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Old 11th November 2007, 09:03 AM   #1
iant187 is offline iant187  United Kingdom
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Default Noob to the Leak ST20

Hello, I'm looking to get a tube amp, however I know just about nothing about electronics and have about the same absence in knowledge of valves. (I'm not generally stupid just ignorant in electronics - if that clarifies things!)

So I'm thinking the learning curve in electronics is going to be too great initially for me to go and build an amp from scratch and I'm a very impatient guy, so I was thinking about buying a Leak Stereo 20 (that worked and was serviced etc.) this would get me going and I could then learn more from there!

However again I've had a quick look at the Leak Stereo 20 and I can't even see what the switch cable is for and why some have changed it for a toggle switch and where the inputs are and whats the connector at the front...... I know this makes me probably sound quite simple to you experienced bunch, but is this a good way forward (buying the Leak 20) and if so is there an instruction book somewhere to show me how its suppossed to be set up.
NB Will the Leak be OK for powering my KEF floorstanding speakers or would I need something in between (or new speakers!)

Many thanks for any help..
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Old 11th November 2007, 10:49 AM   #2
dnsey is offline dnsey  United Kingdom
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I'd suggest that you listen to a decent ST20 before making a decision. For all the hype, they're not all that wonderful IMHO, and are overpriced these days - you might well get a better deal with a modern Far Eastern model, or even a pair of Mullard 5-10s, which sound much better to me.

To answer your other questions, the 'umbilical' connector and switch connector were intended for use with Leak's preamps or similar, most of which are by modern standards very mediocre, and not worth using. You can fit a separate switch across the switch terminals, or bridge them and switch the supply to the amp however you choose. The inputs should go to the phono sockets on top of the chassis, which are paralleled with the umbilical inputs. You may need to update these sockets, or obtain vintage style 'long pin' phono plugs to suit. Similarly, the amp uses difficult to obtain speaker connectors (at least in some versions), and are better changed for standard 4mm sockets / binding posts.

For normal domestic purposes, the ST20 will be quite adequate for KEFs of a similar vintage.
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Old 11th November 2007, 02:35 PM   #3
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iant187,

Something to be aware of about the Leak amp is that it has WAY too much gain to be used with a CDP as the source. Dealing with that issue is not particularly difficult. Since learning how to read a schematic is something you MUST do, I'm linking the ST20 diagram. Look here. Please observe that a 12AX7/ECC83 is shared between the I/Ps of both channels. Replacement of that tube by a 12AY7 will reduce overall gain. Unfortunately, the reduction is likely to be insufficient. Buy a 50 KOhm stereo volume control and a pair of 1% tolerance 47 KOhm resistors. Connect each of the "hot" wires from the CDP to an end of a 47 KOhm resistor. Connect each remaining resistor end to the "top" terminal of the control in each of the channels. The wipers of the control connect to the "hot" terminals of the amp. In each channel: the "cold" (ground) wire from the CDP, the "bottom" of the control, and the "cold" wire to the amp are connected together. Short, LOW capacitance cabling between the volume control and the amp is essential.
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Old 11th November 2007, 03:48 PM   #4
EC8010 is offline EC8010  United Kingdom
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Eli is absolutely right in saying that the Stereo 20 has far too much gain. However, simply replacing the input ECC83 with a lower gain valve will only lower the amount of global negative feedback available, not the closed loop gain. You also need to change the feedback resistors and their associated compensation. And that's rather harder. Frankly, it's not really worth the trouble. You'd do better to treat the Stereo 20 as a chassis with a set of transformers, and build a more modern circuit with a more sensible gain. In the meantime, just use it as is with an attenuator on the input.
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Old 11th November 2007, 06:29 PM   #5
iant187 is offline iant187  United Kingdom
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Oooo thanks for the replies guys.

Typical of me just wanting to jump sraight in and play with something, I'm glad I took the time to post!

In that case perhaps I'll have a go at building a very basic amp first (I guess there must be a load of posts about that somewhere, but if you have any suggestions I'd appreciate any help).

I was thinking as an experiment to get me going in the world of valve amps, that I'd like something just small and simple that could power my iPod for now - as I said above if you've got any circuit diagrams or suggestions or kits then I'd appreciate any help.

Regards
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Old 11th November 2007, 06:42 PM   #6
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Quote:
You'd do better to treat the Stereo 20 as a chassis with a set of transformers, and build a more modern circuit with a more sensible gain.
That's exactly what SHiFTY is doing. Look here. Staying with EL84 O/P tubes is OK, but a small toroid power trafo needs squeezing in to provide B-. Switching to the 6V6 family as "finals" allows the 5 VAC winding to power the B- rail. Switch to a 6BY5 for the rectifier.
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Old 11th November 2007, 09:21 PM   #7
iant187 is offline iant187  United Kingdom
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Quote:
Originally posted by Eli Duttman


That's exactly what SHiFTY is doing. Look here. Staying with EL84 O/P tubes is OK, but a small toroid power trafo needs squeezing in to provide B-. Switching to the 6V6 family as "finals" allows the 5 VAC winding to power the B- rail. Switch to a 6BY5 for the rectifier.
Wooohhhhh.... lets go back to the bit where I said I was a noobie
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Old 11th November 2007, 11:23 PM   #8
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Quote:
Originally posted by iant187
Oooo thanks for the replies guys.

Typical of me just wanting to jump sraight in and play with something, I'm glad I took the time to post!

In that case perhaps I'll have a go at building a very basic amp first (I guess there must be a load of posts about that somewhere, but if you have any suggestions I'd appreciate any help).

I was thinking as an experiment to get me going in the world of valve amps, that I'd like something just small and simple that could power my iPod for now - as I said above if you've got any circuit diagrams or suggestions or kits then I'd appreciate any help.

Regards

Dude,

Before you spend a single penny, please tell us the nominal impedance and efficiency ratings of your KEF speakers. Mating the speakers to the wrong amp will cause you considerable grief.

EC8010 is spot on about not using a 12AY7 in the Leak ST20. I overlooked the fact that the tube is inside a global NFB loop. IF the ST20 is suitable for mating to your KEF speakers, you will build a passive attenuator along the lines previously mentioned. However, the parts values will change. Buy a 10 KOhm stereo volume control and 90.9 KOhm 1% tolerance fixed resistors. A passive attenuator is an excellent 1st project for a complete "noob".

When you feel ready to attempt your 1st scratch build of an amp, may I immodestly suggest "El Cheapo". Read the lengthy saga here.
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Old 12th November 2007, 10:15 PM   #9
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I use the Leak TL12.1 and theya re wonderfull I used to have the LeakTL12+ which are really the mono version on the Stereo 20.

I have had a hard time finding something that sounds better tha my Leak tl12.1, by the way they are muuuch better than the Quad II.

About gain issues: Yes its high, good for passive preamp, or volume control attenuator. I use a DIY 1k shunt type, but the EVS 1k attenuator will work wonderfully, no need for a preamp.
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Old 13th November 2007, 10:25 PM   #10
iant187 is offline iant187  United Kingdom
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Quote:
Originally posted by Eli Duttman



Dude,

Before you spend a single penny, please tell us the nominal impedance and efficiency ratings of your KEF speakers. Mating the speakers to the wrong amp will cause you considerable grief.

EC8010 is spot on about not using a 12AY7 in the Leak ST20. I overlooked the fact that the tube is inside a global NFB loop. IF the ST20 is suitable for mating to your KEF speakers, you will build a passive attenuator along the lines previously mentioned. However, the parts values will change. Buy a 10 KOhm stereo volume control and 90.9 KOhm 1% tolerance fixed resistors. A passive attenuator is an excellent 1st project for a complete "noob".

When you feel ready to attempt your 1st scratch build of an amp, may I immodestly suggest "El Cheapo". Read the lengthy saga here.
Thanks Eli, The speakers I was going to use with the amp actually are B & W 601 S2's, which are 8 ohm and 90db sensitivity - not sure what my Kefs are! but I thought the B & W would be more suitable as prob a bit more sensiive.
I'm perfectly happy with El cheapo for now, just want to get my hands wet so to speak!)
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