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Parasonic 15th February 2012 06:04 PM

Office radio project
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I like to listen to music in mono, and so got the idea to make a larger table type radio for my office. I will use a kit for the tuner part, but wanted to try to do something a little different in the audio section. There's also a bunch of parts around that would be nice to use up, and also, I got some stuff for Christmas to build me a coil winding machine (that's still in progress, but I'll post some pics of it when it takes form), so I wanted to make a transformer or two for the radio.
Sometimes you want some jump from the amp so in order to preserve the "off the wall" nature of this project, I've been playing around with the old "power booster" circuit that transformer couples an amp chip to a pair of power transistors functioning as emitter followers across a center tapped choke. To complicate matters, I have decided to use a 12EH5 tube to drive the booster. I didn't want to contend with a varying impedance reflected from the output choke/ speaker combo into the EF bases, so decided to incorporate a couple of transistors operated common base, which will virtually stabilize the impedance seen by the driving transformer secondary, and since the dynamic input impedance of the CB stage is only tens of ohms, that plus an emitter set resistor (56R in my breadboard setup) will in turn reflect a more stable 3K into the primary for the tube load with the proper turns ratio. Right now all I have is a power transformer for the driver tranny and I'm using my little Grundig battery operated radio into the primary. This scheme seems to work, as the sounds I'm hearing are very pleasing. At first I had red LEDS on the CB bases but it was too much bias and pulled the 2955 bases into too much conduction. The present scheme works with the 1N4005 bias on CB's. (I can't find anything where CB stages were used like this anywhere). Looking forward to seeing how far I can push this. I know you can fry a speaker with a setup like this, but haven't ruined anything yet. Sure it's weird, but if I couldn't pursue stuff like this, I might as well fly a kite. Rough schematic and rat nest photo below.


dangus 26th February 2012 08:41 AM

My first inclination is to suggest using a standard hifi tuner for the tuner. They're virtually free at thrift stores and pawn shops.

If that's too bulky, use a car radio. Factory radios tended to have excellent tuners. Some even feature diversity tuning for FM, or AM Stereo (if there's still any of that on the air). Try autowreckers, thrift stores, or antique car swap meets. If repairs are needed, it's not hard to find service docs for factory radios. Perhaps this is more in keeping with the spirit of your project: use a funky chrome radio from the '60s or '70s. Maybe even with 8-track... it wouldn't be impossible to bypass the 8-track circuits and feed in sound from an external source.

Parasonic 26th February 2012 07:11 PM

Hi Dangus,

Your ideas are well taken. I had thought of doing just that---with the auto radio format quite a few years ago, but got sidetracked. I'm still listening to this setup bread boarded on the work table. I found a great Oaktron alnico 8 inch fullrange driver for $6.99 at a thrift store, which I can't stop listening to. I thought when I brought it home---"ok, another run of the mill paging speaker"---not so, it is very musical and detailed without being annoying. A good match for this particular amp. I already have this FM/AM kit:

Electronics Kits: Amerikit AM/FM Radio Kit

so that will be the tuner portion, and you are right, some extra inputs for other sources will be added as well.

I found my old coil winding machine I built, out in the shed (thought it had disappeared---couldn't find it anywhere) so I really don't have to wait to wind the choke/ transformer, but I think I will wait until the new winding machine is built, as it will be so much better-easier to use.

I appreciate the input---

Parasonic 4th April 2012 11:39 AM

Sheddus Interruptus
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Pardon me while I finish off this "honey do" project.
The radio project will continue as soon as possible, eagerly anticipating resuming it.

Thank you

Parasonic 17th June 2012 05:49 PM

Time out for RF
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RF alignment oscillators/ rf voltmeter under construction. 10.7 Mhz/9 Mhz switchable being built from a generic NightFire Electronics board. 455 Khz w/ modulation being constructed from Lindsay's Impoverished number 5. All necessary before turning attention to audio.

The coil winding machine parts are seen--awaiting somebody to assemble them. ;) The wood is marked out for motor mounting. PS components not shown, but there's counter/ footswitch/ bi-directional motor speed controller/ chuck.

The speaker will (hopefully) be the 6X9 parts shown, a TA Weber frame (thanks TA) w/ Alnico mag with paper cone and probably phase plug. The cone and gasket are just sitting in there for now.

The amp is still under consideration and probably will be experimental for awhile. It won't see any tube amplification til the coil winder is done.
All this stuff takes more time than I'd like but hopefully not too long. I've always wanted a good radio with a personal touch.


Parasonic 18th December 2013 12:09 PM

Project ain't dead yet....
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Just been experiencing a massive reorganization at my company the past year--taking a lot of time to adjust to all the head-spinning changes....have to admit most times I come home and just want to vegetate.


This amplifier came about after playing with some "light bulb" amplifier experiments. One of those experiments a CFP and a Triad autotransformer drove an output transistor with an auto light bulb as a collector load. OK for screwing around a bit seeing if different things have any merit or not, but I know those are not serious amps.

This amp here is about as simple as it gets. I lifted Mike Martell's bjt class A amp for the driver found here:

RASON Projects

..and using the Triad N-1X as a driver for a MJ2955 output. I wound the output tapped choke myself (have a pair of them). Driver biased at approx 56ma.

There is hope this (radio) project will someday be completed but no ideas as to when. The amp is playing through an aperiodic 2 way that has a Tang Band woofer and I must say---this is not a bad amplifier at all.

I want to do a pair for stereo before the radio project continues.

Happy holidays

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