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ARP2500 - 1047 Filter Redraw and Questions
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author ARP2500 - 1047 Filter Redraw and Questions
Laughing
Over the past couple days I've gone through the first extensive redraw I've ever done, trying to get the one module I find most interesting into an EDA tool so I can clone it.

Now that the most tedious part is complete, I've got to ask some questions on some parts of the module, based on what I've got.

What the heck are the SEL resistors? I want to guess that they're cost effective trimming to make up for the terrible 10% tolerance resistors that they had to use, without shelling out for more metal film precision resistors or trimpots.

What are the two pots on the audio input path? I put in my own guesses based on what was put for the others.

What is the capacitor coming from point A from the trig? and hell, why is the trig going to the audio in anyways?

I hope I can get these questions answered so I can move forward with a board. I know Jhulk has the entire 2500 under his wing, but I want to give my own shot at making a clone, too. I'll probably post board files later after I review the copyright notice at the top of the forum.
KSS
Laughing wrote:
What the heck are the SEL resistors? I want to guess that they're cost effective trimming to make up for the terrible 10% tolerance resistors that they had to use, without shelling out for more metal film precision resistors or trimpots.

Correct.

Laughing wrote:
What are the two pots on the audio input path? I put in my own guesses based on what was put for the others.

They are the audio 1 and Audio 2 input attentuators on the panel.

Laughing wrote:
What is the capacitor coming from point A from the trig? and hell, why is the trig going to the audio in anyways?

It conditions the Kkeyboard input pulse. Perhaps calling it a ping input will ring true for you? This was new ground when Dennis Colin did this. Have you read his AES paper on the 1047? See also Dave Rossum's UAF, which is the result of trying to make a filter to match the published ARP1047 specs. Failing to do this, he later discovered the 1047 didn't meet its published specs either! On the E-mu UAF you can see the Percussion/ping input also.

Did you get the 1047 docs from musicparts.com? 35USD answers a lot of questions for those who need it. That's what JHulk is using/copying. You can also find the board layout of Dave Leith's 1047 clone on the web.

What format are you building this for?
KSS
Now realise you were asking for the input pot values. 100kAud. Lin will work too.
Laughing
KSS wrote:
Did you get the 1047 docs from musicparts.com?


No, these are from GuitarFool's website. I'm not really in a position to spend much money at all, not even $35.

I actually e-mailed him and got a response back today, and he directed my to David Leith's website, as well as all the images from the HKHilversum website, which he graciously re-uploaded. David's version of the 1047 is certainly available as an Adobe Illustrator file on that site, though I'm not sure how to move ahead without a free alternative to Illustrator. Inkscape? One of the LiberOffice programs?

I have certainly read the Dennis Colin paper, at least as much as I could without getting out a notebook and referring back to my Signals and Systems scribbles from just last year. I suppose I'll have to read the paper again.

I'll be building this into a 5U style system with the MOTM power connector and +/-15 V supply rails. It would be nice to make it card connected and have those neato 20-select switches on the top and bottom, but no, 1/4" plugs it is.

I'll have to see about getting my hands on an E-Mu modular in a couple months, though... Maybe I can take some dedicated pictures of the board layout if circuit sheets don't already exist.
Cheradenine
I've cloned the 1047 (and the 1004), did my own board layout - too bad photobucket killed all my pics link Dead Banana
Without the service manual it'll be hard to choose the SEL resistors as they're part of the complex calibration procedure.
Some are just parallel to others resistors for fine tuning the value and as such not really needed, but other the filter won't work without (R46 / R48)
Laughing
46/47/48, yeah, I was worried about that... I suppose one awful option would be to put trimmers everywhere. I should probably do some serious circuit analysis, actually.

Cheradenine
Is the 1004 any easier than this?
Cheradenine
The 1004 is slightly harder to clone and calibrate properly, because almost none of the SEL res and cap are explained in the service manual Dead Banana
Cheradenine
Ok now that I'm back home, I can confirm the audio pots are 100k audio and the missing cap value is 0,1uF (designator C33 btw)

You can replace the 1339 opamp with TL071 instead of MC1439, so you can remove the external compensation for these (R19/R21/C4 for A4, R31/R33/C7 for A5 and R43/R45/C10 for A6)
They're for summing purpose, not really crucial for the filter sound (IMO)

Why use LSK389A instead of the LS3954 ?
They are at the core of the filter, you should keep the original part
Linear System still produces them.

The Lamp needs an external +12V supply and is not really usefull...I scratched it; much easier than adapting the circuit for a LED Mr. Green

Finally, the percussion mode needs a trig AND a gate to work correctly
The trig will indeed ping the filter (that's why it goes in the audio IN) and the gate will shape the ringing with the "final Q" pot.
By the way, R1 (resonance/Q) and R4 (final Q) are panel control pots, not trimmers. On your schelmatic you named them "RV1" and "RV4" ?
loki
Cheradenine wrote:
You can replace the 1339 opamp with TL071 instead of MC1439, so you can remove the external compensation for these (R19/R21/C4 for A4, R31/R33/C7 for A5 and R43/R45/C10 for A6)
They're for summing purpose, not really crucial for the filter sound (IMO)

Why use LSK389A instead of the LS3954 ?
They are at the core of the filter, you should keep the original part


Use OPA134s for all the op-amps. Input voltage noise is 8nV/rootHz rather than 18 and 20 VuS slew rate as opposed to 13 Vus for the TL071. Completely eliminates the need for the 2N3954s. If this filter has a distinctive sound it is a product of the discrete transconductors not of the op-amps. You will observe that high Qs at high frequencies cause this filter to oscillate.
Laughing
Now I can catch my breath from moving.

Uploading a new schematic with things fixed that have been pointed out: https://my.mixtape.moe/gunknt.pdf

Wish I knew how to upload a file again.

KSS: The reason I'm using LSK389 is because I have and ungodly amount of them, and when I subbed one in in the sine shaper circuit in my TTSH, it did way better.

Do you have experience using the tl071 in the place of the 1439? I got about 10 at a $1 each, so I'd like to continue to use the prescribed opamp, but there's always room for experimentation, plus there's always room to leave pads open for compensation/etc. Frankly, I'd like to not be using LM301's, and I'm probably going to try NE5534's in it's place, but regardless, my main problem is the issues I have read about and understand regarding input impedance.

The bipolar 1339 is not directly replaceable by the JFET TL071 in some applications. Sometimes it can be traded out, sometimes it can't because the circuit is relying on the ~300K input impedance of the opamp. Again, I'm referring to posts made in the TTSH build threads.

loki: You mean to say that the sound, and problems, of the device comes from the 4005 sub-modules, then? Have you built one and tried them out as well?
loki
KSS wrote:
See also Dave Rossum's UAF, which is the result of trying to make a filter to match the published ARP1047 specs. Failing to do this, he later discovered the 1047 didn't meet its published specs either! On the E-mu UAF you can see the Percussion/ping input also.


I saw the UAF at Dave's apartment in Santa Clara in 1975. It did in fact meet the specs of the 1047. It was a challenge. When he finally got hold of a 1047 it didn't. I don't doubt that Dennis Colin's personal hand tuned unit meet the specs.

Here is Rossum's UAF:
loki
Laughing wrote:
loki: You mean to say that the sound, and problems, of the device comes from the 4005 sub-modules, then? Have you built one and tried them out as well?


To the extent that the 1047 has a "sound" other than its frequency response as a filter, it will be determined by the distortion characteristics of the transconductors.

The two integrator loop state variable filter has a well known problem of Q enhancement because of the phase shift through the amplifiers in loop. Using wider bandwidth op-amps reduces this problem. See the LM318s in Rossum's filter.
Notice, in the upper left hand corner, a small capacitor marked select. If you look up a picture of the module you will see a strand of ribbon cable an inch and half or so in length. It is the tuning capacitor of a phase lead network to tune the loop response of the filter.

Tomorrow I will dig out the letter from Robert Ashely about Colin's filter article and Colin's response and post it here.
Oh, and I built my first state variable filter about 1971,still have it in fact.
loki
Here is the exchange between Ashley and Colin. Notice: I spelled Ashley wrong in my previous post.
The Real MC
Interesting discussion...

loki wrote:
Notice, in the upper left hand corner, a small capacitor marked select. If you look up a picture of the module you will see a strand of ribbon cable an inch and half or so in length. It is the tuning capacitor of a phase lead network to tune the loop response of the filter.


That "ribbon cable" is simply a small capacitor - two conductors separated by a dielectric (the ribbon insulator) whose separation (distance between conductors) and square area (product of separation and length of ribbon) determined the capacitance. You tuned the network by trimming the ribbon cable, which varied the capacitance.

Gotta love those engineering physics types!!! applause
Cheradenine
Laughing wrote:

Do you have experience using the tl071 in the place of the 1439? I got about 10 at a $1 each, so I'd like to continue to use the prescribed opamp, but there's always room for experimentation, plus there's always room to leave pads open for compensation/etc. Frankly, I'd like to not be using LM301's, and I'm probably going to try NE5534's in it's place, but regardless, my main problem is the issues I have read about and understand regarding input impedance.

The bipolar 1339 is not directly replaceable by the JFET TL071 in some applications. Sometimes it can be traded out, sometimes it can't because the circuit is relying on the ~300K input impedance of the opamp. Again, I'm referring to posts made in the TTSH build threads.


My clone uses TL071 in place of 1339 without issues.
In the 1047 the 1339 are used for mixing audio & CV signals, in what seems to be usual virtual earth mixers.
I'm not sure the low impedance input of the 1339 is of any use here.

If you look at the 1005 and 1006 schematics, the 1339 are used in places where it's quite possible the low impedance is part of the design.

Do you have links for the posts you're refering concerning the 1339 opamp ?
I didn't find anything
Laughing
Cheradenine This might be it. Apparently it was due to the fact that the tl071 was too fast. The LM301 had lag, which was compensated for appropriately, but this didn't translate across when the TL071 was put in.
https://www.muffwiggler.com/forum/viewtopic.php?p=2599155&highlight=#2 599155

This might not be exactly the same post I was remembering, though.
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