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Liquids
Joined: Jun 06, 2011
Posts: 23
Location: CT, USA
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 Posted: Thu May 10, 2012 9:07 am Post subject:
Ray's VCO (Current Design) and FETs |
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I did a little correspondence with Ray about this, but I imagine his e-mail box is perpetually bombarded so I wanted to ask some follow up questions here rather than directly back to Ray...and maybe someone else will find this info useful too. 
For starters, my initial question was:
"Ray - what's the best commonly available sub (or ideal characteristics) for the PN9341 NFET in your current VCO design?
Mouser lists it as obsolete. [jameco and smallbear don't sell it either]
I have an array of MPF102, 2N5459, 2n5458, 2n5457, J201s for reference."
His reply was:
"I would go with the 2N5457."
So, now I have 2 follow-up questions:
1a) Is the PN4391 a notably more consistent, or superior device than the 2N5457 is, or is it probably just the one he happened to have a slew of that works properly here, and is the one he designed this around....yet the 2N5457 should be an otherwise identical/no difference substitute?
1b) Or should I expect diminished performance in tuning or waveform purity of the VCO from a 2N5457?
2) If I were to pick a 2N5457 from my stash, should I look for anything (measurable) in particular, or any will do fine?
Any takers? |
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havah1138
Joined: Feb 25, 2012
Posts: 4
Location: United States
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| Posted: Thu May 10, 2012 3:45 pm Post subject:
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I just started building a modular recently, starting with mostly mfos modules. I've used the resources provided in this forum extensively, but due to my inexperience I haven't seen an opportunity in which I would have anything significant to add to the experience and insight provided by the synth diy mainstays on this forum. That being said, I read your post and I can relate to your questions since I was in the same spot a few weeks ago.
The PN4391 is indeed impossible to find, but the 2N4391 is exactly the same thing in a TO-18 metal can package. I ended up picking up a couple on eBay for $1.50 a piece. I used a socket just in case, but it performed just as expected. Just mind the pinout on the datasheet.
I was going to use the 2N5457, but I had the same reservations as you, and I wanted to go with what was originally used in Ray's design. If you do end up just using what you already have, I would love to hear how it turns out. Hope I was of some help. |
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Liquids
Joined: Jun 06, 2011
Posts: 23
Location: CT, USA
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| Posted: Tue May 15, 2012 9:29 am Post subject:
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| havah1138 wrote: | I've used the resources provided in this forum extensively, but due to my inexperience I haven't seen an opportunity in which I would have anything significant to add to the experience and insight provided by the synth diy mainstays on this forum. That being said, I read your post and I can relate to your questions since I was in the same spot a few weeks ago.
[...]
If you do end up just using what you already have, I would love to hear how it turns out. Hope I was of some help. |
Thanks, sir! I'm rarely one to think there is only one part for the job. But of course, some are better than others and that is why I asked Ray.
I imagine one could pick Ray's brain about his reason for using that exact FET. Maybe he didn't even design around it originally. Maybe he could easily suggest tweaks if all one had was an MPF102 FET and a stubborn will about buying more parts....dunno.
I haven't looked at the old revisions - maybe the old ones use a different FET and have some tweaks - I haven't looked.
But as for my own experience, this weekend I finished my resistor-string 5 octave (61 key) keyboard controller, hooked it up to my already-built and working MFOS single buss keyboard PCB, then breadboarded rays VCO (only the part of the circuit that's on the first page, basically, ignoring all the post-ramp wave shaping and listening only to the 'ramp'/sawtooth), using the 2N5457, and it worked quite well.
In fact, it worked quite well even though I subbed a 18k resistor for the HF trimmer and resistor just to get it started, I used 2x 1k tempcos in series since that's all I have right now and my 2k tempcos still have not arrived from Scott, and at that, only one of the 1k tempcos was in contact with the 2sc1583 (or whatever obsoleted 5-pin dual transistor I have that is close to that numbering) I used for the expo pair.
And yet I still got very close tuning from the circuit IMO, given that it was just a hack attempt at hearing the circuit. I didn't test any of the characteristics of that particular 2N5457, either. I just grabbed one from my stash of FETs that I have set exclusively for breadboarding (past present and future), and plugged it in. There was nothing from there out that suggested I needed to do much more than tune it a bit to get a good response, or else I'd have been off to replace the FET and or the HF resistance with a trimmer. Not that one should just throw an 18k in there - I just wanted to hear it and play around, and I was being lazy and impatient when breadboarding it - just wanted to hear some audio, and plopped it in.
Anyhow, so I'd say, the 2N5457 should be used if that's what you have on hand.
havah1138 - since you have the specified part, if you have a 2n5457, and can easily swap it with the component you have, I'd love to hear you report back on your efforts of swapping, re-tuning, and comparing results.
To anyone who is reluctant to get the specified FET, and/or reluctant to build it without one, if you have a 2N5457, I think the level of success I had with tuning in the ballpark, fairly quickly and easily with a 2N5457 while ignoring critical tuning elements like a trimmer for the HF subbed with a randomly chosen 5% resistor value in the middle of the range of the design's set resistor+trimmer, on the breadboard, with tempcos in series and only one in contact with the expo pair should be indication that one need not fear about getting unsatisfactory results with 2N5457, if that is what you have, for example... |
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Mongo1
Joined: Aug 11, 2011
Posts: 411
Location: Raleigh NC
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| Posted: Wed May 16, 2012 12:51 pm Post subject:
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If it matters, I used the 5457s on mine, and that sound great.
Gary |
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