I replaced the output transformer on my power amplifier Antique Sound Lab
AQ1008DT. I wanted to test its performance with a scope and signal generator. I connected a 5ohm resistor as load and at 40V P-P output signal I could hear it loud and clear coming from the output transformer
starting at about 2.2Khz and going up as far as I could hear.
I checked the original transformer and the phenomenon ocurred again at a slightly different frequency.
Is this a mechanical resonance of the transformer?
Otherwise the amplifier performs very well though I am not sure if there is any difference between the original transformer and the new one.
Comments on the issue are welcome.
Regards
Tigro
AQ1008DT. I wanted to test its performance with a scope and signal generator. I connected a 5ohm resistor as load and at 40V P-P output signal I could hear it loud and clear coming from the output transformer
starting at about 2.2Khz and going up as far as I could hear.
I checked the original transformer and the phenomenon ocurred again at a slightly different frequency.
Is this a mechanical resonance of the transformer?
Otherwise the amplifier performs very well though I am not sure if there is any difference between the original transformer and the new one.
Comments on the issue are welcome.
Regards
Tigro
hey-Hey!!!,
That is entorely normal. Change the magnetic flux on the steel core and it changes its shape/volume. The effect is called magnetostriction. Takes about 7% Si to get that effect at a minimum and that alloy is not so friendly either mechanically or magnetically compared to 3.5% Si.
cheers,
Douglas
That is entorely normal. Change the magnetic flux on the steel core and it changes its shape/volume. The effect is called magnetostriction. Takes about 7% Si to get that effect at a minimum and that alloy is not so friendly either mechanically or magnetically compared to 3.5% Si.
cheers,
Douglas
It's not resonance. Resonance occurs at one or more particular frequencies. It's the bobbin moving on the core or the core lams vibrating against one another, etc. due to electromagnetic forces. It could also be the core changing dimension due to magnetostriction I guess.
Potting is usually the cure for a singing inductor. Either yours is not well potted or is being overdriven or maybe that's just what you get. Most of my non-potted OPTs sing at least a little, but the potted ones are quiet (havent tried the 300W ones yet though...)
Potting is usually the cure for a singing inductor. Either yours is not well potted or is being overdriven or maybe that's just what you get. Most of my non-potted OPTs sing at least a little, but the potted ones are quiet (havent tried the 300W ones yet though...)
I may not have this right, but the amp at 100W peak output power and a 5R load would at max be putting out what, 20-25 V p-p. Isn't 40V p-p pushing those OPTs a little hard?
I think if you convert 50V pk-pk to RMS (17.7VRMS)and use VRMS^2/Zload you get about 62 watts.
Not quite 40Wrms.. (based on OPs original 40Vpp)
Incidentally mathematically equivalent - you can square Vp, then divide by 2, and finally divide by the load impedance and that gives you watts rms..
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For an output transformer, this should not be a problem....
You are hearing the wire vibrating... The transformer is just lightly varnished....intead of deeply vacuum impregnated..
Most of the HI-END audio transformers I wind I use very little varnish...
If you deeply varnish the transformer the winding capacitance will go up by as mush as 30%, which can be bad in most but not all cases....
If you are refering to POWER transformer, then by all means that should be well varnished and silent..
Best Regards
Chris
You are hearing the wire vibrating... The transformer is just lightly varnished....intead of deeply vacuum impregnated..
Most of the HI-END audio transformers I wind I use very little varnish...
If you deeply varnish the transformer the winding capacitance will go up by as mush as 30%, which can be bad in most but not all cases....
If you are refering to POWER transformer, then by all means that should be well varnished and silent..
Best Regards
Chris
using the formula VRMS = Vpp / ( 2*sqrt(2) ) I get 17.67 VRMS from 50Vpp
17.67^2/5ohms comes out to 62.43 watts Po on my spreadsheet.
As far as I know, there is no such engineering metric as "peak sinewave watts" (found only in 1970s power amp marketing literature). Think about it... what's the ratio of peak watts to RMS watts for a square wave ;-)
Watts is watts.
Michael
OOps I can calculate but I can't read! 40Vpp (not 50V) into 5 ohms is indeed 40 watts
sorry...
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There are a very transformers I have encountered that don't normally sing except at very high signal levels - I would not consider the singing at 40Wrms output to be an issue - at a couple of watts it might be annoying, with speakers connected however the in room spl would totally mask this phenomena, and in any event this is about -4dB away from maximum power so I doubt the amplifier statistically will be running at this power level for a significant % of the time.
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