THE AMPLIFIER

Today, we are going to look at audio amplifiers. 

I have three needs for a decent amplifier.

1) To replace my Tascam PA30 that I've had for 20 years. (It died a terrible death.)

2) Headphone Amplifier.

3) Cue Amplifier

I do not think I will need a CUE AMP in my mixer, but you may.

With that in mind, let's take a look at some ideas. Maybe we can find a design that will suit all of our needs.

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Radio systems used a large OP AMP (LM2879 & LM4765) in their Cue Amplifier and Headphone Amplifier. 

I always thought that was one of the FLAWS in their design. (The other was the use of mechanical switches) These chips were prone to a ton of HEAT, and that does not work well in a closed environment.

I never saw a Radio Systems RS series output board that the Cue or Headphone Amp chip had not been replaced. (Along with associated burned resistors and capacitors.)

Here is my spare RS Output board. You can see where the chip has been replaced. And if you look closer, you can see a few burned up resistors. (BTW - It came to me this way)

But I did have some chips on hand and decided to prototype a test amp.

These chips are NOT designed for proto boarding. All I ended up with was a screaming oscillation, and a TON of heat.

SO, back to square one. We don't need a high class, complicated amplifier for cueing. We just need to hear the audio.

The same is true for a power amp. I don't need a ton of power. Just enough to drive my two little speakers.

So, I ordered a few LM386 OP Amps. I saw a couple of circuits online that may suit our needs. 

Let's get out the breadboard and see what we can come up with. 

This circuit did not work as advertised. I won't BORE you with the details, they just didn't work.

Then I tried a TDA2058 chip.

This looked good in theory, but it screamed like a cat caught in a woodchipper.

I have a feeling these circuits do not lend well to a proto board. I would need to design a board and have it made just to try the circuit.

Before we go down that road, let's see what else might work.

So, I started to search. And I found this on AMAZON.

This is a Stereo Amplifier KIT that I may be able to modify into a CUE AMP. I won't need it to be STEREO. Nor will I need the ON/OFF switch.

But it only costs $5.00! Dude, you can't BUY THE PARTS for 5 bucks!!!!

Let's start on the kit.

The kit needs +/- 12 V AC. The AC volts is rectified by 4 diodes. With a filter cap the +/- DC is 17 V.

I decided since the mixer will already HAVE +/- 15V, we could bypass the rectifiers and feed the DC voltage direct.

Here is the schematic for the kit.

If you jumper D1 and D4, you can feed the DC voltage directly to the amplifier. You then DO NOT mount D-2 & D-3.

I jumpered the diodes and installed the other jumpers required by the board layout.

I also noticed that J-4 is incorrectly numbered on the board. (J-1 & J-4 are reversed on the board.)

If it sounds good, I may use this as an audio amp in my radio room.

I need to replace my DEAD Power AMP.

Let us continue.

Now we wire in all the resistors and capacitors.

(I always mount the small stuff first.)

Here are the jumpers.

The resistors

Small Capacitors

Large Capacitors. I left the large filter caps in the circuit. (More filtering)

Now the rest of the large components. I did not mount the I.C.'s yet. I want to put power to the board and make sure my jumper idea gives me +/- 15 Volts to the chips. (Without any smoke)

Power worked, so let's solder the chips, hook it all up and see if the magic smoke escapes!!

Well, that was nice. No smoke and Burton Cummings coming out of the speakers,

Wanna hear it???? I know you do.

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The kit went together with very few issues. They failed to give the color code for the 10-ohm resistors, but if you can't figure that out you should NOT have a soldering iron in your hand.

As you could see, I started with the small parts, then moved up to the large ones. I took me perhaps an hour to put together.

HELPFUL HINT: I like to measure each cap and resistor BEFORE mounting. But all the parts in this kit were correct.

The audio is CLEAN with no hum or noise. It easily drives my speakers and sounds just as good as the PA30.

The heat sinks get a little warm but not near as warm as my Power Supply heat sinks. (And they are HUGE.)

Now I think I will order more of the kits, just to get the parts. Then I will craft a CUE and Headphone Amp board.

So, the boards have arrived. The amplifier board also contains the Cue circuit.

Doesn't it look G R E A T!!!!

Left and Right channel all done and tested.

The Cue Amp also went well. 

Now we can put it aside and move on to the next part of our journey.

I still have to design and build a Driver Board for this amplifier. (I ended up putting the driver circuit on the output board)

Plus, I need to build one up to use as a Studio Amplifier. Same design, different board. (This never came to pass, as I found a nice Radio Shack Amplifier at a thrift store. Scored it for 20 bucks and it works like a champ)

Now, back to the Amplifier board.

There were a couple of issues with the board that I need to adjust. 

Here is the final schematic and board design.

This project went very well, once I got over the prototype problems, and is now ready for mounting in my mixer.

Let's update our planning list.

MIXER PLANNING

1) POWER SUPPLIES

+/- 15 VDC XMFR 1

+/- 7.5 V DC XMFR 1

+ 5.0 VDC (CLOCK/TIMERS) XMFR 2 OR COMMERICAL UNIT

SCHEMATICS

CIR BOARDS

CABINET

2) TIMER / CLOCK - SEPERATE UNITS FROM MIXER DUE TO SPACE LIMITS

12 HOUR MOD 60 MOD 60 MOD 12

24 HOUR MOD 60 MOD 60 MOD 23

TIMER UNIT

RE-SET PANEL

DISPLAY BOARD SCHEMATIC CIR BOARD

MAIN BOARD SCHEMATIC CIR BOARD

60 SEC/MIN MOD SCHEMATIC CIR BOARD

23 HOUR MOD SCHEMATIC CIR BOARD

12 HOUR MOD SCHEMATIC CIR BOARD

CABINET

PROTO TYPE

FINAL

3) HEADPHONE AMPLIFIER

DESIGN

SCHEMATIC

PROTO TYPE

CIRCUIT BOARD

4) CUE AMPLIFIER

CAN ALSO USE AS A TEST AMP WHILE BUILDING AUDIO BOARDS

DESIGN

SCHEMATICS

PROTO TYPE

CIRCUIT BOARDS

5) POWER AMPLIFIER (REPLACES TASCAM PA-30)

DESIGN

SCHEMATICS

PROTO TYPE

CIRCUIT BOARDS

6) OUTPUT BOARD

DESIGN

SCHEMATICS

PROTO TYPE

CIRCUIT BOARDS

7) MONITOR SELECT

DESIGN

AVOID OLD STYLE SWITCHES, USE DIGITAL

SCHEMATIC

PROTO TYPE

CIR. BOARD

8) PGM / AUD / MONO SELECT

DESIGN

AVOID OLD STYLE SWITCHES, USE DIGITAL

SCHEMATIC

PROTO TYPE

CIR. BOARD

9) INPUT SELECT

DESIGN

AVOID OLD STYLE SWITCHES, USE DIGITAL

SCHEMATIC

PROTO TYPE

CIR. BOARD

10) AUDIO INPUT BOARD

DESIGN

RS-12 VERSION

SCHEMATIC

PROTO TYPE - NOT DOING A PROTO TYPE

CIR. BOARD

11) MONITOR BOARD

DESIGN BASED ON MONITOR SELECT DESIGN

SCHEMATIC

PROTO TYPE

CIR. BOARD

12) EXTERNAL MONITOR BOARD

DESIGN

SCHEMATIC

PROTO TYPE

CIR BOARD

13) VU METER BOARD - SEPERATE UNIT ON SHELF

ONLY USED BECAUSE IT IS ALREADY BUILT

14) LED METERING - ON BOARD

USE RS-12 CIRCUIT REG LED'S OR BAR GRAPHS ????

SCHEMATICS

PROTO TYPE

CIRCUIT BOARD

15) D.A. AMPLIFIER

DESIGN

SCHEMATIC

PROTO TYPE

CIR BOARD

16) 4 PORT EXPANDER BOARD

DESIGN

SCHEMATIC

PROTO TYPE

CIR BOARD

17) POWER SUPPLY DISTRIBUTION BOARD??

MAY BE NEEDED WITH MULTIPLE BOARDS??

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Now we can move onto the next part of our project!!!  

We are working on the protypes for the LED Metering, the D.A. Amp, AND the input controller Cards. Busy Time.

See you then.

God Speed, Mother Nature.

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