GHR Audio

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

Design files -

Schematic -

Layout -

Fully tried and tested design. I'll post measurements and more pictures later.
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Gerber files here
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Boards for sale
viewtopic.php?t=49
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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

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

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

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.