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Old 23rd October 2016, 01:45 AM   #1
eimis is offline eimis  Europe
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Default Little Bear T8 MM/MC Phono gain modification

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

-t8-png
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...
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File Type: png t8.png (46.6 KB, 571 views)

Last edited by eimis; 23rd October 2016 at 01:58 AM.
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Old 23rd October 2016, 01:53 AM   #2
rayma is online now rayma  United States
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Quote:
Originally Posted by eimis View Post
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.

Last edited by rayma; 23rd October 2016 at 02:05 AM.
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Old 23rd October 2016, 02:05 AM   #3
eimis is offline eimis  Europe
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Haha.. Well those caps were the ones I omitted from my diagram! Now it's the full schematic:
-t8-png
So I guess nothing else can be done to increase gain as the capacitors are already 470uf?
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Old 23rd October 2016, 02:11 AM   #4
rayma is online now rayma  United States
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Quote:
Originally Posted by eimis View Post
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.
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Old 23rd October 2016, 02:13 AM   #5
Waly is offline Waly
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Quote:
Originally Posted by eimis View Post
Haha.. Well those caps were the ones I omitted from my diagram! Now it's the full schematic:
-t8-png
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.
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Old 23rd October 2016, 02:19 AM   #6
eimis is offline eimis  Europe
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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
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Old 23rd October 2016, 02:40 AM   #7
rayma is online now rayma  United States
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Quote:
Originally Posted by eimis View Post
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.

Last edited by rayma; 23rd October 2016 at 02:45 AM.
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Old 6th December 2017, 09:01 PM   #8
TUBBER is offline TUBBER  Netherlands
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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.
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