Flip input to output.

[architect5150]

Could I theoretically take one of the pairs of inputs and flip it to a passive output?

My idea would be to split the channel outs from the volume pot to the tube input stage and the newly passive output pair, so it would almost be a throughput with a volume pot in series, and the rest of the amp would be exactly the same, sans one input. I could leave the 2nd position of the rotary switch open for a double make before break between my two inputs.

The goal here is to be able to passively use it (with power off) with my 300w/channel power amp OR be able to power it on and use it to power my speakers without my power amp, without having to unplug/replug wires.

I built a passive speaker/amp selector switch that I can use to switch between the 2 at that stage.

 

[Paul Joppa]

This question comes up every now and then; you might search the forum to find more.

But the main limitation is that the volume/balance controls are moderately high impedance, and thus not able to drive more than a little cable capacitance, or a low impedance power amp input - solid-state amps often have an input impedance as low as 10K ohms. A lower probability issue is that some inputs are not linear when the amp is off, which may introduce some distortion.

Without knowing more about the power amp, the easiest way to test these possibilities might be to just try it.

 

[architect5150]

It's an Emotiva XPA2G2. Says the unbalanced input impedance is 23.5KR which I believe is kind of middle of the road.

If I split it after the volume pot, before the balance, it will cut down my output impedance to essentially whatever factor of the logarithmic resistance the pot is set at, correct? So at that point, if I just turned the volume pot all the way up it seems like it COULD be feasible?

Seems like the biggest downside to trying it would be an hour or two of time and an couple inches of wire. Might just have to do it and report back.

Thank you Paul.

 

[brightcity]

I don't know but you may want to experiment with using a coupling capacitor to keep the second load from interfering with the first as they have different input impedances.

A low value should work as you are coupling a high impedance to a very low solid state.

In all honesty using a switch would be better: http://www.partsconnexion.com/accessories_qed.html

 

[Paul Birkeland]

You can also just connect your power amp to the speaker outs on the Stereomour.

 

[brightcity]

Cool concept, that would be interesting.

 

[Paul Birkeland]

Cool concept, that would be interesting.

With the SMR2 set up for 8 Ohms, you can get a preamp output from the speaker jacks capable of about 5V of signal output.  You will certainly want the DC filament upgrade installed for this, but the extra load presented by the power amp is negligible in this scenario.

It doesn't really address what you wanted to do from the start though.  If you don't have the shunt regulated power supply uprade, it's possible to use the second half of each 12AT7 to create a buffered output.  I can give you the general "how to" on this, but you'll have to figure out how to get it installed on your own.  Still, this doesn't create a pass through when the SMR2 is powered off.

The MourQuiet attenuator upgrade could be altered to be a much lower impedance control, and that could drive some preamp outputs, but the 23K input impedance of your power amps is, IMO, a little too low for this to be sensible. 

-PB

 

[Paul Joppa]

It's an Emotiva XPA2G2. Says the unbalanced input impedance is 23.5KR which I believe is kind of middle of the road.

If I split it after the volume pot, before the balance, it will cut down my output impedance to essentially whatever factor of the logarithmic resistance the pot is set at, correct? So at that point, if I just turned the volume pot all the way up it seems like it COULD be feasible?

Seems like the biggest downside to trying it would be an hour or two of time and an couple inches of wire. Might just have to do it and report back.

Thank you Paul.

Actually, the balance control comes before the level control. At that intermediate point, I get an impedance of 12.4K - but no control of the signal level.

At the level control output, the output impedance is 12.4K at max volume, around 28K worst case (6dB of attenuation), back to 12.4K at 11dB attenuation, and falling to zero at zero signal output. In the worst case, 140pF load capacitance will give -1dB at 20kHz. But the 12AT7 has about 100pF Miller capacitance, leaving only 40pF for the cable to the power amp. That's about one foot of ordinary cable, possibly as much as two feet of very low capacitance cable. (This paragraph is my quick-and-dirty calculation - I don't guarantee it is precise, but it should be close enough.)

You could of course replace the balance and level controls (and the 33K resistors) with 10K controls and 3.3K resistors, but then the issue is that the input impedance of the Stereomour drops to about 7K, which may be too low for some sources (especially tube phono preamps, which generally want to see at least 50K).

 

[architect5150]

Thank you all for the input/suggestions. Didn't realize balance was before volume!

My other idea would be to use my old FPIII as a low-gain preamp between the two. One output to the SMR2 and one output to the power amp. Volume control to the power amp would be much clunkier as-is but perhaps I can switch the resistors on the channel pots to operate at the upper 50%(log of course) of the stock range to give me finer control at that stage since I will only be using the XPA2 for higher volume listening, and still be able to attenuate in the SMR2 when I want that sexy class A SET life. Maybe I can convert the two channel pots to work more like the submissive/beequiet? Nevermind. I can't see it in my head but I don't believe there are enough poles on each to manage that.

OR!!! Maybe I'll just get a submissive and call it day. The -4db insertion loss MAY be negligible since the SMR2 can be driven to clipping levels anyways, and I don't really ever use all 200w of my XPA-2. I say 200 since I have Zu Omens with a 12R nominal rating. That would be like 120db at a meter. My wife would leave me. I would only need about a half meter to a meter of interconnect length to get to both the SMR2 and the XPA2.

Now I'm rambling.

ALSO: Just to clarify I don't yet own the SMR2, I am just ruminating on what to do to get the results I'm looking for before pulling the trigger.

 

[Paul Birkeland]

I would recommend building the SMR2 stock and comparing it to what you have, then working forward from there. 

Though the webpage says that the Omen can take 300W, that's simply to avoid getting the question from people with 300W amplifiers about whether they can use the Omen with their amplifier.

If we take the 97dB sensitivity and postulate that a reasonable max SPL is around 110dB before the drivers gas out, then 20W is all you're going to be able to use with that speaker.  PJ tends to recommend being able to reach 103dB as a good rule of thumb, though a huge subset of our customers use the Stereomour with sub-90dB sensitive speakers.  This would suggest that 4W is going to get you to live sound levels if the 97dB sensitivity rating is correct.

What this all means is that you need an insertion loss of about -20dB to match your sources to your power amp the combination of the sensitivity of your speakers and the gain of your amp. 

 

[architect5150]

Great recommendation.

I do believe the Omens are very close to their rated sensitivity. Granted it takes more work for any amp to power them since they are higher impedance than a lot of other speakers. I have played them up to 110db at the listening position with around .76%THD and I couldn't go any higher because the mic input began clipping at that point which also thew off the THD. They do get very loud. Not that I would ever try to throw 300W at them.

I built a SEX3.0 with C4S that I have used with them, and even at 2.5w rating I got about 95db at the listening position,about 2.65m, at 10%THD(from my graph it looked like almost all second order) and got up to about 98-100db before really noticing the soft tube-style clipping. Needless to say I am a huge fan of the SEX3.1 with C4S. I sold the SEX as that was the purpose behind getting it was to build it and sell it for a profit. We are in the process of adopting from India so I started building audio gear to help with costs.

Perhaps I can save some money by getting a submissive sex combo (again insertion loss should be negligible since my sources could easily drive the SEX to clipping) or maybe just use a SEX3.0 with my FPIII. I am getting enough tiger maple board feet I could actually build matching bases for both. Attached is the design idea I had for the SMR2 that I could change up to the SEX and FPIII or SEX/submissive.

So many options, but feels like it's hard to go wrong, and instead there are plenty of right ways. Thankful for Bottlehead and thankful for you all and your helpfulness.

 

[fullheadofnothing]

Did you sell your SEX 3.1 to Marty McFly or Doctor Who?

 

[architect5150]

LOL Bill and Ted, actually.

Fixed.