advertisement


John Linsley Hood Class A or similar

With regards to the transition frequency of the transistors 2MHz Vs 10MHz, if you can hear the difference in a class A design you have better ears than a bat.
I was more worried about stability. My ears are definitely not bat-like at all.
 
Just be aware that if you use replacements with a higher Ft it would pay to check the stability of the amp.
 
Thanks for the advice gents, I'm going to buy the blank PCBs recommended earler and populate them with components from Maplin/Farnell. Then I know what I've got.
 
Thanks gents. What do you think about these offerings:
https://www.ebay.co.uk/itm/2016-1969-amplifier-class-A-amplifier-HIFI-Board-high-quality-MOT-2N3055-diy-kit/272270848443?_trkparms=aid=222007&algo=SIM.MBE&ao=2&asc=49138&meid=5d3cbd5c55ad4d1eb481d57df34427c4&pid=100005&rk=5&rkt=6&mehot=pp&sd=172237677755&_trksid=p2047675.c100005.m1851
Of which you would need a pair.
Or is this style better (note different O/P transistors)
http://rover.ebay.com/rover/1/710-5...0001&campid=5338728743&icep_item=141684499920

I don't mind using either kit as a basis and binning any dodgy components in favour of something better when I get it under way.

Here is what I built. Looks like I even mirror imaged the layout :cool::
IMG_2538.jpg


IMG_2536.jpg


IMG_2539.jpg


Looks like I modded the cap multiplier a bit.

I think it gives about 15W per channel in 8R with !q of 1.2A.
Dissipation was 16W per output transistor and 11W in the cap multipler pass transistor.
Judging from the gain number I scrawled on each 15003 I have positioned them for lowest distortion as per the link below

Useful info here:
http://www.angelfire.com/sd/paulkemble/sound3b.html
And I expect you have already downloaded the orginal JLH articles.

If you want to borrow the modules for a play around, I am also in Leeds.

Edit: just realised my dissipation figures don't tally with the voltages I wrote on the schematic. Looks more like ~20W per output tr. and ~2.5W for the pass tr.
 
This site contains affiliate links for which pink fish media may be compensated.
Thanks for the advice gents, I'm going to buy the blank PCBs recommended earler and populate them with components from Maplin/Farnell. Then I know what I've got.
The PCBs linked upthread seem to implement http://sound-au.com/tcaas/jlhesl.htm which is some distance from the original. (FWIW I think they have their numbers wrong on the 57, but that's another thread.)

I think I'd be tempted by http://rover.ebay.com/rover/1/710-5...0001&campid=5338728743&icep_item=322945218390 which look to implement the original circuit and include the single ended regulated PSU, something like S-Mans' arrangement above. Or similar, before digging into the updated and modified versions with bipolar PSUs and more modern input circuitry.
 
This site contains affiliate links for which pink fish media may be compensated.
I agree with Paul... and I wouldn't have dicked about dremeling my own boards if those PCBs had been available.
 
And an opposite opinion.. that the updated version will sound a fair bit better.

S-Man you really need much much bigger smoothing caps there! Add a zero on the end of that 4700uF (will need to be mounted off board) and now we're talking;)
 
Which ones need increasing? C7? C9?

The cap mulitplier does its job properly and results in no audible hum on the amp output. That's an authentic way to build this "classic".
Of course the other way to build the amp is with an unregulated supply with huge smoothing caps.

Are you saying both are necessary?
 
Which ones need increasing? C7? C9?

The cap mulitplier does its job properly and results in no audible hum on the amp output. That's an authentic way to build this "classic".
Of course the other way to build the amp is with an unregulated supply with huge smoothing caps.

Are you saying both are necessary?

C9 only. Not actually necessary but should improve performance.
 
I did consider upping C9 (and moving it offboard), but the JLH was quite a long way behind my Obelike (my version of an Ion Obelisk) in sound so I didn't bother playing around with it.

BTW there's quite a difference between:
S-Man you really need much much bigger smoothing caps there! Add a zero on the end of that 4700uF (will need to be mounted off board) and now we're talking;)

and
C9 only. Not actually necessary but should improve performance.
 
I would expect a good JLH with all the modern upgrades etc to beat anything on the lines of an Ion Obelisk...
Compare with £1500+ commercial amps...
 
The 2003 version of the jlh boards are on ebay I would recommend those, that is the dual version so no output cap.
 
This site contains affiliate links for which pink fish media may be compensated.
Thanks gents, that’s an interesting debate with plenty to think about. For now I think I’ll get the original PCBs and use 2N3055s, complete with output capacitor etc. This will at least be a marker, and it is the original. I like the thought of starting at the source. As regards future improvements, the construction and installation of 2 new boards won’t be the biggest job in the world, if I am sensible about the way I lay it out. I’m thinking that I’ll use an existing (old) case that houses an elderly and unreliable DVD player, and cut and shut a heatsink for it. The PSUs will be on separate boards (easy if I start with SMPS from computers or printers) and everything else is gravy. I’m looking forward to it.
 
Morning all, further progress here, in the form of a case and probably a PSU. I've been chatting to a mate and he has a couple of dead AV amplifiers from way-back-when that aren't dead so much as just disused. They are lurking in some dark corner of his houseful of hifi, and as nobody is buying Pro-Logic AV amps any more he's offered the castoffs to me. I call this a considerable win. So before I go poking around the back of his outhouse (ooer) with a multimeter to check out power rail voltages, what do people recommend? I'm going with the original circuit, 2N3055s, apparently with the regulation as described by S-man above. I imagine the AV amps are a fairly conentional 30-0-30V, if I can just steal this PSU along with the case will the JLH handle this? If not then a rummage round will probably generate an alternative, if I know what to look for.

Edit - it looks as if I have been a bit dim. On looking at the schematics again the -ve rail seems not to be a -30V rail but a 0V rail, so the thing seems to run on (say) a 30V-0V supply. S man's version shows 35V-0V at the exit from the rectifier, if I am reading it correctly, regulated down to 33V by that transistor arrangement at the top. The -ve end of the speaker output therefore runs off the 0V rail, with the +ve speaker terminal coming off the floating point generated by the 2 transistors. So I only need a single rail supply, hence the presence of only 1 BR in the PSU diagram, which should have been a bit of a giveaway. Is this now corect?
 
In the original 1969 version it's just one rail yes. You still only need one bridge rectifier even with two rails. The problem you will find with transformers (TX) from old amps is that they will have excessive voltage but often insufficient current. Very roughly, the max output power of any SS amp is determined by the PSU voltage and therefore only a TX taken from an amp of 10-15W output will be right for an original JLH... problem is that being class A the JLH draws a hefty current at all times and so any TX taken from a class A/B amp would likely be too weedy current wise.

Sometimes you may be lucky and find one with separate windings for the + & - rails ie no centre tap. You can then use these in parallel. Another way, though not as good, is to use just half the secondary of the TX.....

For 35V you will be wanting a TX secondary of 27V.

If you get stuck I believe I have toroidals available which would be ideal in two VA ratings, large ones suitable for one TX powers both channels or the smaller type for monoblocks.
 
If you find a 22-0-22 transformer, so for +-30 something rails you could half wave rectify it to produce one +30ish. Whether this would 'help' I really don't know.

IOW two diodes, pointing at the top of the reservoir capacitor, other ends to each end of the transformer, centre tap to 0v. What Naim (used to?) do, but with two centre tapped windings, one per rail.

Additionally if the output devices can stand 15v each at the standing current, then a series pass transistor can too.

Although free AV amps PSUs may be unhappy about the continuous demand of the class A amp. The worst that can happen is failure.
 
If you find a 22-0-22 transformer, so for +-30 something rails you could half wave rectify it to produce one +30ish. Whether this would 'help' I really don't know.

IOW two diodes, pointing at the top of the reservoir capacitor, other ends to each end of the transformer, centre tap to 0v. What Naim (used to?) do, but with two centre tapped windings, one per rail.

Additionally if the output devices can stand 15v each at the standing current, then a series pass transistor can too.

Although free AV amps PSUs may be unhappy about the continuous demand of the class A amp. The worst that can happen is failure.

Good idea! It would in fact still be full wave rectified when done like this. This could allow TX's from old amps to successfully be used for this yes.
Ideally then 27- 0 -27 secondary which suggests to me that a TX taken from an amp of around 50- 60 WPC should be about right when using this bi-phase rectifier set up.
You will need some pretty meaty rectifier diodes >10A and at least 200V.
 


advertisement


Back
Top