Leak Stereo 20 - Johan's Square Wave Theory

[bulgin]

Hello Johan

Here we are again

My little Leak gives me a lot of pleasure - so much that listening hours are limited to only a few hours per week. I usually use my Troughline 11 tuner at the same time, switch the lights all off and watch the glow...

bulgin

[Family_Dog]

Hi Bulgin,

I use my Troughline Tuner with a Leak Stereo 30+30 s/s amplifier, equally gratifying

-F_D

[bulgin]

Hi FD

But of course Now if only a Radford would come knocking on my door

bulgin

[ray_moth]

Bulgin,

Now if only a Radford would come knocking on my door

As this is a diy forum, may I make the obvious suggestion that you build a Radford for yourself? You should be able to get suitable iron from the various sources that exist - and don't forget Johan's local transformer maker.

[planet10]

Originally posted by bulgin
Hi FD

But of course Now if only a Radford would come knocking on my door

bulgin

A diy Radford inspired TL (coincidentily using a kit from the owners of the Radford tradename,

http://wduk.worldomain.net/forum/showthread.php?t=2800

and elctronics are shipping or very near...

http://www.radfordaudio.co.uk/

dave

[Family_Dog]

Why not build the original, upon which all these "aftermakes" are based? The book on building Valve Amplifiers from Mullard is still available, and you could alter them as per Byrith if you so wish.

The best thing about building your own amplifiers is that you can specifiy decent ratings for the mains & audio output transformers instead of the only-just marginal products that the Manufacturers of the time seemed to prefer.

Some good reading from the two articles here:
http://www.lundahl.se/claus_b.html

-F_D

[EC8010]

The key thing about the Radford amplifiers was their excellent output transformers - the HF resonance was beyond 300kHz. They saved money by putting their small components on a PCB.

[Johan Potgieter]

Bulgin then appeared to be interested in my heresy of finding something the matter with the Leak Stereo 20 .... but starts wishing for Radford only 3 posts down. Do I rest my case?

But seriously, to my own amazement I found that the Leak Stereo 20 was lacking in design, to the extent that I had to change certain conponent values so as to get optimal performance. I say this with care, and more so as I do not find any other comments on this on several other sites where refurbishing was discussed. But the problem was quite evident on checking with signal generator and scope, on all of 6 amps that came my way for refurbishing (3 stereo units). Also the relevant components were obviously factory fitted and not the result of tweeking. Perhaps others could comment?

The first matter that struck me was a rather starved operating point for the input triode: About 160V over the tube, only 60V over the load resistor R5. (All of the circuit diagrams I could find used the same sketch and component designations.) Also, checking voltage and current values given in the schematic, the anode current (through R5) is 0.6mA, but through R3 it is 0.56 and 0.65mA (with and without pre-amp connected). This was not serious but did set one wondering.

I ended up with R3 = 1K5 and about 120V on the anode, giving some increase in 1st triode gain. (I was not looking for that, but it did seem to indicate that the triode was now on a rather better operating point. Also, the anode signal amplitude is less than 1V for full output so no problem there - but why work in a region of comparatively high 2nd harmonic distortion?)

With R2 = 22K as given, h.f. open loop bandwidth was less than audio; cutting R2 to 5K6 and with R3 = 1K5, moved it comfortably outside 20 KHz.

Another problem to me with the original was some quite over compensation in the NFB line. The square wave rise was severely jagged and a little oscillatory. As a result of the above increase in gain I had to increase R18 from 12K - 18K for the same NFB factor, and decrease C9 down to 68pF before the square wave was almost perfect. (The equivalent capacitor for 12K would have been 100pF, not 200pF.)

Those are the points I am wondering about. Not serious audio-wise otherwise there would not have been so many satisfied owners; but the question does beg: Why less than optimal component values? I am also not commenting on an ECC81 being significantly better in the 1st stage; that would be a design change, although I did change to that in one instance.

Bulgin caught me rather quickly with this thread; I was going to post square wave pictures of what I am talking about but will have to borrow the amplifier (back) to get those done. (As said, not my own.)

So until then, am I missing something? Who else has done this?
(Or am I a compulsive tweeker? No - other amps that I have refurbished as was were Quads, Dynacos and Leak TL12s.)

Regards.

[ray_moth]

I get the feeling that I must have wandered into the wrong thread - it seems to have some strange history of which I am unaware.

Those are the points I am wondering about. Not serious audio-wise otherwise there would not have been so many satisfied owners

I wouldn't bet on that, Johan. When those things came out, hardly anyone understood much about audio, and not too many Leak customers did any comparisons with other gear to find out what they might be missing.

Why less than optimal component values?

I doubt if it was considered important - after all, who would know the difference? Like the lack of a PS choke, the overheating power tranny, the . . . I can't go on, I get depressed by it all!

[lumanauw]

Hi, Ray_moth,

Anda ada hubungan dengan TubeLover Jakarta? Sebetulnya saya mau tanya ini PM, tapi PM anda non-aktif.