Over time, I've gradually upped the quiescent current of a small Class A amp (my 1st venture into Class A) to better drive lower impedance loads. Before increasing it a tad more, I checked current draw from the transformer, and it is 1.7A from a 2A transformer. Ripple current has increased as a result. A larger filter cap (already 10mFD) will lower the ripple some, but I'm concerned about the xfmr. It definitely runs warm now.
I replaced it with a 4A xfmr but of course the xfmr is not loaded as much now and the voltage is too high. So back to the original one.
Is it OK to run the xfmr continuously this close to its VA rating?
I am beginning to see the value/attraction of SMPS for Class A amps...
I replaced it with a 4A xfmr but of course the xfmr is not loaded as much now and the voltage is too high. So back to the original one.
Is it OK to run the xfmr continuously this close to its VA rating?
I am beginning to see the value/attraction of SMPS for Class A amps...
A small warning: How do you measure the secondary current?
Or to be more precise: What instrument do you use - is it equipped with TRMS (true rms) for AC measurements?
If not, then the actual RMS current will be higher than shown on the meter as it will then only be calibrated to measure correctly with sine-shaped current.
Cheers,
Martin
Or to be more precise: What instrument do you use - is it equipped with TRMS (true rms) for AC measurements?
If not, then the actual RMS current will be higher than shown on the meter as it will then only be calibrated to measure correctly with sine-shaped current.
Cheers,
Martin
What kind of rectifier topology is it? There's a DC current factor from the AC rating that depends on it.
https://www.hammfg.com/electronics/transformers/rectifier
https://www.hammfg.com/electronics/transformers/rectifier
I wouldn't do that. Usually should derate by 10% or more, rather than uprate.
Especially for class A, which is constant full loading.
Especially for class A, which is constant full loading.
Thanks for catching that. Looks like a 3A xfmr is needed for this. Interesting that the xfmr is putting out higher-than-rated ACV even under these conditions.
Perhaps I should dial the bias back on this one, and start afresh on another one in a better-ventilated chassis to drive the lower loads. After running for an hour, temps inside are ~43ºC.
Perhaps I should dial the bias back on this one, and start afresh on another one in a better-ventilated chassis to drive the lower loads. After running for an hour, temps inside are ~43ºC.
How high is your AC line voltage running? Is it a 115V or 120V transformer?
I've always thought that class A amplifiers should have the electrolytics mounted
so they are outside of the chassis.
I've always thought that class A amplifiers should have the electrolytics mounted
so they are outside of the chassis.
It's 121.4 today, pretty much always in the 120-121 range.
Indeed, the xfmr is 115V.
'Lytics outside the chassis has artistic possibilities, I suppose 🙂
Indeed, the xfmr is 115V.
'Lytics outside the chassis has artistic possibilities, I suppose 🙂
the thing to watch out for is core temperature rise...1.7 from a 2A rated traffo winding is peanuts,
you can draw twice or thrice that amount, only copper losses and temperature rise will happen but the caveat there is duty cycles, how many seconds, minutes or hours limits your power draw...
That would do it, but no room for it in this chassis.
This is the raw DC for a regulator that feeds the amp modules. I think I'm gonna replace it with a switcher before upping the xfmr current.
I had OEM-grade transformers which ran too hot to hold for over a decade.
And cheap junk that died in a year.
It WILL die someday. Can you afford it? Yes, it is a real option to say "Cross that bridge when I burn it."
And cheap junk that died in a year.
It WILL die someday. Can you afford it? Yes, it is a real option to say "Cross that bridge when I burn it."
Class A amplifier=100% duty cycle, as many hours as it´s ON every day.
A transformer which is happy with a Class AB amp and Music program will be stressed (not over-stressed) in Class A, where program becomes irrelevant.
I would be more worried about exceeding diode ratings than trafo winding ratings. RMS current will be higher than that 1.7 amps - a 1 amp diode is overloaded by more than a factor of two. Use 1N540x in the bridge.
OK, I assumed the current would be shared among the diodes. I have some 1N5401 here, I'll swap them in.