[sdiy] Using both OTAs in 13700 as one OTA

[sleepy_dog at gmx.de]

Assuming you use windows, if you have a not too shabby ASIO soundcard, 
it means a program can get sole ownership of the card and windows mixer 
not polluting your output signal. So you could do audio range SNR and 
THD measurements with looping the soundcard output, going through your 
test hardware, back to the input, and using appropriate software. In the 
confines of the distortion & noise of the soundcard used itself, 
obviously, but I suspect that DIY analog synth circuits will be noisier 
than a decent ASIO soundcard / external USB audio interface, my 
Roland/Edirol UA-25ex served me well enough.

Other than using existing software, you could implement it yourself, I 
forgot where, but there are some app notes by TI or National explaining 
how to use an FFT and generated test signals to measure this. I have 
done it, but am not sure how well I did it as to let my crappy little 
program loose on you :-D Unless you insist, hah.
It was done on one weekend, though, using FFTW library C# wrapper. So 
not too difficult. If you are into software dev that is.

There is a better option: one nice, free of charge program which does 
all those things, and generate frequency response curves etc - If you 
are interested, I will ask my colleague next week, because I forgot its 
name, alas.

You would need some level conversion for testing synth hardware that 
way, I guess. At least protection for the soundcard input. But (warning, 
I'm an analog semi n00b) I guess that operating typical synth circuits 
with inputs of only +/- 1V or whatever comes out of a soundcard might 
not do the noise figures justice?

I'm sure some of the experienced people here can comment on that, how to 
best do this. I'd be interested myself, for I have not build any test 
bench for synth DIY circuits in this regard (interfacing with PC 
soundcard *the proper way*) myself, but would like to.

As for many circuits with LM13700 not using the diodes, I would guess 
that many were designed for the CA3080 and when that became unavailable, 
people dropped in the 13700 as-is? (please do correct me ;) )

Steve


Am 28.02.2016 um 09:13 schrieb Quincas:
> Hi guys
>
> This thread got me thinking a lot about LM13700 VCAs, so I went to the datasheet and noticed they suggest using the linearizing diodes to reduce harmonic distortion. So I breadboarded it, changed some values to fit synth levels and added a voltage to current converter as seen in Thomas Henry's VCA1. I don't have adequate tools to measure THD or SNR, but it sounds dead clean to me! No noticeable noise and no audible distortion using the nice sine wave from my Norton filter in self oscillation. Is there a reason none of the designs discussed above use the linearizing diodes? Here's my shabby hand drawn schem....
> http://s1070.photobucket.com/user/Quincas_Moreira/media/image_zpsu5cm6fz8.jpeg.html
>
>
>
> Sent from my iPad
>
>> >On 10/02/2016, at 6:57 a.m., Simon Brouwer<simon.o at brousant.nl>  wrote:
>> >
>> >Hi Mattias,
>> >  
>> >I mean the circuit like this:
>> >http://www.bernacomp.com/elec/lockbox/lockbox_vca2.gif
>> >  
>> >  
>> >So, no.
>> >  
>> >Best regards
>> >Simon
>> >  
>> >
>>> > >Op 10 februari 2016 om 12:31 schreef Mattias Rickardsson<mr at analogue.org>:
>>> > >
>>> > >
>>> > >Simon,
>>> > >
>>> > >what about the temperature dependencies in the transistor case?
>>> > >Is that control circuit more sensitive for temperature variations?
>>> > >
>>> > >/mr
>>> > >
>>> > >
>>> > >On 10 February 2016 at 09:08, Simon Brouwer<simon.o at brousant.nl>  wrote:
>>>> > > >Yes it was.
>>>> > > >
>>>> > > >However, this circuit still uses one opamp, a resistor and at least two
>>>> > > >diodes.
>>>> > > >The more straightforward approach adds two resistors, and replaces one diode
>>>> > > >by a
>>>> > > >transistor.
>>>> > > >So this doesn't add much in complexity and cost, and you can control both
>>>> > > >halves of an LM13700 independently.
>>>> > > >
>>>> > > >Best regards
>>>> > > >Simon
>>>> > > >
>>>> > > >
>>>>> > > >>Op 9 februari 2016 om 20:20 schreef Mattias Rickardsson<mr at analogue.org>:
>>>> > > >
>>>>> > > >>
>>>>> > > >>
>>>>> > > >>Aha!
>>>>> > > >>It*was*  a beautiful trick.:-)
>>>>> > > >>
>>>>> > > >>http://electronotes.netfirms.com/AN399.pdf
>>>>> > > >>
>>>>> > > >>/mr
>>>>> > > >>
>>>>> > > >>
>>>>> > > >>On 8 February 2016 at 19:09, Ian Fritz<ijfritz at comcast.net>  wrote:
>>>>>> > > >> >The reference to the original Jung article is JAES, 23, 1975. This
>>>>>> > > >> >method
>>>>>> > > >> >was widely used in EN projects. I looked at using this method with the
>>>>>> > > >> >LM13700 to aid people who want to build off of these projects, but who
>>>>>> > > >> >can't
>>>>>> > > >> >obtain the CA3080. It's also an easy way to improve S/N by 3dB, as
>>>>>> > > >> >pointed
>>>>>> > > >> >out by Serge T. in his EN article from when the chip was new.
>>>>>> > > >> >
>>>>>> > > >> >I couldn't wade through Aaron's article either. It sounded like negating
>>>>>> > > >> >then subtracting? Isn't that the same as adding?
>>>>>> > > >> >
>>>>>> > > >> >Ian
>>>>>> > > >> >
>>>>>> > > >> >
>>>>>> > > >> >
>>>>>> > > >> >On 2/8/2016 2:41 AM, Mattias Rickardsson wrote:
>>>>>>> > > >> >>
>>>>>>> > > >> >>On 4 February 2016 at 05:52,<ijfritz at comcast.net>  wrote:
>>>>>>>> > > >> >>>
>>>>>>>> > > >> >>>Good grief! Just connect them directly in parallel. An example,
>>>>>>>> > > >> >>>complete
>>>>>>>> > > >> >>>with the W. Jung trick:
>>>>>>>> > > >> >>>
>>>>>>>> > > >> >>>
>>>>>>>> > > >> >>>http://electro-music.com/forum/viewtopic.php?highlight=jung+ota&t=50884
>>>>>>> > > >> >>
>>>>>>> > > >> >>
>>>>>>> > > >> >>What does it do? What is the "trick"?
>>>>>>> > > >> >>I feel like one of the answers in that thread:
>>>>>>> > > >> >>"I hadn't heard of this trick before, is there an article somewhere
>>>>>>> > > >> >>that explains the benefit of drawing bias current from V-? I feel like
>>>>>>> > > >> >>there's something obvious that I'm missing."
>>>>>>> > > >> >>
>>>>>>> > > >> >>I spontaneously found Aaron's idea potentially better than nothing,
>>>>>>> > > >> >>but I haven't plunged into it.
>>>>>>> > > >> >>
>>>>>>> > > >> >>/mr
>>>>>>> > > >> >>
>>>>>> > > >> >
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