SR45 amplifier

debk

debk

New member
I am thinking of building the SR45 amplifier from the schematic that is posted elsewhere.

Are the boards that were used in the upgrade kit still available?  I can get the components elsewhere, but I would prefer not to have to design boards for the project. 

From what I have read this was a fantastic sounding amp.  How does it compare to the Paramounts modified for 45's

Thanks
Debra
 
I haven't done a 45 Paramount. There would be the advantage of direct coupling over the original SR45 however it wouldn't have the shunt regulated output stage. So it's hard to say which you might like better. I'm thinking it might be possible to do an SR45 on a Stereomour chassis. If it's feasible and there is enough interest perhaps we could do another limited run for advanced builders who don't need dot to dot instructions (we can't justify taking many man hours to write a step by step manual for a product that we might only make five or six copies of). At any rate, yes, I'm sure we have PC boards that could be used for the purpose.
 
The original SR45 used the PT-2 power transformer, long since out of production. That transformer had the 2.5v heater winding.

I always thought that the refinement of a fully shunt regulated 45 amp was compromised by the hum from the AC heater supply. I do have sketches and notes in development for a more modern version with DC heater supply. The critical PC board is the current source feeding the shunt regulator, and I am revising the Crack Speedball board to cover such an application (and to use a slightly larger heat sink). That board should be in production in a few months if things go as currently planned. This summer looks like a good time for prototyping!

In both designs, the driver is cap-coupled. At the time, we thought (and I for one still think) that the advantage of shunt regulation is greater than that of direct coupling, especially if you are using really high quality coupling caps (teflon V-caps are I believe still at the top of the list). Direct coupling pushes the power supply voltage higher, to beyond the ratings of currently available the solid-state components in the shunt reg and current source. At that point, the safety issues get really hairy too, so I haven't looked into the necessary re-design issues.
 
Small correction - Eileen has just advised that the PT-2 can be had, for a price, in small quantities.
 
If I were to build this I would like to do it with DC heater supply.  I am sure I can get a power transformer from the usually sources or have one wound that will work.

I am in no hurry, having just built a pair of 300b DRD amps,  but am  I am interested in getting a couple of the boards when you have them available.

Thanks

Debra
 
The version I built, single chassis and power supply, like a Stereomour. As to hum I use AC and  have inaudible hum. I centered the hum pots when I wired them up and have never needed to adjust them.

One thing I did. 2.5V transformers were out of stock when I built mine, so I took a 5V Hammond with a center tap and did a little surgery. turns out it is 2 separate windings, joined at one end to make the tap. I separated them to make 2, 2.5V windings on a common core. not sure if this bucks the hum or not.

I would try AC first, unless you have 100db speakers, I don't think you need it         
 
Debra,

I'd go with the AC filaments on the Orcas -- you should hear nothing like hum from  that pairing.

In fact, I'll test it out for you here as soon as I have a preamp as I just unboxed my SR-45s this morning.

-- Jim
 
I just measured mine, I have less than 2mv at the outputs with the input shorted. How much less, is hard to say. I didn
 
To measure small levels of hum, I twist the meter leads together - it minimizes pickup of magnetic fields, typically from the power transformer but they are everywhere in a modern house. Ideally you would use coax.
 
Paul Joppa said:
To measure small levels of hum, I twist the meter leads together - it minimizes pickup of magnetic fields, typically from the power transformer but they are everywhere in a modern house. Ideally you would use coax.
Click to expand...

We've been doing some empirical research in the lab with cables that past few days. The improvement in magnetic field rejection of coax vs. twisted pair is pretty amazing. We've been plugging various interconnect cables into a S.E.X. amp and listening thru headphones while moving the cable around very close to the power transformer. Some cable made with inexpensive Belden coax stomped everything else we tried for killing magnetically induced hum.

Before everyone runs out and changes over to cheapo Radio Shack coax interconnects (which worked great too), the downside is the coax we tried didn't sound as good as our hand made twisted pair in braided shield cables do. But I think it might be interesting to look into using coax for power supply wiring near a power transformer.

Perhaps the simple lesson from this is don't run your interconnects close to power transformers.
 
Put me down for a pair of the PT2 transformers as well.

Do we have a count yet and how would we go about making this happen?

If we need somebody to coordinate a group buy, I'll be3 glad to do it, but I probably should wait until we are moved into the new plac ce -- say late August sometime.

Thanks,

Jim
 
I actually want to build a pair of paramour IIs for 2a3s, so yes, would need an appropriate c4s/driver board for that amp as well.

This way I can leave the stereomour in the other system and not have to worry about moving it between systems.

-- Jim
 
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