Valve mic preamp design incoherent rambling

[analag]

Edcor 1:8 into 6sl7 with minimal feedback sound just about right. A compromise between the warmth of 1:5 and the pointed sound of 1:10

 

[abbey road d enfer]

Define "pointed".

 

[Winston OBoogie]

I only see one Edcor 1:8 for mic pre duty on their site and, since it's M6 steel and not in a mu shield can, I'll make a guess as to how it might be "pointed" in any mic pre I built... I'd point that 1:8 sucker straight in the bin.
No disrespect to Analag or Edcor.

 

[Winston OBoogie]

I've been thinking (always a dangerous thing) and I wouldn't mind seeing if there's any benefit to loading either a low noise cascode or a pentode with a constant current and operating the first tube as a transconductance amplifier. Linearity and gain would go up. With a 1:5 - 1:8 input transformer, there's probably enough gain in the transformer and transconductance amp for most things. Use a linear, high enough current follower after the 1st amp to drive your output and return your feedback. Similar to Solkatten's hybrid, or my E88CC scheme I posted earlier,. but with an active load.
Dunno? More incoherent ramblings...

 

[analag]

I only see one Edcor 1:8 for mic pre duty on their site and, since it's M6 steel and not in a mu shield can, I'll make a guess as to how it might be "pointed" in any mic pre I built... I'd point that 1:8 sucker straight in the bin.
No disrespect to Analag or Edcor.

Take a look my friend. Nickel core. I use the 1:5 as 5:1 in a few SS microphones. I use the 1:2 in a few M1 style mic pres. I actually use a stereo mic pre with those 1:2 amping my passive summing box. The 1:8 is in a FET and octal tube mic pre. No mu metal can just a foil shield, but sensible placement can work satisfactorily.

Define "pointed".

Think 20 to 20khz and visualize sloping ends of the spectrum with the usual mid range saturation

 

[Tubetec]

Ive found it beneficial to use a Mu metal screened mic step up transformer backwards in a tube mic , with voice or acoustic instruments it makes no odds , but for micing up guitar combo's with mains tx's in the vicinity definately less hum and hash . An unscreened I/P transformer on a pre-amp might just work with an off board psu but with nearby mains transformers on the same chassis induced hum becomes dominant even more so if you've really gone to town on sources of noise within the tube itself(ie quiet psu) . Mssr's Neumann probably never envisaged their U47 being used to record electric guitar amps up close and personal way back when . Ok if your micing up a 100w Marshall through a big cab ,cranked all to hell, a dynamic is most likely what you want . Smaller lower powered tube amps at moderate levels can definately benefit from tube micing tecniques though.

Ramble on my friends , let the coherence/incoherence be in the ear of the beholder.

 

[solkatten]

I have been thinking of an easy cost effective cascade, jfet + 6n6p. A 1:10 IPT to a jfet and a 6n6p on top and 6n6p CF to 2:1 OPT (vtb2291). By changing the trafos ratio you end up with the same amount of gain as the planar v72... With 1 tube per channel its easy to build mutiple channels

 

[solkatten]

I´ve been playing around winding my own IPT on nanocrystalline toroidal cores. Vitroperm 500F. I think this type of core has the right propertys to handle small singnals and subjectively it works great. If you lay the primary windning as a singel layer first it ensures very good noise rejection because of the perfect symmetri of winding, it actually worked whitout sheild in the same chassi as the PT by placement and orientation. But even better with a shield. It takes a couple of hours to hand wind a toroidal core but I encourage you to try. Pri: 200-300 turns, secondary depends on step up ratio. By sectioning the secondary you have ex. 80 turns and 30 section, its easy to count and to have an even spread of the secondary winding.

I think it could be a good DIY hack to lower the cost of a mic preamp

https://www.mouser.se/datasheet/2/599/W911-240893.pdf

 

[gyraf]

Nice - around here, we're also experimenting with winding our own toroid amorph's for microphone inputs (for a upcoming G25 recording amp, probably). Still fighting assymmetries in secondary capacitance distribution, but fun and a great way to learn..

/Jakob E.

 

[Winston OBoogie]

Take a look my friend. Nickel core. I use the 1:5 as 5:1 in a few SS microphones.

My bad, I clicked the wrong Google link which took me to a steel core 1:8.

I´ve been playing around winding my own IPT on nanocrystalline toroidal cores.

+1 on Jakob's "Nice"

 

[solkatten]

Nice - around here, we're also experimenting with winding our own toroid amorph's for microphone inputs (for a upcoming G25 recording amp, probably). Still fighting assymmetries in secondary capacitance distribution, but fun and a great way to learn..

/Jakob E.

Nice I dont have any good measuring tools. So I cannot determine were it self resonance... THD... , but the details are great and it has an egde on the noise perforamnce side. I´ve seen some hifi guys winding SUT on similar cores. You reach a resonable primary inductance with few turns on this cores, and that really hepls with all the trafo parameters... The sectioned secondary is layed in a progressive maner... and you just circle the toroid ones...
And I think having a single layer primary really helps the cmrr

 

[Tubetec]

Interesting, I hadnt thought of toroids as input transformers . I have tried re-winding tiny toroids used in wah wah pedals , takes time and patience but it can be done.

Have you tried REW ?

 

[solkatten]

REW?
This new amorph/nanocrystalline material have low losses och high permability and high b-sat. The challange is the winding...

 

[volker]

What primary impedance at what frequency and what input level are you aiming for? From quick calculation the way I would set it up I would estimate your primary turns are too low by maybe a factor 2, but that depends on your chosen constraints.
Also if you only wind secondary straight on the primary I think you might run into problems with high-end, particularly at a high ratio like 1:8. Look at the sectioning of the V72 input transformer in order to avoid capacitance between the windings or other dissections from CJ of more conventional input transformers.

 

[solkatten]

What primary impedance at what frequency and what input level are you aiming for? From quick calculation the way I would set it up I would estimate your primary turns are too low by maybe a factor 2, but that depends on your chosen constraints.
Also if you only wind secondary straight on the primary I think you might run into problems with high-end, particularly at a high ratio like 1:8. Look at the sectioning of the V72 input transformer in order to avoid capacitance between the windings or other dissections from CJ of more conventional input transformers.

If you have 260 turns on the primary you got 4H at 10K, that should be even higher at lower freq. Secondary is then 2000 turns. Maybe easier to go for 1:5. 260T:1300T? The goal would be to do a easy to DIY toroid IPT... To do layers of pri and sec is to hard if you wind by hand.... The "semi" progressive as I described is probabably the best/simplest to do it without machinery...

https://www.mouser.se/datasheet/2/599/W911-240893.pdf

Im calculating primary inductance this way:

http://www.diygallery.de/DIYsites/inductor.html

 

[gyraf]

If you have 260 turns on the primary you got 4H at 10K, that should be even higher at lower freq.

Yes and no. This is very true for traditional nickel-based high-AL cores like mu-metal and radiometall - but for (most) nanochryst materials there is not this raise at lower frequencies, so we need to incorporate ALL the needed primary inductance for lower frequencies..

/Jakob E.

 

[Tubetec]

REW = room Eq wizard . A fantastic and free software capable of audio measurements ,bandwidth and distortion etc. It works with your existing audio hardware seamlessly . You can generate a test signal , feed it through what ever you want to test and get a graph of frequency responce .
As mentioned by Abbey and Winston input transformers rely on capacitive balance between sections to get good HF responce , thats hard to achieve with hand winding . There was one Polish company winding sectionalised toroids but he was keeping tight lipped on his methodology , he seemed to be spacing out the sections equally around the circumference of the ring core.

 

[solkatten]

REW = room Eq wizard . A fantastic and free software capable of audio measurements ,bandwidth and distortion etc. It works with your existing audio hardware seamlessly . You can generate a test signal , feed it through what ever you want to test and get a graph of frequency responce .
As mentioned by Abbey and Winston input transformers rely on capacitive balance between sections to get good HF responce , thats hard to achieve with hand winding . There was one Polish company winding sectionalised toroids but he was keeping tight lipped on his methodology , he seemed to be spacing out the sections equally around the circumference of the ring core.

I will look that up I have MAC at home hope it works

 

[solkatten]

Yes and no. This is very true for traditional nickel-based high-AL cores like mu-metal and radiometall - but for (most) nanochryst materials there is not this raise at lower frequencies, so we need to incorporate ALL the needed primary inductance for lower frequencies..

/Jakob E.

Im actually going to get a better LC meter so I can measure at diffrent freq. What are your thoughts on minimum primary impedance. A bare minimum seems to be 2H but many IPT has much higher inductance. And you are trading primary inductance against all the other parameters. I have 2 options. A singel layer with 0,2 mm wire=4H 260T or 0,15 mm = 7H 360T?

I will try to make some measurments on my IPT and I hope they perform well, at least they reject noise very well

 

[gyraf]

Aiming at something realistic like 1200 Ohms and a -3dB point of 20Hz, we need somewhere around 9.5H primary.

Getting a 7:1 from here is far from trivial, but it's a great learning curve..

/Jakob E.