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Messages - J M Fahey

Thanks, Teemuk and Darwin. Well, it MAY work, and could be built as a curio, but if simplicity is the name of the game, the bias can be simplified (and bettered) by pulling the zener (which does NOT track the output stage temperature) and recalculating the bias resistors , replacing the unnecessary FET "current source" by a 2k7 resistor, and some other minor changes. The "power stage" has only a gain of 6 (100K/15K),and isn´t properly stabilized. An LM3886 is as simple and much better.
Schematics and Layouts / Re: Dean Markley
May 01, 2008, 10:23:32 AM
Great amps. The K-15 is great as a killer sound homemade amp, check it!!
Dear Mensur. Thanks for posting these pictures, but I´m afraid they're not enough. Would you be so kind as to post your MG100 schematic? That would help a lot. Thanks.
Schematics and Layouts / Re: Baldwin Professional
April 29, 2008, 11:45:58 PM
Thanks for sharing such a beautiful piece of history. There's a very minor error in the schematic drawing: in the 6 output transistors, emitters and collectors are swapped, that´s to say, emitter "arrows" should be touching the R .47's , and pointing "upwards" towards the "-43". Mr Martínez pictures were also very good.
Well, oliveaudio seems to have dissapeared from the net. Can anybody post those schematics, just for a curious look? Thanks.
Dear Teemuk. I *think* I understand what the CA3094 does here. It´s a buffered OTA, very similar to the LM13600, great for compressors or VCFs. My doubt was: if that´s so, where´s the diode ? (or rectifier). I think I´ve found it (or at least something that functionally imitates it). The guitar signal is fed into pin#2 (suitably attenuated 15k/1k); it develops across the OTA's load (10k+20k trimmer), and is buffered by the right half Darlington pair, leaving through pin #6. The signal (480mV) is fed into pin#5 of the 3094. The audio *voltage* is symmetrical, but I think that the *current* is not, the pin 5 surely does not source or sink the same amount of current. In fact the datasheet says it sinks a few mA (the gain, or more precisely transconductance-controlling current), fed by R169 (33k) , but says nothing about sourcing current, in fact it isn´t necessary at all. I think that the non-symmetric current handling capability creates a D.C. component which modulates gain with the audio signal. Not the best "diode" in the world, but it has no threshold, and makes the output signal non symmetrical to boot. What do you think ? J M Fahey
Dear Bismark. An easier option is to  build a power supply with "over the counter" components. You´ll need a 24V center tapped 2 Amp transformer (that means 3 secondary wires), 4 1N5402 diodes (or a 3-4 Amp bridge) and two 2200 or 4700 uFd x  25V. electrolytics. It´ll give +-16Volts (32 total) and will allow your amplifier to supply around 15W, which is an excellent practice power. These are VERY common (and relatively cheap) components.
Amplifier Discussion / Re: TDA2003 bass amp
April 29, 2008, 07:35:02 PM
Dear JoeCool. I think thay your BIG problem was not so much with the amp chip itself, but with the speaker. Guitars work anywhere, bass guitars don´t. Please try it again (powered by the 12V battery) with two 6 or 8 inch woofers, 4 ohms each, in parallel (yes, the optimum load is 2 ohms), in a closed box at least 10" deep (WxH depends on speakers used).  You´ll see that the result is more than usable. (No, not with a drummer ) Please try it and post the results . MP3s and pictures would be most welcome.
Amplifier Discussion / Re: Gibson G-80
April 27, 2008, 01:08:44 PM
Hi. Please scan and post the schematic . Don´t change parts at random (specially IC´s) hoping to find the bad one. Very probably the output transistors are shorted (that "hum" is going to kill your speaker). Bye.
Don´t waste your time. With 12 Volts nothing you do will beat a TDA2030 or similar. You´re not trying to build a real "Tube amp".
Amplifier Discussion / Re: Holmes MiniPicker 12
October 02, 2007, 09:56:20 PM
3055 possible but waaaaay overkill. Tip 31 are already better and Tip 41 most bang for the buck, while still TO220
Amplifier Discussion / Re: little rebel layout
October 02, 2007, 09:49:49 PM
Well, I´ve seen the schematic. It´s a Dist pedal , about 90% of a ShredMaster/Guv´nor, etc.,  without the contour, straight into 2 LM386.   Looks cool though. I think the samples were recorded using some kind of Celestion box, which sure helps anybody. Sorry but I think that the Canadian speaker, with its whizzer cone glued to the voice coil will sound buzzy. Good luck wuth the project.