Ace And Steve: Reviving The Leak Stereo 20 (Part 1)

Ace and Steve’s Big Vintage Summer Adventure: 

Update No. 1

Leak Stereo 20 Progress, and I Share a Testing Tip 

by Steve Graham

The Leak Stereo 20 is up and running.  I installed the remaining caps and resistors (Ace had already replaced about 75% of them).  I also made a couple of supports for the new B+ filter capacitors (A in picture below) mounted on the underside of the chassis.  The mount and caps are held in place with sticky-as-all-get-out, double-sided tape that only gets stickier in the presence of heat.  The original filter caps, protruding through the top of the chassis (see picture in first article of this series), have been disconnected but left in place for cosmetic purposes only.

Ace chose all of the parts, mostly new production resistors and caps with the exception of half a dozen NOS, USSR-made, paper-in-oil coupling caps (B in the picture below). 

A gentle power-up using a Variac, a few voltage measurements to confirm no freaky faults and the Stereo 20 was ready to go. Connected to Triangle speakers and a portable digital player source the joint was jumpin’ within about five minutes. The Leak’s noise level was commendably low, in line with contemporary tube amps and better than some very expensive amps.

I’m going to put 100 hours or so on the Leak then have a shootout between it and the prototype SPP EL84 project amp that dutifully performs on a near-daily basis in my workshop.

Steve’s not-so-secret noise “measurement” device

And how, do you ask, does Steve quantify noise level? With expensive test equipment perhaps? No chance. As shown in the pictures below all that’s needed is a pair of shorting plugs (make your own, about 10 bucks), a pair of inexpensive low impedance, high efficiency headphones the sort often used with portable players (50 bucks or less), a cheap jack (5 to 10 bucks) and some wire. As an option a pair of 5 to 25 ohm, 3 to 10 watt wirewound resistors (about 5 bucks) may be used or just leave your speakers connected to the amp you are “testing”.

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Disconnect the preamp or line stage from the power amp and install the shorting plugs.  Turn the amp on and wait a minute or two.  If hum and/or hiss is audible with an ear about six inches of the speakers, there is likely a problem already.  If the speakers are high efficiency horns a little hum or hiss may be acceptable.  Otherwise, with most normal efficiency speakers, nothing should be audible. 

Leave the speakers connected or switch the amp off, disconnect the speakers, connect the wire wound resistors to the speaker terminals and turn the amp back on.

Plug the headphones into the adapter jack/wire rig.  Put the ‘phones on your head.  Touch the wires from the jack across the speaker terminals and listen for hum and/or hiss.  Check both channels.   

I admit, this is a subjective test. But once you’ve checked three or more amps you will get a feeling for the hum/hiss noise level and how it might relate to what can or cannot be heard through your own speakers. Certainly, I would expect a new tube amp to have almost zero hum or hiss as heard through this test rig. If noise is more than slightly audible, I would question the efficacy of the amp’s design and/or build.

Amps I’ve “tested” so far, ranked for noise:

The WoS Tubelab SPP project amps, both the prototype and the amp detailed in the build series.

Score: Virtually noiseless.*

*This only applies to the version WITH the (~$15) power supply filter choke installed. Without the choke there was hum audible when connected to speakers.

The Leak Stereo 20.

Score: Just a very minimal amount of hum and hiss. Not audible on speakers of moderate efficiency and likely of little consequence when used with high efficiency ones.

My Audio Research Reference 110.

Score: It pains me to report that this amp (through headphones) is rather noisy. Nothing is audible when connected to medium efficiency speakers, but it bugs me all the same.

Audio Research VS60.

Score (before power supply caps replaced): About the same as the Ref 110 above.

Score (after power supply caps replaced): Somewhere between the Ref 110 and the Leak Stereo 20.

That’s it for now. Look for an update on the Fisher amps coming soon.