Transformerless Feedforward Vari-Mu

[Heikki]

Transformerless vari-mu based on the U73 compressor/limiter.

Design files

notU73.zip

(10.92 MiB) Downloaded 180 times

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Schematic

73.pdf

(148.42 KiB) Downloaded 246 times

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Layout

73layout.pdf

(2.37 MiB) Downloaded 215 times

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Fully tried and tested design. I'll post measurements and more pictures later.
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Gerber files here
viewtopic.php?t=75
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Boards for sale
viewtopic.php?t=49
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not73.JPG (3.72 MiB) Viewed 19614 times

[Heikki]

Specifications

Input impedance ≈ 12k ohms, balanced line-to-line *

S/N > 86 dB re +4 dBu, 22 kHz BW, unity gain

Frequency response = 20 - 20 kHz, ±0.5 dB

Gain = 0 dB at limit mode, 6 dB at compress mode

Output impedance = 50 ohms, balanced line-to-line
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* INA2137 Internal resistors are ratio matched but have ±25% absolute value

[scott2000]

I can make out U5 on the DCDC board pic. What model Recom is used?

[Heikki]

Recom R12-150B

Regulators for the DC-DC converters are 12V and regulator for the heaters is 5V.

R1 = 1 ohms

R2 = 2.2 ohms

R3 = 4.7 ohms

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[Script]

Would 6BA6 tubes work ? Read that they are not fully equivalent to 6K4P, something to do with internal resistance.

[Heikki]

6BA6 is essentially equivalent to 6K4P. Only somewhat significant difference is that in 6KP4 suppressor grid is internally connected to cathode. Difference in internal connections won’t matter since tube pins 2 & 7 are connected together on the board. Any vari-mu pentode with same pinout as 6BA6 or 6K4P will work, with other tubes R13 might need to be changed.