StephenGiles

FET phono preamp
« on: October 31, 2004, 11:15:17 AM »
Is this interesting? - looks like a lot of FETs
http://www.vacuumstate.com/images/RTP5_rev_1.GIF
Stephen
The best place to find a helping hand is at the end of your arm!


Re: FET phono preamp
« Reply #1 on: October 31, 2004, 12:04:22 PM »
Quote from: "StephenGiles"
Is this interesting? - looks like a lot of FETs
http://www.vacuumstate.com/images/RTP5_rev_1.GIF
Stephen

Lot of FETs and lot of noise.
Input noise resistance of 2SK170 is 25 ohms (I measure
it by my own hands),
2SK147 is nearly the some.
If you connect four times resistance series to gate,
why you use 2SK147 ?  You can use tube itself
with nearly the some results.
Emmiter resistors the some. Is better to select device to
optimal DC current, that emmitter resistors.
This resistors are not only white - noise current sources,
they are affected  by DC current and then have low 1/f noise.

Ommit 100 ohm  input resistors and emmiter resistors.
It is pity, that you can not use
use miller feedback resistors {with capacitors in series}
from plates of first tubes to the input.
to ommit input # resistor {other noise source} .
You have RIAA correction as plate-load impedance.

At the second stage, ommitt 25 k gain pots,
They can be noise-source
and use simple 25 k pot with series capacitor
between the plates. It is nicely symetry to input plate - load RIAA.
have a fun
             xvlk

StephenGiles

FET phono preamp
« Reply #2 on: October 31, 2004, 02:09:55 PM »
It must be crap then!
Stephen
The best place to find a helping hand is at the end of your arm!

Marik

FET phono preamp
« Reply #3 on: October 31, 2004, 03:00:28 PM »
Quote from: "StephenGiles"
It must be crap then!
Stephen


No it is not. Not at all!!! The input section is similar to Kevin's phono pre and is a cascode. I use this circuit in my mic pre. It has high gain and low noise, and sounds wonderful. The gate (100 Ohm) and source resistors should be there. When Kevin gets back from AES I'll email him so he could probably  shime in with more detailed description of the circuit.
Samar Audio & Microphone Design

www.samaraudiodesign.com

The Art of Ribbon Microphones

rafafredd

FET phono preamp
« Reply #4 on: October 31, 2004, 03:35:35 PM »
Here you can see the CCS used:

http://www.vacuumstate.com/images/PP-1C_ps.gif

what resistors should I change to vari current? I guess the 330k from gate to ground?

StephenGiles

FET phono preamp
« Reply #5 on: October 31, 2004, 03:56:34 PM »
Marik - do you know Cedar Breaks? We spent a week there at 9500 feet a few years ago. I suffered with altitude sickness for 6 out 7 days we were there in the form of a permanent headache. The folks in Cedar City were fascinated by our English accents!
Stephen
The best place to find a helping hand is at the end of your arm!

FET phono preamp
« Reply #6 on: October 31, 2004, 04:46:12 PM »
Quote from: "rafafredd"
Here you can see the CCS used:

http://www.vacuumstate.com/images/PP-1C_ps.gif

what resistors should I change to vari current? I guess the 330k from gate to ground?

This can be practical,  the smaller resistor, higher current.
Resistor is high, and its noise effect is leaked by capacitor.
It can be 1 M pot with 100 k series resistor.

But this way only to 50 % current change.

{100 % = 0 Ohm and condenser can not perform}
For bigger current change, change source 470 ohms,
twice the value = half of current and vice versa.
                xvlk

FET phono preamp
« Reply #7 on: October 31, 2004, 04:59:21 PM »
Quote from: "Marik"
Quote from: "StephenGiles"
It must be crap then!
Stephen

 I use this circuit in my mic pre. It has high gain and low noise, and sounds wonderful. The gate (100 Ohm) and source resistors should be there.

Yes, for 200 ohm microphone {symetric} you adds 2* 100 ohms =
200 ohms. = 3 dB noise figure. It is not much for mic amp,
but there was better amps  for last 60 years.
For 35 ohm moving coil pick up this is adds 8.2 dB by these resistors.
Transistors itself have 2*25 ohms noise resistance. It is 3.8 dB. Some improvement?
And that 8.2 dB was  without emmiter resistors.
Yes, noise of the vinyl plate is far masking this effects.

                             Marik,  have a nice week
                                          xvlk

Marik

FET phono preamp
« Reply #8 on: October 31, 2004, 05:58:07 PM »
Quote from: "xvlk"
Quote from: "Marik"
Quote from: "StephenGiles"
It must be crap then!
Stephen

 I use this circuit in my mic pre. It has high gain and low noise, and sounds wonderful. The gate (100 Ohm) and source resistors should be there.

Yes, for 200 ohm microphone {symetric} you adds 2* 100 ohms =
200 ohms. = 3 dB noise figure. It is not much for mic amp,
but there was better amps  for last 60 years.
For 35 ohm moving coil pick up this is adds 8.2 dB by these resistors.
Transistors itself have 2*25 ohms noise resistance. It is 3.8 dB. Some improvement?
And that 8.2 dB was  without emmiter resistors.
Yes, noise of the vinyl plate is far masking this effects.

                             Marik,  have a nice week
                                          xvlk


Usually MM pickups have much higher impedance. MC use step-up transformers.
I did it as a single ended (i.e. only upper half of schematics) with 1:4 transformer input.
Samar Audio & Microphone Design

www.samaraudiodesign.com

The Art of Ribbon Microphones

rafafredd

FET phono preamp
« Reply #9 on: October 31, 2004, 06:54:16 PM »
1:4 transformer will render the noise insignificant, I think.


FET phono preamp
« Reply #10 on: October 31, 2004, 07:24:54 PM »
Quote from: "rafafredd"
1:4 transformer will render the noise insignificant, I think.

... But several 2SK170 single ended in paralel can have better noise figure, than transformer 1:4.
1:4 transformer changes source self noise resistance by 4**2 = 16.
16 * 35 = 560 and  then Marik s  100 ohm resistor noise is insignificant.´

But you can still use direct input tube with 200 ohm input resistance,
which is still insignificant, if you use input trafo. You can not use 2SK170
                           xvlk

rafafredd

FET phono preamp
« Reply #11 on: October 31, 2004, 07:27:16 PM »
I guess it´s a matter of subejective sound difference if you use the FET at the input...

PRR

FET phono preamp
« Reply #12 on: October 31, 2004, 08:16:56 PM »
> Is this interesting? - looks like a lot of FETs

If you like to see a lot of FETs and tubes, it is groovy.

If you want to listen to records, a 3-BJT design can be excellent, and fit inside the turntable.

If you need something a bit sexier, get the Hagerman Bugle. I would not design a phono preamp that way (I like to "coax 60dB out of one opamp"), but as I keep saying: it isn't the design but the details. Hagerman has detailed this low-price kit as if he really cared about it.

I have notes somewhere for several low-price ($20-$60) phono boxes. Unless they changed AGAIN, the Radio Shack is best hacked with a very large hammer. The ART phono box is almost-good: my main objection is a LARGE amount of sub-sonic random flutter which trips-out the "DC Protection" on many speaker amps. If I find my notes, I can tell you how to swap some caps around and make the ART less fluttery and a bit more accurate, but if you can swing a solder-iron that well then the Bugle is a better bet.

rafafredd

FET phono preamp
« Reply #13 on: October 31, 2004, 10:26:31 PM »
Well, as a micpreamp, I´ve seen more than one people swear that this input stage RULES!!! I´m talking about the single ended version, with a 1:4 to 1:7 transformer at the input. But I never heard one myself thought.

There are people in this forum that LOVES the this FETcascode-tube thing.

adrianh

FET phono preamp
« Reply #14 on: October 31, 2004, 11:07:06 PM »
Yes this stage does rule.
SE with input and output Xformers it is great!
See K&Kaudio website for info.

The 2SK170 will give the best noise unles you use a better fet or a lot of them in parallel.
In a real world the tubes cannot be that low noise. Kevin of K&K
for 6 years built RIAA preamps and after 1 year the tubes would get noisy.
Yes even 417A. The FETcode thing is very good, look it up on the
internet or better yet build it.
No scrap that thought do not build let me build it an I will use it in my studio
while all you people who complain will not have it. But I will have it and be making money with it while you go hungry.
Marik is right build it and listen.

rafafredd

FET phono preamp
« Reply #15 on: November 01, 2004, 12:02:46 AM »
Adrianh, did you tryed it in the balanced configuration?

Maybe it would be nice with a lundahl 1+1:4+4 at the input.

adrianh

FET phono preamp
« Reply #16 on: November 01, 2004, 07:41:22 AM »
No I have not.
I have heard the SE version.
I am one of those loosers I talk about as I have yet to build it.
Another guy Dave D in the audio club has done some push pull stuff, but always with input and output iron.
see http://www.raleighaudio.com/
Kevin C builds these for him.

kevinc

FET phono preamp
« Reply #17 on: November 01, 2004, 01:54:51 PM »
The FET/tube cascode offers pretty low noise, pretty high gain, and has the less edgy (than many solid state and some tube high gain stages)sound I attribute to tubes properly used.  The noise isn't as low as some other configurations, but low noise isn't everything.  The SE hybrid stage Marik is talking about at the input of the mic preamp he built using the K&K Audio phono preamp circuit has about 45-50dB of gain.  Not bad for a small handful of discrete parts, but it can hardly compete with an op-amp, if you like that sort of thing!!  Add a transformer coupled output stage with another 15dB of gain and an input transfomer with anywhere from 4 to 20dB and that should be enough gain for most any mic!  Sometime next year I will adapt the phono preamp design to actually include phantom supply, gain adjustment, etc., to make it straightforward to implement as a mic pre.
Kevin Carter
K&K Audio
www.kandkaudio.com

FET phono preamp
« Reply #18 on: November 01, 2004, 05:37:48 PM »
Quote from: "rafafredd"
I guess it´s a matter of subejective sound difference if you use the FET at the input...

Yes, tube have semicubuc, fets parabolic ...
Quote from: "PRR"
3 BJT

... and BJT exponential. ´

What about to fade between 3 BJT and tube amp to adjust subjectively
the best sound ?
                xvlk


 

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