Build A Headset Amplifier, Sept 2013

Post Reply
EPA III
Posts: 39
Joined: Tue Aug 17, 2004 1:01 am
Location: Beaumont, TX
Contact:

Build A Headset Amplifier, Sept 2013

Post by EPA III »

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
Paul A.
User avatar
CeaSaR
Posts: 1949
Joined: Sat Nov 08, 2003 1:01 am
Location: Phoenixville, PA USA
Contact:

Re: Build A Headset Amplifier, Sept 2013

Post by CeaSaR »

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
Hey, what do I know?
UhClem
Posts: 13
Joined: Fri Dec 28, 2012 4:19 pm
Contact:

Re: Build A Headset Amplifier, Sept 2013

Post by UhClem »

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
David Schultz
User avatar
CeaSaR
Posts: 1949
Joined: Sat Nov 08, 2003 1:01 am
Location: Phoenixville, PA USA
Contact:

Re: Build A Headset Amplifier, Sept 2013

Post by CeaSaR »

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
Hey, what do I know?
User avatar
oldtechie
Posts: 16
Joined: Sat Sep 08, 2007 9:24 am
Location: Indiana - USA
Contact:

Re: Build A Headset Amplifier, Sept 2013

Post by oldtechie »

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.
User avatar
CeaSaR
Posts: 1949
Joined: Sat Nov 08, 2003 1:01 am
Location: Phoenixville, PA USA
Contact:

Re: Build A Headset Amplifier, Sept 2013

Post by CeaSaR »

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
Hey, what do I know?
ronwande
Posts: 1
Joined: Thu Sep 12, 2013 4:15 pm
Contact:

Re: Build A Headset Amplifier, Sept 2013

Post by ronwande »

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
Post Reply

Who is online

Users browsing this forum: No registered users and 5 guests