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Messages - Littlewyan

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Amplifier Discussion / Re: Old SS Project Rebuild
« on: March 28, 2021, 11:11:00 AM »
Quick update, turns out the TDA2050 has blown! Hence the funky stuff I found on the power rail. I’m going to order an LM1875 to replace it and see what happens. I don’t think the chip blowing was the issue, I think it blew because of the issue.

Amplifier Discussion / Re: Old SS Project Rebuild
« on: March 28, 2021, 07:23:00 AM »
And here are scope pics of the oscillation I measured. Test12 is the speaker output and Test13 is the power rail.

Amplifier Discussion / Re: Old SS Project Rebuild
« on: March 28, 2021, 07:22:03 AM »
Photos at last!

Amplifier Discussion / Re: Old SS Project Rebuild
« on: March 28, 2021, 07:17:41 AM »
Well the amp is built, I'm waiting for a new power switch to arrive, but for now I'm using a terminal block on the power wires. Unfortunately, I'm having weird oscillation issues! I've attached the schematic. Now, let me tell you a story!

First of all, I have tested the power amp by itself absolutely fine and then tested again later with the preamp on a breadboard. It worked ok, but had a lot of noise (expected as it's on a breadboard). I built the pre amp board, fitted it into the chassis and wired up, on first switch on the amp oscillated with the volume on 0. I measured frequencies all over the place, from as low as 500Khz to 2/3Mhz. I moved the volume pot ground from the preamp board to the power amp board and it went away, however when the volume was above 7 it would come back (This is without any signal on the input). On my scope this appeared as an almost perfect sine wave?! I could also see this oscillation on the ground points around the boards, however resistance between these points was no higher than 0.1ohms. Odd.

It gets weirder, when testing this morning, without changing anything the oscillation started happening all the time again. It's the power amp board, even isolated (Pre completely disconnected and PA input grounded) it oscillates and is no longer a perfect sine wave. I scoped the power rail and the oscillation signal is present and the same on the power rail as it is the speaker output. What on earth could this be?! The only change I made between breadboard testing and now is the feedback resistor on the PA board, I changed it from 22K to 18K, however it's still above 24dB gain so should be ok. I'm not sure that would cause the oscillation to be present on the power rail though. Any ideas anyone? I'm at a loss.

Amplifier Discussion / Re: Old SS Project Rebuild
« on: March 27, 2021, 07:37:37 AM »
Here are photos of the before. This was an amp that guys on this forum helped me build years ago. I knew practically nothing about electronics at the time, I barely knew ohms law, yet everyone was still very patient with me :). One in particular was Roly who I sadly understand is no longer with us. I would have loved to have shown him that his patience at the time was not wasted. I went back through those posts (I think back from 2013) and now with a better understanding was able to use the various equations and advice that he gave to me at the time to rebuild this amplifier into something better (And safer).

Amplifier Discussion / Re: Old SS Project Rebuild
« on: March 24, 2021, 11:14:46 AM »
I decided to drop the Mixed Mode Feedback circuit in the end and just stuck with the standard Voltage Feedback circuit from the datasheet. I read a few articles (Written by teemuk I think...) whereby various feedback circuits were compared and I wasn't confident that I'd have room on my board for the appropriate sized resistors. Perhaps in a future project I'll attempt this again!

The amp is currently half built. The PSU and Power Amp boards are complete and working, I have successfully breadboarded the Pre Amp and have just started to put the board together for it. Pictures will be uploaded soon!

Amplifier Discussion / Re: FET Self Biasing Method
« on: March 24, 2021, 10:49:15 AM »
I think you're right. Unfortunately in my case I designed my preamp around the FETs have 18K Drain Resistors, however I had to bump up to 33K to get the bias right in the actual circuit which increased the AC gain too much. I tried adjusting by Source Resistor but this didn't help much either. In the end I had to lower the supply voltage by 10V so I could stick with the 18K Drain Resistor. Luckily the tone wasn't affected by this (It's a completely clean preamp anyway).

Amplifier Discussion / Re: FET Self Biasing Method
« on: March 23, 2021, 09:14:03 AM »
I realise now that IDSS is actually the maximum current the JFET will draw.

Amplifier Discussion / FET Self Biasing Method
« on: March 23, 2021, 08:39:54 AM »
I've been building myself a simple amplifier (Of which I will post more details about in a separate thread) which uses a TDA2050 chip of the power amp and a few JFETs and NPN transistors in the pre amp. I've breadboarded it and it works fine, but I don't believe the bias of my FETs is correct. I've googled and googled, read various websites and have gotten nowhere, everyone seems to have different methods of finding the correct bias. Can someone please help? I'm using J202 JFETs, datasheet is here:

My supply voltage is 35VDC. I'm currently using 33K Drain and 1K Source. My VDS is 20V which is roughly ok, but the JFET is only drawing 0.46mA. The datasheet states it should be between 0.9 and 4.5mA.

Amplifier Discussion / Re: Old SS Project Rebuild
« on: March 18, 2021, 07:26:43 AM »
Ah sorry, ignore those, rather embarrassingly I don't have those diodes as models in LTSpice yet....I'll get around to adding them one day :D. So yeah they'll be 1N4001 in the final build.

Amplifier Discussion / Re: Old SS Project Rebuild
« on: March 18, 2021, 06:37:05 AM »
I may have actually just solved this.....possibly. Found an old post from J M Fahey on another forum where he suggested this to someone. It seems to have the desired affect. The only issue I have now is working out what wattage resistor I need to use for the 0.33...

Amplifier Discussion / Old SS Project Rebuild
« on: March 18, 2021, 05:35:49 AM »
Hello all!

It's been a rather long time since I posted here, but recently I decided to get back into building Solid State amps. The last one I built was......well no other way to describe it, a death trap! So I've gutted it and am starting again applying all the lessons I've learnt building various valve amps, mainly in safety! I will post project photos etc to show progress but I need some help please with applying current feedback to my design.

You'll see in the attached screenshot that I'm basically using the standard TDA2050 (Ignore the fact the model says TDA2030) circuit from the datasheet, but am trying to add current feedback to it. As I'm using it in a Non Inverting configuration I'd have thought it should be applied to the voltage feedback loop, but this gives me positive feedback? Is this right?

Amplifier Discussion / Re: Two Channel Amp Design
« on: December 08, 2014, 01:15:57 PM »
Thanks guys. I still haven't got round to this yet. Been tinkering with my valve amps lately as I've been using them a lot for band practices and just trying to get the tone right. Hoping to do this project over xmas.

Amplifier Discussion / Re: Two Channel Amp Design
« on: October 14, 2014, 06:11:24 PM »
I now have all of the components so I can finally design my board layout. I'm planning on having separate boards for the PSU and Amp, however I'm unsure about the Pre Amp filtering. Do you think the Pre Amp will be ok if I keep all of the filter caps on the PSU Board and just have jumper wires going over to the Amp board? I mean I've done this kind of thing before in a valve amp but unsure about solid state amps.

Also what would be the easiest way to wire up my pots? Solder wire permanently into the board or perhaps have some sort of plug/socket system to make it easy to take pots out?

Amplifier Discussion / Re: Two Channel Amp Design
« on: October 06, 2014, 01:27:55 PM »
O yes forgot about the potentiometer symbols!

Right I see, so basically make sure the input grounding doesn't get mixed up in the high current grounding by giving it a separate trace back to the star ground?

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