[sdiy] 2SK170 FET alternative

[Seb Francis]

Well I've managed to get it working with the 2SK30A without it 
oscillating any more :)
It took a change of R34.  The timing of the attack and release doesn't 
seem right though so I think I will need to rework some other values to 
compensate for the different FET.

Definitely good advice about breadboarding it though .. I'm getting the 
impression that the circuit as it is just won't do what I want it to 
do.  I guess it's meant to be one of those 'transparent' compressors or 
something .. I want it to SQUASH drums and cause all kinds of audible 
artifacts!

Thanks again,
Seb


Seb Francis wrote:

> Thanks Jay, this is helpful, but looking at the circuit it seems to me 
> that the transfer function is very non-critcal ..
> http://www.thatcorp.com/datashts/dn115.pdf
>
> The capacitor C18 charges or discharges to whatever voltage is needed 
> to get the output of the FET (-ve comparator input) to match the 
> compression level voltage at the +ve comparator input.  At the point 
> when these voltages are equal the comparator is 0V, so both the the 
> lower (attack) and upper (release) current mirrors output flows only 
> to the comparator and not to/from the capacitor.
>
> When the compression level voltage changes, the comparator 
> sources/sinks the current to/from the lower/upper current mirror, 
> 'steering' the current from the other mirror to the capacitor C18 
> which charges/discharges at a rate determined by the current mirror 
> (i.e. attack or release rate).  This occurs until the output voltage 
> of the FET stabilises to the compression level voltage.
>
> Aaaah!  Having just written this I now think I see the problem!  The 
> gate-source voltage must always be less than a diode drop away from 
> 0V, otherwise the diodes don't work to steer the current away from the 
> capacitor C18.  This would explain why I'm getting a nice high 
> frequency oscillation going on in my circuit when using a 2SK30A.
>
> So I think I can make it work by just changing the resistor R34 to 
> allow sufficient voltage swing at the FET output without the VGS 
> getting too negative ... hmm, just off to calculate some values!
>
> Seb
>
>
> jays at aracnet.com wrote:
>
>> Seb,
>>
>> There are two things that I think that you need to look at. First is 
>> where the linear range is of the JFET and next is the amount of 
>> signal on the input. The 2SK170 is higher gain so the slope of the 
>> transfer function is steaper so it's input requirements are less than 
>> a 2SK30. It's input goes from ~0.5V to 0.0V VGS (off to on) so if you 
>> pick the center point in its VGS curve ~0.15V. For the 2SK30 its VGS 
>> range is ~-2.8V to 0V with a center at ~-0.8V.
>>
>> So you'll have to change the input level and the bias point.
>>
>> Also I don't know how they are using the JFET in this case. It could 
>> just as a buffer or it could be more. They maybe relying on the fact 
>> that JFETs don't clip as hard as bipolar transistors and using that 
>> as part of the compression.
>>
>> When I do something like this I just breadboard that part of the 
>> circuit and get it to the point where I think it works. Then I go on 
>> and do the rest and see how it really behaves. What looks good on the 
>> scope may not sound so good.
>>
>> Hope that makes sense.
>>
>> Jay S.
>>
>> Seb Francis wrote:
>>
>>  
>>
>>> Thanks Jay, those articles look very useful.  I also went to bed 
>>> last night with The Art Of Electronics chapter on FETs, so I'm 
>>> feeling a bit less ignorant today!
>>>
>>> The 2SK170 is used as a buffer with the input voltage connected to 
>>> the Gate, V+ connected to the Drain, and a resistor between V- and 
>>> the Source.  Am I right in thinking that to compensate for using a 
>>> JFET with a different transfer curve (VGS) I can just change the 
>>> resistor value?
>>>
>>> Seb
>>>
>>>
>>> Jay Schwichtenberg wrote:
>>>
>>>   
>>>
>>>> They have different transfer curves (VGS) so I wouldn't consider 
>>>> them exchangeable.
>>>>
>>>> Here are a couple of good articles on JFETS:
>>>> <http://www.borbelyaudio.com/special_articles.asp>
>>>>
>>>> Check out JFETs, The New Frontier Parts 1 and 2.
>>>>
>>>> Jay S.
>>>>
>>>>     
>>>
>>
>>
>>
>>  
>>
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