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Messages - Littlewyan

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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?

Amplifier Discussion / Re: Two Channel Amp Design
« on: October 05, 2014, 06:58:03 PM »
Ah ok I see. I always knew you should decouple every two stages but didn't know why :D.

In the notes for the TDA2050 it mentions having separate grounds for the two inputs and the speaker output in case of high current. I'm guessing it is basically telling you to avoid a ground loop and to keep the grounds as far apart as poss?

Amplifier Discussion / Re: Two Channel Amp Design
« on: October 05, 2014, 06:49:49 AM »
Ok I've done as you said and did some more decoupling in the PSU. So basically transistor amps have the same rules as Valve Amps, decouple the supply every 2 gain stages. I didn't bother with much decoupling originally as I saw other manufacturers didn't but then again they were using Op Amps and they were probably skimping on parts.

For the Power Transformer I've bought a 2x18VAC 30VA Toroidal. Next size up from that was a 2x25VAC 100VA Toroidal but I didn't think the very small gain in volume and large gain in heat was worth it.

Managed to find some Transistor sockets on ebay as well, 10 for £2.50. The amp is coming together :D. Just need to draw out the layout of the PCB and then order the PCB making kit.

Amplifier Discussion / Re: Two Channel Amp Design
« on: October 04, 2014, 09:27:47 AM »
What size transformer would I need for this? I read on another forum that someone recommended a 100VA Transformer with a 20V Secondary. Thats 5 Amps! Sounds a bit extreme to me. I was thinking more 20VA. What do you guys think?

The Quiescent drain current of the TDA2050 is 90mA Max with 25+/- supply. I'm using a 25V DC Single Rail Supply so I imagine it'll be a bit lower than that.

Amplifier Discussion / Re: Two Channel Amp Design
« on: September 30, 2014, 06:08:02 PM »
I was looking at the Source Follower just before the TDA2050 and do you think I should have it connected to the PSU before or after the 1K Resistor? Reason being if its after the 1K resistor I'm thinking I might run into issues when I use the Distortion Channel as that has it's own 1K Resistor and we might get issues with the two channels interfering with each other.

Edit: Also do you think I'd have issues with interference if I crossed the Power Track with a resistor? So basically the signal path crosses the supply path.

Amplifier Discussion / Re: Two Channel Amp Design
« on: September 29, 2014, 06:52:06 PM »
I've just added that resistor in and it made a huge difference. Tried the values you mentioned but they seemed to cut a lot of high end even with the pot on 10. So I've played around and for the moment settled on 1K + 10K Pot + .022uF Cap. But obviously the real test will be once I've built it and hearing how the control affects the amp. Its all fine and dandy looking at simulation results or doing equations but at the end of the day it all comes down to the sound. Thanks for pointing me in the right direction though :). I think the design is pretty much complete, just need to check it all over again and have a think to ensure I haven't missed anything.

Amplifier Discussion / Re: Two Channel Amp Design
« on: September 29, 2014, 01:31:18 PM »
Unfortunately I don't have any trim pots for this, however I will be breadboarding the Pre Amps so won't be too difficult to switch out resistor values and I do have a fairly good selection............he says.

Anyway here is the updated schematic and LTSpice file. The cut control works and the master volume works (in the simulation anyway). I chose a 5K Pot for the Cut Control as going up to 10K didn't seem to make any difference but we'll see when I build it for real. What do you guys think?

Edit: Roly its funny you should mention your JFETs being used with simulation values as last time I used JFETs (2N3819 I believe) the source resistor was 500ohms off simulation values. I mean that as in 5.6K in the simulation gave the same result as 4.7K in the actual amp. Maybe things have gotten better.

Amplifier Discussion / Re: Two Channel Amp Design
« on: September 29, 2014, 03:42:54 AM »
That Randall RG100 is interesting, nice to see JFETs being used instead of Op Amps.

I'll see what I have for the source follower as I have a collection of JFETs and some BJTs.

Had a quick go on the simulation and it seems the master volume control messes with the cut control. So I'll move the master to before the source follower and it should be fine then.

I also thought of a good idea for squeezing the 6 controls onto the front of the amp. As the tone control for both channels is just a treble I was thinking I could stick them both onto a stereo pot.

Amplifier Discussion / Re: Two Channel Amp Design
« on: September 28, 2014, 07:04:37 AM »
Just played through that amp again and it seems alright compared to before. I think the harsh treble was possibly due to me having it too loud in a small room. The cut control I can't implement as it loads down the tonestack, although I could put a source follower just before the TDA2050 which would help seperate the tonestack from the input of the chip and I could then put the cut control in. Will have a think.

Have to say the amp as it is sounds quite nice clean tone wise, hasn't got a tonestack so has a nice Fender Champ sound. Might remove the tonestack from the clean channel when I rebuild it but we'll see.

Just need to draw out the stripboard and once I've done that I can start ordering parts  :dbtu:

Amplifier Discussion / Re: Two Channel Amp Design
« on: September 27, 2014, 05:48:56 PM »
Ah ok I see now. Well my plan of action was to solder the power amp onto the stripboard, then breadboard each pre amp to ensure they work correctly, then I am going to use little 3 pin sockets for the transistors so I can swap them easily in the final test.

One thing I found with the first solid state amp I built is that the attack was VERY harsh. The only cause that I could think of is that I'm using a voltage regulator for the Power Supply and as a result there is no sag. As a result the initial attack of the note being played isn't softened at all and is thus very fast and harsh. Does this sound right?

Amplifier Discussion / Re: Two Channel Amp Design
« on: September 27, 2014, 03:01:21 PM »
Thanks for your reply J M

1) The JFETs that I will be using are J201s, except for the stage before the source follower which is a J202 so thats why it is biased differently. Reason for using a J202 here as it shouldn't distort as quick as a J201. The source follower is a 2N3819 as I have a few of these and thought it'd be a good chance to use one.

I think I've only about 10 at the moment, but I think J201s and J202s are dirt cheap so I'll just order a load off ebay.

I definitely won't use trimmers, not faffing about with that, I'll just experiment like you said. One thing I need to ask though, how do I test the JFETs? I read about people testing them to find out their properties as I know they are very inconsistent but how do they do it? Do they just build a circuit like I've got and test them that way?

2) That could be an issue as currently my gain is about 3......So I've made a few adjustments as per your recommendation. I have made a voltage divider just before the TDA2050 and brought the gain back up to 18. Will this be ok?

I've attached a revised schematic and LTSpice file.

I should add that I may try and use current feedback on the TDA2050 but am struggling to simulate this in LTSpice, so it will have to wait until I start to build the amp.

Amplifier Discussion / Re: Two Channel Amp Design
« on: September 27, 2014, 03:17:07 AM »
No problem Roly. The only part I'm unsure of is the distortion pre amp. I know I could easily do this with valves but with transistors would I get blocking distortion at all doing this?

Amplifier Discussion / Two Channel Amp Design
« on: September 26, 2014, 03:36:55 AM »
Hi Guys

I'm rebuilding an amp I built ages ago into a Two Channel Clean/Distortion amp! I've put together a design and I just want to know if anything looks like it won't work to you guys. I've attached a screenshot of the LTSpice Sim that I've made and the Sim itself. Both channels in the sim give out near enough the same output, although I can obviously measure and adjust this when I build the amp. The chip amp is a TDA2050.

I've made the distortion side near enough like the Marshall JCM800 2203 Pre Amp, mainly because you'll get distortion with it at all levels on the gain knob. The clean is just a two stage pre amp with a fender tonestack. I've biased the FETs using a guide from to make them sound more valve like. Thats why they aren't bypassed at all and they don't need to be anyway as there is plenty of distortion.

I should mention that the input will always be connected to both channels, the switching will be done just before the master volume.


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