JLH headphone amplifier
I'm looking to build a JLH headphone amplifier, the second circuit at <a href="http://www.gmweb.btinternet.co.uk/jlhphones.htm">http://www.gmweb.btinternet.co.uk/jlhphones.htm</a>. I need to substitute different transistors for the BC184 and BC214, since I can't get those easily. I was looking at BC549 and BC559, which have slightly lower minimum hfe than the BC184&214. Do I need to check the BC549&559 to make sure they have at least the hfe of 150 mentioned in the article?
Also, the article say the emitters of Q1 and Q2 sit at -0.55V and +0.55V, which I guess is the voltage across a PN junction. Are C4 and C5 the right way around in the schematic?
You can go ahead and use the alternate transistors without any problem. In most cases the devices have much higher hfe than the minimum stated value. That has been my experience. You could also look for the 'B' devices , eg. BC546B which have a higher min/max hfe.
About the caps C4/C5. Looks like you are right. Q1 emitter is
at -0.55V wrt to gnd and hence the cap C4 should be -ve there with the positive going to ground. C5 also would be reversed.
I would have easily overlooked this error !
Don't forget a decent heat sink on the BD output transistors. They should get pretty warm ( about 1.5 watt dissipation on a 1.5 inch thin Aluminum plate ) - hot!
You can use BC549B/559B min 250 hfe or
BC549C/BC559C min 400 hfe.
They ar BOTH low noise transistors, like BC184/BC214.
I would think BC549B/BC559B, would be perfect.
I myself have some BC549B, at home
Also you can use BC550B or C and
BC560B or C. And BC548B/C + BC558B/C
has high hfe 400-600 and are lownoise.
That is good.
Nelson Pass says he tries to find
input transistors with as high as possible hfe.
So do I!
Do not use BC546/556 or BC547/557. They are not low noise.
Have somewhat higher input noise.
/halo - knows about BC and BD transistors. :cool:
Ashok and Halo: Thanks very much for the replies. I like it when the answers are what I wanted to hear.
Now I have another question:
I can get BD139/140 transistors cheaper than BD135/136. These are the same family, but 139/140 have higher voltage ratings. Cheaper + higher voltage sounds too good to be true to me. Can I substitute these transistors without affecting sound quality?
Yes you can use the 139/140. These are also quite cheap over here but I suspect many of them (here) are just stamped that number and just about meet the specs. But they all work.
Since this is not a preamp, even very low noise devices may not be absolutely necessary . I might even stick out my neck and say that for the BC54X and 55X if you used any variety , the circuit will work fine and you may not even be able to tell the difference ( between 546, 546A, 546B ).
So take any ones you get and build it. If you have problems we can help you fix it.
If you are very new to all this, it might be a good idea to put a small value resistor in the power supply rails to avoid a shorted or excess current situation. Remember you will need a capacitor after the resistor also. If you goofed badly you will probably only lose the resistor. If it works fine, you can remove the
resistor ( you must !).
Value of the series resistor = about 0.5 volts/ max dc current
Maybe 0.5/0.3 = 1.8 ohms . So anything less than 5 ohms should be OK. 1/4 watt is best as it would be working at over 50% of rated wattage and hence blow if you had too much current. It could catch fire ( small fire!) So be careful when you turn on the power. If you have an over current situation it might burn a couple of seconds and go open circuit!
Thanks again, Ashok. You advice about the resistor in the power rails sounds good, as I've never built a wall-powered amp before (batteries only until now).
does this JLH headphone amplifier invert phase?
JLH headphone amplifier
I don't believe it does. I've built two of these amps. Excellent sound quality when driving senn. HD 600 phones. Quiet & flat, an easy listen.
There is a problem with the ckt. as drawn. I will check my drawings & post the correction.
There needs to be a 2.2nf film capacitor connected between the junction of c4 & c5 & the output; between the collectors of the output transistors. Without this the amplifier will oscillate at an audible frequency.
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