Doug Self Preamp from Linear Audio #5

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blacknoz

Member
Joined 2019
bk856er said:
Some pics below of the "finished" product. I went with a 16x12x2" alodine par-metal enclosure to match with some other projects on my rack. Neglected to account for the larger shaft nut on the Lorlin and stole some larger black knobs for now. The front/rear panels are temporary until I get something from FPE with labeling, but this allowed me to exactly confirm the layout. Grounded xlr pin1 to the chassis with short wires. Needed a tantalum cap on the I/O board to get 12V. Added a rotary mute switch (the inner black knob) - heck, had to do something with the minimum order qty 5 Lorlin switches!

BK
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blacknoz said:
Dear BK,
sorry to get this part of the thread back to life, I was in the same situation as you and Hicoco and followed the recommandations of Jan (R5/R6 at 100R and R9/R10 at 1K) as I wanted to get the preamp powered with 17V and as my main voltage is around 230-240V.
Everything is running smoothly although I find the heatsink a little bit hot (around 50°C).

May I ask what cap you used exactly and where you put it precisely and maybe why it helps getting 12V? (I currently measure 15V in this part of the I/O board )
Second question, hope I'm not disturbing too much, shall I definitely ground the pin 1 of the XLR to the chassis per your suggestion or is it something you prefer doing but not mandatory. No cap needed here? Just a short?

Last but not least, @Carl, MANY THANKS for all that stuff, you gave me the power of Doug and I really like what I ear.

Thanks all.
Raph
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Ok, ok, sorry I disturbed you all with that.
I think I found what you were talking about and finally sorted it out.

I'm just updating things here so that other noob like me can move on :)
Everything is written in the datasheet o the TL750L12CLP (page 5 and 6 indeed): a 10uF tantalum cap with a small ESR is required between OUT and COMMON/GROUND to prevent the output from oscillating and so getting the correct voltage. Just done some basic testing here on a breadboard, the TL750L12CLP was varying between 10,3V and 15V while feeded with +15VDC, I managed to get 11,8V with some cap.

One more time, thank you all for all of that stuff.
Raph - Never too late too learn :)
 
H

Hicoco

Member
Joined 2018
Paid Member
blacknoz said:
Everything is written in the datasheet o the TL750L12CLP (page 5 and 6 indeed): a 10uF tantalum cap with a small ESR is required between OUT and COMMON/GROUND to prevent the output from oscillating and so getting the correct voltage. Just done some basic testing here on a breadboard, the TL750L12CLP was varying between 10,3V and 15V while feeded with +15VDC, I managed to get 11,8V with some cap.

One more time, thank you all for all of that stuff.
Raph - Never too late too learn :)
Click to expand...
We used 22uf 35V this one TAP226K035CCS AVX | Mouser France
 
F

Freecrowder

Member
Joined 2007
Connecting up the balanced output to balanced input amp. Ran into an issue with the L-/ L+ as seen in attached scope shot. They are 180 phasing but not equal amplitude. One is 1.25vac and the other 1.09. This causes one channel in the bridged amp to clip before the other. Just assuming will have to go through gain stage of pre and see where the imbalance occurs. I'm assuming U8a/b.
 

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hadihf

hadihf

Member
Joined 2007
blacknoz said:
Ok, ok, sorry I disturbed you all with that.
I think I found what you were talking about and finally sorted it out.

I'm just updating things here so that other noob like me can move on :)
Everything is written in the datasheet o the TL750L12CLP (page 5 and 6 indeed): a 10uF tantalum cap with a small ESR is required between OUT and COMMON/GROUND to prevent the output from oscillating and so getting the correct voltage. Just done some basic testing here on a breadboard, the TL750L12CLP was varying between 10,3V and 15V while feeded with +15VDC, I managed to get 11,8V with some cap.

One more time, thank you all for all of that stuff.
Raph - Never too late too learn :)
Click to expand...

Post #1067 already solved this issue.
 
T

tcpip

Member
Joined 2003
tcpip.dhandanought.org/
HaroldHill said:
I'm picking up a professional power amp that has balanced inputs. Nice, because I can hook it up via XLRs directly to my Doug Self preamp. But I don't have a balanced DAC to feed my preamp. Is it possible to feed the preamp using a SE DAC with RCAs?
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I would use a balanced cable from DAC to preamp. The male XLR plug of the cable will go into the preamp. At the DAC end, use an RCA-to-female-XLR adapter. That way, I'd get the benefit of separating the shield from the signal-ref-ground for most of the cable length.
 
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