I made a perf layout for the Dorsey circuit. Any obv. future noise problems?

[midwayfair]

Dorsey's  modified Shoeps circuit:

Layout:

EDIT: REMOVED. Do not use that layout, the noise is awful!

I'm using a 990 housing, so there's more room if it's necessary to spread anything out more.

I tried to make sure that the wires for the capsule didn't have to be right next to each other, and the places where the audio line has to cross over other things is perpendicular rather than parallel to other audio lines.

I've seen some layouts/PCBs that do something special with the 1G, though, where it's lifted off the PCB. Is that to keep it away from a ground plane, or is there something else at play that I haven't considered?

(Why perf? With this few parts, I really didn't feel like making a PCB layout.)

 

[PRR]

A "1Gig", mounted on cheap perfboard, will be a few hundred Megs at best (board leakage).

Mount 1Gig and JFET Gate on a GOOD insulator. Teflon is classic. A scrap of brittle Polystyrene with two grommets can work.

 

[midwayfair]

A "1Gig", mounted on cheap perfboard, will be a few hundred Megs at best (board leakage).

Mount 1Gig and JFET Gate on a GOOD insulator. Teflon is classic. A scrap of brittle Polystyrene with two grommets can work.

Huh. Even if I keep it physically off the board? How far away does it "leak" into the board? Where it the point where it's leaks -- the body of the resistor or the leads (not that the latter makes much sense to me)?

Not sure i have any teflon lying around the house but my dad might have some ...

 

[abbey road d enfer]

I tried to make sure that the wires for the capsule didn't have to be right next to each other, and the places where the audio line has
I've seen some layouts/PCBs that do something special with the 1G, though, where it's lifted off the PCB. Is that to keep it away from a ground plane, or is there something else at play that I haven't considered?

That's because the insulating material is not perfect, particularly the phenolic perfboard is made of; its conductance may seriously alter the value of a 1 Gig. Even with FR4, any contamination alters performance. Lifting the sensitive components is asimple effective way of avoiding problems.

 

[midwayfair]

Cool. Thanks for the help! I'll try keeping the 1Gs about 2-3mm off the board and put some teflon under them just to be safe.

 

[abbey road d enfer]

Cool. Thanks for the help! I'll try keeping the 1Gs about 2-3mm off the board and put some teflon under them just to be safe.

It's not really the resistor itself that must be lifted, it's the sensitive connection. You should have the 1st 1G connected to the perfboard ONLY at the top, the 2nd 1G connected ONLY AT THE BOTTOM, the gate of Q1  and the 820pF connected only to the lifted end of resistors.

 

[midwayfair]

Cool. Thanks for the help! I'll try keeping the 1Gs about 2-3mm off the board and put some teflon under them just to be safe.

It's not really the resistor itself that must be lifted, it's the sensitive connection. You should have the 1st 1G connected to the perfboard ONLY at the top, the 2nd 1G connected ONLY AT THE BOTTOM, the gate of Q1  and the 820pF connected only to the lifted end of resistors.

Yikes. You mean like a PTP connection, physically floating on top of the board? That sounds ... a little dodgy.

Maybe use a couple ceramic standoffs as turret posts for the R9-C6 and C6-Q1G-R10 nodes? I might be able to find a couple small enough to fit.

EDIT: Or maybe I should just not be cheap/impatient and do a PCB layout. It's not like OSHPark takes that long to deliver ...

 

[abbey road d enfer]

Cool. Thanks for the help! I'll try keeping the 1Gs about 2-3mm off the board and put some teflon under them just to be safe.

It's not really the resistor itself that must be lifted, it's the sensitive connection. You should have the 1st 1G connected to the perfboard ONLY at the top, the 2nd 1G connected ONLY AT THE BOTTOM, the gate of Q1  and the 820pF connected only to the lifted end of resistors.

Yikes. You mean like a PTP connection, physically floating on top of the board? That sounds ... a little dodgy.

Maybe use a couple ceramic standoffs as turret posts for the R9-C6 and C6-Q1G-R10 nodes? I might be able to find a couple small enough to fit.

That's the nice way of doing it; PRR suggested teflon. When you don't have teflon or ceramic, use air.

EDIT: Or maybe I should just not be cheap/impatient and do a PCB layout. It's not like OSHPark takes that long to deliver ...

Same caveats apply to PCB, even if FR4 has much better insulation than phenolic. Contamination is the major issue. the residues must be cleaned thoroughly.

 

[midwayfair]

Well, I built it and it makes noise. Lots of noise. Self noise (hiss) was really bad. I matched the PNPs pretty well and cleaned everything with alcohol and it was still just terrible. Replaced the FET once (tried both 2SK170 and MPF102) but there was no improvement in hiss, and it's definitely the circuit because it was still very hissy even with the capsule disconnected.

Oh well, can't win them all!

I might give it another go with a circuit board some time in the future, but for now I popped the original circuit back in with the replacement capsule (took out some screens too) and I'm pretty happy with it. It's not like there's anything really wrong with the original circuit anyway.  I think this'll be a touch better on female vox than the MK4 that's my go-to. It's a bit more midrangey than the MK4 that's my go-to mic, and a little less crispy and the capsule is still a substantial improvement.

 

[Chris_V]

What king of cap did you use for C6 ? You must have a very low leakage cap, PS or PP.

 

[midwayfair]

What king of cap did you use for C6 ? You must have a very low leakage cap, PS or PP.

Kemet box cap, 1n/100V. (FWIW, the input cap on the SMD circuitboard is still a box film cap.)