tube pre has extra gain ?

[kambo]

i was messing with 8 ch summing mixer with different mic pres....
and when i use to connect slowblow mic pre to summing mixer,
it was always 4db -6db short... now, i have to pull the gain down !!!

slowblow mic pre has 45db of gain now. it use to be 36db.  of gain 

where is that extra gain came from...  any idea...

 

[ruffrecords]

what is the input impedance of the summing mixer?

Cheers

Ian

 

[kambo]

balanced NY Dave design. 20K in 200r out.

 

[ruffrecords]

OK, so that is not a significant load so it cannot account for the gain change. Has anything else changed?

Cheers

ian

 

[shabtek]

I gonna guess loading. is unloaded gain reduced now; does summing expect low z balanced drive;

 

[kambo]

nothing else changed.

 

[ruffrecords]

How are you measuring the gain and between which points?

Cheers

ian

 

[kambo]

feeding 1k sine from DAW to micpre (with pad or summing mixer ) ,
and measuring back in DAW.

 

[ruffrecords]

feeding 1k sine from DAW to micpre (with pad or summing mixer ) ,
and measuring back in DAW.

OK so this depends on the calibration of your DtoA and AtoD. Have these possibly been changed?

Cheers

Ian

 

[kambo]

nothing changed... except the gain of slow_blow... 
everything else measures the way they should.... 

 

[ruffrecords]

Are you measuring the gain from the DI input or the mic input? What is the setting of the level control? What signal level are you feeding into the input?

Cheers

Iani

 

[kambo]

Are you measuring the gain from the DI input or the mic input? What is the setting of the level control? What signal level are you feeding into the input?

Cheers

Iani

Are you measuring the gain from the DI input or the mic input? :
mic input

What is the setting of the level control? 
full gain

What signal level are you feeding into the input?
line level signal, 40db padded with 200ohm shunt resistor before mic input

 

[ruffrecords]

Looking at the circuit the mystery is why it was ever as low as 36dB. The 1:10 transformer has 20dB of gain. The 12SN7 stage probably has about the same gain and the 5687 output stage could well have another 20dB making about 60dB in total. It would be normal to have a step down transformer at the output which would probably have a 12dB loss making the overall gain close to the 45dB you measured. However, with the gain control full up and no output transformer, I am surprised this circuit has only 45dB of gain.

Cheers

ian