arp avatar 4075 filter probem fixed!

[steven thomas]

Hi Joachim,
   well I managed to fix the problem with the 4075 filter. 
I carefully unsoldered the old LM3900 opamp, put in a dil socket and 
a new chip. I connected the 4075 to the main board and the buffered 
inputs/outputs of the 40235 module as you suggested.
It now works really well! Sounds really smooth..not perhaps as powerful as the 
4035 4-pole filter, but rather nice all the same.
Now I have a three way slide switch mounted on the front panel near the 
VCF freq/res sliders, which switches the outputs of the 4023/4035/4075
filters as desired. (including the series resistor needed to improve 
resolution of the resonance slider).
 Next I will add white legend on the front panel next to the 3-way switch to 
complete the job. 
Future plans for this project will be two add about 40 jacks to the lower 
front panel (on the avatar there's lots of room) as a way of modularizing it, 
and mounting it vertically in a cabinet.
I certainly Don't look forward to drilling 40 holes in pretty thick steel 
sheet though!!

I hope to add pictures of all this plus other synth projects on some webpages 
when i get time.

Anyway thanks for all your help..I am pleased with the results.

cheers steve

>===== Original Message From Joachim Verghese <jocke at netcontrol.fi> =====
>Hi Steven,
>
>> Things are much better now..less noise (or perhaps more signal?) than
>> before and better cv control of the filter--the 40235 that is.
>
>Ok, that's good to hear.
>
>> I am beginning to think this filter wasnt functioning properly from the
>> start. So I will take it out and try replacing the LM3900. I hope nothing
>> else got damaged. Well, we shall see.
>
>Things to check on the 4075:
>
>If your module doesn't have the series diode on the negative supply
>line (CR1), one or both of the supply conditioning transistors (Q15,
>Q16) may be damaged. This kind of fault usually makes the 4075 totally
>silent, so this might not be the problem in your case.
>
>Replacing the LM3900 is easy, so doing that is a good idea. Given the
>fact that your module has problems with CV feedthrough, you might
>try replacing Q11 and Q12 (both 2N3906) if changing the 3900 did not
>help. These transistors need not be matched/selected.
>
>> By the way I really found your website of great value..I am just wondering
>> whether you ever had in mind designing boards for other arp filters?
>
>Yes, I've had quite a few requests for the 4012, as this is perhaps
>the most famous ARP filter. Although it's quite similar to the 4035,
>it is DC-coupled, which means that you can use it to process control
>voltages in addition to audio. How useful this is is of course a
>matter of opinion, but I believe the DC-coupled nature of the circuit
>gives it characteristic of its own, even with audio signals.
>
>Some users of the model 2600 synth have expressed some interest in
>a 4012/4023 dual filter. This is something I might do later this
>year, as I am currently busy with an other (also ARP-related)
>project, and I'd really like to get some updates done for the website.
>
>> I guess using LM394's for the matched pairs. Is this worth while do
>> you think?
>
>The LM394 is a very good part, but it's not widely available.
>Matched pairs aren't always easy to find, and the ones that are
>aren't very good (CA3046/3086). The 4012 is a "high-end" filter,
>so to speak, so it really calls for something like the LM394 or
>MAT-02.
>
>> Anyway will let you know if I mange to resurrect the 4075!
>
>Please do!
>
>> At least I can be thankful that its not 'potted'!!
>
>Right.
>
>
>Best,
>
>-joachim

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