EHHA Rev A - Interest Thread
Sep 22, 2010 at 9:28 PM Post #572 of 1,752

jdkJake

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Do you measure the offset across a load (headphone), or with no load resistance other than the DMM ? Does it make a difference ? The worst case scenario for offset is into low impedance, high efficiency cans like Grados. Do you get that kind of offset spike on turn on, measured across a 30 ohm load or only across an open circuit ?





Well I certainly did not perform those measures with my pristine HF-2's plugged into the amp! I suspect the results would be the same, but , perhaps Alex can chime in with the correct answer.

So, to answer your question, I did not measure under load. Now, if you want to loan me your grado's, I would be more than happy to repeat the tests under load for you. :wink:

I suppose I could slap a resistor across the output to simulate, but, I would rather not and just take the open measure as a baseline reference. I don't believe it is a big issue with this amp, just don't plug in your phones until it has a chance to settle. Either that or build a couple epsilon 12's and rest assured.
 
Sep 23, 2010 at 1:24 AM Post #573 of 1,752

sachu

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Quote:
Thanks. That's three that have landed. I sent sachu's to the wrong address and ujamer and cobalt are waiting on Canadian post. Just checking.


hah..well seems the boards got forwarded to a friend's address. Gonna receive them tomorrow and the parts should get delivered same day as well. word up!! 
beerchug.gif

 
Sep 23, 2010 at 1:54 AM Post #574 of 1,752

bada bing

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Quote:
Well I certainly did not perform those measures with my pristine HF-2's plugged into the amp! I suspect the results would be the same, but , perhaps Alex can chime in with the correct answer.

So, to answer your question, I did not measure under load. Now, if you want to loan me your grado's, I would be more than happy to repeat the tests under load for you. :wink:

I suppose I could slap a resistor across the output to simulate, but, I would rather not and just take the open measure as a baseline reference. I don't believe it is a big issue with this amp, just don't plug in your phones until it has a chance to settle. Either that or build a couple epsilon 12's and rest assured.


I've been "burnt" by offset issues a couple times, and I have a policy to measure offset under load on any amp that exhibits a power on spike unloaded. The "load" I use is 4x120ohm 1/4 watt resistors paralleled ~ about $.25 worth of resistors. The loaded offset will always be at least a bit under the unloaded measurement and in some cases significantly less. My whole idea is to get some notion of what a pair of grados will see before plugging them in. I certainly wasn't suggesting testing a new amp with anything of value jacked in. I probably should have been more clear about what I was interested in.  
 
 
Sep 23, 2010 at 9:34 AM Post #576 of 1,752

BoilermakerFan

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Quote:
 

hah..well seems the boards got forwarded to a friend's address. Gonna receive them tomorrow and the parts should get delivered same day as well. word up!! 
beerchug.gif

LMK when your eXStatA boards and the Toshiba FETs show up via email.  I've been absolutely slammed at work so this is my first time on H-F in about 3 weeks. 
 
Beta builds look great guys.  Where did the "Thumbs Up" smiley go?  That's the one I need man...
 
 
Sep 23, 2010 at 8:59 PM Post #579 of 1,752

sachu

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Got half the resistors soldered down. Am planning to actually socket even the BJT and MOSFET resistors and got transistor sockets for the output devices. This way i can switch to either configuration. Also, I might lay off from populating the boards till next thursday...got some other things going on.
 
Sep 23, 2010 at 10:07 PM Post #582 of 1,752

bada bing

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Quote:
If you don't mind me asking, what happened? Did you lose a decent set of phones?



I destroyed a channel of a KSC75 in a rush to hear music during a DynaHi build.
Something happened to the servo op-amp, it was maybe defective.
Nothing else on the amp was hurt. The KSC75 really isn't that hard to destroy.
There was no loud sound, the driver housing just suddenly got rather warm.
 
I permanently damaged a set of SR60 during a "burn in" of a set of output caps on a Bijou.
I was rolling different caps and left an oddball set of "motor run" caps from Ebay playing overnight.
One channel was audibly damaged on the SR60's, nothing happened to the amp. Post mortem
revealed there was a slowly varying offset averaging about 50ma on the damaged channel, cap related.  
 
None of that is really relevant to this build thread, except be careful with good phones on new
builds till they are well proven. Everybody probably knows that already, but I bet there's a few other
stories of cheap cans making the ultimate sacrifice in other people's experience. Knowing better
doesn't always mean doing it, at least in my experience.
 
Sep 24, 2010 at 7:10 AM Post #584 of 1,752

cobaltmute

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Anyone have any good reason not to use CMF55 for all the resistor choices?  1/2W at 70C like the 1/2W BOM resistors, exactly the same part as the RN55 and you can get a slightly better tempco part for slightly cheaper than the BOM RN55.
 
Sep 24, 2010 at 7:18 AM Post #585 of 1,752

jdkJake

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Anyone have any good reason not to use CMF55 for all the resistor choices?  1/2W at 70C like the 1/2W BOM resistors, exactly the same part as the RN55 and you can get a slightly better tempco part for slightly cheaper than the BOM RN55.





Are they the same size? Some of the pads are pretty tightly spaced. As long as you can make them fit I see no reason not to use them.
 

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