12AX7 + LM3886 schematic

[danerius]

Hi. Just got my amp from eBay and theres no schematic included. Ive mailed the seller repetedly but to no avail. Can any of u fellas help? Planning to mod the feedback loop and too Im thick to find it the right resistors? Need a schematic?

The logo on the board says: Ai HI-FI, aibiwen

Regards /Bo

[Rodeodave]

Just draw the circuit from looking at the board. Take a sheet of paper, a pencil and an eraser, draw the chips and the tube first, and then fill in the blanks. The capacitors usually have their value written on them in plain text, the resistors as a color code: 5 Band Resistor Color Codes.
For chips' and tube's pinout see:
http://www.national.com/ds/LM/LM3886.pdf
12AX7 - Wikipedia, the free encyclopedia

And please post your schematic, I'm sure there are more than a few people interested.

[Rodeodave]

If you need help identifying parts just take a picture and post it here, we'll help. While we're at it, e few more pictures would be nice anyway

[Rodeodave]

This might help: 12AX7µç×Ó¹ÜÇ°¼¶+LM3886ºó¼¶¹¦·Å

[danerius]

Originally Posted by Rodeodave

This might help: 12AX7µç×Ó¹ÜÇ°¼¶+LM3886ºó¼¶¹¦·Å

Hahaha... thats it No info on the LM3886s feedback loop though. But I found this:

Theres 0.2ohm 2w resistors on my output as well

Regards /Bo

[danerius]

Originally Posted by Rodeodave

If you need help identifying parts just take a picture and post it here, we'll help. While we're at it, e few more pictures would be nice anyway

Took these. Hope it helps. Need any other angles just let me know

Regards /Bo

[Rodeodave]

The circuit of your actual amp must be different from the one you found. The feedback resistors will be close to the output, I'm guessing 22k and 1k. Look here: http://img03.taobaocdn.com/bao/uploa...OU8_101122.jpg. Just reverse-engineer the circuit, you'll learn a lot.
What type are the small transistors?

[AndrewT]

Hi,
check if your circuit matches the posted schematic.

The posted schematic has problems.
Does your circuit work properly?

The feedback of the inverting chipamps is 200k & 10k.
The input bias currents flow through the 10k into each other. This will upset the input offset voltages and the output offset voltages.
The three 10k are in parallel to the 47k giving an input impedance of ~3k11
The gain is 20times (+26dB)

Your source must be able to drive that 3k11.
A valve at the input used as a buffer to drive that 3k11 is in the wrong place. The buffer should be back at the output of the source.

[danerius]

Rodeodave: Upon further inspection ur right. Theyre different. Ive mailed hifibar.net regarding the schematic. The amp was bought off some dude on eBay. If theres no reply from them I'll reverse engineer it.

That PCB-board looks spot on. Any further info on it? The transistors says: S9014, C331

AndrewT: Amp is not up and running yet. Still need to order transformer and some other stuff. Still gathering info on what is need to complete the amp.

Speaking of that. Im thinking a 24-0-24 3.33A transformer? Any thoughts on that?

btw: Ive never built a power amp before.

Regards /Bo

[Rodeodave]

Hi,

No, no further info. I just googled for "12AX7 LM3886" and switched to image search.

Regarding the amp-building...Read a lot of articles here: DIY Audio Articles (especially the Power Supply section) and here: DIY Audio Projects - Hi-Fi Blog for Audiophiles. A lot of info on gainclones can be found here: Gainclone chip amp index page.