Little Bear T8 MM/MC Phono — gain modification

[eimis]

Hi,
So I'm trying to use the Little Bear T8 phono, but the gain is a bit too low with the Grado cart I use. Which resistor should I replace and what value? I can read schematics but my knowledge basically ends there - not sure which resistors are best to replace. Please advise

Did my best with the schematic, this is only the right channel, MM signal path. "In" is where cartridge connects, "Out" is the RCA output...

 

[rayma]

the gain is a bit too low with the Grado cart I use. Which resistor should I replace
and what value? 470uf each.

You could try bypassing the source resistors of the two gain stages.
Ai least 100uF to 470uF across each of the two 680R should help.

You can eliminate the input coupling capacitor, it isn't needed.
The jfets are usually shown "upside down" compared to this drawing.

 

[eimis]

Haha.. Well those caps were the ones I omitted from my diagram! Now it's the full schematic:

So I guess nothing else can be done to increase gain as the capacitors are already 470uf?

 

[rayma]

I guess nothing else can be done to increase gain as the capacitors are already 470uf?

Each of the two gain stages would have to be redesigned for somewhat higher gain.
This might require different jfets and resistor values.

 

[Waly]

Haha.. Well those caps were the ones I omitted from my diagram! Now it's the full schematic:

So I guess nothing else can be done to increase gain as the capacitors are already 470uf?

You may increase a little RR8 and RR14.

 

[eimis]

Well I was only thinking of replacing some resistors as I really haven't got the knowledge to redesign it. Will play with RR8 and RR14 tomorrow...interesting

 

[rayma]

Well I was only thinking of replacing some resistors as I really haven't got the knowledge
to redesign it. Will play with RR8 and RR14 tomorrow...interesting

Don't increase them very much, or the second stage will more easily clip half of the waveform.
Perhaps you could get away with as much as 5.1k for each. This should increase the overall gain
by something like 4dB.

 

[TUBBER]

Did you eventually succeed to improve the gain? I bought this phone preamp and i was looking for the schematic. it was nowhere to be found but you made it yourself. Thank's fout that. I am prettig disappointed with this amp. It sounds dull and flat.