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Old 19th April 2003, 04:01 PM   #21
trwh is offline trwh  United Kingdom
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Thanks Andy and Jonathan for your replies.

Jonathan, what you say about the power supply reservoir capacitors is interesting. Having looked at their ground paths more closely I understand what you mean. I hope that my PCB in its present incarnation will offer stable performance. I think double sided boards with more carefully defined ground planes are in order in the future!

Andy, considering I only want one PCB, I think PCB Pool works out as (just) being cheaper. They also offer a shorter lead time. However, from what you say, Minnitron would seem to be much cheaper for more than one PCB. Thanks for the info! FWIW, PCB Pool once sent me double the number of boards I asked for free of charge, all of them fine.

Thanks again,
Tim.
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Old 19th April 2003, 04:47 PM   #22
ALW is offline ALW  United Kingdom
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Default Tim

Sounds like PCB pool did you proud.

It's quite normal to get extra boards, as the company would usually overmake to account for any rejects. They often send those extra boards, particularly to obvious one-off's or DIY types.

If nothing else, at least it's another avenue to try!

This place is great isn't it - where else can you get critique from well-known audio designers

Andy.
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Old 19th April 2003, 04:54 PM   #23
trwh is offline trwh  United Kingdom
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Quote:
ALW said:
This place is great isn't it - where else can you get critique from well-known audio designers
Yeah, I appreciate all the insight!

Just did a Google search with "Jonathan Carr" - I hadn't realised!

See ya,
Tim.
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Old 21st April 2003, 05:56 PM   #24
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Quote:
Originally posted by PMA
peranders,
all of the BUF634T's that I have bought or obtained as samples have had TO220-5 case with five in-line pins, that is why I use in-line mount.
Yes, I know (by experience) but it's very easy to bend two pins...
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Old 21st April 2003, 06:25 PM   #25
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Tim, don't want to critizise too much I just want to give you some food for thought:

The groundplane is very sliced up and the return currents go a little everywhere. This may not be a problem (you could get reduced performance) but you could make jumpers in stategic places in order to get better and more predictable current paths. One trick to make a better groundplane is to let traces go closer and by that get wider ares with ground.

Using a 2000 V/µs device is can be tricky but since this only is a buffer I'll guess it's not so tricky....
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Old 21st April 2003, 09:51 PM   #26
PMA is offline PMA  Europe
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peranders,

combining BUF634 and opamp it is an opamp's transient response what you get as a transient response of the combination.... unless the opamp is very very fast...

Pavel
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Old 22nd April 2003, 09:04 AM   #27
trwh is offline trwh  United Kingdom
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Hi Peranders,

"This may not be a problem (you could get reduced performance) but you could make jumpers in stategic places in order to get better and more predictable current paths."

Thanks for the idea - I shall do that. For instance, I could place a jumper across the power trace running under C10 and C17.

"One trick to make a better groundplane is to let traces go closer and by that get wider ares with ground."

I have my design rules set for a minimum track-track distance of 13th (~0.33mm). It just looks a lot more in the screen shot I posted.

Thanks a lot for your input,
Tim.

PS - I wish I had posted here _before_ I ordered PCBs
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Old 27th April 2003, 12:46 AM   #28
rljones is offline rljones  United States
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Peranders (or anyone else),

On your schematic of your headphone amp (very nice looking headamp, BTW), I noticed the CCS after the input opamp and before the BUF634. In reading over various threads, such as the Best Opamp, the concensus is for a 2mA CCS at this point, and this is what you designed.

While this would seem like a good option for you to have, I'd a little confused as to why anything is different at these CCS settings. My confusion concerns two items: the opamp idle current and the opamp output current.

The idle current of most opamps is over 3mA (the AD8610 is about 3mA; the AD845 is 10mA; and most are somewhere in between). I've read that you want the CCS to be about 20% of the maximum output. In the case of the AD8610, it's maximum is 50mA, so 10mA would be a good goal. For the AD845, its output is also 50mA, so it's output is probably already close to 20%.

Does this mean that the AD845 won't be improved since it is possibly already in class A mode? Does it also mean that the AD8610 should be run closer to 10mA rather than 2mA?

Finally, if the idle for the AD8610 is 3 mA, with probably most of this in the output stage, than it would seem to already being running at about 2mA on the output stage. So what would providing a 2mA CCS add? (If the answer, is the AD8610 should have a CCS of 20% of its output, or 10mA, then a CCS on the output starts to make sense.)

Thanks, Robert
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Old 27th April 2003, 01:04 AM   #29
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"bell-clean" !

Quote:
Originally posted by peranders

I have only one prototype and I switched (slowly!) between opamps. With this method you will forget how it exactly sounded, I know, but still, every opamp sounded splendid. Electrically I liked AD8610 the most. "Klockrent" performance!

Can anyone give me an english word for klockrent?, Bull eye maybe? :bulleye:
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Old 27th April 2003, 08:48 AM   #30
ALW is offline ALW  United Kingdom
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Default CCS

It would be interesting to know whether the CCS brings anything to the AD8610 / BUF634 combination.

In the early days, before the complementary bipolar processes used these days the PNP's used were terrible and largely responsible for the poor performance of the o/p stages.

With the CB, and particularly XFCB process used in the AD8610 and the like, they are much higher quality. Although not class A in operation, the output stages produce very little distortion, particularly when running buffered, i.e. virtually unloaded as is the case here.

I wonder if the difference is audible, and hence whether the extra dissipation is justified.

Have you tried it without the CCS Per?

Andy.
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