convert Stereomour to a preamp.

[drewh1]

I have built a bunch of Bottlehead kits, Kaiju, Stereomour, Toad, and Headphone amp.  I now am not using my Stereomour for much and would  like to convert it to use as a preamp with an Ayre solid state amp or mono-blocks. 

Is something that can be done?  I am of course aware you sell a pre-amp kit but I have this piece with lots of tubes already so I want to make it into a project while social distancing.

thanks,

drew.

 

[Paul Birkeland]

It's not particularly difficult.

 

[drewh1]

Hi  Paul - long time no chat!  well shows how much I know - I would have thought the output of the speaker terminals would exceed the line input voltage.  So seriously, I don't need to do anything but hook it up?

 

[Paul Birkeland]

3.5W into 8 ohms is about 5V. That will rise a little with no speaker on the binding posts, but that's still a rather reasonable output for a linestage preamp.

 

[drewh1]

thanks so much for your help.  I'll give it a whirl.  BTW, still loving the Kaiju.  Recently upgraded my Green Mountain speakers with Raal Ribbons, quite an improvement. I think this is the longest I have had and enjoyed the same amp and speakers.

drew.

 

[Paul Birkeland]

The Raal ribbons are excellent.  If the guy I sold mine to ever offers to sell them back to me, I'd certainly like to have them back!

 

[drewh1]

So I tried the experiment Made some connectors and hooked the Stereomour up to the McCormack.  There was a constant low level hum, like a short but nothing was shorted.  The music sounded fabulous besides the background hum (low and mid frequency),  but I could only turn the volume up to about the second notch before it was very loud. seems  like just too much gain? If you have any ideas (impedance mismatch?), they would be appreciated but I am thinking it is just not going to work.

BTW, you can contact Alex at Raal, I think he would be willing to sell direct.

drew.

 

[Paul Birkeland]

If this is a Stereomour I, then yes, there will be plenty of hum coming out of the Stereomour when used as a preamp.  If you have a Stereomour II with the DC filament upgrade, I would imagine it would be plenty quiet.  Impedance matching isn't a factor here, and the output impedance of the Stereomour is so low that it doesn't really matter what amp you plug it into.

If you have a Stereomour I, I would pad down the outputs.  I would use an 8 ohm 5W resistor with one end connected to the red binding post, then the other end in series with a 1 ohm 1W resistor that connects to the black binding post.  The preamp output would then be available across the 1 ohm resistor and would have vastly lower noise and considerably lower gain as well.

 

[2wo]

How about pulling out the 2A3 and tapping an output from pin 3 (grid)? The first stage is set to deliver a lot of voltage gain but with a low enough input or pad, it might work. Let Paul comment before you try in but I suspect your hum is from the output stage...John

 

[Paul Birkeland]

You would need to add something like a mosfet follower to bump up the drive current and lower the output impedance.  Without the step-down of the output transformer, gain would be a lot higher than it would be from the speaker taps.

 

[drewh1]

Ill double check, I thought I had done the Filament upgrade to my Stereomour, I have done so many upgrades, I cant keep track.  I can easily try to pad the output as suggested.

I also wondered about a ground loop issue as both units are sharing a ground but it only starts humming when the tubes warm up so likely not.

thanks for the help!

 

[drewh1]

Nevermind - filament upgrade to Kaiju, I don't think you offered one for the ST1.  So will try to pad it down.

so just to make sure I understand this,  i am adding two resistors in series between the black and red speaker output posts.

Black post to 1 ohm to 8 ohm to Red post, all other connections remain as is.

 

[Paul Birkeland]

Yeah, so S1 will have a few mV of noise at the speaker terminals.  The 8R/1R pad will knock that down enough that you should be in good shape.  That will give you a 19dB reduction in gain and noisefloor, which will turn the S1 into a bit less than unity gain preamp, which ought to be quite lovely for your setup.

 

[maryc27182]

Nevermind - filament upgrade to Kaiju, I don't think you offered one for the ST1.  So will try to pad it down.

so just to make sure I understand this,  i am adding two resistors in series between the black and red speaker output posts.

Black post to 1 ohm to 8 ohm to Red post, all other connections remain as is.

But make sure you do what Paul said, and take the output from the ends of the 1 ohm resistor. You don't want to use the red binding post as an output node once you've made a voltage divider.

 

[drewh1]

My apologies for being so dense, but that last post really confused me.

Please see my drawing, so once I have put the resistors in between the posts in series, you mean I should take the positive output to the amp from the Right side of the 1 ohm resister and not from the red binding post?  In other words, i should just disconnect the red post lead, connect it to the resistors take to ground and then connect the red post to the right side of the 1 ohm so I can use my standard interconnects.

My original understanding was to add the resistors in series and continue to use the posts as is.

 

[Paul Birkeland]

Like this.

 

[drewh1]

Ha, I just looked up voltage divider and get it now!  Thanks again Paul, you are a very patient man.

drew.

 

[Paul Birkeland]

Now is a good time for cheap experiments.  ;D

 

[drewh1]

Got the parts and took about 10 minutes to put in. I put them across the binding posts and ran a lead to the plus on the 3d set of RCA's ( I only use one input anyway)  so I assume I can use it either way without modifying anything, speakers or padded outputs.

It works great!  I am running at about half way on the volume pot. 

Thanks so much, now I can spend hours comparing it to the Nelson Pass Buffered Passive Preamp I just built!  thanks goodness for hobbies. Hope you are all well at Bottlehead.

drew.

 

[Paul Birkeland]

Awesome, I'm glad that worked out for you.  If you need more or less gain, you can mess with the resistor ratio to fine tune things.