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PostPosted: 26 Dec 2017, 12:18 
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mwhouston wrote:
The second build of the simple headphone amp runs with a cooler tranni. Same tranni same build but tranni is noticable cooler. All I can think of is the limiting resistors before the diodes, direct from the tranni secondaries, are larger. 270 from 220ohms.


Ref series resistance and R/C time constant.
Charge pulse current limitation<<has to be traded off against sag during transient pulse charge time.
It's always been an interesting talking point with transient recovery time an next transient event also linked to discharge time/ transient delivery current.
Obviously linked to ESR/induction supply values and recovery.

Interesting link to charge pulses and valve rectifiers.

A bit like diode junction and voltage operation being in reverse bias after initial charge pulse.

But that's life.. :beerchug:

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PostPosted: 26 Dec 2017, 17:32 
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My problem has been with both builds is that the HT is way in excess of what I need. To have some limiting resistors, not only drop final HT to required level but take the strain off the power tranni is a win-win for me.

Fortunaly with the amp only drawing 50mA and 280uf of storage sagging may not be a problem when driving 32ohm headphones. If I can't get the HT down there is NO amp. Not a lot of choice.

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PostPosted: 26 Dec 2017, 18:33 
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Quote:
Fortunaly with the amp only drawing 50mA and 280uf of storage sagging may not be a problem when driving 32ohm headphones. If I can't get the HT down there is NO amp. Not a lot of choice.


How much B+ are you dropping?
If your resistors are before the rectifier in the AC then there should be little impact from sag.

It is more compounded if its between capacitor sections because the PSU loses the ability to dump current from the supply into the load.
Ie the first capacitor holds the charge but the transient current comes from the 2nd cap in the supply which is drained faster because the total cap discharge to load from 1st cap is restricted.
This is also true with choke supplies because transient represents change in current. The more stable the DC the less effect from induction.
Pretty obvious really. But the reverse is also true in that the recovery of supply on the caps after transient is restricted.

Regards
M. Gregg

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PostPosted: 26 Dec 2017, 19:39 
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The final limiting resistor is about 750ohms. @ 50mA is about 38V. Ps is 270ohm-(diodes)-100uf-5H-180uf-750ohm-100uf.

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PostPosted: 27 Dec 2017, 15:43 
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Quote:
The final limiting resistor is about 750ohms. @ 50mA is about 38V. Ps is 270ohm-(diodes)-100uf-5H-180uf-750ohm-100uf.


I don't know if you are still playing/building this, but what effect do you get if you swap the 180uf and 100uf either side of the 750ohm (Does it hum )?
or if you put a 10uf film cap across either the 180uf or the 100uf.

Something for fun you can try is the B+ reducer shown here:
Bottom of the page
http://www.geofex.com/Article_Folders/m ... tfolly.htm

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M. Gregg

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PostPosted: 27 Dec 2017, 17:08 
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It has been finished for some weeks. On some more effcient phones there is a very slight background hum. I also found this on my Beyerdynamic DT770 Pro 250ohm. On the 32ohm version of these phones, nothing.

If I use a 12AU7 and not a 12AX7 equivilant there is even less hum. The hum doesn't change with volume. When the music is playing only the music can be heard. I'm staying with the E88CC Gold Lion because it sounds so much better than other tubes I have tried but brings up a little more noise than tubes with less gain.

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PostPosted: 27 Dec 2017, 17:24 
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Greg, took a gander at the link. Not sure there is anything there for me but thanks.

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PostPosted: 30 Dec 2017, 19:09 
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A little off thread but changing the E88CC for a 12AU7 (less gain) dropped the hum and allows me to use my more sensory phones.

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