SR45 amplifier

[johnsonad]

Just a ping for an anticipated project. We are awaiting it with baited breath PJ!

Half of my custom iron has arrived for this project already.   In the mean time I've been enjoying a Firstwatt SIT 2 amp (impressive). I anticipate that the SR45 will trump in in sound but not in power.

[Paul Joppa]

Thanks Aaron, I need the push. There's another project just ahead of it but I should get back to it shortly.

[xcortes]

Great Paul!

[Natural Sound]

Where might I pick up a schematic for the SR45?

[Paul Joppa]

The SR45 was a one-time design, never a full kit, and is not currently supported. I am working on a replacement to resolve some of the issues with the first one. I discourage spreading the design around (beyond original purchasers) for several reasons, both technical and professional. For the latter, see post #58 in this thread.

[Natural Sound]

The SR45 was a one-time design, never a full kit, and is not currently supported. I am working on a replacement to resolve some of the issues with the first one. I discourage spreading the design around (beyond original purchasers) for several reasons, both technical and professional. For the latter, see post #58 in this thread.

I understand and respect that, Paul. I'll wait patiently until the replacement arrives. Please include me in the category of interested and considering purchase.

[Jim R.]

Tom, YGPM...

Also, I tried the SR-45s with my preamp and the Orcas and while a tiny bit better than with the nagas in terms of max volume, the fact is that there simply is not enough gain in this system, so I'm afraid I'm going to have to give up on the SR-45s.  I'm afraid it will have to be the 2a3 paramounts that will poewr my blumenstein Nagas -- not that's a bad thing, but I was looking forward to the SR-45s as my mainstay amp.

PJ, I assume the new design will not have any significantly higher gain?

Thanks,

Jim

[braubeat]

Just another vote for this project. The minute it's available I

[Paul Joppa]

... PJ, I assume the new design will not have any significantly higher gain? ...

So far, it has been my plan to repeat the choice of driver tube, with a fairly low gain. However, I think we'll have to convene a brainstorming session and revisit the gain structure question in a more consistent fashion - our preamp gains are less than they were a few years ago.

For what it's worth, the SR45 needs a nominal 1.65v to generate full power. The BeePre has a gain of about 2.5, so an input of 0.65 volts should generate full power from the SR45. A redbook digital source with 2vFS should then be able to drive the SR45 into clipping by 10dB.

For comparison, Paramount 300B requires 1.56 volts but makes 8 watts instead of 1.5 watts. SEX and Stereomour take around half that, 0.75 volts for full power of 2 watts and 3.5 watts respectively.

However, some modern recordings may utilize more headroom than traditionally used, and some source devices deviate from the redbook output standard.

[Jim R.]

Thanks, Paul.  I'm a little baffled by what's happening with my sr-45s then as in theory at least, a unity gain preamp fed from a 2v nominal cdsource should get the amps pretty close to clipping, and with the Blumensteins, they just don't appear to be anywherre near that.

Tomorrow I'll try my 3 watt LM mini amp that is much more sensitive.  Wish I had some good test equipment to test all this stuff and see what's going on with the waveforms.

BTW, for those who haven't seen it, I'm looking at selling my SR-45s:

http://www.bottlehead.com/smf/index.php/topic,3924.0.html

Thanks,

Jim

[2wo]

This may one of those times when theory meets practices. I have the same experience as Jim. A "2V" source, without an additional line stage will not drive my SR-45 to full power but will drive any other amp I have (many), plenty loud. Now these amps are not only probably more sensitive but have more power. My system with Horn Shoppe Horns, needs all the power a 45 can deliver and with a line stage it is plenty.

I did briefly try to increase the gain of the SR-45 by subbing a 12AT7 for the driver, I only tried it at the 6CM7 operating point. I got a bit more gain but didn't like the sound as much as the 6CM7, what was a bit of a welcome surprise.

 All in all, the SR-45 is my favorite amp, so if I need a line stage so be it...John         

[tsingle999]

The squeezebox 2 powers it up nicely for me but the ipad/phone are too low. I did get a big jump in volume from switching the coupling cap from a russian teflon to vcap.

[Jim R.]

Teran,

YGPM/EM

-- Jim

[Paul Birkeland]

I did briefly try to increase the gain of the SR-45 by subbing a 12AT7 for the driver, I only tried it at the 6CM7 operating point. I got a bit more gain but didn't like the sound as much as the 6CM7, what was a bit of a welcome surprise.
   

A 12AT7 will burn up (somewhat quickly) if you rewire the socket for a 12AT7 and drop it in, as the shunt regulator section dissipates over 4 Watts, and the 12AT7 is rated for 2.5 Watts.

Also, with 8 Volts of bias and 36mA, your plate voltage will drop way, way low, and you'll run out of swing in one direction (along with raising the heat dissipated by the driver C4S).

[Jim R.]

I'm going to take Teran's advice and try a new coupling cap.  The amps do have KK teflons in them now, and I do have a pair of .1 uF vcap CuTF caps I can try. Tomorrow we're supposed to get a healthy dumping of snow, so we'll be mostly contained at home for the weekend, so it will give me something interesting to try.  I don't feel I need all that much more volume, so this may just put me in the ballpark, if not for all listening, at least a good bit more of it, and then the 2a3 paramounts can be changed in for the heavy lifting.  Funny to think of 3 watts as heavy lifting, but there you go :-).

I'll report back on this later in the weekend.

-- Jim