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Wessel Dirksen
 
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Default Advice for grounded grid input stage

Hi RATs,

I'm considering a grounded grid stage as the first input stage in my
continuating first power amp project. Considering that my soundcards
(Lynx 2 and Terratec EWX-24/96) both have low output impedances (100
and 200 ohm resp), it seems that this would be possible from a load
perspective. I am considering a rather high gain config with a 12AU7
that according to my calculations should yield an input load of about
1.4K. So a few questions:

1)Seems like a reasonable impedance matching for soundcards but will
this also be compatible for other pre-amps ect?
2)I believe that the input would see the low DC bias voltage of a few
volts. Can I direct couple this without the soundcard getting unhappy,
or would a soundcard need to be shielded from this small voltage?
3)I want to run feedback to this stage. It seems to me that you could
maybe put a small resistance between the grid and ground and put the
feedback between them. Is this possible?

Thanks. I'm coming along in this tube stuff. Lots of fun.

Wessel

  #2   Report Post  
Patrick Turner
 
Posts: n/a
Default



Wessel Dirksen wrote:

Hi RATs,

I'm considering a grounded grid stage as the first input stage in my
continuating first power amp project. Considering that my soundcards
(Lynx 2 and Terratec EWX-24/96) both have low output impedances (100
and 200 ohm resp), it seems that this would be possible from a load
perspective. I am considering a rather high gain config with a 12AU7
that according to my calculations should yield an input load of about
1.4K. So a few questions:


How did you calculate the Rin to a 12AU7 with grounded grid was 1.4k?



1)Seems like a reasonable impedance matching for soundcards but will
this also be compatible for other pre-amps ect?


No.


2)I believe that the input would see the low DC bias voltage of a few
volts. Can I direct couple this without the soundcard getting unhappy,
or would a soundcard need to be shielded from this small voltage?


Use a coupling cap to isolate the tube DC from the sound card output.

What value C will you need to get a LF pole at 7 Hz?


3)I want to run feedback to this stage. It seems to me that you could
maybe put a small resistance between the grid and ground and put the
feedback between them. Is this possible?


Yes, this is possible, and phase of the FB has to be correct.

The Rin to the cathode will rise, and so will the required
input voltage.



Thanks. I'm coming along in this tube stuff. Lots of fun.


Before you award yourself a medal for your achievements,
be sure you know what you are doing :-).

Patrick Turner.



Wessel


  #3   Report Post  
Wessel Dirksen
 
Posts: n/a
Default


Patrick Turner schreef:
Wessel Dirksen wrote:

Hi RATs,

I'm considering a grounded grid stage as the first input stage in

my
continuating first power amp project. Considering that my

soundcards
(Lynx 2 and Terratec EWX-24/96) both have low output impedances

(100
and 200 ohm resp), it seems that this would be possible from a load
perspective. I am considering a rather high gain config with a

12AU7
that according to my calculations should yield an input load of

about
1.4K. So a few questions:


How did you calculate the Rin to a 12AU7 with grounded grid was 1.4k?


As it turns out calculated incorrectly. I have rp+Ra/mu+1 as the input
impedance formula and I'm only getting about 300 ohms or so now.
Disappointed though. I was hoping I was on to something.



1)Seems like a reasonable impedance matching for soundcards but

will
this also be compatible for other pre-amps ect?


No.


For future reference, what is a typical output impedance for a preamp
or CD player?


2)I believe that the input would see the low DC bias voltage of a

few
volts. Can I direct couple this without the soundcard getting

unhappy,
or would a soundcard need to be shielded from this small voltage?


Use a coupling cap to isolate the tube DC from the sound card output.

What value C will you need to get a LF pole at 7 Hz?


Ok, also for future reference then. I guess you are suggesting that it
is a good idea to cap couple the input. Primary 1st order pole at 7 hz
would seem like a good filter point. Good LF bandwidth but not too
rediculous high value of C.



3)I want to run feedback to this stage. It seems to me that you

could
maybe put a small resistance between the grid and ground and put

the
feedback between them. Is this possible?


Yes, this is possible, and phase of the FB has to be correct.

The Rin to the cathode will rise, and so will the required
input voltage.


Humm, this would raise Zin then as well but not enough to help Zin
without killing all the gain.

Oh well . . .




Thanks. I'm coming along in this tube stuff. Lots of fun.


Before you award yourself a medal for your achievements,
be sure you know what you are doing :-).


Considering my nonsense calculation of 1.4k for Zin, I can understand
this caveat.

Yet the addage " . . . know just enough to get yourself into trouble"
is something I thrive on during the learning process.

Thanks for your input Patrick.

Wessel


Patrick Turner.



Wessel


  #4   Report Post  
Patrick Turner
 
Posts: n/a
Default



Wessel Dirksen wrote:

Patrick Turner schreef:
Wessel Dirksen wrote:

Hi RATs,

I'm considering a grounded grid stage as the first input stage in

my
continuating first power amp project. Considering that my

soundcards
(Lynx 2 and Terratec EWX-24/96) both have low output impedances

(100
and 200 ohm resp), it seems that this would be possible from a load
perspective. I am considering a rather high gain config with a

12AU7
that according to my calculations should yield an input load of

about
1.4K. So a few questions:


How did you calculate the Rin to a 12AU7 with grounded grid was 1.4k?


As it turns out calculated incorrectly. I have rp+Ra/mu+1 as the input
impedance formula and I'm only getting about 300 ohms or so now.
Disappointed though. I was hoping I was on to something.


If you can't remember all formulas, think in basic terms.

Say your grounded grid input stage has 1/2 a 12AU7 with 47k anode load and
2.7k Rk, and you wish to connect to the cathode.

The tube open loop gain is µ x RL / ( RL +Ra )
For 12 AU7, A = 14 approx.

So for 47 volts of change at the anode, you'd get 1 mA of of I change.
Since A = 14, you'd need ( 47 / 14 ) v at the cathode, = 3.35, and since
there *must* be 1 mA change there too the Rin = ( 3,35 / 1.0 ) k ohms =
3.35k

In other words, Ri = LL at a / A.

BUT, we also have 2,7 k in parallel with Rkin, so Ri = 3.35k parallel to
2.7k
= 1.498k, or say 1.5k.

If you were to use a CCS from oV to the cathode, then Ri at k
is simply 3.35k.






1)Seems like a reasonable impedance matching for soundcards but

will
this also be compatible for other pre-amps ect?


No.


For future reference, what is a typical output impedance for a preamp
or CD player?


Ro = low = 600 ohms typically,

Ri = high = over 10 kohms typically.

Output Z should be low so the voltage output is unaffected
by the load of something connected to it.

All devices should follow the rule of low Ro and high Rin.




2)I believe that the input would see the low DC bias voltage of a

few
volts. Can I direct couple this without the soundcard getting

unhappy,
or would a soundcard need to be shielded from this small voltage?


Use a coupling cap to isolate the tube DC from the sound card output.

What value C will you need to get a LF pole at 7 Hz?


Ok, also for future reference then. I guess you are suggesting that it
is a good idea to cap couple the input. Primary 1st order pole at 7 hz
would seem like a good filter point. Good LF bandwidth but not too
rediculous high value of C.


-3 dB point at LF is where the recatance of the cap in ohms
= Rout + Rin.

If you have 600 ohms feeding 1.5k, you have a total R = 2.1k.

To get a reduction of signal current by 3 dB you need C =
1,000,000 / ( 2,100 x 6.28 x 7 ) uF = 10.8 uF.

So you'd ask for a 100 volt rated 12 uF cap at the counter of the parts
store.







3)I want to run feedback to this stage. It seems to me that you

could
maybe put a small resistance between the grid and ground and put

the
feedback between them. Is this possible?


Yes, this is possible, and phase of the FB has to be correct.

The Rin to the cathode will rise, and so will the required
input voltage.


Humm, this would raise Zin then as well but not enough to help Zin
without killing all the gain.

Oh well . . .


Your power amp should be configured to make full power at a volt
input including NFB.
The soundcard can make up to a volt output, no?




Thanks. I'm coming along in this tube stuff. Lots of fun.


Before you award yourself a medal for your achievements,
be sure you know what you are doing :-).


Considering my nonsense calculation of 1.4k for Zin, I can understand
this caveat.

Yet the addage " . . . know just enough to get yourself into trouble"
is something I thrive on during the learning process.

Thanks for your input Patrick.

Wessel


In time, you will become use to sorting out all the factors affecting a
simple
circuit.
Learning the basics about triodes and some RLC network theory
are the most important things when building analog amp stages.

I tell Mr Yeager that all the time, and finally I hope to not have to tell
him anything;
he'll always get it right.

Patrick Turner.






Patrick Turner.



Wessel


  #5   Report Post  
Wessel Dirksen
 
Posts: n/a
Default


Patrick Turner schreef:
Wessel Dirksen wrote:

Patrick Turner schreef:
Wessel Dirksen wrote:

Hi RATs,

I'm considering a grounded grid stage as the first input stage

in
my
continuating first power amp project. Considering that my

soundcards
(Lynx 2 and Terratec EWX-24/96) both have low output impedances

(100
and 200 ohm resp), it seems that this would be possible from a

load
perspective. I am considering a rather high gain config with a

12AU7
that according to my calculations should yield an input load of

about
1.4K. So a few questions:

How did you calculate the Rin to a 12AU7 with grounded grid was

1=2E4k?

As it turns out calculated incorrectly. I have rp+Ra/mu+1 as the

input
impedance formula and I'm only getting about 300 ohms or so now.
Disappointed though. I was hoping I was on to something.


If you can't remember all formulas, think in basic terms.

Say your grounded grid input stage has 1/2 a 12AU7 with 47k anode

load and
2.7k Rk, and you wish to connect to the cathode.

The tube open loop gain is =B5 x RL / ( RL +Ra )
For 12 AU7, A =3D 14 approx.

So for 47 volts of change at the anode, you'd get 1 mA of of I

change.
Since A =3D 14, you'd need ( 47 / 14 ) v at the cathode, =3D 3.35, and

since
there *must* be 1 mA change there too the Rin =3D ( 3,35 / 1.0 ) k ohms

=3D
3.35k

In other words, Ri =3D LL at a / A.

BUT, we also have 2,7 k in parallel with Rkin, so Ri =3D 3.35k parallel

to
2.7k
=3D 1.498k, or say 1.5k.

If you were to use a CCS from oV to the cathode, then Ri at k
is simply 3.35k.


This is the stuff I like. Appreciated.





1)Seems like a reasonable impedance matching for soundcards but

will
this also be compatible for other pre-amps ect?

No.


For future reference, what is a typical output impedance for a

preamp
or CD player?


Ro =3D low =3D 600 ohms typically,

Ri =3D high =3D over 10 kohms typically.

Output Z should be low so the voltage output is unaffected
by the load of something connected to it.

All devices should follow the rule of low Ro and high Rin.


10 to 1 good enough then? Or is some overkill still a good idea?





2)I believe that the input would see the low DC bias voltage of

a
few
volts. Can I direct couple this without the soundcard getting

unhappy,
or would a soundcard need to be shielded from this small

voltage?

Use a coupling cap to isolate the tube DC from the sound card

output.

What value C will you need to get a LF pole at 7 Hz?


Ok, also for future reference then. I guess you are suggesting that

it
is a good idea to cap couple the input. Primary 1st order pole at 7

hz
would seem like a good filter point. Good LF bandwidth but not too
rediculous high value of C.


-3 dB point at LF is where the recatance of the cap in ohms
=3D Rout + Rin.

If you have 600 ohms feeding 1.5k, you have a total R =3D 2.1k.

To get a reduction of signal current by 3 dB you need C =3D
1,000,000 / ( 2,100 x 6.28 x 7 ) uF =3D 10.8 uF.

So you'd ask for a 100 volt rated 12 uF cap at the counter of the

parts
store.







3)I want to run feedback to this stage. It seems to me that you

could
maybe put a small resistance between the grid and ground and

put
the
feedback between them. Is this possible?

Yes, this is possible, and phase of the FB has to be correct.

The Rin to the cathode will rise, and so will the required
input voltage.


Humm, this would raise Zin then as well but not enough to help Zin
without killing all the gain.

Oh well . . .


Your power amp should be configured to make full power at a volt
input including NFB.
The soundcard can make up to a volt output, no?


Yes that is what I strive for. I have gone to direct connection from
quality soundcard because a preamp is simply not necessary.

The Lynx is amazing. +/-100 ohm Zout. Yet apparently designed to
function perfectly driving 1V into a 600 ohm load or higher.




Thanks. I'm coming along in this tube stuff. Lots of fun.

Before you award yourself a medal for your achievements,
be sure you know what you are doing :-).


Considering my nonsense calculation of 1.4k for Zin, I can

understand
this caveat.

Yet the addage " . . . know just enough to get yourself into

trouble"
is something I thrive on during the learning process.

Thanks for your input Patrick.

Wessel


In time, you will become use to sorting out all the factors affecting

a
simple
circuit.
Learning the basics about triodes and some RLC network theory
are the most important things when building analog amp stages.

I tell Mr Yeager that all the time, and finally I hope to not have to

tell
him anything;
he'll always get it right.

Patrick Turner.






Patrick Turner.



Wessel




  #6   Report Post  
Patrick Turner
 
Posts: n/a
Default


All devices should follow the rule of low Ro and high Rin.


10 to 1 good enough then? Or is some overkill still a good idea?


10:1 is only 600 to 6k.

Overkill is better. Tube amps have typically 100k input Z and
CF output from preamps = 600 ohms.
SS pres have opamps, maybe Ro = 20 ohms, but usually
they place a 470 ohm R in series to the output terminal which
stops the opamp fusing itself dead if a shorted output occurs.



Your power amp should be configured to make full power at a volt
input including NFB.
The soundcard can make up to a volt output, no?


Yes that is what I strive for. I have gone to direct connection from
quality soundcard because a preamp is simply not necessary.

The Lynx is amazing. +/-100 ohm Zout. Yet apparently designed to
function perfectly driving 1V into a 600 ohm load or higher.


If that is possible, then its ok to drive low impedances of 600 ohms.
It is very easy to make a robust output stage after an opamp
which will have low thd and able to put maybe 5 vrms into 100 ohms.
Its only 50 mA rms of current, and easy from a pair of class AB bjt
transistors
after an opamp and included in the loop of NFB.



Patrick Turner.

  #7   Report Post  
John Byrns
 
Posts: n/a
Default

In article , Patrick Turner
wrote:

Overkill is better. Tube amps have typically 100k input Z and
CF output from preamps = 600 ohms.
SS pres have opamps, maybe Ro = 20 ohms, but usually
they place a 470 ohm R in series to the output terminal which
stops the opamp fusing itself dead if a shorted output occurs.


What is the typical input Z of a modern SS amp. both pres and integrated?


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/
  #8   Report Post  
Patrick Turner
 
Posts: n/a
Default



John Byrns wrote:

In article , Patrick Turner
wrote:

Overkill is better. Tube amps have typically 100k input Z and
CF output from preamps = 600 ohms.
SS pres have opamps, maybe Ro = 20 ohms, but usually
they place a 470 ohm R in series to the output terminal which
stops the opamp fusing itself dead if a shorted output occurs.


What is the typical input Z of a modern SS amp. both pres and integrated?


Quad 405 Rin = about 20k; many SS amps tend to be around this figure,
including preamps, and this keeps them a little quieter, since noise is worse
with
high Rin with nothing connected.

Rout is typically less than 0.1 ohms from power amps because
often there is 110 dB of local and global NFB applied,
and what actually makes up the 0.1 ohms may be the LR zobel network
and the fuse soldered into the output circuit if there is one.
( it does need to be soldered in, not clipped in, since fuse clips and fuse
holders are notorious
for increasing thd from 0.001% to to 0.01%.

Rout from preamps is often between 220 and 600 ohms, and is made up with the
opamp
with Ro less than 100 ohms, and a series R added to protect the opamp from
shorts.


Patrick Turner.





Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/


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