Success! (And a question) (And a problem) (And solved)

danox574 · 2938

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Offline danox574

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on: September 17, 2013, 05:47:00 PM
My build is complete, and I've been listening to Sennheiser HD650s all evening with expected results.  Very enjoyable.  Need to let it burn in for a while but definitely don't feel like I need to go rolling tubes, it sounds pretty excellent right out of the box.  I did have two cold solder joints that I found during the final check before powering up and everything is rather ideal - balance seems perfect, all volume settings behave as expected, all the voltages were as expected.  None of the other problems reported are here.

I have a question about the resistance across the inputs.

I have Crack and a Stereomour - each with a 100k volume pot and 100k input impedance.  I've added an additional set of RCAs on to my Stereomour that allows me to connect the Crack in parallel with whatever Stereomour input is selected - reducing the input impedance to 50k.  Paul Joppa said this was fine.  It allowed me to switch between headphones/speakers without wiring anything around and select the input on my headphone amp easily with the Stereomour selector.

However, I've noticed that the Mainline input impedance is between 20k and 40k depending on the position of the fine attenuator (coarse attenuator doesn't matter).  Meaning, if I connected it to my 'aux output' on the Stereomour as I did the crack, the input impedance could be as low as 16k for the system.

I suspect my SACD player and streamer can handle that as they are newer solid state devices and are probably not picky, but my phono stage is a Seduction, which if I recall was designed around 50k input impedance.

So what happens if I hook up a device to something with input impedance this low and the device can't handle it?  Is it a risk for any of the equipment or just potentially a reduction in performance?  Can the seduction (w/ C4S) handle that low 16k input impedance?  Is there any risk to connecting it this way?
« Last Edit: September 17, 2013, 07:09:38 PM by danox574 »



Offline Doc B.

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Reply #1 on: September 17, 2013, 06:36:54 PM
You won't damage the Seduction. It may just sound like it has a bit less gain. I had this experience last weekend when i tried to run a DAC circuit that is very similar to the Seduction in parallel with a very low output impedance mic preamp. I couldn't get the DAC loud enough until I installed a Crack after it, used as a tube buffer. Crack's much lower output impedance handled the low output impedance of the parallel mic preamp easily.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline danox574

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Reply #2 on: September 17, 2013, 06:45:59 PM
Actually, I've run into a problem.  I realized I forgot to bias the tubes with my excitement of finishing so I went back to the bench to take care of it.

I left the amp running for about 10 minutes so everything was up to temperature, connected to pin 20, and adjusted the B side pot until I had 145VDC.

The pin 30 adjustment, however, only shows 78VDC regardless of the position of the pot.  I'm not sure what's wrong but I thought you might have a suggestion on where to look first.

-Dan



Offline Doc B.

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Reply #3 on: September 17, 2013, 06:51:45 PM
start by swapping tubes and see if the low voltage follows one tube. if it does, let the tube burn in for several hours and check voltages again.
« Last Edit: September 17, 2013, 06:53:19 PM by Doc B. »

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline danox574

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Reply #4 on: September 17, 2013, 07:09:27 PM
interesting, i was poking around the bias board with the tubes plugged in and IA and OA were reading as shorted, but not on the B side.  I looked around for a while and didn't see anything that concerned me so I came back up to check the forum again.

By your advice, I then swapped the tubes and now both sides are showing 157V and 168V and I am not reading a short on Ia/Oa or Ib/Ob.  When it warms up I'll adjust them both.  I can't figure out what would have caused it - anything that could have shorted near the tube sockets or the bias board that would have caused that?



Offline Doc B.

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Reply #5 on: September 17, 2013, 07:14:22 PM
Very difficult to say without being able to look at the amp. It could simply be that the tube needed to run for a bit to settle in.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline danox574

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Reply #6 on: September 17, 2013, 07:22:11 PM
No problem.  I was just wondering if any adjacent pins on the tube coming into contact could have caused it, but thinking about it and the pinout, I don't think so.  That's the area where I have the closest proximity of stuff.  Anyway, it seems great now.

Thanks for your late night responses.