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I have a problem with a Schematic

Started by Conguito, February 25, 2007, 11:20:51 PM

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Hello I've tried to do this guitar amp: http://www.redcircuits.com/Page52.htm
I've separated the preamp stage from the power amp stage and the problem that I have is that the preamp works correctly without any problem but when I connect de power supply to the power amp stage the fuse (1,6A) blows quickly, I've measured with the Ammeter and it gives me a measure of 6A!!!!! I have to say that the transistors are all in the correct position and with a good heatsink (designed for 20W)........who can be the problem?
Anyone has tried to assemble this amp? I'm start to think that the schematic is not correct......


Personally, I'd avoid any design on that page. In my opinion most of them are very, very bad designs. So what's up with that... This is likely the most basic power amp setup: Bootstrap/bias circuit loaded common emitter is buffered with a push-pull emitter follower current amplifier. DC stabilization and gain set is handled by feedback through C11, R13 and R14.

If you have shorted that you have either

a) built it incorrectly
b) destroyed one of the transistors i.e. while soldering
c) set up the bias incorrectly.

Replace ALL transistors in the power amp because a tester will not likely reveal which are faulty now - except under loading.

Check the condition of the bias trimmer: In this configuration a lifting wiper will blow up the power stage. Wiper may lift even during an adjustment.

Check that the circuit is built correctly and power it up with the bias trimmer set to minimum resistance between collector and base. Follow the instruction of adjusting the bias.

Do yourself a favour: Replace the two prong cord with a three prong cord that has a safety earth and fit in a PRIMARY FUSE. D2 and D3 could (and should) have a greater current handling capacity as well.

Edit: The design should work with right component values and since the guy is selling it as a kit then likely it works. If it doesn't I wouldn't be surprised. Some of the designs on the concerned page use components with insufficient voltage/current ratings, badly designed bias circuits and tone stacks. Most of them are serious safety hazards with their two prong cords and only secondary fusing. Well, this design filled all those clauses.


Well, I will work a few days with this schematic, but if it still doesn't work........┬┐Anyone has other schematic for a power amp made with transistors? I prefer a transistor power amp and if I can I don't want to use a chip amp, my idea is to make a 5-10W power amp, I don't need too much power to play at home.


Ok. Some points you should tweak: Where are the emitter resistors? Looks like the amp is begging for thermal runaway. The power amplifier should be buffered since in this configuration the volume potentiometer will not control gain logarithmically. If you want to stick with the design then you should at least correct these drawbacks.

I ran a SPICE sim of the circuit and it works - however, the power ratings are pretty much bloated. It's more like 5W to 8 ohms and 10W to 4 ohms. If you did not get this circuit to work you have built it incorrectly. The bias pot should likely read about 1.9k to get rid of the crossover distortion.

I know that the used topology itself is not that bad: It is actually very stabile as the feedback goes straight to voltage amplifier stage. The capacitor coupling provides DC protection for the speaker but increases distortion - especially if you use low quality components. Those are about all the merits this simple circuit can provide.

If you want to try another design...

I have to ask: Why not a chip amp? Feature-wise they are pretty good and most certainly beat simple discrete designs in distortion figures and reliability. If this is for a reason that you think that simple designs sound better I can assume that it is not so. Anyway, there are good discrete designs but they WILL be more complex than basic circuits like the concerned one. That's the price you have to pay for quality.

If you want to find another discrete design why don't you just do a google search? Or search this forum? There are couple nice schemas on the schematic section of this forum - like the Princeton amp. It is about 10W and discrete. Joe had some designs too if the links still work. Also, It hasn't been a while ago when someone asked for PCBs and schemas for 10-20W amp. I provided some links back then.


Well, what I really want to do is a power amp stage because i want to use different preamp stages with the same power amp, I will use a switch to change from a preamp to another......and I think that with an chipamp I can have more problems that whit an transistor power amp, but if you think (I see that you are more experienced that me with amp construction, this is my first amp construction, but not my first electronic work, I have done many PCB's without any problems at this time) that with an chipamp I will have no problems with the preamp switching........you are welcome to give me your tips  :tu:
I am looking for a clean sound power amp, I will make the distortion or crunch tone with different preamps has a chipamp a cleaner sound that an discrete power amp?


An amp is an amp whether it is fabricated on a chip or built out of conventional, discrete components. Making a switching arrangement you described should be as easy with both if you understand how the concerned circuits work. If this is troublesome then you simply have to study more.

In my opinion, a simple, low power amp is about the worst choice if you want a clean sound.