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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Doug Self Preamp from Linear Audio #5
Doug Self Preamp from Linear Audio #5
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Old 12th July 2019, 06:21 AM   #1401
Hicoco is offline Hicoco  France
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Doug Self Preamp from Linear Audio #5
Quote:
Originally Posted by bk856er View Post
24.5V after the rectifiers, 4.1V across R5/6, 1.4V across R9/10.

BK
I just see in my notes that i had the same values, but when i past 200mA with 60R load value, who is the preamp intensity consummation in the positive Rail the tension start to drop to 15V...you should test it to see..
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Old 12th July 2019, 06:49 AM   #1402
bk856er is offline bk856er  United States
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Doug Self Preamp from Linear Audio #5
Quote:
Originally Posted by Hicoco View Post
I just see in my notes that i had the same values, but when i past 200mA with 60R load value, who is the preamp intensity consummation in the positive Rail the tension start to drop to 15V...you should test it to see..
My understanding is the load from the preamp is ~150mA. Therefore I am happy with the PS behavior as load tested with 170mA. I will confirm proper output voltage after assembly, and if necessary address any issues at that time.

BK
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Old 12th July 2019, 06:59 AM   #1403
Hicoco is offline Hicoco  France
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Doug Self Preamp from Linear Audio #5
Quote:
Originally Posted by bk856er View Post
My understanding is the load from the preamp is ~150mA. Therefore I am happy with the PS behavior as load tested with 170mA. I will confirm proper output voltage after assembly, and if necessary address any issues at that time.

BK
Yes you are right for the preamp board, but you have to ad the values of the I/O Board
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Old 12th July 2019, 07:28 AM   #1404
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by bk856er View Post
24.5V after the rectifiers, 4.1V across R5/6, 1.4V across R9/10.

BK
That's cutting it close. From 24.5 input you need: 2V headroom for the opamp, 4.1V for R5/R6, 1.4V for R9/R10, getting down to 17V. Get a dip in mains and it drops out of regulation.

That R5/R6 is too large and R9/R10 is too small, for me that is a design oversight.

Jan
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Old 12th July 2019, 07:35 AM   #1405
bk856er is offline bk856er  United States
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Doug Self Preamp from Linear Audio #5
Quote:
Originally Posted by jan.didden View Post
That's cutting it close. From 24.5 input you need: 2V headroom for the opamp, 4.1V for R5/R6, 1.4V for R9/R10, getting down to 17V. Get a dip in mains and it drops out of regulation.

That R5/R6 is too large and R9/R10 is too small, for me that is a design oversight.

Jan
Understood. I'll grab some of the values you recommended earlier.

BK
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Old 12th July 2019, 08:43 AM   #1406
Hicoco is offline Hicoco  France
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Doug Self Preamp from Linear Audio #5
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Originally Posted by jan.didden View Post
Well done, yes use 47 ohms, or higher, up to 100 ohms should be OK. With 1.2V across the 1k, even 100 ohms only eats 0.12V from your overhead. 100 ohms may be better for stability.

Jan
With 100R for R5/R6 and 1k for R9/R10 with preamp board and I/O board connected for 1h..no overheat and stable at 17V, 200mA for the positive Rail and 150mA for the negative.
Perfect......Thanks a lot Jan
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Old 12th July 2019, 09:18 AM   #1407
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Hicoco View Post
With 100R for R5/R6 and 1k for R9/R10 with preamp board and I/O board connected for 1h..no overheat and stable at 17V, 200mA for the positive Rail and 150mA for the negative.
Perfect......Thanks a lot Jan
Yes, there should be no difference in dissipation. The dissipation is (Vin-Vout)*Iout and that doesn't change.

Jan
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Old 12th July 2019, 09:22 AM   #1408
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Hicoco View Post
I just see in my notes that i had the same values, but when i past 200mA with 60R load value, who is the preamp intensity consummation in the positive Rail the tension start to drop to 15V...you should test it to see..
The main reason for that is that the input voltage drops with higher load, and the ripple voltage increases with higher load. Then there is noi enough headroom left for all that other stuff we discussed.

It does also mean that with a larger transformer it may work up to larger loads. Untill your neighbour switches on his sawing machine and the mains drops 10V ;-)

Jan
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Old 13th July 2019, 03:43 AM   #1409
bk856er is offline bk856er  United States
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Doug Self Preamp from Linear Audio #5
Quote:
Originally Posted by jan.didden View Post
That's cutting it close. From 24.5 input you need: 2V headroom for the opamp, 4.1V for R5/R6, 1.4V for R9/R10, getting down to 17V. Get a dip in mains and it drops out of regulation.

That R5/R6 is too large and R9/R10 is too small, for me that is a design oversight.

Jan
I replaced R5/R6 with 100ohm and R9/R10 with 1kohm.

Still have stable +/-17V with 170mA load, but now with 0.144V across R5/R6 and 1.41V across R9/R10. Temps are the same.

I had no issues, but made the change on Jan's suggestion as good practice. Thanks everyone for the good discussion!

BK
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Old 13th July 2019, 07:35 AM   #1410
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Quote:
Originally Posted by jan.didden View Post
It does also mean that with a larger transformer it may work up to larger loads. Untill your neighbour switches on his sawing machine and the mains drops 10V ;-)

Jan
In the third world, yes, not in the North EU
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