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Old 4th July 2010, 12:47 AM   #51
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Default BUFv2 schematic!

annnd the new schematic and bill of materials. opinions?

~ brad.
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File Type: pdf v2.pdf (31.8 KB, 113 views)
File Type: txt bom.txt (1.8 KB, 17 views)
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Old 4th July 2010, 01:26 AM   #52
Gordy is offline Gordy  United Kingdom
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Donít know enough to comment deeply on the circuit, except to say that your op-amp needs a path to ground for the positive input to draw current.

Also, consider using a similar type of FET for the CCS, as this should theoretically help drift and give the op-amp a better chance of maintaining zero output offset.

Your positive shunt power supply makes sense, but you donít really need the negative supply shunt because the amp circuit CCS already provides good PSRR to the negative supply. I think a simple three terminal regulator should be fine in the negative side. I know the supply will Ďlook unbalancedí, but the amp circuit already is, so donít be concerned!

Ampslab seems to have parts: 2SK1058 RENESAS/Hitachi Audio Power Mosfet - Buy Online @ AmpsLab
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Old 5th July 2010, 05:17 AM   #53
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Quote:
Originally Posted by Gordy View Post
Donít know enough to comment deeply on the circuit, except to say that your op-amp needs a path to ground for the positive input to draw current.
i'm not exactly sure what you mean. the opamp output has a (high-impedance) dc path to ground through the mosfet gate and an ac path to ground through the 33pF capacitor...?

Quote:
Originally Posted by Gordy View Post
Also, consider using a similar type of FET for the CCS, as this should theoretically help drift and give the op-amp a better chance of maintaining zero output offset.
i'll be sure to simulate a FET in the CCS position... an interesting thought.

Quote:
Originally Posted by Gordy View Post
Your positive shunt power supply makes sense, but you donít really need the negative supply shunt...
well, i suppose a bit of overkill can't hurt, eh? :P (i just read this as i finished the first revision pcb...)

speaking of the pcb, i've attached it. it's a first-run, so it's not perfect, i'm sure... any glaring mistakes that i've missed through my all-day pcb brainstorm?

thanks,
~ brad.
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Old 5th July 2010, 02:32 PM   #54
Gordy is offline Gordy  United Kingdom
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Your positive op-amp input needs to be able to source current from somewhere, and at the moment that is from whatever source you use, but only if it is DC coupled. When you disconnect the signal input of your circuit, or if the source is AC coupled, then there is no-where to source current from and that may result in a large DC offset at the output of the op-amp.

Every non-inverting op-amp circuit that I have seen has a permanent way for the positive op-amp input to receive current, normally via a resistor to ground (to the left of your series input resistor).

I think this is less of an issue with FET input op-amps, but still a consideration.
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Old 1st August 2010, 03:07 AM   #55
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Smile input resistors

alright, here's the updated schematic, layout and bom with input resistors. any comments? the pcb feels like a hack-job to me, for some reason, though i'm confident in the schematic design (and bom for that matter).

any layout nazi's out there?

~ brad.
Attached Images
File Type: png v2b-tmp.png (63.0 KB, 302 views)
Attached Files
File Type: pdf v2.pdf (32.8 KB, 40 views)
File Type: txt bom.txt (3.0 KB, 10 views)

Last edited by geekysuavo; 1st August 2010 at 03:11 AM. Reason: forgot attachments, lol
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Old 5th August 2010, 08:50 AM   #56
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geekysauvo,
nice build log. i am planning on building a bog standard Szekeres with the intent to tweak the circuit as you have. i'm going to start off with a regulated single rail 24V PS (o11 The σ11 Regulated Power Supply), and then convert to a DC coupled with the remote dual-rail unregulated PS from my F5 and a o22.

i have an M3, B22, and Bijou (tube) as well.

thanks for taking the effort to detail your project
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Old 15th August 2010, 11:28 PM   #57
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Quote:
Originally Posted by fishski13 View Post
thanks for taking the effort to detail your project
thanks for the interest! let me know what you think of the szekeres.

ok, here is a redesign of the current circuit. circuit and component changes are minimal aside from different zener diodes and half the number of regulator ccs fets.

i spent a bit more time trying to make the layout better, so hopefully it's less awful this time around. i'll be ordering parts pretty soon, as long as i can bring myself to spend the money. until then, please add any advice or ask questions.

~ brad.
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Old 17th August 2010, 02:55 PM   #58
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Lookin good. I was just reading proffered op amps. Seems like some new ones to try.
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Old 17th August 2010, 02:57 PM   #59
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Oh my. The Szekeres amp is over 25 years old !!
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Old 28th August 2010, 07:50 PM   #60
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Quote:
Originally Posted by gszekeres1 View Post
Oh my. The Szekeres amp is over 25 years old !!
a grand (perhaps late) happy birthday to the szekeres amp and many happy days for all its siblings and children! thanks for the encouragement! i'm honoured that you've read the thread.

i just started the parts ordering process for this design and i'll be tweaking the pcb as they come in, to make sure everything will fit.

some quick simulation values:

THD@1kHz/1Vpp(in): 0.00014%
THD@20kHz/500mW(out)*: 0.0047%
Phase shift@20kHz: -0.5 degrees
Freq Resp@0dB,-3dB: 0Hz-2.5MHz
Gain: 3V/V

* = The maximum input power for my HD595's

these values are surely overzealous, since they come from a simulation, but i'll be happy if the amp performs an order of magnitude worse than the simulated THD specs, honestly. :P

~ brad.
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