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Old 24th April 2011, 01:35 AM   #71
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i'm glad there's interest in the thread.

well, here's my first try at the layout for an isolated amp section. i went with a ground plane. it's my first attempt at such a thing, so let me know if there are any glaring mistakes.

thanks,
~ brad.
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Old 29th April 2011, 05:13 AM   #72
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Smile interesting...

after playing around with the amp in a number of ways, i found that the hum left when an 8 ohm speaker load was connected instead of the 32 ohm headphones. i know i'm exposing my ignorance, but what could cause this?

with the 8 ohm load, the sound is very detailed and sharp. the bass is more impactful and controlled than you'd expect with such a low-impedance load. with that in mind, i simulated the THD for 0.5W into 8 ohms to be 0.0018%. not bad.

just thought it was interesting. makes me anxious to get the design working with headphones. this thing should sound really good.

~ brad.
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Old 16th May 2011, 01:14 PM   #73
regal is offline regal  United States
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Quote:
Originally Posted by geekysuavo View Post
after playing around with the amp in a number of ways, i found that the hum left when an 8 ohm speaker load was connected instead of the 32 ohm headphones. i know i'm exposing my ignorance, but what could cause this?


~ brad.
Grados are have much more sensitivity than any 8 ohm speaker
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Old 29th May 2011, 02:30 AM   #74
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Grados are much more sensitive than any 8 ohm speaker
Ah, makes sense. Thanks.

I've built the single amp channel design and it measures well into a 20R dummy load, at least on what I can measure, using a lab bench power supply. No evidence of oscillation from DC to 30MHz. Frequency and phase response seem to adhere reasonably well to the design predictions. Noise at the scope's baseline and transient response is OK.

I'm more than happy to run extra tests if anyone has recommendations, but I am limited by my test equipment...

~ Brad.
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Old 29th May 2011, 04:53 AM   #75
regal is offline regal  United States
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Originally Posted by geekysuavo View Post
Ah, makes sense. Thanks.
I'm more than happy to run extra tests if anyone has recommendations, but I am limited by my test equipment...

~ Brad.
I have a hard time measuring hum that shows up in Grados with my inexpensive equipment, so I'd say the one of the best tests for a headphone amp is to plug in a pair of grados set the volume a little louder than normal listening and run a silence track in a very quiet room.
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Old 29th June 2011, 05:34 PM   #76
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alright, it's been a while, i know. i'm a bit gun-shy after the previous revision.

nevertheless, here's the new board! i really need opinions on layout here. be as critical as you want; i'd rather have to fix the design now than after i spend more money on parts and wire it up...

i did my best to focus on grounding. notice the two 'star' points on the groundplane for earth, the capacitor bank, the regulated rails, and the headphone return. i connected the small-signal grounds straight to the groundplane. the +/-15V rails do follow semi-circuitous routes to their destinations, but i think it's OK.

also worth mention is that the board is designed for a hammond 1455-series enclosure, though which one escapes me at the moment.

~ brad.
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Old 29th June 2011, 07:14 PM   #77
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Hi all,I'm getting quite high THD on mine(0,7%)....
Has anyone tried to put the Szekerez inside an opamp FB loop ?
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Old 29th June 2011, 07:25 PM   #78
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Has anyone tried to put the Szekerez inside an opamp FB loop ?
hahahaha! well you're in the right place.

you're quite right. the open-loop szekeres can be tweaked for ~0.1% THD, but bass response is poor due to input and output coupling capacitors, and phase shift is high in the khz range.

just go back a few posts to the last schematic i posted and you'll see an example of just what you propose. simulations show 0.0014% THD and flat phase/frequency response to ~50khz. (it sounds pretty good too, but i can't tell you specifics as i haven't listened using high-quality headphones and a good low-output impedance power supply.)

~ brad.
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Old 17th April 2012, 12:34 AM   #79
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Post it's been a while...

alright, figured i should finally wrap this project up; i revisited the board layout and tweaked the power and ground traces, trying as much as possible to isolate high- and low-current ground returns. here's the revised board, in horrible neon...

i'll be ordering a pair of pcbs soon. build log and results to come shortly!

~ brad.
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Old 17th April 2012, 12:27 PM   #80
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You might be interested to read through the PRR "TORI" thread.
This is pretty much what you are trying to build.
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