6N1P Bass DI - Revisited

[jonasnoble]

Hey everybody, this thread's been going for a couple years...

I been wanting to build this (or something based off it) and I have transformer questions. I really want to buy a few of CJ's, but he's been absent. And I don't know if he would sell to new members anyways, so I've been talking with Cinemag. He keeps steering me to smaller, 7:1 transformers, the CM-27101, so I think I'm not communicating something correctly.

What I really want is the REDDI transformers, which he won't sell me for obvious reasons. But there's another big OT that I think would be perfect, but he seems to think it's not what I want. There's no datasheet, so here's the info. It can get used in a 12AU7 push-pull circuit, with DC current.



Has anybody built this circuit with a Cinemag transformer? And if not, can somebody help me figure out which one's going to give me the fat round bass tone I'm looking for?

 

[beatnik]

Those Cinemags are not what you need, the 9600 is for a push pull output stage and the 27101 doesn't seem to take DC and the ratio is somewhat higher than required for two 6N1P triodes in parallel

Specs of CJ transformer is 8K/600 so 3.67:1 and most importantly the core should be gapped type to take DC through the primary, schematic specs 8.36mA current.

You could ask Cinemag again something with the specs above, or perhaps look into Lundahl LL2763 wired 4:1 the stock version is rated for 7mA but they should be available to make it with other values.

Alternatively Sowter 1367 or 9915 wired 5:1 should work as well and they come with a stock rating of 15mA

 

[jonasnoble]

Those Cinemags are not what you need, the 9600 is for a push pull output stage and the 27101 doesn't seem to take DC and the ratio is somewhat higher than required for two 6N1P triodes in parallel.

That was my concern also, but the confusion is probably where I asked for REDD47 transformers. Those look pretty good for that.
He did ask what my DC current requirements were, implying he could set the CM-27101 to handle some DC, but that doesn't fix the ratio issue.

What about a CM-2810? It's 9.6k:600 or 4:1, and I could ask him to set those for 8mA DC current.

But these are still small (like the 27101) compared to what I would expect to see in a REDDI, doesn't that limit low frequency response? I'm assuming the REDDI magic comes from the super-sized core.

 

[My3gger]

That was my concern also, but the confusion is probably where I asked for REDD47 transformers. Those look pretty good for that.
He did ask what my DC current requirements were, implying he could set the CM-27101 to handle some DC, but that doesn't fix the ratio issue.

What about a CM-2810? It's 9.6k:600 or 4:1, and I could ask him to set those for 8mA DC current.

But these are still small (like the 27101) compared to what I would expect to see in a REDDI, doesn't that limit low frequency response? I'm assuming the REDDI magic comes from the super-sized core.

In my test 27101 with steel core didn't do that well in bass. See if UTM from Poland, or even better local Edcor can make transformer with big steel core working well in 6N1P DI/line amp.

 

[beatnik]

It's not all about the size but also the core material.
From my understanding a transformer with mu metal / high nickel laminations can have same primary inductance as a steel core but with less primary turns, so it can be physically smaller but have same bass response.

You could try Cinemag CM-9589 it has the right turns ratio and available with gapped core and hi nickel laminations.

 

[jonasnoble]

Cinemag CM-9589 on order! Thanks for all the help guys.

 

[My3gger]

It's not all about the size but also the core material.
From my understanding a transformer with mu metal / high nickel laminations can have same primary inductance as a steel core but with less primary turns, so it can be physically smaller but have same bass response.

You could try Cinemag CM-9589 it has the right turns ratio and available with gapped core and hi nickel laminations.

Interesting discussion,do they make CM-9589 with 50Ni or HiNi cores for over 8mA DC?
I have CM-27101 for 7mA with 50Ni core stacked 2Nix2Fe (can't remember how this stacking method is called), if used up to 10mA it needs gapped steel core with all E's in one direction and I's one the top making the gap. In my experiences very big cores tend to sound different in bass from smaller cores when inductance of both is high enough, if better depends on what one wants.
Page 9 of this paper has interesting comparison of various lamination materials:
https://www.jensen-transformers.com/wp-content/uploads/2014/08/Audio-Transformers-Chapter.pdf

 

[L´Andratté]

Wasn´t a central spec of CJ´s xformers the super high primary inductance (like 40H IIRC)?

 

[jonasnoble]

I screenshot one of his posts yesterday that said his should measure 175H. I don't know exactly what that means for the construction of the transformer, but I think I'm going to have to start winding my own to really learn.

 

[CJ]

I'm here, they keep changing the login window every week. I was logged in for 10 years straight without ever having to log in. Now I have to log in every week, wtf, jus saying..


I have enough endbells for two transformers right now

The usually drag about 1 ma or under, average is 0.75 ma at 20 hz and 22 volts rms.

22/0.00075= 30 k impedance

30k/6.28 x 20 = 238 Henries

These things are unique. They have a weird bump at 12 kc which I think is what gives them the funk.

You can build a xfmr with the same core and inductance but it won't have the magic. Not that I'm great, nothing to do with that bs, I just got some dumb luck on my side for a change.

Another weird thing. The famous Peerless K 241 D transformer that sells for 1 grand , well, it has a bump at


12kc

Interesting thing is that on the 5000 turn primary, if I am off by plus 20 turns on one side of the split coil making me minus 20 turns on the other side, then the 12kc bump moves down to 10 kc, and the hi freq bump moves down from 120 kc to 100 kc. So these transformer coils are pretty touchy. Ask Sowter if you don't believe me. Balance. It is all about the balance.
.

 

[jonasnoble]

I have enough endbells for two transformers right now

CJ, I would love to buy these if you were willing to wind them up. I have an order in with Cinemag, also, but there's a 5 week lead time. And I think yours are going to work better for what I'm doing.

 

[Migs 31]

I was logged in for 10 years straight without ever having to log in. Now I have to log in every week, wtf, jus saying..

It's for YOUR OWN SAFETY CJ! Don't you get it?
Someone like your wife or children could enter your account and post about lams and transfos!
I know, WTF!?! Anyway...

Thanks for the valuable info on these transfos and their turns etc, Sir.
Really really appreciated.

M

 

[jonasnoble]

I have a question about the output transformers for this schematic. The datasheet for a 6N1P says max dissipation is 2.2W per triode, which because we're paralleled should give us 4.4W. When I run the calculator I'm using, setting the power level to 4.4W makes me stack my EI100 core laminations to 1.25".

I'm wondering do I need that much headroom? And if I do, but under-stacked the core for slightly underpower, what would be the adverse affects? I would like to conserve materials if possible and make this wind repeatable. And of course I'll experiment, but I hate to waste money and create trash unnecessarily.

 

[jonasnoble]

Well, anyways, my Cinemags should be here Wednesday, so I'll try them out. First prototype is coming along. I'm leaving most things unmounted, so I can play with the placement for minimal noise. These Amtek toroids are nice and cheap.

I'm leaving space to add another gain stage with James tone circuit, as I've seen somebody do. And my first couple faceplates are going to be less than perfect, as I learn the process and capabilities of my CNC rig. (pictured is my first attempt: too aggressive and got some wobble on the lettering.

 

[jonasnoble]

This prototype is assembled. I've been playing through it and it sounds pretty good to my ears. Noise floor is low, I get some juicy meat in the low end, and some sparkly things up top.

It's a little sloppy right now, so I haven't photographed the guts. I haven't really tested/measured it extensively. And I have a lot of work to do on the faceplate. There's a lot of blank space on the front where I'm going to add more controls. And I haven't played with the mic input transformers, something I'm just putting in for fun and experimentation. It'll probably never make a great vocal box, but you never know when you want some tubey goodness on a snare or something.

I'm concerned these Cinemags are a little more high-fi sounding than CJ's transformers would be, but they sound good enough for a first attempt.

 

[jonasnoble]

Something I had a question about regarding a well timed post from MisterCMRR:

If you use a bifilar output transformer driven by a single-ended output stage (as in consumer gear, for example), you will notice that the signal on the transformer secondary is not symmetrical with respect to ground (and become less symmetrical as frequency increases)

Pins 2 and 3 of my output jack are exhibiting something like this behavior. With 1vp-p input, I get 3v on pin 2 and 12v on pin 3, unloaded. It varies with the volume control, and is symmetrical at the lower settings. Is this something anybody has experienced with this circuit? In his post, he goes on to add that the asymmetry is not a problem, I'm just curious if what I'm seeing is common and is the same thing he's talking about.

 

[Philbo King]

there is an older thread that got kind of cluttered so we are starting a new one for purposes of clarity,

here is a schematic for a Bass DI box that uses the Russian 6N1P dual triode.

these DC voltages were taken from an actual 6N1P running at 170 V-dc plate supply,

using one cathode resistor instead of two gave the measured results from the actual unit,
so we are simplfying the possibly erroneous schematic shown in the other thread,

a DIY OPT was used,

here ya go>

Looks like the Aux Out ought be labeled Aux In.

 

[jonasnoble]

Not sure what you mean, would I not label the output of the preamp as "Out"?

This is a real real sloppy first attempt and is no way anything close to where I am taking it. But all ideas are appreciated, of course.

edit: oh you're talking about the original schematic. I believe that's a pass through jack that REDDI added on later models, to allow the bass signal to go to a stage monitor/amp, and the balanced line out goes to the FOH.

 

[L´Andratté]

Aux Out = Signal "Through" (like on a mic splitter or you see it wit MIDI)

 

[jonasnoble]

Okay, I couldn't leave it like it was. I cleaned it up some and it's slightly less embarrassing to look at.