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April 06, 2020, 11:15:25 PM

Author Topic: convert Stereomour to a preamp.  (Read 263 times)

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Offline drewh1

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convert Stereomour to a preamp.
« on: March 22, 2020, 10:05:36 AM »
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.
J-River on Custom built Music Server in Silverstone Case
Ayre QB-9 USB DAC
Kaiju
Stereoumour
Diy Cotton wrapped wire interconnects and speaker cab!es
Green Mountain Audio EOS HDx speakers
Crack with Beyerdynamic T1
Shunyata Diamond Back Power Cable
DIY Sub with Seas L26Roy Driver

Offline Paul Birkeland

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Re: convert Stereomour to a preamp.
« Reply #1 on: March 22, 2020, 10:12:14 AM »
It's not particularly difficult.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline drewh1

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Re: convert Stereomour to a preamp.
« Reply #2 on: March 22, 2020, 11:47:34 AM »
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?
J-River on Custom built Music Server in Silverstone Case
Ayre QB-9 USB DAC
Kaiju
Stereoumour
Diy Cotton wrapped wire interconnects and speaker cab!es
Green Mountain Audio EOS HDx speakers
Crack with Beyerdynamic T1
Shunyata Diamond Back Power Cable
DIY Sub with Seas L26Roy Driver

Offline Paul Birkeland

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Re: convert Stereomour to a preamp.
« Reply #3 on: March 22, 2020, 12:00:03 PM »
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.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline drewh1

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Re: convert Stereomour to a preamp.
« Reply #4 on: March 22, 2020, 03:06:26 PM »
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.
J-River on Custom built Music Server in Silverstone Case
Ayre QB-9 USB DAC
Kaiju
Stereoumour
Diy Cotton wrapped wire interconnects and speaker cab!es
Green Mountain Audio EOS HDx speakers
Crack with Beyerdynamic T1
Shunyata Diamond Back Power Cable
DIY Sub with Seas L26Roy Driver

Offline Paul Birkeland

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Re: convert Stereomour to a preamp.
« Reply #5 on: March 22, 2020, 04:05:02 PM »
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!
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline drewh1

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Re: convert Stereomour to a preamp.
« Reply #6 on: April 04, 2020, 02:47:21 PM »
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.
J-River on Custom built Music Server in Silverstone Case
Ayre QB-9 USB DAC
Kaiju
Stereoumour
Diy Cotton wrapped wire interconnects and speaker cab!es
Green Mountain Audio EOS HDx speakers
Crack with Beyerdynamic T1
Shunyata Diamond Back Power Cable
DIY Sub with Seas L26Roy Driver

Offline Paul Birkeland

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Re: convert Stereomour to a preamp.
« Reply #7 on: April 04, 2020, 03:26:08 PM »
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.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline 2wo

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Re: convert Stereomour to a preamp.
« Reply #8 on: April 04, 2020, 04:49:04 PM »
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
John Scanlon

Offline Paul Birkeland

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Re: convert Stereomour to a preamp.
« Reply #9 on: April 04, 2020, 06:31:45 PM »
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.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline drewh1

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Re: convert Stereomour to a preamp.
« Reply #10 on: Yesterday at 09:13:38 AM »
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!
J-River on Custom built Music Server in Silverstone Case
Ayre QB-9 USB DAC
Kaiju
Stereoumour
Diy Cotton wrapped wire interconnects and speaker cab!es
Green Mountain Audio EOS HDx speakers
Crack with Beyerdynamic T1
Shunyata Diamond Back Power Cable
DIY Sub with Seas L26Roy Driver

Offline drewh1

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Re: convert Stereomour to a preamp.
« Reply #11 on: Yesterday at 09:54:27 AM »
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.
« Last Edit: Yesterday at 10:25:04 AM by drewh1 »
J-River on Custom built Music Server in Silverstone Case
Ayre QB-9 USB DAC
Kaiju
Stereoumour
Diy Cotton wrapped wire interconnects and speaker cab!es
Green Mountain Audio EOS HDx speakers
Crack with Beyerdynamic T1
Shunyata Diamond Back Power Cable
DIY Sub with Seas L26Roy Driver

Offline Paul Birkeland

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Re: convert Stereomour to a preamp.
« Reply #12 on: Yesterday at 10:41:24 AM »
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.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline maryc27182

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Re: convert Stereomour to a preamp.
« Reply #13 on: Yesterday at 05:33:32 PM »
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.
« Last Edit: Yesterday at 05:36:36 PM by maryc27182 »
DIY 2-ways (TAD TD-2002, AE TD15M), Stereomour 45
Fostex TH-500RP, S.E.X. 2.1
Eros Phono

Offline drewh1

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Re: convert Stereomour to a preamp.
« Reply #14 on: Today at 04:01:29 PM »
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.
J-River on Custom built Music Server in Silverstone Case
Ayre QB-9 USB DAC
Kaiju
Stereoumour
Diy Cotton wrapped wire interconnects and speaker cab!es
Green Mountain Audio EOS HDx speakers
Crack with Beyerdynamic T1
Shunyata Diamond Back Power Cable
DIY Sub with Seas L26Roy Driver