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Author Topic: 25W lm1875 Harp Amp with Professor tweed preamp  (Read 7903 times)


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25W lm1875 Harp Amp with Professor tweed preamp
« on: October 26, 2008, 06:23:36 PM »
Hi all, These are the schematics I used to build my solid state Harp Amp. I should say a couple things about it:
1) I used mainly scavenged parts for the power supply (except the 10000uf 35V filtering caps and the 12v regulator for the preamp power), so that's the reason you see two transformers. You should note that the trannies are wired in series such that there are +, -, and ct taps on the secondaries. It's vital to get the proper phasing here, so you must test the output with a multimeter before permanently wiring here, or you will get phase cancellation (effectively producing 0v out either tap). Figure 1
2) The prof tweed was tweaked with some info I gained from the DIY stompbox forum. The following changes were made and are shown as notes on my schematic: I changed all the DC coupling caps to 0.1uf. I changed the caps in the speaker sim section to 5n8. I changed the cap in the feedback loop to 0.01uf. The first two mods allow for more bass, a mod that is normally done to guitar amps to help "voice" them for harp. The last mod actually increases the gain a bit, which I found necessary to make this circuit work properly as a preamp. Also you should note that I am powering the thing at 12v, which means the fets should be biased at around 6v (it was originally powered by 9v). Of course, I biased the fets "by ear", which I always find more satisfactory. Figure 2
3) The lm1875 poweramp board is a kit obtained from qkits.com. I won;t paste the schematic here, but this is the link to where you can purchase it from qkits: http://store.qkits.com/moreinfo.cfm/QK50 The kit is high quality: I found the board to be really well designed, and the components to be of good quality. If you think it is necessary, you could always use higher quality components (1% metal film resistors instead of the stock 5% carbon ones, tantalium caps instead of the stock electrolitics), but who knows how much difference that would make. I really appreciated the on boarde fuse protection. You will need to provide your own heatsink, and make sure you isolate it from the rest of the chassis (the chip is not insulated). Also, there is an on board zobel network, which is prevents oscillation when driving difficult speakers or when pushing signal over long lengths of cable. No schem

Finally, please note that this amp was designed and intended for amplified Harmonica tone. That means it's really quite dirty, although I plan to add a clean channel in the near future. It may work for guitar or bass, but I don;t play either, so I can neither confirm nor deny it's efficacy in either case. Also, I decline to show schematics of the jacks and switching, as I did not make any, and these are often a matter of personal choice anyway. I wired a true bypass switch to the preamp as a place saver for a clean channel switch, and I made a effects send/return loop before the master volume control, but these are not requirements for a working circuit.

The amp can be seen and heard here: http://www.youtube.com/watch?v=RiV78_rUv8c
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