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Serious Blocking Distortion

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

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stormbringer

Ok, just checking to make sure you have both positive and negative supply. (+/- 12 or 24 v), as if you used a single supply you would need to offset the voltage to 12v if using a single 24 supply, and also the "ground" would need an offset to the virtual ground at 12v.  The red circuit regardless how bad, says +/- 24 v. 48v transformer with center tap.

Littlewyan

Ah now it is a single supply as I'm only taking the positive side of the AC Waveform coming from the two 30V windings on the transformer, which gives me a 30V positive rail that then goes into the voltage regulator. So I don't have a negative supply, just a ground.

stormbringer

That might explain some of the problems with the first Circuit aswell. There will be no headroom avaliable at the negative side of the signal which will distort bad.

Littlewyan

Well I have changed C10 for a new capacitor in case the other one was leaky, also changed the output transistors and now I find I get the blocking distortion ALL the time. Tried increasing the voltage to 26 volts but exactly the same issue occurs. Measured the voltage on the emitters of the two output transistors and it is 13 volts, on the base of Q3 its 0.7 volts exactly. I think these voltages are correct, so perhaps its the preamp causing the issue?

Littlewyan

I should add that C5 is always connected in my circuit, the 47uF capacitors in the Preamp are actually 22uF which should make the situation better in theory and the line coming from R13/R14 has got a ground line going over the top of it on my PCB, however both are covered by heatshrink.

J M Fahey

Quoteso perhaps its the preamp causing the issue?

Is this a joke?   :o

There's 2 pages of posts telling you this power amp is cr*p and explaining why and you still think it's something else?

Please re-read the entire thread.

As many times as it's necessary  :cheesy:

stormbringer

#21
As jm says, you should probably abandon that amp for a better suggestion, i just wanted to make the supply configuration clear. As a dual supply with center tap referenced ground is not the same as a single supply.

Edit: you can still split a single supply, but you will need to add a virtual ground, and remove the dc offset created that way. It wont just replace the psu out of the box.

Littlewyan

I understand that, however others have also said the power amp should work to an extent. I just wanted to see if I could get this to work, then I was going to use the TDA2030. Does the TDA2030 overdrive nicely btw? As in when its pushed?

stormbringer

#23
Havent heard it myself, but if you got the power supply and a heatsink already, its really cheap to build the single supply circuit jm suggested from the datasheet. 7 caps, 4 resistors, the ic and 2 diodes. :)

In my opinion i think you will benefit more from overdriving the preamp when it comes to solid state circuits.

Littlewyan

Well in that case I think I'll just get hold of the parts and use the chip, as it sounds like the only thing I'll learn from this circuit is how to NOT build a power amp.

By the way, how would I overdrive the Preamp?

J M Fahey


Littlewyan

Sounds good, I'll get onto the parts buying on Wednesday and I'll let you guys know once shes finished. Thanks for all your help. I'll stay clear of RedCircuit in future and will spread the word!

Littlewyan

What were the two diodes for by the way? Distortion?

stormbringer

The diodes looks like polarity protection or something like that, cant say for sure. Distortion should happen in the preamp, while the power amp should be as clean as possible.

QReuCk

I'm with stormbringer there:
If you think about it, the real point of SS amps aside from the price and maintenability is the scalability. If for some reason you have your power amp distort, then you miss the biggest tonal advantage of SS amps: they sound almost as good at low level as at near-full volume. The only difference is introduced by psychoaccoustics (our ears do not compress bass and trebbles the same way depending on volume), room reverb (I meen "real" room reverb, you know sound wave bouncing on the walls and the ceilling) and feedback (sound waves from the amplifier exciting guitar strings).
Tube amps sound better at high volume because tube power amps are push-pull stages clipping slowly and symetrically while tube preamps are single-ended triodes clipping kinda slowly and asymetrically (which can sound good to an extent, but doesn't sound quite the same).
Fortunately enough, you can build a fairly pleasing soft symetric clipping into a solid state preamp. You can also put some asymetric clipping if you need it and even design the possibility to mix them together. So there is really no point in trying deliberately to have a solid state power amp sound different depending on the outup volume.