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In Reply to: Can anyone show me a Preamp Schematic with 30X gain using Mini Tube, the best/good ever build for your expereience ? posted by calico on March 26, 2007 at 18:14:05:
How about this?
No Transformer and about 35 gain.DanL
Follow Ups:
I noticed in the circuit diagram coax is being used to shield ground connections, for example on the input. I can see a possible advantage if the shield were used as a low impedance connection, but as a shield I can't see it. Can someone explain how this works?
...puts a shielded cable after a 100k attenuator?Goodbye bandwidth, goodbye microdynamics at anything other than full volume or less than 1/10 volume.
Regards, Allen
how many Pf's per foot and how many feet are you assuming?Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi.This is a triode I/P.
Yes, short lengths of coaxial or shielded hook-up wire is often used from the I/P jack to the triode grid though I never touch it. I don't believe in any shielding for signal paths, even for phonostage.
Let's take an example of a minature coaxial cable, Belden 8218 (R59/U) used as the I/P cable. It gets 20.5pf/ft. Cgp of 6N1P is 3.1pF & amp factor A=35.
So the Miller effect capacitance alone is Cgpx(A+1)=3.1x36=111.6pF.So the total I/P capacitance is sum of (grid I/P stray capacitance due to ideally no longer than 1/2 ft coaxial cable=10.25pF + Cgp)x (A-1) plus Cgk. Neglecting the very small Cgk (a couple pF?), we have total grid I/P capacitance not less than 480pF !!!!
c-J
I understood that the Cgp is multiplied by the gain of the tube. I didn't know the C of the coaxial cable would be multiplied by the gain of the tube?I would think, in your example, that the total shunt C would be 111.6pF plus 10.25pF or 121.85pF total.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi.RHD4 stated in "Input impedance & Miller Effect":-
"The total input capacitance also includes the capacitance from grid to cathode plus (A+1) times ANY stray capacitance from grid TO plate".
My interpretation is ANY stray capacitance from grid TO plate (not between grid and plate) also includes capacitance of the hook-up wires etc etc.
"Stray capacitance" is defined as "Undesirable capacitance btween circuit wires, btween wires & chassis, or between components and the chassis of the electronic equipment".
Correct me if I were incorrect.
The whole idea is to minimize any stray capacitance added to the grid circuit. Coaxial cable is a ready source of stray capacitance.
I would use only twisted pair withOUT any overall shield from the I/P jack to the triode grid. ALL my SS & tube phonostages never ever use any shielded signal cables.Technically, the inter-conductor capacitance of a tightly twisted pair of wires of EQUAL size & IDENTICAL material is around 5pF, but the conductor-to-shield capacitance is in the order of some 40pF for EACH conductor, depending on the distance btween conductor & the shield.
For a coaxial cable, C=7.36xe/(log10 D/d) where e is the dielectric constant of the conductor insulation, D is the overall diameter of the insulation & d is the diameter of the conductor.
As Mark said it right, too much shielding capactance can only ruin the sound. This is the very reason I step away from any wire shielding.
For really EMI/RFI noisy environment, use a 100% metal chassis.
c-J
Stray capacitance should be minimized, always. I agree. But..."The total input capacitance also includes the capacitance from grid to cathode plus (A+1) times ANY stray capacitance from grid TO plate".
"My interpretation is ANY stray capacitance from grid TO plate (not between grid and plate) also includes capacitance of the hook-up wires etc etc."
NO, the capacitance of the hook up wire would be grid to cathode.
The C of the hook up wire to the grid is not Cgp but it does add to the tubes Cgk which is not multiplied by the gain of the tube.Take a look at fig. 12.1 that shows the "stray capacitance" between the grid and the plate. I think this "stray capacitance" IS the interelectrode capacitance of the tube but even if the book is talking about outside stray capacitance it would still be stray capacitance between the grid and plate not between the grid and ground (cathode).
Total input capacitance of a tube is Cgk + (Cgp * (A+1)) not Cgk +Cgp * (A+1).
Cgk would include capacitance which is native to the tube plus any external stray capacitance in the grid to cathode circuit.
Cgp would include capacitance which is native to the tube plus any external stray capacitance in the grid to plate circuit.
Take care.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
my passive RIAA filter. The .034uf cap, that's shunting to ground, multiplied by the second tube's gain is gonna leave me with no highs at all.:-(
Tre'
P.S. R1 = 22000ohms (including output impedance of driving stage)
It seems funny that Gary Pimm would use these values?Please understand that this is all sarcasm. Please don't take it personal. Just having some fun.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
AllenCalico was looking for a preamp
with ~30 gain and no transformer.
This was the ONLY one that I know of.
So I proposed it to Calico -
gain of about 35 and 200 Zo.DanL
Nice circuit, but Allen is Right (pardon the pun), you'll need to get rid of the shielded hookup between the attenuator and the grid. So layout will be important for low noise.Also- the grid input needs a grid stop resistor (about 1-5K) as close to the tube as possible! I would also recommend playing with the 680 ohm cathode resistor- offhand given a mu of 35 the value seems a little low, but OTOH a lot depends on the semiconductor plate load. If it were me, there would be no way I would allow the semiconductor devices into the circuit....
That semiconductor or any semiconductor?Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
for plate loads and the like I prefer a good resistor, but I have no problem using semiconductors for regulation if they are properly designed.
This design is not mine - just one I saw.
I have never used this design nor plan to.
My 12B4 is quite fine for me 8^D
Here is the site I got it from:http://www.cognitivevent.com/av_foreplay2.html
DanL
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