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Deleted member 550749
More lower emitter resistor also a reason more power draw, u can see in this scheme used 0.22ohm or 220mili ohm,
All I want to know is why is the voltage different when there is a load connected and does it even matter ?
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Deleted member 550749
Because without load not conducting and with load starts conduction few , so this is normal and happens in every amp,
But without a load the voltage is the same does that mean that the quiescent current is balanced on both transistors?
Maybe Jefh01 has a valid point with the size of those heatsink.
You are running -/+39 volt rails so at 3.4 volts rms output into 8 ohms (1.45 watts in load) you have a current draw from the supply of 15 watts calculated as 78*0.45*load current which is 0.425amp. So I make that 14.9 watts. Subtract the load dissipation and that give 13.45 watts or approx 6.7 watts per output transistor.
So thinking about it and maybe it really is down to the heatsink. Your rail voltage is very high for a small amp and that means the power dissipation is higher than you might expect.
You are running -/+39 volt rails so at 3.4 volts rms output into 8 ohms (1.45 watts in load) you have a current draw from the supply of 15 watts calculated as 78*0.45*load current which is 0.425amp. So I make that 14.9 watts. Subtract the load dissipation and that give 13.45 watts or approx 6.7 watts per output transistor.
So thinking about it and maybe it really is down to the heatsink. Your rail voltage is very high for a small amp and that means the power dissipation is higher than you might expect.
That suggests that the dc offset on the output is not zero, or that one half of the output stage is oscillating.
Back to my earlier suggestion about the dc current, IIRC the amp draws about 35-50 mA when idle. At 1W (steady tone) it should draw about 350mA. If the current is way more than that there is a problem.
The current draw on both the rails should be fairly similar.
I’ll hook mine up tonight if I get time and share my measurements.
Back to my earlier suggestion about the dc current, IIRC the amp draws about 35-50 mA when idle. At 1W (steady tone) it should draw about 350mA. If the current is way more than that there is a problem.
The current draw on both the rails should be fairly similar.
I’ll hook mine up tonight if I get time and share my measurements.
At the moment I have unequal current draw one used of the voltage rail sags faster than the other ! I believe that to be a problem . I can’t see any oscillation on the scope under load one rail drains much faster than the other . The dc offset is about 49mv ! Yes I do believe those heat sinks are not up to the task . I will try and get my hands on a dead receiver and harvest the heatsink from it
I don't think it is a problem 😉
I never like advising to measure rail current directly but you could try adding a 10 ohm (which I think you have) in series with each rail to the amp itself.
(It must be in the rail supply to the board, not anywhere else)
With NO load measure the volt drop across each to get be able to work out a true current reading.
I didn’t add a 10ohm in series with the amp. Reason I know it’s on the amp is because A I’ve measured the psu itself as it’s draining and they are balanced B on the amp before I would not get any current on the ground but now I do which suggests that there is an imbalance . But as soon as I get some parts in I will text the other amp as well to see if it is balanced or not
The 10 ohms would give a very good idea of exact current... I still don't think there is a problem with the rails though 🙂
Hey guys how can I lower the dc offset ? Also will lowering the dc offset destabilize the amplifier?
To reiterate @Mooly ’s calculation above. At 1.5 watts output the power transistors are dissipating about 13 watts. That is roughly 10% efficient. The thing to note is that as the power output increases so does the efficiency, however, it doesn’t increase fast enough to reduce the dissipation as the power output increases until you get towards maximum power, only then are you putting more power into the load than you are into the output transistors and you are dissipating a lot!
On the asymmetric power draw, the circuit is not symetrical either side of the power rails, so there will be some asymmetry built in. I have noted power rails collapsing at different rates on pretty much everything I have built including opamp circuits and chip amps.
I might get to my mx50 at the weekend, so I’ll take some measurements for comparison.
On the asymmetric power draw, the circuit is not symetrical either side of the power rails, so there will be some asymmetry built in. I have noted power rails collapsing at different rates on pretty much everything I have built including opamp circuits and chip amps.
I might get to my mx50 at the weekend, so I’ll take some measurements for comparison.
With difficulty on an existing design. You could add a 220 ohm trimmer in place of R19 and R21 in the diagram above with the collector of Q3 going to the wiper.
You haven't an issue with offset based on your measurements.
Nope.
So I have to match r19 to r21 the both have to be the same value correct ? I just want to get it as close to zero as possible