T&L Publications

While reading this article I noticed in the schematic that one of the output transistors, Q11 apparently does not have any connection to the signal on it's base. Instead, it appears to be biased at about a fixed -10 Volts via a 2.2K resistor and an LED.

Is this correct? If so, can the author or someone please explain exactly what this output transistor does and how it does it?

TIA

It is controlled by Q8, which, when on, turns "more" negative.

More disturbing is the designations in the parts listing vs. the schematic of Q1 through Q9.
Anyone else see a problem here?

CeaSaR

Q11 is operated as a current source.

In other words, this is a Class A amplifier. If I was going to go Class A for a headphone amplifier I would probably base it on something from the Nelson Pass DIY site. Like: http://headwize.com/?page_id=105

CeaSaR wrote:...More disturbing is the designations in the parts listing vs. the schematic of Q1 through Q9.
Anyone else see a problem here?

Since no one has replied, might I point out that, in the BOM, Q1-Q5 are listed as 2N5551 NPN transistors and
Q6-Q9 are listed as 2N5401 PNP transistors. Those are the correct designations for those particular part numbers.
However, in the schematic, Q1-Q5 are shown as PNP transistors while Q6-Q9 are shown as NPN transistors. The
BD139's are correctly depicted.

Perhaps there was a mixup? Mr. Anderson? Mr. Bergeron? Anyone?

CeaSaR

I'm still hunting for those controls shown in the picture on page 28. Bass, Mid, Treble -- not found on the schematic.

Also notice that the exact same picture is used in August 2013 on page 30. Mr. Anderson makes reference to the photo being wrong and says it is of a headset amplifier. (notes on page 28 of Sept issue) He seemed to omit those extra controls from his circuit as shown in figure 2 etc.

oldtechie wrote:I'm still hunting for those controls shown in the picture on page 28. Bass, Mid, Treble -- not found on the schematic.

Lots of places to look out there. Try Red Free Circuits for something that can be adapted. Obviously, it must be doubled to be stereo...

CeaSaR

Sorry all that I didn't get back to this forum a lot sooner. First of all, you got me! The small transistor designations are reversed. No end of proof reading seems to be good enough for me. The PNPs are 2N5401 and the NPNs are 2n5551. The schematic shows the PNP and NPN correctly but the text is wrong.

As to the picture, I should have been more specific here. The article describes building one channel of the amplifier. The box shown in the picture is my final product, two channels plus a two channel tone control. I should have stated that the tone control was not part of this article. I would be pleased to supply an article about the tone control should the editor (Bryan) want it.

There was some confusion about the function of Q11. One of the responders above correctly identified it as a current source (actually a current sink). The LED voltage is about 2.1. With the 47 ohm emitter resistor, the current is set at about 35 Ma. This is the load for the emitter follower Q10.

Ron Anderson