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Arp 2600 4027-1 question
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Benjamin AM



Joined: Nov 04, 2010
Posts: 83
Location: Boise

PostPosted: Wed Nov 09, 2011 3:26 pm    Post subject: Arp 2600 4027-1 question Reply with quote  Mark this post and the followings unread

I know other people have cloned this circuit, as I myself have recently cloned a few copies of the of this VCO along with the 2600s waveshaper from the service manual. I am experiencing a possible problem that is driving me crazy. It is that the hi freq trim doesn't seem to be tracking 100%. I have used both a 2n5910 and 2n3906 for Q5. Both seem to give similar results. I was curious if anyone has messed with the resistors in the hi freq comp. circuit and have come out with perfect results? I myself could not find any combination to be perfect. I am also curious if the original circuit(Arp 2600) was not 100% accurate. Has anyone else replaced this section of the VCO with a more accurate hi freq trim?
Also, I am thinking about removing R3(1k resistor leading from the mixing node to the expo converter) from the circuit. It seems like it may have the potential to be temperature sensitive. I know that this resistor was not present in the Odyssey. Can anyone think why I should keep it there.
-thanks
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Benjamin AM



Joined: Nov 04, 2010
Posts: 83
Location: Boise

PostPosted: Thu Nov 10, 2011 3:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

Solved. The problem was not with the circuit but with my controller. More specifically the active buffer on my multiple jack. What a headache over nothing!
Anyhow, I'm still curious if it is worth getting rid of R3. I'll test in 2 different VCOs over the next few weeks.
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Tim Servo



Joined: Jul 16, 2006
Posts: 924
Location: Silicon Valley
Audio files: 11

PostPosted: Sat Nov 12, 2011 5:26 pm    Post subject: Arp 2600 4027-1 question Reply with quote  Mark this post and the followings unread

I'm curious: what was the problem with the buffer in the multiple? Was it not set for gain of 1?

Tim (needs mulitple multiples) Servo
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Benjamin AM



Joined: Nov 04, 2010
Posts: 83
Location: Boise

PostPosted: Tue Nov 15, 2011 9:48 am    Post subject: Reply with quote  Mark this post and the followings unread

Tim. A little background. I am currently using a Moog LP with CV outputs as a controller for my homemade modular system. Out of the box, the Phatty does not work well as an external controller. The keyboard control voltage source degrades over a stretch of patch cord no long than my arm. Therefor requiring a buffered multiple to be used if one decides to control more than one module. Not a big problem, but a bit discouraging finding out that Moog is designing there equipment to very low standards these days. I guess I should have known, after all the Moogerfooger series don't track at 1V/oct. Anyhow, I could go off on that subject forever but to explain the buffer situation. I made a 8x buffer using two TL074. Standard design, signal entering + with output tied to -, ending in a small output resistor. I initially thought that this solved my problem until I tuned my Arp Oscillators with a frequency counter. Higher frequencies tracked well whereas low frequencies/ voltages not so much. Anyhow the solution was to prebuffer the signal before I sent it to the multiple buffers.
Also during this process I messed around with the 2n5910 and the high freq tracking circuit. After fixing my voltage source problem, I came to the conclusion that the 2n5910 functions a little better as a drop in component than a 2n3906. Although I can verify that changing the values of the two 61.9k resistors in this circuit does change the compensation amount. I didn't get the correct values because I was lazy and had a few 2n5910 sitting around.
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Jerry_100



Joined: Nov 18, 2011
Posts: 6
Location: Bavaria

PostPosted: Fri Nov 18, 2011 2:48 am    Post subject: Jerry_100 Reply with quote  Mark this post and the followings unread

Hi, I´m new here but sneaking around in the forum for a while.

Some time ago I was prototyping a 4027-1 replacing the passive resistor summer with an Op. It turned out to be rock stable over a huge range, it´s my favourite vco since. I did temp. compensation with a 1k KTY 81-120 following the Rene Schmitz approach. I am very glad with the results.
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Benjamin AM



Joined: Nov 04, 2010
Posts: 83
Location: Boise

PostPosted: Wed Feb 22, 2012 9:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Jerry,
sounds good, I decided to use this approach as well. I was curious if you replaced the pnp/npn expo converter with a matched npn current sink? I've been experimenting with this to allow for a linear input.
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Jerry_100



Joined: Nov 18, 2011
Posts: 6
Location: Bavaria

PostPosted: Mon Feb 27, 2012 6:08 am    Post subject: Reply with quote  Mark this post and the followings unread

Only by simulation and it seems to work o.k, but honestly I am very happy with the pnp/npn combination, at least it works good for me. Getting rid of the passive summing resistors was a much valuable improvement. Now I´m planning to substitute the auto zero level shifter with an OP approach, which means that there will be no need for the CA3086 (replacing the remaining npn´s by single trannies). Another step would be implementing a stable reference voltage......there is so much room for tweaking. Laughing
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Benjamin AM



Joined: Nov 04, 2010
Posts: 83
Location: Boise

PostPosted: Mon Feb 27, 2012 9:37 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Now I´m planning to substitute the auto zero level shifter with an OP approach, which means that there will be no need for the CA3086 (replacing the remaining npn´s by single trannies).

I'm not exactly sure which section of the circuit that your referring to. I'm assuming it is Z1c-e and the associated components. I've never fully understood this section of the schematics. As for replacing the ca3086 with transistors, I can confirm that it works perfectly fine. I have done this in my design as well. I used 2n3904s and handmatched the ones that share common emitters.
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Jerry_100



Joined: Nov 18, 2011
Posts: 6
Location: Bavaria

PostPosted: Mon Feb 27, 2012 10:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Exactely, it´s the last stage. The circuit simply pushes the output of the core to 10 V and sets the signal pretty nice on top of the base Line. As I told you, I am trying to replace it with an op amp circuit I saw somewhere in the forum. Just tweak the resistor values and your done. But, I am not sure how it will change the sound, because maybe you noticed that the higher frequencies are very slightly distorted and I´m pretty sure that this contributes to the famous punchy 2600 Sound.
The whole circuit is a piece of art made by men they fully understood analog computer design.
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Benjamin AM



Joined: Nov 04, 2010
Posts: 83
Location: Boise

PostPosted: Mon Feb 27, 2012 12:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Interesting, now I'm seeing it. Seems like that section could simply be replaced by an inverting op amp with and a negative bias. Values would be similar. Let me know how it goes and thanks for the insight!
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