Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 06:57:48 AM

Login with username, password and session length

Recent Posts

 

Two Channel Amp Design

Started by Littlewyan, September 26, 2014, 03:36:55 AM

Previous topic - Next topic

Littlewyan

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 runoffgroove.com 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.

Ryan

Roly

Not ignoring you; looks interesting, just a bit preoccupied ATM, keen to see how it turns out.  :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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?

J M Fahey

No blocking distortion.
It will roughly distort the way tubes do.

A couple details:

1) simulations are fine, but real world Fets are spread all over the map, so in a protoboard setup a 25V supply, 1K and 33K resistors source and drain, gate grounded, and plug there your Fets before actual soldering in the PCB, so you preselect the ones which bias reasonably close.
And also use 1K/33K at the tone stack driver, there's no reason for that 820 ohms resistor there.

Buy 2X what you'll actually use and select.

Or if you think you'll use them later, buy 50 or 100 (no kidding)  , measure and separate them in groups.

ROG "avoids" this by using a trimmer at each drain, which is horrible ... and expensive.

2) you need gain 20X or greater at the TDA20xx or it becomes unstable.

If too sensitive you can always attenuate input a little.

Littlewyan

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.

J M Fahey

QuoteDo they just build a circuit like I've got and test them that way?

That's exactly what I suggested, using a free and of a protoboard just for testing, afterwards you disassemble it:
Quoteso in a protoboard setup a 25V supply, 1K and 33K resistors source and drain, gate grounded, and plug there your Fets before actual soldering in the PCB, so you preselect the ones which bias reasonably close.
Then you separate them in little labelled boxes or simply tape them to a piece of cereal box cardboard and write the actual "plate" voltage you got, which with a 25V supply should be between 10/15V .

If most are either too high or too low, you must choose another value of drain resistor.

One problem with simulators is that they use an idealized transistor model.

The actual part you buy may be far from it, specially in Fets..

Littlewyan

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?

J M Fahey

Transistors are very fast, hence "harsh".
The PSU regulation will be noticed when clipping, not a "clean" levels, so in practice it won't be noticed.

By the way all SS guitar amps have standard unregulated supplies.

What speaker you were using?

Check the Randall RG100 schematic, their "Presence" is actually a "cut" type control, Pres. max is actually flat and min. is cutting highs.

Adapt it to your circuit, rising pot value (they use 2K) and lowering the cap by the same factor.

I guess something 10X higher and lower (say 10K or 22/25K pot and corresponding 5/10X smaller cap)  will work well to tame shrillness but be able to restore bite at higher power or in some horrible room.

Or put a rotary switch with various cap values to ground after Tone Control, same thing.

Littlewyan

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:

J M Fahey

QuoteThe cut control I can't implement as it loads down the tonestack,
You can add just 1 humble FET like Randall (RG100ES) does.

In fact, easier and cheaper, you can use almost any NPN small transistor there, same biasing and emitter/source resistor.

I use BC547C (Hfe~500) , search for any NPN with Hfe >200 and you'll be fine.

I'd use 10X higher impedances, so 1K + 2K pot (hard to find) + .1 ceramic would become 10K + 20/25K pot + .01 ceramic .

Littlewyan

#10
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.

Roly

Quote from: J M FaheyThe actual part you buy may be far from it, specially in Fets.

While this is certainly true my experience with MPF102's of late is that I can just bang one in to the simulated circuit values and they work fine, the spread in practice today seems to be much closer than the 10:1 on the datasheet.

I remember my early encounters with MPF102/2N3919's where I would by a bag of 10 to get the one I needed; did that a while back and got 10/10 usable. {tighter process control?  Or maybe I was just lucky?}


Quote from: LittlewyanI definitely won't use trimmers

Nuthing wrong with a little trim between friends where something might change during the course of the amplifiers' life, such as output bias or offset if the output transistors have to be replaced.

A trimmer can be valid to determine the correct value during construction, then replace with a fixed resistor.  I think the main objection to the ROG mess is that the adjustments have been placed in the Drain rather than the Source.  To me this is a loud warning bell that the "designer" didn't actually know what they were doing.  {"You never find just one roach in a kitchen"}

If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

#12
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.

J M Fahey

You forgot the resistor after C24 . (check the Randall control an my earlier post)
Either 1K + 2K pot (5K may work) +.1uF cap OR 10K + 10/20K pot + .01 cap

You missed the R in the RC network.

It ytill works, sort of , (and you needed the larger value cap)  just because the Fet follower does have some internal resistance ... but it's a random not well defined value, which will change amp to amp.

Better use predictable parts.

Littlewyan

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.