First bass amp building project!

[Jerzy Drozd]

HI guys!

Which amp you suggest me to build for my first project?
I want to build 50 watts amp.
any suggestion? and If you can give me link to the schematics

thanks!

[R.G.]

www.ssguitar.com

Build yourself a chip amp from the LM3886. 50-68W depending on how you do it.

If I may suggest, bass needs lots of power, probably in the range of 200-500W. Think about making a modular setup with one or two LM3886's driving each speaker. Four Lm3886's in bridge-parallel would provide about 200W to a single 15. Make as many of these boxes as you need, and put signal in from a preamp.

[Steve Conner]

For bass, I'm checking out the Class-D modules from Coldamp (and not just because they gave me an OEM discount! ) They cost a few hundred dollars, but I think they are good value for money when you consider the huge amount of work (and the huge heatsinks, cap cans and iron core transformers!) they replace.

http://www.coldamp.com/

You get 400W RMS into 4 ohms from something about the size of two VHS tapes, including a switching power supply. I'm currently working on a hybrid bass amp that uses the Coldamp power module with an Alembic/Fender style tube preamp and a small dummy push-pull "output" stage.

[Magic]

I read about Your idea for a bass-head and as I have the same idea I would like to hear how (or if!) You completed this project.

My plan is to use either the Coldamp or then an integrated PS-AMP from Cadaudio in Denmark.

[Steve Conner]

Well, I had something of a bad experience with it.

I got the SPS30 power supply and BP4078 amp module. On my first prototype, the SPS30 shut down early, limiting the output to 250 watts. While investigating this at home, I managed to explode the SPS30, spitting out pieces of blown transistors. I am an electronic engineer with previous switched-mode experience, and I checked with a friend who used to design SMPS and inverters for a living, so I'm pretty sure I wasn't doing anything stupid.

Coldamp sent me a replacement SPS30, and it shut down early just the same. I investigated more and found that the problem was the higher UK mains voltage compared to the rest of Europe. When I ran the SPS30 off a variac, reducing the mains to 220V, it worked perfectly and met the Coldamp spec.

So that was that problem solved, but then one day I was playing with the setup on my workbench. I cranked it to 400W output into a dummy load and forgot to turn on the cooling fan for the BP4078. It blew a MOSFET, shorting the output to the +60V rail, which would have cremated any speakers I'd had connected.

This was probably my fault for forgetting the cooling fan, but it was the last straw. I'd now need a speaker protection relay if there was any risk of that happening again, and part of the point of buying modules in was that I wouldn't have to bother adding stuff like that.

There was also the problem that my BP4078 had nasty clipping behaviour (sounds like someone treading on corn flakes when the clip light comes on) and what seemed to me a slightly aliased high end. You'd probably never hear this with properly bandlimited input signals, it was just the harmonics in my experimental overdrive preamp that brought it out.

I heard that Hypex Class-D modules are good, but I've never tried them. I think I'm going for a Plan B involving a bank of 6550s and a lot of iron Or a new hybrid amp design that I've been working on too.

[ocnarf]

The problem that Steve highlight is due to the fact that the amp he is using is not equipped with a good limiter, I suppose. In the amps I design for Epifani I placed an automatic soft limiter system that prooved to be very efficient.
Best regards to everyone.

[Steve Conner]

Well, I managed to get a soft limiter circuit working that stopped the BP4078 from clipping. I didn't publish it here in case I want to sell it to anyone in future.

It was the self-destruct functionality of Coldamp's gear that made me abandon the whole project. In particular the way that it threatened to take speakers out with it, unless I used a DC protection circuit that would take up even more space.

Also, I question the whole need for a limiter. If a power amp can't be overdriven without sounding bad, that should count against its rated wattage, according to the whole "Tube watts are bigger than solid-state" argument.