EMRFD Message Archive 10430

Message Date From Subject
10430 2014-11-09 04:50:08 jliving2001 Audio Amplifiers
I have an old Ten Tec Corsair. The receiver has much more hiss compared to my Onmi D, a problem commented on many times by aficionados of these rigs. I thought about replacing the audio amplfiier. I remembered the discussions in Solid State Design and Doug Demaws Notebooks regarding cross over distortion in op amps. One high performance receiver in Solid State Design described on p 132 used a cool preamp, audio filter, and 2N5880/2N5881 class B amplifier driven by an LM301A, working as a phase spliter. Is there any place nowadays for designs like this. Yes, I do have a dog eared copy of EMRFD, but I am just curious about your thoughts on audio amplifier design for modern transceivers.

Joe W3GW


10431 2014-11-09 06:30:20 David Re: Audio Amplifiers
After doing a little research. I do not see any particular reason that the
LM380/LM383 used in the Ten Tec Corsair would be inherently noisier than the
alternatives except for poor power supply rejection of the amplifier.

A larger problem may be all of the cascaded RC4136 stages. The impedances look
acceptable (except for the notch filter) so excessive resistor and current noise
are not a problem but that is only because these operational amplifiers have
such high voltage noise anyway. I found a couple of references online saying
considerable improvement being made by replacing them although this is not
trivial because of their uncommon pinout.

The OPA2134 suggested here to replace the RC4136 is acceptable although I might
have chosen something else:

http://corsair.wikidot.com/corsair-opamp

The configuration of the audio amplifier you mention is an obscure way to get a
higher gain-bandwidth product, a faster slew rate, and more power out of a low
performance operational amplifier. It uses the output stage as an additional
common base gain stage with current feedback which has the effect of
decompensating the operational amplifier for better high frequency performance.
The voltage gain of the discrete output transistors is controlled by the current
feedback into the output pin of the operational amplifier.

On 09 Nov 2014 04:50:08 -0800, you wrote:

>I have an old Ten Tec Corsair. The receiver has much more hiss compared to my Onmi D, a problem commented on many times by aficionados of these rigs. I thought about replacing the audio amplfiier. I remembered the discussions in Solid State Design and Doug Demaws Notebooks regarding cross over distortion in op amps. One high performance receiver in Solid State Design described on p 132 used a cool preamp, audio filter, and 2N5880/2N5881 class B amplifier driven by an LM301A, working as a phase spliter. Is there any place nowadays for designs like this. Yes, I do have a dog eared copy of EMRFD, but I am just curious about your thoughts on audio amplifier design for modern transceivers.
>
>Joe W3GW
10432 2014-11-09 15:22:19 kb1gmx Re: Audio Amplifiers
They are not all that noisy.  Its the Opamps used earlier in the audio chain that 
contribute a fair amount of noise being older parts.  I prefer the LM5532 
low noise opamp for general use even for driving headphones (32ohm or higher).

The LM38x however do have a fair amount of distortion most of it crossover
but there are tricks to improve that.  They generally work better if they are run at 
Vcc approaching the upper limit for that version (386 come in -1 for 12V max 
and -4 for 18V max) so using them down at 5-6V tends to have more distortion.


Allison/KB1GMX
10434 2014-11-09 19:51:35 Jerry Haigwood Re: Audio Amplifiers

Hi Joe,

    Go to http://qrp-popcorn.blogspot.ca/.  This is Todds blog AKA popcorn QRP.  Todd discusses several discrete AF amps.  I have laid out his amplifier number 3.  It is a low distortion 1 W amplifier.  I sell the board for $6 + $2.20 shipping.  Also, look into replacing the RAY4136 op amps with lower noise types. 

Jerry W5JH
"building something without experimenting is just solder practice"


10435 2014-11-10 07:33:41 Bob Thompson Re: Audio Amplifiers
Thanks for the link, Jerry. Lots of interesting stuff there!

Bob, K1AO

On 11/9/14, 10:51 PM, 'Jerry Haigwood' jerry@w5jh.net [emrfd] wrote:
 

Hi Joe,

    Go to http://qrp-popcorn.blogspot.ca/.  This is Todds blog AKA popcorn QRP.  Todd discusses several discrete AF amps.  I have laid out his amplifier number 3.  It is a low distortion 1 W amplifier.  I sell the board for $6 + $2.20 shipping.  Also, look into replacing the RAY4136 op amps with lower noise types. 

Jerry W5JH
"building something without experimenting is just solder practice"