Old Project Gallery

Some of the projects of Bob Richards - somewhat chronological order

My SWTPC Universal Tiger Poweramp, Tube Type Dynaco Preamp and 1962 Empire Turntable - 1973


 

1976ish. This one was inspired by my brothers McIntosh ML-1 (A very good sounding speaker, even by today's standards).


15 inch JBL sub-woof with satalite - 1982

 

These had the polypropylene six inch Peerless (sealed cabinet), with the 1-1/2 inch Long (brand) Dome, and a Bozak type arrangement of two Peerless 2 inch papercone tweeters, which are very similar to the ones in my brothers McIntosh ML-1's. The crossovers are built on heavy guage circuit board material with banana-plugs such that they can plug right on to the banana-jacks mounted in the speaker cabinet.


Above is a preamp I designed and built in 1983, while working in video engineering at Tektronix. It uses all 5534 op-amps carefully phase equalized for stability. It sounds very good. Even Matt Kamna likes the way it sounds. It also features an excellent RIAA phono preamp I designed using an LM394 differential front-end device driving a diff-amp second stage, which uses a norton current mirror to provide the complimentary drive for the push-pull output stage (the separate board in the rear corner). There's four inputs, stereo reverse, mono, and three external processor ("tape monitor") loops. Perhaps the best feature of this preamp is the four section Baxandhal tone controls that I developed myself. They allow you to dial in just the amount of reverse Fletcher-Munson "Loudness Comp" that works best in a given acoustic situation, for each channel independantly. There's Low, Low-Mid, High-Mid and High. There's also a bypass switch for the tone control circuit in case you're a nerd.

If I ever build a preamp again, I will definitely lay out a circuit board that will have all connectors and front panel pots and etc. mounted on it, with very few exceptions. The wiring was a bitch.


The chassis with the active crossover, woofer EQ and Carver Hologram re-optimization - 1985ish

Under that is a stereo mixing console I designed and built for recording or playing of musical instruments.

Under that is the finished "double-tube" speaker system (more below). Some people will recognize the 7 inch ScanSpeak driver with the kevlar cone.


 

The double-tube speaker design - 1993ish

The diaphrams of these two 7 inch Kevlar Scanspeaks would be at the edge of a circular cavity (two cavities actually- one on each side), which according to the Audio Cyclopedia, is a less reactive enclosure shape than any variation of a rectangular shape cabinet. The curved walls on the front were made out of MDF with slots that go 90% of the way thru, which were then filled with RTV silicone rubber glue and lead buckshot, to add mass and thereby dampening to these walls. It was tricky, and is not recommended. On the inside, the curved walls were made of 'Sonotubes', which you get at the mason supply house. Sonotubes are used as moulds for pouring concrete by construction workers that are making pilons for freeway overpasses or whatever. The end to end resonance is broken up both by the seal half way down the cabinet, and the leaky but somewhat effective angled disks of MDF. The insides surfaces of all of my cabinets are covered with glued in, high quality felt-like under-carpet padding. When I can find some decent looking felt for the outside of the front, I will cover that too. I glue it on with a glue that never dries to the point of being brittle. Liquid nails or silicone rubber cement has been working well for me.

 

Way too much work to build. The walss may not be rigid wnough, and the tweeters were way too low relative to a listeners ear.

A "mistake" is an opportunity to learn.


My first ultra low-noise stereo mic preamp - 1986

I used the Analog Devices ultra low-noise mic pre chips and my four section Baxandhal tone system - absolute top notch!


 

The Scan-Speak 9300 tweeter with Kevlar 7 inch - 1996

A more conventional approach. Note separate woofer cabinet on left. Again, the tweeter is too low relative to the listeners ear. The tweeter should always be at least three feet off the ground no matter what else that means.


One of my best speaker designs (1985) :

Quad-amped 4-pole 4-way calibrated

+/- 2dB fom 125HZ - 18kHZ

These really needed to be higher up off the floor to show their potential. Very good sounding. Notice the 3/8 inch thick wool felt on the front and front-sides of the cabinet. Diffraction is comparatively very low.

I used Dynaudio 3/4 inch and 2 inch domes, with KEF five inch drivers. The woofer is a Dynaudio 11 inch in a sealed cabinet that is electronically forced to be flat down to 25 HZ at the listener position (with active EQ). See active crossover board below.


4 Pole 4 Way active crossover circuit for satalite/subwoofer arrangement.

EQ in the woofer section makes the bass relatively flat down to 25 HZ at the listener position.

Bottom of hand soldered active crossover board


A smaller version of the above cabinet style done years later, for a tri-amp'd system.

Small enough to hang from the ceiling.

Two five inch drivers with a one inch dome between them.

These would have been great, but I decided that I really don't want speakers hanging from the ceiling. You need a bigger room for that.


A late 1990's system that is designed to allow experimenting with the "D'Appolito" arrangement.

I guess I needed to remind myself how bad sounding a rectangular midrange cabinet is, even with very good internal damping, and double thickness (1.5 inch MDF) fronts and backs. I found that it sounds much better to have the tweeters on top, rather than having them in between the two five inch drivers. Not so much because the D'Appolito arrangement isn't good, but because the tweeters need to be higher up off the floor to sound good. If you arrange the drivers D'Appolito style (tweeter in between the two mid drivers), and have the tweeter at least 40 inches off the floor (very highly desirable), you end up with a speaker that is too obtrusive for me. Your wife or girlfriend will take you out back and beat you. I don't want the overall cabinet to be that tall. At the bottom is an 8 inch Focal brand kevlar cone woofer (very nice) in a sealed box with active EQ (which has since been moved over to the side wall where it interacts much better with room acoustics).

I think I need a new Lava Lamp...


This is a four section Baxandahl tone control circuit that I developed myself. It allows one to compensate for the usual problems, and to create a reverse Fletcher-Munson (Loudness Compensation) curve. This is the right way to do "tone controls". Same circuit as in my 1983 preamp above.