670 signal amp/control amp woes

[skal1]

Ok this what i have set up for the GR amp.

Signal gen out is .245 mVrms to primary of 600:10k tx , voltage across secondary is 1.016 Vrms  ,this  is injected into the grids of the GR amp which has a set down Ratio of 4:1 ,the  voltage across the  primary is 24 Vrms and the secondary voltage is 6 Vrms , this is without a load across secondary of transformer.

How would one calculate the db gain of the GR amp, please let me know if i am using the wrong terminology for the voltage , don't want anybody getting confused.
 

regards

skal

Quite easy, since we know the input and output levels:
I guess you meant 0.245V or 245mV for the input level, so the actual overall gain is 6/0.245 or 24.48 or 27.8dB.
Since the input and output xfmrs have the same ratio (in reverse), the actual grid-to-plate gain is the same.

yes 0.245V,  so whats 245mv is micro volts i'am guessing,  where did you get the  6 from in this  formula ( 6/0.245) and why did  it get divided by the input level.

regards

skal

 

[abbey road d enfer]

Ok this what i have set up for the GR amp.

Signal gen out is .245 mVrms to primary of 600:10k tx , voltage across secondary is 1.016 Vrms  ,this  is injected into the grids of the GR amp which has a set down Ratio of 4:1 ,the  voltage across the  primary is 24 Vrms and the secondary voltage is 6 Vrms , this is without a load across secondary of transformer.

How would one calculate the db gain of the GR amp, please let me know if i am using the wrong terminology for the voltage , don't want anybody getting confused.
 

regards

skal

Quite easy, since we know the input and output levels:
I guess you meant 0.245V or 245mV for the input level, so the actual overall gain is 6/0.245 or 24.48 or 27.8dB.
Since the input and output xfmrs have the same ratio (in reverse), the actual grid-to-plate gain is the same.

yes 0.245V,  so whats 245mv is micro volts i'am guessing,  where did you get the  6 from in this  formula ( 6/0.245) 

You said yourself the output voltage is 6V. 

and why did  it get divided by the input level.

Gain is the ratio between the output and input voltage;

 

[skal1]

so if i load down the output to say 2.45V the overall gain is (2.45/0.245 ) or 10db with a load  , so the load on the output tx would be the pad and a device connected to the output in parallel , correct me if i'am wrong.


regards
skal

 

[skal1]

so it looks like i have 20db of reduction without a load and 8db headroom ,so if i load  the output to have an overall gain of 10db would i have reduction of 10db and no headroom?

cheers
skal

 

[abbey road d enfer]

• so if i load down the output to say 2.45V the overall gain is (2.45/0.245 ) or 10db with a load  , so the load on the output tx would be the pad and a device connected to the output in parallel , correct me if i'am wrong.
  
  
  regards
  skal

  By virtue of heavy negative feedback, the output impedance of the control amp is quite low ; if you load the output to drastically reduce the level (10dB is drastic in the circumstance) the amp will distort and clip. The load et the output is:
  • The constant load defined by the 3.9k across the secondary
  • The dynamic load presented by the timing capacitors
  The amp has been designed and calculated to behave linearly into this complex load. I would think the attenuation presented by the actual load does not exceed a fraction of dB (a few percent) during normal operation.

 

[abbey road d enfer]

so it looks like i have 20db of reduction without a load and 8db headroom ,so if i load  the output to have an overall gain of 10db would i have reduction of 10db and no headroom?

cheers
skal

No. It doesn't work like that. You have to understand the effect of ratio. You may be right in having lost headroom in the control amp, but not for GR.

 

[DaveP]

I am confused ???

There are two amps, signal amp and CV (control voltage amp).

We started off talking about the signal amp TX as the best one to buy,

Which amp are we on now? ???

DaveP

 

[abbey road d enfer]

I am confused ???

There are two amps, signal amp and CV (control voltage amp).

We started off talking about the signal amp TX as the best one to buy,

Which amp are we on now? ???

DaveP

Back to the control amp since then http://groupdiy.com/index.php?topic=59681.msg781253#msg781253

 

[skal1]

no lads i was talking about  the GR amp my apologies

skal

 

[abbey road d enfer]

no lads i was talking about  the GR amp my apologies

skal

Hell! That changes everything...
The GR amp has an intrinsic high impedance, that is stabilized at about 600r by the permanent load created by the attenuator (R145-150) and the reflected impedance at the primary of T103.
In order to drop the level by 10dB, you would need to strap a 300r across the output. Why would you want to do that?
Headroom would drop by 10dB also, however headroom relative to nominal output level would be the same (-10dB in max level, but -10dB in operating level => same dynamic range).
However, the compression threshold would be 10dB higher, resulting in very little or no compression at all...
Pls explain your idea.

 

[DaveP]

I understood it as what CV would be required to give you 10dB of Gain reduction. :

That's why I asked.
DaveP

 

[abbey road d enfer]

Brains overload...emergency reset.
What amp?
Restate question.

 

[skal1]

Ok lets stop it here , this is how i am thinking about this hole loading thing.

according  to Davep  and  his invaluable  research,  see jpg http://i42.tinypic.com/25q9lio.jpg  ,to get 50v out of the cv control amp you need 10db out of the GR amp , which is 2.45v.

So my thinking was  ,to create a GR amp that could output 6V with a 0.245V  AND then , pad the GR amp output level down to 2.45V ready for the cntrol amp input tx...

Am i mad in my thinking

cheers

skal

shooting in the dark :-[

 

[DaveP]

This is why I said to check the voltage gain through the CV /GR amp.  If your IPT to that amp is not giving you x17 like the original, then you need to have a different pad or a different output from your Signal amp, so that the final 50V ends up the same.

DaveP

 

[skal1]

so last night i did an experiment with the control amp  input transformer for Ref T203 .

Signal gen out is  2.45V  to primary of the control amp input tx  and  output  is 32V at the secondary (not load).


is the gain calculation  out/input ?.

regards

skal

 

[abbey road d enfer]

so last night i did an experiment with the control amp  input transformer for Ref T203 .

Signal gen out is  2.45V  to primary of the control amp input tx  and  output  is 32V at the secondary (not load).


is the gain calculation  out/input ?.

regards

skal

As I said before Gain is Vout divided by Vin, so 32/2.45=13
It should be more than that...

 

[skal1]

so looking  at this point now ,i can see that the  transformer i am using for (Ref T203) has a gain of x13 which is below the recommended level, of x17 as DaveP has pointed out, so i'am thinking, can i compensate for this short fall in gain ,in the GR amp ?

I'am learning but slow thanks to everyone who's helping me out.


regards

Den

 

[abbey road d enfer]

Sorry, I misunderstood. I thought that was the overall measurement of the amplifier.
OK, so this is the measurement of the transformer alone. input transformer, is it? So it's a little hotter than the original (1:9 instead of 1:13). That's about 3dB more. Since there is an attenuator on the front, it should be no problem.

 

[skal1]

yes the side chain input transformer the one after the attenuator (R145-150).


regards

skal1

 

[abbey road d enfer]

yes the side chain input transformer the one after the attenuator (R145-150).


regards

skal1

So, it's the input xfmr for the control amp. It is supposed to be 1:17.
Since it has a very high nominal secondary impedance (170kohm), it is difficult to measure.
How are you checking the voltage, DMM? Anyway, if it is actually 1:13 instead of 1:17, the difference is only about 2dB. It should be easily compensated by adjusting the threshold pot (R115).