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Topics - incurably_optimistic

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Amplifier Discussion / Hartke B90 - disappearing sound
« on: November 15, 2016, 02:40:06 PM »
Hello everyone,

Recently I've purchased the Hartke B90 from my friend for cheap and I used it at my rehearsal space. The sound was alright, but after some time a weird problem occured - one day, after 15 minutes of playing, the sound abruptly cut off and you couldn't hear anything. 5 seconds after turning off I've heard this "capacitor discharging" sound that was not there when I turned it off during normal operation.

If I turned on the amplifier after the discharge sound, it would work normally for 30 seconds or so, before shutting down again.

At first I thought that the power-amp was a chip-amp of some sort and what happened was some thermal protection shutdown, but when I touched the back of the chassis it didn't even get warm, and as you can see on the schematic, it is made of discrete components.

I also suspected a cold solder joint, but after I opened it up and took the PCB out, I couldn't find anything suspect. I did reflow most of the bigger solder joints, but the problem is still there.

I am new to amp troubleshooting, so I post here hoping that some of you could guide me or maybe someone more experienced has already seen something like this and knows what to do.

The problem with doing measurements is that the problem usually occurs after 5-15 minutes of loud playing, and for obvious reasons I cannot do this at home. I don't have any dummy load either.

Amplifier Discussion / best build for 18V laptop supply
« on: June 28, 2016, 12:45:21 PM »
hi everyone, it's me again - after building myself a "real" 3886 amplifier with the real power supply, I accidentally laid my hands on a 18.5V, 3.5A SMPS from a broken laptop. I figured I could use it to build a small chip power amp in the 5W-10W range so I have a few questions:
  • What chip would be the best for this application? I have or a LM1875 somewhere on my shelf, but while looking at the datasheet I found that this chip operating on a single 18V supply would provide even less  than 5W into 8ohms, so it's not ideal. Maybe using a chip designed for a low voltage single supply power supply would yield better results? Like one of those car chipamps (for example TDA2003) - do you have any suggestions which chip would work best in this situation? 
  • What about the class D amps? Take for example the TDA7492 - it should be able to provide more than 15W into 8ohms on 18V PS and 1%THD. Does anyone have any experience with them - are they any good?
  • And speaking of the laptop SMPS - I've read a lot about how noisy the switching power supplies are.  Would it really be a big problem in this build or will it work just fine?

hello everyone!
I posted this thread on diystompboxes site because I was more familiar with that messageboard, but I didn't get any responses (because, well, it's a stompbox board, not an amp one) so I figured that this will be a better place to post this stuff:

Hello everyone!

This is my first post so i guess i should introduce myself - I've been making guitar pedals for quite some time but I have never worked with amplifiers, so now is the perfect time to change it I guess! And please excuse me for my poor english as I'm from Poland so I apologize in advance for all my mistakes.

I want to build myself a guitar amplifier. As a first project I would have chosen something small, quiet and simple, but I already have a small 8W amp to play at home, so I thought that maybe I could make myself a practice amp loud enough to play in a band setting? I did some searching and googling and it turned out that a LM3886 chip would be perfect as a power-amp section to get something around 50-60W into 8ohm speaker (but please correct me if it's an overkill or wrong on some other level).

I've had an old & broken solid-state amp laying in the basement, so I disassembled it, threw out the board (after salvaging some parts like 2200uF capacitors or LM1875 chip) and now have myself a nice chassis and neatly wired AC section of the amp. Unfortunately, the transformer included in the amp is not powerful enough for me to use it - it only gives +-17V AC and has a max rating of 45VA, so I'll take it out and use it for some other project.

But back to the topic - now that I've decided on using the LM3886 chip as a power section (I've read that it's loud and clean and that's all that you want from the power section of an amp, right? oh, and it has lots kinds of protections from various failures) I have to choose a preamp to go with it, but I don't really have too much knowledge in this area, so I would like to ask you:

what are the good sounding and simple (as this is my first build it's kind of important) pre-amps to use with a lm3886 power-amp? I was thinking maybe of some low-voltage (or maybe high-voltage?) tube pre-amp - I've never owned a tube-amp so this is kind of a dream of mine, but if there are some great solid-state preamps out there I'm open to suggestions!

I would like it to have a clean and distorted channel, as well as some simple tone control (low, mid and high frequncy controls).

I also have an accutronics spring reverb tank taken from some broken amp that I would also like to throw in there.

For example, I found this project - and it looks nice for me, but what the hell do i know - what do you think about this one?

Let's say I decided to build the preamp from the link above - can I use one big transformer with secondary voltages 24/0/24 AC to get the +-35VDC needed for LM3886 and then use the +24/0 winding for the pre-amp (as the power supply on the schematic indeed uses a mains to 24V transformer)? Or should I use two separate transformers, one for the power amp and one for the pre-amp?

Sorry for this long post and thanks in advance for all the answers!

after not getting any replies I bumped with own thread with another bunch of stupid questions:

Sorry for bumping my own thread, but I found a schematic for a preamp based on the Fender Twin Reverb amp and it looks really simple and, more importantly, it uses two cheap transformers (one is 12v*0,25A=3W and the second is 12*0,15=1,8W).
Here's the link:
I was thinking about designing the power supply for the whole amp - does it make more sense to use separate power supplies for the preamp (as it uses two small power 230v-12v transformers) and the power amp (bipolar 230Vac primary +-24Vac secondary, power, say, 150VA) or should i use this big transformer to get the +-24Vac needed for the LM3886 and then some other 24V to 230V transformer to crank up the voltage for the tubes? Which solution is better/cheaper/more practical?

I also have a 600ohm accutronics reverb tank taken out from a broken fender amp that I would like to include in this amp, is this preamp powerful enough to drive the tank? If so, then i think i should use some kind of recovery stage, should a single op-amp operating on, say, +-20v be enough for this?

There are a lot of questions in those two posts, so if anyone could answer any of them I would be really thankful!

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