B1 with Korg Triode

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#3,201
I did comment on "You can add a voltage divider to adjust the gain" ....... where voltage dividers are usually L pads

what I meant is simple - every signal attenuation pointt is most likely point of decrease of Signal Energy .... not having better expression

pretty much already well explained in article I linked to , same as my musings written here : Iron Pumpkin(s) and other smaller vegetable animals and in following posts
 
#3,202
brian92fs said:
The efficiency of you speakers isn't relevant for determining the pot value. The pot loads the source and contributes to noise. It's a balancing act between a value high enough for your source to drive without problems but not so high as the add unnecessary noise. 50K is often a good compromise (as seen by the value recommended in the B1K article).

In addition, the source can load the pot if your source impedance is high. A rule of thumb is that the pot value should be at least 10x the source impedance. This usually isn't an issue unless you have a high impedance source.

Finally, the pot itself is subject to loading from the following stage. What the pot is driving should ideally have a high input impedance or it may load the pot in a way that adversely affects it's taper (Hence the B1 buffer in this design after the pot and before the Nutube).

Assuming your pot is a audio or log taper, you'd ideally want you typically listening position to be in the middle third of the rotation. With your high efficiency speakers, you may find you have too much gain and end up with a pot position in the lower third of the attenuation band which may be too sensitive. If this is the case, you'll want to adjust your gain structure somewhere.

You can add a voltage divider to adjust the gain, just be aware of the loading affects it may have on the pot (as discussed above). Another buffer stage may help.

Rod Elliott has a great article on this: Beginners' Guide to Potentiometers
Click to expand...

Thanks Brian I needed a starting point as I'm about to order one I was looking all over for a figure on the forum and bom.

My valve amp has 470K pots (Volume and Balance) almost impossible to find now.

As for the beginners guide that's something to read after breakfast with plenty of coffee.
 
#3,204
Volume Control after Nutube output?

My understanding is that the distortion level of the Nutube varies with output voltage. Doesn't this mean that the distortion levels with vary with volume pot position? If this is true, is there any validity to placing the volume pot after the B1K and adding another buffer stage? With this topology, the Nutube's contribution to the distortion profile would remain constant. Or would the "amplify then attenuate" approach comprise the signal-to-noise ratio to an unacceptable level?

If you used this setup, you could retain the pot at the input as a tool for adjusting gain structure.
 
#3,205
It varies with voltage, but that input or output voltage isn't constant as we are considering music and not a fix signal. So input voltage varies regardless if you have a variable volume adjustement or not, and so does output voltage as it is input x fixed gain.

So although I do understand what you want to do, I am not sure what you want to do would make any real technical sense... but on top anyway in our "music" case it wouldn't cure your initial concern at all IMHO.

Sorry...

Claude
PS : BTW, most power amps (and they don't have a pot / gain variation) also display non linear raising distorsions with power levels, we all seem to live with it quite well up to a certain point I guess (?)
 
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#3,206
If you put a volume pot after the Korg, you do have some control over the amount
of distortion, that is to say you can increase it, as it goes up with level before the
pot. If you put gain after the Korg, or use an amplifier with more gain, you will
decrease the distortion.
 
#3,207
I have to say I am quite pleased with the “pioneer” kit from the diyAudio.com store that consisted of the Completion Kit and Chassis Kit. Once the microphony issue was resolved, the preamp really challenges my all tube Audio Research LS7 line stage preamp.

If I do anything else, it might be replacing the Alps Blue Velvet volume pot with a TKD volume pot. The preamp is worth the extra investment.
 
#3,211
which schematic exactly you're using?

Papa's schematic is having cap on input and cap on output

input cap is having resistor (volume pot) on input side , so no possible floating , which voltmeter can show as DC offset

same applies to output - there is resistor (220K) shunting output side of cap to gnd , so voltmeter again can't confuse you

so , you can have output DC offset only if you did something from audiophoolery trickery arsenal
 
#3,217
Getting exited now!

Lots of stuff ordered and on it's way - Complete kit from the store minus the chassis from US to NZ, Galaxy 2U Chassis without holes on the front to match ACA from the store Italy to NZ. Combined Khozomo Selector and relay Volume control with remote Poland to NZ. New soldering Iron tips, extra RCA sockets knobs and switches from Jaycar NZ and 0.9mm solid core Gold plated OFC hookup from Hi Fi collective UK. It's going to be like Xmas.

I've been using Rhino 3D to layup different layouts with a viewing window for the Nutube - I want it front and centre (Isn't that part of the fun of valves to have them on display?) plus playing around with various options for toggle V rotary on off switches ect. I'll have some very careful drilling and filing to do on the front pane to make it look good 🙂
 
#3,220
well , you still didn't convince me that cable was culprit 🙂

complementary Toshiba JFet buffer is having more than enough Cojones to drive even ridiculous length of screened cable , so culprit must be ...... somewhere else

care to post some pics of your build , maybe it'll be of some help....
 
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