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Serious Blocking Distortion

Started by Littlewyan, July 05, 2013, 10:54:13 AM

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Roly

Point #1; as a general rule the load of a following stage should be at least about five times the Anode/Drain/Collector load of the driving stage, thus with a 27k Drain load the following resistor (pot) should be at least 100k.  (When we look at this first stage in AC terms the Gain pot winds up effectively in parallel with the 27k Drain resistor).

Point #2; your 5k pot isn't DC isolated from the transistor base, so when turned down it will shunt all the bias to ground and turn the transistor off.  A blocking cap required.

I think you also need a small value decoupling resistor in the supply rail directly above your tone control so that the preamp stages are isolated and protected from signals impressed on the supply by the power amp chip (or you may get some form of instability).  Calculate resistance for a 10% drop in supply voltage at whatever current the first two stages draw.


"376" ohms?  Boy, where did that value come from?  The normal E12 10% value progression is;

10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, 100.

Outside precision instrumentation you simply don't need resistors that are more accurate than this almost universally available range.  With the exception of precision audio filters and oscillators, the use of 1% resistors in audio circuits is simply not required and comes under the heading of "spurious accuracy" - an advantage which is purely an illusion, and frankly often just to impress.

When you consider the device it is biasing will have a spread of around 30%, and a FET may be anywhere between 3:1 and 10:1 spread, ordinary caps are 10% and electro's can be as wide as -50/+100%, then using 1% resistors is simply an absurd waste in the home workshop.

{and it also suggests that the person who originally specified these values didn't actually know what they were doing.}
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

That was just the value i got from my equation, i will obviously put a std resistor in there i just couldnt think of what value at the time. I shall take a look later if i have time, got band practice first. Thanks guys for all your input on this.

Littlewyan

Ok I've redone the schematic again. If I didn't have you guys I would of seriously gotten nowhere  :o

Roly

Those clipping diodes may still need some work, but it's getting down to suck it and see time.   :tu:
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan


stormbringer

There's alot you can do with clipping diodes to experiment and find what suits you best. Right now you have "symmetric" clipping. it clips both positive and negative side equally, by putting one extra diode in series with one of them, or using for example 2 different diodes (maybe a LED and a standard Silicon diode) you might get interesting results. But start out easy, and then you can try out different configurations. :)

Littlewyan

Ah right yer I see. Thanks guys :) I'll order the parts now and will hopefully start building next week

Roly

...aaaaand also, other possible variations (depending on what you find in practice) are;

- insert an electro in series with your clipping diodes so the stage can be biased without the diodes conducting,
- placing a resistor in series to soften the clipping,
- placing the diode pair in different positions, such as across the base to ground, or, again with a series cap, across the signal path from collector to ground,
- switching diodes in/out.

These depend on how quickly the stage goes into clipping and what you desire, what sounds good to you.  You could spend a month just trying out different diodes in different positions.  (A little point about LED's - just be aware that they are also light sensitive.)

{And this is where rolling your own has it all over just going out and buying somebody else's compromises - you get to tweek, diddle, and futze with it until it pleases you, and if you ever change your mind you can get into it and give it a rewiring it'll never forget.  ;) }
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

This is very true. I want to get this amp just right so i can use it at practice if my other amp dies or even at small gigs or even recordings.

Littlewyan

Would it make a big difference if I used a 220uF Cap on the Output instead of 1000uF?

stormbringer

It only affects the low cutoff according to the datasheet, and my calculation says 16 hz original, around 70hz with 220uF. You can try it, and if you later feel you lack low end you could increase the capacitor later.

Roly

Quote from: stormbringerIt only affects the low cutoff according to the datasheet

The problem here is that the chip datasheet isn't wrong, just incomplete.

Electrolytic caps are rated for capacitance, for maximum working voltage, but they also have a lesser known rating for ripple current, or maximum AC current that may pass through them without excessive heating.  The ripple current rating is loosely related to the capacitance so it rises as the capacitance rises.

This amp chip is rated for a 35 watt output into 8 or 4 ohms.  We can work out what the required AC current is for this power in both loads;

P = I2R

P/R = I2

I = Sqrt(P/R) = (P/R)0.5

I = (35/8)^0.5 = 2.09 amps

For 4 ohms;

I = (35/4)^0.5 = 2.95 amps

So there is actually quite a lot of AC current flowing through this output coupling cap at full power.

Looking at the RB caps listed in the Jaycar catalogue a 220uF/50V has a ripple current rating of only 430mA, even 1000uF/50V is rated at only 900mA, and we don't find a rating in excess of 2 amps until we get to the 4700uF/50V.  The danger with using a cap that is being consistently driven in excess of its ripple current rating is that it may literally explode.

This is also an important consideration with power supply caps in high powered amplifiers.

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

stormbringer

I really dont intend to hijack the thread, but just for the sake of it, how would parallell caps act of used for DC blocking? :) I have been taught for example that often 2 smaller caps can be a better choise than one bigger for example as filter capacitors after the bridge rectifier in a Power supply, due to the lower ESR. What if you would use, let's say 2x470uF in parallell instead of 1x1000?

Roly

This question is spot on.  You will notice above that if you use five 220uF caps in parallel you will get the datasheet recommended value of ~1000uF and a ripple current capacity of 5x 450mA;

Strictly;

5 * 220 = 1100uF, and

5 * 450 = 2250mA or 2.25 amps

A particular reason why multiple parallel caps can be better than a single big one is the ratio of volume to surface area which is much larger with multiple caps, and therefore can get rid of heat better.  (they may also be easier to physically arrange in a build)
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

Well guys just to give you an update, the amp is now built..........................and it works! Mostly. I couldn't turn it on for very long as it was half 11 at night when I finally finished it, but I did have it on long enough to find that at full volume it wasn't quite as loud as it should be.

Now I just realised that the 1M Resistor on the Gate of the JFET is actually a 220K at the moment, I know this probably isn't the cause but I guess it doesn't help either. Also I have left out the tone control as well as I'm not too bothered about having one at the moment, may add one later, so that should surely help give a bit more volume as tone controls tend to lower the gain of the circuit. I have also got the PSU still set to 24 volts, so I'll try raising this to 28 volts tomorrow night as the TDA2030 datasheet specifies 28volts (14+/-) for 8/9 watts.

Once the volume is sorted I may try to add a bit more overdrive as it didn't sound like it had much, but one step at a time. I'm just thrilled that it worked first time, as you all know most circuits tend not to work at first due to things being missed!