K89 / TLM 193 Project

[svyet]

LATEST (as of March 14, 2023):

My original objective starting out was to build a DIY TLM 193 (K89 capsule with transformerless circuit). However, I've since acquired an actual TLM 193 with some initial problems. It should be an easy fix, but I have nonetheless wanted to deliberate which circuitry to use in case of emergency. Accordingly, this project has now evolved into more of an inquiry and insurance endeavor in case my OEM circuitry does eventually fail. The goal would be to have a PCB design that can be easily bought and populated with minimal components and dropped directly into a TLM 193 in need of a circuit replacement. This thread can also double as a ready-made pool of ideas for any future DIY K89 build (or any capsule for that matter) all in one thread, saving time for non-engineers like me and/or people who might not know where to start with their first build.

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ORIGINAL THREAD POST:


Hello all,

Before reading on, please be kind and consider that this inquiry, however blind, is out of genuine interest in the feasibility of such a project, even if it only ends in discussion.



PREFACE

After some messing around with my NT1 for a while (big shout out to @kingkorg for the flat IR that works very well: Rode NT1 Kit (the black one) Mods) and loving the results, I still have the itch to build a flat-response workhorse from scratch. Particularly a cardioid Neumann U 89 / TLM 193.

I found out about the TLM 193 on Audio Test Kitchen (ATK), and it sounded so super smooth and neutral to me. Probably boring for most people, but I really need a high-quality platform that takes EQ especially well (good dynamic range and high SPL handling should go without mention). Found out it’s basically a transformerless (KM 100 circuit) cardioid-only version of the U 89i. They have the same frequency response in cardioid, and the TLM 193 is half the price of the U 89i.

Here are the frequency response curves for the U89 and TLM 193 respectively:





The U 89 / TLM193 seem to have a following with classical recordists, but it suited the variety of tracks just fine on ATK. I love its unexaggerated character. However, there’s not much at all on either the TLM 193 or the U89 on GroupDIY. Seems like most people love the U87 which now sounds way too hyped compared to the U89 for my purposes. However, I did find a schematic for the TLM 193 (posted below), which is already a big step forward.

In my search on the forums, an additional – and quite possibly the biggest – glimmer of hope: only a few months ago @soliloqueen alluded to wanting to pick apart a K89 capsule after some interest from a few other members (Soliloqueen's k87(k67) and k47 capsules). Judging by everything I’ve read from the other members about her work on other capsules, that would be a much-welcomed offering for a handful of us

All of that said, here is my reasoning for the project, basic build info to get started, and then finally preliminary notes on parts. While I can follow directions and solder very well, I’m not an engineer and therefore need a lot of help and pointers on deciding and sourcing the needed parts.



REASONING

Technical:
A cardioid-only U89 with simple circuitry (i.e. TLM 193)
Flat/smooth/natural frequency response
Simple, robust, discrete, serviceable

Financial:
Not spend $1,500 on a mic
Not spend $1,500 on a mic that is not serviceable

Intangible:
Satisfaction of building my own serviceable workhorse microphone
Satisfaction of saving money by building my own workhorse microphone



BUILD INFO / PARTS:

TLM 193 schematic deleted from this thread and posted in the proper schematic forum HERE
(thanks to @Moby for originally posting it HERE)


Parts:

- K89 capsule (@soliloqueen? Anyone else?)

- PCB (anyone who can design a board, anyone who prints them reasonably)

- Component list (need help)

- Body with suitable headbasket
(I'm not sure where to start and what would be comparable to a U 89 or TLM 193)
(also depends on PCB size, or rather PCB size would depend on donor body?)
Alternatively, source a genuine headbasket (pretty essential) and retrofit it to a donor tube



INITIAL QUESTIONS

1. Even with such a simple circuit, is there any special biasing/adjustments that need to be made? How much of a headache would this be?

2. Is this ultimately a lost cause, and should I simply try to find a reasonably-priced used TLM 193, and consider simply replacing the PCB inside if/when there is a failure?

3. I’m concerned most about consistency and close replication, not a mere approximation to the TLM 193. What I have with the NT1, Flat IR, and some EQ, it’s a very reasonable microphone as is. Perhaps that's reason enough to go the used TLM 193 route and replace the internals if/when there is failure?



Looking forward to everyone’s thoughts!

 

[soliloqueen]

A k89 would need a very high preorder load to meet the MOQs for the various parts, I would need just over 100 preorders at $199...probably not super feasible in the short term, but if there's interest, there's interest.

 

[Khron]

Probably easiest for the electronics to be something like @rogs ' OPIC? Especially if you need it to be cardioid-only. Well, unless you wanna go fancy with some low-cut and/or padding as well...

 

[kingkorg]

Every now and then that capsule shows up for sale. Currently Peluso is selling one "refurbished" at sky high price. Maybe find a Sennheiser dealer/service/technician and approach them carefully for a spare one. Sometimes they are willing to order and sell stuff. There are people who know people...

+1 what Khron said, no biasing required.

 

[soliloqueen]

I have one here already. It's pretty straightforward. same principle as charging and taking signal off of the diaphragms, but with isolated backplates inside the grounded body, offset from grounded diaphragms instead. makes it basically impossible for it to break, pretty clever. pairing it with an RF circuit was one of my first thoughts. if they worked well together, the resulting microphone would be practically impervious to humidity and very easy to weatherproof.

 

[kingkorg]

I have one here already. It's pretty straightforward. same principle as charging and taking signal off of the diaphragms, but with isolated backplates inside the grounded body, offset from grounded diaphragms instead. makes it basically impossible for it to break, pretty clever. pairing it with an RF circuit was one of my first thoughts. if they worked well together, the resulting microphone would be practically impervious to humidity and very easy to weatherproof.

And useable without any grille. For the brave...

 

[soliloqueen]

And useable without any grille. For the brave...

not even just that! from what I can tell, it'd technically work underwater, if it were fully sealed. this design concept is very robust. i was planning to integrate how the termination works into an original capsule design later, along with some other fun tricks.

 

[svyet]

Wow, thank you all for the info. That simplifies so much.

I assume with the electronics, it just needs to be a simple/linear circuit like the OPIC and that's it? Nothing fancy? As for adding any additional features that @Khron mentioned, I'm not interested at all in complicating anything. I'm just interested in raw capture.

Thanks @soliloqueen for the warning, but also the hope that someday you might have something of your own on offer! As far as the robustness, I believe that is one of the selling points of these mics. That they can really take a beating. That also has its appeal to me.

As far as an RF bias circuit is concerned, would the KM 100 circuit qualify as such or no? I'm not sure what I'm looking at in the schematic. All I know is that many of the big names are using RF circuits in the modular small condenser mics that often see outdoor use (Sennheiser, Schoeps, etc.). In the same thread where I found the TLM 193 schematic, I believe the OP was having an issue with the board and encountered a lot of protective coating - not specifically mentioned in anything I've read, but Neumann did brag about the protection on the circuit, and if I recall correctly, some reviewers have had 193's since they came out in the 90's and they're still going without any problems.

Seems like if I want to do this any time in the near future, I'm best to keep my eye out for a genuine capsule and go the project route, or keep it simple and hold out for a beat up TLM 193 for the right price and worry about tinkering later.

Thanks all, for helping me come back full circle for the time being

 

[Екатерина]

Oh, when you study this circuit, you will find that they are not simple. Those two parts are not standard parts and there are no parts to replace them. They just package the circuit into parts, which is naughty

 

[bremusound]

The TLM193 is cardioid only but nevertheless Neumann put some volts on the rear backplate. That‘s the most unusual thing about the circuit.
The impedance converter is identical to those which are used in the KM100 series active capsules.

EDITED, there‘s no voltage on the rear backplate.

 

[Khron]

The TLM193 is cardioid only but nevertheless Neumann put some volts on the rear backplate. That‘s the most unusual thing about the circuit.

They did? How and where? I see it goes to ground through a 16ohm(?) resistor.

 

[bremusound]

They did? How and where? I see it goes to ground through a 16ohm(?) resistor.

Oh, you are right… i remembered wrong! Sorry

 

[Khron]

Speaking of the schematic though, is the front diaphragm grounded through a 49.9 ohm resistor, or a 49 gig one? That "49E9" marking is a bit weird...

 

[svyet]

Oh, when you study this circuit, you will find that they are not simple. Those two parts are not standard parts and there are no parts to replace them. They just package the circuit into parts, which is naughty

I'm not an engineer, so perhaps I don't see what you see, but it doesn't look too entirely complicated other than the two proprietary IC's, the first of which could be a "High Dynamic Range MMIC Mixer" while the other seems like a Texas Instruments Op Amp (though if only I knew how to read the values to figure out which one haha). which is probably what you are talking about. Those would definitely need some investigation. Pictures of TLM 193 internals can be found in this other thread: Neumann TLM193 circuit help?. Everything else looks pretty standard, though. I actually wondered how the older style KM circuit (e.g. from KM84) compares to the newer KM 100 circuit above (e.g. KM 184), and whether or not the older circuit could act as a replacement for the new one and what effect that would have (especially given the transformer)? Kits for the KM84 circuit are readily available.

The TLM193 is cardioid only but nevertheless Neumann put some volts on the rear backplate. That‘s the most unusual thing about the circuit.
The impedance converter is identical to those which are used in the KM100 series active capsules.

EDITED, there‘s no voltage on the rear backplate.

From the TLM 193 product description: "This unit is mated to the proven FET 100 circuitry of the KM 100 series."

Speaking of the schematic though, is the front diaphragm grounded through a 49.9 ohm resistor, or a 49 gig one? That "49E9" marking is a bit weird...

Looks like 49.9K

https://www.mouser.com/c/?q=49e9

Of particular interest to @kingkorg and @soliloqueen, perhaps by divine providence, I've already sourced an older TLM 193 in rather "ugly" cosmetic condition with a connection/grounding issue that seems easily resolved. It's likely related to the beat up XLR connection tube (it's missing a piece of the tube..something I've seen before elsewhere). Works and sounds great if the mic is held on it's side or upside down according to the seller (who has an excellent rating and was quite thorough in their description). At half price, I figured it was worth the risk, and worth it just for the capsule, head basket, and body alone, even if it needs a new chassis bottom (fingers crossed it's removable from the rest of the chassis) and/or circuit. Won't be able to work on it for quite some time, but I'll report back with pictures and a summary when all is said and done..assuming I won't be coming this way with problems haha. But, it really just looks like the mic maybe needs a new chassis bottom if there is any issue with the cable (working that out with the seller right now). Otherwise, a good cleaning, tightening some screws, and maybe a touch of solder somewhere should do the trick. Fingers crossed!

Would be nice to work out the circuit in the meantime out of curiosity and future reference in case of repair need and/or enough interest eventually arises in a @soliloqueen K89 capsule

 

[svyet]

Update:

Confirmed with the seller that the XLR connector is not the issue, and the connection port/tube holds cables just fine. One less thing to worry about, and can be repaired if I really feel concerned about it.

Seller has also confirmed the capsule can't be the problem, and as a plus it looks very clean and sounds great (when the mic is working).

The problem seems to be a weird grounding issue somewhere. Maybe someone can take a crack at guessing without seeing anything. Here are the conditions:

1. When mic is upright, there is intermittent noise similar to a noisy potentiometer in a guitar, mixed with normal signal from the capsule
2. Problem above also responds to squeezing the mic body, which produces the noise in correspondence with the squeezes (this is the intriguing part)
3. When mic is held horizontally or upside-down, problem is gone and mic operates as usual
4. One of the three headbasket screws is missing; could this be a factor at all?

So overall, it seems like a grounding issue affected by gravity or something.
The squeezing is bizarre too, and makes me think the mic may have taken such a beating that on impact, something internally had bent just enough to break a grounding connection somewhere. The body itself looks fine though.

Ultimately, I won't know until it's in my hands and I can take it apart, but part of me doesn't feel too worried.

On that note, I have a question for @Moby based on your TLM 193 repair thread from where I found the schematic (Neumann TLM193 circuit help?): what is the order of operations for disassembling the mic?
EDIT: Found disassembly instructions and some points about the screws and their vital role in grounding at this prosoundweb thread. Reponse was from Neumann Mic Development. See post #17 below.

@Moby, I'm still curious about the cost of MH102 and/or MH660. All the service links in your TLM 193 repair thread are dead. Thanks in advance!

 

[svyet]

Probably easiest for the electronics to be something like @rogs ' OPIC? Especially if you need it to be cardioid-only. Well, unless you wanna go fancy with some low-cut and/or padding as well...

I have one here already. It's pretty straightforward. same principle as charging and taking signal off of the diaphragms, but with isolated backplates inside the grounded body, offset from grounded diaphragms instead. makes it basically impossible for it to break, pretty clever. pairing it with an RF circuit was one of my first thoughts. if they worked well together, the resulting microphone would be practically impervious to humidity and very easy to weatherproof.

And useable without any grille. For the brave...

What about @rogs ' AMX circuit that @Khron did the PCB for? Would it also be suitable for use with a K89 capsule? Any foreseeable differences in how the K89 would behave when comparing the AMX and TLM 193 circuits? And if so, what would need to change? Thinking of the benefits that @soliloqueen and @kingkorg have alluded to regarding an RF bias circuit + K89 combo. In principle, an appropriate AMX circuit could pair nice with a K89 for a future build and/or for repairing my TLM 193 if and when that time would come (I would happily commision a re-design of the PCB to fit the TLM 193 chassis).

 

[svyet]

Found disassembly instructions that included some points about the XLR screw in particular and its vital role in grounding at this prosoundweb thread.

Reponse was from Neumann Mic Development:

Hello,

the TLM193 opens up from the top, not from the bottom, like U87.
Before thinking about RF, please indeed the grounding aspect, as you're certainly talking about standard mains hum (50Hz EU, 60 Hz US).
The TLM193 housing is grounded via the XLR screw, so if you loosen that screw, the TLM193 *has* to produce hum, like any unshielded electronic circuit. Retighten that screw really solidly, making sure that there is the washer ("scratching disc" style) below the screw head. Additionally you might scratch off, with a sharp pointed tool, any remaining lacquer there is at the bottom of the screw hole, to assure proper ground connection.
If you should have dismantled the head grille as well, you might free the 3 conical holes at the side of the head grille as well, of any lacquer inside the conical drilling, and retighten the screws firmly.

Best regards,
Martin Schneider / Neumann Mic. Development

This has me highly suspicious of the XLR screw now. If it's not tight enough, then it's highly possible that it's simply not making enough contact to ground the chassis to the body/base when the mic is in the upright position (i.e. the screw head would be down and away enough from the base). Tilting the mic to the side or upside down could of course cause the weight of the chassis to tilt or pull the screw back into place enough to make the proper contact, hence normal operation in those positions.

Fingers crossed it will be as easy as tightening everything down!

In the meantime, I've contacted Neumann about getting a hold of the headbasket screws as well as the XLR screw and scratching disc washer just in case something is missing or damaged.

 

[svyet]

What about @rogs ' AMX circuit that @Khron did the PCB for? Would it also be suitable for use with a K89 capsule? Any foreseeable differences in how the K89 would behave when comparing the AMX and TLM 193 circuits? And if so, what would need to change? Thinking of the benefits that @soliloqueen and @kingkorg have alluded to regarding an RF bias circuit + K89 combo. In principle, an appropriate AMX circuit could pair nice with a K89 for a future build and/or for repairing/upgrading my TLM 193 if and when that time would come (I would happily commision a re-draw of the PCB to fit the TLM 193 chassis).

Alternatively, as I read and compare, I might actually be more interested in doing a traditional U 89 circuit in cardioid only, as I don't foresee using the TLM in any particularly humid conditions (or outdoors at all), and I really do like the idea of a more classic Neumann sound. Studio will be the primary application. The U 89 is a proven circuit and the output transformer may complement the sound a bit better for use as a studio mic with a little less edge (I've recently worked with TLM 102/103's in a friend's studio and they are too crisp and sibilant, and I almost always need to make some significant EQ cuts; I can say the same about the NT1 sounding a little crispy too despite having a relatively flat frequency response, and even flatter with @kingkorg 's flat IR).

There should be no fitment problems either, as the TLM 193 is 3mm larger in diameter and only 10mm shorter in overall length. Given the slightly larger diameter, height clearance on either side of the PCB for all the same discrete components should be a guarantee; and given a simpler cardioid-only circuit, the PCB can be a little shorter and components spaced out more without consequence. Thoughts? I'm really liking this idea a lot.

The only technical "downsides" would be a smaller signal to noise ratio of the U89 circuit (77db A-weighted in the U 89 vs 84db A-weighted in the stock TLM 193), but practically that matters very little to me and my primary use cases. There would also be a lower max SPL in the U 89 circuit as well (134db vs 140db)..but 130db is a pretty standard target. As long as the actual sound is improved and smoothed out a little. My main concern is a flat response, and the U 89 and TLM 193 share the same reponse in cardioid. That's why I think it could be a great direct swap to add a little more "richness" given the discrete components and output transformer. Modern classic Neumann.

Such a conversion would scratch the DIY itch and become something truly mine, breathe a new life into the mic, and I will have the stock circuitry as an emergency backup

As a funny aside, the fact that my TLM 193's red badge paint is nearly gone (see attached pic) could be a sign that it has already prepared to relinquish its status as a transformerless microphone

 

[Moby]

@Moby, I'm still curious about the cost of MH102 and/or MH660. All the service links in your TLM 193 repair thread are dead. Thanks in advance!

I replaced the front impedance converter circuit with the TLM103 one, but the rest of the circuit was OK so I never went deeper into this microphone circuit.

 

[svyet]

Doing some more digging around and have something more to share.

Per forum rules, I've posted a U 89 schematic in the proper forum HERE that I sourced with higher quality than what is in the forum and what Gyraf has on their website (@gyraf , perhaps you guys can replace it with this one - by the way I love your design and gear philosophy. I can only dream of hardware right now, but I keep a close eye on what you guys are up to).

I also posted the TLM 193 schematic in the proper forum and have deleted it from this thread to save server space per the forum rules.

Anyway, the U 89 schematic I found was originally a decent picture from a Reverb.com listing for an older U 89 with all the papers, so I took a screenshot at full res and did some perspective/contrast/dodging adjustments to make it more clear and even all over.

The number on this one is "1 1247 901 04" as opposed to "1 1247 901 02 . 5" found on the Gyraf site and elsewhere on the net. Seems like the numbers in bold equate to some kind of revision or clarification. Just at first glance, D7 in the 04 version has three asterisks with a corresponding note at the bottom that doesn't exist in 2.5. Additionally, DR1 has some extra letter markings at each of the three connections. Not sure how significant those changes are..only an EE here could tell me what I'm looking at.

If that weren't enough, I've contacted someone else about a high quality scan of the same set of U 89 documents that happen to contain a schematic with a "1 1247 901 05" designation - again, whether or not that last number is really significant, the scan should at least be of much higher quality than even the one I'm attaching here. Should make it much easier and certain to map out a cardioid-only circuit