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JLH Headphone Amp
JLH Headphone Amp
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Old 7th March 2018, 05:21 AM   #751
Detectit1 is offline Detectit1
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Join Date: Jan 2018
Thanks for clarification.

I don't have a 12v transformer atm so I will try with 15V and look further on that.

Can you tell me which points on the board I have to measure and which value approx I have to read?
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Old 7th March 2018, 05:41 AM   #752
solder slinger24 is offline solder slinger24  United States
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If measuring DC-offset, you'll simply connect a DVM across the left and right outputs and set your meter to the 200mV scale.

Very simple to measure, but you'll find trying to adjust the trimmers to get the lowest reading will be a tedious and to no avail process since the offset will always be fluctuating by a wide margin because of the design.
My JLH amps would have anywhere from a headphone frying 2V! DC-offset when cold to 40-50 mV when trying to use the supplied trimmers and no servo.
It was kinda like a dog chasing his tail when trying to adjust the supplied trimmer. Just when you think you have the trimmers set where it's the lowest, it goes up considerably when you first apply power to the amp.

That's why I designed a small DC servo PCB that keeps the offset within a few millivolts.

Be careful! You can easily fry a nice pair of headphones without a servo to control the DC at the outputs.

Good luck with your build.
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Old 7th March 2018, 05:55 AM   #753
Detectit1 is offline Detectit1
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Join Date: Jan 2018
Quote:
Originally Posted by solder slinger24 View Post
If measuring DC-offset, you'll simply connect a DVM across the left and right outputs and set your meter to the 200mV scale.

Very simple to measure, but you'll find trying to adjust the trimmers to get the lowest reading will be a tedious and to no avail process since the offset will always be fluctuating by a wide margin because of the design.
My JLH amps would have anywhere from a headphone frying 2V! DC-offset when cold to 40-50 mV when trying to use the supplied trimmers and no servo.
It was kinda like a dog chasing his tail when trying to adjust the supplied trimmer. Just when you think you have the trimmers set where it's the lowest, it goes up considerably when you first apply power to the amp.

That's why I designed a small DC servo PCB that keeps the offset within a few millivolts.

Be careful! You can easily fry a nice pair of headphones without a servo to control the DC at the outputs.

Good luck with your build.
Hahaha that last comment is very reassuring...

I bought this one to drive my Audeze Sine Planars.

I will finish the build this evening and investigate how to implement a DC Servo.

Pretty new to all this. So thanks for your help.
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Old 10th March 2018, 05:27 AM   #754
solder slinger24 is offline solder slinger24  United States
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You're quite welcome.

Let us know how your build goes.
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Old 10th March 2018, 02:41 PM   #755
anti is offline anti  Slovakia
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A low-cost intermediate solution for the DC offset problem would be to use a bi-polar electrolytic between the amp output and the headphone socket..
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Old 14th March 2018, 04:55 AM   #756
solder slinger24 is offline solder slinger24  United States
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Quote:
Originally Posted by anti View Post
A low-cost intermediate solution for the DC offset problem would be to use a bi-polar electrolytic between the amp output and the headphone socket..
I agree that would be a low-cost solution, but in my opinion the best output coupling cap is no cap at all.
Others here can disagree with that statement all they want, but I prefer DC-coupled designs when possible.

A simple servo takes care of the JLH's excessive offset without one having to resort to the old adage "which brand and type of coupling cap sounds best".

If someone is looking for a simple servo PCB for the JLH HA, shoot me a PM. I'm pretty sure that I still have a few left.
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Old 28th March 2018, 12:37 PM   #757
mod_sonic is offline mod_sonic  United Kingdom
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Location: London
Default Servo Boards

Hi Solder Slinger24,

Looks like I can't send you a PM ?

Thanks

Mark
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Old 28th March 2018, 12:44 PM   #758
mod_sonic is offline mod_sonic  United Kingdom
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Join Date: Nov 2005
Location: London
Default DC Offset

Using the resistor changes proposed by Miles Campbell way back in this thread and leaving the trimmers in place, my JLH 1969 has a DC offset at switch on of about 70mV. That drops to c. 30mV in less than a minute and is down to zero plus/minus a couple of millivolts in a couple of minutes.

A servo would solve this problem, but I just wait a few minutes before plugging in the headphones using the amp !

I don't know if a dummy load plug in the headphone socket would get the offset down faster.
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Old 23rd June 2018, 03:39 AM   #759
edwtsui is offline edwtsui  Hong Kong
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Hi folks,

I am going to build circuit 2, i just see there is a 4.7uF input cap, may I know if I can skip this one??
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Old 23rd June 2018, 03:56 PM   #760
edwtsui is offline edwtsui  Hong Kong
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Quote:
Originally Posted by edwtsui View Post
Hi folks,

I am going to build circuit 2, i just see there is a 4.7uF input cap, may I know if I can skip this one??
Anyone can help???
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