Guys,
I burned my bass amp out last night, this is a thing that I built back in the 1980's, using some 2N301 power transistors as the finals and I'm surprised it's lasted this long.

I now have a new design that I'm pondering, it has a pre-amp, where as the older one didn't.
What I'm enquiring about is this, I need a potentiometer volume control, I can't for the life of me remember whether you use an A taper (Linear) pot or a B taper (Logarithmic)style thing for this application.

Any takers?

Hey there Mike;

The Volume Pot should be one with an "Audio" Taper - a Logarithmic increase or decrease in Resistance value, so go with the type "B" Taper Potentiometer.

If the type "A" Linear Taper Pot was used, the perceived increase, or decrease in sound level will not be "Linear" while the Pot handle is rotated.
A Linear Taper Pot would result in a gradual perceived increase in Volume Level for the first 40% to 45% of the rotation, then appear to quadruple the Volume between 45% and 60% rotation.
At the end, it would appear to double the Volume for the final 40% (from around 65% to wide open 100%).

The Linear Taper Pot could be used on the Preamp (or at the Output of the Bass Guitar) if you cannot find an Audio (Logarithm) Taper Pot available with the Resistance & Power Values you need.
Probably not an issue finding an Audio Taper Pot, with a Value of 250K or 500K Ohm, 1/2 to 1 Watt rated, and a decent Tolerance (5% and less).

-- Scott

Hey Scott,
Thanks a million, mate.
For once in my life, I've elected to make things reasonably simple on myself and have gone with a kit-set approach.
Jaycar down this way make a pretty good 50 Watt Mono amplifier that is based upon an LM 3876T chip.
(In case you folks were wondering, 50 Watts is more than any amount for a Bass amplifier, anyone that tells you any different, is trying to sell you something you don't really need)
I've bought all the bits for it (sans speakers) and I was having a wee bit of a set-out session here the other night.
This kit requires a dual-rail power supply of +/- 35V and for this I needed a centre-tapped transformer, the only one I could find was a toroidal type and unfortunately, it takes up half the case.

Here's a little picture of how far I've got so far:



When Labour weekend hits here the weekend after next, I'm going to be into this like a rabid dog.
Not so much the electronics construction, but building the speaker cabinet that will hold 4 x 8" speakers and a base port.
These speakers are rated at 200W a piece (RMS) and from the specs, they should do the trick.

One thing I was wondering about though, do you need to have a pre-amp to have any sort of "tone shaping" with an amplifier, what I mean is (very loosely), Treble, Mid-range and Bass pots.
A bass guitar gives you rather limited control over how your signal sounds on the output of an amp.
Is there a simple circuit I could use, to effect some sort of top-end or low-end cut/boost?

Mike,

Awesome job with the Power Amp!

I forgot that you play Bass, as well as Drums.

Regarding the Preamp, you could place Tone Controls between the Instrument Signal Input, and the Power Amp, via a basic Tone Control type OPAMP based Driver, or as part of an actual Pre-Amp.
The Tone Control block is a simple Pre-Amp, so either way you will have a Pre-Amp'ed Signal to the Power Amp.

If you trim the Signal's Tone with Pots only (no Pre-Amp), the Signal might be noticeably lowered with Trim pot adjustments.

I will search through my Circuit Archives for a simple Tone Control Schematic, designed around a common OPAMP.
If successful, I will let you know, so as to E-mail the Schematic to you.

Until then, here is a Link to a nice On-Line Electronics site:

Epanorama

From the Main Page, under the AUDIO Section, select the "Music" link. This area has several Instrument Oriented Schematics.

Good luck!

-- Scott

Woah!,
A lot of good info there Scott.

I think what I might do, is build this amp and speaker unit up and see how it sounds, before I go changing anything.
You never know, it might sound great from the start (that would be a first though).

Thanks for your help, mate, it's most appreciated.

More pictures will follow, once I've actually done something with this kit.

Scott,
While I've got the front and rear panels fitted out, I have a question for you, that I hope you can answer.

There have been documented cases of guitarists (and bassists) being electrocuted by touching the strings on their guitars (which are inherently steel, because we need things like pick-ups).
Is this a breakdown of the amplifier circuitry that has caused this to happen?

See this amplifier works off the 230V mains here and the way I intend to build it is that so parts that need to be "grounded" within the amp, will be tied to mains Earth.

I'm not sure what your experience is with musical gear, but any advice you can give me would be most appreciated, as I don't really want to electrocute myself just yet.

Oh and could you please type slowly, as you know that I don't read very fast.

Mike,

I have seen Guitarists get Shocks from various connection points.
From the looks on their Faces (and the swear words immediately following...), the Shocks were Intense and quite Painful!

Luckily, none of these Faults grabbed and held anyone, nor did I have to pull anyone off of Live Equipment, or treat a Victim for Electric Shock Trauma.

Mostly the results were Bruised Egos.
There were a few Ankle Sprains (from tripping over patch cords / falling off the riser / etc., after the Shocking experience...)

These Shock Scenarios were caused by the Infamous term:

---"GROUND LOOPS"---



The most common Shock was between a Guitarist and a Microphone. Right through the mouth, too!
Loop Currents would flow when Guitarist held Strings (playing a Chord), and touched Microphone (Singing) at the same time.

Other Loop Current Shocks were found when plugging in Patch Cords to Auxiliary Equipment - i.e. at the PA.

To correct these issues, all Sound Equipment EGCs were bonded to a single Star Point.
The AC Power Cords from the various Audio Equipment (Amps, Effects, PA, etc.), connected to a Break-Out point at the same location.
Several 20 Amp Branch Circuits were brought to this Break-Out point, via a dedicated home run.

Where XLR Connections were used, the Shell was Ground Bonded at (1) End only (at the PA).

If needed, I can draw a basic schematic of the Star Point concepts, and reduction of Loop Currents on Instrument Cables + Equipment.

In addition, please let me know if the questions were answered satisfactorily
(I am half asleep while typing! )

-- Scott

I think these things are caused by people not investigating defective equipment. They have a fault that puts a hum in the system and they immediately start cutting ground wires to make it go away.

Mike,

How is everything coming along with your Instrument Amps?

-- Scott

Mate,
To say I've made any real progress with this project, would be a lie.
I've got the case all drilled out for switches, sockets and the volume pot.
I also have the stand-offs in for the main board.
Here are some pics of it thus far:



Above is a pic of the little I have done to the base of the amp case.



Above is the front panel, with a 230V DPDT switch that has a neon in it, the lower hole to the right is the input socket, the hole above that is the Master Volume pot.



Above is the rear panel, on the right is where the 230V feed will head into the amp, via a cable gland, to the left will be the output to my speaker cabinet.
In the middle is the hole for the screw that will tie the Power amp IC to the heat-sink on the outside of the case.

The one thing (aside from work) that has held me up on this project, is that you can't buy plywood here at the moment and I need this to build any sort of a strong speaker enclosure.

More pics will happen when I advance through this project.

I read this the other day until I got a headache! http://www.edn.com/article/520749-I...alizer_filter.php?cid=NL_UBM+Electronics
Notice that nobody made any comments.

Then I thought of your project Trumpy, and thought you might want to add some tilt to it.
Joe

Joe,

I like that filter!!!
Kind of looks like 1/3 of a Parametric Equalizer.

Mike,

I have a similar problem regarding personal Projects (the "Not Work Related / Fun Projects"), being that there is always some pressing task, which comes along right after I begin working on something non-work based!

Kind of sucks about the Plywood - more appropriate, the 'lack-of' Plywood.
Is MDF also difficult to obtain?

-- Scott

Scott,
Now if he had only come up with the Excel spreadsheet so that we could just plug and chug! I know, I know, There's probably already an "App for That!" I have absolutely no use for that filter but it might be a fun way to try out those digital pots I bought without having a use for them. (One of the ones that stores the last count in NOVRAM on power down.)
Joe

Hi Scott,
I got the plywood late last night, however with my hectic work schedule (think coming to the end of the "fitting season" here), it's going to be a wee while before I have saw-dust on my workshop floor.

A couple of weeks back, I managed to get the PCB loaded:


The green wires are all earth points taken back to a common ground and will be tied via a crimp lug to mains earth on the body of the case.

Mike,

The PCB looks sweet! Nice Soldering work on the Ground Leads, too!

Maybe you will get Snowed in at Home; severe enough to allow 3 days of paid vacation..., which could be more like Power Amp / Enclosure Construction time.
Several Gallons of Fuel, an adequate Genny, and an understanding Spouse would be perfect!

For me and others North of the Equator, the Season is inverted as would be Winter in the Southern Hemisphere is Summer here.
Two Weeks after the Summer Solstice, the hot stuff kicked in with a vengeance!

Three Heat-Related outages were experienced since the Vernal Equinox (March 20-22), with Two occurring since July 1st!

One of the Outages was due to a Primary Feeder failure on a Pole, located 2 Blocks away.
The ACSR Conductor failed at a Dead-End Insulator, and that Single Ungrounded Conductor fell to a freshly paved Parking area below!
I was not there to witness the Fireworks, but judging from the Flash Points, and the actions of the Security Guards standing near the downed Conductor, it must have been sparky and loud!

When the PoCo re-closed the Breaker (attempt to "blow free" the fault issue) , the resulting "BOOM!!!" was loud and clear at my House!

Outage lasted about 3 Hours.

Anyhow, keep us informed to the progress!

-- Scott (EE)

These chips sure do make building amps easy. I remember building my first audio amp with tubes. It was probably about 5 watts output with more like 20-30w in, just to keep the tubes warm.
That LM3876 is rated at 56W

Yeah Greg,
I have quite a large heatsink in mind for this project and if all goes well, it shouldn't even be running anywhere near rated wattage.

I have to agree in saying that these chips do make building an amp pretty easy, especially with a minimal amount of field components.
When I first decided to buy this kit, I checked out the LM3876 chip to see what it's parameters were, for the dollar spent, this seemed like the best value for money.
Although I would have loved a valve (tube) amp, I'd still be in the design process and this kit was a means to an end.

I suppose the next part of this project will be placing the toroidal transformer, getting the bridge rectifier and filter capacitors mounted to the base of the case and then wiring them up.

Guys,
I'm in two minds about the speaker enclosure, what I've decided on is 4 x 8" drivers, I bought a bass port tube and some foam to go inside the enclosure.
I've got enough plywood and timber (for the sub-frame), to make the box any size I like.
It's the actual dimensions that have me really up in the air, I mean you could probably make the box any size you liked (as long as the speakers fitted), but speaker enclosure design is not something I've had a lot to do with.

Any thoughts there fella's?

Then you are going to be looking for crossovers ;-)

I did this some years ago and ended up with some pretty nice speakers but they were big honking things that I had a problem finding a place for.
They did not make the trip into my new entertainment center.
I gave them to a church here that got robbed and lost all of their audio equipment.

What voltage is that amp running on?

Cross-overs?
Why?
Although I can sort of understand what you're saying, I've never actually needed them before.

This amp is powered by 230V mains, fed into a 160VA transformer that has 2 x 25V secondary windings, that will be wired in series to give a 50V center-tapped output fed into a bridge rectifier.
This then feeds 2 x 2200uF 63V electrolytic capacitors, giving 2 balanced supply rails of +/-35V

If you are building a speaker box with multiple speakers you really want to split out the highs, mid range and lows.

Passive Crossovers are simple to build, and as Greg stated above, when a single enclosure contains multiple Speakers, splitting and directing specific Frequencies per Driver type is almost prerequisite.

Not only do these Filters direct Frequencies to specific Speakers, they also keep the overall Load Impedance steady, as seen by the Power Amp (Power Amp drives into a more "Steady" Impedance throughout the spread of Frequencies).

I have an Application for designing Passive Crossover Networks, which may include Notch Filters, Impedance Equalizers, L-Pads, etc. for 1st through 4th order Networks built around Two Way and Three Way Loudspeaker Arrays - including Dual Voice Coil types.
There are some examples in the Tech Section, way at the end (some of the first drawings to be posted in the Tech. Reference area).

Send the Loudspeakers' Parameters (Data), along with where you prefer to roll Frequencies at (Crossover Points), and I can produce Xover Schematics for you.

You may wind the Inductors yourself, as that is the most effective way to achieve values close to the Design.
Capacitors may be stacked in Parallel, to achieve the Design Values.
Polypropylene or Mylar may be used.
I would stay away from the cheap Electrolytic Non-Polarized Caps.

The Xover Application may be used in conjunction with an Enclosure Design Application I also have, which makes for a complete Design Package.

As to building Pre-Amps and Power Amps with Op-Amps (i.e.: LM386 / LM387), I built a few Pre-Amps around these Op-Amps, and it was so much easier than the Phonograph Pre-Amp I built using Discrete Components!
It was for a Magnetic Cartridge Phonograph (35mv signal), typical RIAA Equalization "Reversal", with 3 level Tone Controls, Subsonic, Notch & Rumble Filters, and solid Line Level output.
Took eons to build and tweek!

Don't know what happened to that Pre-Amp. Someone took off with it back in 1993, as there were a few "Wannabee Electrical Engineers" hanging around where I used to practice at, and they liked "Fixing Things" which they deemed inoperable... you know the types!
These guys were acquaintances of the other Band we shared the Studio with, and probably attempted to use the RIAA Pre-Amp for something other than a Mag. Cartridge Phono (I am thinking Tape Deck).
When the Audio sounded bad, it was time to "Fix The Pre-Amp".

Things of that nature occur often when sharing a Studio!

Anyhow, good luck with the Project!

-- Scott (EE)

Powered PC speaker systems are getting so good these days, that is about all I use. We have them everywhere and I just distribute line level outputs and pick it off where we want the sound (small spaces at a time). I have one built into my LaZboy, my wife has one behind her chair, there is one in the kitchen and one out in the back room. I still have a regular amp and some big sansui speakers out in the tiki bar.

Mike (Trumpy),
I should have posted a response to the Speaker Enclosures first!



Enclosure Designs Criteria:

Construct the Enclosures to be as solid as possible. An Enclosure should have a dead sound to it, when knocking on it with your fist.
I have seen Enclosures with a thin Layer of Sand between Two Layers of Plywood.

Build Boxes which are Rectangular - not Square or equally cubed!
Square Enclosures - like a 12" x 12" x 8" Box, will have bad sound.
A Cube type Enclosure - like a 10" x 10" x 10" Box, would be the Absolute Worst sounding Box!

An Enclosure's size will be related to the Woofers' size.
For example; (1) 8" Woofer on a Three-Way Speaker has a Minimum Enclosure Volume of 0.56 Cubic Feet; with Box Dimensions of 16.5" High, 10" Wide, 6.125" Deep.
The higher end Volume is 0.98 Cubic Feet, and the nominal Volume is 0.75 Cubic Feet

Damping materials should be mounted securely to the Enclosure's Bottom, Sides and Back. 1" Thickness is typical.

Mount the Crossover Assembly on the Bottom of the Enclosure, or even better, outside of the Enclosure.
My Xovers are mounted under the Enclosures, on the Boxes' Stand; separate sealed Speaker Leads enter the Box from below.

Regarding the Tuned Port Enclosure:

A Tuned Port will allow you to use a relatively small Enclosure, and still obtain deep Bass response.

Length and Diameter of the Tuned Port is figured per the Volume of the Enclosure and the Tuning Frequency of the Enclosure.

There is a lot of Science and Math involved with Enclosure Designs, but the basics are easy to grasp.
I may be of assistance with the Designs of various Enclosures, so feel free to contact me if needed.

-- Scott (EE)