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 FatMan VCA ADSR, Audio Output connector 
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Joined: Sun Nov 09, 2008 2:49 pm
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Post FatMan VCA ADSR, Audio Output connector
>>>>> Dash Badcock wrote:
>>>>>
>>>>>> Hi Scott,
>>>>>>
>>>>>> I appreciate that you probably get tons of emails asking for help when troubleshooting the kits so I'll apologise in advance for yet another one.
>>>>>>
>>>>>> I've just finished building my fatman and have encountered what seems to be quite a small problem. The synth works perfectly.....except for the fact that there is a very low level constant tone always emitting from both oscillators of the synth. When I 'play' the synth it behaves as expected, but as soon as I release a key the low level tone is present.
>>>>>>
>>>>>> I think I have traced this problem to the ADSR. When I probe the ADSR output I read a DC voltage of ~0.3V when no key is pressed and 6.3V when a key is pressed. The manual states that this should swing from 7-0V. Am I right in thinking that the 0.3V that is always present at the ADSR output is causing the VCA to remain very slightly open at all times, leading to the constant output?
>>>>>>
>>>>>> Can you give me any pointers as to how I can troubleshoot this? I've checked the soldering and polarity of the ADSR components - there are a fair few diodes and all seems ok. Is it possible to trace a signal through the ADSR itself so I can isolate the problem further?
>>>>>>
>>>>>> Or am I barking up the wrong tree and the 0.3V at the output is symptomatic of another problem?
>>>>>>
>>>>>> Many thanks in advance for your help,
>>>>>> Dash
>>>>>>

>>>>> Hello Dash.
>>>>>
>>>>> I'm not sure how old your FatMan is, but over the years there have been some updates that affect leakage due to the VCA ADSR not going as near zero as it should. Check to see if the VCA ADSR Sustain control has a 220ohm red-red-brown-gold resistor soldered to it's outer lugs and if so, clipping it off will help with the leakage. R99 at the input of IC13:C was updated to be a 330k vs 470k. Maybe a rogue 1M got in the 10M bin and the green band on it looked blue for the resistor installed at R100? Look to be sure the Release control R96 has good connections and the 100ohm brown-black-brown-gold R95 is as specified--a higher valued resistor here would prevent all the voltage being removed by Q11 for the release phase of the ADSR.

>>>>> Thanks. --Scott

>>>
>>>> Hi Scott,
>>>>
>>>> Thanks for the fast response.
>>>>
>>>> My fatman is a new one - I received the kit last week. I have followed your suggestions - including pulling out the resistors below and rechecking their values and everything seems to be correct. I am still getting the same issue though. I've thoroughly checked the panel wiring and the connections are all solid so I'm at a loss.
>>>>
>>>> Note that I don't have to actually play a note for the sound to start. It starts as soon as I power on the unit. If I play a note then, it plays fine - and the adsr works - but the noise remains after the release phase of the note.
>>>>
>>>> Do you have any other suggestions? Are there any components I should look to swap out or check further?
>>>>
>>>> Thanks
>>>> Dash

>>> Hi Scott,
>>>
>>> I just want to add to this that I'm seeing about 31db difference between the output when played normally and the level of the leakage sound. This is trivial for me to gate out.
>>>
>>> This is my first analogue synth - am I just being a bit demanding? :-)
>>>
>>> Dash

> On Saturday, 12 September, 2009, at 06:27AM, "Scott Lee" <scott@paia.com> wrote:
>
>> Hello Dash.
>> Hmmm.....maybe I haven't ever quantified the signal to noise (leakage) ratio, but will and see how this compares to what you're seeing.
>> Just to be sure....double-check the 1/4" output connector to see that the ground is to the sleeve and the audio is to the tip. We have had some connectors that have a different terminal arrangement that what is shown in the illustration.
>> Thanks. --Scott
>>


Dash Badcock wrote:
> Hi Scott,
>
> Thanks so much for your help - it looks like this has fixed it. I'm getting great performance now I've switched the wires on the output connector.
>
> Many thanks
> Dash
>


Hello Dash.

Great! Thanks for letting me know. I checked the one here and see about 30dB too, but apparently 30dB above the noise floor is much easier to live with than that same amount below.

Thanks again. --Scott

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Scott Lee
scott@paia.com


Tue Sep 15, 2009 11:38 am
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