Hi,
Andy Evans who's a member, once started a thread on his mods for the leak 20.
While I haven't had the courage to search for it locally, I did find a webpage describing the work.
I also have a circuit diagram with Glen Croft's mods of that circuit. In case you're interested I could post it here for you.
ANDY AND CHRIS TACKLE THEIR LEAK.........
Cheers,
Andy Evans who's a member, once started a thread on his mods for the leak 20.
While I haven't had the courage to search for it locally, I did find a webpage describing the work.
I also have a circuit diagram with Glen Croft's mods of that circuit. In case you're interested I could post it here for you.
ANDY AND CHRIS TACKLE THEIR LEAK.........
Cheers,
Hi,
I happen to own a Leak Stereo 20 that was heavily tweaked by Steinmusic "http://www.steinmusic.de/" here in Germany.
I don't know what were the exact changes but knowing Holger Stein for some time he sure replaced everything not sounding to his liking. That includes using liberal amounts of his favourite ERO caps (mostly again bypassed) and also MKT in some places. Tweaking Leak's has been part of his business for the last 20 years. Maybe you'd like to give him a buzz......
rgrds,
Oliver
I happen to own a Leak Stereo 20 that was heavily tweaked by Steinmusic "http://www.steinmusic.de/" here in Germany.
I don't know what were the exact changes but knowing Holger Stein for some time he sure replaced everything not sounding to his liking. That includes using liberal amounts of his favourite ERO caps (mostly again bypassed) and also MKT in some places. Tweaking Leak's has been part of his business for the last 20 years. Maybe you'd like to give him a buzz......
rgrds,
Oliver
One of the best things you can do with a Leak is to improve the output transformer. Most loudspeakers these days are nearer to 4 Ohm than 8 - especially those using a pair of bass drivers. But if you set a Leak to match to a 4 Ohm load, you only use half of the secondary.
If you have the courage to take the shells off the output transformer, you will find that the individual windings are terminated on cardboard strips glued to the coil. You can then reconnect them as shown in the right hand diagram so that you use both windings - which makes for a transformer with reduced leakage inductance - a better transformer, and it cost only a little time. You will probably go one step further and connect new wires directly to the transformer and throw away the phenolic tagboard at the base of the transformer and its associated old wires.
Having tweaked the secondary, you'll probably look at the primary and see that it's possible to replace the wires there too. The PVC-covered wires solder to little solder tags inside the paper windings, and it's perfectly possible to desolder them and replace them with longer wires that go directly to the valve sockets.
Although it's highly desirable, reducing the gain of a Stereo 20 can't be done easily. You'd probably do better rebuilding it completely with a different driver. A differential pair with CCS might be nice. There was a thread about this recently.
If you have the courage to take the shells off the output transformer, you will find that the individual windings are terminated on cardboard strips glued to the coil. You can then reconnect them as shown in the right hand diagram so that you use both windings - which makes for a transformer with reduced leakage inductance - a better transformer, and it cost only a little time. You will probably go one step further and connect new wires directly to the transformer and throw away the phenolic tagboard at the base of the transformer and its associated old wires.
Having tweaked the secondary, you'll probably look at the primary and see that it's possible to replace the wires there too. The PVC-covered wires solder to little solder tags inside the paper windings, and it's perfectly possible to desolder them and replace them with longer wires that go directly to the valve sockets.
Although it's highly desirable, reducing the gain of a Stereo 20 can't be done easily. You'd probably do better rebuilding it completely with a different driver. A differential pair with CCS might be nice. There was a thread about this recently.
Attachments
OK, in that case, I'd agree with EC8010; a nice CC diff amp would be perfect. With EL84s, you don't need huge amounts of swing, making the circuit easier- maybe cascoded ECC88s. Morgan Jones recommended a two tube circuit using a concertina, and while that would certainly work well, a cascode CC diff amp has a certain symmetry and elegance that I find irresistable. If you want to go low feedback, a simple CCDA will work well.
Once you get rid of that big tagboard, a lot of room becomes available above and below the chassis; room for an HT choke and a big HT capacitor for the output stage. By comparison, a transformer to provide a -15V supply (which could also be used for grid bias of the output valves) is trivial. Give it some thought. Without the negative supply, your options are more limited.
I've done so many mods to Stereo 20s I've lost track. EC8010 is right on the button here. The transformer isn't great as it is - I prefer even a 6.6K Maplin one. You have to do something about the power supply. I'd take out the tag board and put in chokes for a start - I use two Maplin ones 10H, 100mA. these actually bolt in directly under the OPTs using the same holes. I've also managed to get all polypropylene PSU caps in there, and I'm looking to add two more chokes, one per side, for the input section. I went over to zero global NFB a while ago and don't regret it. I drilled out the first valve base to take an octal socket and took out the cap the other side to put another octal socket in. I'm not happy with any of the 9 pin input tubes, and I'd rather use 6SN7/6J5 types. I ran 2C22 into 12b4 concertina for a while - that was good. Now I've gone over to a simple 6SN7 diff pair in the input and it sounds just as good. This is the way forward. I have 47K on the anodes of the diff pair, and 500R bypassed with a polypropylene 47uf on the cathode. Not ideal, but sounds good anyway. Anode voltages are surprisingly close. Grid 2 grounded. coupling caps are Russian Teflon - that's not negotiable in my book unless you like exotic paper in oils. Simple 2 stage, no global feedback. Makes the original circuit look positively schizophrenic, and sounds miles better. and now to look at the transformer mods suggested!! Nice post. Andy
leak
Hi All.
As someone who has advised Andy on many occasions on his '20', it seems that many people miss the vital modifications that provide a sonic step up.
One of the first modifications I did to the amplifier, was to DC couple the first and second stages, this provided 3 important levels of improvement.
1. To remove an interstage capacitor.
2. Remove resistors on the second stage that were causing sonic
negatives.
3. The ability to change both valves at will without biasing
problems due to the now floating second stage.
We found, just this simple mod improved matters substantially and was simple to do.
Playing about with output transformer impedances is a little dangerous, not only in terms of not completely knowing the final loading on the output valves but also in terms of providing a known stable base.
Personally, the output transformer used in the '20' is not up to current standards but it is something that took a fair amount of time to develope and in this, extreme care is advised and must be respected.
When it comes to output transformers, you need to know a lot more than the schematic provides, each winding will have its own relationship and interaction to its position on the core and changes here can seriously affect the working of the amplifier if not measured properly or monitored.
It is a sad fact that a lot of tweakers have very little knowledge in electronics or lack the very basic requirements in test equipment and this has to pointed out.
Also, no one but yourself can be responsible if the amplifier blows up or causes harm, as have been found with several home built units I have repaired.
If you want to discuss this amplifier, specify the changes and I or other knowledgeable people can advise on the best route to take.
Regards and a safe new year
Chris
Hi All.
As someone who has advised Andy on many occasions on his '20', it seems that many people miss the vital modifications that provide a sonic step up.
One of the first modifications I did to the amplifier, was to DC couple the first and second stages, this provided 3 important levels of improvement.
1. To remove an interstage capacitor.
2. Remove resistors on the second stage that were causing sonic
negatives.
3. The ability to change both valves at will without biasing
problems due to the now floating second stage.
We found, just this simple mod improved matters substantially and was simple to do.
Playing about with output transformer impedances is a little dangerous, not only in terms of not completely knowing the final loading on the output valves but also in terms of providing a known stable base.
Personally, the output transformer used in the '20' is not up to current standards but it is something that took a fair amount of time to develope and in this, extreme care is advised and must be respected.
When it comes to output transformers, you need to know a lot more than the schematic provides, each winding will have its own relationship and interaction to its position on the core and changes here can seriously affect the working of the amplifier if not measured properly or monitored.
It is a sad fact that a lot of tweakers have very little knowledge in electronics or lack the very basic requirements in test equipment and this has to pointed out.
Also, no one but yourself can be responsible if the amplifier blows up or causes harm, as have been found with several home built units I have repaired.
If you want to discuss this amplifier, specify the changes and I or other knowledgeable people can advise on the best route to take.
Regards and a safe new year
Chris
Chris has raised some points about the output transformer, and I've been trying to email EC8010 to clarify. The diagram seems to show two windings, joined in the centre where the 4R tap is. I'm presuming that the bottom of winding one (going upwards) is 'start' and that to stay in phase this should be joined to the bottom of winding 2 (upper winding) where it presently joins the 4R tap. The other thing I'm unclear about is that if you parallel the two secondaries, do you still get the same impedence ratio? I wonder if EC8010 could just clarify these details, plus give any indication of benefits in sound quality having done the mod? Andy
An alternative view
I just don't get it: why would anyone want to mod a classic well sought after Leak?
Let's be realistic - the Leak is not a bad amp but will never really fly with the best. Seeing how much collectors pay for mint amps, why mess around and destroy the collectors value? With the proceeds of an ebay sale one can buy real transformers, real caps and real tubes and with very little effort build a real amp.
I just don't get it: why would anyone want to mod a classic well sought after Leak?
Let's be realistic - the Leak is not a bad amp but will never really fly with the best. Seeing how much collectors pay for mint amps, why mess around and destroy the collectors value? With the proceeds of an ebay sale one can buy real transformers, real caps and real tubes and with very little effort build a real amp.
I just don't get it: why would anyone want to mod a classic well sought after Leak?>
Because it's a cherished item I bought new with my father, and started my interest in hi-fi. I want to keep it but I also want it to sound as good as possible - my other requirement for an amp I listen to.
I agree that rarer classics should be left original, but the Stereo 20 is so common that one hardly feels any guilt at doing whatever one wants with it.
Because it's a cherished item I bought new with my father, and started my interest in hi-fi. I want to keep it but I also want it to sound as good as possible - my other requirement for an amp I listen to.
I agree that rarer classics should be left original, but the Stereo 20 is so common that one hardly feels any guilt at doing whatever one wants with it.
Update on the output transformer from the Leak group. There are three output windings. Two are already in parallel. For the third, EC8010 is right. The third winding - looking at EC8010's useful diagram - starts at the 4 ohm tap and goes up to the 15 ohm tap. this winding should be re-connected, the 4 ohm side to negative and the 15 ohm to positive. I hope this is clear. Haven't tried it yet.
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