Replaced capsule in 3 pattern mic but only figure 8 is working

[DFischer]

I grabbed a couple of these while they were on sale, unfortunately they are back up to $200 at the moment. Out of curiosity, I traced out the schematic to the best of my ability. I haven't done a whole lot of work digging into mic circuits, so I don't recognize the source design. I'm also curious if this circuit has any de-emphasis on it, and I haven't done any measurements yet. I seem to remember a thread about replacing the capsule with a capacitor and doing a frequency sweep with REW, but I need to look that up before I can get started.

Edit: Corrected schematic posted later in the thread. Triple checked the connections, corrected the capacitor values, and changed the layout to more closely resemble the U87ai schematic.

 

[157audio]

I grabbed a couple of these while they were on sale, unfortunately they are back up to $200 at the moment. Out of curiosity, I traced out the schematic to the best of my ability. I haven't done a whole lot of work digging into mic circuits, so I don't recognize the source design. I'm also curious if this circuit has any de-emphasis on it, and I haven't done any measurements yet. I seem to remember a thread about replacing the capsule with a capacitor and doing a frequency sweep with REW, but I need to look that up before I can get started.
View attachment 142822

Oh cool, I’ll look at that diagram and hopefully learn something! And wow, too bad the sale price is over. I picked up 2 tac700’s as well for $40 each, and another that I gave to my assistant as a gift. Glad I got that in under the wire or I’d be a little sadder about messing up that first one!

 

[micolas]

I grabbed a couple of these while they were on sale, unfortunately they are back up to $200 at the moment. Out of curiosity, I traced out the schematic to the best of my ability. I haven't done a whole lot of work digging into mic circuits, so I don't recognize the source design. I'm also curious if this circuit has any de-emphasis on it, and I haven't done any measurements yet. I seem to remember a thread about replacing the capsule with a capacitor and doing a frequency sweep with REW, but I need to look that up before I can get started.
View attachment 142822

An interesting scheme

 

[micolas]

Oh cool, I’ll look at that diagram and hopefully learn something! And wow, too bad the sale price is over. I picked up 2 tac700’s as well for $40 each, and another that I gave to my assistant as a gift. Glad I got that in under the wire or I’d be a little sadder about messing up that first one!

Too bad it got expensive.
It's a good platform for mods.

 

[DFischer]

I think Khron nailed the polarization scheme in post #16. I'm just a hobbyist, so I have no idea how the DC/DC voltage section works and I can't tell if the circuit is flat or de-emphasized by just looking at it.

 

[Khron]

Out of curiosity, I traced out the schematic to the best of my ability. I haven't done a whole lot of work digging into mic circuits, so I don't recognize the source design. I'm also curious if this circuit has any de-emphasis on it, and I haven't done any measurements yet.

Yes it does - very, very close to the U87A, albeit i'm not 100000% sure you reverse-engineered the capsule connections quite right. Pretty sure "pin 4" should go to ground (too), though.

But the rest is indeed very U87A-like - C16 should govern the high-frequency attenuation.

 

[DFischer]

The metalwork is excellent. I love the die cast frame and the 87 style angled grille. There's also no ringing in spite of the steel body. It seems well damped by the construction.

 

[DFischer]

Pretty sure "pin 4" should go to ground (too), though.

It sure does! There are a few very small plated thru holes on these boards that can be easy to miss. Schematic edited.

 

[micolas]

Yes it does - very, very close to the U87A, albeit i'm not 100000% sure you reverse-engineered the capsule connections quite right. Pretty sure "pin 4" should go to ground (too), though.

But the rest is indeed very U87A-like - C16 should govern the high-frequency attenuation.

But the sequence C14 15 16 does not change the phase of NFB?
Shouldn't C15 be connected to Drain jFET?
C16 of 220nF seems huge.
R16 20Mohm?

***
Are you sure the values are written down correctly?

 

[DFischer]

C16 should govern the high-frequency attenuation.

Would removing it result in a flat circuit then? And I assume that increasing it's capacitance would result in more roll-off, like on the AKG Perception mod?

 

[DFischer]

Shouldn't C15 be connected to Drain jFET?
C16 of 220nF seems huge.
R16 20Mohm?

I checked C15 again and I think it's location is correctly shown.

C16 is a Wima marked" 220/100-". I think that R16 in parallel is keeping me from getting a good measurement in situ.

R16 is marked "206" and measures about 20Mohm.

 

[micolas]

I checked C15 again and I think it's location is correctly shown.

C16 is a Wima marked" 220/100-". I think that R16 in parallel is keeping me from getting a good measurement in situ.

R16 is marked "206" and measures about 20Mohm.

Wima 220/100 means 220pF/100vcc

 

[soliloqueen]

this is one of 797's stock mics, which are kinda good. this is the cr86
View attachment 20230911_38c180ac4f7abf55_427364498796_mp4_264_hd_unlimit_aliyun.mp4

 

[micolas]

this is one of 797's stock mics, which are kinda good. this is the cr86
View attachment 142825

I noticed it too. It is more difficult to obtain here in the EU
I also received an invitation to the 797 presentation on their Facebook group

 

[DFischer]

I've replaced the previous schematic with an updated one. I'm pretty sure this is mostly right now. There's enough bad info out on the internet without my contributions.

 

[Khron]

I've replaced the previous schematic with an updated one.

Where?

 

[DFischer]

 

[Khron]

I wonder how you measured MINUS 90V, despite the direction D4 & D5 are pointing. And are you 10000000% sure the voltage line from the R17-20 divider goes to the R12-C11 node?

 

[DFischer]

Thanks, corrected! The DC/DC converter is a bit of a black box to me. I'm pretty sure about the divider to R12-C11, but perhaps I got something else wrong.

 

[pderbidge]

First post here from someone who knows absolutely nothing (so take my opinions with a grain of salt) But I've been learning a lot from the good folks here about the importance of circuits and circuit design vs the magical "sound" of a unicorn capacitor that automatically makes your mic into a $1k microphone, or the $15k mic capsule that is better because it's $15k. Anyways, thanks for all you guys contribute.
I'd like to thank the OP for posting pics of this mic and those who have been able to decipher the circuit. I've had my eye on this mic for a while in hopes that it might have a negative feedback circuit to make it a good simple mod platform for someone who wants a u87 type of headbasket but with the ability to tailer the high end. Seems like a good alternative to a p420/220. Seeing all those quality caps inside the TAC1100- after all, they're WIMA right I took the plunge and bought one since it's back on sale for $69. I have to say that it "feels" solid.
For those interested in the out of the box sound, I think it sounds a LOT like my Studio Projects B3, which makes sense since they seem to share the same 797 capsule. Based on the only review on Adorama (obviously paid for review) I would have thought the TAC1100 would be less bright but nope, it sounds almost identical to my SP B3. Having said that, I don't see a need to change out the capsule, given this has negative feedback that can tailor the "high end" to ones personal liking. Since I have no need for two mics that sound the same, I'm thinking of taking my 77JC K47 (which I like the sound of a lot) and putting it in my B3 since it has that more cylindrical headbasket and a flat circuit, more suited to a U47 headbasket and "sound". I could then use the capsule from the B3 and put it in my P420, which capsule has a dead diaphragm in the back, so currently is only good in cardiod mode. This "might" give me a u47 like sound in my B3 and an even more u87 like sound with my TAC1100 over my P420 since it's headbasket is more U87ish (not that I have a U87 mic to compare nor do I have a U47). Anyways, Just thought I'd chime in on the sound for those who have heard an SPB1/B3. They sound pretty similar.