Bottlehead Forum
Bottlehead Kits => Stereomour II => Topic started by: drewh1 on March 22, 2020, 10:05:36 AM
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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.
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It's not particularly difficult.
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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?
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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.
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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.
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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!
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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.
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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.
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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
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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.
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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!
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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.
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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.
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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.
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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.
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Like this.
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Ha, I just looked up voltage divider and get it now! Thanks again Paul, you are a very patient man.
drew.
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Now is a good time for cheap experiments. ;D
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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.
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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.
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Just smoked one of my RAAL Tweeters. I was going to change out the volume control on the Stereomour. Inputs and outputs were connected. I turned the power off the Stereomour but not the power amp. When I pulled the power cord from the Stereomour to disconnect it, got a loud hum and one of my tweeters went up in smoke. Fried the Ribbon.
Thankfully everything else seems fine including the tweeter on the other channel.
All of my power is from a Panamax 5300 power unit. Supposed to have all sorts of safety stuff.
Things had all been working fine and sounding great prior to this disaster. Any ideas what might have happened?
thanks - Drew.
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The turn on/ turn off procedure is: power amps; turn on last, turn off first. With the power amp left on, you probably gave the tweeter the full power capability of that amp instantaneously.
Jamie
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Well thanks - but the preamp was already powered off. Nothing happened till I pulled the plug. Don't see why that would have caused a problem unless there is some kind of ground short.
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Grounds are involved. The inputs and outputs are of course grounded to the chassis, which in turn is grounded to the safety ground through the power cord. You might look into whether the sources and/or power amp are safety grounded. It could get complicated to figure out exactly what happened, but it may well have to do with disconnecting the safety ground connection.
Ribbons are fragile! Normally a second or higher order passive crossover is needed to protect them. We've learned this lesson the hard way some years ago.
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Looking over the specs, the power manager you mentioned appears to have a built in power on/off sequencer that does what we are talking about regarding preamp on first/amp on, amp off first/preamp off.
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Thanks for feedback -
Paul - Alex at RAAL helped me with the final circuit and values for the tweeters, so I am guessing he would have caught a problem in the xover. and it definitely has to be some problem with grounds. No other damage but the tweeter. From now on, that power amp is definitely getting shutoff and disconnected before I do anything else but i would like to figure out if there is something else that needs to be fixed.
If you have any other ideas let me know.
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Drew,
No disrespect intended. I have learned the hard way, myself, about turn off procedures. I have F’d up tweeters too, so I got religious about the “turn off “. Sorry about your RAALs. I have never used them but I hear they’re very nice.
Jamie
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No worries - I appreciate your help.
drew.
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I think I found the problem - I reused two of the input RCA connectors for the Voltage divider output. The problem is although they had ground wires connected, they only had ground when the selector switch is in the correct position. Which of course it wasn't, it was on the input RCAs.
Therefore, there was no ground to the RCA connectors going to the power amp. So I imagine that when I unplugged the safety ground, the capacitors discharged directly to the only available path - through the RCA's to the power amp (which was still turned on)
Does that seem like a possibility? At any rate, these need to be grounded directly to the chassis, so that is happening now.
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Drew, to be clear, I would not disconnect any component until everything is powered down. Also, it wouldn’t hurt to wait a few minutes after power down to begin disconnecting stuff.
Jamie
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excellent advice - too bad I am learning this the hard way, as usual.
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Also per Doc's comment - I now have the power amp and preamp plugged in to the appropriate circuits, so that the delay times in the power unit power things up and down in the appropriate order.
Now I just have to try to locate a replacement ribbon for the RAAL, Only available from Serbia unfortunately :)
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Yeah, that could definitely cause an issue.
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Did you ask Solen if they have replacement ribbons? I know have sold RAALs in the past.
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Thanks for the suggestion. I'll check. Unfortunately, the 70-20xr is an OEM only product so usually only manufacturers get them and so Madisound and others don't carry parts. I was able to get them because I was finishing a speaker project that Roy Johnson started before his death last year. So Aleks was willing to help me out. Still can't believe I smoked it, I guess you guys have seen a lot of smoke over the years though.
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Drew, I know Madisound doesn’t carry that model, but a call or email to Adam Johnson couldn’t hurt. He has gotten me stuff that no one else could, so a reach out to him might pay off. If you are interested PM me for his email. The great thing about the guys at Madisound is that they service the DIYer with the same care as given to the OEMs.
Jamie
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Nah, we never have any accidents.
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Doc - You are the MAN! Solen had them but don't put anything about the parts on the website. Got some ordered and sent Fed Ex. I think all problems are fixed so maybe I can get back to listening. Not going to mess with anything for a while.
drew.
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The folks at Solen are super nice and very prompt in communicating and shipping.
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They sure know who you are Doc!
If anyone is interested, attached is a photo showing what happens when you fry a ribbon.
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Some duct tape will fix that right up. My scare moment with the RAALs was when a new employee was putting one of the foam diffusors on the tweeter. There was a little piece of steel wire stuck into the foam, that is attracted to the tweeter magnet to hold the foam in place on the tweeter flange. He inadvertently brought the foam up to the face of the tweeter at a right angle and the magnet pulled the wire right out of the foam. It shot into the ribbon opening and grazed one side of the ribbon. Another millimeter over and it would have punched right through it.
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those magnets are powerful, glad I don't have any metal piercings :)