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Benjamin AM

Joined: Nov 04, 2010 Posts: 83 Location: Boise
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Posted: Wed Nov 09, 2011 3:26 pm Post subject:
Arp 2600 4027-1 question |
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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
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Posted: Thu Nov 10, 2011 3:05 pm Post subject:
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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
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Posted: Sat Nov 12, 2011 5:26 pm Post subject:
Arp 2600 4027-1 question |
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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
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Posted: Tue Nov 15, 2011 9:48 am Post subject:
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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
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Posted: Fri Nov 18, 2011 2:48 am Post subject:
Jerry_100 |
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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
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Posted: Wed Feb 22, 2012 9:27 am Post subject:
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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
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Posted: Mon Feb 27, 2012 6:08 am Post subject:
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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.  |
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Benjamin AM

Joined: Nov 04, 2010 Posts: 83 Location: Boise
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Posted: Mon Feb 27, 2012 9:37 am Post subject:
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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
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Posted: Mon Feb 27, 2012 10:17 am Post subject:
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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
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Posted: Mon Feb 27, 2012 12:49 pm Post subject:
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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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