Leak Stereo 20 gain

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Michael L
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#1 Leak Stereo 20 gain

Post by Michael L »

Could anyone suggest the best way of reducing the gain of my Leak Stereo 20.
I can't use the turntable because volume is a bit too high on lowest preamp volume.
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IslandPink
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#2 Re: Leak Stereo 20 gain

Post by IslandPink »

What's the input to the Stereo 20 look like ? ( circuit ) . Should be pretty easy to mod some sort of voltage-divider on the input to the first valve's grid , even assuming it doesn't have a volume control on the input.
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Michael L
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#3 Re: Leak Stereo 20 gain

Post by Michael L »

Image

Here's the schematic showing one channel
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IslandPink
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#4 Re: Leak Stereo 20 gain

Post by IslandPink »

I need to do a sketch ; but basically, form a voltage divider, by :
... reduce the 1M grid-leak resistor to eg. 330k, and put a resistor eg. 470k to 1M on the way from the input to the junction where the 1M comes down. Have a look what resistors you have. Carbon composition preferred.
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Michael L
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#5 Re: Leak Stereo 20 gain

Post by Michael L »

IslandPink wrote: Sun Feb 12, 2017 10:21 pm I need to do a sketch ; but basically, form a voltage divider, by :
... reduce the 1M grid-leak resistor to eg. 330k, and put a resistor eg. 470k to 1M on the way from the input to the junction where the 1M comes down. Have a look what resistors you have. Carbon composition preferred.
Ok thanks. I didn't think it could be that easy. Will see what I have.
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Mike H
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#6 Re: Leak Stereo 20 gain

Post by Mike H »

What Mark said, including being carbon composite, as the type of resistor may colour the sound. Depending on your personal taste.
 
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Michael L
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#7 Re: Leak Stereo 20 gain

Post by Michael L »

Mike H wrote: Mon Feb 13, 2017 12:22 pm What Mark said, including being carbon composite, as the type of resistor may colour the sound. Depending on your personal taste.
I changed most of the resistors and all the capacitors. Just a few of the hard to get at resistors still in place.
If I remember correctly all the replacements were carbon film. I had read that metal film didn't sound as good.
Are carbon composite different?
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#8 Re: Leak Stereo 20 gain

Post by IslandPink »

Carbon composition (aka Allen Bradley ) are essentially amorphous and non-inductive. I prefer them for small-signal work, especially in phonos. Carbon films have a spiral cut into the film that form the resistive element ( like metal films ) but the spiral has fewer turns and is less inductive than metal films. Or .. maybe it's the way the end-caps are attached.
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Mike H
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#9 Re: Leak Stereo 20 gain

Post by Mike H »

I use these:

https://www.rapidonline.com/carbon-composition


I also use carbon film.
 
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#10 Re: Leak Stereo 20 gain

Post by Michael L »

I used 470k grid bias resistors and 1M resistors in series with the source. I also swapped out the remaining resistors I hadn't finished doing when I changed the caps. Cleaned up the valve pins with a fibre pen. I have a 4W 100 ohm soldered in such a way as it would hopefully drop out if it overheated though I would prefer fuse protection.
I put some nice new phono sockets in too. It's sounding much better and the volume reduction is just right.
I only had carbon film resistors but I'm happy with the sound.
I'm still unsure about the speakers I'm using with it - World Audio KLS2. With some material they sound a bit bright though generally they are pleasing. Has anyone used/made any of these? I bought them on t'bay.

Update Sunday morning - I realise the amp is still a tad loud on my pre's 1st click. Funny how things seemed different after a couple of beers. Need to experiment some more.
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#11 Re: Leak Stereo 20 gain

Post by IslandPink »

Ok, good work - looks like 330k or 270k to ground then.
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#12 Re: Leak Stereo 20 gain

Post by Michael L »

IslandPink wrote: Sun Feb 26, 2017 12:15 pm Ok, good work - looks like 330k or 270k to ground then.
Is there any problem with dropping to 270k when the original resistor was 1M?
I was going to increase the value of the resistors going to the phono sockets.
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Mike H
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#13 Re: Leak Stereo 20 gain

Post by Mike H »

No I wouldn't want to increase that any more.

Actually 750k -> 220k might preferable, closer to the original 1M input impedance.
 
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#14 Re: Leak Stereo 20 gain

Post by Michael L »

Mike H wrote: Sun Feb 26, 2017 4:15 pm No I wouldn't want to increase that any more.

Actually 750k -> 220k might preferable, closer to the original 1M input impedance.
Mike does this mean that I don't have to worry about the value of R1 as long as the input resistance total presented to the source remains around 1M?
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Mike H
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#15 Re: Leak Stereo 20 gain

Post by Mike H »

If you want to keep to its original input impedance.

But to confuse things even further, if you know or can find out what load impedance the source can drive, then I would think about making it that.

So if for example the source can drive 100k (quite a common value for input impedances) then I could think it terms of making the resistor divider 75k and 22k.

As it stands, having such a large value resistor in series with the first valve grid might be a problem with Miller effect, i.e., the input capacitance of the valve.
 
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