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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
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Posted: Tue Oct 08, 2013 5:37 am Post subject:
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That would be my first guess for the expo converter, but not sure whether it would work, having not done it. I tend to overlook things like loading effects so I didn't want to assume it would work.
So at this point I should not be looking at the first schematics you posted?
I think this is probably a case where you will need to build on a breadboard (assuming you have one) and tweak things from there My main concern would be the FET and whether it's fully functional, or the comparator. You might try measuring the signal at the gate of the FET to see what it's doing.
What should happen is that the FET should be pinched off while the cap charges, then the level should get to something that matches the comparator which then opens the FET and drains the cap. With that not working (I would assume) I'd verify the voltages into the comparator are what I expect them to be, and maybe replace the comparator (or at least take it and test it with some other controlled inputs). |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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Posted: Tue Oct 08, 2013 7:32 pm Post subject:
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Thanks elmegil.
Yes, I updated the schematic and the board design to reflect the original ENS 76 design, and that's when I bought some CA3130 op amps and some FETs
I think my next step is to build test circuits for each component, to find out exactly what parameters I'm working with. I know FETs are notoriously unreliable as to their specs, and I want to be able to test comparators and op-amps too.
I've realised now that I have to build each circuit with regards to the actual parameters of the physical components - an "ideal op amp" approach, or even using the datasheet specs simply will not do.
...but right now I have a 2-day arduino workshop to prepare for starting on Thursday, so it'll probably be the weekend before I get back to tinkering |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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Posted: Fri Nov 08, 2013 6:29 am Post subject:
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Don't know if anyone is following this any more but...
I found the culprit. That is really the last time I buy any ebay purchases from China. I don't know what they are supposed to be, but after building a tester for dual transistors I found neither the LM394 nor the AD2210 chips I bought are in fact, working transistor chips.
So, I finally shelled out for a couple of MAT12s from a reputable UK seller.
£18 a piece. Whew
Ed |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Fri Nov 08, 2013 6:53 am Post subject:
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That sucks :/
I was doing some troubleshooting of a circuit with CA3080's a couple weeks back, and couldn't get the darn thing to work on a breadboard. I resorted to testing, and sure enough--every single one of the 25 3080's I bought from EBay/China 18 months ago was bad. Not "it doesn't do anything" bad, but not an OTA, certainly.
I generally won't buy from China unless it's a final resort and the part just is not available anywhere. I did get lucky with a few parts early on, but mostly it's been a bad experience. |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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Posted: Sat Nov 09, 2013 5:51 pm Post subject:
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Yeah, I seem to have been fairly lucky with My CA3080s so far, but I'm really burnt now with the matched transistors.
I want that vintage sound, but I'm willing to sacrifice that with the view to reliably being able to build circuits. I've started trying to transpose everything to LM13700 chips, but have yet to test the sine shaping circuit I designed yet.
I'm also wondering when to use the buffers on the LM13700 and when not to, when re-designing old circuits. |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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Posted: Sat Nov 09, 2013 5:54 pm Post subject:
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I think... Of course the darlington in an LM13700 is just a current buffer to lower output impedance, so I'm inclined to think that it's never a bad idea. I notice the OSCar filter uses them for the filter part, but not the resonance/feedback (using an LM13600).
And, I read some vitriol from someone saying that the differential pair input was compromised on the LM13700 when compared to the CA3080, but looking at the schematic for the LM13700 I don't think this is the case. I guess the sine-shaping circuit (adapted from a Thomas Henry CA3080 design) will test this... |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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Posted: Fri Dec 27, 2013 4:07 pm Post subject:
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OK, once again I am an idiot, but I've been teaching myself about this for a while, and mistakes are only problems to be solved. After reading more of Horowitz and Hill I understand my mistake.
The drain and source of my JFETs were all the wrong way around in the circuits I built. Now I realize that because the op-amp of the integrator is used in an inverting configuration, the output of the op-amp will always be a lower voltage than the input (I think). This may be obvious to some, but I naively thought that it did not matter which was the drain and which was the source. Is this right?
Well, I'm not in full-on work mode as I just got back home from Christmas with my family and two young boys. Looking forward to testing my Ray Wilson core I built. Can the rest of you just put a little D,G,S onto your schematics?
Thanks,
e |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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Posted: Sun Dec 29, 2013 3:37 pm Post subject:
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UPDATE: after removing and re-fitting the FET in my Ian Fritz sawtooth core, with just the scope probe connected and no exponential converter connected, the output ramps up from 0V to about 4V and then RESETS
It takes about 8 minutes to do this! Time to build a working exponential converter. |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
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Posted: Thu Jan 02, 2014 6:48 pm Post subject:
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Has anyone explained why the "way around" matters? My understanding of FETs (admittedly not a DEEP one) would lead me to expect it shouldn't matter. |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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Posted: Thu Jan 02, 2014 7:06 pm Post subject:
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I really don't know, and the topology doesn't give me any clues as it looks completely symmetrical to me and doesn't have any PN or NP junctions int the way of the current path...BUT it's just occurred to me - perhaps the nature of the channel (N, P) dictates this. After all, N-channel has holes to be filled with electrons from a negative charge, and P-channel has spare electrons to be sucked up by a positive charge.
Still don't know. I should probably read this in more detail:
http://www.colorado.edu/physics/phys3330/phys3330_fa11/pdfdocs/AN101FETintro.pdf
I'll admit my ignorance for now. |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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Posted: Thu Jan 02, 2014 7:08 pm Post subject:
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PS I think MOSFETS can handle being swapped around. Maybe that's why my MOSFET VCO worked, while my JFET ones didn't. I just etched a board for the MOSFET prototype so I'll report back when it's built and I have some results. |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Thu Jan 02, 2014 8:22 pm Post subject:
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That looks like a great reference, thanks |
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gdavis
Joined: Feb 27, 2013 Posts: 359 Location: San Diego
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Posted: Thu Jan 02, 2014 8:24 pm Post subject:
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MOSFETs can't be swapped because of the internal body (substrate) connection. MOSFETs are more like 4 terminal devices but connecting the substrate to the source is so common that the connection is almost always made internally.
If you connect the drain of an N-channel MOSFET to ground and the source to V+, you forward bias the diode between the substrate and the drain.
Apparently most JFETS can be swapped but some can't (don't know why). _________________ My synth build blog: http://gndsynth.blogspot.com/ |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
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Posted: Thu Jan 02, 2014 8:27 pm Post subject:
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I was just going to post this quote from that "Introduction to FETs" and speculate about enhancement mode vs depletion mode:
Quote: | There are, however, some fundamental performance dif- ferences between MOSFETs and JFETS. JFETs, by na- ture, operate only in the depletion mode. That is, a reverse gate bias depletes, or pinches off the flow of channel cur- rent. A MOSFET, by virtue of its electrically-insulated gate, can be fabricated to perform as either a depletion- mode or enhancement-mode FET. Quite unlike the JFET, a depletion-mode MOSFET will also perform as an en- hancement-mode FET. |
I only vaguely remember the terms from my school days 25 years ago, but perhaps this isn't it.... |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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Posted: Sat Jan 04, 2014 10:49 am Post subject:
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UPDATE: The I got my MOSFET VCO prototype out again - it's still on my breadboard. I was surprised to find out that swapping the MOSFET around stopped it working. Hence the two FETs in the 4016/4066.
Swapping the direction of the JFETs on my Rene Schmitz sawtooth core, and the Ian Fritz, seems to have got them working. For now I'm happy just to know which way around to put a FET in an integrator+reset ramp VCO. couldn't get the Hutchins VCO to work however, but that's OK 2 out of 3 isn't bad.
Now to build some waveshapers! |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
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Posted: Sat Jan 04, 2014 1:55 pm Post subject:
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So which way round is it? |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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frijitz
Joined: May 04, 2007 Posts: 1734 Location: NM USA
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Posted: Sat Jan 04, 2014 9:36 pm Post subject:
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synchroma wrote: | Fundamentally like this, for a JFET-based integrator+reset-based ramp generator |
You have the JFET backwards on the drawing. What device are you using?
Ian |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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Posted: Sun Jan 05, 2014 8:22 am Post subject:
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frijitz wrote: |
You have the JFET backwards on the drawing. What device are you using? |
Really? This is where the mystery deepens in my head.
I'm using the 2N4391.
For me, this has worked for the Rene Schmitz VCO 3 core, and it seems to be working for your core too.
I only noticed this arrangement after I built the Ray Wilson core (with a PN4391): http://www.musicfromouterspace.com/analogsynth_new/VCO20120618REV0/images/vco20120615wsoic_page_1.gif
This is the one in the image posted on 2 Jan (where you can see the oscilloscope trace). |
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frijitz
Joined: May 04, 2007 Posts: 1734 Location: NM USA
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Posted: Sun Jan 05, 2014 10:58 am Post subject:
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synchroma wrote: |
Really? This is where the mystery deepens in my head. |
Yes, really. No big mystery. If you look at the ENS-76 schematic you will see the FET terminals clearly (and correctly) labeled.
Most JFETs are symmetrical, and it doesn't matter which way they are oriented. That's why their terminals often are not marked on schematics, and why they almost always work the same with either orientation.
Well, you can believe Ray or you can believe me and Bernie. Your choice.
Ian |
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synchroma
Joined: Oct 18, 2011 Posts: 72 Location: London
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Posted: Sun Jan 05, 2014 4:09 pm Post subject:
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frijitz wrote: |
Yes, really. No big mystery. If you look at the ENS-76 schematic you will see the FET terminals clearly (and correctly) labeled.
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Yes I see that. But, at the moment of discharge in Bernie Hutchins' design, "the capacitor voltage is +20 volts (-15 from the comparator, and +5 fed in from the ramp through R12)" (page 2, paragraph 3 from http://www.synthsource.com/ens76/vco2.htm).
Therefore the voltage at the Drain of the JFET is positive +20V relative to the voltage at the Source +5V at the moment of discharge.
As an experiment I tried swapping around the MOSFET I'm using in my own design, connected with the Drain at the input from the Exponential converter. It's an enhancement mode N-Channel MOSFET (2N7000). When I put it the other way around, the circuit stopped working. Of course the gate in my circuit goes from 0V to +15V, rather than -15V to 0V.
I'm not trying to start a fight, just trying to prove that you are both right and trying to figure out why
Ed |
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