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"Outboard" Amplifier Project

Started by n9voc, November 06, 2008, 09:33:55 AM

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Hello, fellow solid state amplifier buffs!

Been awhile since I've posted, and I conceived of an amplifier project – and thought it would interest the members of this forum to see the process of the project from inception, conceptualization, design and final build.

Thus, I started this thread.  I'll be posting to it as the project progresses – with photos, schematics and commentary – I hope that you find it enjoyable as well as informative to "get inside the head" of this builder and player for this project.

As I describe each step, I am aware that there are probably better ways to go about the design, and ways to get "more" out of the design.  However, I am not designing this for production, but rather for performance and ease of build using components on hand as much as possible – in other words, with as little cash outlay as possible and still make a serviceable amplifier.  With those caveats out of the way, here we go!

The Project:

I want to build an amplifier that will be the bottom of a "half stack" configuration with my 3 watt "Classie Lassie" amplifier I built earlier.  I want to be able to add 20 to 30 watts of capability to the "Lassie", and have the amplifier stack look good, like the "Classie Lassie" amplifier does.  (See attached pictures of the "Lassie" below, and further information on this amplifier can be found under my post of "my favorite practice/small group amplifier")

First – The "footprint" of the upper amp (the "Lassie") is 17 inches long by 11 inches wide.  The "footprint" of the amplifier she'll be sitting on must be at least this size, to match in looks.  Height of the bottom (outboard) amplifier will be determined later.

I dug into my junk box and came up with the parts shown in the picture below, from left to right I have laid out:  (some materials to work with!)

A power transformer pulled form a defunct 150 watt power mixer, dual center tapped secondary with dual primary for configuring to 120V or 240V.

Some "project board", LED's, IEC power connector, a fuse holder, some bridge rectifiers and a good set of filter capacitors for the power supply. The big blue one is 18000 uF at 25VDC rating, computer grade.  The others are either 3300 uF or 4700 uF.

Two FK607 boards (an amplifier module based on the TDA2004/2005 chip),  a couple of TDA2003 amplifier chips, a heat sink, some ¼" jacks, several NE5532 op-amp chips, and cabinet hardware pulled form the aforementioned power mixer.

Speakers:  An 8", 8 ohm 10 W speaker from a JWDavis ceiling fixture, a couple of 5", 6 ohm speakers with big magnets, pulled from a defunct studio monitor set – estimate at least 50 watt capability – and an aluminum plate to mount controls upon.

From this array of parts, an idea of triple PA (one power module per speaker),  op-amp preamp amplifier design suggested itself.

I am now ready to begin conceptionalizing the amplifier, sort of pre-design work based upon the materials at hand.

**** Continued Next Post ****



Hello Again!

If I'm going to use the parts seen in the last post, there are few items to consider that will immediately affect design parameters.  I do so here in this post, and will end up with a preliminary block diagram of the amplifier at the end.

The FK607 modules are based around the TDA2005 chip, and are in Bridge Tied Load configuration (see schematic below).  The TDA2005 has a maximum supply voltage of  18 VDC, and power rating of 15 watts.

The Power transformer must supply sufficient voltage and current to run the amplifier, without over voltage issues, or voltage "sag".

First the FK607s:  I will use the two modules each to drive a 6 ohm speaker.
I applied power and an input signal of 0.1 volts peak to peak to the input of module, and read an output voltage of 1.5 volts peak to peak.  Therefore the modules have a voltage gain of 15. (an important note – to be used later in design parameters)

I found the transformer, if the dual primary is series connected, has two outputs: the output with the biggest wire (about 16 gauge -highest current carrying capability) gives me 25 volts with a center tap (12.5V/CT/12.5V).  The other secondary give (with a smaller wire (about 20 gauge) gives me a voltage of  20 volts with a center tap (10V/CT/10V).  Being that this transformer powered a 150 watt powered mixer, I feel fairly certain that it will not "sag" much when my 20-30 watt amplifier is being driven by it!

Because I have several NE5532 op amps, using these for the preamp stage with a bipolar power supply of the 20 volt winding will give me approximately +/- 12VDC.
From my prior work in the power supply industry, I knew that if I took the RMS voltage and divided it by 0.85, it will give me a fair estimate of the DC voltage out when using a full wave rectifier, given adequate capacitive filtering (RMS/0.85≈DC).  Using a full wave rectifier assembly on the 25V CT winding  will give me approximately 15 VDC.  Ideal for the power amplifier modules that I will be using.

Basic power supply conception done, I drew up a power supply schematic, as seen below.

Now as to the amplifier proper.  Because I have some pre-built modules I want to use, they will determine initial amplifier considerations.  In BTL configuration, with 15 VDC applied, the output signal of the FK607 module can be up to approximately 30 V peak to peak before clipping.  This gives me a maximum output level, with a planned input of 2Vp/p based upon the gain of 15.

(30 V p/p into 6 ohms = about 18.74 Watts RMS – using a sine wave, very close to the maximum power rating of 15 Watts for the FK607, and well below the maximum power rating for the speakers in hand)

I have measured my electric guitar on an oscilloscope, and when all coils are in series, the output level from the guitar can hit 750 mV p/p when strummed hard.  For input to the amplifier signal purposes, I am going to initially consider 250 mV p/p as my input signal with full volume to get 30V peak to peak output  (This is subject to change as the preamp evolves).  Total maximum voltage gain of the entire amplifier should then be 30V/0.25V = 120.  (anywhere's close to this will work for me!)

Looking at the gain of the FK607 module of 15, the pre-amp must have a voltage gain in it to achieve a system gain of 120.  That gain is figured by 120/15=8  (in*8*15=in*120)
Thus my pre-amp assembly, consisting of NE-5532 op-amps with a bipolar power supply of +/- 12 Volts,  must have a voltage gain of about 8 to make the system work.

With the bipolar power supply above, the absolute maximum the signal can reach in the preamp is 3 volts p/p before clipping.  This being the case, it is probably good to split the preamp gain between before the volume control and then a small gain after the volume control. Thus, I have more headroom for larger signals on the input without distortion.

Our preamp gain is a total of 8, and we look at the third speaker in our set – the 8 ohm, 10 watt unit from JW Davis.  At 10 watts RMS, and 8 ohms impedance the voltage level to achieve this is 8.9 VRMS ( by  - SQRT{P*R}=V).  8.9 VRMS translates to 25 Volts p/p.

I saw in my earlier calculations that a gain of 15 - similar to FK607 -  is a bit much for my 10 watt, 8 ohm speaker (30 V p/p gives me approximately 14 WRMS - a bit much for the speaker)  I ran some preliminary numbers, and determined that a stage gain of about 12.5 would deliver 25Vp/p to the speaker with the same input as before.  This works out to about 9.7 WRMS - within speaker rating.

I will be using the two TDA 2003 chips I have configured into an H-Bridge, with a total gain of 12 to 12.5.  With an input voltage of 2V p/p (same as FK607 modules) this will give us an output voltage of 25 Vp/p - perfect for the speaker as seen above.

Thus my basic amplifier conception shows me that I need:
Input signal design level: around 0.250 volts p/p
Preamp Gain of 8, to be split in two sections one before volume control, one after.
FK607 units to 6 ohm speakers - as supplied in as built.
FK607 with modified gain to about 12.5 for  8 ohm 10 watt speaker.

By no means have I maximized the capability of the components involved, but I am looking for good, clean, UNCOLORED sound out of the amplifier, and am not that particularly concerned with maximum power output.  These are my conceptualization goals for the electrical portion of the project.

Now I can draw up  a block diagram of the amplifier, based upon my conceptualization. (see below for my initial block diagram,)

This being done, I am ready to start designing the remainder of the modules
Specifically, the preamp/buffered output section and the TDA2003 H-bridge section.

*** In the Next Post, I'll present my preamp design, how I made an error within the design, found it during testing,  and how this error caused me to modify my block diagram, and amplifier specs – happily to a better amplifier***



Good Day, solid state amplifier buffs!

In my first two posts, I described the amplifier project, initiated power supply design, and put together a block diagram of the proposed outboard amplifier.

For my preamp section, I needed an amplifier with a total gain of 8.  I decided to split the amplifier gain into a section before the volume control and then after the volume control.  The gain before the volume control was to minimize the effects of any "noise" that might be introduced by the volume control, but had to be limited to keep my ceiling before preamp clipping high.  The section after the volume control would complete the gain.  I also wanted a buffered output that could serve to tie the amplifier to a "house" sound system or another amplifier.

My initial design is seen in the figure "input1" below. 

(before I continue, please be aware that I now know I made an error in this design – I address that with figure "input" – addressed third paragraph from here!)

In input1, I have my input impedance from 29 kohm to 250 kohm – this is based upon the specifications of the NE5532 input impedance, as pulled from the data sheet.  The input buffer feeds both the buffered output and the first gain stage.  In the buffered output, the resistor R6 protects the 5532 from shorts, and the output signal on J2 will be approximately the same voltage level as that of the input at J1.

I am using the standard non-inverting amplifier configuration for the remaining two gain stages.  I calculated the gain of stage one at 4.7 , and stage two at a gain of two.  The 4.7 gain allows me to have an input signal as high as 5 V p/p, without going into preamp based clipping.

When I built the preamp assembly, I found the measured gains of the amplifier stages were as follows:  Stage one = 5.8,  Stage two =  3.3.  I was a bit taken aback – instead of a total gain of 8, I had a total gain of 9.14!  "What did I do wrong?" I thought.  Then it hit me.  The gain of an INVERTING op amp amplifier is : R(feedback)/R(input).  Whereas, the gain of a NON-INVERTING op amp amplifier is calculated as: R(feedback)/R(input)+1 !

R17/R11 = 220k/47k = 4.68  whereas (R17/R11)+1 = 5.68.  The difference between the calculated gain of 5.68 and the measured gain of 5.8 is accounted for by the ±5% tolerances of the resistors actually used in building the circuit.  Similar figures work out for the second stage of the amplifier section.

Ok, I had my preamp section all built, and initial testing showed me the error in my gain calculations.  Now I had a problem – the amplifier was not performing as originally designed.  I had two choices: A) I could pull the gain resistors and substitute values as needed to achieve the original gain calculations or B) see what the amplifier specs will be with these new gain figures.
If I was "hard and fast" to the fact that the preamp MUST have a gain of 8, I would have been forced to go with option A.  However, this is a hobby project, and as such, the design can have a certain fluidity to it.  Besides, I really didn't want to pull apart my newly built circuit if I didn't have to!

Looking at the block diagram, substituting the measured gains -  I came up with the following:

1) Maximum voltage input prior to preamp clipping: 4 V p/p (4V*5.8 = 23.2 Vp/p)
I can live with that - my electric guitar puts out approximately 1 Vp/p maximum
under HARD strumming with all coils in series.
2) Instead of 250 mV p/p output for full power out, I now have 100 mV p/p for maximum
     power out before clipping (0.1 V * 5.8*3.3*15 =28.71 V p/p)
I can live with that - on some testing of a Fender Champ (different time, different project), I found out that a 30 mV p/p signal gave me full power out with volume at maximum, thus a  100 mV p/p signal input for full power out is an O.K. level -  probably make the amplifier perform better.

Since the preamp clipping voltage and input level for maximum power output was O.K. by me, my conclusion was to go with option B – modify my design expectations!
Thus, I modified the block diagram of the amplifier to reflect the design realities. The modified block diagram is shown below (ampblk).  Note total max  of the preamp is now 5.8x3.3=19.14

The lesson I took away from this is to REMEMBER and use the CORRECT gain figure calculation when designing a NON-inverting op-amp amplifier circuit!  It was also gave me a bit of a warm fuzzy to see the measured results being very close to the theoretical results as predicted by the calculation. ;D

This brings me to block diagram version 1.1 and preamp design 1.1, both shown below.

While building the preamp, I added some additional power line filtering, this is reflected in the schematic called "input" below.

I also have included a picture of the completed input/preamp module board for examination.

Sometimes, when designing for yourself, you can take the "lemon" of a mistake, and turn it into lemonade!

Just a couple notes:
1) If I wanted to put a preamp gain control, to drive the amplifier into preamp clipping, I would substitute a potentiometer (variable resistor) , probably about a 5-10 kilohm value, for R17 in the schematic.
2) You may wonder why VR1 is a 100K pot and I have R13, 22k in parallel with part of the pot.  The explanation is simple: I have several 100K LINEAR pots in my junk box, but no "Audio Taper" (LOGRYTHMIC) pots.  By running the resistor in parallel to the 100K pot, this simulates the action of a 20 K log (audio taper) potentiometer. Paralleling a resistor with the wiper of a linear pot (resistor should be about 10-20% of the value of the pot used) is an old trick to make a linear pot act like a lower value audio pot.

This works in a circuit like this one, where U2A amplifer really doesn't mind an output impedance shift from 50Kohms (resistor and full value of pot in parallel) and about
15 Kohms  (100K resistor, 100K pot, 22k resistor all in parallel).

The volume pot, being an off-board device, is still subject to potential change, depending upon parts availability.  If I have either a 10 K audio pot, or a 10K ten turn pot, I may use one of them for the volume control – we'll see!

**** In the next post, I will detail design and construction of the TDA2003 amplifier module***



Hello again!

Before I discuss the TDA2003 BTL module, just an update and minor change to the power supply schematic is in order.

In the "Classie Lassie", I installed a pair of duplex receptacles for 120V (wall plug assemblies) on the back of the "Lassie" and found this exceptionally useful.  With one plug from the mains, I now had the option of plugging in four accessories into the "Lassie" – rather like a built in extension cord.  I've added this to my basic power supply schematic shown below, called powrsup1.jpg.  – Just a convenience for me.

Now, on to the TDA2003 BTL module.  To re-iterate, we want this module to have a gain of about 12 so that when the FK607 (TDA2005) modules are putting out full power into their speakers, this module will be putting a tad less than 10 watts into it's 8 ohm speaker.

First let us pull up the TDA2003 H-bridge configuration from the TDA2003 datasheet.  This is seen in "ORIG2003" below.  A few notes about the "H" bridge design:  Because it is using the two chips connected so that one chip is in inverting amplifier input, and one chip is in non-inverting amplifier input, the output can swing up to 2x the rail voltage peak to peak.  Think of it as one chip amplifies the "positive" half of the wave and one chip amplifies the "negative" half of the wave (for visualization purposes only).

The gain of the left – noninverting – side is calculated at: 2x((R24/R21)+1) = 27

The gain of the right – inverting – side is calculated at : 2x(R23/(R22+R21))= 26.8

(Originally I thought the gains would be  13.5 and 13.4 respectively, however, investigation with an operational circuit proved the above equations to be accurate for these chips in this circuit configuration.  You learn something new every day!)

To offset the slight difference in gain, C19 is slightly larger than C20.

The total module gain is both sides added together: 53.8

In other words: 0.5 Vp/p into the system produces  26.9 Vp/p output.  Max wattage is then determined by speaker impedance and current capability of the chips!

In the H-bridge configuration, the circuit can produce 20 watts RMS into a 4 ohm load, or with the same voltage out – 10W into an 8 ohm load.

Now, we DO NOT need a total gain of 53.8, we need a total gain of 12!  To achieve this, we start playing with the values of R3&R4, noting that it is probably important to maintain R5 and R6 at equal values.

Along with this gain change, I want to stick with "standard value" resistors when I re-design this circuit – so I can use "off the shelf" components.  To maintain circuit stability,  I want to stay relatively close to the values for R3 & R4 as seen in the manufacturer's datasheet.

My first modification is seen in "2003vers2" below.

When I built the unit, I found the gain to about 12-13 as expected, however, I found that the p/p output started "flattopping" at about 20 Vp/p instead of close to the 30 Vp/p I expected with the 15VDC supply voltage.

I note that I have departed greatly from the original design values for R5 & R6, which seems to have a negative effect on the design.  I have read where some chips like a gain of at least 20 for best operation, when you bring the gain down below about that, the circuit gets either less stable, or has reduced operational characteristics.

I looked at the FK607, which has a total system gain of 15.  I noted that there was a 4.7 K and 10 K resistive voltage divider at the front end of it – the chip gain is about 22, with the voltage divider in the front dropping the signal to make a system gain of about 15.

Good trick, thought I'd try it.

The final result of this experimentation is the seen in "2003vers3" seen below.  I started with a 4.7 k and 10 k divider, but found the system gain to be about 15.  So, I changed out the 4.7 k for an 8.2 K resistor, and found that by moving the voltage input closer to ground, I ended up with a system gain of 12 – right where I want it to be.

I found that the output, into 8 ohms, got as high as 28Vp/p before flattopping out.  This meant that I would be overdriving my speaker before I got into the flattop range of the amplifier output.  (28 Vp/p = 9.9 Vrms ; (9.9^2)/8 = 12.25W )

A second advantage is that the overall circuit is more stable because the input is "closer" to ground at the chip level.  Finally the preamp would be driving a total resistance of about 5 kohms when the three final amplifiers were attached to it – WELL within the capability of the preamp to drive it!

The original "H" bridge circuit works well, and can be driven by the levels put out by an electric guitar – as is done in the "Bug Eyed Monster" design I posted elsewhere.

By keeping the configuration gain above 20, but not TOO high, I reduce noise percentage introduced by the amplifier into the signal.  The resistive divider provides a level of resistive isolation between the power amplifier sections, and between the P.A. sections and the preamp assembly.

A photo of the completed board is seen in "PA3" below.

There you have it – power supply, preamp assembly, triple P.A. assemblies –
Now I need to pull it all together in a nice enclosure! – But that is for next post!



Good day to all! :D

I have completed building the preamp and amplifier modules, and have a proven – though simple- power supply design.

Now I need an enclosure to house all the electronics and speakers!  In this first of a two part post on the enclosure, I'll be putting together the basic enclosure from stock lumber.

Since I want it to be a compliment to my existing "Classy Lassie" combo (as seen elsewhere in this forum – and shown in earlier posts), I took the time to measure the "Lassie"'s dimensions once again.

Found on the wall of an old Babylonian carpenter shop were these words (post translation) "Measure Once, cut Twice. Measure Twice, cut Once".  Never so true as with this.  Originally I measured the Classie Lassie "footprint" at 17 inches by 11 inches.  I used the old "trick" of starting the measurement at the 1" mark on my measuring device when I took these – but, apparently I forgot to subtract 1 from the measurements.  For when I measured again, I found the "Lassie" footprint to be 16 inches by 10 inches.  The width was a combination of a 9 ½  inch board and a ½  inch back plate.  9 ½ inches wide is the width of a "1x10", so to facilitate the main body I went down to the local lumber yard and picked up an 8 ft 1x10.

I decided to match the "Lassie" in footprint, which means the new amplifier had to be 16 inches long by 10 inches wide total.  Because the "Lassie" has no controls on the front panel, I decided to match that look with this outboard amplifier.  This means that I must put the control panel upon the back plate, if I am going to have the "Lassie" sitting on top of the outboard amplifier.  Thus, front panel will be speakers only, top is plain wood and controls and inputs are to be mounted on the ½ inch thick back plate – should work out well.

Please see the drawing "concept1.jpg" for a visualization of this.

We interrupt this thread entry for an "Aside" or "Sidebar":

Now, before I go any farther, I want to dispel the myth that you need a professional woodshop to make a nice enclosure.  The tools I use to manufacture my enclosures are simple handheld power tools and hand tools.  Specifically: A circular saw, a saber saw, a cordless drill/driver, drill bits, countersink bit, screwdriver bits, ¼ sheet palm sander, small handsaw, hammer, tape measure, and carpenter's square.  With these common handheld power tools, some careful measuring, wood glue and screw hardware,  I have built the enclosures for all the amplifiers I have posted schematics for elsewhere in this forum.  Every permanent wood to wood joint is glued, then screwed down – usually with a drywall screw. 

Now we take you back to our regularly scheduled thread entry:

I determined to use a 1x10 (8 ft) as the base wood for my enclosure. Actual measurements of the 1x10  board are 9 ½ inches by ¾ inches.  The total length of the "Lassie" is 16 inches, and subtracting two board widths from that gives me a front (speaker) panel width of 14.5 inches.   The question was then, could my speakers I want to use fit within this size board?  I laid the speakers out on a board, and found that I could fit all three speakers in a 12 inch square, with 1 inch boarder on all sides – 13 inch square: perfect!

I cut a 14.5 inch square board, marked off an offset boarder of 1 inch, and laid the speakers out on it.  You can see a picture of the speaker plate labeled "spkrpnl1.jpg".

My back plate board will then be 16 inches by 16 inches.  I cut this from a scrap piece of ½ inch thick OSB I had around the house at the time.

With this speaker panel, and the ½ inch back plate, the final dimensions of the "outboard amplifier" will be 16 inches long, 16 inches high and 10 inches deep. 

The speaker plate can be attached via screws into the front plate, or sandwiched between a border wood and an interior wedge.  Since the Classie Lassie is set up in the latter method, I will follow suit with this unit to visually match.

The pieces of 1x10 needed for the enclosure are 2 – 14.5" long and 2- 16" long.   To create the wood frame that keeps the speaker panel from falling out the front, I took a furring strip ( 1 ½" x ¾") and rip cut it down the middle for 48 inches of the board.  This gave me 8 feet of  approximately ¾" x ¾" "trim board".

I carefully attached the "trim" pieces the full length of the 14.5" boards and centered a  13 inch piece of trim onto the "front" edge of  both 16" pieces of 2x10.  This allows the trim pieces to form a "frame" for the speaker panel to rest against.  (check Assy1.jpg for how this fits together).

Examine "initwood.jpg" to see all of the pieces for the enclosure as described laid out.

Next, I took one of the 1x10 pieces and scribed a line on the end of each 16" board, on the "inside" showing how thick the 1x10 was.  Then I drilled two holes from the "inside" to the "outside" on each end, and used the countersink bit on the "outside" for the mounting screw head.  See "drilled.jpg" for this step.  This is a critical step to insure a clean and accurate assembly.

I put a bead of wood glue on the end of one of the 14.5" pieces, laid the 16" piece on it and ran screws through the pre drilled holes into the end of the "side" piece.  Attached the other side, and then affixed the bottom in a like manner.  The screws provide the clamping action for the glue, and assist in keeping it all together.  Careful examination of "concept1.jpg" below will give you the manner in which the box is assembled.

In "Assy1.jpg" below, you can see how the speaker panel is butted up on the inside of the frame, and the main box is now ready for sanding – once the glue is dry!.

In my next post, I'll be cutting out the speaker panel and prepping it for assembly, as well as preparing the back panel with control and I/O locations. :tu:



Sorry folks - I just noticed the date is over year off on my digital camera!, I never reset the date after last battery change -- oh well!  :loco


Hello Again!

I've been putting in some hours on this project in this holiday season, which is why the posts came closer together!  This post, I'll show the front and back panel work.

First, I cut out the speaker holes and drilled the mounting screws for the speakers in the front panel board.  (see barfrnthw8.jpg)  below.  Next, I installed mounting bolts - 8x32 x 1.5 inch - with the screws bolted to the board, then slid the speakers over the mounting bolts for a test fit (see skrbrfntmy2.jpg).

I then pulled the speakers off, and sprayed the front down in black so that it would not show up as bare wood under the speaker grill cloth. (blkfrntcl8.jpg).

For the grill cloth, I had a plastic mesh I had picked up at some surplus place for a song.  I have used colored burlap and other "open mesh" fabrics for grill cloth before.  I wrapped my mesh around the speaker panel, installed speakers and slipped the assembled panel into the box for a test fit. (See  skrinstbktn6.jpg)  The grill cloth was stapled to the back side of the panel after it was pulled around the edges.  The next picture (spkinstfntwg9.jpg) shows the front view of the box with panel in a "test fit".

Now, I move on to the rear panel.  (See backplatenz8.jpg) I carefully measured and cut holes for the control panel assembly (top middle), the power input jack and fuse holder (left bottom) and for the two duplex 115 receptacles with outlet cover (bottom right).  Finally, I drilled three mounting holes on each side to screw this backplate onto the main box for assembly.  I sanded the unit, put a polyurethane clearcoat on it, sanded lightly, then I put a coat of black over the clear, sanded again - removing the black from the high spots -  and then a final clearcoat.  The effects on the backplate appearance are seen in hndlviewyg6.jpg.  I have installed the Duplex assembly onto the backplate to assist me in placement of large components within the box (specifically the power transformer).  You also see the handle on the top to the amplifier box.

In feetfitsb0.jpg, you see the "feet" I attached to the box, as well as the four mounting holes for the power transformer.  The power transformer, being both heavy and the largest component to go into the box, needed careful placement to keep it physically away from the mains lines as well as not interfering with any controls or operation of the amplifier.  From the holes, you can see I elected to put the transformer in the middle of the bottom panel of the amplifier - keeps center of gravity low as well.

Finally, for this post, in spekerfitcg8.jpg, you see the method for insuring the speaker panel is affixed within the main box.

From here, it is a matter of taking our handfuls of parts and modules and integrating them into an operational amplifier assembly.

That comes NEXT post!



Hello Again!

Well, after much work and testing -  the amplifier is completed and functioning to my satisfaction!  Originally planned as an "outboard" amplifier, its sensitivity is such that I can plug the output of my chorus pedal directly into the front end, and have very satisfactory results.  To say I am well pleased is an understatement.  This amplifier has become my new favorite for "living room" and small gig work.  It has a high end of about 45 watts, and is "transparent" in that it does not color the sound - suitable for my guitar work, or, with a mic preamp, vocals work.

Here was the integration of a handful of parts and modules into the case and then a complete combo amplifier is born.

First, I began with the power supply.  In the first image (pwrsuplie0.jpg), you can see the basic configuration of the transformer bolted to the bottom of the amplifier.  The +15V filter diode and filter caps on the right, and the +/- 12 V supply on the left.  I used a silicone based caulking compound to mount the capacitors - eventually dries clear.  I added a dual duplex outlet to the back panel, this can bee seen in the second image (inner4lj7.jpg).  You will note that I used #14 solid wire to connect the duplex to the "IEC" input power plug.  I did not fuse the "built in extension cord", but will run the power for the amplifier proper through the fuse. The image (inner3ix8.jpg) following this shows the power supply assembly in the final build configuration.

Next, I integrated in the preamp and control assembly.  This is seen in the next image (inner1es3.jpg).  You will observe that I routed the +/- 12V through a shielded cable, that the control panel (aluminum) is tied to ground and the frame of the power transformer is tied to the ground plug of the IEC connector.  I found out through trial and error the ground from the +15 Volts is tied to the ground from the +/-12V only at the board.  A second connection between these two grounds produced a ground loop buzz.  This goes along with the single point "star grounding" philosophy that is often mentioned on other DIY audio sites - ground loops can give you fits!

I ended up not installing a power LED indicator, the position of the switch tells me whether the unit is on - and that eliminated some additional wiring "hanging out" inside the amplifier to pick up hum.  (Might or might not have occurred.  In my case, the "power on" indicator was not an essential item - so I left it out).

Next, I installed the power amplifier modules (seen in inner2jm0.jpg).  You will note a couple of things: 1 - the outside modules are the FK-607 modules from my kit and the center module is the twin TDA2003 amplifier previously build. 2 - the input line for the power modules comes from the preamplifier board through a shielded pair cable.  The positive and return lines are connected as normal, but the outside shield of the wire is connected only at the preamp ground end - to minimize stray signals, and prevent another ground loop.  Finally, note the heatsink orientation and the fact that the center module is mounted by the heatsink - to reduce strain on the TDA-2003 chips - that much heat sink is HEAVY!
You will note that each speaker has its own power amplifier module, per the original design.  It is critical to have the speakers all in phase (the high side of the amplifier output to the "+" terminal of the speaker, for example).  If one of the speakers is wired backwards to the others, there will be some sound cancellation.  By wiring all three effectively  in parallel, they reinforce one another, for an excellent sounding output.

Ok, I've got all the innards put together and ran final tests to make certain all is working as planned - now as to the final look at the outside.

The front panel is shown in (dout2pr8.jpg).  Very basic, classic and clean design.  The Pekingese on the front is the "Astro-Peke", my dog and mascot to a small repair business I started and closed this past year.

The rear panel is shown in (dout1uy2.jpg).  The on switch is on the far right, the input between the switch and the volume control and the buffered output port on the far left of the control panel.  A fuse holder, IEC power in and duplex AC outlet assembly complete the rear panel.

The wood is sanded and covered in 3 coats of clear polyurethane - the sound is awesome. The design is for crystal clear sound, no distortion at all and it DOES shake the windows when turned up!

A final word:

As you can see, there is much more to building an amplifier than just getting a schematic.  Case, assembly, lead layout and control layout as well as speaker placement and case materials and construction all affect the final sound.  I could alter the sound slightly by porting the cabinet, but I like it with the "closed back" sound, and have no intention of changing it.

I hope reading this process was informative, and gives you an idea as to what goes into designing and building a combo amplifier - solid state style.

As with any of my designs, this unit is not totally optimized out as far as performance,  However, it is designed to run trouble free for a VERY long time, and it fits my needs for a combo amplifier perfectly.

Have a great day, and keep on making music!



Great project, thanks for sharing it with us through all of the steps!
Life is what you make it.
Still rockin' the Dean Markley K-20X