Bottlehead Forum

Bottlehead Kits => Moreplay => Topic started by: corndog71 on May 25, 2022, 09:22:16 AM

Title: Potential output impedance issue
Post by: corndog71 on May 25, 2022, 09:22:16 AM

I’m considering getting the Moreplay but I’m still a little fuzzy on potential impedance issues. 

I want to run one pair 15’ rca interconnects to my tube amps which have an input impedance of 275K Ohms.  At my tube amp inputs I will use an rca splitter and run another 3 foot pair of rca cables to my dual mono sub amps which have an input impedance of 30K Ohms. 

So, if I’m figuring this right the Moreplay will see a total of 305K Ohms input impedance. 

 My rca cables have a capacitance of 11.5pF per foot.  I understand this might be a factor but am unsure.

Does this sound doable or should I look for a different preamp?

Title: Re: Potential output impedance issue
Post by: Paul Birkeland on May 25, 2022, 09:59:28 AM

What is the specific model of the tube power amp? (It can matter a lot!)

Title: Re: Potential output impedance issue
Post by: Doc B. on May 25, 2022, 10:05:16 AM

Your lucky, the brain trust is here having a meeting today. Parallel impedances don't add. It works out to about 27K.

Biggest potential issue though will be the Miller capacitance of the input tube on your tube amp. That will add to the 170 or so pF of your 15ft cables.
BTW, what cables are you using that are only 11.5pF/ft?

Title: Re: Potential output impedance issue
Post by: corndog71 on May 25, 2022, 10:09:58 AM

My tube amps are VTA M-125 monoblocks and my sub amps are Rythmik A370PEQ.

Input tube of the M125 is a Tung-Sol new issue 12AU7.

Interconnects are BAV by Iconoclast.  Designed by retired Belden engineer, Galen Garies.

Title: Re: Potential output impedance issue
Post by: Doc B. on May 25, 2022, 10:26:45 AM

Haha, what goes around comes around. Seems that the air dielectric cables are a take on the old Ampex 300 repro head cables, which were IIRC, about 8pF/ft. Basically a bare wire slipped into a plastic tube.

We generally recommend 6ft max for cables to guarantee flat HF response. That said, with the 12AU7 input tubes in the amp the cable capacitance will pretty much dominate.

Title: Re: Potential output impedance issue
Post by: corndog71 on May 25, 2022, 10:40:44 AM

So when you say the capacitance will “dominate” that means what?  High frequency roll off?

Glad I asked.  I still might buy the Moreplay for a different stereo because it has a readily available tube set.  My old Foreplay 2, Quickie, and Smash wave hi!

Title: Re: Potential output impedance issue
Post by: Paul Birkeland on May 25, 2022, 10:45:55 AM

Yes, you are right at the point where you wouldn't want to push it much further.