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Post by ccoleman on Sept 25, 2006 6:46:52 GMT -5
Guys I am seriously considering fitting a FET preamp but I have this one question for those of you who have already done so or have gave it some thought.. How do you handle the chance that when you're playing live, and the battery dies.. do you : a) add a bypass switch that feeds the volume control directly to the output so you can keep on playing, or.... b) install a second phone jack for the passive output that is always available.. and if the battery dies in mid-gig, just switch jacks and keep on playing!! c) something else  ? |
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Post by JohnH on Sept 25, 2006 7:39:18 GMT -5
My view is, build it carefully and then dont worry about it. If well designed, an FET preamp takes very little current, and batteries do not suddenly die, but they fade slowly, and the sound is not affected until they have decayed significantly. They are much more reliable than your strings, or probably your tubes. On my guitars, I test them occasionally with a DVM, which I do through the jack socket by plugging in a stereo jack plug.
John
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Post by ccoleman on Sept 25, 2006 10:07:07 GMT -5
Thanks for the tip John, I guess it's true , a 9V battery really does last 6 months or a year in this application... In your opinion does the following kit, that I just found on ebay, have the right type of JFET and the right values of other components to make a high quality onboard FET guitar preamp ?? If not I'm not going to get it.. cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320010755136 |
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Post by sumgai on Sept 25, 2006 15:05:19 GMT -5
cc, I don't know what this eBay seller is up to, but his description leads me to believe, correctly or otherwise, that he is including a butt-load of components with the kit. For $8.50, the parts for the circuit that he shows off to the side (including a PCB) would be a good value, but if you get all those other parts too...... no brainer! ;D So just in case I'm wrong, I looked at some his other stuff.... Wow, 476 items in his eBay store! Lots of packages of raw components (you know, 10 of this, 20 of that, 5 of something else), and most of them are over $10, but I kept looking around. Guess what? He has many kits of the exact same nature as your desired JFET amp (Kit #1) - but they all have the exact same description, and the same schematic! The only thing changed from one item to the next is the name of the JFET he's offering. They all come with that same butt-load of parts, which I find attractive, but they omit one important point - - no 2N3904 transistor! If you wanna build that particular circuit, then you've got to buy either the single part, or another kit, presumably from the same joint. Not a bad idea, so long as one is aware of what one is getting into, cost-wise. Now, outside of all that, to answer your question more directly..... His suggested circuit uses a 2N3904, a general purpose small-signal amplifier, and which is considered a bit noisy by today's standards. And no bi-polar transistor can ever be called miserly about power, they are all juice hungry, compared to an FET. This circuit will eat 9V batteries much quicker than 6 months, more like 6 to 8 weeks, even if you don't use it more than 10 or 12 hours a week.  Have you given thought to any of John's own circuits, published in various threads on this forum in the past half-year? Try this one for starters. Take a look at the far right side of the schematic.... note that there are only three resistors and three capacitors, plus the FET. Not bad - cheap, easy, compact, and very long battery life - what's not to love? ;D I remember seeing some other schematics here too, but I can't find them at the moment. Perhaps John can help here.... the Search function is malfunctioning on this topic. HTH sumgai |
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Post by UnklMickey on Sept 25, 2006 15:11:19 GMT -5
... the Search function is malfunctioning on this topic. ... more than likely, the searcher is not aware of how to manage the nuances of the search function. what seems to be giving you difficulties? |
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Post by JohnH on Sept 25, 2006 15:49:33 GMT -5
That circuit of mie that Sumgai pointed to is my favorite on-board preamp. It is just a buffer, and it cleans up the signal. It does not provide extra gain or adjustment, but I think it is a great and reliably simple thing to enhance the clarity of the guitar signal. I recomend it if that is what you want, but what are you looking for your preamap to do? I may be able to suggest alternatives.
John
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Post by sumgai on Sept 25, 2006 15:55:16 GMT -5
unk and John, I searched on "fet", and "all posts by member: JohnH", and got back lots of stuff, but I was sure John had posted more than one schematic, that he'd also posted a simple version, sans guitar paraphernalia. John, am I imagining things here?  And I'm pretty sure I'm not just thinking of the 'in-cable' amp that you once linked to, but maybe my recollection is a bit off target on this one. Still, I'd think you'd have mentioned 'fet' in the body of the message, or at least somewhere in the thread from whence it came, and the Search function gives me zippo. Perhaps unk is right, it is my inability to search correctly......  sumgai |
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Post by ccoleman on Sept 25, 2006 16:10:29 GMT -5
John to answer your question, I just want to be able to flick a switch and make my guitar immune to the various unpredictable amounts of cable capacitance in my different cables and varying values of input impedence in my different amp and preamp inputs... basically at will, convert it into a Lo-Impedence output instead of high-impedence...... And Sumgai... right on, the 3904 is missing from that ebay kit, but I dont care because I wouldnt want it, I would prefer to build john's JFET circuit since it is much more miserly with the electrical power and long lasting on a single battery... and bonus like you say it would be lower noise. For me, battery life concerns are the prime downside of installing an onboard FET preamp, plus accessibility to be able to change the battery easily (gotta install a battery door on the back of my axe!) and low probability of running into problems during any one gig / recording session.. gotta use a rechargeable 9V and carry the 9V battery charger + a spare 9V IN MY GUITAR CASE.. and even at that, Im 99% sure I will install an emergency bypass switch in case battery power drops too low during mid-gig !! The thing I like about that ebay deal is getting ALL the parts needed in one shot... I have no time to shop for individual parts !! price is great too.. !! UPDATE: there is also this onboard FET preamp item... fully assembled and ready to install in your guitar... sorta cheap price.. what do you guys think of this one ? cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=250026948897UPDATE 2: Guys, Check it out.. this guy modified a don tillman onboard fet guitar preamp to make it more linear response.. veroboard layout diagram included. Whaddya think ?? johannburkard.de/blablog/music/effects/2006/08/27/JFET-Preamp-Splitter.html |
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Post by UnklMickey on Sept 25, 2006 16:11:08 GMT -5
Sumgai,
in this case, i think the search is doing what it's supposed to.
you might be thinking of the "fetzer valve" thread where John was working with Nightowl.
i think John didn't post an additional diagram, just outside links.
John,
since this vendor also has a J-201 kit, as well as 6 J-201s for a buck,
would you recommend those over the J-174 kit?
a novice will be able to find much more information on a J-201.
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Post by UnklMickey on Sept 25, 2006 16:26:41 GMT -5
"It uses a MOSFET transistor for the amplifiacation instead of an Op-Amp, which is ususally used for the high-boost applications. The FET has a much warmer, more musical sound and if it begins to clip it does so gracefully, without sounding terrible. Using a quiet J201 JFET transistor, it is a very low noise circuit, and will amplify your signal all the way up to 9V." that might be just fine, but he'll never get a dime from me. the J-201 is a jFET not a mosFET. they are not built the same. it won't go to 9v, without distorting. (not that it matters, i just don't like incorrect claims.) and that 9volts would be peak to peak, not peak or RMS. i prefer to not do business with someone who can't get their facts straight. but that's just me. |
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Post by ccoleman on Sept 25, 2006 16:31:44 GMT -5
true unk... MOSFETs are audio output power transistors.. !!!!
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Post by UnklMickey on Sept 25, 2006 17:10:06 GMT -5
i think there might be a wiring error on the board. (unless there is a break in the middle trace of the board that we can't see.) if JohnH has the time, he might be able to confirm or refute this. also, many FET output transistors are mosFETS. but a mosFET doesn't necessarily have to be an output transistor. the terms mosFET and igFET are generally reserved for FETs with insulated gates. jFETs have a "junction" between the gate and the channel. unk |
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Post by JohnH on Sept 25, 2006 18:06:50 GMT -5
Hi again. Im at work now. Im looking forward ti following those links, but probably won't get to it until tommorrow. The JFET circuit on the LPmax is my only posting of a buffer that I have designed, and it does exactly what ccoleman describes - no more and no less. One thing to bear in mind - because it protects the signal from cable capacitance, it clarifies the top end - ie it sounds a bit different (better IMO) - its like having a very short cable. So by all means, use a bypass switch if you want to ring the changes with this effect however.
John
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Post by ccoleman on Sept 26, 2006 0:15:56 GMT -5
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Post by JohnH on Sept 26, 2006 1:41:35 GMT -5
The Tillman design is fine, and is the most well known preamp of this type. My version has more headroom, and uses less than 1/10 of the battery power.
On the burkhard design, Unk is right - it wont work without a break in the copper trace which he has not shown. No problem though, but a bit sloppy.
You could build a Tillman, from that kit. But I would not buy the kit, because there are alot of parts you dont need, a poor range of capacitors, and only one board, so you can only build one thing. It would be better to buy packs of resistors, caps and some JFETs, and there may be suitable packs on his site -Ill take a look
John
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Post by ccoleman on Sept 26, 2006 7:01:19 GMT -5
OK John thanks for offering mate, yes I would appreciate your offer to check out that seller's parts kits on ebay to find what is most appropriate for an ultra low power JFET onboard guitar preamp, because it would really make life easier to get all the parts required at once...
Also, in your opinion, is the tillman J201 design really lower noise?? If yes, how would you modify his design to use 1/10 the current like your design does..?? Or is it impossible because it is the fact that he uses a different J201 JFET that would be the reason why his circuit is eating 10X more current ??
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Post by JohnH on Sept 26, 2006 7:52:58 GMT -5
OK, to buy packs of caps and resistors etc would cost more, so only worth while if you want to get into electronics, beyond just making a buffer. If you get the J201 kit, you can make a buffer, maybe using a couple of caps together to compensate for missing values in the kit. You could make the Tillman, or you could make one like mine using a J201, and I can suggest which resistor values to use to work with the J201. My design is very low noise, and is a better one for ultra low power, because it achieves this while still having very low output impedance. The Tillman one has a bit of gain, which mine does not.
BTW, I dont really regard the buffer I use as 'mine' - it is just a standard configuration of a JFET, adapted for this job.
With the J201 kit, apart from the board, youd be getting enough parts to make one of each design, or two the same. You could cut the board in half if you wanted to try this.
But you could also order just what you need from a supplier like Mouser.
John
EDIT - just noticed you only get one battery snap!
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Post by ccoleman on Sept 26, 2006 10:07:48 GMT -5
Here is a guitar fetish onboard pre... $27 shipped off ebay... it DOESNT say if it's using a jfet or opamp !! But the part looks like a JRC4250 which I dont have any info on... Does J mean that's definitely a JFET part ?! What do you think of the noise performance and probable battery life ?? Ever heard this one in action before ?? cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7358107271 |
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Post by UnklMickey on Sept 26, 2006 10:28:57 GMT -5
.... Does J mean that's definitely a JFET part ?! ... no. i'm pretty sure the JRC prefix came from a brand name. (Japan ... something) since JRC-4558 was an op-amp used in tube-screamers, it is likely JRC-4250 is also an op-amp. unk |
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Post by ccoleman on Sept 26, 2006 11:04:51 GMT -5
ah OK unk. It's just weird there are apparently no free datasheets on the internet (that dont require you to log in somewhere) for that JRC-4250 surface mount chip. But here is why I still have a doubt: Since the tube screamer is a world renowed sweet sounding distortion pedal... I would not be surprised if it uses the JFET technique (rather than biopolar) to get its sweet tube-sounding overdrive. Is there such a thing as a JFET opamp ? I bet that Yes, it should be possible that these exist... What do you think ?? |
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Post by UnklMickey on Sept 26, 2006 11:14:38 GMT -5
...Is there such a thing as a JFET opamp ?... yes, but they didn't exist until about 25 years ago. |
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Post by ccoleman on Sept 26, 2006 12:40:00 GMT -5
ah..yes.. interesting unk... I just found out that the TL072 is a FET input opamp cousin of the 4558.. to keep in mind for future designs.
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Post by ccoleman on Sept 26, 2006 12:43:30 GMT -5
just noticed you only get one battery snap! John ... the kit ALSO has NEITHER a stereo 1/4 inch jack (to save battery) NOR a bypass switch for when murphy's law strikes and the battery dips below 7Volts mid-show.. |
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Post by sumgai on Sept 26, 2006 15:21:54 GMT -5
cc, Those kits you're finding on eBay are cleverly devious, designed to give you enough parts to do all, or nearly all, of one job, but with so many left over, you fee like you "just gotta" order some more so's you can make another device. As John said, if you're gonna get serious about spending the winter months with a soldering iron in your hand, this ain't a bad way to go. Just exercise a little care and caution, and ask questions here if you're not sure. But if you just want a 'plug and pray' solution, the more expensive pre-assembled kit might be your better choice. Besides, if it doesn't work, you don't have to take the blame for it! ;D Just ship it back, or whatever it takes to get right with the seller. A tip for ya. When looking at IC packages, the rule of thumb is: 3-legs, either a discrete device, a very tiny voltage regulator, or a reference standard 4-legs, probably an opto-isolator 6-legs, possibly two discrete devices, pretty rare. 8-legs or more, a large bunch of devices packed into a real tiny space! I'm not so sure that the 4250 designation is the chip type. Usually, the first line of a package's numbering is the date code. That's not hard and fast, mind you, but in most cases, this is so. It might explain why you can't find anything under that number in the datasheet repositories.  Go back and try it again with 6007A, and see what you can find. And JRC stands for Japan Radio Corporation, an abbreviation not often seen in the USA. And just to put the finishing touches on your masterpiece of confusion, bear in mind that tiny design/manufacturing houses like this one are prone to using "house numbers", or to put it in plain English, they obscure their product by purposely not using standard part numbers. They have the chip maker inscribe them with numbers according to their own (the designer's) scheme of things. That'll throw a monkey wrench into your search!  The TL-07x series are low-power, low-noise packages. The -071 is a single device package, the -072 has two devices per package, and -074 has 4 devices per package. (Actually, those are not just discrete devices, but complete circuits inside there. But you can treat each one as if they were a single active device.) There is also a TL-08x series, which, generally speaking, can be interchanged with the above. The trade off is, you get a slightly higher noise figure in return for greater bandwidth. The TL-07x is more desirable for our purposes, but when you buy them in "hobbyist" quantities (less than 1,000 piece lots) they are more expensive than the TL-08x. And finally, bear in mind that the JFETs are only used in the front end of each circuit. The rest of the solid-state junctions are bi-polar. Don't let that scare you though, too many successful designs have used these things for you (or anyone) to dismiss them summarily. The datasheet is here. Just a bunch of FYI's.  sumgai |
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Post by ccoleman on Sept 27, 2006 12:11:16 GMT -5
I ordered the J201 parts kit........ it should arrive in about 1-2 weeks... it comes with about 5 J201's too.... Hot diggety !
While we're waiting for it to get here...... Any suggestions on how to modify the JohnH design the take a J201 or modify the DonTillman design to run with lower power would be totally welcome.
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Post by UnklMickey on Sept 27, 2006 15:33:26 GMT -5
THERE'S NO SUCH THING AS A FREE LUNCH! Tillman's design is a common source configuration. it inherently has some voltage gain. Tillman designed it for modest (3dB) gain, reasonably low noise, reasonably low output impedance, and low power consumption. the configuration John chose is a common drain (aka source follower). it can only have unity (or less) voltage gain. ie. the output signal will be the same amplitude as the input signal. it is a BUFFER. the reason to use it, is the inherently low output impedance. because of it's inherently low output impedance, John was able to increase the value of the source and drain resistors. this increases the output impedance, but it is still reasonably low. but the power consumption is extremely low. if you scale up the source and drain resistors on Tillman's design, that will: -- decrease the power consumption. -- increase the output impedance -- increase the noise only the first one of these is a good thing. you might get away with it, but my advice is, if you need a little gain, use Tillman's circuit -- AS IS. only tweak the values (slightly), if needed, to balance the circuit. and stop being jealous that you only have low power consumption, instead of extremely low power consumption. if you don't need gain, then use John's configuration and: ...You could make the Tillman, or you could make one like mine using a J201, and I can suggest which resistor values to use to work with the J201.... so the only question that remains unanswered is: which do you want? a little gain? or ultra-low power consumption? pick ONE. cheers, unk |
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Post by ccoleman on Sept 27, 2006 15:55:38 GMT -5
hmm... good point unk.. thanks !!!!
I don't know WHICH one I want... but I guess that I could affort to get a little gain, but let me ask.. why would I really WANT a little 3dB gain ?! what do I get during performance by sending a bit more signal thru my pedals and rack FX units ?!
Did you ever play like that, and if so, what benefits or drawbacks did you notice, as far as level matching, being able to overdrive inputs, letting your playing stand out over the RF, or anything else...!?
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Post by JohnH on Sept 27, 2006 15:56:42 GMT -5
ccoleman - Yes - what Unk says is right. From what you have said, the buffer type that I use will suit you better. It is not hard to rework it for the J201 - in fact I could tell you the likley values now. But it is better to test. I have some J201's, and i can mock up a buffer at the weekend, using values only from that kit. Then I think you would be good to go.
John
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Post by ccoleman on Sept 27, 2006 16:01:11 GMT -5
thanks john... man, this board is so full of helpful people, I can't believe it !!
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Post by JohnH on Sept 30, 2006 0:02:18 GMT -5
ccoleman - just to let you know that Ive finished checking out the buffer design for the J201, and its all good - it will work very well. Ive tested it to simulate it driving a low impedance input, through about 100' of cable, and with a decayed battery down to about 4.5V, while thrashing with my loudest 2Hb in series pup setting! It has that desirable quality of being almost a perfect device. Full posting will follow.
John
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?
Go back and try it again with 6007A, and see what you can find. And JRC stands for Japan Radio Corporation, an abbreviation not often seen in the USA. 
