BertBert37
Member
I own a number of B-5s which work well. However 2 of them show low output - obviously due to low capsule voltage supply. In order to check the built in oscillator circuit I need a pertinent schematic.
Looking for Behringer B-5 schematic
- Thread starter BertBert37
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Internal photos (well-lit, well-focused) could help...
[Later edit]
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQv5f9XLqDzCR52BUpqAHzMJVAMjLuVDRkn4Q&usqp=CAU
It's on the small side, but looks very much like a Schoeps circuit to me. Two green inductors next to each other there in the middle, two (output) transistors nearer to the XLR output, and a JFET near the capsule end.
[Later edit]
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQv5f9XLqDzCR52BUpqAHzMJVAMjLuVDRkn4Q&usqp=CAU
It's on the small side, but looks very much like a Schoeps circuit to me. Two green inductors next to each other there in the middle, two (output) transistors nearer to the XLR output, and a JFET near the capsule end.
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BertBert37
Member
I have similar pictures taken. I also know that the output is a single sided (asymmetrical) schoeps type circuit. The signal path works OK. To check the oscillator circuit on all parts, I would like to have an complete schematic that could tell me the critical parts. I assume that a leaky capacitor or a bad rectifier is the culprit, which results in a low voltage for the diaphragm bias.
Is there no schematic available ?
Is there no schematic available ?
You "know" the output is single-ended? That's not a Schoeps then. Photos of the actual mic you have, as i mentioned / hinted at in my first reply, would help clear things up.
An internally shorted ceramic capacitor is a very real possibility. A couple years ago i revived a friend's Rode NT5 by swapping out one of the output filter capacitors from the capsule bias supply. And earlier this year, a film(!!!) capacitor in the capsule bias supply of my Rode NT2-A.
An internally shorted ceramic capacitor is a very real possibility. A couple years ago i revived a friend's Rode NT5 by swapping out one of the output filter capacitors from the capsule bias supply. And earlier this year, a film(!!!) capacitor in the capsule bias supply of my Rode NT2-A.
RuudNL
Well-known member
BertBert37
Member
Many thanks to RuudNL ! This helps a lot, although there might be an error in the output stage - however that is not my problem. I was not able to perform a complete reverse engineering myself but could identify parts on the backside layer. That is too boring ! Now I see how the bias is generated !
It is a matter of dialectic, if a schoeps style output that serves only one branch with a pnp emitter follower is still a schoeps circuit. To me the main difference is the eliminiation of a transformer. It is sure that a single sided output is more sensitive to hum and noise on the phantom supply. but it saves one transistor ! For that matter I had to revise my TASCAM DR-680 for cleaner phantom power.
It is a matter of dialectic, if a schoeps style output that serves only one branch with a pnp emitter follower is still a schoeps circuit. To me the main difference is the eliminiation of a transformer. It is sure that a single sided output is more sensitive to hum and noise on the phantom supply. but it saves one transistor ! For that matter I had to revise my TASCAM DR-680 for cleaner phantom power.
Attachments
I have two B-5s, one of them probably having the same issue. But I never bothered to find the root cause of the failure as I was going to replace the electronics anyway with my own design. If I find some time, I might give it a try and repair it.
When I got the mics a few years ago, I drew up the schematic of the impedance transformer and output sections. I then found the same schematic RuudNL shared and found several differences, which you may have noticed yourself in the meantime. E.g. the 22uF elcap on the right is 100uF on my board (and yours too), there is a 24V zener (D2) parallel to this cap, C11 on the schematic is C5 on my board and there are caps on the output terminals. Maybe there are other differences as well, but I didn't do a complete check. Or maybe there are different versions of the B-5 around? Anyway, I guess the oscillator will be identical, as this is quite a common design. Btw, personally, I don't like this type of oscillator. I've heard many times it is not oscillating, even when the inductors were placed correctly with the hot sides as depicted in the schematic and were tightly coupled.
Zooming in on your picture, I see quite some (charred?) flux residues. Don't know if they were already there, but I'd give this board a good clean first with some IPA before trying anything else. While cleaning and handling the board, wear unpowdered nitrile gloves to avoid conductive sweat to disrupt normal operation or cause noise.
Wishing you good luck with the repair!
Jan
When I got the mics a few years ago, I drew up the schematic of the impedance transformer and output sections. I then found the same schematic RuudNL shared and found several differences, which you may have noticed yourself in the meantime. E.g. the 22uF elcap on the right is 100uF on my board (and yours too), there is a 24V zener (D2) parallel to this cap, C11 on the schematic is C5 on my board and there are caps on the output terminals. Maybe there are other differences as well, but I didn't do a complete check. Or maybe there are different versions of the B-5 around? Anyway, I guess the oscillator will be identical, as this is quite a common design. Btw, personally, I don't like this type of oscillator. I've heard many times it is not oscillating, even when the inductors were placed correctly with the hot sides as depicted in the schematic and were tightly coupled.
Zooming in on your picture, I see quite some (charred?) flux residues. Don't know if they were already there, but I'd give this board a good clean first with some IPA before trying anything else. While cleaning and handling the board, wear unpowdered nitrile gloves to avoid conductive sweat to disrupt normal operation or cause noise.
Wishing you good luck with the repair!
Jan
BertBert37
Member
Again thanks for the input. I heave already noticed some differences such as the 100 µF electrolytic capacitor providing the internal supply voltage. I expect more to find. Since I have another 6 pc of B-5 in good condition, I have no hurry to make the repair and postpone it for the dark winter season. The flux residues are not on the high impedance side, but I will remove them anyway.
Beyond that I currently work with my own built ribbon prototypes which have proven a remarkably better resolution than condensers. The reason for that is an advanced design and the use of 0.5 µm ribbons. This is at the cost of extreme handling care since only some crosswind on a location may result in an instant catastrophy !
Bert
Beyond that I currently work with my own built ribbon prototypes which have proven a remarkably better resolution than condensers. The reason for that is an advanced design and the use of 0.5 µm ribbons. This is at the cost of extreme handling care since only some crosswind on a location may result in an instant catastrophy !
Bert
From another thread about the B-5 on this forum I found the answer to my own question whether there are multiple versions. The answer is yes. The schematic RuudNL shared is of the new SMT version. Mine and Bert's versions are old THT versions. I've almost finished drawing up the schematic and will share it here once finished.
I also found the issue on my broken B-5 PCBA. It was the 2N5551 oscillator transistor. I replaced it with a new one and worked like a charm. Measured -48.5V on C14. The other mic I have measured 51V. I tested the removed part on a transistor tester and it seemed to be ok and had a gain of 183. Yet, it failed to work in the oscillator... Well, it did oscillate, but with low amplitude on the output.
Btw, the circuit is very similar to the t.bone SC140, which is made by 797 Audio. Although it was not printed on the PCB, I assume this mic is also from 797 Audio.
I also found the issue on my broken B-5 PCBA. It was the 2N5551 oscillator transistor. I replaced it with a new one and worked like a charm. Measured -48.5V on C14. The other mic I have measured 51V. I tested the removed part on a transistor tester and it seemed to be ok and had a gain of 183. Yet, it failed to work in the oscillator... Well, it did oscillate, but with low amplitude on the output.
Btw, the circuit is very similar to the t.bone SC140, which is made by 797 Audio. Although it was not printed on the PCB, I assume this mic is also from 797 Audio.
I have uploaded the schematic of the Through Hole version to the Technical Documents section. In case of any questions or remarks, you can post them here or PM me.
https://groupdiy.com/threads/behringer.84857/#post-1103385
Jan
https://groupdiy.com/threads/behringer.84857/#post-1103385
Jan
Bert37
Member
Hi Jan
Again thank you for the pain to evaluate a complete and well structured reverse engineered schematic. I appreciate this help very much. As stated before, I will resume the work with my faulty B-5 only in fall or winter. So far I have only a few remarks:
Again thank you for the pain to evaluate a complete and well structured reverse engineered schematic. I appreciate this help very much. As stated before, I will resume the work with my faulty B-5 only in fall or winter. So far I have only a few remarks:
- I could not find R3, R9, C5, and C9 on the schematic nor on the picture. R5 exists twice, but I assume the one with 6k8 might rather be R9 connected to R11 ? C5 is only visible on the picture, but might be across C4 ? If C9 exists it might be 15 nF across C10 ?
- There are also some obscure details found in the schematic that have to be checked and give a chance for improvement:
- The signal path by C12 (10nF) and R5 (150k) form a high pass with edge frequency of approx. 100 Hz. This is against a good low frequency behavior !
- The output stage seems to be similar to a one sided schoeps stage. However the schoeps circuit uses the feed resistors (6.8k) in the phantom circuit as emitter resistors and provides a large output swing capability. The current schematic has a complicated network consisting of R1, R4, and C4 that lowers the available voltage swing and provides some low frequency boost near 150 Hz which counteracts to the above mentioned high pass. This is hard to understand.
- R2, C1, and C3 care for symmetrical load of phantom power and silence on the XLR-2 signal line. However it causes signal loss on XLR3.
- The 10 dB pad uses a voltage divider in the source circuit. This only reduces the level to the output stage, but does not care for lower distorsion in the input FET stage. A better solution would be to parallel the capsule by a specified capacitor. This would also reduce noise.
RuudNL
Well-known member
C12 = 10 nF? Really?
Uhmm, no 100nF... Well noted, thanks! Actually, that C12 capacitor should be C5. There are more errors, which I will discuss in another post.
What's equally surprising, to me at least, is the fact that C1 and C2 are of different value. Double checked it, but they are 15 and 10nF respectively. This causes impedance imbalance, reducing CMRR at high frequencies. But CMRR is poor anyway with this circuit topology. See the CMRR plot I uploaded in MicUllis thread about microphone measurement setups.
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Thanks for reviewing the schematic, Bert. Despite all the checks I made, I made some mistakes. I'll go through them one by one and I'll adjust the schematic and PCBA picture accordingly.
- There is no resistor on the PCBA that I could positively identify as R3. Where should it be located? Value? Maybe I missed it...
- R5 6k8 on the left should read R9.
- C5 is the capacitor from the switch to the base of Q1, which I erroneously named C12. And it should read 100nF iso 10nF. I made the same error with C6, which should also be 100nF.
- I could not find C9. It is probably the 470pF input capacitor, but there's no ref des printed for this cap on the PCB. Given that C7 and C8 are also associated with the JFET buffer, let's call it C9 instead of C19.
- The obscure 100Hz HP mystery was solved. See item 3 above.
- Indeed, output swing is not optimal with just 3.2V across Q1. This has been improved in the SMT design of the RuudNL shared by adding base-emitter resistor R13. The voltage drop across Q3 in that design is 7.4V.
The circuit comprising C3, R1, R4, C4 and R2 is not that difficult to understand. R1 and R2 provide DC bias current to Q1 and keep both outputs at the same DC level, which is especially important for transformer coupled inputs on mixers. C4 and R4 form the AC coupling path to the output. C3 keeps the positive output at GND level for AC signals and is a poor attempt to make the output impedance balanced. This output has a very poor CMRR. Personally, I prefer symmetrical outputs without capacitor coupling and CFP (Sziklai) output configuration. - The 10dB pad by means of a voltage divider could be considered adequate for this circuit. This is a Source follower circuit with lots of headroom. The traditional Schoeps has roughly half the headroom of this circuit and will start to distort at lower SPLs. A capacitor parallel to the capsule might reduce noise, but I guess this will be least of a problem at high SPLs. What's more important: the capacitor adds significant harmonic distortion. I've attached a document describing this phenomenon. In my designs, I use a CMOS based oscillator that generates several output voltages from 5 to 60V. This reduces capsule generated distortion, mic electronics distortion and prevents clipping of your mixer preamp. No other pad solution does all that at the same time. Some may argue that a lower bias voltage will affect the capsule resonance frequency and its voicing, but I have yet to prove that once my mic measurement setup has been completed.
Cheers, Jan
Bert37
Member
Magnificent – the new revised circuit diagram seems to be completely correct ! I also completely endorse all the explanations except the last one about the effect of a capacitive 10 dB pad.
To be true, I did not care about the input to a transformer coupled mixer that needs zero potential difference between XLR-3 and XLR-2. The original schoeps circuit provides this feature by symmetrical output and therefore can use direct connection of the emitters to the phantom resistors of 6.8k usually found in 48 V phantom supplies.
The 15k resistors used in the B-5 considerably reduce the maximum output swing, but could probably be reduced to remarkably lower value. This might be simulated via pspice, but unfortunately at this time I have no license left.
I propose to eliminate the strange output network (R1, R2, R4, C4) completely. Instead, there should a similar transistor as Q1 hooked up with its base also via 150k at the crossing of R12, R5, C10 exactly as Q1, but no signal feed (blocking capacitor to ground). This transistor would only act as a bleeding path for XLR-2, carrying no signal. Both emitters may be connected directly or via low value resistors to XLR-3 and XLR-2. The cost of an additional transistor may well be paid off by the loss of C4 ! The gain in output swing and according reduction in distorsion may be considerable !
I am much more worried about the statement that a pad capacitor parallel to the capsule might increase distorsion. This is a complete nonsense and I am going to prove it. Since this needs to edit some mathematical relations which is tediuos in writing, I will add it in a later contribution.
Bert
To be true, I did not care about the input to a transformer coupled mixer that needs zero potential difference between XLR-3 and XLR-2. The original schoeps circuit provides this feature by symmetrical output and therefore can use direct connection of the emitters to the phantom resistors of 6.8k usually found in 48 V phantom supplies.
The 15k resistors used in the B-5 considerably reduce the maximum output swing, but could probably be reduced to remarkably lower value. This might be simulated via pspice, but unfortunately at this time I have no license left.
I propose to eliminate the strange output network (R1, R2, R4, C4) completely. Instead, there should a similar transistor as Q1 hooked up with its base also via 150k at the crossing of R12, R5, C10 exactly as Q1, but no signal feed (blocking capacitor to ground). This transistor would only act as a bleeding path for XLR-2, carrying no signal. Both emitters may be connected directly or via low value resistors to XLR-3 and XLR-2. The cost of an additional transistor may well be paid off by the loss of C4 ! The gain in output swing and according reduction in distorsion may be considerable !
I am much more worried about the statement that a pad capacitor parallel to the capsule might increase distorsion. This is a complete nonsense and I am going to prove it. Since this needs to edit some mathematical relations which is tediuos in writing, I will add it in a later contribution.
Bert
Your proposed mod of the output stage makes sense. It improves the dynamic range and gets rid of the elcaps in the output which ruin low-frequency CMRR as they never have exactly the same ESR and capacity. See the CMRR plot of the B-5 which I shared in another thread. Actually, with this mod, you are just a Drain resistor and coupling capacitor away from a basic Schoeps-like circuit... OK, OK, the Pad and HPF circuits will become more complex too, but you're almost there.
With respect to load capacitance induced capsule distortion, there was thread on GroupDIY: How does condenser mic capsule load affect THD?
Actually, I'm also interested in measuring the real distortion's increase or decrease with a capacitive pad versus different capsule bias voltages. But I haven't made the setup yet to measure mic distortions. I want to measure mic distortions using two tweeters and then do a difference tone measurement. This is a well known method that eliminates the need for an ultra-low distortion speaker. But as I always tend to have more plans and ideas than time to materialize them, don't expect these measurement results anytime soon.
Regarding PSpice: why not use free LTspice?
Jan
With respect to load capacitance induced capsule distortion, there was thread on GroupDIY: How does condenser mic capsule load affect THD?
Actually, I'm also interested in measuring the real distortion's increase or decrease with a capacitive pad versus different capsule bias voltages. But I haven't made the setup yet to measure mic distortions. I want to measure mic distortions using two tweeters and then do a difference tone measurement. This is a well known method that eliminates the need for an ultra-low distortion speaker. But as I always tend to have more plans and ideas than time to materialize them, don't expect these measurement results anytime soon.
Regarding PSpice: why not use free LTspice?
Jan
Bert37
Member
Bert37
Member
Thank you for proposing LTSpice. It is already on my PC but I was too lazy to create my environment. May be a matter of age - approaching 87 ! I also fully approve the thread cited above. I have made my own, which goes very close to it and holds the same result. Since I fear scrambing, I include the text as *.pdf. All together, I think we are going to present a very good revision for the B-5 amplifier which is worth the nice capsules !
