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Old 9th February 2009, 12:56 AM   #1
jetbat is offline jetbat  United States
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Join Date: Nov 2008
Location: Maryland
Default Building a discrete CA3094

Hi. I have a guitar amp that uses CA3094 operational transconductance amplifier chips. These chips have no substitutes that can be used (the CA3080 can be used as a replacement in some circuits but not others, and it is the same quality chip) and while they can still be found in limited quantities, I would like to have a higher quality chip. Since there are none, I was thinking of building a discrete OTA. I have a schematic of the chip, but since I am fairly new at electronics, I am not sure about a few things. On the schematic it list the resistor values, but that is it. The diodes I figure can be 1N4148 since they are used in the amp with the CA3094. The thing I'm not sure of is the transistors. I see that there are pnp and npn transistors, but I'm do not know what ones I should use since there seems to be quite the selection of different ones. I am trying to make the circuit as low noise as possible since I am using high quality op amps, resistors, capacitors in the guitar amp. Is there any way to figure this out?
Attached is a picture of the schematic.
Here is a link to the data sheet for the CA3094 if needed.
http://experimentalistsanonymous.com...ets/CA3094.pdf

Thanks for any help
Scott
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File Type: gif ca3094.gif (40.1 KB, 243 views)
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Old 9th February 2009, 03:10 PM   #2
jetbat is offline jetbat  United States
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For some reason it did not post the attachment. Here is a link to it.

http://www.mudpods.com/CA3094.gif

Thanks
Scott
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Old 14th February 2009, 04:03 AM   #3
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Please take a look at On Semi # NE5517.

http://www.onsemi.com/pub_link/Collateral/NE5517-D.PDF

- Richard
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Old 14th February 2009, 05:43 AM   #4
jetbat is offline jetbat  United States
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Join Date: Nov 2008
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The CA3094 is a single channel, 8 pin chip. The NE5517 is a dual, 16 pin chip.

I got some parts today.
2N5210 NPN
2N5087 PNP
1N4148 diode
2k & 47k resistors
Going to try to build one tomorrow. I'll post the results.
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Old 16th February 2009, 12:45 AM   #5
jetbat is offline jetbat  United States
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Join Date: Nov 2008
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I assembled the pc board yesterday and plugged it in. And... It works! When sound came out I was so happy. This is my first circuit that I have built. I have been working on my guitar amp for about 5 months now. It started as a recap job and turned into something fun.
The transistors were taken off of some non-working guitar preamp boards, the diodes and resistors were new. The board sounds at least the same quality as the chip. Its a little slow switching between the two because of the connectors from the board. I need to find a better connector for the board. The noise floor does not seem to have changed. Still a very low hiss. It may be that any changed in noise is being masked by something noisier. When I switched op amps the noise dropped considerably. When I switched the carbon pots for conductive plastic, there was another noticeable drop. I have more parts to upgrade before I'm done. I'm trying to make a ultra quiet reference quality guitar amp.

Anyway these are the parts used.
6 - 2N5087 PNP transistors
7 - 2N5210 NPN transistors
6 - 1N4148 diodes
1 - 2k resistor
1 - 47k resistor
1 - radioshack PC board 276-148

The board is connected with 25 to 30 Ga. wire (Not sure on the size, but it is thin) with pins soldered to the ends. (I used lead trimming from resistors I have used) The pins plug into the DIP socket. Attached is a low quality layout picture was done in Photoshop to help me figure what went were. The red dots are the PNP. The green are the NPN. The blue are the diodes. The purple are the resistors. And the yellow are the wires.
Here is a link to some photos of the board.

http://www.mudpods.com/CA3094_board.html

I'm going to look into making another with some higher quality parts and definitely find a better way to connect it to the socket.

Scott
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File Type: jpg layout.jpg (61.7 KB, 308 views)
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