Changing input sensitivities on a Leak Varislope 2 Stereo

[JohnM73]

Hi.

I know general consensus is Varislopes are VERY much off the pace by modern standards, but I have one which was left to me by my Great Uncle which has sentimental value to me. I rebuilt his Stereo 20 - sticking to the original stock component values - a couple of years back and remain astonished at how good it sounds. It would be nice to also get his Varislope working too, or as well as its design allows. Whilst I have a good quality passive pre-amp/phono stage, it’d be fun to hook up the Varislope now and then with the amp it was purchased with for a bit of nostalgia.

Sadly just about all the components are out of spec. So I’ve been replacing all resistors with modern metal films, polypropylene ‘Orange Drop’ coupling caps, and polystyrene for the lower value ones, on the main circuit board.

To make component replacement less of a nightmare for this section, I have desoldered the input selector switch assembly (SWI/SW2 on the schematic), to make it easier to replace the input sensitivity setting resistors/caps.

It occurred to me that whilst it’s out anyway (and I only want to do this job once) it would be worth seeing if the input sensitivity values can be tweaked a tad to better match the sources I’ll be using, without changing the overall design (i.e. it’s still a Varislope 2 Stereo for good/bad, just with tweaked input sensitivity values).

Whilst my soldering and constructional skills are very good, maths was never my strong point (even less so since illness alas), so I’m looking for any suggestions as to suitable component changes to tweak the inputs.

[JohnM73]

Posted below are the changes I’m ideally looking at per input, with attached circuit images of each input highlighted in yellow. Ideally I’d also like to remove the input adjusting pots and replace with resistors to lower a source of potential noise a tad. I’ll be leaving the Tuner input values as stock, incase I ever fancy using my Troughline Stereo tuner in the future.


1. ‘Mic’ = Aux 1. Convert for use with a DAC or CD, so around 0.7-1V input sensitivity required. Could I simply copy the 1V input sensitivity values from the ‘Extra/Lo switch’ original stereo Varislope (the earlier stereo model before my grey one), which is 2M2 across each input socket, with 150k resistance on the input switch wafers? (stock input circuit image posted below highlighted in green). Is 2M2 too high an input impedence these days? Apologies for being utterly lost at sea when it comes to these calculations.

2. ‘Tape Head’ = Aux 2. Convert to a 2nd line-level input by duplicating the values from the above. I assume one would also remove R47 (47M-ohm) between the Pickup and Tape Head switching gear?

3. ‘Tape Amp’ = Tape Replay. Is this input suitable for ‘modern’ (i.e. 80s/90s) cassette deck playback using the stock circuit values (125mV input, 70-100k input impedence)? My tape deck’s playback output = 400mV @ 800 ohms.

4. ‘Record Output’ = Hi/Lo output levels selected by switch. Stock values: Hi = 125mV. Lo = 12.5mV. According to the Varislope manual, it requires 100k input impedance or more on the tape deck’s record input. My tape deck’s record input = 60mV @ 47k. Will the mismatch between the 100k expected input impedence vs my Tape Deck’s 47k record input be an issue?

5. ‘Pickup’ = Lightly tweak to better match modern MM cartridges. Can I simply replace the 100k pot on the input with 47k resistors across each channels phono input socket instead? Would I need to add a 100pF loading capacitor across each phono input socket too?


Have posted excerpts from the Varislope Stereo, and the Varislope 2 Stereo’s schematic below that show the stock circuit values per input. As the full circuits are available at www.service-data.com, I believe it’s against forum rules for me to post the full Leak Varislope schematic here, but excerpts are OK? (Apologies if I have misunderstood this).

Thanks in advance for any help/advice.

John.

[Jez1234]

I partially rebuilt mine with modern 1% low noise resistors, polypropylene caps etc and can report that although it still ain't the worlds best preamp or anything it is MUCH better especially in terms of noise and also of course the frying bacon and crackles from the awful old resistors is gone.

I wouldn't connect up "Tape Out" unless you are making a recording. Whilst it won't do any harm it will increase the distortion and pull down the output level a little. It won't do this if switched to low level tape out but there won't be enough output for any modern recorder either....

If you have a switch for two record out levels then that suggests it is a Varislope 2 hence it should have variable input sensitivity rather than just high and low on the Varislope 1.

[JohnM73]

Thanks Jez. Yes I likely wouldn’t be using tape very often anyway, so think I’ll leave the Tape Amp and Tape Out with the stock circuit values, albeit with new resistors.

Yes it’s a Varislope 2 Stereo, with variable (70k-100k) pots for Tape Amp, Tuner and Phono, and a Hi/Lo switch for Record Output. I wish Leak would have noted in the manual what the input sensitivity figures are with the pots at their min/max settings. Actually the manual doesn’t mention the pots at all.

[vidjoman]

Originally Posted by JohnM73

I wish Leak would have noted in the manual what the input sensitivity figures are with the pots at their min/max settings. Actually the manual doesn’t mention the pots at all.

Leak would have quoted the figures with the pots at maximum. No point quoting at minimum as the slider is grounded.

[JohnM73]

Originally Posted by vidjoman

Leak would have quoted the figures with the pots at maximum. No point quoting at minimum as the slider is grounded.

Thanks for clarifying that.

[JohnM73]

Thanks to a member here emailing me (cheers John!), I’ll be able to make some of the mods I mentioned in my first posts. Looks like I’ll be changing the Mic and Pickup inputs only now, as the others are more complex to alter, and in the case of modding the ‘Tape Head’ input, it appears it shares some components with the RIAA stage.

I could remove the RIAA stage completely and free up two more inputs for line level use, but I rather like the idea of still being able to use the Varislope’s phono stage, despite the fact it’s not considered all that good today.

So I’m erring on the side of keeping it simple. I’ve been advised to change R11 to 27k on the ‘Mic’ input, which will allow this to be more suitable for CD/Dac use. I would have simply used the Tuner input instead, but am keeping that for if/when I get my stereo Troughline serviced. On the subject of the altered Mic / line level input, do I also need to add any resistors across the phono inputs to set impedence, in addition to changing R11 to 27k?

Lastly, for the revised Pickup input, I’ve been advised to remove the input level pot, and instead add 75k resistor to ground across each rca socket, to better match a modern MM cartridge, along with whatever loading capacitance is recommended for the cartridge (I’ll likely only add 50pF or so).

Unless anybody else has any further suggestions, I’ll proceed with ordering the resistors needded to revise the Mic and Pickup inputs. Thanks,

John.

[Jez1234]

A quick tip on the FM tuner front. Get a Leak Stereofetic tuner to go with the varislope. The styling matches it and it is a much better tuner. In spite of all the stuff one may read on the internet about what a wonderful tuner the Troughline Stereo is well I'm afraid it's not. Not by a long chalk in fact. The early and crude Mullard stereo decoder in it is dire and overall results sound much like MW radio sound quality but with lower background noise and some stereo effect.

[JohnM73]

Before I go and order the resistors to convert the Mic and Phono inputs, can anybody let me know if I need to do anything other than simply changing the Mic’s R11 to 27k please? Do I need to add any resistors across the phono input socket too to set impedence?

And for the Phono alteration, anything else I should do in addition to removing the input pots, and replacing with a 75k resistor across each phono socket? Ta.

[Jez1234]

I assume you are wanting to make both line level inputs here? You need to remove the capacitors in the phono EQ C4 and C5 (you can just short out C4 with soldered wire link) and change value of R8 to suit sensitivity required. It's never going to be super low noise due to the size of R1 120K as the input resistor of the inverting shunt feedback topology but you could, assuming using modern sources that don't need to see a super high impedance, make it 47K and scale the other feedback resistors proportionally for a useful improvement in S/N ratio.

[JohnM73]

Thanks for that Jez, but the phono/pickup stage is now being left as-is (back in post 7), but just tweaked to be better suited to modern MM cartridges (will save your suggestion incase I do decide to change phononto line level in the future though, ta).

To tweak the phono stage a tad, it was suggested for me to remove the 100k input pots for the phono stage, and to instead solder in a 75k resistor across each phono input socket, along with whatever loading cap pF value is suitable for cart in question, so around 50-100pF. I was told this would better suit a modern MM expecting a 47k load?

The Mic input is the one being changed to better suit line level. Here it was suggested that I change R11 to 27k. But I was wondering if any resistors needed to to across each Mic phono socket to set impedence?

P.S. Noted about the Troughline Stereo too. But it belonged to my Uncle along with the Varislope and Stereo 20, so will be using it from time to time just for a bit of sentimental nostalgia.

[Jez1234]

Ah right gotcha. That should be just fine as you suggest then. You don't need to add resistors across the mic inputs no. 27K will of course make that input much less sensitive than the others but that's prob a good thing with modern source levels.

[JohnM73]

Thanks for confirming Jez.