More On Mike Matthews Dirt Road Special - Schematic Found

A blog reader has requested a schematic for the EH-1313A amplifier.  I looked for one and found a post from 2009 where you can scroll down a bit and find a link to a schematic.  I am copying the image here in the event the above links stop working:

EH-1313A Schematic

I have no idea whether or not this is accurate.  It is the best one I could find after a brief search.  I hope this posting will make this schematic easier to find in the future.

An Official Welcome To My Klipsch La Scala Loudspeaker Pair

I thought it might be nice to share the sound of part of my system playing part of a piece by Dave Matthews and Warren Hayes, their rendition of Neil Young's "Cortez".  The video clip below is just under a couple of minutes long.  This is as close as I can get this medium to sound like my setup does "in person".  I hope you enjoy listening :)

I first heard the Klipsch Heritage Series at "Music Box" in Wellesley Massachusetts in the mid 1970s.  That experience has stayed with me to this day and I am really pleased to have the opportunity to let you to hear what a fully horn loaded loudspeaker system can sound like inside a small room.

Note:  The Nakamichi Receiver 3 provides the preamplifier.  An original TriPath Class "T" digital amplifier by Lepai, modified by removing all analog circuitry except what is on the digital chip, provides the power amplification.  The source is a wav file of part of the sound downloaded from a YouTube video recommended on the Klipsch Owners Facebook Group page.  The sound is streaming from USB through a Mini USB DAC by SignStek Audio.  The Nakamichi Receiver 3 has the best sounding preamplifier I have been able to find so far.

Lepai Power Amp and Mini USB DAC

The system is so efficient it requires less than 200 milliWatt of power from the power amplifier to fill the room with sound.

Barn Door Horn Speakers

I turned the eight-foot horns around to face the barn door opening.  Here are two views of the setup.

Barn Door Horn Speakers

Barn Door Horn Speakers

The original sliding door is quite heavy with wheels that ride on an iron track set in the floor.  I had to leave a foot wide clearance in order to operate the door from inside.

I have not played this system very loudly yet so as to avoid complaints from neighbors :)

Sound Transformer Matched Pair

These are 50% longer than the originals.  This version improves both efficiency and smoothness of frequency response.  That is a "Class-T" amplifier in the lower right corner.

Matched Pair

The Latest Find: Outdoor Party Speaker Pair

Well, I have been on the look-out for highly efficient speakers that will play at reasonably high levels outside.  While I was mowing my lawn a few days ago, I noticed a yard sale in progress just up the street at a neighbor's house that had recently sold.  Somewhat impatiently, I finished mowing the lawn and then went inside to get my camera just in case anything of interest turned up at the sale.

Instead of taking pictures, I wound up acquiring, through an exchange of equal value, a pair of owner-designed and built bass-reflex cabinets.  Inside each cabinet, I found an Electro Voice SP12B, a Calrad CN-2 crossover, and an Atlas Sound HR-3 horn tweeter.




 Atlas HR-3 - Calrad CN-2 - Electro Voice SP12B

The Atlas HR-3 tweeter appears to date the bass-reflex project at sometime after 1958.  Here is some advertising from the magazine Audio, February 1958:



Atlas Sound HR-3 Horn Tweeter (Audio February 1958)

 

The Atlas HR-3 tweeters in these cabinets look like this:



Atlas Sound HR-3 Horn Tweeter 1



Atlas Sound HR-3 Horn Tweeter 2

Here are images of the Calrad CN-2 crossover:



Calrad CN-2 crossover label



Calrad CN-2 crossover schematic

The Calrad CN-2 pictured is a series dividing network.  As you may already know, this means that the woofer and tweeter are wired in series.  I verified this by drilling out the four rivets holding one of them together and peering inside at the innards.  The ferrite ring core inductor and leads are potted into the case so you cannot see them.  The inductor is tapped and there are two capacitors so that you can change from 2500Hz to 3500Hz by shorting or un-shorting a pair of screw terminals.  They are found configured at 2500Hz in this instance.

The SP12B woofers are the same as you will find in a previous post, Corner Speaker Pair with ElectroVoice SP12B Speakers.   Here is some advertising from Hi-Fi Stereo Review, February, 1962, courtesy of http://vintagevacuumaudio.com/:



SP12B Ad in Hi-Fi Stereo Review February 1962 - p1



SP12B Ad in Hi-Fi Stereo Review February 1962 - p2

Here are images of the interior of each cabinet:



Bass-Reflex Cabinet Interior 1



Bass-Reflex Cabinet Interior 2

And here they are set up outside for listening tests:



Bass-Reflex Pair Outside For Testing

These have a very lively sound and will definitely make great Party Speakers.  I moved these with the wheelbarrow pictured.  I am thinking of adding the Jensen tweeters found in the Magnavox speakers featured in the post,  More on Plywood Horns and Poor Man's Horn Driver, to the Corner Speaker Pair with ElectroVoice SP12B Speakers.  

Mike Matthews Dirt Road Special - Speaker Upgrade

Several months ago, I posted about a repair on the Mike Matthews Dirt Road Special.  A few days ago, I had the good fortune to run across an exact replacement, electronically and acoustically, for the Celestion 12 Inch original speaker for this amplifier.  For example, the new Fender 12 Inch replacement speaker has almost exactly the same free-air resonance frequency.  The two sound nearly identical under a "tap" test.  By removing the GE (antique) temporary replacement and installing the new Fender, it is now possible to re-install the original power supply transformer to get the original 70 Volt DC power supply level.  The small stone phase shift effect is almost infinitely adjustable with the full 70 Volt supply.  Here is the Mike Matthews Dirt Road Special with new Fender replacement speaker:



Mike Matthews Dirt Road Special with Fender Speaker



Corner Speaker Pair with ElectroVoice SP12B Speakers

This pair of corner speakers arrived with a little dust but otherwise in almost new condition.  Here is how they looked when I first got them:


Corner Speaker Pair with ElectroVoice SP12B Drivers

Of course, I took them apart to see what was inside and discovered SP12B Drivers in pristine new condition.  EV manufactured several versions of these over the years.  Here are some photos of the ones in these boxes:



ElectroVoice SP12B Speaker

ElectroVoice SP12B Speaker Label



ElectroVoice SP12B Speaker Cone and Whizzer

ElectroVoice SP12B Speaker Voice Coil Dust Cover



ElectroVoice SP12B Speaker Terminals

 

I weighed one of them on a spring scale:



ElectroVoice SP12B Speaker Weighs 12 1/2 Pounds

Here is what the back of the cabinets looks like:


Corner Speaker (Back)

The terminals on the back look like this:


Corner Speaker Terminals

To sound best, they should be positioned in a corner:



Corner Placement For Best Sound

These are among the best sounding speakers I have heard.