6N1P Bass DI - Revisited

[Wavelength]

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?

Sorry just seeing this. I have done a number of tube things with Cinemag. The S217 is fracken killer in both single ended and PP. But if you are doing single ended you need a loading choke. I did a audio line preamplifier for a guy in Italy with a 12J5 (basically a 12 volt 1/2 of a 6SN7) loaded with a 220H low dcr choke at 10ma and cap coupled (parallel feed is what it is called) and the S217 from Cinemag was killer. I am thinking about doing an all out DI with that after I finish up a couple of custom projects I am working on (acoustic preamp with optical compressor and FX loop and a pedalboard amp + di + overdrive + 9V/2A supply). BTW if you ask Dave and tell him what your looking for he will make honest suggestions. But remember these smaller outputs are better off running parallel feed rather than direct feed as the inductance for this stuff needs to be really high or your phase error in the low end is going to suck.
Gordon

 

[jonasnoble]

Man, you guys have knowledge and experience light-years beyond mine. Thanks for the info, I'll certainly put that transformer on the list to experiment with.

You said some words I didn't really understand:

But remember these smaller outputs are better off running parallel feed rather than direct feed as the inductance for this stuff needs to be really high or your phase error in the low end is going to suck.
Gordon

How much is enough inductance to run direct? Is it related to gapping the stack for DC?

And phase error. I'll spend some time reading the technical meanings of that phrase. But what does it sound like here? A loss of low end?

 

[jonasnoble]

Hook it up to a lamp cord

Put a ammeter in series.

Use power strip so you have a circuit breaker.

Measure current.

If using 120 AC then use this formula.

Primary inductance is


120/6.28 x 60 Hz x I or simply

Henries = 0.318/I

Example 1 ma will equal 318 Henrys

Be careful you do not hook the 120 up to the secondary.


There is also a small fuse in your ammeter.

A Variac is nice but I use the wall socket on a regular basis.

Using 120 will simulate real world levels and frequency.

Inductance will be higher at 20 Hz but not by much in a gapped core like you have.

I tried several different methods to measure inductance on these CM-9589's, CJ's quoted here is the only one of gotten to work. I've taken a few sets of measurements and they all come out to >200H. I would think that's high enough to run direct, but this is new territory for me.

(I could also be doing something stupid, of course, which is making my numbers wrong.)

 

[My3gger]

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

You made it very nicely, soldering looks good at most points.
Play with it, get your friend try bass, running mastered albums through can also be interesting for comparison (might not sound good on this type of circuit), synths are another option to try.
I should not mention small/big core so much because there is a lot more to transformers than how big the core is. MisterCMRR's remark explained that non symmetrical signal at the output of multifilar transformer when driven from single ended sources is not a problem, mismatched impedances are. This is about an old myth saying both voltages must be the same for balanced output...
One thing; where is IEC's ground pin connected, is ground connection a few cm to the left of IEC soldered to proper ground connector? This part is important in the long run and for safety. Good job!

 

[Wavelength]

First designing audio transformers is a skill in diminishing returns. Too many windings leads to too much capacity and resistance which leads to poor top end and high losses. General rule of thumb on primary is that the voltage drop should not exceed 20Vdc. So I*Rpri < 20V.... Inductance for direct feed has to be done with current passing through it. Plus using a tube make its hard because of the Rp and secondary loading. I think there is a pretty good section in the RCA Radiotron Handbook on that stuff. Also what core material and air gap has a big overall effect on inductance. When I have amp outputs made I usually use M6 (Iron quality, higher numbers have lower quality) as core material. When I am do line or DI type work then I use high nickel for a couple of reasons. It has higher perm and it tends to reflect 50/60Hz noise.
Now the air gap thing someone else will have to explain but with parallel feed (i.e. cap coupled) the output has no air gap so the inductance is pretty straight forward. The loading choke is in parallel so there is some interaction there. I don't design transformers but I have worked with some good ones.
One note on sound and HiFi (I do a lot of that as well). Caps have a big impact on the sonics of any design. Mixing power supply types and coupling can totally change the character of the sound. I typically will use a good film cap from Mundorf in the early stages and then V-Cap ToneCaps (Oil & Paper) later on and that mixture tends not to sound HiFi but doesn't kill the top end like some Oil and Paper caps do.
Ok so here is a simulation my buddy Dave Slagle did for me on a loading choke for a USB DAC I make with 71A tubes for Audio. The lower graph is current and the lines are related to the air gap applied to the choke and the resulting inductance. I was using 9.4ma (basically 2x 4.7ma current regulating diodes on each half of the filament) so we opted for 0.0032 air gap for the design because I wanted as much inductance as possible. The output fed a step down autoformer in high nickel which basically had an inductance of 4000H. You can also see the losses were low so 9.4ma * 541ohms =~ 5V.
Gordon

 

[Pinokio]

Hi CJ. Is it possible to buy two transformers for a stereo build?
I've dm'd you in the past but can't seem to dm (anybody) atm...

Would be great, I'm in Europe but also got contacts in the US.

CJheers!

 

[jonasnoble]

You made it very nicely, soldering looks good at most points.
Play with it, get your friend try bass, running mastered albums through can also be interesting for comparison (might not sound good on this type of circuit), synths are another option to try.
I should not mention small/big core so much because there is a lot more to transformers than how big the core is. MisterCMRR's remark explained that non symmetrical signal at the output of multifilar transformer when driven from single ended sources is not a problem, mismatched impedances are. This is about an old myth saying both voltages must be the same for balanced output...
One thing; where is IEC's ground pin connected, is ground connection a few cm to the left of IEC soldered to proper ground connector? This part is important in the long run and for safety. Good job!

I haven't checked in for some time but figured I would offer an update. This simplified prototype worked wonderfully. My Nashville friend absolutely loves it, and is putting all his bass, synths, and guitars through it.

Knowing the output concept works, I've moved into adding things in the front end. Mic input transformer/instrument input will go to a DC coupled cathode follower, which will drive some passive tone controls which feed into the 6N1P output stage.

I'm still waiting on CJ to spin up his next batch of output transformers, but for now these Cinemags are spectacular. Also, finally got the bobbins and bells in to attempt my own wind in the next couple of weeks.

I'm on vacation now, but will hit it hard when I return and I'll keep you guys updated. I'll have a ton of questions about how to use a mic input transformer, phantom power, etc.

 

[Murdock]

I finally managed to build this DI with this unknown heavy hitter iron.
I was worried about the low frequency performance because of it's rather low inductance.
But it's the other way around...
Low frequency performance is great! -3dB at around 10Hz.
But the high frequency drops really fast and is -3dB down at 10 KHz or worse depending on where the output pot is...
I thought this whole section winding thing is made to improve high frequency performance? I measured the primary leakage inductance and it is rather high at 130mH at 7.9KHz.
With a 10k load response is really great, over 140KHhz.
So I would like to change the output configuration somehow as my inputs have an input impedance of about 5k Ohm.
Is it as easy as replacing the two 300 Ohm resistors with, let's say 2200 Ohm?
Or is there any other way to improve high frequency response?

 

[ruffrecords]

I never like the arrangement of the output pot which gradually shorts the secondary of the transformer. You could just replace it with a regular 2K pot wired normally. This will have an output impedance of no more than 600 ohms.

Cheers

Ian

 

[Murdock]

I never like the arrangement of the output pot which gradually shorts the secondary of the transformer. You could just replace it with a regular 2K pot wired normally. This will have an output impedance of no more than 600 ohms.

Cheers

Ian

Thanks for your input Ian.
But what do you mean with “regular 2k pot wired normally”?
Still a log pot or linear?
And do you mean to keep the two 300 ohm resistors but putting them before the pot?
Something like that?
What would be the input impedance of that circuit?

 

[merlin]

You don't need the 300 ohm resistors in that configuration

 

[Murdock]

You don't need the 300 ohm resistors in that configuration

Thanks! I tried it that way with the 1k log I still had here. It's definitely better. When loaded with 10k ohm the -3dB point now shifted to 21KHz.
Would a 2k log or even 5k log be even better? What does this arrangement do with the output impedance? Is the pot in parallel with the output impedance?
It still puzzles me that it behaves so "badly" with all those sectioned windings. I thought this is beneficial for improved high frequency response?

 

[merlin]

What does this arrangement do with the output impedance?

Differential output impedance will be one quarter of the pot resistance (plus the output Z of the transformer itself), with the wiper at the -6dB position. Less at all other settings.

 

[Murdock]

Differential output impedance will be one quarter of the pot resistance (plus the output Z of the transformer itself), as a maximum. Less at all other settings.

Thanks, that’s great to know!
So a 4k7 pot would be quite a lot, probably too much.
So I’ll try a 2k pot as recommended.
Thanks!

 

[ruffrecords]

Thanks for your input Ian.
But what do you mean with “regular 2k pot wired normally”?
Still a log pot or linear?
And do you mean to keep the two 300 ohm resistors but putting them before the pot?
Something like that?
What would be the input impedance of that circuit?
View attachment 128053

Maybe I am confusing this with another project but that is not the schematic of the output pot that I remember. It is much better. You can probably dispense with the two 300 ohm resistors whicxch is what I had in mind.Merlin is spot on with his calculation of the output impedance.

A LOG pot is probably best.

Cheers

Ian

 

[dmp]

The schematic on the first post by CJ has the log pot before the 300 ohm resistors. Read EMRR's crtisicm and suggested alternative a few posts down (this has already been discussed).
Presumably the original design did something the designers liked (i.e. distort the sound) by shorting the transformer secondary
But if you DON'T like it do something more textbook correct
I don't remember what I ended up doing in my builds

 

[CJ]

It is a bass DI,

 

[Murdock]

It is a bass DI,

You’re right, and -3dB at 21Khz is probably good enough, maybe even overkill… but for me it is more a matter of learning and trying to understand what’s going on.