Whats the right size transformer for the original dynahi PSU

[sbelyo]

I built my original Dynahi 8 years ago and I'm going to re-case it to match my other builds and to fix some hum because the transformer is in the same case.

I built a stereo version with two one channel boards powered by one PSU. I do not remember how I arrived at the value of the transformer I have now but I think it's 160 VA.

The original BOM for the PSU says 250VA. Should I go up to the 250 VA spec'd in the BOM? Or is the 160 VA enough? I was thinking of trying to reduce the size of the transformer because of the hum but now I see I may have undersized it.

[UFN]

As I recall, the recommended transformer for a non-balanced original Dynahi was 2x30V/80VA+. Each board draws around 300mA, so 80VA would be roughly twice the current draw of two channels.

//UFN

[sbelyo]

Thanks...  Looks like I'm over-sized.  So reducing to 100 or 80VA would help reduce the hum correct?

[luvdunhill]

Thanks...  Looks like I'm over-sized.  So reducing to 100 or 80VA would help reduce the hum correct?

Nope. It might help reduce some heat, but shouldn't really reduce hum, unless you have a poorly designed transformer.

[UFN]

Agree. Hum could occur because of a poorly designed transformer, an undersized transformer, interference (due to suboptimal wiring), grounding issues etc. Just moving the transformer to a separate enclosure isn't guaranteed to fix the issue.

 

//UFN

[sbelyo]

I thought I read somewhere the over-sizing your transformer is not recommend because something happens with the magnetic field that induces hum.

However, I'm sure the wiring is not optimal at all.  That is one of the reasons I'm putting it in a new case.  My guess is that fixing the wiring and shielding the transformer should stop the hum.

[Pars]

Bringing this thread back.
Was playing with my dynahi/s22 combo this weekend and put larger sinks on the s22 as things were approaching 70C.
Started measuring stuff and loaded with a pair of dynahi boards set at 75ma the transformer is putting out 37Vac. SumR 120VA, 2 x 30. Regulation seems to be kind of poor even though it is lightly loaded.


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[sbelyo]

So the PSU is not performing well then right?  I'll be swapping out the original dynahi PSU for a GRLV in a week or so.  I could send you that one.  I'm curious as to how it would measure.  Feel free to keep it.

[Pars]

No the PSU is performing just fine, but running a bit hot because of the higher input voltage. S22 requires more overhead on its input voltage than a GRLV does.
Since the transformer is putting out ~37Vac, the raw DC after the rectifiers is ~51Vdc, giving around 20+ V overhead which is dropped on the pass devices. I believe the Antec 20V transformer I am using on my Dynalo is putting out right around 20Vac, so just under 27Vdc for 6.x V drop with a 20V output. Runs fairly cool.
Anytime you use a larger transformer than needed, it will run at higher voltage than spec (part of the regulation spec given). The Antec is a 50VA while the SumR is 120VA. Supposed to be 14% regulation but this one is outside of that.
I am going to build a GRLV for the Dynahi and might grab a 100VA Antec and get rid of the SumR.


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[sbelyo]

oh ok, I see what you mean

[sbelyo]

I got rid of the hum...  Yay!!!  Two layers of a coiled steel band around the diameter of the transformer did the trick.  The best part was that Avel Lindberg sent it to me for free.

[Pars]

Cool!


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[sbelyo]

Yeah, it was awesome.  The amp's always had some hum since I built it in 2008 but figured that's the way it was gonna be.  Now, if I can find a cheap source for grain oriented steel.