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# evengravy

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1. ## Help with voltage conversion

Hi I am creating a project with the new arduino due and I need to convert the voltage output (0.55v to 2.75v) from the DAC into a range that is linear from -5v to +5v I have looked through national semi app note 31 and in the upper right corner of page 6 there is a circuit for voltage conversion but I'm really stuck of where to go from here, I have no idea how to calculate the offset I need and the gain required. http://www.ti.com/ww/en/bobpease/assets/AN-31.pdf I have an accurate and stable +10v and -10v supply to work with, Can anyone help? I'd really appreciate any input, Thanks and regards
2. ## Help with a Tube EQ circuit

Hi, I have been given a circuit diagram of a tube based EQ unit and I need some help if anyone could help me I'd be gratefull. Basically the circuit is in two parts a Tube base amplification stage and a "passive" inductor based EQ circuit that "appears to me" to be in the feedback path of the Tube amplification stage allowing for both boost's and cuts to be applied at the set frequency ranges. The EQ section is fairly well understood and I have worked out the multi tapped inductor values based on the given frequencies and capacitor values. I am having some issue understanding the Tube topology however, I am not at all experienced with Tube designs. My initial analysis from staring at the circuit for some time now is that the tube amp circuit is essentially forming an op amp. What I would like to do is replicate the EQ unit but replacing the Tube amp stage with a solid state amplification stage keeping the EQ pretty much as is. Could anyone help me understand how to do so? could I possibly use standard opamps in some configuration? or could someone help me "decode" the circuit so that it could be replaced with BJT's of JFet stages? Sorry for the long post, thanks in advanced, even
3. ## hv transformer wiring

hero, thanks very much, even
4. ## hv transformer wiring

Thanks hero, I've been working with hv dc ozone up until now so sorry for the confusion, just so I'm clear could you check my wiring diagram? Thanks, even
5. ## hv transformer wiring

thanks hero, one question, does it matter which wire of the secondary is grounded? there are no markings on the secondary wires at all, and they are both the same colour. am I correct in thinking one secondary wire is + and one - that should be grounded? if so how would I identify which is +? my thinking is that the turns ratio is 1:13 so if I apply mains voltage to the secondary side and measure the primary side as it produces a safer measurable voltage this way, I can determine the + and - wires on the secondary. Not sure if this is even necessary? Thanks, even
6. ## hv transformer wiring

My current thinking is to wire the secondary to the electrode like the image attached, Since one side of the secondary is contained within a sealed pyrex tube I am presuming this is correct but I would appreciate input/corrections, thanks.
7. ## hv transformer wiring

hi, I am building an ozone generator using a high voltage neon sign transformer and I am a little unsure of wiring. The transformer I have is brand new and runs off 230v and produces 3kv@9ma. I want to be 100% sure of how to wire this safely for obvious reason so I would appreciate your assistance guys/gals. Basically the nst that I have bought is like in the image attached. The ozone generator electrodes will consist of a scroll of stainless steel mesh that is pushed into a Pyrex test-tube where it unfurls against the walls and a lug on the end makes connection to the circuit through a nut and bolt through the screw-on lid. A stainless steel mesh tube on the outside of the test-tube acts as the other mesh electrode and is connected to ground via the terry clip mountings that hold the assembly in place. The secondary side of my transformer is not marked, do I simply ground one side of the secondary (which will be the outer electrode) and tie this to the case also?

"A mic preamp used an input transformer about 30 years ago but now they use an electronic circuit." Yea, I understand what you mean but there is a certain tonal advantage to using a transformer also, I see what you are saying about the impedance, adding a transformer would be wasting the high input impedance of the Jfet, If I were to choose a transformer however what specs would be suitable? impedance in/out and ratio however, in order to achieve the electrical balancing, could I use similar spec matched jfet pair (2SK-369) and the same circuit doubled, one circuit for positive input and one for the negative? then merely add phantom power and dc blocking caps to the input?

audioguru: "its input impedance is 10M ohms so why ruin it with an input transformer" I really need to make this ciruit balanced and add to add phantom power, which is why I thought of adding a transformer, is there another simple way of achieving these functions without the input transformer? I know that it is possible to add dc blocking caps to allow phantom power to be blocked from entering the input, but is there a way to work with the balanced input that emulates the transformer function. even.

thanks again hero999, even

I think I may have been wrong with the output impedance values that I posted, basically the circuit (well two in series will make up the mic pre) will need to be suitable for connection to a line level mixer, after some more reading it seems as if "The impedance of the line input is high -- about 10K to 1 Meg ohms" http://www.tape.com/resource/impedance.html

so, if i understand what you're saying correctly I need to increase current through q4 such that it can drive the low impedance load but such that it is low enough so as it wont overheat. Is this correct? I have no idea how to go about running output impedance simulation, I am using multisim. also in regard to the input transformer, I understand that the input impedance is high with the jfet input, but i'm unsure how to add phantom power using dc blocking capacitors, this is why I thought a transformer would be easier, but i'll scratch that idea. thanks guys.

I know it has been a while since I posted on this circuit but I have been flat out with other projects until now, so, I have modified the circuit to include diodes between the base of Q4 and ground as you suggested and this has reduced the current to just below 6ma, I'm not sure how to simulate for output impedence, It will need to drive a 600ohm load max (i think around 75Ω @1kHz) how would I go about simulating for this, I have been searching online for answers but I can't seem to find any relative info. any help is greatly appreciated, also the ac coupling hasn't been added as I intend to incorporate a transformer on the input, I will get to changing this and the bias resistor values soon.

thanks a lot for this i'm glad i have at least biased correctly and i will work on the current draw on Q4 and post updates as/if required, you've been a massive help cheers

im simulating with multisim 10 with a specific 2sk117gr model, which are the ones I have. some voltages of circuit in no signal condition Q1 drain voltage is around 17.3V. gate sits at 20.2mV source 133mV Q4 base 17.3V collector 36v emitter 16v Q3 base 7.5v collector 16v emitter 6.81v i've took a print screen of probe measurements from throughout the circuit. I wonder if the multimeter in multisim draws current? im using a probe this time and seem to be getting different values.

I thought you calculated current by the following: (I = Current/V=Voltage/R(t)=Resistance Total) I = V / R(t) I = 36/(8.5 + .060) I = 36/8.56 I = 4.2(mA) this is close to the value i'm measuring in the simulation also, so now i'm really confused.

"Why have you insisted on a non-standard value for R1" basically I am a little confused as to bias the fet correctly, my take on it from my reading was to aim for a figure close to the measured Idss of the measured device (gate and source connected and voltage applied with no signal) with a 50ohm source resistor the drain resistor value to bias close to 0v just happened to be 8.5k. I think I can choose a standard resistor value close to this value without significant change to the bias. "4.332mA through Q1 seems too high" maybe i'm measuring the current incorrectly. I'm measuring from Q1 drain to ground, is this correct. current measured is 4.322mA (18.5v at drain) which is close to the Idss value I have measured for the Fet. oscilloscope measurements of the amplified output are swinging about 0 almost perfectly, with and without the source follower section attached so I took this to mean my biasing was at least somewhere close to being correct. "add a couple of diodes between Q3's base and 0V" ok, what type of diodes would be suitable for this application? and would you add two in series? sorry for so many questions, i really appreciate your help.

"I'm not sure if you understood when I talked about E96 and E24 values" oh i understood ok, just wondering whether metal film E24 values are superior to carbon E24 values in terms of noise. "You have sketched a class-A heater.." haha, might be usefull its freezing here in ireland ok, heres the full first stage with part numbers, Q4 is a Darlington which was chosen merely due to its voltage rating and I was under the impression that using a darlington would minimise loading on the jfet (higher input impedance) the bc635 is a generic npn, again chosen due to its voltage rating suiting my supply voltage (technical I know). really just trying to achieve a source follower for impedance. can you recommend part values and arrangement at all? thanks again guys, really helpfull.

"What does the xlr grounding pin do?" Not sure to be brutally honest, I'll have to look into this, although it is on almost every preamp schematic that I have seen so far. "What load is the pre-amplifier driving?" Hmmm........well it will be driving a professional line level sound card, impedance is approx 20kΩ balanced and 10k unbalanced. "An ordinary silicon diode connected between the input and 0V will clamp the voltage at -0.6V" thanks for this, will the orientation be the other way round to the zener currently in the circuit? "It needs to have a significantly higher impedance than 4.02k" I think i have achieved 47k with the adjustments made in the image attached, one amplification stage shown for clarity. The current draw may be quite high however. comments? "E24 is normally good enough" understood, would there be any difference in terms of noise produced with metal film resistors vs standard E24 ones? Note: I have the actual schematic on my own laptop at home and don't have access to it right now so please excuse the image which i have drawn.

"think Mr Evengravy records electric guitars so he likes the even harmonics distortion caused by Jfets and vacuum tubes. He also likes overdrive but not the severe clipping distortion produced by opamps. " pretty much spot on, the pre will have DI input for guitar plus a transformer to allow mic input/phantom power and pad switch on secondary of transformer. He is lucky that the Japanese Jfets are binned into fairly low ranges of conductance. I bought so many of the toshiba parts for this very reason, shame they don't make them any more. :( "The gates of both transistors are biased at 0V which will bias them too far on" thanks, I thought this would be an issue, after some reading today I'm pretty confident I know the correct way to select a source resistor to bias correctly. "Is R9 supposed to be the load? The output impedance is 4.03k so it won't be able to drive a 49.9R load." R9 is connected to the xlr port grounding pin 3, to be honest I found this value is common on many preamp outputs so this is why it was chosen. Is this inncorrect then? "What purpose does R3 serve when it's in parallel with a 10k resistor?" Oh yeah, this should have been removed, I began by designing a single stage and was left over after the copy/paste of first stage if i remember correctly. "What does D1 do? It looks like a clamping diode to protect the input. Why use a zener rather than a normal silicon diode? A normal silicon diode will do." Since I intend to use the unit as a guitar DI as well as a Mic preamp this zener diode was to protect the input Jfet from voltages from accidental hot plugging, so a silicone diode will work in this case, I think the value should be closer to 1v (if these are available that is) "How much of the amplifier design is going to be JFET?" well i've been doing some more analysis today with average mic output data obtained from spec sheets: Max dBu (approx 140dB spl) = +10dBu (2.45 Vrms, 3.46Vpk ) High dBu (approx 130dB spl) = 0dBu (0.77 Vrms, 1.08Vpk) Loud dBu (approx 120dB spl) = -10dBu (0.245 Vrms, 0.35 Vpk) Average dBu (approx 97dB spl) = -36dBu (0.016 Vrms, 0.023Vpk) with these figures the 2sk117gr can handle one stage of amplification achieving approx 34dB of gain (if my analysis is correct) so..... I will have to rethink this. I was thinking of following the first jfet stage with a bjt pair, this will give me a better impedance for the output. Although the single stage will be out of phase. Any other suggestions are very welcome. thanks guys I appreciate your comments/suggestions

The overload characteristics are exactly why I want jfet, I can design an opamp preamp quite easily and don't want to go down this road again as I already own some very low noise opamp pre's, some based on the ina217 which I built recently, these are very very good but opamp designs from a personal sonic perspective always sound somewhat clinical. Clean sound is not always what is required during a recording session. Although in electronics design you aim to reduce distortion as much as possible, in my case the distortion characteristics of the jfet, which are similar in respect to tube designs, is exactly the attraction. Also I want to learn how to properly design Jfet circuits. The aim for me is to have a circuit which can be offer clean amplification at average gain settings but can also be overdriven slightly if that overloaded tone is warranted. Appologies for sticking to my guns on this one, but I am looking to learn these technologies also.

Firstly thanks for the responses guys, Audioguru: "Why use Jfets? Why not use an audio opamp instead?" i am looking to avoid opamp designs for personal reasons, I hate the fact the signal goes through so many stages, i love the sound of jfets and their gracefull distortion/overload characteristics. "Why use an expensive condenser mic that needs a 48V supply?" well... I work in a professional recording studio as an engineer and working with phantom powered mic's is completely unavoidable. "A resistor from the source to ground will add negative feedback to reduce the effect of the wide range of IDSS" The idss is 4 - 6ma for the 2sk117gr that I have so the spread is not so wide in comparison to other JFETs although still a wide enough variation I guess, I will look into this, thanks, how do i calculate the source resistor and drain resistor to bias the FET correctly? "A gain of 59dB is almost 1000 times so talking at a normal level to the mic (10mV output) will result in an output from your preamp that tries to go to 28V peak-to-peak." ok, so I would need to limit the gain somewhat in the first stage, I was thinking this would be an issue, how would I go about this, resistance on input? also would increasing the source voltage to 36V help in this regard? thanks for responses, also sorry about the image, it is perfectly clear when previewed on my machine....... Hero99 "Does it have to be all J-FET?" well I would like to keep it that way really yes, the toshiba parts are beautiful sounding to me. "The phantom power supply needs to be DC to a transformer can't be used" not sure what you mean by this could you clarify please "An AC coupling capacitor can be used to connect the microphone if its floating at a different DC voltage to the amplifier." I am aiming to have two independant power supplies, one at 48v for phantom powering and a seperate supply at either 24v or 36v for the preamp. thanks again guys

Hey, I have been slowly teaching myself audio electronics over the past few years (part time). My knowledge is somewhat lacking to say the least, so please excuse this. I am trying to design a simple twin stage JFET mic preamp with components I have laying around, I have plenty of these (100 x 2sk170bl, 150 x 2sk117gr) so matching for Idss shouldn
24. ## gating rf frequencies

quote: The signal being switched should always have a voltage within the supply voltage range ---- Ok so maybe i should look at a solid state relay instead? the function generator output signal is sine and varies from -1v to +1v at 3-5mHz and i need to gate this signal at 25% duty (on) 75% off at a rate of 1330Hz. Any suggestions for solid state relays rather than a cmos type switch? I have been looking at solid state relays: Sharp S202SE1, but these are rated at 8A 230V ac, total overkill? could you recommend a low voltage ssr at all? I have also been toying with the idea of using n and p mosfet pair? any suggestions? i dont mind the cost as long as it functions well. I also want to completely avoid regular "clicking" relays. quote: An opamp at the function generator ouput will provide high impedance during switching. The PWM device can be used to control the opamp output. I may look into this thanks, but i would prefer to use either a switch or a relay if i can. thanks for responses guys
25. ## gating rf frequencies

hi, thanks for responses, i have been looking at the 74hc4006 since your reply, not sure if response time is fast enough yet untill i study data sheet in a bit more detail but it surely appears to have the bandwidth for my application, thanks. am i correct in assuming that in order to use this bilateral switch that the gate trigger signal from my 555 timer should also swing bilaterally? (eg. -5v off to +5v on) as it swings dc to Vcc at present (or there abouts).
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