Small signal audio transformer design for a vacum tube microphone - Page 1
Forum section: electronics / projects

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 15, 2020, 01:40:41 am
content: So I have been planning to build the GIC microhpone for quite a while (here is the link to the page about this microphonehttp://www.gyraf.dk/gy_pd/g7/gic.htm) and I already have the capsule and so on... Where I got a bit stuck is the transformer. Sadly I am still just a student and not a milionaire so I cant afford really any transformer that would be quality certified for this job (and before you tell me "you already bought the expensive capsule just pay up the transformer" well no i bought the cheapest possible double sided capsule on ebay which was 30 bucks and thats already expensive for me).(before you dive deep into this I just want to warn you I am not native and my grammar will probably not be on the spot. Its also 3:40 AM at the time of writing this)Since buy isnt an option here I went with the "well what can go wrong just wind one" option.I am not an expert on this so dont burn me please. So I pretty much have the impedances and turns ratio with that.. It would be 15K to 600ohm (and no DC current would ever pass trought the transformer itself). For me to figure out how many turns I need on a given core with a given permeability I need to know what kind of inductivity I need to pass audio. I figured since I already have bought some 1:1 600 ohm transformers from china I should be able to just measure them and go with that value. It turned out about 1.2H (which sound alright to me). I have done some checking arround and I also found other microphone transformers that have the same 600 ohm output impedance which are about 1.8H on average.I assumed it would be the right inductance to go with.I was checking arround on TME and I found a pretty decent ferrite core (AL 8700nH or rellative permability 2700) and with this one I would need arround 130 turns of 0.1mm wire to achieve arround 1.8H (I just used a online calculator for this and I know it does not take layer thickness into account and neither the bobbin itself). Since I know the ratio should be 25:1 then i should just multiply 130 by 25 and bam I got my secondary turns ratio which is 3250 turns. At this point I think to myself great math checks out and I have now a number to go with. However I just had a slight thought of how about the primary tube side inductance. I know it will be a whole lot larger but I did not expect it to turn out to be 1200H. I am almost certain I messed up somewhere in my maths and 1200H just doesnt right. Then again I never ran across such a special purpose transformer in my life and didnt measure it so I dont really know.Best I could figure is to check other tube transformers and so I did check my power transformer for a EL34 push pull amp(I know its wildly not the same as what I am trying to create). It has a 6k6 ohm winding and it turned out to be arround 590H so I regained some comfidence. I dont want to go ahead yet on this because I am a bit worried about this very high inductance and I wouldn want to wind and unwind 3250 turns of wire (nope to happening). I just want confirmation on this if I am doing this right, if the 1200H sound right for you at all. I have never come across anything with that high inductance ever before let alone in a transformer.All I want to know is if the inductance of the primary 15K ohm winding (which is 1200H) is it sounds right to YOU. I will appreachiate any advice and opinions about what could be done if its right or if not.Technical parameters I look forward to achieve :Primary impedance : 15KSecondary impedance: 600ohmsBandwidth 20Hz-20kHzPower: just signalMy results: (RM core AL=8700nH/relative permability 2700, not gapped, code: B65815E N30, link to datasheet:https://www.tme.eu/Document/4a01e423b117af0ac949e52794f2a2d2/rm_12.pdf)Primary:Impedance: 15KTurns: 3250Wire: 0.1mmInductance (estimated without accounting for layers and bobbin): arround 1200H(!)Secondary:Impedance: 600ohmsTurns: 130Wire: 0.1mmInductance (estimated): 1.5H-2HTo anyone who is wondering why I did not buy the transformer that costs 60 dollars. I am sorry I am a student and with limited finances I will gladly go down the route of more effort, besides its gonna be a great adventure to turn that wire round 3250 times... and educational aswell.(also if someone could link me a PDF that would contain all the required math for this I would be gratefull. I have only used online calculator tools which are good only so far.)Adam.

author: Cerebus
author_rank: Super Contributor
timestamp: April 15, 2020, 02:18:20 am
content: Before you go any further I suggest you go back and check your understanding of the basics. You've got a turns ratio of 25:1andan impedance ration of 25:1 - that's a problem as the impedance ratios of transformers go as thesquareof the turns ratio. So your 25:1 turns ratio has an impedance ratio of 625:1. To get a 13k:600 (25:1) impedance ratio you need a turns ratio of 5:1.Remember that a transformer scales voltage and current in opposite directions simultaneously. So say you have a 1:2 turns ratio transformer and put 1V at 1A into it (an impedance of 1 ohm) on the primary, on the secondary you'd take out 2V at 500mA (an impedance of 4 ohms).And yes, 1200H sounds way out of the ball park for a mic transformer. I've no idea what would be a reasonable inductance, but I know enough to know that 1200H is alargetransformer. You shouldn't be aiming for any particular inductance here, but for a particular magnetic flux level in your core. You have to know that you're not saturating it, equally you need to know that your signal is going to use a reasonable amount of the magnetic flux available to you in that core. You further need to aim to be working on a nice linear part of the core's BH curve, and you need a core with low magnetic hysteresis.I don't want to dissuade you, but designing low level signal transformers is not the easiest of tasks and I'd want to go and re-read all my materials on transformer design before I tried it. By all means go ahead, but I'd suggest that you have a few days reading of textbooks in front of you if you want to produce a competent design.

author: trobbins
author_rank: Frequent Contributor
timestamp: April 15, 2020, 03:58:33 am
content: Also suggest you visit groupdiy.com and search on transformer teardowns and design.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 15, 2020, 03:34:14 pm
content: Quote from: Cerebus on April 15, 2020, 02:18:20 amWell man...That was the problem. I forgot to realise that impedance is squared. Thanks for opening my eyes. I did build three tube amplifiers with microwave transformers and there I did have to correctly estimate and calculate (and thankfully no rewinding was needed). I dont have an idea how could have I missed that.In that case the primary 15K ohm side would end up being 650 turns and the inductance of that would be 46H. That makes much more sense. Core saturation wont be an issue in my case. Its such low power and on top of that just by the shear size you can tell its not gonna happen. I can still fit the transformer into the body of the mike but its larger than any comercial transformer. But to be hones this is just guessing here. If you could link me to some theory I would be glad. The text books they gave us in school are useless beyond trying to make a astable oscilator.I will quite possibly post updates on actually physically making the transformer here.Many thanks, Adam.

author: sam[PS]
author_rank: Contributor
timestamp: April 15, 2020, 03:36:12 pm
content: Have you seen the video series Dave made few years ago with former Rhode microphone head engineer ? If you get into microphone design that's a must see.Note what he says about transformer :-they have lot mojo so good for the sale (other than that not so good)-they are not needed-most of the time they are detrimental to the quality/signal integrityIf you are on a budjet i'll suggest you to not use transformer at all. Instead inspired by his work at Rhode just consider using a simple resistor as equivalent load for your tube circuit's output folowed by a BD139/BD140 pair as a output buffer and balance the output by matching the output impedance with a resistor between the ground and the S- terminal. Not only that works but most of the time this solution will be better than a transformer. It will cost you just couple $ and easy to buil on a small veroboard. I do recon it kinda kill the mojo, but when you'll get the money for a nice jensen or carnhill transformer it's easy to swapp.Just my 2cts...

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 15, 2020, 05:02:12 pm
content: I agree with you Sam.I did watch the video a while ago and I actually found it very interesting. I could do what you described and to be hones I dont really care about the mojo of the whole thing. I just like to do tube stuff ine general. Truth be told a temporary solution would turn into a tempermanent one. Besides transformers is the only thing I havent really done so adventure path it is. If it turns out I just wasted hours on winding something useless thats fine too we live and learn.Will see what the results will be of this. If it turns out to be meh then I may just at some point buy a transformer or do the transistor method. Funny its actually not just the transformer that is difficult to conserve price on but the 1G ohm resistor is possibly the worst part of the entire microphone hah.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 15, 2020, 06:59:46 pm
content: Why guess at required inductance, from an inferior component (generic audio transformers are usually phone bandwidth, if that)? It's easier to calculate from first principles:Lmin = Zo / (2 pi Fc)15kohm needs 120H at 20Hz cutoff (-3dB), and 600 ohm needs 4.8H. The turns ratio of course is sqrt(15k/600) = 5:1.Classic transformers used permalloy cores, having very high permeability and modest saturation flux density. This gives the most impedance for small signals, which is the desired goal. You probably won't find one of these handy at the usual suspects, but nanocrystalline cores can offer comparable performance (and even lower core loss, not that that matters here) and are widely available.There's some priority to keeping the winding wire length short, as this factors into the HF cutoff of the component. I have a few of these transformers that are about thumb-tip sized. I forget what signal level they're rated up to (which would be limited by saturation at Fmin). Keeping the overall size down, also keeps wire length down.Tim

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 16, 2020, 08:48:14 pm
content: Well Tim that makes more sense now.With a 120H for primary I would need about 1050 turns and secondary 210 turns.You were stressing wire lenght a lot as that is the limiting factor for the HF cutoff. Well I just did some looking arround and now I understand why wire lenght limits the HF cutoff.The primary would turn out to be with 1050 turns roughly 60 meters long. The secondary 210 turns would end up being roughly 9 meters long. I will be using 0.1mm wire.Doesnt sound all that bad to me. When I get the core I will measure its permeability so I get most accurate possible on the inductance. And then lets begin the adveture of winding...Cores with higher permeability would be welcome but I cant have everything. I will try my best and see how good it will end up. At some point once we return to school i will be able to measure the -3dB point.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 19, 2020, 06:26:01 pm
content: I bought the wire and the transformer. (Forgot the clips but pff next time more luck).You mentioned saturation could cause a problem with bigger signals.How can I measure it the easy way? All I have is a RLC meter, vtvm regular voltmeter dual channel digital scope. I guess I could give it a signal until I see noticeable distortion and find out where is the limit but if there is another easy way to measure it I would like to know it and try it

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2020, 03:01:34 am
content: Saturation is easily seen on a squarewave test; the waveform droops suddenly at some point. Easiest to see with a source impedance much lower than the coil reactance.Tim

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 21, 2020, 09:42:31 pm
content: well seems like my calculations are rubbish.. I wound 222 turns of wire in two layers (110 and 100on top of it insulated with capton tape) and the meter shows me only 1H. Gapping the cores seems to raise the inductance that the meter reads (MIC-4070D LCR METER). I dont know really.. I measured the permeability with 26 turns of 0,51mm wire and the relative premeabillity was spot on 2700 and even a little overachieving. Now when I wound the real deal 600 ohm winding assuming my 222 turns same parameters the relative permeability calculates to be 800: rather low. I toook out the battery and powered it with my bench power supply because I thought it was acting up on me but apparently it isnt. I dont have another tester like this availableI dont know what to do next. Oscillator test? so I can determine based on the resonance the inductance? And even if and the numbers end up being the same, what then?. The 600 ohm winding isnt all that long(9,5meters long at worst).Its the same core same everything as I said in my previous post. (RM core AL=8700nH/relative permability 2700, not gapped, code: B65815E N30, link to datasheet:https://www.tme.eu/Document/4a01e423b117af0ac949e52794f2a2d2/rm_12.pdf)Or my meter is acting up I cant really tell.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 21, 2020, 11:37:54 pm
content: At what, 1kHz and 1V or less?I like how it insists it is "some of the most advanced", yet doesn't actually say what it's doing on the highest inductance ranges, or even have selectable frequency or amplitude, or Kelvin leads I guess; all of which are features I would expect from something claiming to be.https://webbuilder.asiannet.com/ftp/810/MIC-4070D%20User%20Manual.pdfAnyway, at relatively high frequencies, you'll be limited by winding capacitance, and at low amplitudes, you'll be in the initial permeability regime. Test with a signal generator and you should find it's in spec.Make sure also that the core faces are clean, planar and well clamped.Tim

author: The Electrician
author_rank: Frequent Contributor
timestamp: April 22, 2020, 01:12:23 am
content: SK_Caterpillar_SK, you might find this useful:https://www.jensen-transformers.com/wp-content/uploads/2014/08/Audio-Transformers-Chapter.pdf

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 22, 2020, 05:25:51 pm
content: Yeah I just lost all my trust in the meter. The 200mH range said it's too much the 2H range says it's 400mH and the 20H range says it's anywhere between 2 up to 7H (absolute bs) and the 200H range shows it's 20H Soo...I guess pick your own.I am already hunting for a old TESLA BM591 or BM595 or even a BM591 or one of those older vacuum tube LC meters.. Will see where this goes. I am going to make a oscillator out of it by just putting a cap in parallel (3uF foil brown dip) and look at the frequency on the scope and roughly calculated the inductange. I estimate it should come out roughly @40 some odd Hz.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 22, 2020, 08:15:12 pm
content: alright well..it might have been correct about the 400 some odd mH. I am completely burnt out. I used a 3,3uF cap (measured 3,271uF) and with the coil in paralel on the scope I measure 125Hz (spot on).That woul mean its only 495mH. Definitely far far far away from what was expected.. It should have been 5H.I have also done this with a 1uF wima 630V capacitor and the frequency comes to be 227.3Hz and that means the inductor is 490mH.I have definitelx done goofed up somewhere. I am starting to suspect they may have given me wrong cores or something.. One is labeled N30 all over and the other one isnt. When you order you order a set not piece by piece. And I also have to say I forgot to buy the clips but I held the transformer very firm and pressed both half cores together where the clips originally are.Same results arround 490mH1uF cap gets 227.3Hz which is 490mH XL=700ohms220nF cap gets me 500Hz which is 460mH XL=1.447k ohms10uF cap gets me 71.9Hz which is 489mH XL=221 ohms

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 22, 2020, 09:20:24 pm
content: well...turns out either the core isnt what it should be but the permeability is not 8700nH... Or I am just somewhere goofing up again and I might need kilometers of wire...I may just got with the transistor output here.. It appears this might be just too much for me.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 22, 2020, 09:52:53 pm
content: What signal level?Did you accidentally get a gapped core set?Tim

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 22, 2020, 10:41:14 pm
content: no, the cores are perfectly flush no light visible trough the center..i was doing this test very crude with a 9V batt and very quickly just touched it hping the scope captures it in single mode. the permeaility is more like 270 than 2700. then the calculation fits reality..

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 22, 2020, 10:47:41 pm
content: Quote from: SK_Caterpilar_SK on April 22, 2020, 10:41:14 pmif 270 is the real deal then my secondary should be 700 turns and primary 3500turns to get me the desired inductance. I guess I could have been better off buying it or not trying to build this thing at all. stuff like this really shows how bad I am at electronics

author: sam[PS]
author_rank: Contributor
timestamp: April 23, 2020, 01:09:00 pm
content: Dont give up SK_Caterpilar_SK, that doesn't show you are bad, that shows you are doing somethingThat's what real world electronic is about. You try one thing, it fail, you adjust what's needed, it get's closer, rince, repeat, and eventually at some point you end up with something close enough. And yes sourcing the right part is a much bigger part than most imagine and often you have to adapt your design to the part you got. That's what this game of real world electronic is about.So get few days off the project then put "the eye of the tiger" loud on your stereo and stand back up on the ring

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 23, 2020, 05:05:35 pm
content: It's been a painfull 2 hours of trying wind 6 layers. I have ended up with 4.1-4.3H. it's not what I wanted but I'll take it...The 0.118mm wire ripped twice..I almost broke into tears. It's been 700 painfull turns (I think I was going layer by layer so it really depends on luck but if perfect it should be 125 turns a layer. Now it's time to do at least 30 layers...I don't have the proper equipment too wind it to count the turns. In the process I just went on AliExpress and bought one 600:15K transformer for 16 dollars.I may just not finish this at all.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 23, 2020, 09:51:31 pm
content: Alright well..secondary turned out to be 4,2H and primary 64H.. The wire ripped at the last 200 turns or so and that was a rathe emotional moment after hours of carefully winding. It shouldnt really matter cause It would barely fit onto the bobbin..with previous winding failures atempted repairs to it with capton tape which only reduced more space from the bobbin.. I might have been able to wind more bravely with a coil winder and actually knowing the exact amounth of turns instead of finding out how many turns is one layer and average them out layer by layer...this way the wire was perfectly wound at least but I used a LOT OF IT. I dont think the HF performance of the transformer will be any good.Oh and because of it ripping now I have a 1:4 turns ratio which is 1:16 impedance wise...not what I wanted but it will do. I was supposed to include one more secondary that would be the feedback winding but guess what didnt fit and I dont have the counter so I simply just gave up on it. I may try the thing with this transformer and see if its any good but its likely I will eventually spend the hundred dollars on some other proper tranformer premade from somewhere in the world. Or go with the tube-semic output option..But my labor I have done to get these results was not worth it, rather dissapointing.

author: H713
author_rank: Regular Contributor
timestamp: April 25, 2020, 08:20:23 am
content: Winding a microphone transformer is no trivial task... and one that probably requires more manual dexterity and patience than I have.You might look at what Edcor has to offer. While they aren't a Jensen, they also don't cost $100 each.Also, having measured quite a few, good audio transformers are pretty wide-band. I've measured Jensen transformers from about 20 to beyond 50 kHz. The distortion is really pretty tiny, less than .01%.The main reason transformers have fallen out of fashion for microphones and preamps is the cost. Really good microphone transformers are in the $100 range, so their use tends to be limited to the more expensive equipment. Putting a $100 transformer on each channel of a 40 channel mixing console will add a considerable amount to its price. It has been done, however, once again, it's limited to the more expensive equipment (Neve, Trident, API, MCI, Harrison...)Microphones are tricky, especially with tubes, so persistence is required.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: April 26, 2020, 11:01:16 pm
content: Quote from: H713 on April 25, 2020, 08:20:23 amWell all is true.. I do have the dexterity but not the luck with this try. Once I save up a little I will buy the winder and I will atempt this silly idea again. Surprisingly the one I have wound worked rather nicely. What I did is just built the circuit and applied signal to the grid of the tube manually. (Because I simply dont have a high value resistor for the microphone capsule and that would be at least 1G ohms). I could not mesure the frequency repsonse but by ear trough a buffer...its pretty dang close to what I would like. But I have not done any measurements yet. I will need to set it up for proper testing. But it worked however its not perfect because I couldn finish winding it because the wire snapped. Better luck next time with more proper gear so I can have a fighting chance. I might find some way to put the roll of wire onto ball bearings and just break it with felt and stuff... I will do what I can