UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 18th Feb 2012, 7:09 pm   #1
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 986
Default JLH Class 'A' Amplifier

It has always been my intention to have a go and build this amplifier, but have never got around to it. Spurred on by a thread in the 'wanted' section of this forum It reminded me that I had made a start a couple of years ago by mounting some 2N3055's on heatsinks. After these had been found, a concerted attempt was made to complete the project.
Why build this circuit in the first place? Simple! Much has been written about Class A amps, so I wanted to see what all the fuss was about. It is also the simplicity of the circuit which grabs my attention. A mere handful of bog standard components cannot compete with amplifiers which contain tens more, surely.

I decided to follow the original 10 Watt circuit, but to use modern transistors. In truth these had to be devices that were to hand. BC212, BFY50, and 2N3055 outputs. Luckily, I had to hand a switched mode power supply that delivered an output of 28 volts at 4 amps, so I could make the amplifier into a fairly compact unit.

I initially knocked up a pair of channels on the bench on some matrix board, rather untidily it has to be said, to see what the sound was like. I was blown away with the performance of this simple circuit. It really does sound great. l can hardly believe that 4 transistors and a small handful of components can produce an amplifier of such quality.

After such a surprise, I deemed the circuit worthy of a case! This consists of a galvanised electrical knock-out box, furnished with a couple of chrome handles that were bought for another project that did not quite get off the ground!

If anybody is thinking of making this circuit, I highly recommend that you do, you will NOT be disappointed!

Cheers. SimonT.
Attached Thumbnails
Click image for larger version

Name:	JLH amp1 001.jpg
Views:	4544
Size:	101.9 KB
ID:	62715   Click image for larger version

Name:	JLH amp1 002.jpg
Views:	6883
Size:	87.4 KB
ID:	62716   Click image for larger version

Name:	JLH amp1 007.jpg
Views:	3403
Size:	113.0 KB
ID:	62717  
__________________
The honesty of imperfection..........
thermionic is offline  
Old 18th Feb 2012, 7:14 pm   #2
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 986
Default Re: JLH Class 'A' Amplifier

Here are more shots of the internals. Please excuse the untidy wiring, I wanted to get it finished so I can enjoy listening to it!
Attached Thumbnails
Click image for larger version

Name:	JLH amp1 005.jpg
Views:	3039
Size:	116.3 KB
ID:	62718   Click image for larger version

Name:	JLH amp1 006.jpg
Views:	1961
Size:	94.2 KB
ID:	62719  
__________________
The honesty of imperfection..........
thermionic is offline  
Old 18th Feb 2012, 7:17 pm   #3
Brian R Pateman
Nonode
 
Brian R Pateman's Avatar
 
Join Date: Jun 2005
Location: Western Lake District, Cumbria (CA20) - UK
Posts: 2,136
Default Re: JLH Class 'A' Amplifier

A nice project. I particularly like the innovative use of an adaptable box for the chassis.

I always had a soft spot for JLH designs. Unlike many of the designs which appeared in the magazines of the period they usually tended to work more or less as published.
__________________
Brian
Brian R Pateman is offline  
Old 18th Feb 2012, 7:46 pm   #4
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 986
Default Re: JLH Class 'A' Amplifier

Here is a link to the Class 'A' site . http://www.tcaas.btinternet.co.uk/

I have just hooked up my amp to the signal generator and 'scope. The picture on the left is the amp driving an 8 ohm load @ 1kHz square wave, generator output at the bottom, amp output at the top,driven to the point of clipping. The picture on the right is the same set up but at 20kHz. Not bad at all!

Power output with my 28 volt rail into an 8 ohm dummy load, delivers 10.56 Watts with a 1kHz sine wave, driven to the point of clipping.

SimonT.
Attached Thumbnails
Click image for larger version

Name:	JLH amp1 004.jpg
Views:	1302
Size:	61.0 KB
ID:	62726   Click image for larger version

Name:	JLH amp1 003.jpg
Views:	1429
Size:	70.3 KB
ID:	62727  
__________________
The honesty of imperfection..........
thermionic is offline  
Old 18th Feb 2012, 8:12 pm   #5
Whaam68
Hexode
 
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 454
Default Re: JLH Class 'A' Amplifier

Looks great.....glad I spurred you on to finish it
I am going to use Chinese pcbs as they were only about £15 with the parts and postage. I have built a class A set amp with 6B4Gs so am curious to see how this circuit compares. I've got an 807 amp to finish before I get going on trying to teach myself about the JLH
Mike
Whaam68 is offline  
Old 18th Feb 2012, 8:29 pm   #6
Silicon
Nonode
 
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,152
Default Re: JLH Class 'A' Amplifier

Well done. Your choice of a switch mode power supply is interesting.

Most advice is to stay well clear of them because they generate a lot of noise.
Do you know what equipment the power supply originally worked with?
Silicon is offline  
Old 18th Feb 2012, 8:53 pm   #7
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
Default Re: JLH Class 'A' Amplifier

Are the heatsinks suitably overspecified? You've reduced their potential performance a bit with convection airflow restriction into the fins from mounting them tight to the chassis.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 18th Feb 2012, 11:28 pm   #8
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 986
Default Re: JLH Class 'A' Amplifier

Hi Silicon.

The SMPS was one that I had to hand, one of those things that you accumulate over the years! The output was slightly noisey, hence my inclusion of additional filtering in the form of a in-line mains filter unit, but fitted on the DC output. This killed the small amount of hash on the DC line, you can see it in one of the pictures, strapped to the top of the SMPS.

Hi Herald.

The heatsinks were all I had available. I do think they could be slightly larger. However, I have drilled holes in the chassis below the fins and have fitted feet to the underside of the case and have included vents in the bottom cover so this will allow convection up through the heatsink fins. Time will tell - if the 2N3055's die then I will have to think again. It has been running now for some 5+ hours, and I have been enjoying its output. You certainly don't suffer listener fatigue with this amp.

SimonT.
__________________
The honesty of imperfection..........
thermionic is offline  
Old 18th Feb 2012, 11:50 pm   #9
terrybull
Octode
 
Join Date: Jan 2011
Location: Southwell, Nottinghamshire, UK.
Posts: 1,779
Default Re: JLH Class 'A' Amplifier

I used to have a contact that dealt in HiFi with a 'bent' towards vintage equipment. He was kind enough to allow me act as a tester so I have had some very nice gear to play with, much of which was way out of my price range. My own amp was a Leak Stereo 20. I built a JLH to the original 1969 design many years ago and was blown away that such a simple design could sound so nice. I preferred it too and soon sold the Leak and even in comparison with some pretty expensive kit I had listened to it holds its own. I still use mine to this day. In fact it is in the process of a re-build as I never did tidy the rough prototype build as I was so happy with it. I hope your heat-sinks are up to the job. I use a large computer type fan running slowly to cool mine, as it got too hot with sinks not much smaller than the ones you are using. What current per channel have you set it at?

Last edited by terrybull; 18th Feb 2012 at 11:59 pm.
terrybull is offline  
Old 19th Feb 2012, 12:26 am   #10
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 986
Default Re: JLH Class 'A' Amplifier

Hi Terry.

I have set the standing current at 1.9 amps. This was arrived at purely by observing the response of the amp at differing test frequencies and output levels, not very scientific, I know! Mine is also mainly built to the original article, but using different transistors. I followed the advice regarding the Hfe of the output pair. I have used the lowest Hfe of my batch of 2N3055's as the upper transistor, to try and reduce the harmonic distortion to below 0.1%

I have also included the roll off filter, comprising 2 resistors and a 330pF capacitor in the input circuit.

I do have a Radford Distortion Measuring Set lurking somewhere, so it would be interesting to dig this out and take some measurements at some point. I agree that this amp sounds great. I am a great believer in keeping things as simple as possible, this simplicity only enhances the attraction of this amp.




(Simple) SimonT!
__________________
The honesty of imperfection..........
thermionic is offline  
Old 19th Feb 2012, 1:10 am   #11
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
Default Re: JLH Class 'A' Amplifier

I missed the holes

If the transistor cases are less than spit sizzling hot, they should be OK (provided dissipation is less than about 40W/device).

There's some heavy duty incarnations out there, though.....

http://electronics-diy.com/electroni...tic.php?id=489

__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 19th Feb 2012, 1:30 am   #12
terrybull
Octode
 
Join Date: Jan 2011
Location: Southwell, Nottinghamshire, UK.
Posts: 1,779
Default Re: JLH Class 'A' Amplifier

Hi Simon. Have you used the standing current setting modification that JLH published in a follow up to his 1969 design. I assume that is what the 2 pots are underneath. I have toyed with making mine split rail to do away with the output capacitor (1996 design) but I like the simplicity as it is and it sounds fine.
terrybull is offline  
Old 19th Feb 2012, 11:51 am   #13
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 986
Default Re: JLH Class 'A' Amplifier

Hi Chris. The dissipation seems to be fine - certainly not spit sizzling hot! The amp in your link certainly is heavy duty!

Terry, I have kept to the basic design of a pot for the standing current setting, rather than using the later modification. The old wirewound pots you see on the brackets are for this purpose. I have however, included a 100k preset to replace one of the 100k fixed resistors in the first transistors base circuit so that I can easily tweak the mid point voltage.

My worry with the split rail circuit is one of the output transistors going short circuit and dumping lots of DC through the speaker. I know a fuse here could help, but......

The audiophools would not entertain the output coupling cap,I'm sure! To my ears, the amp sounds great as it is. I would like to make a higher power version one day, perhaps not quite as beefy as the one in Chris's link, though.

SimonT.
__________________
The honesty of imperfection..........
thermionic is offline  
Old 19th Feb 2012, 11:58 am   #14
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 986
Default Re: JLH Class 'A' Amplifier

Quote:
Originally Posted by Whaam68 View Post
Looks great.....glad I spurred you on to finish it
I am going to use Chinese pcbs as they were only about £15 with the parts and postage. I have built a class A set amp with 6B4Gs so am curious to see how this circuit compares. I've got an 807 amp to finish before I get going on trying to teach myself about the JLH
Mike
Good luck with this, Mike. I will be interested to see these PCBs, when you get them.

SimonT.
__________________
The honesty of imperfection..........
thermionic is offline  
Old 19th Feb 2012, 2:57 pm   #15
Whaam68
Hexode
 
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 454
Default Re: JLH Class 'A' Amplifier

Hi Simon

I went for these as the quality looks good

http://diyhifishop.com/free-shipping...-kit-p-13.html

I also got a pcb part kit for a single rail power supply from eBay (
Jims audio) for about $19. This arrived and the pcb is really thick. Can't moan for the price.

I might tap you up for some advice at a later stage

Regards

Mike
Whaam68 is offline  
Old 19th Feb 2012, 8:07 pm   #16
thermionic
Heptode
 
thermionic's Avatar
 
Join Date: Apr 2003
Location: Cornwall, UK.
Posts: 986
Default Re: JLH Class 'A' Amplifier

Hi Mike. Those kits seem ok! I would rather mount the output transistors straight to a heatsink, though.

I wonder why they think it necessary to include a 4.7uF input coupling cap?

Cheers. SimonT.
__________________
The honesty of imperfection..........
thermionic is offline  
Old 19th Feb 2012, 8:25 pm   #17
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: JLH Class 'A' Amplifier

As the amplifier has DC coupling throughout, if the signal source happened to put out some DC, you could fry your loudspeakers. Having an input cap protects against this, at least!
kalee20 is offline  
Old 19th Feb 2012, 8:53 pm   #18
Whaam68
Hexode
 
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 454
Default Re: JLH Class 'A' Amplifier

Hi Simon
I'm assuming I could mount the transistors off board and run wiring to the pcb?
Power supply board for the lazy/stupid (read me!) are here
http://www.ebay.co.uk/itm/2208261904...84.m1439.l2649
Mike
Whaam68 is offline  
Old 19th Feb 2012, 8:59 pm   #19
val33vo
Retired Dormant Member
 
Join Date: Apr 2008
Location: Banffshire, Scotland, UK.
Posts: 191
Default Re: JLH Class 'A' Amplifier

You are on the right track with this amp, you cant beat the old 2n3055,s for ultra cheap power, saves mucking about with expensive ponsy FET,s
val33vo is offline  
Old 19th Feb 2012, 9:01 pm   #20
Whaam68
Hexode
 
Join Date: Feb 2007
Location: Luton, Bedfordshire, UK.
Posts: 454
Default Re: JLH Class 'A' Amplifier

Cheapest 1969 kit I could find ....

http://www.ebay.co.uk/itm/2pcs-JLH-1...#ht_2020wt_689
Whaam68 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 7:25 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.