Gyraf Audio's Obscure Schematics

Here's some schematics and information that can be hard to find elsewhere. If you have some schematics that you think belongs among these, please do not hesitate to contact me.. I hope to be able to extend this little collection with time, so check back for updates.

NOTE: Some of the larger .gif-files dosen't show correctly in some browsers. If you have that problem, instead "right-click" on the link and select "save target as.." to save to your harddisk - and view it from there.

Mic Preamps:

Neumann_PV76 Mic Preamp

Neumann V476 Mic Preamp

Disa 91b21 Microphone preamp (!!)

Tube-Tec MP1A Preamp

NTP 179-270 Mic pre and Compressor

Amek M-2500 Mic pre stage

Calrec UA8000 mic/line input stage

Calrec 1061 input channel w/ EQ.


Neumann PEV Equalizer

Ear 825 "Passive" tube eq

Sontec 430C EQ

Sontec MEP250a

Pultec PE1A Equalizer

Pultec HLF3 Passive High/Low-cut Filter

Pultec MEQ5 EQ Section

Amek M-2500 EQ-stage

Langevin EQ251A Passive - Thanks to mr. Rick Chinn

Klein & Hummel UE-400 EQ

Klein & Hummel UE-100 Tube EQ:

KH_UE100_equalizer.pdf - Thanks to Zebra50!

Klein & Hummel UE-1000 Discrete EQ:



Siemens U273 Diode-based Limiter


DISA 91N02 Limiter - inspiration source for Manley's Vari-Mu

CCA LA-1D limiter/AGC w/ 6386 tube

Drawmer 1960 (original version) (Removed by request from the manufacturer)

NTP_179-140_Manual.pdf (1.8Mb)

NTP_179-160_Compressor.PDF (1MB)

NTP_179-300_Limiter_card.pdf (700Kb)

Drawmer DS-201 Gate (temporarily removed)

Drawmer DS-202 Gate (temporarily removed)


AKG SolidTube schematic (Electrect!)

AKG C414 (The old version)



AKG C422 - Dual 414



AKG C535EB (Eltrect!)

Neumann Handheld Microphone

Neumann KM74i

Neumann KM73 - KM74 - KM75


Neumann KM86

Neumann KM88i

Neumann U89

Gefell MT71S

Radiometer Microphone (Danish)

Sennheiser MKH-105

Sennheiser MKE-401

Sennheiser MKH-405

Sennheiser MKH-435

The "Thermo bottle"

Schoeps N-221-B w/AC701

Senheisser MD421 mechanical drawing





A Japanese article about the Sony C800 Microphone:

Sony C800G Schematic - with component values..

Sony C800 Page 01

Sony C800 Page 02

Sony C800 Page 03

Sony C800 Page 04

Sony C800 Page 05

Sony C800 Page 06

Sony C800 Page 07


SSL 4000E Mixer:

SSL 4000E Input amplifiers - early version, 1979

SSL 4000E Input amplifiers - recent version

SSL 4000E Quad VCA, inserts, Compressor time constants

SSL 4000E Bus compressor time constants, quad fader, autofade

SSL 4000E talkback compressor

Neotek Elite Mixer:

Input channel "A" board

Input channel "B" board

Mixbus amplifiers

Aux mixbusses

Controlroom, Studio and Solo

Balanced outputs

Power supply

Mackie CR1604 VLZ

Allen&Heat (1980'es)

Other devices of interest:

Neumann PV15 Isolation Amplifier

Neumann PV46 Line Amplifier

Neumann W444 active fader module

Neumann 472 Isolation amplifier

Neumann 482 Distribution amplifier

Neumann V475 Mixbus amplifiers

Neumann WE66 RIAA playback equalizer

Bridged attenuators layout

NTP paper describing NTP 179-120, 179-160, 179-230, 179-400, 182-100, 182-200, and old Pricelist

VCA's and discrete opamps:



VCAs_Ben_Duncan.pdf (an article)

DBX 2150 Application notes

Dbx 202 VCA - schematic and data

NTP M-100 Discrete opamp, 1978

More versions of NTP M100 Schematics.pdf

NTP M-100 datasheet page 1

NTP M-100 datasheet page 2

RadioDesigners Handbook - RDH:

RadioDesigners Handbook - Ch.5 - Transformers

RadioDesigners Handbook - Ch.1 and 2 - Introduction to the radio valve

Other stuff:

Voltage multipliers with CMOS gates.pdf


RTW1227E PPM schematics

Behringer ADA-8000 8-ch AD/DA converter analouge sections

Westlake BBSM-10

Revox G36 tubed tape recorder Schematic

Beyer Dynamics catalouge page - audio transformers, 1969

Updated 15-02-2006

since 10-2002