Project GORD 01 - KM84 for C480B Frankenmic Thingy

A project to see if we can stuff the electronics of a KM84 into a C480B

The C480B contains electronics of a truly ludicrous complexity. There are about 24 transistors in it, compared to the one to five in most microphones. The capsules used on this mic are nice.

Gord Basa contacted me to see if I could see a way to get some more reasonable electronics inside it. He particularly likes the idea of putting the Neumann KM84 type of circuit in it. He wants to get something like a KM84, but at less cost. Also we will get a more practical microphone with greater reliability as many KM84s are showing their age. We will have the option of using the best modern components for lowest noise. I have a few modifications to suggest for the circuit to take advantage of these, but also want to design the PCB so that the original circuit topology can be built unchanged, as I'm sure many people will want to do that. It will be difficult, mainly because of the transformer output, but I think it's worth a try.

If a similar idea had occurred to me, I think I would have chosen to base the design on the Oktava MK-012, a simple, elegant and popular circuit. It has two transistors and a transformerless, impedance-balanced output like the C480B. But we will have a look into Gord's idea first, and maybe have that as a backup plan. There is no reason not to try both.

What we will get is probably something rather like a KM84 and I hope better in some ways. It will not be cheap, but will I think be a very useful microphone.

Transformer

The first problem is to find a transformer that will do the job and also fit inside the tube.
Some possibilities:

http://cinemag.biz/mic_output/PDF/CM-2510.pdf Cinemag CM2510, looks good.

http://www.tab-funkenwerk.com/id133.html AMI T13 looks good.

Lundahl

All the Lundahl transformers that might do the job seem to be a little too large to fit.

Another possibility is to have one specially manufactured.

To see full-size images, right-click > view image > click on image

	Redrawing of the original Neumann schematic
	rt-gord01.sch.png Here I have simplified the circuit slightly. We might as well take advantage of modern components and design technique and get rid of a couple of RC stages and the zener diode.
	Pad and HPF switches. This version of the circuit is an attempt to make use of the C480 switches for pad and HPF with my modifications. Maybe they can be incorporated into the Neumann circuit. The HPF method here is simpler than the AKG, having only a first-order response, and there will be some small effects of leakage inductance and also a response dependent on the input characteristics of whatever preamp the mic is connected to. However, not in my experience a reason to discard the idea. What is lost in steepness of slope and accuracy in this type of circuit is often compensated for by a cleaner-sounding low-frequency response.
	I want to be able to reproduce the exact Neumann KM84 circuit, or optionally include my mods. This schematic shows how that can be done. If the PCB goes on sale, it will suit people who think the Neumann circuit should remain unmolested, and also those who want to try a few simple mods .
	Preliminary PCB design V1.
	Photo from Gord comparing the layout with the AKG PCB. The new PCB design is numbered RT-48084.
	In this build I am going to use my modifications to the schematic. The resistor R13 will be omitted. The three pairs of solder pads which have been blobbed here directly connect the ground of the circuit to pin 1 of the XLR.
	A couple of photos from Gord showing AMI T8 can be fitted properly.
	I constructed this transformer for a 1:7 ratio, with 300 turns on the secondary and 2100 on the primary. The wire used is 7 mil diameter for the secondary, but only 2.5 mil diameter for the primary. The very fine wire is difficult to handle. This is a fairly rough job, done without setting the machine to the wire diameter.
	I have glued it in, but I think it would be better to use silicone gel. But this is for prototype purposes, so I will most likely refit it after testing.
	Assembled for testing. In this version I am using a J113 FET, and R3 has been selected as 8k2.
	Electrolytic capacitors are on the back of the PCB.

Component selection

Fortunately no rare components are needed and good quality candidates are available from Farnell, Mouser and Digikey.

If you are going to use the KM84 drain current of approx 0.25mA, you will need to select a value of R3 to give about 10V on the FET drain. I feel that it might be better to rearrange things for a larger current, however I have tested a small variety of FETs at the original current and found these results for R3:

J112		13k
2N3819		17k
U1898/J111	11k
J113		8k

Obviously, YMMV.

Gord reports that he has used a LS846 successfully and it requires a 3k6 resistor.

Capacitors 4p	Farnell order code	Notes
MULTICOMP MC0805N4R0C500A2.54MM CERAMIC 4PF, 50V, C0G/NP0	1694276
15p
AVX SR151A150JAR Multilayer Ceramic Capacitor, C0G/NP0	1100498
MULTICOMP MC0805N150J101A2.54MM C0G/NP0	2112750
33p
KEMET C315C330J1G5TA7303 Multilayer Ceramic Capacitor C0G/NP0	2575632
AVX SR151A330JAR Multilayer Ceramic Capacitor, 33 pF C0G/NP0	1100370
470p
WIMA FKP2D004701D00JSSD Film Capacitor, 470 pF, PP (Polypropylene)	1005988
1n
MULTICOMP MCPBSFC-1J102KA45 polyester	1685470
KEMET R82EC1100Z350J Film Capacitor, 1000 pF, PET (Polyester), 100 V	1679411
WIMA FKP2D011001D00HSSD Film Capacitor, PP (Polypropylene)	1890176	Also see AMCH film capacitors for C1 http://www.partsconnexion.com/prod_pdf/amch.pdf
10n
KEMET C315C103K1R5TA Multilayer Ceramic Capacitor, Gold Max X7R	1457642
100n
EPCOS B32529C0104K000 PET	9750878
WIMA MKS2C031001A00KSSD Film Capacitor, 0.1 µF, PET (Polyester)	1006030
VISHAY BFC237021104 Film Capacitor, 0.1 µF, PET (Polyester)	1215515
220n
KEMET SMR5224J63J03L16.5CBULK PPS	2529177
KEMET R82DC3220DQ60J Film Capacitor, 0.22 µF, PET (Polyester)	2429338
1/25
VISHAY 489D105X0025A1VE3 Tantalum Capacitor, 1 µF, 25 V, 489D Series	1753978
1/63
MULTICOMP MCGPR63V105M5X11	9451455
KEMET ESK105M063AC3AA	2069032
MULTICOMP MCMHR63V105M4X7	9452737
MULTICOMP MCRH63V105M5X11	9452001
4.7/25
PANASONIC ELECTRONIC COMPONENTS ECEA1EKS4R7	1902002
22/63
PANASONIC ELECTRONIC COMPONENTS EEUFC1J220	9692495 **
PANASONIC ELECTRONIC COMPONENTS ECA-1JHG220	1848558 **
Resistors
1G
TE Connectivity Manufacturer Part No: RGP0207CHK1G0	N/A	Mouser Part No: 279-RGP0207CHK1G0 Digikey A106006CT-ND ***
Stackpole Electronics Inc. HVA12JA1G00	N/A	Digikey HVA12JA1G00CT-ND **
OHMITE SM102031007FE	1550765	??
Ohmite Thick Film Resistors 1/4watt 5% MOX-200001007JE	N/A	Mouser Part No: 588-MOX-200001007JE
10M
TE CONNECTIVITY LR1F10M	2329991
MULTICOMP MCMF006FF1005A50	2401834
100k
TE CONNECTIVITY LR1F100K	2329987
56k
TE CONNECTIVITY LR1F56K	2330186
47k
TE CONNECTIVITY LR1F47K	2330158
2k2
TE CONNECTIVITY LR1F2K2	2330087
10k
TE CONNECTIVITY LR1F10K	2329990
47
TE CONNECTIVITY LR1F47R	2330160	Also see Vishay VAR series film resistors partsconnexion.com
10
TE CONNECTIVITY LR1F10R	2329993
FET
2N3819 obsolete, old stock available various places, Farnell suggested equivalent is J112	1017712
J201 ditto PF5102	2454063
J113	1017713
Zener
FAIRCHILD SEMICONDUCTOR BZX79-C24	9844554
Switches
C & K COMPONENTS OS203011MS1QP1 Slide Switch, DP3T	2435093 **
E-Switch SWITCH SLIDE DP3T 200MA 30V EG2305A	N/A	Digikey Part Number EG1920-ND
Transformer
TAB-Funkenwerk T8
Cinemag CM-5722
Cinemag CM-2510		cinemag datasheet pdf

Message from Gord:

> I've made up a bill of materials for all the parts. In general, these are
> the parts I tried to use:
>
>   - C0G(NP0) ceramic capacitors
>   - Elna Silmic II and Panasonic FM electrolytic capacitors
>   - Wima MKS2 film caps
>   - Vishay/Dale CMF55 resistors
>   - I'm stealing the 1G resistors from my old AKG circuit boards.  They're
>   expensive to purchase!
>   - LS846 JFET from Linear Systems:
>   https://4donline.ihs.com/images/VipMasterIC/IC/LISI/LISIS00305/LISIS00305-1.pdf
>   - TAB/Funkenwerk T8 Transformer

List from Gord of his preferred components:

With Switches   Robert's Values         Original KM84   Gord's Values           Seller          Part Number

C1              (1nf) 470pf             470pf           470pf                   PartsConnexion  AMTRANS-73780
												(alternatively Wima FKP2 ?)
C2              4pf                     4pf             4pf                     Mouser          RCE5C1H4R0C0DBH03A
C3      	22/63 (4.7/25) 		4.7/25  	22/50   		Mouser  	EEU-FM1H220
C4      	22/63 (1/25)    	1/25    	22/25   		Mouser  	RFS-25V220ME3#5
C5      	22/63 (4.7/25)  	4.7/25  	22/50   		Mouser  	EEU-FM1H220
C6      	100nf   		100nf   	100nf   		Mouser 		MKS2D031001A00JO00
C8      	4.7/25  		4.7/25  	22/50   		Mouser  	EEU-FM1H220
C9      	15pf    		15pf    	15pf    		Mouser  	RCE5C1H150J0A2H03B
C10     	220nf (100nf)   	100nf   	100nf   		Mouser  	MKS2D031001A00JO00
C11     	33pf    		-       	33pf    		Mouser  	C315C330K1G5TA
C12     	1/63    		-       	1/50    		Mouser  	RFS-50V010ME3#5
C13     	220nf   		-       	220nf   		Mouser  	MKS2-.22/63/5T

R1      	1G      		1G      	1G      		Mouser  	MOX200001007FE  Use old ones from AKG 480
R2      	1G      		1G      	1G      		Mouser  	MOX200001007FE  Use old ones from AKG 480
R3      	Bias    		Bias    	Bias
R4      	47k     		47k     	47k     		Mouser  	CMF5547K000BER670
R5      	0 (10M) 		10M     	10M     		Mouser  	CMF5510M000FHEK
R6      	0 (56k) 		56k     	56.2k   		Mouser  	CMF5556K200DHR6
R7      	100k (10k)      	10k     	10k     		Mouser  	CMF5510K000BEEA
R8      	10M     		10M     	10M     		Mouser  	CMF5510M000FHEK
R9      	2.2k    		2.2k    	2.2k    		Mouser  	CMF552K2000FHEB
R10     	2.2k    		2.2k    	2.2k    		Mouser  	CMF552K2000FHEB
R11     	47r     		47r     	47r     		Mouser  	CMF5547R000FEEK
R12     	47r     		47r     	47r     		Mouser  	CMF5547R000FEEK
R13     	0 (10r) 		10r     	10r     		Mouser  	CMF5510R000BEEK

T1      	J201 FET        	J201 FET        J201 FET        	http://www.nacsemi.com/ LS846 or LSK170
										or https://www.trendsetter.com
GR1     	Zener Diode     	Zener Diode     Zener Diode     	Mouser  	BZX79C24
    
Switches        E2305       		E2305        	E2305        		Digikey 	EG1920-ND
										Mouser		612-EG2305
    		E2305   		E2305        	E2305        		Digikey 	EG1920-ND
										Mouser		612-EG2305

Transformer                     			T8      		TAB/Funkenwerk  T8