Design Review/Question= Bandpass Filter on a Mic Differential Pre-Amp? More - Page 1
Forum section: electronics / projects

author: t1d
author_rank: Super Contributor
timestamp: August 19, 2022, 10:20:22 pm
content: Goal = Create a Pro-Level, XLR-Type, Pre-Amp for a Sennheiser e835 Dynamic Microphone. Use a proper THAT 1512 op amp in a true differential configuration, for active noise canceling. A-n-d, add a Passive Bandpass Filter on the front end??? I will use this to drive a simple, DIY amp.I think I have suddenly found a problem...My thinking was to add the bandpass filter before the op amp, so that the op amp would not be processing unwanted frequencies, needlessly. However, if I add a bandpass filter on both phases the signal, whatever mismatch there is between the component values in the two halves of the filter will be amplified. Correct? If so, what are the typical solutions, or filter arrangements.Thank you for your help and suggestions.

author: Benta
author_rank: Super Contributor
timestamp: August 19, 2022, 10:41:34 pm
content: First, let's call things by their right name.What you have is a high-pass (the input caps etc.) and a low-pass (the caps before the amp).But you haven't understood the beauty of differential design. Why are your input components connected to ground?The first four resistors can be replaced with one (prolly around 68k, I couldn't be bothered doing the calculation).The 1n caps can be replaced by a 500p cap.Forget the trimpots, they'll drive you crazy.Let the amp do the rest. At most, you might have to do a bit of DC offset compensation.

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2022, 11:39:23 pm
content: Quote from: t1d on August 19, 2022, 10:20:22 pmYou have it right. You can filter the differential signal before the difference amplifier, but if you filter the common mode signal, then mismatch in the filters will convert common mode noise to differential noise.The general solution is to only implement EMI filtering before the difference amplifier, and then filter the signal after the difference amplifier.

author: t1d
author_rank: Super Contributor
timestamp: August 19, 2022, 11:53:25 pm
content: Thank you, Banta, for helping me! Much appreciated. Please clear up my misunderstandings...Quote from: Benta on August 19, 2022, 10:41:34 pmNo, I understand how the diff amp cancels noise, I just don't know how to combine it with the filter section. I am copying a common filter design.Quote from: Benta on August 19, 2022, 10:41:34 pmAs said, I don't know how to combine it with the filter section.Quote from: Benta on August 19, 2022, 10:41:34 pmI could really use some details, here. How is the resistor and how are the caps arranged? You state that they should be untied from the XLR ground. But, that being so, where should they be grounded to? Each other as a virtual ground? In addition to your description of the needed corrections, a drawing would be helpful, if you have the time.And, if the filter will still have two halves, please address my question about their differences being amplified. Edit: David answered this part.Thank you for your effort in sharing your expertise!

author: t1d
author_rank: Super Contributor
timestamp: August 19, 2022, 11:57:16 pm
content: Quote from: David Hess on August 19, 2022, 11:39:23 pmThanks, David. It is really great for you to join in. I don't know much about EMI filtering. Could this be as simple as a Ferrite Bead on the mic cable?

author: Someone
author_rank: Super Contributor
timestamp: August 20, 2022, 12:22:42 am
content: THAT Corporation have some really amazing design resources with explanations and details, why are you not going with one of their designs?"THAT Corporation Design Note 140"

author: t1d
author_rank: Super Contributor
timestamp: August 20, 2022, 12:32:46 am
content: Quote from: Someone on August 20, 2022, 12:22:42 amHi, Someone. Thank you for your post. I am going with a THAT design. See Figure #4, of the data sheet, for the amp circuit portion of my schematic.Thanks for telling me of Note 140; I did not know of it. I will look it up. Cheers!

author: Someone
author_rank: Super Contributor
timestamp: August 20, 2022, 12:55:19 am
content: Not really the same as datasheet Figure 4 (below). Probably best to learn how differential filters work first as suggested by Benta above.

author: t1d
author_rank: Super Contributor
timestamp: August 20, 2022, 02:05:45 am
content: As David did a great job in answering my question, I am marking this thread as solved. Thanks to everyone for joining in and helping.

author: blackdog
author_rank: Frequent Contributor
timestamp: August 20, 2022, 11:08:24 am
content: Hi t1d,Not so fast!We're not done with you yet....Your filter at the input to limit the frequency range certainly works.But there are and quite a few negative characteristics you introduce with those two filters.The That corporation has done their best to make the comonmode suppression as good as possible.With your filter, that good suppression for interfering signals is thrown right out the window!What others also made clear, use the examples in the manufacturer's datasheet.And also DC shifts when you adjust de Potmeters, this because the high Bias Currents of the THAT1512. (thats relativ normal for low noise components)And then another important point, because of your extra applied filter components, the noise number becomes much higher,this is due to the high resistance values you put in series with the microphone.This does not have to be a problem when shouting into the microphone, but it is usually a problem with dynamic microphones due to their low output compared to the type of condenser...With a good microphone technique, you will certainly start to hear more noise from these additional components alone.And then there is the additional interference from the very poor commonmode suppression by the filter components.Choose your gain wise for the THAT1512 wise (not too high, dont let the THAT1512 clip!) and then place your limiter, and then the other filter operations you want to do on the signal.By e.g. choosing a somewhat higher supply voltage for the THAT1512 you can get more dynamic range, for example you can use + and - 19V power supply.The best filtering effect is a bat, with which you hit the microphone user, when he screams and eats the microphone.One singer who had a good microphone technique was David Bowie and of course there are others.Always sing a little further from the microphone to avoid those ugly proximity effects.And when they start singing louder they take more distance from the microphone.But unfortunately, that's hard to teach singers...Kind regards,Bram

author: David Hess
author_rank: Super Contributor
timestamp: August 20, 2022, 11:28:47 pm
content: Quote from: t1d on August 19, 2022, 11:57:16 pmIt could include ferrite beads, but usually amounts to RC filters but with a cutoff frequency much higher than the audio band. Attached below is an example where the RFI filters are 100 ohms and 100 picofarads.

author: t1d
author_rank: Super Contributor
timestamp: August 25, 2022, 03:04:22 am
content: Thank you, to everyone, for helping me!I forgot to add to my original post that I am just a hobbyist. That means that adding details (of both why and how to do it) to replies really helps. I am fortunate to have a rather nice lab, but the equipment is way smarter than I am. So, I can probably make most any measurement that would be helpful. Although a spectrum analyzer would likely be useful, as this is an audio project, I only have a scope. But, it has FFT, which may be useful, to some degree.David's confirmation of my concerns with the filter being before the 1512 led to- moving the filter to after the 1512,- adding back- a) Caps 1, 2 and 3 and- b) The Bias Resistors R1 and 2.Regarding C1/2/3 and R1/2. I had removed these components, because I thought (incorrectly) that the filter, in its former pre-1512 position, was also fulfilling the purposes of these components.So, here are some new considerations.…The Data Sheet does not clearly state that C1/2/3 are simply to filter noise, as would any other decoupling cap. I believe this is their function. Do I understand correctly?Is there significant benefit to using a precision resistor array for the biasing resistors, R1/2, to warrant its approximately $8USD price?Figure 4, of the 1512 DS, clearly shows the use of two types of ground…- "Rake" symbol. This ground is true Earth. It is provided by the XLR Input Connector's case. It provides the exist for (only) C3. Do I understand correctly?- "Inverted Triangle" symbol. This is the null point of the +/-15V supply. All grounds except C3 return to this point and exist viaPin3Pin1 of the XLR Connector. Do I understand correctly?Benta did not expound on why he suggested that trim pots not be used to tweak the filter. I take it that there are two reasons...- The setting is not critical.- Trim pots are subject to thermal drift.Do I understand correctly?If so, I have left the trim pot footprints in place, to provide some options. They may be used for trim pots, a second fixed value resistor to be used in series with the first resistor, to give flexibility in attaining a particular value, or simply bypassed with a jumper wire, if not needed.I would like to add David's EMI filter. See Reply #10/Figure 18.9, of the document that he provided.- I believe it consists of the pairs C3/100pF + R7/100K(not 100R) and C4/100pF + R8/100K(not 100R). Do I have 100K, not "R," correctly?- Which Ground does the EMI filter return to, Earth or Null? Earth is shown.- Exactly where is it inserted into the circuit? After the biasing resistors RBIAS1/2?Again, I am grateful for your help, expertise and effort to reply. We have a great community.

author: blackdog
author_rank: Frequent Contributor
timestamp: August 25, 2022, 08:48:54 am
content: Hi t1d,If you do not have strong HF fields in the environment where you are going to use the microphone preamplifier then the filtering is OK.Leave out the snubber, by the way the impedance you have chosen for this is not suitable for an opamp, it puts far too much load on the opamp.Through RPB2 of 10K the opamp output sees no capacitive load that makes it difficult for the opamp.If you still want some protection against e.g. longer output cables, then add directly to pin 6 of the opamp a 47 Ohm series resistor.Your output impedance on the left side of C4 is around 10K, so the high value of C4 is not needed there at all.Also, because of the high output impedance you may not load the circuit lower than say 150K because otherwise your tilt points will shift,A better solution is a buffer opamp that is DC coupled and leave C4 out at that point.Then your filter is exactly as you want it, independent of the load.If you apply the buffer opamp, again a series resistor directly to the opamp output against generation phenomena and maybe a coupling capacitor.Kind regards,Bram

author: t1d
author_rank: Super Contributor
timestamp: August 25, 2022, 09:48:33 am
content: Thanks, Bram. These are all very excellent considerations.Quote from: blackdog on August 25, 2022, 08:48:54 amI will remove this option. It was just a consideration that I saw on a LM386 project.Quote from: blackdog on August 25, 2022, 08:48:54 amI will add it in.Quote from: blackdog on August 25, 2022, 08:48:54 amI like the buffer idea and I will take a shot at adding it in. But, I am sure that I will need you to guide me. I take it that another 1512 is best to use for it. I will post the corrected schematic and we will go from there. Again, thank you so much!

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2022, 12:57:08 pm
content: Quote from: t1d on August 25, 2022, 03:04:22 amThose are there to filter RF noise which could potentially be demodulated by the instrumentation amplifier.QuoteMismatch between the shunt resistors at the input and the source impedance limit common mode rejection, so the input resistors should be reasonably matched. 1% resistors are typically considered good enough, but 0.1% is not much more expensive.QuoteCorrect, C3 should return to chassis ground close to the connector.QuoteCorrect, and there should be a single ground connection between the chassis and that point.QuoteTrim pots might be used to trim the filter if it is at the input where mismatch of the differential filter would limit common mode rejection. Otherwise the filter cutoff is not usually that critical and adjustments only add the opportunity for maladjustment. Do you need a filter cutoff closer than 5%?QuoteThose are 100 ohms.QuoteThey RF filter returns to chassis ground close to the connector.QuoteThe biasing resistors could connect either before or after, but after has some advantages.

author: srb1954
author_rank: Super Contributor
timestamp: August 26, 2022, 06:44:03 am
content: The output coupling capacitor should connect to pin 3 of RV2 for best filtering at higher frequencies. The way that it is currently drawn the HF filtering will plateau at higher frequencies if the RV2 is adjusted to anything other than 0.The output coupling capacitor C4 is probably not necessary as any DC out of U1 is already blocked by Cbp1. C4 would only be required if the input of the subsequent stage is biased to something other than 0V. To avoid loading the output of the band-pass filter the subsequent stage will have to have a high input impedance, say > 100k, relative to the resistor values used in the band-pass filter. With this high input impedance the value of C4, if required, can be made much smaller, say 470nF, and still obtain adequate bass response. There is no point having the inter-stage coupling designed to pass extremely low frequencies <1Hz if the preceding band-pass filter has already substantially cut off everything below 50Hz.

author: t1d
author_rank: Super Contributor
timestamp: August 27, 2022, 02:23:02 am
content: These are really great responses, David. Clear and concise. Thank you!Quote from: David Hess on August 25, 2022, 12:57:08 pmI matched a pair from my stocks and they are nearly dead on.Quote from: David Hess on August 25, 2022, 12:57:08 pmI thought that the circuit ground was to be fully isolated from Earth ground. So, I am going to repeat back to you your instructions, for verification for certain.At this location,1) the PCB Board's +/-15Vsupply Null Point Return,2) the Aluminum Case Project Box Chassis,3) Pin#1 of the XLR connector4) and the Metal Case of the XLR Connectorare all to be tied. Do I understand correctly?Quote from: David Hess on August 25, 2022, 12:57:08 pmNo, not at all, for my application.Again, David, just excellent. Thank you.

author: t1d
author_rank: Super Contributor
timestamp: August 27, 2022, 02:33:34 am
content: Quote from: blackdog on August 25, 2022, 08:48:54 amThanks, Bram. My thinking is that you suggest the 47 Ohm series resistor as a typical low impedance termination. If I have that correctly, would not the more common value be 50 Ohms? From what I can tell, whatever value I have that is closest to 50 Ohms is okay, as the value is not critical. Even though this is a low amperage preamp circuit, I take it that the resistor's wattage rating needs to be considered.

author: t1d
author_rank: Super Contributor
timestamp: August 27, 2022, 03:11:13 am
content: At this point, I am taking a side-step and building out the DS Figure 4 circuit, with the addition of Bram's resistor. I will play with the assembly as a proof of concept. I am bodging the (errant) original board that included the filter in the wrong position. The input caps and resistors can be fitted nicely.The board needs a cleaning and the camera really shows it. Bram's output resistor will go in the C4 location.

author: t1d
author_rank: Super Contributor
timestamp: August 27, 2022, 03:32:19 am
content: A linger question about the needed value for the gain pot... I stuck a 1K multi-turn trim pot on the board, sort of as a place holder. I will have to learn what math to do to calculate the correct value, but I am shooting for an output of 0-1.5V. IIRC, 1.5V will be close to what is considered "Line Level" for pro audio equipment, with just a touch of extra headroom. Any advice in this regard is welcome.

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2022, 04:43:13 pm
content: Quote from: t1d on August 27, 2022, 02:23:02 amGoing by memory, 1% resistors yield a worst case common mode rejection of 54dB which in my experience is more than good enough. Better resistor matching quickly reaches diminishing returns because capacitive effects begin to dominate, and then the capacitance needs to be trimmed to get better performance.The resistor mismatch *includes* the source impedance mismatch, which is why differential oscilloscope probes have as high an impedance as possible, and why an instrumentation amplifier with very high input impedance performs better with mismatched source impedances than a difference amplifier with lower input impedance.In audio applications this comes up when an unbalanced output is connected to a differential input. One side of the differential input sees the remote ground point, presumably with an impedance of zero, while the other side sees the single ended output which typically has an impedance of a few hundred ohms, although it is possible to design a line level signal output with an impedance of zero which avoids this problem. This mismatch also degrades common mode rejection, but usually not significantly.Douglas Self wrote an excellent book called Small Signal Audio Design on this subject, although I do not remember if he covers common mode rejection as thoroughly as I have described.QuoteIdeally there is a single point ground where the chassis ground and circuit ground meet. In practice this becomes a problem when multiple external connectors are present, so some compromise has to be made. Having the chassis completely separate from the signal ground is not something I would recommend, but maybe some audio equipment does it? Maybe someone in the forum knows.Where trouble starts is how the XLR ground and shield are connected. One recommendation is that the shield be connected to the chassis, and the pin-1 ground be kept separate until it attaches to the single point ground at the chassis. This helps keep currents in the shield from affecting the signal ground as much.I do not know if any audio equipment does it, but in test instrumentation where ground loops are a problem, the input may be completely galvanically isolated from the circuit and chassis. An instrumentation or difference amplifier with a differential input is almost as good in an application like audio, and of course a transformer could be used but that is less practical these days.

author: t1d
author_rank: Super Contributor
timestamp: August 28, 2022, 03:02:31 am
content: Quote from: David Hess on August 27, 2022, 04:43:13 pmIn my original design, which had the bandpass filter on the inputs before the 1512, I had mistakenly tied the filter to circuit ground. We know that the input caps of a properly designed input (DS Figure 4) should be tied to Earth ground. We can infer from Benta's reply question ["Why are your input components connected to ground?"/Reply #1] that he does not think the two grounds should be tied. I will send him a PM and ask him to provide some insight.

author: t1d
author_rank: Super Contributor
timestamp: August 28, 2022, 03:31:17 am
content: Just to recap where we are, these are the/some/all? open questions...- If Earth Ground and Circuit Ground are ever to be tied (at the chassis.)- Calculation of the resistor value of the Gain Pot.- The purpose and value of the resistor that Bram suggests should be added to the output. Or, the use of an output buffer op amp and its circuit design.I have sent PMs to Benta and Bram asking them to join back in, particularly regarding the above issues upon which they have previously commented.I am learning a lot and having much good fun. Thank you to everyone!

author: t1d
author_rank: Super Contributor
timestamp: October 21, 2022, 06:20:56 am
content: Quote from: t1d on August 28, 2022, 03:31:17 amI did not receive any replies to the above. I am posting an updated rough-draft of the corrected schematic. In addition to the above, I could use help in determining how to terminate the circuit's output, in order to have it at the proper output impedance. Please and thank you, as always.It was suggested that I not use trimmers on the pass filter. I have left them, just to have options to tweak the resistor values, with pots, or splitting the total resistor value between two resistors, if a single common resistor value will not do the trick.

author: jonpaul
author_rank: Super Contributor
timestamp: October 21, 2022, 07:51:44 am
content: Just joining this very long and old topic.Application is studio dynamic mic?Never saw a RC single pôle bandpass needed in any mic preamplifierJust RF EMI beads or caps.Best to follow the excellent Les Tyler app notes from THATSuggest that you Omit bandpass and output elcapSet gain as THAT spec sheet reccomendedbest mic preamplifiers use a matching transformer.Just the ramblings of an old retired EEJon

author: t1d
author_rank: Super Contributor
timestamp: October 21, 2022, 08:37:13 am
content: Quote from: jonpaul on October 21, 2022, 07:51:44 amThank you, JonPaul, for joining in.Yes, the project is old. I can only work on it, when my health permits.Background: I am just an EE hobbyist and audio is not really my area. So, I am just stabbing at it. I have been through the data sheet and app note, but, even though folks think they are excellent, I am finding them difficult, as I am not an EE.Your point about the pre-amp is a light that has just become brighter. I was trying to limit the work that the op amp would have to do. See prior version of schematic which had the filter before the op amp. Having it there created problems, so I moved it. My thinking was that having it after the op amp would at least reduce the work of the next section of the amplification chain. But, as you say, maybe it just plain doesn't belong in a pre-amp.So, I will strip out the filter and output cap, as you suggest. IIRC, the CG1 cap is for limiting the bandwidth to a very low value. IIRC, it was 68KuF, which is a big and expensive cap. But, I do not know how to calculate the value needed to go down to say 60Hz. Maybe that would help lessen the work load on the op amp and be cheap enough.That leaves just the 47R resistor that was suggested. What is its purpose? To set the impedance?Speaking of impedance, what, if anything, do I need to do to set it? I know nothing of using transformers for matching impedance. So, I will study on that.The other two questions remain, as well. How to calculate the value of the gain pot, RG1. And, if the circuit ground and earth ground are ever tied together, at any point. The input caps, CD1 and 2, return to Earth ground.Thank you for your interest, effort and help.

author: jonpaul
author_rank: Super Contributor
timestamp: October 21, 2022, 07:45:37 pm
content: Rebonjour Cher T1bbravo pour votre effortscrazy busy brief response1. Gain pot.. all in app note and spec sheet2. bandpass... two RC hp, LP are NOT a BP.The mic spec will show frequency response, eg 20 Hz...14 kHzno filter needed3. most audio amps, preamplifier have AC coupling no output cap needed, offset of I C is tiny.4. design of ground plane, common, earth....see books and app notes.5. mic transformersmatching of Zo of mic eg 68 Ohms ( see mic spec) to input of preamp, eg 50,000 ohms ( tube first stage)A typical UTC, Jensen, Lundahl mic transformer has a step up ratio of 1:30, and a shield.The transformers go directly from the mic connectors (XLR) to the preamplifier inputThe transformers step-up voltage, and give great CM noise rejection.The THAT parts were designed to make a transformerless preamplifier, with reasonable CMRR, but in professional, studio and broadcasting the intense EMI environment may still prefer a transformer input.See the classic designs of fine studio preamps Neve, SST, Neumann, Shure, etc.Bon SoiréeJon

author: t1d
author_rank: Super Contributor
timestamp: October 22, 2022, 11:14:45 am
content: Quote from: jonpaul on October 21, 2022, 07:45:37 pmThank you, JP, for your support. The power of encouragement can not be overestimated and I greatly appreciate yours.Quote from: jonpaul on October 21, 2022, 07:45:37 pmI have briefly reviewed the data sheet and AN138, for an additional time. The app note seems to be the more helpful. While I do not find an exact calculation for cap CG, Table 1 shows its maximum/minimum swing between 5-10K Ohm. So, I will use a 10K Audio Taper pot.Quote from: jonpaul on October 21, 2022, 07:45:37 pmThis is an excellent point and makes sense. I will remove the filter.Quote from: jonpaul on October 21, 2022, 07:45:37 pmI will leave a place for a cap, here, just for the sake of flexibility to add bass boost, if needed. Initially, I will just jump it.Quote from: jonpaul on October 21, 2022, 07:45:37 pmYet to be determined, though I do not recall any written text, in this regard... Just the schematic symbols. I imagine this is left to the engineer, whose training is expected to have included this knowledge.Quote from: jonpaul on October 21, 2022, 07:45:37 pmAs the 1512 may be used without a transformer and my use is not to such a high professional level, I will not be adding a transformer.There is a conflict between the basic circuit diagram of Figure 4/Data Sheet and Figure 2/AN138. The CG symbol in the Data Sheet is non-polar, while the one in AN138 is polar. Which is appropriate? A polarized/6K8uF e-cap/in stock at Mouser is over $4USD. So, if I can find a less expensive alternative, I would consider it.As has been said here, many times, it is becoming clear that the basic circuit in Figure 4 of the data sheet is all that is required. However, being told that and understanding why that is so are two different things. Thank you, to those who have helped me to understand, because understanding equals learning. Thank you, to each and everyone of you, for helping me.

author: jonpaul
author_rank: Super Contributor
timestamp: October 22, 2022, 02:34:22 pm
content: Rebonjour Cher T1,As a retired EE 53 years, I am interested to help the new generation to work with analog, audio, instruments and circuits.1/ a non polar lytics.... recipe - select two caps, of 2x desired C, place in series opposed + to +2/ a series cap is not a bass boost!A bass boost is a shelf filter, not a series C.3/ ré grounds, layout, all parts of circuit are to ground plane, the PS common.Nothing touches châssis or mains earth!A single point star ground is best for audio, to avoid ground loops.See SYNAUD CON, Jim Brown's excellent site for professional audioAlso Jensen Transformers, Bill Whitlock's app notes.PS...A walk down memory lane.....in 2018, IDesigned a valve mic preamplifier for historical reconstruction of WWII SIGSALY 1942 speech scrambling.XLR>>UTC 1:30 matching transformer >> 6SL7>>6SL7 (X30, X30,X30, CF) for 40 V output from dynamique mic. Audio >>doubler detector>>6 léveil flash converter with 2050 thyratron tubes.Recently found 7 vintage 1950s,,1970s mics at flea markets and street sales in Paris. Shure, Philips, Beyer, Akai. Some LN or NIB. had DIN or Tuchel or XLR connections and cablesAverage cost 5..10€ each!Jon

author: t1d
author_rank: Super Contributor
timestamp: October 23, 2022, 01:08:18 am
content: Quote from: jonpaul on October 22, 2022, 02:34:22 pmGreetings, friend!Quote from: jonpaul on October 22, 2022, 02:34:22 pmI am also retired. I took up electronics to keep my mind active and learning.Quote from: jonpaul on October 22, 2022, 02:34:22 pmYes, a good trick. I also considered multiple caps for the 6K8 Ohm/CG cap.Quote from: jonpaul on October 22, 2022, 02:34:22 pmThank you for this clarification. I have extra room on the PCB, so I may leave the series output cap spot, to have options to decouple any stray DC.Quote from: jonpaul on October 22, 2022, 02:34:22 pmThis is in contradiction to what other commenters have instructed (there have been multiple grounding instructions) and the schematic drawings (which use two different ground symbols.) So, I am left confused... I could simply add a jumper on the PCB to be able to tie the isolated Earth return to the common circuit Ground return, if the common return is what is needed.Quote from: jonpaul on October 22, 2022, 02:34:22 pmThank you, for the references.Quote from: jonpaul on October 22, 2022, 02:34:22 pmAs you have direct experience designing mic pre-amps, I imagine your instructions about not tying ground to mains Earth is best.Quote from: jonpaul on October 22, 2022, 02:34:22 pmThis very thing is what initiated this project. I found a Sennheiser e385 and a Shure 157 clone at the thrift store! Funny!Thank you, Jon!

author: t1d
author_rank: Super Contributor
timestamp: October 23, 2022, 01:16:42 am
content: Did we clarify if cap CG/6K8 Ohm should be polarized, or not? Thanks, again.

author: t1d
author_rank: Super Contributor
timestamp: October 23, 2022, 12:09:13 pm
content: I have sent an email to THAT support asking for clarification of the Earth ground on CD3 and whether CG1 should be a polarized.

author: jonpaul
author_rank: Super Contributor
timestamp: October 23, 2022, 03:25:19 pm
content: Rebonjour:I forgot the questions re CG and grounds.Looking again at the THAT specs,1/ The 47 pF cap C3 at input CT to "rake" symbol:For EMI surpression. "rake" is CHASSIS and earth, and ONLY for shielding and EMI.C3 can be omitted if EMI is not an issue.https://www.ranecommercial.com/legacy/note165.htmlhttps://www.prosoundtraining.com/https://groupdiy.com/threads/mic-input-transformer-grounding-questions.72733/page-2All other ground triangle symbols are circuit/PSU return, PCB GRD pl and need NOT connect to earth.2/ Do not omit the CG cap or short it!gain R RG MUST be POLAR 6,600 uF 10V cap with plus to the RG1.Suspect the cap can be smaller for lower gain selection of gain.No idea where you got the non polar idea.THAT specify the cap in Design note 138/140https://sand.thatcorp.com/datashts/dn138.pdfhttps://sand.thatcorp.com/datashts/dn140.pdfBon chance,Jon

author: t1d
author_rank: Super Contributor
timestamp: October 24, 2022, 02:43:01 am
content: Quote from: jonpaul on October 23, 2022, 03:25:19 pmOk; this agrees with earlier comments. I will make it so, on the PCB.Quote from: jonpaul on October 23, 2022, 03:25:19 pmIn Figure 4 of the data sheet, the plus symbol is missing.Quote from: jonpaul on October 23, 2022, 03:25:19 pmI did not have DN140. I will study it. Thanks!Jon, thank you so much! I think that you have cleared up all of the issues that were remaining. Good job! It is always good to make a new friend, too.I think that it is important to contribute in like kind, for all the help I receive on forums. So, I will continue this thread by posting my KiCad schematic and PCB files. They will give other KiCad users a good head start on their own 1510/1512 adventures. I am nearing completion.Cheers!

author: t1d
author_rank: Super Contributor
timestamp: October 24, 2022, 12:25:31 pm
content: Here are my schematic and board. Things to know:For the prototyping/testing, the big 6K8/CG1 cap will lay horizontally on the back of the board. This will help with getting everything to fit into a small project box.I used a ground plane. Star grounding might be better (I don't know,) but I have not learned how to lay it out, yet.If the output cap C4 is not used, it could be jumped, or the output wire could be attached to its positive pad to shorten the trace run.The first boards are not going to be laid out to high standards. I am moving ahead to order them from OSH Park and parts from Mouser.All good fun! Thanks, again to all of you!

author: jonpaul
author_rank: Super Contributor
timestamp: October 25, 2022, 06:21:48 am
content: Latest version: Still lots of issuesBias 1 k use ORDINARY resistors. matching NO needed!* ALL CAPS TO BE POLYESTER(MYLAR] FILM,______Use monolyth ceramic for PS bypass,Mylar film only useful in signal path, even there any plastic and some others are fine.RESISTORS ARE THIN FILM.Useless, just standard metal film/carbon film.+15V/GND/—15V SUPPLIESCheck PSU line rejection, of the THAT 1512. May need better PSU noise/ripple. Any Chinese Wall plug PSU will NOT give good result.LINE PROTECTION DIODES ARERECOMMENDED.Not in most DIY/consumer. see THAT 1512 spec on protectionXLR SIGNAL INPUTSFERRITE BEADOnly broadvcst environmentSWITCH MUTING.Inside mic not at preamp!M" cc1 |s A FILTER CAPWHICH SETS THE LOW END CUTOFFTO 5Hz.NO 5 Hz HP needed. DO NOT omit as the 1512 may need a bias block.BANDPASS FILTER |s ALREADYINCLUDED.Remove no use and not a bandpassSee markupj

author: t1d
author_rank: Super Contributor
timestamp: October 27, 2022, 03:02:19 am
content: All such excellent information! Thank you, Jon!Quote from: jonpaul on October 25, 2022, 06:21:48 amYou are correct. My thinking was that keeping both sides of the input differential signal as true to one another as possible could not hurt. A matched resistor is going to have the best resistance match and best common thermal reactivity. I did not make this a specification of the notes. There should be room within the matched resistor footprint to substitute separate resistors on the board, if so desired.Quote from: jonpaul on October 25, 2022, 06:21:48 amNotes corrected.Quote from: jonpaul on October 25, 2022, 06:21:48 amIIRC, this was a comment in the DS. Removed from notes.Quote from: jonpaul on October 25, 2022, 06:21:48 amYour concerns about wall wart PSUs is well warranted. I have known about their failings for awhile. I will use a proper transformer and linear regulation, with the proper minimum 3V of ripple limiting headroom (not the 1.5-2V dropout headroom that is so commonly specified, in error.)Quote from: jonpaul on October 25, 2022, 06:21:48 amI will correct the notes.Quote from: jonpaul on October 25, 2022, 06:21:48 amI will correct the notes.Quote from: jonpaul on October 25, 2022, 06:21:48 amI will correct the notes.Quote from: jonpaul on October 25, 2022, 06:21:48 amI removed the output cap, as I realized CG1 does that job.Quote from: jonpaul on October 25, 2022, 06:21:48 amRemoved.This post became long and I began to lose track of things. So, I will stop, here, and make anther post within a couple of hours (I hope.) It will have the updated drawings and notes.Extra special thanks to Jon, for graciously sharing his amazing expertise, time, and effort!

author: t1d
author_rank: Super Contributor
timestamp: October 27, 2022, 04:48:54 am
content: Here are the updates...I will see what comments arise, but I think that we a close to finishing the project. As soon as we are satisfied with it, I will post the KiCad files.More thanks and gratitude, to all!Cheers!t1d