Home-made 600 watt amp!

The amp shown above is my newest pride & joy. It has a real 600 Watts to 8 ohms,
A fully programmable Quasi-12 band parametric EQ, MIDI controllability, a 15 page
EQ Memory, lights up pretty colors with the music, and sounds awesome!!
To see how it was built, read on!

A new amp has been on the "wish list" for quite a while over here. So I went to the Carvin site and picked out a nice little mixer/amp, about 350 Watts total, and went to order it. My suspicious better half queried me, "Do you know what the shipping will be?" and I replied, "well the guy said $65 to Canada by UPS, but then taxes on top of that...." Wrong!! UPS, while adored in the USA, is a bad thing here as they do their own customs brokering, which they charge lots for, and then have their own add-on fees. So Gena contacted the fellow at Carvin, and asked if they could ship it US Post. Nope, he says, "it's too big." Ya right. Well UPS was going to charge us $210, which is 2/3 of the price of the amp so I went to plan B.
I have been eyeballing several Chinese companies selling various amp boards and power supplies to drive them. I ordered the 600 Watt amp board (right) , and a 48 Volt, 12.5 A power supply from Sure Electronics. That amperage is nominal because it can draw up to 12 Amps on the 120VAC lines! To make a long story short on those, I ended up just building another amp (they have all the schematics online!) as the main chip is a $15 part from Digikey. Most of the other stuff I already have, including heatsink & a fan. The project didn't end there, (of course!) as the main thing I wanted on the amp is some form of EQ. I found these lovely little audio chips from ST MicroElectronics, called the TDA7419. I've used a similar chip from them before when I built the micro-power stereo for the boat. Somehow I forgot how cryptic the datasheets were. The ones for the 7419 are even worse! Pin functions were omitted, so I had to stumble on to them, and the whole thing was written in broken English so pretty hard to understand at all. The next speed bump was the I2C bus they have. I'm accustomed to SPI so had to re-learn it all.
Below is a basic block diagram of the whole thing. One reason I chose the TDA7419 is it has a spectrum analyzer on it, and I think it's something that's missing on standard mixers/amps. It is a visual aide when tuning an EQ, or at least to me it is. It gives a general idea of the audio coming in, which tends towards the low end frequencies, and in a max-output scenario, pulling up some of the higher bands can be important.

Anyway! One drawback to the TAD7419's is they have only 3 bands. But each band can be shifted 4 ways at will, plus the 2 lower bands have a "Quality Factor" adjustment. This is really just a bandwidth selector, from wide to narrow. There *is* a 4th band, the sub output, which could be mixed back into the chip, but I chose not to use that as on stage, subs get lost unless they are a massive "power-grid" size.
The way I solved this band number problem was to use 2 chips, and mix them after. This gives a possible 12 bands plus Q-Factors. That's enough for a good stage amp.

I had some little speaker cases that I tore speakers out of for my battery powered busking set-up, which I thought would make a nice case for the amp, plus I'd have a spare it case I messed it up. My neighbor gave me some plastic molding that just fit over the edges to protect them.

The power supply has it's own fan (noisy too!) and the amp has a fan, but I thought I'd better put a fan on the back and drill a hole in the bottom to allow air to circulate out of the box. My other neighbor had given me some dark Plexiglas sheets which really came in handy for doing the face.
(Top Right)

The back panel soon became really busy with the 3rd fan, audio sockets, MIDI sockets, relays, and a 24 Volt switching supply I found at the Economy Store for a buck.

If I could do it again, I'd have substituted the back panel (old style press board crap) with 3/8" plywood....but with some epoxy re-enforcements and backing plates it seems strong enough.

Well enough with the mechanical stuff, lets get on to the really interesting part, the electronics!

As I mentioned, the TDA7419 has a spectrum analyzer on it, so I needed to come up with a display. LCD would be nice, but there's something about those square LED's from the 80's that I found at the scrap yard last month that turns me on so in they went! (photo left)

Because I hate using knobs (they're expensive and turn by themselves) or sliders (get full of dust or break off entirely) I decided to use 1 knob and a bunch of tiny buttons to select which band or attribute to adjust.
The first button is to select/program EQ pages, and the next 6 are main band selectors. As you adjust the knob (not there yet!) the LED bars will go up and down. The knob clicks in the middle so you know when you're at center, then the red LED at the bottom lights to show you're attenuating the band as the LEDs again climb.

The two out-of-place lower buttons are for band shift, and Q-Factor, which work with the knob also.

A new life for an old 6" speaker cabinet!

This board is just the display board, the other being the processor board where all the action happens. I chose (yet again!) the PIC16F74, mostly because of the port count, and the built-in USART, but also because I know it so well, I can program it in my sleep. Also I can re-use routines I wrote long ago for similar interfaces.

The PIC chip must perform the following functions to control the amp:

Do a delayed startup of the amp based on:	(1) <7% Audio present on input, as amp chip doesn't like loud starts! (2) Right Speaker isn't shorted or <4 ohms (3) A warm-up time (2 secs) to allow pre-amps to stabilze (4) An LED test and speaker load display
Load Last EQ settings from an EEPROM or load default settings if no EEPROM data.
Display updates on the LEDs for:	(1) Spectrum Analyzer Display (2) EQ Adjustments (3) Q-Factor Width display (4) Shift a Band Left or Right
Connect with a PC via MIDI in that:	(1) MIDI is sent thru via MIDI Out#1, less channel 7 data (2) MIDI is received to control all functions of the amp plus extras (3) MIDI is send thru MIDI Out#2, or software selectable to return special local data to the PC (i.e. EQ updates, Play trigger )
Control 2 Audio Processors and save/load 16 EQ /Shift/QF pages to/from an ATMEL Flash EEPROM The list was shorter to start, but that's the fun of building your own stuff, if it's missing something, just add it! As can be seen in the photo of the main board, everything is surface mounted. I find it easier to do this way with the board jumpers (or whatever) on the back. The two boards are stacked, joined by ribbon cable. Here is the last schematic update ( I keep adding stuff!! ) which is how it is now (May 24th 2013)

The AN0-4 on the PIC are configured as ADC's to read levels on the front panel knobs (EQ, Left, Right) , the spectrum analyzer (SA) input, and Speaker test levels.
The SA is kind of weird in that it's output impedance is well over 100K, which is far beyond the PIC's max ADC recommendation, I slowed the ADC some in the program and it seems to work fine, which is a miracle really! The Main Output level pots are on AN 2&3. I used 50k pots, but this is ok as a .1uF cap makes up for the PIC's 10k ADC cap charge limitation.
The speaker test input is basically a voltage test from a 1.2V source into a unknown speaker through an 8 ohm resistor. 8 ohms=.6V 4ohms=.4V (r1/r1+r2)*E

The LED display also doubles as the button sampler. PortB is loaded with the LED bits (high = on) and Port E selects the row it will be in through a 74LS138 3-8 demux. (Open collector outs)
The same sink to ground on the 74LS138 will signal if a button is pressed via RD,5 input (pulled high) The "Page" button though has it's own port input RA,4 which makes software easier for a hold and press 2nd button scenario, a secure method which is used for programming an EQ page.

You may have noticed there's a lot of 3.9K and 10K resistors coming off ports. This is because of pain-in-the-butt 3.3V devices. I still cling to 5V everything as a habit and a principle!
These connect the PIC to an Atmel EEPROM (SPI bus) and the TDA7419's (I2C) serial data lines. The first pair of 7419's I cooked! They didn't mention the voltage limitation in the very confusing datasheet. Nor did they mention that the "clock" must be set to internal, or that the Mute input doubles as a clock input. The audio flow diagram is also misleading, it appears the audio can be fed out to the attenuators after the Bass EQ, but nope, it goes out of the chip to be AC coupled back in. It took a while to figure it all out!

Anyway, the MIDI in is the standard optically isolated setup with a 6N137, but the MIDI out evolved into a dual, one just MIDI though (less channel 7) and the other MIDI through, or a MIDI out to send data back to the PC. The reason I wanted that was mainly for a MIDI "Play Trigger". Once the level on a selected band exceeds the preset level, and the trigger is "armed", a MIDI Play will be sent. This would be good for starting a song with a guitar, or just a mic. Yay! No more foot pedal!

The Voltages on the board are 5V, 8.6V, and 24V. The 24V comes from a switching supply that comes on with the power switch, and is also used to drive various relays and even the fan.

These are the 2 boards I photo-etched, one being the display / UI board, while the other is for the uP and related circuitry. Notice how the connectors for the ribbon cable line up so they can be stacked.

You can see the SOT sized TDA7419 footprints and how they are wired directly over to the PIC chip with pull-ups to 5V. Smoke! Also all the traces on top going to ground had to be cut to add those coupling caps in. If you want to build this, follow the schematics, not the board ;)

All of the relay boards were hand-done with a felt pen, so are a bit ugly!

I even used some of that breadboard (proto-board?) I found at the dump a couple weeks ago, for the sockets fro the plugs and pots on the face panel. I don't much like it. Drawing it up and etching is way easier.. for me anyway.

The time came (at last!) to put everything together and stuff it into the tiny speaker box.

It barely fit, but I managed to get the back on with *some* force hehe. I hooked up the speakers, and some inputs from my mixer board, flicked it on and everything worked, no smoke!!

I've never heard my tower speakers sound so good, ever. Finally they have some "real" power to boom it out. Right away (while listening to a somewhat loud DJKJ mix) I started to make a FL Studio Dashboard to test the MIDI stuff.

I played with it all afternoon then took it over to show off to the neighbors (as they contributed some stuff right?) I'm happy! Much better than just buying stuff.

That said, I now have realized I need to pull it apart and fix / change / add a few things:

1) Flip the speaker +/- around! I forgot I had inverted the signal with the 4560 op-amps!

2) Make a MIDI-Thru "power off" bypass. Sometimes I want MIDI after the amp without turning
it on. It's pretty late in the chain: Korg, Casio Synth, Strip-Lights, SDS300+300, LightBar,
Light16... but sometimes I just want the lights and my little 'puter sspeakers.

3) After making the "Dashboard" pictured above, I realized that when I change an EQ page, the
   buttons on the bottom, the EQ sliders don't update! Well how could they right? I do have
   them linked to my UC-33 MIDI controller's page 2, but need to output position updates too.
   Luckily, FL Studio allows any number of controllers to control a single knob.

4) I want to add a MIDI-controlled "power-down" that will both power down with a MIDI note
and if there's no level on the spectrum analyzer for X hours. A standby I suppose.
This of course means a "power up" upon receiving an EQ page selection!

5) Need to get some labels or lettering, this came out nicer than I thought and using a paint pen
just seems like an injustice!

Well there you go! It was a bit of a hair puller, but it's my new baby, and I'm sure it'll be
rockin' the island this summer! Yaaaa!

There's a zip download below that includes the .asm, the owners manual and schematics + info.

Cheers!

	SDS 300+300 600Watt Amp by Sandy Sims


May28 2013