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Direct Conversion Audio Amplifier Chain

Tue, 01/06/2009 - 15:10 — NT7S

This circuit was designed to work in a direct conversion receiver. With the exception of a volume control, mute circuit, and filtering, this is a complete audio chain. The amplifier is designed for a 50 Ω input impedance, which makes it suitable to follow a diode mixer. Total voltage gain is nearly 100 dB, which will provide enough gain for a reciever that contains a mixer with conversion loss. The final stage provides more than enough power to drive headphones, and might be capable of driving a small speaker. The circuit is still a work in progress, so consider it experimental at this point.

8 January 2009 Update

I was getting too much RF into the audio chain due to the inadequate high frequency roll off of the amplifier chain. A few strategicially placed 10 nF caps were installed to kill the high frequency response a bit. Now the amplifier is better behaved under bad conditions.

Schematic: 
Creator: 
Jason Milldrum, NT7S
AttachmentSize
NT7S DC AF Amp Chain.asc7.02 KB

Comments

Tue, 02/17/2009 - 22:43 — DanT

NT7S DC AF AMP

This is a nice audio chain.  However, this is where I really appreciate op-amps.  A cheap, common, low noise op-amp like a LM5532 provides a similar open loop gain to your circuit.  However, in order to improve distortion characteristics, this open loop gain can be set to a much lower closed loop gain such as 25 db with the excess unused 75 db gain providing distortion reduction as compared to open loop operation.  When using four stages of a LM5532 to provide the same 100 db of total gain, 300 dB of gain can be used to squeeze out distortions.  Thus, the resulting audio will sound much cleaner especially when there are multiple signals in the audio passband.

Do not try to misunderstand me, I am not trying to knock what you are doing here.  It is outstanding to see someone experimenting and understanding how to do things like this. I am just trying to get you to the next point to understand why feedback is important to a really clean sounding audio chain.

- Dan, N7VE

 

Sun, 02/22/2009 - 00:20 — NT7S

I appreciate it

Thanks very much for the feedback, no offense taken at all. I love to hear comments like this. No doubt you are 100% correct about op-amps. I've been on a "popcorn" kick to try to build as much as I can "IC free" lately. I don't have anything against ICs, just a personal goal of mine. I'll have to try that trick with four stages of op-amp @ 25 dB closed loop gain per stage. I bet that sounds extremely good.

My goal with this amp was to provide a discrete component amp that could be build out of very common parts that would make for a better replacement for the typical LM386 (not hard to beat that). I think it sounds pretty decent (definitely better than the 366), but there's no way it would measure up to the op-amp AF amp you described. Still, for a casual use rig, I think this circuit holds its own.

Thanks for taking the time to comment!

Fri, 01/23/2009 - 17:53 — DL6MBI

How to upload .asc files?

Hi Jason,

how did you manage to upload a SwitcherCad .asc file as an attachment? I'd like to upload an .asc attachment but I got a error message.

73's
Rolf, DL6MBI

 

 

Fri, 01/23/2009 - 23:09 — NT7S

Uploading files

There is no provision to upload attachments in comments, but when you create your own page, blog post, etc. there is an attachements section below the main body of the content where you can upload files.

Tue, 01/27/2009 - 08:46 — DL6MBI

Jason. One more question.

Hi Jason,

 

thank you for the information. I tried to send a LTspice .asc file. When I try to upload the file I get an error message:

The selected file Oszillator.asc could not be uploaded. Only files with the following extensions are allowed: jpg jpeg gif png txt doc xls pdf ppt pps odt ods odp zip 7z tar gz.

I searched the "help" to find out how to upload an .asc attachment but can't find any hint. I tried to follow your information but I obviously make a mistake. Can you please tell me how to upload this kind of attachment?

73's,
Rolf, DL6MBI

 

Tue, 02/17/2009 - 21:13 — NT7S

Sorry

Rolf,

Sorry for the very long delay in getting back to you. I have been struggling with just trying to get the site working correctly. I did not have the file upload settings correct, but I have now fixed them. You should now be able to upload .asc files.

73,

Jason

Tue, 01/06/2009 - 19:51 — KB9BVN

Here we go...

Jason...on the plot...what is the dotted line representing?  I think I understand the soild line.

Wed, 01/07/2009 - 00:28 — NT7S

Should have labeled it better

The solid line is the amplifier gain, while the dotted line is the phase response of the circuit. When you do a AC Analysis in LTSpice, you always get a magnitude plot and a phase plot.