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Two Channel Amp Design

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

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Littlewyan

#15
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.

Littlewyan

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.

Roly

Quote from: LittlewyanSource Follower

...goes into the non-inverting input of the chipamp, so I'd put in another leg of decoupling, a 1k between R5 and J7, and another 100uF to ground.

Similarly I notice that you have three cascaded stage inversions on the same supply rail in the overdrive section.  Again I'd be inclined to add another decoupling leg of 1k between R12 and R23, with another 100uF to deck.

Quote from: LittlewyanAlso 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.

No problem.  Your supplies appear to be well bypassed so there shouldn't be a lot of AC on the supply rails, just don't make them too cosy.

Quote from: LittlewyanWhat size transformer would I need for this?

The output stage is 50% efficient, so you need a tranny rated for double the required power output (generally given in Volt-Amps, VA, read "watts").  So a 50 watt amp requires a 100VA transformer stack.


A 25V single supply rail with an 8 ohm load will give;

25Vpeak-peak
12.5Vpk
12.5/1.414 = 8.8Vrms

P = E^2/R

(8.8^2)/8 = 9.7 watts out, requires a 20VA stack.


Voltage
25/1.4=17.9
25/1.1=22.7

So an 18 to 22VAC winding @ about 1 amp.

HTH

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

Littlewyan

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.

Roly

In an amp using transistors or valves each stage generally inverts the signal.  This means that with two cascaded stages any unwanted feedback from the anode/collector of the second stage to the first is negative, so the worst outcome is less overall gain than expected.  But when you add a third stage the stage current is now back in phase with the first stage, and there is gain in between.

Positive feedback and gain means what?

So as the decoupling cap on this rail ages and its impedance (ESR) rises the positive feedback will increase until something obvious happens, such as low frequency instability, putt-putt-putt "motorboating".  Before that you may have transient or parasitic instability, damped oscillations triggered by signal edges, or depending on certain signal conditions to excite them (making them easy to miss).


You can get lax about bypassing with op-amps because they are designed with a very high power supply rejection, typically about 120dB, which single stages aren't (and really can't be while they are single-ended rather than balanced).  In contrast our typical single-ended gain stage will have a supply rejection of only about 6dB.

This can lead to the seriously mistaken idea that op-amp supplies don't need bypassing.  They may not need as much bypassing at audio frequencies, but having monumental open loop gain and wide bandwidth they sure as hell need high frequency by-passing, and right across the chip supply pins if you can, but close because we are dealing with very high gains and frequencies in the supersonic to RF that make stray capacitance circuit couplings significant.
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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?

Roly

Well we need to be a little more specific about a couple of things.

It's not just two stages, but two inverting stages.  It's when you get the third inverting stage that the troubles start.

In your circuit you have a Source-follower J6 coupled to inverting gain stage J5.  This follower does not count as a "stage" because a) its current is already in phase with J5, and b) as a follower it has, by definition, less than unity voltage gain, so J5 and J6 count as a single stage.


The idea behind this split grounding is that the high currents in the output/speaker circuit do not share common traces with grounds in the input side - we are trying to avoid ground resistances in common that would cause voltage drops due to output currents being injected back into the input.

The entire path from the transformer, rectifier, filter caps, output stage, speaker, and back to the tranny via the rectifier again, should be treated as a high current path with thick low resistance wiring (and the power supply loop from tranny through reccy, caps and back to the tranny, in particular; very high impulse charging currents flow around this loop)
If you say theory and practice don't agree you haven't applied enough theory.

Roly

Forgot these attaches;

Yahoo Groups LTSpice group is simply stuffed with all sorts of useful models (you just have to figure out the user interface to find them  ::) ).
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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?

Roly

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

Littlewyan

#25
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?

Roly

Bypass caps should really go right across the stage(s) they are bypassing, while the series resistors can go anywhere (generally back in the PSU).  The HT and ground lines should radiate from the PSU, not daisy-chain if you want to avoid supply paths in common and unwanted coupling.

Solder the pots in.  Unless you are particularly ham-fisted they should only require replacement every 10-50 years, and over that time span any sliding connector will also have a significant failure rate of its own.  In general, avoid if possible; these are to facilitate mass production.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

Try to fit bypass caps right at the preamp (at least for the first stages) , mainly to avoid having long ground paths which late will drive you crazy.

Littlewyan

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.