Leak Stereo 20 rebuild questions

[gkargreen]

Thanks snowman, the caps across the 12K resistor is a 150 pfd, I am guessing the cowboy didn't realize the twp silver caps were across the 12K and just made up the feedback loop with what he had, same with the 1 meg, the schematic shows 1 meg but as you correctly identified the output trannies, I have replaced the 1 meg with 470K resistors. Radio, I agree with making changes to the feedback loop, we do have a guy in the states that actually does quite a bit of work on feedback loops and frequency shelving in those amps that need them reworked from the original, like the Eico HF87, various Heathkits, etc. He goes into quite a bit of detail on those changes and follows up the before and after with scope pics, I think that Dave Gillespie knows what he is doing, but not the cowboy who worked on this one!

[Radio Wrangler]

I used to design frequency synthesisers at HP, which was pretty much the deep end of tailoring the responses of feedback loops. Some audio amplifiers are designed fairly close to the border of stability problems and it isn't directly visible just how thin was the ice they were skating on. This goes for some well-renowned amplifiers. They were in fact working at the limits of valve technology, short of the big step to output transformer-less designs.

David

[gkargreen]

Quick question, for the 3 watt 100 ohm resistor, how is it soldered in place? I heard that it is basically a safety resistor such that if it gets too hot, it will melt the solder and fall out. Does that mean it should be just tack soldered in place? Thanks

[bikerhifinut]

In a word yes.
The tags should be cut in a way that if the resistor gets hot enough to melt the solder, then it falls away and cuts the HT feed.
It's crude, and very debatable as to whether it actually works but it's the authentic way to do this.
There has in the past been a bit of debate as to whether this was done on all the Leak Stereo20 amps as I remember it on the original, now defunct, Yahoo Leak Group.

Andy.

[turretslug]

That "drop-off" scheme was the original idea, I'm not sure how well it works in practice but it seems reasonable in theory. Apparently, it's not unusual to find that they've been through rebuild at some point in the past where someone has diligently wrapped the leads round the terminals thinking that the original construction was sloppy- it can go against the grain for folk who spent years working with the principle of the "secure mechanical joint"- but it defeats the safety interrupt function.

[gkargreen]

Thanks guys, I will check it carefully and solder it so that it can fall away if it gets too hot. Right now, the "cowboy" has 2x 200 ohm 5 watt wrapped around the terminals for a total of 10 watts of never falling off, what a joke!

[GrimJosef]

I'm in the 'it's far too crude ever to work' camp, I'm afraid. Resistors make bad fuses in just the same way that fuses make bad resistors. If we feel a fuse is required then we should fit a fuse. It will work more reliably when needed and cause less trouble when not needed.

The 100R is run at well over its rated temperature under normal circumstances, and while this wouldn't be too bad for the amp's expected life (5-10 years of heavy use, 10-20 years of lighter domestic use ?) after 55-60 years it will have scorched the tagboard above it and rendered itself much more prone to failure.

cheers,

GJ

[gkargreen]

Thanks again guys, I have this up and running in the system, boy does it have grunt in the low end, much more than I would have expected from a 10 watt amp! One problem I seem to have is the input gain, running my HH Scott 310 preamp down in the low end of the pot I have a bit too much non-linearity between the channels at normal operating values, if I goose it up, the channels balance much better but then I get yelled at to turn it down! Wondering if I substitute a 5751 for the input tube (12AX7) if that is OK as the 5751 is a lower gain and supposedly similar to the 12AX7, what do you think?

[Radio Wrangler]

No. Don't.

There have been several discussions over reducing the gain of the stereo 20 and the other Leak amplifiers. Search around and read them.

These amplifiers are very sensitive compared to most modern ones. This sensitivity is designed into them and its consequences run through all the original design calculations.

12AT7 and 12Au7 are pin compatible with the 12AX7 and have lower gain. BUT all the valves are in a feedback loop. The loop will attempt to restore the designed value of overall gain if you fit one of these lower gain devices. This will change the dynamics of the overall amplifier, its frequency response and its distortion and its noise floor and thee output impedance presented to the speakers.

To get the stereo 20 to what it is, a lot of careful maths was done. Certainly, the amplifier can be rejigged to make it less sensitive without having to change any valves... just resistor and capacitor values. Again, there will be consequential changes to the dynamics, frequency response and all that, but by careful design... about as much work as Leak put into designing the thing in the first place... it should be possible to make it quite good.

No-one has done this... at least no-one has done this properly. The reason being that when you've finished you have modded stereo 20, and they are worth an awful lot less on the open market than a virgin one.

There are plenty of people prepared to do all sorts of mods for you. They all make extensive claims for the improvements rendered. The biggest improvement is to their bank balances. The problem is that they wipe out most of the resale value of your amplifier. You can never get back originality, at least in the eyes of the people who pay the big bucks for them.

The best ploy is to leave it alone and just fit an attenuator in each channel's feed from your preamp. This way the sound of your amplifier is not disturbed.

It's quite common that the channel to channel matching of stereo potentiometers is poor at lower levels. The attenuators will mean your volume pot needs to be higher up where it's better matched.

One thing, having a system with too much gain and only needing to open the volume pot a crack has a psychological effect. It makes people think, 'wow that amplifier must be powerful!' once you've put attenuators in and got smoother action in the volume control, you might feel the amp is less dramatic. This is just your perceptions tricking you, the amp will be the same.

When their power amplifiers were designed, Leak made a big splash about 0.1% total harmonic distortion. THey were only just able to do this on some of their amps. To do it, they needed to use stronger feedback with more loop gain than other designers and to keep the amplifiers stable with that amount of loop gain took quite some doing. So the design is a bit like walking on a knife edge. It can be changed, but you have to know just what you're doing, and it will be a fresh design at the end. No longer a real Stereo 20. No longer having the reputation. If done properly, it will be OK, but no-one will believe you.

David

[GrimJosef]

If you've restored the input components to their original circuit values then the easiest and best solution, as David says, is to plug inline attenuators into the amp's input sockets and then to put the input signal leads into the attenuators. Personally I'd be looking for 15dB attenuators (10dB often isn't enough attenuation and 20dB might be too much). I couldn't immediately find any US ones, although I'm sure you must have them over there. Here are some 12dB ones from Harrison https://www.hlabs.com/products/attenuators/ as an example. They'd probably be OK. Other manufacturers will exist, as they say.

Cheers,

GJ

[bikerhifinut]

To add to this, and RW david has it bang to rights.

I made a connecting lead up with my own attenuator fitted into the RCA plugs on the input. If you use a decently sized plug here you can easily fit a couple of quarter watt metal film resistors inside for a nice neat job.
I'd first rig a 50 or 100k potentiometer up into the input and then rotate it until you find your optimum attenuation for use with your preamps. Then its a simple matter of the multimeter and get a air of the nearest preferred valuse to make the permanent attenuator.

This has reminded me that I scribbled out a plan to use a small project box that I could somehow fit a pair of RCA plugs spaced just right to plug into the leak phonos and have a pot inside the box connected to a pair of RCA sockets. It stumbled due to scratching the head on how to fit the plugs but i think its do able if the correct nuts can be found to screw the plug bodies in to the box.

Andy.

[bikerhifinut]

photo of homebrew attenuator lead.

A.

[Radio Wrangler]

Just remember to label your attenuator lead VERY clearly. It could cause you a lot of confusion sometime in the future when you've forgotten about it. It will lie in wait for some moment when it can cause maximum trouble.

David

[bikerhifinut]

Originally Posted by Radio Wrangler

Just remember to label your attenuator lead VERY clearly. It could cause you a lot of confusion sometime in the future when you've forgotten about it. It will lie in wait for some moment when it can cause maximum trouble.

David

A fair point.
Mind you as the only leads I use with those jumbo sized RCA plugs are the attenuated jobs I reckon I've still got enough marbles left to remember that!

A.

[bikerhifinut]

To add to David and GJ's excellent advice, I did once try a 5751 in the V1 position and unsurprisingly it had no effect whatsoever on the overall sensitivity of the amp. in any case the gain difference isn't that much different to an ECC83/12AX7. There are those who claim it gives a "darker" sound whatever that means. To me it means the frequency response has been skewed to cause an earlier than normal HF roll off...................... Add to that the exponential increase in the price of NOS 5751 in recent years and thats a lot of mugs of coffee whilst enjoying the music.

I've been using the stereo20 into a pair of Quad ESL989 for a week or two now and in its bog standard form so to speak, it's doing the job handsomely and when you consider these speakers need about 3W up their terminals just to make a normal (i.e. non deafening) room filling sound, then on transients the amps probably giving its full 12W before clip at times and it doesn''t sound stressed. As I type this I have plugged in an old mitsubishi power amp with LED bargraph power meters which on the final section of mike oldfields Ommadawn LP is telling me that its hitting between 8 and 13W on transients. Its lying of course but all the same.............

If you are thinking to use the Leak as your main amp its worth experimenting with a "pot in a box". Granted if you like tone controls and a balance control then you seem more than capable of knocking up a circuit based around a couple of op amps with a baxandall feedback tone control.
Or have a shuftie on the internet auction sites like ebay and you'll find a plethora of modules either ready built or in kit form that actually work rather well.

A.

[gkargreen]

Thanks to all for these helpful and insightful replies, they are most appreciated!

[gkargreen]

Again, thanks to all who answered questions and posted info here. I was able to get my Leak Point One stereo preamp connected to the Stereo 20 and the difference between that and my Scott 130 was not subtle. Even though the Point One is said to be an inferior preamp, I have found it to be far superior with better balance allowing my Stereo 20 to play with no noise, very nice combination!

[TonyDuell]

And that, surely, is the important thing. It doesn't matter what others claim are 'better' or 'worse' units. It doesn't matter what the technical specifications are. If a system produces a sound that you enjy listening to, then that's what you should use.

[bikerhifinut]

Originally Posted by TonyDuell

And that, surely, is the important thing. It doesn't matter what others claim are 'better' or 'worse' units. It doesn't matter what the technical specifications are. If a system produces a sound that you enjoy listening to, then that's what you should use.

Hear, hear.

Andy.

[bikerhifinut]

Originally Posted by gkargreen

Again, thanks to all who answered questions and posted info here. I was able to get my Leak Point One stereo preamp connected to the Stereo 20 and the difference between that and my Scott 130 was not subtle. Even though the Point One is said to be an inferior preamp, I have found it to be far superior with better balance allowing my Stereo 20 to play with no noise, very nice combination!

I'm not surprised.
I recently did a quick service on my Point1 stereo, just the Paper in oil and black hunts capacitors replaced for plastic films and a check over on the original resistors and voltages which all checked out. I do confess to having altered the feedback on the first stage so that all inputs are now at the same line sensitivity as the tuner and "extra" inputs but that can be reversed if ever needed. (doubtful).
The point One stereo has a nice touch with the input attenuator pots on 3 of the inputs which I find handy for balancing different source levels.
I think that having the volume and balance pots right at the the output has the effect of reducing perceived noise as you are attenuating the noise of the electronics along with the amplified signal rather than feeding an attenuated signal into the electronics so that noise levels would always be at their maximum.
It's not an issue as the wiper of the 100k volume pot is connected directly across 1 megohm of input resistance so there's no loss of response etc.
Even the Tone controls aren't too severe and can be very useful on occasion.
My only real gripe with it, is that the phono stage is frankly not up to the job given the abilities of the rest of the pre and power combination, and also the disc input is across a 100k pot which isn't best ideal for matching but whether it makes a hill of beans difference we can argue ad infinitum.
But we must remember it is a thing of its time, and for me a thing of beauty and who can't love the almost infinite serviceability compared with something modern stuffed full of microscopic SMD etc.

And as previously said, if it floats your particular boat then who really gives a ****** about anyone elses opinions.

Enjoy

A.