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"WHAMMY" Pass DIY headphone amp guide
"WHAMMY" Pass DIY headphone amp guide
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Old 14th March 2019, 11:34 AM   #1251
vvs07 is offline vvs07  United States
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Join Date: Jun 2011
"WHAMMY" Pass DIY headphone amp guide
Quote:
Originally Posted by monacelli View Post
Hi guys, I am planning my Whammy build, and I was wondering how critical is the value of the gate resistor on the output MOSFETs? For whatever reason the 499R RN55D resistor is $1.28 at Mouser, whereas the 511R is $0.16. Small potatoes, but still would rather save where I can. But more broadly, what are the key design considerations to keep in mind here?
This value is not critical. The gate resistor along with input capacitance of mosfet form the low pass filter to prevent the high frequency oscillations.
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Old 14th March 2019, 07:04 PM   #1252
monacelli is offline monacelli
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Quote:
Originally Posted by vvs07 View Post
This value is not critical. The gate resistor along with input capacitance of mosfet form the low pass filter to prevent the high frequency oscillations.

Thanks for your reply. It seems like these oscillations typically manifest themselves north of 50 MHz, so a corner frequency 1 decade below that is maybe a good target (5 MHz corner to ensure -20 dB attenuation at 50 MHz). For the FQP3P20/FQP3N30, the minimum listed input capacitance is 175 pF, which suggests that a gate-stopper value of ~180R is maybe a good lower bound (-3 dB just north of 5 MHz). This is inline with the guidance in [this post], which suggests that gate-stopper values between 47-220R tend to have a minimal impact on sound quality.
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Old 17th March 2019, 08:29 AM   #1253
MadHatterRyu is offline MadHatterRyu
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Click the image to open in full size.






So I thought I was done with the basic build.. went n fired it up.. the 47R resistors (36 and 35) suddenly went boom... they caught smoke too.. any ideas?


p.s. I tested the power supply post building. Seemed ok. was getting around 17.5

Last edited by MadHatterRyu; 17th March 2019 at 08:34 AM.
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Old 17th March 2019, 08:55 AM   #1254
MEPER is offline MEPER  Denmark
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You may have a short on the op-amp board?
Or the opamp itself have "gone south" and made a short?
From the schematic the 47R is part of the opamp circuit.


You could try to use a "normal" opamp after exchange of the 47R and see if that helps.
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Old 17th March 2019, 09:07 AM   #1255
MadHatterRyu is offline MadHatterRyu
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Just built mine and fired it up..

Two 47R resistors were smoked immediately..


Update.. I had the opamp in reverse.. I'm dumb
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Old 18th March 2019, 05:31 PM   #1256
wayne is offline wayne  United States
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"WHAMMY" Pass DIY headphone amp guide
Those 47 Ohm resistors serve multiple purposes and letting you know your op amp is bad or miss stuffed is one.
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Old 18th March 2019, 06:52 PM   #1257
jhofland is online now jhofland  United States
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Default WHAMMY variant

I posted some information in post #388 about my try to make a reduced size WHAMMY using surface mount components for much of the design and I may have followed up with some information about having a high frequency oscillation that subsequently got solved.
After working on the design for awhile and testing a FET switch based stepped attenuator I designed with the board I ended up not using the attenuator design.
I then did an iteration on the main board design to move to the Talema toroid transformer instead of the Hammond transformer I originally had in the design. I also added a DS1882 attenuator to the board for volume control. The attenuator is controlled by an MSP430FR2433 processor mounted on a small front panel along with a small OLED display.
The design includes a board mounted input jack and board mounted power input module. It all fits into a Hammond 1455 series chassis that's a smaller version of the original WHAMMY. And there are no wires.
A couple pictures showing the board in the enclosure and the front panel display.
Attached Images
File Type: jpg whammyJr2_in_chassis.jpg (931.1 KB, 258 views)
File Type: jpg whammy_jr2_in _chassis_display.jpg (829.2 KB, 448 views)
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Old 18th March 2019, 07:28 PM   #1258
cubicincher is offline cubicincher  Germany
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Default answer to jhofland #1257

Hello Jhofland,


very nice and compact build! Very good implementation of volume control with
display.
I like it! Listen and enjoy.

Greets Dirk
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Old 19th March 2019, 02:26 PM   #1259
MadHatterRyu is offline MadHatterRyu
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Quote:
Originally Posted by wayne View Post
Those 47 Ohm resistors serve multiple purposes and letting you know your op amp is bad or miss stuffed is one.

I made a mistake

That's the OPA2156 converted to DIP8.. The white dot is where I was supposed to orient the SOIC op amp towards for soldering. I used the white dot yet again as a means of identifying orientation of op amp.. just put it facing the notch on the PCB.

It was supposed to be the other way around..

Currently waiting for the replacement resistors. They'll be here soon hopefully. I've measured the other resistors and they seem to still measure alright. Hopefully nothing else went bad.



I did have some SMD 47ohm resistors that I tried soldering using a solder bridge and for some reason I got really high DC between channels and ground.. (R36 when measured during running seemed to show much higher resistance like.. 80+ ohm.. 35 was still around 47.



The power supply stage still shows around 16V though.. Let's see if proper resistors fix this.

EDIT: I am currently using the RC4580 that came with the kit
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Old 19th March 2019, 04:31 PM   #1260
PC997 is offline PC997
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Quote:
Originally Posted by jhofland View Post
I posted some information in post #388 about my try to make a reduced size WHAMMY using surface mount components for much of the design and I may have followed up with some information about having a high frequency oscillation that subsequently got solved.
After working on the design for awhile and testing a FET switch based stepped attenuator I designed with the board I ended up not using the attenuator design.
I then did an iteration on the main board design to move to the Talema toroid transformer instead of the Hammond transformer I originally had in the design. I also added a DS1882 attenuator to the board for volume control. The attenuator is controlled by an MSP430FR2433 processor mounted on a small front panel along with a small OLED display.
The design includes a board mounted input jack and board mounted power input module. It all fits into a Hammond 1455 series chassis that's a smaller version of the original WHAMMY. And there are no wires.
A couple pictures showing the board in the enclosure and the front panel display.
I love this.. Any chance you will make this board available?

Thanks
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