[sdiy] LM13600 Gain Question

[Scott Gravenhorst]

Thanks Tim,
I thought the question was lame at first, but apparently not so.  I'm very
much used to experimenting on an SBB first, then transferring the fine tuned
result to stripboard.  The answers about messing with input and output
resistors make sense now.  The reason this came up for me was in looking at
the FatMan SVF schematic (LM13600 based) I see the input signal is attenuated
100:1  but the FatMan's output is fairly substantial in comparison. 
Apparently, the signal amplitude is restored by the VCA.  I want to add a new
filter (VC Sallen-Key which will also have a significantly attenuated input
stage) connected to a VCA/ADSR, but I wanted to calculate the VCA part
values.  SBB.

<tim102 at tstinchcombe.freeserve.co.uk> wrote:
>> I'm looking at the LM13600 data sheet and fig. 4 (stereo volume 
>> control) has a formula to calculate the gain given Iabc as:
>> 
>> Vout
>> ---- = 940 X Iabc
>> Vin
>> 
>> Why 940?
>
>I too had been troubled by this expression a few years ago: I have more
>resources at my disposal now, and (hopefully!) a better understanding of
>such things, so I decided to have another look at it.
>
>Running a few simulations was enough to convince me that their main
>expression Iout=Is*2*Iabc/Id is out by a factor of about 2. The culprit
>appears to be the assumption that the voltage splits in the diodes as
>Id/2+/-Is: in fact Id/2+/-Is/2 would be much closer to reality (making
>Iout=Is*Iabc/Id). I then wrangled a few expressions and ended up with a
>nasty non-linear one relating Is to the +/-'delta' current flowing in the
>diodes. Using Mathematica to solve the expression gave me very similar
>figures to the simulation, i.e. that the 'delta' is about half Is, which
>acted as independent confirmation I'm on the right track. Conclusion: the
>940 was either obtained a) empirically, b) by simulation, or c) by some
>heavy maths using goodness-knows-what approximations.
>
>As already noted, using it as a starting point and experimenting is going to
>be the order of the day. Incidentally, the 0.94 does seem to arise from the
>component values in Fig 2 (with Vs at 15V); using Fig 4 values, the
>simulation gives the factor around 1.05 or so (Iabc in amps).
>
>Tim
>
>__________________________________________________________
>Tim Stinchcombe 
>
>Cheltenham, Glos, UK
>email: tim102 at tstinchcombe.freeserve.co.uk
> 
>
>
>

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