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Forums | Rap N Tap Clubhouse - Basic Crystal Radio Fun! | Radio Shack Mini Amplifier/Speaker Post Reply Send Topic To a Friend
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RobertCPosted - 3 November 2011 21:51  Show Profile  Email Poster  Edit Message
Hi all,

It seems to me that the weak link in this hobby for beginners is the headphone situation. One must either use the cheap piezo earplugs which are fussy and prone to malfunction or one needs to find a good pair of vintage 2000 Ohms headsets. Well there is a third option.

Radio Shack makes a "Mini Amplifier/Speaker" (part number 277-1008C) which can take the place of vintage 2000 Ohms headphones in any radio circuit. Radio Shack specs list the impedance of the Mini Amplifier/Speaker (MAS) at 5000 Ohms. I measured the DC resistance across the input terminals at 5000 Ohms. That remained constant whether it was turned on or off. It uses one 9 Volt battery and has a volume adjustment.

It seems to have about the same impedance as a set of vintage 2000 Ohms headsets. When using it with a typical xtal set circuit it likes to be placed at a tap of about 20 to 40% of the number of turns from ground. It also likes to be used with 1N34a type diodes. The MAS does not detect, it is an audio amp/speaker so you still need a diode off the coil. Just place the MAS in the circuit just like 2000 Ohm headsets.

One thing I noticed is it does NOT like the typical fixed headphone capacitor placed across its terminals like many wiring diagrams call for. If your diagram calls for that capacitor, just leave it off.

I tried it with the "simplest xtal set" with an indoor antenna and cold water pipe ground. I simply wired the following: antenna, diode, ground. Then I clipped the leads from the MAS to each side of the diode placing the MAS in parallel with the diode. I could hear a combo of several stations and plenty of 60 cycle hum (my house is awful about that).

The input to the MAS is via a 1/8" (3.5mm) mono headphone jack (Radio Shack part number 274-0287). (The MAS has a female 1/8" jack receptor.) What you need to do is get some Test/Jumper Leads (Radio Shack part number 278-1157), which are nice to have anyway, and cut one of them in the middle to make two leads with alligator clips at the ends. Then, strip the ends of the wire and solder them to the 1/8" Phone Plug. There are two terminals in the Solder-type Phone Plug, one for each wire. If you get another type of 1/8" phone plug make sure you get the two terminal "mono" type (not stereo type.)

I think these mini amp/speakers are a neat little tool for most anyone from beginner to folks like me with crummy ears!

My local Radio Shack carries all of these parts or they can be ordered online at . I have also used the MAS in leau of headphones in several amplified circuits and it works well. Just remember that it has a DC path across the lead terminals whether it is on or off, so it may run down your battery in your radio circuit if it is left connected depending upon how your radio is wired.

The price at my local Radio Shack for the MAS was about $15, the 1/8" plugs ran about $3 dollars for a pack of two, and the test/jumper leads ran about $8 for a pack of 8.

Edited by - RobertC on 11/3/2011 10:06:27 PM

homebrewPosted - 3 November 2011 23:1  Show Profile  Email Poster  Edit Message  
I build a lot of simple circuits and use one for hands free operation when I'm selecting and positioning components.
Or to ID a unknown station while I'm doing other things.
golfguruPosted - 4 November 2011 3:56  Show Profile  Email Poster  Edit Message  
Updated "Simple Set" circuits, incl. amp, choke and double diode variations.
Uploaded at RobertC's request.

Edited by - golfguru on 11/4/2011 3:56:32 AM

Garry NicholsPosted - 4 November 2011 6:59  Show Profile  Email Poster  Edit Message
Squeaky Wheel here.

At the risk of becoming annoying, I will beat the drum one more time about "tuning up" crystal earplugs.

I make this post because last night I was listening to my new simple set with my two earplugs wired in series and a 50k pot across them when suddenly the set went "dead". No pops or crunches, just a very rapid fade to everything on the dial.

So I tapped the earplugs on the large tuning knob. First one -- no result. Second one --bingo! Easily copied stations again!

It seemed to respond better to having its back tapped against the knob, but it could have been a random event.


RobertCPosted - 4 November 2011 10:8  Show Profile  Email Poster  Edit Message

Edited by - RobertC on 11/4/2011 9:53:24 PM

RichardPosted - 4 November 2011 10:19  Show Profile  Email Poster  Edit Message  
RS mini amp's work very well for amplifying crystal set outputs but they do have one drawback, a pesky 10 khz background squeal from adjacent channel signals that can be annoying to sensitive/ younger ears. ;)
golfguruPosted - 4 November 2011 17:4  Show Profile  Email Poster  Edit Message  
RE: earplug problems.

I posted a thread at TRB on this. Some of the modern ones are piezo elements on aluminum discs (not the usual brass).

The centre contact is solderd but the other one is just bent over and glued to the aluminium. May go HR.


Edited by - golfguru on 11/4/2011 5:05:54 PM

gzimmerPosted - 4 November 2011 18:59  Show Profile  Email Poster  Edit Message
Here's a circuit of the RS Mini-amp

RobertC: I can't see how the input would be inductive. No input transformer. I think your meter is telling fibs..

Richard: Increasing the little feedback cap on the first transistor would give more top-cut. Would be a useful mod. Maybe via a little toggle switch.

In my part of the world, Radio Shack no longer exists, so sadly these little amps are no longer available here..

............ Zim

Edited by - gzimmer on 11/4/2011 7:01:48 PM

gzimmerPosted - 4 November 2011 19:11  Show Profile  Email Poster  Edit Message

Regarding having to tap Crystal Earpieces....

When I was young and foolish, I always put an inductor (one winding of an audio transformer) in parallel with a Crystal earpiece, rather than a resistor. It seemed to work better.

Have you tried that? I'm wondering if the DC path would prevent the earpiece charging up... Just an thought...

Maybe it might then need a Bennie in series with the whole thing...

............. Aim

golfguruPosted - 4 November 2011 19:18  Show Profile  Email Poster  Edit Message  
>>>> RobertC: I can't see how the input would be inductive. No input transformer. I think your meter is telling fibs.. >>>>

Wire wound resistor? (Not likely.)

My AADE reads from 0.2uH up to 30uh at some resistance value on a carbon pot.


RobertCPosted - 4 November 2011 21:52  Show Profile  Email Poster  Edit Message
Hi all,

Zim I think you are right that my meter is telling a fib. I don't see how it could be inductive looking at the wiring diagram. I am going to delete my post above where I state it measured inductive so someone won't be confused.

If you are interested in the mini amp/speaker, they are available on Radio Shack's web site. Thanks for the wiring diagram!

gzimmerPosted - 5 November 2011 0:1  Show Profile  Email Poster  Edit Message
> If you are interested in the mini amp/speaker, they are available on Radio Shack's web site

A while ago I tried to buy some RS TV ribbon, but they wouldn't ship overseas (old fashioned TV ribbon has almost completely disappeared BTW).

.................. Zim

homebrewPosted - 5 November 2011 8:4  Show Profile  Email Poster  Edit Message  
The flat line audio cable makes a good sub for TV ribbon. It comes in all flavors. I picked up two rolls of "gold plated" audio ribbon at a garage sale and have been trying to come up with a way to incorporate it a xtal set coil.
gzimmerPosted - 5 November 2011 8:17  Show Profile  Email Poster  Edit Message

> trying to come up with a way to incorporate it a xtal set coil.

Just wind it as per normal and then put the two windings in series (in to the first winding, out the other end, back to the start and in the other winding, then out the other end). It will have a little more inter-winding capacitance, if that matters any.

Or use the two windings as a tightly coupled Bi-filar transformer. Maybe use the second wire to drive the diode.

.............. Zim

Edited by - gzimmer on 11/5/2011 8:20:05 AM

RichardPosted - 5 November 2011 8:33  Show Profile  Email Poster  Edit Message  
> trying to come up with a way to incorporate it a xtal set coil.

A Mystery set comes to mind.

Edited by - Richard on 11/5/2011 8:37:12 AM

krystalloPosted - 5 November 2011 16:36  Show Profile  Email Poster  Edit Message
Hey All,

I REALLY LOVE these mini amps.IMHO they are one of the BEST products RS has ever sold.

I have used these amps over many years for quite a number of different applications : crystal set audio amp (especially with the low Z horn), as a stand alone VLF lightning /"Natural Radio " /"sferic" "receiver",a 60 hz "pick up" coil amp, as a small "PA" system along with a mic and big speaker,etc, etc.

Especially for VLF circuits I have even run two in series- the first at low level as a "pre amp" and the second cranked up.
The dismal Z mismatch of such a set up didn't seem to hurt anything.Quite the opposite,it works quite well. Haven't tried this on a crystal set though.

I also use a Mini Amp as a part of my "Lightning Bug" VLF "receiver" (along with graphic equalizers, a big coil w/ choke, a boom box CD input and other parts).

I have at least three of these in my shop and one built in to a bi directional loop "Red Soxs " crystal set for it's speaker.


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