Threshold SL10 modified
 

Hello,

recently I got a Threshold SL10 and was wondering why it had a loud noise on one channel using the MC input. I investigated a little bit and found this Nelson Pass designed input stage with the 8 parallel transistors. And I realized that my SL10 is not original state. There are some modifications, pls see the picture below.
Now my question is: what do you think about these modifications, are they "improvements"?

The noise thing is another theme, it was caused by a bad contact inside my MC cartridge and the DC offset on the SL10 input.

Thanks in advance
Gerhard

Without knowing the actual brand/type of capacitors it's impossible to say if they are "improvements"

Thanks for the quick response.

The 1uF (in place of 47uF at the collectors) is a foil type "Audiophiler KP Kondensator 1uF 250V", I suppose MKP type.
The 220uF (in place of 47uF at the bases) is an electrolytic SME 47uF 25V.
The 47K resistor in place of the 15K is a metal film type.
I enclose a picture.

Thanks
Gerhard

I wonder why the brand and type is essential for answering my question.

Isn't it more important that the original nominal values of the parts have been dramatically changed?



Regards,
Gerhard:confused:

Yes, of course it is. With those mods, specifically the 1uF cap at the output, you greatly increased low cutoff frequency. Perhaps this helps with rumble noise, which I found a little bit high in the SL10 (it is a DC coupled design in MM mode), I don't know. But I think that a new, good, high quality 47uF electrolytic in place of that 1uF would be better, especially when you consider that in the original unit IIRC there were tantalum units.

A friend of mine did this (replacing all coupling tantalum, and all old electros, with new high quality electrolytics) with his SL10 and the sound greatly improved...

Hi Giaime,

thanks for your reply.

That was I thought first, too, when I realized the mod with the 1uF. The original 15K was increased 3 times, I suppose for compensating the smaller capacitor. And the 4 times bigger capacitor at the bases, what effect does it have?

I cannot hear a lack of deep bass, maybe the frequency goes deep enough.
Can anyone here calculate the theoretical frequency cut off?
:confused:

I think I listen a couple of days to it and then restore the original state. I'll keep you informed.

Thanks,
Gerhard

I'll see if I can without committing too many errors :D

Let's make a rough calculation. The stock unit has 47uF, and it sees a 15k resistor, then a 22k pot in parallel (let's forget about the 1.5k collector load for the bjts). For low settings of the volume pot, total resistance is about 9k. The -3dB frequency is about 0.4Hz.

With those mods, total resistance "seen" by the cap is about 15k, so the cutoff frequency (with 1uF cap) is ~11Hz.

Useful if you have rumble problems, absolutely: but if you haven't, I would use a larger cap, at least 10uF. Voltages in the SL10 are pretty low, so no need for 250V caps (it looks like 250V), and I bet you can find a commercial grade 10uF 63V mylar capacitor that has the same physical dimensions of that audiophile capacitor ;)

Hi Giaime,

thanks again for your kind support.

So it seems that the modifications are really well-thought, not only "improvements" following blind some "replace all electrolytics!" religions.

I'll follow your advice after hearing a few days and getting accomodated. In my opinion it sounds pretty good in the current state. Maybe it's even getting better..........:D

Can you imagine the sense of the 220uF? Is it also a subsonic filter function?

Cheers,
Gerhard

Well, personally I don't want -3dB at 11Hz. That means that 20 Hz is also affected, albeit less.

Sure this won't really affect audible frequencies, since I can't imagine records with 20 Hz frequencies (groove would be insanely deeply and widely cut), but as a matter of principle I won't like it.

If you have a reasonable turntable - and I'm sure you have - you don't need a rumble filter at all.

At least for me the questionable benefit of those Mundorfs is not worth the frequency trade-off. I would get a MKP with larger capacity.

On the other hand if you don't care, it seems to be nicely done, so why not stick with it?

Cheers, Hannes

EDIT: oh and I forgot: what would be the sense of adding another subsonic filter?

Hello all,

the green 10uF MKT sounds pretty good. As good as the (Mundorf) 1uF cap......;-))

I cannot hear a difference.

Hannes, I agree!

Thanks to all
Gerhard