K89 / TLM 193 Project

[GeorgeToledo]

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.

Very cool! Good luck on that. I often mention the different termination and the advantage, with the hope one day some maker would delve further into it.

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

I think the TLM193 and 170 sound better than a U89. That capsule pairs well with the TLM family circuit.

 

[GeorgeToledo]

I think awhile back, within the past couple months, someone at GS posted pics of a TLM193 they had gutted and placed a vacuum tube circuit in. Just fwiw…I thought that was pretty wild since you don’t really see people use these as a mod platform.

 

[svyet]

I think the TLM193 and 170 sound better than a U89. That capsule pairs well with the TLM family circuit.

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

I think awhile back, within the past couple months, someone at GS posted pics of a TLM193 they had gutted and placed a vacuum tube circuit in. Just fwiw…I thought that was pretty wild since you don’t really see people use these as a mod platform.

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.

 

[svyet]

So, in my limited knowledge of circuitry, I made an attempt by induction and deduction to map a no-pad/no-cut/cardioid-only circuit over the original U 89 schematic. I thought it would be prudent to simply trace over the original schematic than try to redraw it. I re-drew switches to correspond with the circuit needs (no-pad/no-cut/cardioid-only). Yellow indicates the target path and components, whereas grayed-out connections and components indicate what is not needed. I did it this way so that mistakes would be easily visible, and corrections can be made. I have everything contained in a Photoshop file in such a way that I can make those changes very easily.

The only thing that was unclear to me was whether or not switches (S2 and S3 specifically, but also in general) have a resistance value (and would therefore need to be / not need to be compensated for with corresponding resistors). I was also a little unsure about the Ue>C30>R3>GE>etc. path (or any portion of that) should be included. It was unclear to me on whether or not that is exclusive to the omni pattern path. Other than those, I think I made a decent first attempt.

That said, I'd appreciate it if anyone would be kind enough to check my highlights and get back to me with necessary changes

 

[svyet]

Found another schematic of higher quality that seems to be newer than the 04 (1981?) I posted and the 05 (1982?) I was hoping to receive a scan of. This one has no revision numbers or anything. Extracted from a U89 owner's manual dated 1990. Found here on Recording Hacks, and now posted over in the Neumann schematics thread HERE.

A quick scan through each component, the differences I found between 04 and 1990 are as follows:

1. On the lower left of the transformer, there is RS (meaning pink) instead of VIO (likely meaning violet)
Is this at all meaningful? Simple change in wiring color standard? Indication of different transformer style?

2. C28 and C29 are 330p, instead of 220p in 04 version. They are also 330p in the 05 version of the schematic.
Is this a significant change? How significant? What purpose and effect would this have?

 

[Gus]

It is almost the same as the fet47 charge amp circuit
search for microphone charge amp

I would use the 45VDC with a RCRC filter after it for the capsule voltage. The yellow lines from the DC to DC are at 0VDC?
Good luck if you want to build DR1.
It is a charge amp circuit so C4, R4, C2 and the capsule have a lot to do with the gain.

 

[Drosselmeier]

this right

 

[Gus]

I would connect at the collector of the pass transistor and add RC filtering

 

[Екатерина]

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.



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



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

The old circuit can't replace the new circuit. They only have similar names. They can all get good sound. I like the circuit of km84 very much. It is simple and efficient, and the sound is good. The front determines the rear. The key to determine the overall sound quality is the capsule. If the circuit is not EQ, it is actually the same

 

[svyet]

this right

Thank you. I had wondered about that part of the circuit. I appreciate your time!

It is almost the same as the fet47 charge amp circuit
search for microphone charge amp

I would use the 45VDC with a RCRC filter after it for the capsule voltage. The yellow lines from the DC to DC are at 0VDC?
Good luck if you want to build DR1.
It is a charge amp circuit so C4, R4, C2 and the capsule have a lot to do with the gain.

I'm assuming by "good luck" you mean it will be a pain. I'm not an engineer, and I don't want to get in over my head. My objectives are for a clean, simple, and serviceable circuit. I've looked at FET47, but also now thinking about a KM84 circuit to keep things simple and unobtrusive, and it's merely a step "back" to the transformered predecessor of the circuit that is currently in the TLM 193.

I would connect at the collector of the pass transistor and add RC filtering

As I'm not an engineer, that doesn't translate. I do understand the filtering though. Does this method replace existing filtering, or add an additional stage of filtering? I really want to avoid altering the stock frequency response of the capsule.

 

[Khron]

I'm assuming by "good luck" you mean it will be a pain. I'm not an engineer, and I don't want to get in over my head.

Well, since there are no specs for that coupled inductor anywhere... Good luck indeed. Might wanna try something like this (second schematic), though.

As I'm not an engineer, that doesn't translate. I do understand the filtering though. Does this method replace existing filtering, or add an additional stage of filtering? I really want to avoid altering the stock frequency response of the capsule.

Filtering of a power supply voltage is different than filtering the audio signal coming from the capsule.

 

[svyet]

The old circuit can't replace the new circuit. They only have similar names. They can all get good sound. I like the circuit of km84 very much. It is simple and efficient, and the sound is good. The front determines the rear. The key to determine the overall sound quality is the capsule. If the circuit is not EQ, it is actually the same

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:

The CM1084 circuit drives a 2:1 output transformer not a 10:1 like the KM84 required. The low frequency distortion and response gets much better when you drive a transformer from the lower impedance of the emitter.

Now, in theory you can use anything between a 5:1 ratio and 9.5:1 ratio in a KM84/U87 whose circuits are identical except for the pattern circuitry and there is no de-emphasis in the KM84 circuit. Matt said, "he found a 6.5:1 transformer seemed to be optimum".

With the 5:1 ratio you get a 200 ohm output impedance and with the 10:1 you get a 50 ohm output impedance. The 200 ohm impedance will yield 6db more output into a 1K ohm load or greater than the 50 ohm setting.

The difference between a 5:1 transformer and a 2:1 transformer is 8db. The difference between 10:1 and 2:1 is 14db. The 10:1 have 14db greater loss.

This means the class "A" emitter follower circuit has to do between 8db and 14db less work plus you have the advantage of driving the transformer from a much lower drive impedance.


The lower the transformer drive impedance the lower the LF distortion. Take a Cinemag CM2510 which you can find all the spec's for on the Cinemag site. The CM2510 has an 8:1 ratio just for example.

When you drive the Cinemag from a lower impedance (600 ohms) not only is the phase response at the high frequencies better but the distortion is lower in the low frequencies.

When the CM2510 is driven from 600 ohms there is .6% distortion at 20hz with a level of +18dbu. When you drive the CM2510 from 20K ohms the the distortion rises and +12dbu will produce .4% distortion.

So, you get almost a 6db improvement in low frequency distortion by driving the transformer from the lower impedance.

A LDC capsule also has much more output than a 22mm SDC capsule but the single FET circuit in the U87 has been successful. However, in my experience I would like to see a bit more headroom from an LDC circuit than a single fet provides.

The maximum output of a U87 is -6dbu. The maximum output from our microphone is +14dbu minimum. The KM84 circuit is very simple but adding a class "A" emitter follower circuit adds one capacitor, one transistor and one resistor for a 14db improvement.

Cheers, Dave
aamicrophones.com

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?

 

[svyet]

Well, since there are no specs for that coupled inductor anywhere... Good luck indeed. Might wanna try something like this (second schematic), though.


Filtering of a power supply voltage is different than filtering the audio signal coming from the capsule.

Thanks, @Khron. Seems to confirm perhaps the U 89 circuit is too complicated and not the way to go. Any thoughts on the above KM84 route utilizing Dave Thomas' suggestions?

 

[Khron]

Why not go with the both circuits on @rogs' page then?

That being said, adding a capacitance multiplier as in the U87 / U89 couldn't hurt anyway.

 

[svyet]

I like the simplicity of @rogs' designs, but there is a part of me that would still like to experiment with a transformered circuit and end up with something more serviceable than the TLM 193, but with similar character of the U 89 without all of the complication of its circuit. I have time to figure all of this out anyway, so I'm exploring all available options.

Another big question I have is in regard to the K89 capsule itself and the fact that "the exposed surface of the microphone capsule is at ground potential." I can see how each surface is connected in the TLM 193 and U 89 schematics (at the very least directly to ground, as well as to the switch side of the circuit via a resistor). How would that work in any other circuit?

 

[Khron]

There's nothing to forbid you from connecting a (low-ratio) transformer to the output of the opamp there (ending up with, more or less, Dave Thomas' use of an emitter follower to get lower output impedance).

Regarding the capsule, do keep in mind that the capsules themselves are "dumb", they don't care which part is connected to what.

"Head amplifier" / impedance converters (such as the circuits we're talking about here) are also "dumb", and they only amplify (if there's gain set in the circuit) whatever voltage gets presented at their inputs.

 

[svyet]

Thanks, @Khron! That really helps me to start untangling things. I really do like the idea of using an opamp and foregoing having to mess with FET bias and all that. I had thought maybe the U 89 circuit would be a good way to keep it “distinctly Neumann,” but since the capsule is genuine, I’m already pretty much the whole way there as far as I understand. I have no fascination with purely vintage circuits for their own sake, and there is no reason to overcomplicate anything. So, thanks!

While continuing to dig around the forums, I saw that @Gus actually built an OPIC (or something like it) with a transformer. Found it here where the lot of you were talking about opamp mic circuits for a while already, pre-OPIC it seems. Perhaps the both of you could offer advice on whether or not the aforementioned Cinemag 8:1 or something similar would be suitable, or does low ratio imply something even lower? I’m interested in how closely a transformer should ultimately be “matched” to a circuit. Seems like if the OPIC has good output, I could still use something like the Cinemag, unless you think a lower ratio would be better suited to the circuit.

As for the capsule, I assumed that the leads to the capsule surfaces are important enough to connect them to the right place. Otherwise, couldn’t I lose the benefit of the additional claimed resistance to contamination due to the ground potential? Perhaps @soliloqueen or anyone else with experience with the K89 can shed some specific light on that. Asking as a layman.

Thanks again to everyone for all the input and help.

 

[soliloqueen]

Thanks, @Khron! That really helps me to start untangling things. I really do like the idea of using an opamp and foregoing having to mess with FET bias and all that. I had thought maybe the U 89 circuit would be a good way to keep it “distinctly Neumann,” but since the capsule is genuine, I’m already pretty much the whole way there as far as I understand. I have no fascination with purely vintage circuits for their own sake, and there is no reason to overcomplicate anything. So, thanks!

While continuing to dig around the forums, I saw that @Gus actually built an OPIC (or something like it) with a transformer. Found it here where the lot of you were talking about opamp mic circuits for a while already, pre-OPIC it seems. Perhaps the both of you could offer advice on whether or not the aforementioned Cinemag 8:1 or something similar would be suitable, or does low ratio imply something even lower? I’m interested in how closely a transformer should ultimately be “matched” to a circuit. Seems like if the OPIC has good output, I could still use something like the Cinemag, unless you think a lower ratio would be better suited to the circuit.

As for the capsule, I assumed that the leads to the capsule surfaces are important enough to connect them to the right place. Otherwise, couldn’t I lose the benefit of the additional claimed resistance to contamination due to the ground potential? Perhaps @soliloqueen or anyone else with experience with the K89 can shed some specific light on that. Asking as a layman.

Thanks again to everyone for all the input and help.

There are no leads to the capsule surface. Everything but the isolated backplates is literally grounded through the mounting screws from what I can tell from the capsule alone, but I haven't seen it installed so I wouldn't know for sure

 

[svyet]

Oops, let me try to clarify myself using terms I understand, and please forgive me if I'm butchering it a little:

According to this picture of a mounted K89 and referencing the U 89 schematic for color coding, there are two wires (red+ and green-) coming off the capsule body, and then a black (ground) lead coming off of the metal mount (where the capsule surfaces are ground according to the schematics).

In the U 89 schematic, the black lead connects directly to the ground path. If you look at the TLM 193 schematic, you can see what I assume to be the black ground lead connected directly to the ground path but with a single resistor. Likewise, what I assume to be the negative lead from the capsule also connects to the ground path via a resistor, but after the resistor that the black lead connects to. I don't know how to make sense of the differences (did I mention I'm not an engineer?), but there must be a reason for it?

All that to say, first and most obviously in the case of trying to use a K89 with the OPIC circuit, I'm wondering how to account for these three leads when the OPIC schematic only accommodates two. I.e., where to connect the black lead into the rest of the circuit? I'm sure the answer is obvious, but I'm not sure enough to make any conclusions

 

[Khron]

All that to say, first and most obviously in the case of trying to use a K89 with the OPIC circuit, I'm wondering how to account for these three leads when the OPIC schematic only accommodates two. I.e., where to connect the black lead into the rest of the circuit? I'm sure the answer is obvious, but I'm not sure enough to make any conclusions

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...