Questions about JLH for headphones

Nisbeth

Member
2001-01-29 9:58 pm
Denmark
I've been working on a PCB-layout for the headphone-version of the JLH-amplifier (fig. 1 from http://www.tcaas.btinternet.co.uk/jlhphones.htm) and I have a couple of questions.

1) Is is OK to use BC550/BC560 and BD139 transistors instead of the ones specified?
2) How much power is dissipated in the resistors R11 and R13 (I imagine 1/4W resistors are more than sufficient for the rest, but I'm not sure about these two)


Help and comments are much appreciated!


/U.
 
The transistors you propose to use should be fine.

The required rating for R11 and R13 depends on the quiescent current (R11) and your headphone impedance (R13). IIRC, the Iq is about 110mA which gives a dissipation in R11 of just over 60mW. The dissipation in R13 is unlikely to exceed 100mW with 'normal' impedance 'phones.

Please note that simulation indicates the need for more diodes in the D1/D2 chain in order to obtain an output dc offset close to zero. I would prefer to see a means of adjusting the output dc offset, either by providing a variable voltage from these diodes (perhaps by using a zener with a preset in parallel) or by omitting D1, D2, C2 and R3, connecting R1 to ground and then adding the ccs to inject the required current into the emitter of Q4 as per the 'update' version on my website.

I have a headphone version of the full 'update' schematic (mainly changes to some of the resistor values) which has been built by TimA who commented on completion "what a delightful way to squirt music into ones ears". Send me an email if you would like a copy of this schematic.
 

tlf9999

Disabled Account
2005-05-25 2:25 am
none
Geoff said:
The required rating for R11 and R13 depends on the quiescent current (R11).


What then determines the quiescent current? What does that answer mean for the original question?

R13's selection is quite tricky. the worst case scenario dissipation on R13 can be quite high when the output terminal is shorted. I would use R13 as a "fuse": it should burn open if the output is shorted under maximum output.
 
alcar said:
The circuit is different fom the one in your link, but the output stages are more or less the same.

Sorry to disagree with you but, to avoid any confusion, I must point out that the output stage of the Chiara is not more or less the same. It is an emitter follower with a constant current emitter load (ie unity gain, single ended Class-A). All the voltage gain in the Chiara circuit occurs in the preceeding op-amp

The output stage in the circuit to which Uffe has linked provides voltage (as well as current) gain.
 
tlf9999 said:


What then determines the quiescent current? What does that answer mean for the original question?

The quiescent current is set as follows. Q5 compares the volt drop across R11 with a voltage reference obtained from D3/D4 and adjusts the dc current fed to the bases of Q1 and Q2 to achive the desired quiescent current. C7 filters out the ac signal variations across R11 by applying the same ac signal to both the emitter and base of Q5.

The original question related to the power dissipation in R11 and my reply stated that, from memory, the dissipation would be in the order of 60mW



R13's selection is quite tricky. the worst case scenario dissipation on R13 can be quite high when the output terminal is shorted. I would use R13 as a "fuse": it should burn open if the output is shorted under maximum output.

Agreed, a fusible resistor could be used in this positon but it is not necessary. One of the advantages of the JLH output stage (as with single ended Class-A output stages) is that the output current is limited to a specific value and a short across the output will not cause this limit to be exceeded. Therefore, provided adequate heatsinking has been provided for normal use, and the SOR of the output transistors has been observed, an output short will not cause any damage.
 

tlf9999

Disabled Account
2005-05-25 2:25 am
none
Geoff said:


The quiescent current is set as follows. Q5 compares the volt drop across R11 with a voltage reference obtained from D3/D4 and adjusts the dc current fed to the bases of Q1 and Q2 to achive the desired quiescent current. C7 filters out the ac signal variations across R11 by applying the same ac signal to both the emitter and base of Q5.

The original question related to the power dissipation in R11 and my reply stated that, from memory, the dissipation would be in the order of 60mW


the quiescent current is determined by the forward voltage drop over the diode, and R13. So it is backward, logically, to say that the power dissipation over R11 depends on the quiescent current.

The power dissipation over R11 depends on the forward voltage on the diode and R11. In this case, it is 0.7*0.7/5.1=100mw (or lower if you use 0.63v for the forward voltage).