S250 - ELA M250 inspired Cardioid microphone with 5840//EF732

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ln76d

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[Currently in construction]

Cardioid/Omni
Edge terminated capsule  - AKG  2072 Z 2009
Haufe T14/1 transformer
5899 Tube.
All carbon Beyschlag resistors - same type as in ELAM 250.
Capacitors also same type - two tubular ceramic and two wet tantal.

Would be great to get voltage specs of original (or even clone) unit with AC701k.
For 100% there would be need to change cathode resistor value but for that - original voltage specs are needed.
I started separate topic about the voltages, but  feel free to put any info here ;)


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ln76d

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Quick question - advice needed :)

Two microphones one PSU, one power transformer.

What's  better?

- two separate heater and two B+ circuits parallel to AC side
- two microphones parallel connected to the DC side (for both - heater and high voltage supplies)
- mixed
 

granger.frederic

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May 22, 2010
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451
dual mono for better crosstalk performance and SNR...

about your headamp: personally i would place a filtering capacitor before R : 1 uF bypassed by a 10pf
also i would rise R4 and R1 to 100M min (less distortion) and C2 to 47uF , unless you want to decrease the low end
I would increase C1 to 10pF min as well
I would decrease R5 to 100K with a 5840 / T14/1

my 2 cents
regards
 

ln76d

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Hey granger.frederic,

thanks for info.
What do you mean by dual mono?

For me dual mono was always related with two transformers - and if you had that in mind - there's no option due to costs and PSU enclosure size.
If two separate heater and two B+ circuits parallel to AC side - this would be possible.
Then i think could be resonable to make regulated common heater supply and non-regulated double B+.
 

granger.frederic

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May 22, 2010
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if for cost reason you can't use 2 PSU , the best option is to separate the B+ rails and check if the heater can handle 2 filaments in parallel ...
 

ln76d

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For two separate i could use two chinese PSU with modified circuits but i want make a pair of microphones running from one box.  Price  is not only reason! It's usefull at the recording situation - less boxes on the floor - and i like to running tube mikes from short cables like 3-4m inside recording room.
There's no problem to make common heater with power reserve. Also this would be usefull - without any problem i could run one microphone - powering off B+ second section.
Also!
I have nice box from Gefell PSU for use - maybe this is the real reason :D :D :D
 

granger.frederic

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just be careful that the power transformer can handle 2 B+ rails and 2 H+ , check the temperature after 1 hour : max 40°c
 

ln76d

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Thanks!

If i'll be decided for single psu, then i'll use proper transformer (not chinese) so from this side it would be safe
 

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