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Old 22nd December 2011, 11:06 PM   #51
anatech is offline anatech  Canada
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Hi Gaetan,
The extreme resistances I have seen were encased in glass. They were pretty long too. I once had some 70 K Meg (yes, that was the value) for Geiger counters. As I recall, the resistors inside the glass were painted red. This had to be for voltage breakdown, but also getting the leakage path to be long enough. Normal humidity might very well decrease the value of those resistors, and a finger print would be your undoing.

Teflon standoffs are used to eliminate the "K effect" from PCB material as well as extend the leakage path as previously mentioned. The PCB material would become a capacitor, presenting a loss and doing funny things with charges at those impedance levels. Noise would be expected as a result - never mind distortion with a weird quality. With the Teflon standoffs, conductors now form a lower value capacitor using air as the dielectric. That means a good capacitor. Since the "K" of air is only slightly higher than that of a vacuum, the resulting capacitance is lower than you would get with a material between conductors. They do offer PCBs with a Teflon substrate, but you don't want to know how much. Actually, I don't even know how much that stuff is. It would be perfect for Mic, RIAA or general signal preamps. The main use is supposed to be for RF circuits I think.

If we don't post together before then, Merry Christmas to you as well. Do you think there will be snow in Quebec for Christmas??

Hi Scott,
Thinking along this line, would this be the reason for the small sound differences between the DIP-8 and metal 8-pin packaged op amps? To the best of my knowledge, the die is identical, the bond wires might be the same material, so that leaves the lead frame and material it is encapsulated with. The DIP-8 lead frame would also travel a much greater distance inside the material.

Personally I have never compare the two. I have no expectations at all beyond hearing other reports on this. You be da man to ask if there is anyone around here that could answer.

-Chris
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Old 23rd December 2011, 12:46 AM   #52
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Hello Chris

It could be fun to try to made some diy 1 G ohm resistors and put arround it some silicone glue.

I remember many comments from various diyers and audiophiles about a better sound from metal 8-pin packaged op amps. Yes maby the shorter wires inside the metal can, but maby also because the metal can do a total isolation from external rf noise.

I live in mountain in the Laurentides, here we have 2 inches of snow with ices everywhere on the snow and on the road.

Thank and Merry Christmas to you to.

Bye

Gaetan

Last edited by gaetan8888; 23rd December 2011 at 12:54 AM.
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Old 23rd December 2011, 04:36 AM   #53
anatech is offline anatech  Canada
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Hi Gaetan,
I figured you would have snow. Believe it or not, we don't have any. It's fairly warm outdoors still. Let's see what colour Christmas we get.

-Chris
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Old 24th December 2011, 10:17 PM   #54
1audio is offline 1audio  United States
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Default Photos of mike preamps

Here are pictures of the inside construction of the HP 1/2" mike and a B&K 1" tube mike. I could not figure out how to open the B&K preamps I have and the open ones I have are not here. The B&K has the ntube under the PCB. You can almost see it in the side view. Enjoy.
Attached Images
File Type: jpg HP preamp.jpg (766.6 KB, 163 views)
File Type: jpg B&K tube 2.jpg (818.0 KB, 155 views)
File Type: jpg B&K tube 1.jpg (678.4 KB, 154 views)
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Old 3rd January 2017, 06:57 PM   #55
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I realize this is an Old Thread, but just curious if the B&K 2671 could be made running ?

Or if anyone ever has seen a schematic ? From there the required supply could most likely be determined.

Thanks!
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Old 9th January 2017, 03:20 AM   #56
1audio is offline 1audio  United States
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Is this what you are referring to? https://www.bksv.com/en/products/tra...mplifiers/2671

That family of preamp/interfaces is quite different from the older stuff. All you need is a 28V supply and it modulates the output current. No mike bias so you need prepolarized mikes.
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Old 9th January 2017, 08:46 AM   #57
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Thanks for the response. Yes, that's indeed the one & has the pre-polarised 4155 capsule.

I found this, so I'll build a 4mA current source Project 134

I guess I can testdrive it (checking for basic functionality) simply by that 28V-osh voltage and a resistor & AC-coupling cap.

Was a bit surprised I couldn't find any relevant schematics on the interweb, they seem well guarded.

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Old 9th January 2017, 11:14 AM   #58
1audio is offline 1audio  United States
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B&K has morphed like most of its competitors into a more closed operation. I doubt any of these things get fixed today. In the corporate world they just get replaced. The current mode interface has become quite common and makes it easy to move from transducer to transducer in different applications. With a "small" sacrifice in performance.

Nice link with lots of useful info.
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Old 9th January 2017, 11:24 AM   #59
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Quote:
Originally Posted by 1audio View Post
B&K has morphed like most of its competitors into a more closed operation. I doubt any of these things get fixed today. In the corporate world they just get replaced.
I see, so less/no need for service documentation. The simplest 1 channel interface for such a mic-pre does ~$500 2nd hand (asking price) so that makes it a no-brainer, as in: DIY a 4mA current source myself.

BTW, note that one of the suggested supplies (WB-1372) 'does' 3mA +/-20%, so less than the assumed to be the typ value of 4 mA.

FWIW, I assume it might be related that this WB-box is battery-powered.

Quote:
The current mode interface has become quite common and makes it easy to move from transducer to transducer in different applications. With a "small" sacrifice in performance.
Curious to learn, which performance-sacrifice ?

Quote:
Nice link with lots of useful info.
The ESP-site a valuable resource!
For convenience, several Projects have also PCBs available.

Best regards
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Old 9th January 2017, 07:01 PM   #60
1audio is offline 1audio  United States
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A battery powered version would make a lot of sense. It would not be hard. I would use a cheap DC-DC converter from China + a Lipo battery. Then probably a depletion mode mosfet plus a selected current diode for the current supply. Supply current does have an effect on the output voltage. With the B&K mikes they don't need gain so much less effect on the output. The output is high enough that you can use a follower inside and get a low output Z.

LM2587 DC-DC Boost Converter 3V-30V Step up to 4-35V Power Supply Module MAX 5A | eBay

I could sketch out the rest later if you want.
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