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PostPosted: August 7th, 2015, 4:07 pm 
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FerdinandII wrote:
I don't think the Plinius does have a volume control......
Roscoe
[/quote]


Correct - the Plinius has no volume control


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PostPosted: August 7th, 2015, 5:32 pm 
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If you want to insert a volume control between the DAC and the map, you might try one of the units on ebay. They come with different values of motorized ALPS pot and a remote. Put it in a small box with a wall wart power suppl;y and you are good to go.

Tom

http://www.ebay.com/itm/2CH-Remote-Moto ... 3aaab4ae80


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PostPosted: August 10th, 2015, 8:40 am 
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Location: Potomac, MD
It is my understanding that you live near me. It appears that your composite desired load for your preamp would be about 4 k ohms. The 2.2 uF output capacitor I see in your photos is too small to drive this load with full bass response. At a minimum, I would suggest 10 uF. As to whether your output stage can deliver enough current to drive the output linearly at the required voltage depends on the idle current through the tube and the configuration as to basic compliance. Adding extra output jacks is straightforward mechanical, but determining linearity of output drive requires expertise and suitable test equipment. Since you apparently live near me, I would be happy to evaluate what you have and a logical path to get you to where you want to go.

David Berning


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PostPosted: August 10th, 2015, 9:19 am 
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dberning wrote:
It is my understanding that you live near me. It appears that your composite desired load for your preamp would be about 4 k ohms. The 2.2 uF output capacitor I see in your photos is too small to drive this load with full bass response. At a minimum, I would suggest 10 uF. As to whether your output stage can deliver enough current to drive the output linearly at the required voltage depends on the idle current through the tube and the configuration as to basic compliance. Adding extra output jacks is straightforward mechanical, but determining linearity of output drive requires expertise and suitable test equipment. Since you apparently live near me, I would be happy to evaluate what you have and a logical path to get you to where you want to go.

David Berning


If space is at a premium you could do 5-ufd and that would get you down to about a -3db point of 7-Hz. What you have now puts you at about 17-Hz. which believe or not is too high because of what happens a decade higher -- say at about 170-Hz.

My two cents.

Walt

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PostPosted: August 10th, 2015, 9:48 am 
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Without getting too complicated here, I'd suggest a slight variation of my original idea:

Leave the existing output alone. Add a second output using a MOSFET (selected for drain-source voltage 100V+ greater than your B+; and minimal input capacitance) as a follower (source, instead of cathode). Connect the drain to B+; the gate to the plate of the gain stage, with a protection diode from drain to gate, then a big resistor to ground, from the source. Select the resistor for 10mA or so. Source connects to a series capacitor (your new 10uF cap) the new output jack.

Virtually zero load on the triode (and BTW, the 6BQ7 is a very nice sounding tube!), new output has no effect on the existing circuit, so long as the power supply can handle another 20mA or so.

The existing cap and it's sonic signature are left in place. The new output coupling cap can be compromised a bit if needed to control cost, since this output is intended to drive a bass amp only. Metallized polypropylene should be just fine, just avoid Solen Fast Caps.

In the picture attached, you will have BOTH outputs. One, a triode, to your high frequency amp, and one with the MOSFET to your bass amp.

On the subject of gain pots: none are perfect, good ones are expensive (P&G or autoformers), or difficult to implement LED optocouplers). If you don't need one, don't add one. Crap ones, and even good ones WILL degrade your system sound, it's just a matter of degree.


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MOSFET FOLLOWER.gif
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PostPosted: August 10th, 2015, 10:10 am 
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Location: Potomac, MD
Stuart's idea is good in principle, but caution is in order. It appears that there is a muting relay for the output with perhaps a timer that allows the voltages to stabilize in the preamp before allowing the output to be active. Without muting, there is a risk of destroying ss amps and speakers as the output voltage wanders around during warm up. If you do something like Stuart is suggesting, you probably would need a second relay for the additional outputs.

David


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PostPosted: August 10th, 2015, 10:25 am 
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dberning wrote:
Stuart's idea is good in principle, but caution is in order. It appears that there is a muting relay for the output with perhaps a timer that allows the voltages to stabilize in the preamp before allowing the output to be active. Without muting, there is a risk of destroying ss amps and speakers as the output voltage wanders around during warm up. If you do something like Stuart is suggesting, you probably would need a second relay for the additional outputs.

David


Thanks, David. You're right! I assumed tube rectification and long TC filtering! Forgot to look. Muting outputs during turn on/off is always a good idea!

On the subject of output coupling cap size, and for what it's worth, I prefer output cap sizing to keep F3 below 3Hz, even below 1Hz if practical. In my system, I can hear the difference.

Trouble is, those caps are big, expensive and not as good sounding in the upper registers. So the dual output approach allows smaller caps in both locations as well as optimizing the HF cap for best performance.

Stuart


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PostPosted: August 10th, 2015, 11:52 am 
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If the heater supply is 12v rather than 6v, you could probably do a 2Sk170 source follower off the heater supply...

Roscoe

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PostPosted: August 11th, 2015, 2:16 pm 
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David/Stuart,

Sincere thanks for the recommendations. That being said, I want you to know that I don't understanding 1.0% of what is being suggested (and for that I apologize) :wtf: .

I am happy to bring the pre-amp to your residence (or welcome you to mine's) if you would like to take a closer look to see that is possible.

Kind regards,
Dhar


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PostPosted: August 12th, 2015, 11:02 am 
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Just to circle back:

Spoke to Mr. Berning yesterday. Turns out, I may be able to get by a a simple Y-adapter.

While speaking with Mr. Berning, I realized that I will be utilizing a Behringer DCX2496 or a MiniDSP (balanced) between the pre-amp and the pro-amplifiers to low-pass the signal at 150hz - 200hz.

The Behringer DCX2496 has an input impedance of "approx. 20kOhm at 1kHz"

The MiniDSP has an input impedance of 20kOhm (see need to confirm).

Given the aforementioned input impedance for the crossovers, I should be (theoretically) fine.


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