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In Reply to: RE: I don't get this... posted by Al Noakes on October 16, 2014 at 03:54:18
Hi,
I generally do not like any sand in my amplifier, especially not in the current loops.
I'm not using just 'any' resistor as bias resistor...(see pic). These are silver wire wounds made by Audio Consulting (Switzerland).
Yes, it generates still some noise, but my RIAA it's more quiet than the $250K Kondo M10 phono I once had ;). Noise of my record are generally higher ;)
NC
Follow Ups:
"I generally do not like any sand in my amplifier, especially not in the current loops."
But isn't there "sand" in your heater/filament supply?
I guess it's "shunted" by a 3 ohm resistor so maybe my question is moot.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
That is going to be a huge waste of money just like anything from Audio Consulting . Don't they sell knobs that cost $250 each ? Are you a sandophobe as you used sand and not liked it or just averse to using sand devices full-stop ? I'm no purist and tend to use whatever sounds best :) I'd suggest trying some schottkys in the cathode , it will be cheap and easy to find out if these devices as cathode bias are to your liking :)Al
Edits: 10/16/14
Its soo lovely to see that everyone has an opinion about everything in this forum :)
Did you try the knobs? ;) (me neither).
But I'm very happy with using filaments bias. No sand, keeps Zout low, no need for a cathode bypass cap etc. Only adds a small high quality resistor (in my case) in the signal path.
Enjoying the music...
NC
Yes I have an opinion :) The heater supply in filament bias places any ripple or noise onto the grid as it is referenced to the cathode , I also believe the rejection to common mode noise is also reduced . There is sand in that supply as it must be DC . Seriously , do yourself a favour and rip that bias scheme out . By all means use DC on the heater but get that heater supply out of the signal path . It should make your amp quieter and probably also sound better .Filament bias is best suited to small , low-current DHT fils , but for me , has very limited appeal and I don't use it anymore . I went through a phase of using filament bias on everything a few years ago including bastard-filament valves such as 3C24 and 826 . It's bulky , can be noisy and adds a sound of it's own . Schottky diodes or LEDs provide a workable , cheaper alternative , to be honest I also believe film caps/resistors sound better
Al
Edits: 10/16/14
Hi Al,Thanks for your opinion Al :)
So, to be sure... diodes will not raise the Zout of the stage like an Rk does?
I use a VCCS... Lotsa sand, but no idea about what is and what's not in the input current loop.
Another problem I see with diodes.... It looks difficult to get the exact amount of bias you need. Especially with high mu tubes.
NC
Edits: 10/16/14
You can pad down the diodes with small value resistors to get your required bias point .
Why worry about Rk ? Are you driving something that needs a low impedence such as an LCR filter ?
Al
But thats only in the first stage. The second stage drives an IT, which drives an TVC.
NC
I'd say : try it ! Should be easy to revert back if you don't like what it does . I'd suggest on the input stage first
Al
Did you see my RIAA stage?
Its quite a bit of work, but I will try it once... Thanks for the tip!
(still curious about the theory of the diodes influencing Zout though)
NC
It takes an appreciable amount of resistance in the cathode to affect OP impedance significantly.
The equiv circuit of diode is a voltage source (by definition zero impedance) with a series resistor, which is typically only a few ohms at the milliamps of current typical of a gain stage.
Seriously, try Schottky, they sound great and are dead quiet. SiC will give you around 0.8V and standard Schottky runs about 0.2V in these applications so a combination will give you flexibility up to a few volts.
Naz
It's not the boutique resistor I would worry about so much as any noise that is inherent in your filament supply. If you have time on your hands one day, just try using a Schottky to bias the cathode. I got the idea from Naz and others like Al, and it does work a treat; there will be LESS noise, not more noise, sand-o-phobia notwithstanding. (By the way, a Schottky is not made of sand, anyway.)
Well, its quiet enough for me, but I will try it one day to see if it will be better sounding.
Still not sure how the diode will affect the Zout though...
Thanks,
NC
Naz
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