[sdiy] Working with the ASM1 VCO.. weird MAT-02 behaviour...

[Oren Leavitt]

I've found that the MAT-02 and others seem to do best if they get their 
18mV/Oct drive voltage directly from a low impedance source (e.g. an op 
amp output) rather than from a resistor divider. That'll eliminate the 
effect of any protection diode reverse leakage.

On 1/16/20 2:55 PM, René Schmitz wrote:
>
> Hi Jean-Pierre,
>
> Maybe you loose current through the protection diodes the MAT-02 and 
> LM394 has between emitter and base. (Base of output transistor?)
> Thats one of the differences to the 2n3904s.
>
> Best,
>  René
>
> On 16.01.2020 17:48, Jean-Pierre Desrochers wrote:
>> Hi list,
>>
>> I'm working on the Elby-Design ASM-1 VCO
>>
>> https://www.elby-designs.com/webtek/asm-1/vco/vco-asm1-cct.pdf
>>
>> Im want to use it as a 1v/oct LFO (.05hz to 20hz)
>>
>> I first mounted the circuit on my protoboard with success.
>>
>> at first I used a matched pair of 2N3904 for Q1A & Q1B
>>
>> instead of the MAT02 shown in the schematic.
>>
>> I could reach very low frequencies (as low as .04hz).
>>
>> But to do so I needed to replace R9(56k) for a 33k temporarily. Fine.
>>
>> This morning I replaced both 2N3904 with a genuine MAT-02
>>
>> and started the same frequency measurements as yesterday.
>>
>> I bumped into something weird..
>>
>> Here are the actual measurements I first did using 2x2N3904:
>>
>> mVolt at 1k TEMPCO resistor vs Sawtooth period
>>
>> 158.0mV => 0.050sec
>>
>> 176.0mV => 0.100sec
>>
>> 193.8mV => 0.200sec
>>
>> 212.0mV => 0.400sec
>>
>> 229.8mV => 0.800sec
>>
>> 247.5mV => 1.6sec
>>
>> 265.5mV => 3.2sec
>>
>> 283.2mV => 6.4sec
>>
>> 301.7mV => 12.8sec
>>
>> 319.7mV => 25.6sec
>>
>> (18mV steps, nice exponential response!)
>>
>> Now here are the actual measurements with a MAT-02:
>>
>> mVolt at 1k TEMPCO resistor vs Sawtooth period
>>
>> 157.0mV => 0.050sec
>>
>> 175.0mV => 0.100sec
>>
>> 193.0mV => 0.200sec
>>
>> 211.0mV => 0.400sec
>>
>> 229.0mV => 0.800sec
>>
>> 247.0mV => 1.6sec
>>
>> 271.0mV => 3.2sec
>>
>> **** Now trying to go further increasing the TEMPCO mVolts*
>>
>> **** the oscillator starts backing up and re-increase its frequencies 
>> !!!*
>>
>> **** I could not go further lowering the frequency. *
>>
>> I tried 3 different MAT-02's and even LM394's
>>
>> and they all behave the same.
>>
>> Using different matched pairs of 2N3904 worked perfectly..
>>
>> I suspect  MAT-02’s minimum base to emitter current reached (???)
>>
>> What am-I missing here ??
>>
>> By the way, all the IC's I'm using here are genuine one's
>>
>> (TL082, MAT-02, CA3140, LM311).
>>
>> JP
>>
>>
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>
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