Little singled ended amplifier - UK Vintage Radio Repair and Restoration Discussion Forum

sexton_mallard · 3rd Sep 2017, 3:45 pm

I acquired this curious little amp from a relative of SHMBO years ago. It came with an EAR 5-10 amplifier which I sold long ago along with an EAR tuner. A leaflet (scanned) for a EAR 'Seven-Fifty' which came with the items appears to fit the description for this amp. The case appears to be a homebrew made from chipboard and some expanded aluminum grill. The construction appears to be of a factory standard but the chassis looks crudely cut which leads me to suspect it could also been salvaged from a (say) a record player. Looking at the illustration of the metal case, the layout of the components on my specimen is too tall to fit the shallow profile of the metal case.

After a few basic checks the amp powers up with reasonable sound quality. A recap of all waxies and some servisol is next. Has anyone come across these little EAR amps?

paulsherwin · 3rd Sep 2017, 3:49 pm

It certainly looks like a repurposed record player amp, maybe used as a workshop amp or signal tracer.

Edward Huggins · 3rd Sep 2017, 6:47 pm

Yes, this amplifier was used in a large 1955 EAR Portable Record Player which had a 10" x 6" speaker and Collaro RC54 atochanger. It sounded extremely good with plenty of gain.

kirstyd · 3rd Sep 2017, 7:30 pm

I have an E A R record player with this amp in it. Looks very similar. A few new caps and it was back up and running. With the big speaker it sounds rather good and will play very loud. The label in the lid says it's a EAR microgram and it only has a single play Collaro deck. The two transformers are mounted elsewhere in the cabinet.

sexton_mallard · 5th Oct 2017, 7:43 pm

Following a recap, I let the amp run on a lamp limiter with a old smartphone providing a source. 5mins on and 5 mins off at first about 3 times with a 40w bulb, the mains can was warm but not too hot. I decided to give it a longer run on a 100w bulb. After twenty minutes the 820R resistor between the reservoir and smoothing sections started to give up its smoke! Sound was distorted and garbled and the HT was down to 110v. A quick switch-off and the 2 section can was boiling hot. I left this in place on top of the chassis and treated the amp to 2 450v 47mfd electrolytics under the case and a 7w 820R ceramic resistor I found in my 'bits' box.

Valve lineup is the presumably common EZ80 EL84 and ECC83. There was a 8mfd 275v electrolytic remaining. This was also a bit distressed, probably from being baked to death from the 3 closely spaced valves on the chassis top. A 10mfd 450v went in. The amp now sounds good with plenty of volume. I'm not sure what the speaker impedence should be (15R?) but it runs Ok with a 8R Hi Fi speaker. I now plan to test this with the electric tonearm on the wind up gramophone I described in another thread here: http://www.vintage-radio.net/forum/s...d.php?t=136729

Edward Huggins · 6th Oct 2017, 10:04 am

I feel sure that the loudspeaker impedence will be 3 ohms. In the original 1955 EAR "Microgram" record player it fed a 10" x 6" speaker. Both triode sections of the ECC83 will be used.

kalee20 · 6th Oct 2017, 1:52 pm

Almost certainly 3Ω.

If you have a bit of time, a signal generator, a few load resistors, and a 'scope, you could play around to find out what it likes best.

Connect a load resistor. Drive the amplifier at a middle-sort of frequency (suggest 400Hz) while monitoring the output waveform with the 'scope. Wind up the volume until it starts to distort. It'll probably clip more on one half-cycle than the other.

Now play around with the load, readjusting the volume each time, so that clipping just starts. You'll find that low-value loads will make one side clip; high values make the other side clip. Somewhere in between, you'll get symmetric clipping. This is the optimum load.

sexton_mallard · 21st Oct 2017, 9:07 am

The little amp is back in its chipboard case and it's a belter! I will treat the case to a coat of varnish to stop it turning into weetabix. I do have a audio signal generator and a 'scope and will do some matching experiments when I get time.

kalee20 · 21st Oct 2017, 12:00 pm

Basic little amplifiers like this can give a very good account of themselves.

They're not hi-fi, but neither are they just adequately good. They are well up in the gap between. They can be used for serious listening and give immense pleasure. They tend to have lowish amounts of negative feedback, meaning that any distortion is going to be low-order and this does not give listener fatigue.

Well done on saving it and taking an interest! Look forward to measurements!

3rd Sep 2017, 3:45 pm	#1
sexton_mallard Heptode Join Date: Sep 2013 Location: Ellesmere, Shropshire, UK & Co. Cork, Ireland. Posts: 501	Little singled ended amplifier I acquired this curious little amp from a relative of SHMBO years ago. It came with an EAR 5-10 amplifier which I sold long ago along with an EAR tuner. A leaflet (scanned) for a EAR 'Seven-Fifty' which came with the items appears to fit the description for this amp. The case appears to be a homebrew made from chipboard and some expanded aluminum grill. The construction appears to be of a factory standard but the chassis looks crudely cut which leads me to suspect it could also been salvaged from a (say) a record player. Looking at the illustration of the metal case, the layout of the components on my specimen is too tall to fit the shallow profile of the metal case. After a few basic checks the amp powers up with reasonable sound quality. A recap of all waxies and some servisol is next. Has anyone come across these little EAR amps? Attached Thumbnails __________________ Dom Less snakes...more ladders!

3rd Sep 2017, 3:49 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,943	Re: Little singled ended amplifier It certainly looks like a repurposed record player amp, maybe used as a workshop amp or signal tracer.

3rd Sep 2017, 6:47 pm	#3
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,327	Re: Little singled ended amplifier Yes, this amplifier was used in a large 1955 EAR Portable Record Player which had a 10" x 6" speaker and Collaro RC54 atochanger. It sounded extremely good with plenty of gain. __________________ Edward.

3rd Sep 2017, 7:30 pm	#4
kirstyd Heptode Join Date: Mar 2014 Location: Leeds, West Yorkshire, UK. Posts: 902	Re: Little singled ended amplifier I have an E A R record player with this amp in it. Looks very similar. A few new caps and it was back up and running. With the big speaker it sounds rather good and will play very loud. The label in the lid says it's a EAR microgram and it only has a single play Collaro deck. The two transformers are mounted elsewhere in the cabinet. Attached Thumbnails

5th Oct 2017, 7:43 pm	#5
sexton_mallard Heptode Join Date: Sep 2013 Location: Ellesmere, Shropshire, UK & Co. Cork, Ireland. Posts: 501	Re: Little singled ended amplifier Following a recap, I let the amp run on a lamp limiter with a old smartphone providing a source. 5mins on and 5 mins off at first about 3 times with a 40w bulb, the mains can was warm but not too hot. I decided to give it a longer run on a 100w bulb. After twenty minutes the 820R resistor between the reservoir and smoothing sections started to give up its smoke! Sound was distorted and garbled and the HT was down to 110v. A quick switch-off and the 2 section can was boiling hot. I left this in place on top of the chassis and treated the amp to 2 450v 47mfd electrolytics under the case and a 7w 820R ceramic resistor I found in my 'bits' box. Valve lineup is the presumably common EZ80 EL84 and ECC83. There was a 8mfd 275v electrolytic remaining. This was also a bit distressed, probably from being baked to death from the 3 closely spaced valves on the chassis top. A 10mfd 450v went in. The amp now sounds good with plenty of volume. I'm not sure what the speaker impedence should be (15R?) but it runs Ok with a 8R Hi Fi speaker. I now plan to test this with the electric tonearm on the wind up gramophone I described in another thread here: http://www.vintage-radio.net/forum/s...d.php?t=136729 __________________ Dom Less snakes...more ladders! Last edited by sexton_mallard; 5th Oct 2017 at 7:57 pm.

6th Oct 2017, 10:04 am	#6
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,327	Re: Little singled ended amplifier I feel sure that the loudspeaker impedence will be 3 ohms. In the original 1955 EAR "Microgram" record player it fed a 10" x 6" speaker. Both triode sections of the ECC83 will be used. __________________ Edward.

6th Oct 2017, 1:52 pm	#7
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Little singled ended amplifier Almost certainly 3Ω. If you have a bit of time, a signal generator, a few load resistors, and a 'scope, you could play around to find out what it likes best. Connect a load resistor. Drive the amplifier at a middle-sort of frequency (suggest 400Hz) while monitoring the output waveform with the 'scope. Wind up the volume until it starts to distort. It'll probably clip more on one half-cycle than the other. Now play around with the load, readjusting the volume each time, so that clipping just starts. You'll find that low-value loads will make one side clip; high values make the other side clip. Somewhere in between, you'll get symmetric clipping. This is the optimum load.

21st Oct 2017, 9:07 am	#8
sexton_mallard Heptode Join Date: Sep 2013 Location: Ellesmere, Shropshire, UK & Co. Cork, Ireland. Posts: 501	Re: Little singled ended amplifier The little amp is back in its chipboard case and it's a belter! I will treat the case to a coat of varnish to stop it turning into weetabix. I do have a audio signal generator and a 'scope and will do some matching experiments when I get time. Attached Thumbnails __________________ Dom Less snakes...more ladders!

21st Oct 2017, 12:00 pm	#9
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Little singled ended amplifier Basic little amplifiers like this can give a very good account of themselves. They're not hi-fi, but neither are they just adequately good. They are well up in the gap between. They can be used for serious listening and give immense pleasure. They tend to have lowish amounts of negative feedback, meaning that any distortion is going to be low-order and this does not give listener fatigue. Well done on saving it and taking an interest! Look forward to measurements!