the circuit as 
given has unequal volumes for left and right sides. Adjustment of all 
resistors going to -15V is needed. I estimate about 130.5K for the 133K. Of 
course, if you adapt this circuit to 12V supplies then you need to adjust 
some more.


On Sunday, January 1, 2017 6:24 PM, Richie Burnett 
mailto:rburnett at richieburnett.co.uk wrote:

You could always generate the two VCA control signals with the appropriate 
pan law using a micro... Low cost pic with dual output PWM perhaps? 
Pre-calculated values in lookup table on a low end micro, or use  quadratic 
fit approximation if you're already using a dspic.


Sent from my Xperia SP on O2

---- Tom Wiltshire wrote ----

>That's a fairly involved circuit, but I don't think I've ever seen an 
>analog panner with a switch to select different pan laws. Very impressive!
>I'll have a look over it and see if I couldn't use the gist of it in a 
>simpler way.
>On 1 Jan 2017, at 20:14, Tom Bugs <admin at bugbrand.co.uk> wrote:
>> Not LM13700 based, but the details within the THAT app note here may be 
>> some use:: http://www.thatcorp.com/datashts/dn120.pdf (VCAs in pan pot 
>> apps)

>>>>> -----Ursprüngliche Nachricht-----
>>>>> Von: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] Im Auftrag von 
>>>>> Tom
>>>>> Wiltshire
>>>>> Gesendet: Sonntag, 1. Januar 2017 18:07
>>>>> An: Synth-diy at synth-diy.org
>>>>> Betreff: [sdiy] Panning with LM13700
>>>>> Hi All,
>>>>> Happy New Year to everyone! Here's hoping you all have a great year 
>>>>> with
>>>>> many (completed) synth projects.
>>>>> I've got  quick question - what's the best way to do equal power 
>>>>> panning (-
>>>>> 3dB in the centre) with the LM13700? I'm assuming that such a thing 
>>>>> has
>>>>> been done already, so it's seems silly to reinvent the wheel, but I 
>>>>> can't
>>>>> find anything that isn't a simple linear crossfade (which is ok for
>>>>> crossfading, but not so good for panning).
>>>>> Any tips/pointers appreciated.
>>>>> Thanks,
>>>>> Tom

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