Integrating Stereomour and Crack

[danox574]

I recently ordered a Stereomour and a Crack.  I am familiar with electronics assembly.  I am a little knowledgeable about electronics but only solid state before this effort.

I'm intending on using my Stereomour's three inputs for various sources -- on day one, probably a turntable with preamp, a CD/SACD player, and a USB DAC with fixed line level coming in.  I'd also like an output running to the crack for whatever input is selected.  I have found a few discussions, some related to the Foreplay, on how to do this, and read what I can, and it seems like a few others have at least talked about it, but no reported results.  Has anyone done it?

What I'm looking for is the absolute best way to integrate these, not necessarily the simplest.

My initial thought was just to take the output from the Stereomour's input switch and run the signals and ground over to a new set of output RCAs and connect those to the crack's input, and then power up the Stereomour for speaker listening and the Crack for headphones.  I understand that the 100K volume pots in each device would be in parallel and create a 50k resistor for the inputs, which I have seen posted here would be fine for 'most sources'. 

* What sort of sources might have a problem with this? 
* If I swapped out the volume pots on both devices for 200K versions putting the resistance of the inputs back at 100K while connected in parallel, would I eliminate any risk?
* If I did substitute 200K pots, and then I disconnected them and used the crack separately, would the increase from 100K to 200K across the volume pot potentially cause any issues?

I have also seen from the Foreplay threads that putting a 1K protection resistor between the two outputs is advised.  What would be the upside/downside to doing this on the Stereomour (1K in line with the wire headed to out jacks from the input selector)?  Is it necessary?  Is there a better way to isolate these outputs?  In some other circuits I have used a op-amp buffer but i don't believe I have DC under the hood here (haven't seen the manuals yet) and sonics were not critical in those applications, would like to keep this closer to stock and not route the signal through anything unnecessary.

Is there a better way to do this that I haven't considered?

Your opinions are appreciated.

[Paul Joppa]

For the Stereomour/Crack, I think your proposed approach is excellent.

A few tubed source devices will not deal well with a low impedance load; the main one I am aware of is the classic RCA phono circuit, which needs to see at least 250K. But almost everything will handle 50K, and many things can feed 10K with no problems. In our lineup, the phono preamps prefer 50K, though then can perform quite acceptably down to 10K-15K in our experience - I'm just cautious about pushing the limits in specifications.

Don't increase the 100K pots; that will compromise the HF response due to Miller capacitance. It's probably OK, i.e. nearly inaudible, but it's cutting it too close for my comfort.

As an aside, it's a more difficult question for those who want to add a sub, and need an output which is controlled by the volume control. I just add this comment in case this thread shows up in somebody's search later.

[danox574]

Excellent, thank you Paul, I appreciate the response.  My phono pre is a Seduction.  If it especially likes 50K, I'm sure my solid state devices can handle the resistance, I'll just wire them in parallel as I've described and never worry about it unless I'm connecting something out of the ordinary.

Most likely I'll create a 3.5" stereo jumper between the two for quick disconnect if I want to relocate the headphone amp for any reason instead of a RCA pair.  Will be easier to find room on the Stereomour panel for a single, tiny jack.

The grounds between the two devices will only be connected when the jumper is in place, I guess, I wouldn't connect or disconnect them when powered.  Any issues with that?  Any risk if both amps are inadvertently turned on simultaneously or grounds between the two units are made/broken when powered accidentally?

What about that bleeder 1K resistor from the Foreplay I mentioned?  Is it a safety feature on the Foreplay and would it provide anything here?

 

[Grainger49]

Danox,

To be sure I understand, you want to put an RCA jack fed from the output of the selector switch.  The signal will continue on to the volume control in the Stereomour.  The output you add will feed the one input of the Crack.  Is this right?

If so, Seduction will be fine with the 100k pot of the Stereomour in parallel with the 100k pot of the Crack.  Your solid state sources will also be fine with that load.  The only worry for SS sources is a low input resistance that drives the output device into current limit. 

The Crack and Stereomour will also be grounded together at the wall.  The incoming safety ground is attached to the chassis of each.  It should keep them quiet.

The 1k resistor you have read about is between the main, first, output hot and the second output hot for each channel.  It adds a bit of isolation between the input impedances of the two pieces that the FP III feeds.  You can put 1k resistors in the feeds to your "Crack outputs." 

[danox574]

Yep, you got it right, although I probably will do it with a stereo 3.5" jack instead of RCAs for space on the panel. 

If the 1K resistors are for isolating impedance of the feeds on the FPIII outputs, would it actually do me any good on this application?

[Grainger49]

With the two impedances being 100k, the 1k is not significant. 

[Paul Joppa]

I originally put that 1K into the Foreplay (which had two sets of output jacks) because it is widely done and regarded as "good practice". Since then I have been unable to find out where this came from or why it is done, and I am starting to have doubts about the necessity in most situations.

A long time ago, early transistorized devices (for example, tape deck inputs) had inputs with low and nonlinear resistances when the device was turned off; feedback raised and linearized that impedance. That led to a practice of protecting outputs from the following load in case that load was a cause of distortion. It's possible that's where this habit come from, but I have not heard of the problem in recent decades - I have no idea whether it is still a problem at all. And if it is a problem, most normal tube-based preamps would need a much larger resistor for effective protection - and the larger resistor would kill the high frequency response due to cable capacitance. In any case, the Crack, and pretty much any tube equipment, will have a stable, linear input resistance - in this case, the 100K ohm pot. 1K ohms will not hurt anything, but I don't think it will do anything useful either.

[danox574]

Thanks guys!  Will report back on these when the kits arrive.

[crackaddict]

Hi,

Not sure of the protocol on replying to a very old thread, but this is the only one I found which addressed my question/idea/challenge precisely.

I'm about to build a Steremour to go alongside my Crack (and then the Reduction I ordered -- can't wait for this all to come together!). However, I will have 2-3 audio sources (ARCAM rDAC, turntable+Reduction, and possibly a future FM receiver or a CD player), and now two separate amps. My initial idea was to build a box with 3 inputs and two outputs, and use a switch like on the Steremour and an A/B toggle for the output. But it would be way easier, and much neater on the shelf, and much cheaper, to do as proposed above. I prefer to find a place for two RCA jacks on the main plate, however, rather than use a 3.5mm jack, though if space is a real issue, I could be convinced otherwise

I don't follow all the talk of impedance, resistors in the path, etc, so my plan is simply to wire a second set of L+R outputs to the RCA jacks.  As there was no followup above, I'm wondering if there is any new thinking on how to do this? Is the approach above OK to do?

From a noise perspective, is there any preferred location and/or routing for that extra L/R out, or some location/routing that should completely be avoided?

Has anyone else done this, or solved the problem in some other way?

Thanks!
Derek

[Doc B.]

Well, it's not a second set of line outputs, it's the only set. Other than that nothing has changed, you take signal from the output of the Stereomour selector switch and split it into two legs (wires). One leg of the split goes to the Stereomour volume pot just like the stock kit, and the other leg of the split goes to the output jack, which feeds the Crack.

[borism]

Here is a relatively recent thread that is dealing with your question and has good pictures:
 
http://bottlehead.com/smf/index.php/topic,5297.0.html

Good luck!

[crackaddict]

Thanks Doc & Boris. This gives me some ideas. I like the idea of turning RCA input 3 into an output, which I believe would necessitate swapping out the 3-position input switch (I wouldn't like having a position with no effect). What would be a good (or Stereomour-equivalent) two-position selector switch?

The other idea I'm thinking about is to add a mini toggle switch up front, at which I could split out the second output, making sure both sets of outputs are never active at once. What should one look for in such a switch? DPDT or similar designation?

This will be fun!
Derek

[Grainger49]

A DPDT switch allows you to switch the hots to two sets of outputs.  If you go with 4PDT you can switch the grounds too.

[Paul Birkeland]

Thanks Doc & Boris. This gives me some ideas. I like the idea of turning RCA input 3 into an output, which I believe would necessitate swapping out the 3-position input switch (I wouldn't like having a position with no effect). What would be a good (or Stereomour-equivalent) two-position selector switch?

There is a tabbed washer on the stock selector switch that you can move to the "2" hole to make the switch into a 2 position switch.

-PB

[crackaddict]

Great info! I think I have everything I need now to make this work for me. Will turn the switch into a 2-position switch for now, and integrate the amps like that. When I eventually need the third input, I'll add the extra set of RCA jacks and a 4PDT switch.

Thanks again for the quick replies.
Derek