K89 / TLM 193 Project

[ccaudle]

So you want cardioid-only, right?

1) The black wire (both diaphragms) goes to ground.

I had wondered if any mics with separate backplates did that. So both diaphragms are grounded, but the backplates have different voltage, correct?

 

[GeorgeToledo]

I think "better" is subjective without a specific philosophy of use. I think they just sound a little different. The 193 circuit is cleaner for sure, but I would choose either mic ultimately for the capsule and frequency response, which the TLM 193 and U 89 both share. It needs to take EQ well. Those are the most important conditions for me. I then decided on trying the classic U 89 circuit out of interest and sound, knowing that it still responds the same, and EQ's well. Objectively it can help smooth and round out the sound (including the saturation/non-linearities that are typically more natural and musical to our ears), especially since my current signal chain is purely digital. Sometimes transformerless can feel like it has too much of an edge, even when EQ'd in post. Sometimes it's the right thing. Both have their place, and both have plusses and minuses. For me it depends on which style I can get more and different uses out of, and - intangibly - which one I would prefer to work with. I think if I use as transparent of a transformer as can be found, it might be the right middle ground between the two. Other intangibles: breathing a new/different life into an old beat-up microphone with a proven, classically-Neumann circuit, spending time in research and problem-solving to produce something I can truly call my own. These all in turn can make it more enjoyable to use. Despite being a more objective forum, there is still plenty of passion and emotion involved in the different projects we choose to spend our time on



That is wild, and way further than I plan to go! But yeah, the capsule is the star of the show here, and modding an existing Neumann until there is a good third-party offering is the only way to go unless you can find a capsule for sale at a decent price. But considering the capsule prices vs the price of a used TLM 193, it makes more sense for the money to buy the whole mic used and mod the internals. I'm also hoping that everything I am doing now will ultimately contribute to care and potential conversions of TLM 193's by their owners (maybe boost TLM 193 sales in the process as an affordable and more true U 89 alternative), as well as help pave the way for decent and affordable future clones for people to enjoy building and using once there is a good third party capsule available.

Well, the U89/47fet amplifier circuit is a bit veiled sounding, like, sluggish on resolving transients and not in a way that works particularly well with the capsule.

The TLM193 is a great, under rated mic. Kinda sad to see one is probably going to get destroyed to make something that is not likely to be thought of as better.

Yes, it’s all subjective. I just think there are better things to try to mod. When something already punches above it’s price point, there is less value in modding. But I guess it’s not a big loss either. Plenty of them out there.

I suspect that the KM84 circuit will lead to a feeling of there being too much bass in the result, but I guess there’s only one way to find out for certain.

 

[Khron]

I had wondered if any mics with separate backplates did that. So both diaphragms are grounded, but the backplates have different voltage, correct?

Different voltages between the two of them, yes (except in omni).

 

[Екатерина]

Thanks for your input. I am actually starting to think a bit more about the KM84 circuit. Very simple indeed. I'm not interested in an intentionally EQ'd circuit, just something that lets the capsule shine.

After reading a bit more on it, I've found some notes from Dave Thomas at AA on another forum.

He suggests adding a class A emitter follower circuit (one capacitor, one transistor, and one resistor) for a 14db improvement, and driving the transformer from a lower impedence for much better low frequency response and lower distortion. He also makes some suggestions on transformer type, and uses the Cinemag CM2510 (8:1) as a solid example. Here is the full quote from post I found here:



This seems like a good way to get the most out of the KM84 circuit, should complement the K89 capsule well, and has a transformer with good distortion performance. Anyone have any thoughts about Dave's suggestions and doing the project this way?

Everyone has their own understanding of these amplification circuits. In my opinion, a high ratio transformer is a compromise to the output of the electronic tube. As long as the inductance is large enough and the impedance is appropriate, the 1:1 output can also have a good sound

 

[svyet]

So you want cardioid-only, right?

1) The black wire (both diaphragms) goes to ground.
2) Look what the pattern-select switch does, in the two "cardioid" positions.
(Spoiler: it connects the green wire to ground. Granted, through a 1G resistor, but since there will be no other bias voltage applied in your case, i'm not 10000% sure that's necessary there anymore - R1 or R2, in fact)
3) Since no other patterns than cardioid are needed, C30 is made redundant.
4) You'll want 60V applied to the red wire, if you want to get any sort of signal out of the capsule.

I more than welcome corrections from wiser members here than i, though...

Thanks, Khron! On point number two, I did a facepalm haha. It was so obvious Thanks for that and the other pointers. This definitely helps me continue untangling things.

Well, the U89/47fet amplifier circuit is a bit veiled sounding, like, sluggish on resolving transients and not in a way that works particularly well with the capsule.

The TLM193 is a great, under rated mic. Kinda sad to see one is probably going to get destroyed to make something that is not likely to be thought of as better.

Yes, it’s all subjective. I just think there are better things to try to mod. When something already punches above it’s price point, there is less value in modding. But I guess it’s not a big loss either. Plenty of them out there.

I suspect that the KM84 circuit will lead to a feeling of there being too much bass in the result, but I guess there’s only one way to find out for certain.

Yeah, I see what you mean regarding the U 89 circuit from your comments and others I've read. Regardless, it's a complicated circuit and certain parts would require too much trial and error to get right. It's not for me, and I've therefore abandoned that circuit.

The TLM 193 is definitely great, and definitely underrated, and definitely punches above the price I paid for it. That's why I was so interested in it (and why I ultimately bought one). Destroyed, however, is a strong word especially considering my thought process here. The purpose of this thread started out as an idea for a DIY TLM 193, but since I now have one, the thread has evolved into more of an insurance pursuit. It's not 100% that I'm going to the mod the mic at all. The mic has an issue that I hope is a simple grounding issue. If so, I'll have a perfectly-working TLM 193 and I won't go forward with any modifications. Let's say, however, there really is a serious circuit issue. I'd like to be able to continue using the mic (capsule and body/headbasket being the most important parts here) without paying an arm and a leg for OEM circuitry. I would like it to at least work and sound close to original. Not necessarily going for better. As for KM84 and too much bass, just a few posts back I've already settled on a simpler and cleaner OPIC circuit from @rogs, as @Khron has patiently suggested multiple times in this thread already. The only modification would be the addition of an ultra-transparent transformer. Part of my reason for adding the transformer is technical, the other part is purely out of curiousity and interest. The good that can come from working this out now is so that I will have a ready and available option for repair, as well as a platform for a third-party k89 capsule (or any other capsule for that matter)

Everyone has their own understanding of these amplification circuits. In my opinion, a high ratio transformer is a compromise to the output of the electronic tube. As long as the inductance is large enough and the impedance is appropriate, the 1:1 output can also have a good sound

Thanks again for the input. I'm exploring all of my options. Based on Dave Thomas' quotes above, it seems there is an optimal range of transformer types for any given circuit, so I want to be prudent in choosing the right one!

 

[neowalla]

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 had not heard of that OPA mic project, but I built these from Jules Ryckebush and they both sound very good though as far as I know, no one has made frequency response measurements. One is a cardioid only and the other is a dual output mic with a 5 pin XLR for playing with polar patterns in post. I might someday replace the capsules with Ariene's flat K47 and K87 respectably.
Now if you'll excuse me, I gotta go check out this @rogs OPIC project.....

 

[neowalla]

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.

Sounds like a great capsule for nature/environment recording.

 

[Icantthinkofaname]

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.

This is a capsule I'd be very interested in.

 

[svyet]

Update:

Sent the mic to Sennheiser for an initial overhaul so I can just focus on using it and maintaining it. Everything was fine apart from a loose connection on the capsule side (explains the short) and the missing screws, worn XLR washer, and a beat up XLR rubber ring. Left the XLR tube alone though I was presented with the [expensive] option to replace that. Mic went through a thorough cleaning inside and out (capsule included), mesh cage was bent back into place the best the tech could do (was also presented with an option to replace the mesh cage, but I didn't feel it would be necessary or worth it). Wait time was a little brutal but typical based on independent techs (three weeks or so after they received the mic). But, once it was on the tech's desk, it was quick work to approve the invoice, pay, and for them to ship it out. I was also shocked at the cost..it was nearly half of what I was quoted by some independent repair shops for the same work. One could argue a difference in quality or attention, but some of the notes and explanations on the invoice made me think the tech was very thorough. All that to say, I have a perfectly fine TLM 193 that I will put it through its paces and not give any thought to altering. @GeorgeToledo can sleep at night haha

- - -

Some other info of interest:

Interestingly, the tech said the capsule was very dirty and therefore performed a cleaning. I assumed this is common practice no matter the capsule since it's done with simple distilled water without solvents. But, that would seem to conflict with the information in this recently-posted doc on mic cleaning from Neumann regarding the U 89 and TLM 170 capsules (K89) (thanks @Whoops):

Neumann

Neumann VAB 84 Vertikal Amplituden Begrenzer (german language)

groupdiy.com

Anyone have thoughts on that? Given the underlined "should not be cleaned", it makes me think it's more of a command than a suggestion.. Any clarity on that matter given your deconstruction and analysis, @soliloqueen?

Also, of interest, the mic was manufactured in 1994, and serial number is below the number where an apparent change in the circuitry occurred. Anyone with extensive TLM 193 usage experience have any experience or thoughts on this? @maarvold?

Quotes/paraphrases below are from various forums (I didn't save the links, but I'm sure an internet search can bring you to the rightful poster). Wanted to post them here for anyone interested so everything can be found in one place:

The changeover occurred as of serial number 5825. There was no change in the sound quality of the microphone, but about a 4 dB increase in its overall sensitivity. At the same time, the maximum output voltage of the amplifier circuit was increased such that the maximum SPL for the microphone remained about the same as before. Correspondingly, the newer microphones draw about 25% more current from a 48-Volt phantom power supply.

Neumann did change the internal circuitry of the microphone a bit after serial number 5825. I have A/B the two and while the new circuit is louder with 4.25 dB more sensitivity (and quieter) it's not as pleasing to my ears. So if you can find a serial number before 5825, possibly on eBay, I'd go for that one

The electronics of the TLM 193 were changed starting with serial number 5825; microphones after that point have about 4-1/4 dB greater sensitivity. The newer microphones draw 3 mA rather while the older ones drew 2.4 mA. All other specifications and characteristics remain unchanged.

The lower draw at least should theoretically help me as I operate primarily off of battery power to stay mobile and/or avoid mains noise. As for any changes in sound properties, I'll likely never know. Either way, it's nice to know I may have gotten lucky in at least one concrete way.

- - -

Thanks again to everyone who has contributed to this thread. I think it's a good body of info to help someone learn about the TLM 193, and narrow down a DIY TLM193/U89-like project in the future, especially if there is enough interest to produce a K89 from our very own @soliloqueen in the future.

In the meantime, if anyone has any questions regarding any tests, pictures, or measurements (aural/electrical/physical) of any part of the TLM 193, feel free to ask. I'm happy to accommodate those requests the best I can.