JLH Headphone Amp

[blackdod]

I´ve got Ebay´s old version (green board) and the sound is not bad, no noise or HUM, but the offset is a really nightmare¡¡¡

I can´t keep this parameter controlled around 50mV.

 

[oz7aff]

How about building at servo for it ?

 

[bkercso]

Try to use 4x1N4148 instead of 2.7V Zener.
If you trim the DC offset to 0V after warm up you get about 100mV when amp is cold (and vice versa). Try to trim around 50mV after warm up and when cold get -50mV (or vice versa? ).

 

[blackdod]

Thanks guys

I've been working for a very little time in the amp because I´ve got another headphones amp (B22...)

I´ve made some changes on the original JHL,like this:

A) Trimpot 10k (20 turn off)
B) Nichicon Non polar (1000uF/25volt) on the out put.

I haven´t got any DC Servo.

As much the temperature grows, the offset change too.

 

[blackdod]

Try to use 4x1N4148 instead of 2.7V Zener.
If you trim the DC offset to 0V after warm up you get about 100mV when amp is cold (and vice versa). Try to trim around 50mV after warm up and when cold get -50mV (or vice versa? ).

Well, the board have got 6x1N4148 but if you take a look, two others missing

An externally hosted image should be here but it was not working when we last tested it.

 

[oz7aff]

B) Nichicon Non polar (1000uF/25volt) on the out put.

Because you got these I think that you don't have to care about the offset as long it stayes below something like 100 mV

 

[blackdod]

Thanks Max

Well, the offset is around 140 mVolt when the my amp is warm (with the Non Polar Cap) and I don´t like this. IMHO, 140 mVolt is very dangerous for a headphone. I supposed that with this Cap, the offset would have be around 0 mVolt.

I´m thinking change the input cap for a Wima 1uF or add other Non Polar Cap on the output (on series) ¿?

 

[oz7aff]

Do you have schematic of the Amp ?

My headphones have a resistance of 160 Ohm's, so with 140 mV offset the current will be under 1 mA and that will not harm the phones in any way.

Ps: is the headphones connected to the amp through a capacitor ?

 

[blackdod]

Ps: is the headphones connected to the amp through a capacitor ?

Yes; amp out put--cap--headphones

Do you have schematic of the Amp ?

My headphones have a resistance of 160 Ohm's, so with 140 mV offset the current will be under 1 mA and that will not harm the phones in any way.

Yes I have got the schematic and a fantastic article by Papa Nelson

But reality, I don´t want to spend much time on this amplifier.

I have some headphones, from 32 Ohm to 600 Ohm.

Regards

 

[oz7aff]

Yes; amp out put--cap--headphones

Then there should be no offset at all, the cap must be bad, must have a very large leakage.

Ps: the hole meaning of capacitors is that you can't put DC trough them, only AC.

 

[blackdod]

Well, I don´t believe that the problem be the capacitors... I used a pair of Nichicon electrolytic capacitors on seríe and the result is the same.

An externally hosted image should be here but it was not working when we last tested it.

The offset on this ampli is a "russian-mountain"

Regards

 

[oz7aff]

The offset on this ampli is a "russian-mountain"

I don't know the meaning of "russian-mountain".

But I know that if you'r setup is like this:

There shall be no DC offset to measure with the VDC meter.

 

[blackdod]

Thanks for your answer.

Yes, is my setup, but I never take the DC values with the headphones connecting.

Regards

PD: "russian-mountain"; when something mimics the movement of a roller coaster.

 

[oz7aff]

Then you can't use the measurement, try to connect a 1 kOhm resistor as a dummyload.

 

[blackdod]

... try to connect a 1 kOhm resistor as a dummyload.

Thanks Max, I´ll try.

Regards

 

[bismuth]

Thanks Max

Well, the offset is around 140 mVolt when the my amp is warm (with the Non Polar Cap) and I don´t like this. IMHO, 140 mVolt is very dangerous for a headphone. I supposed that with this Cap, the offset would have be around 0 mVolt.

There is maybe something wrong elsewhere with your board, mine is barely around +-25mV Offset at startup, and it goes down to around +-3mv after around 10 minutes when warm, and I have NO capacitor on output, only 2,2uF on input.

 

[blackdod]

Thanks guys

If I conecct a resistor on the output, the offset is 0 mvolt, but I think that my board have any problem because when I start it up (with out dummy load) the offset goes down to 200 mVolt to around 0 mVolt and when it warns up goes to 140 mvolt

I will check the rest of the board.

Regards

 

[oz7aff]

As I said before, the measurements without the resistor is of no value, measuring on a capacitor without connection on both side is a waist of time.

 

[blackdod]

As I said before, the measurements without the resistor is of no value, measuring on a capacitor without connection on both side is a waist of time.

Thanks Max. Do you think the amp It´s ok?

Regards

 

[oz7aff]

If there is no Offset voltage with the 1 kOhm Dummy load, then I am shure that the amplifier is working OK.