Humfree Splitter (from RG's ABY) with questions and schematic

[Auke Haarsma]

That would indeed be bad.  But I think that the freq range is increased because the xformers are driven by the opamp. As RG says:

You might like to use the transformer isolated output of the hum free splitter at GEO (http://www.geofex.com). It uses a $2.50 transformer available from Mouser.com and drives it from an opamp to extend the frequency response to 60Hz-28Khz.

(source: http://www.diyaudio.com/forums/showthread/t-52105.html)

[R.G.]

Auke in the datasheet of the Mouser Xformer 42TM018  it says that they have a freqency range of 300 Hz tot  3.4 KHz within 3 db  only !

In my opinion it narrows down the performance too much especially in the low end.

That's a common mistake among people who don't understand transformers and datasheets, alf.

First about datasheets. Datasheets present the numbers that the manufacturer is willing to stand behind, in the sense that if his parts don't do at least that well, he'll give your money back. Some datasheets give min and max values of certain specifications, but many only give either a minimum (or maximum) and a typical value. For "typical" you should understand that the maker is saying "we think most of them are close to this. We hope the ones you get are close too. But we won't give your money back if it's not.

The specifications on bandwidth of transformers in general, and these in particular is a minimum of 300Hz to 3kHz. Why that? That's voice bandwidth on telephones. The actual transformers you get are almost certain to be better than that minimum - but how much? I did the obvious thing and measured it. Right off the shelf, the Xicon transformers I measured were about 150Hz to 22kHz within +/-3db. The manufacturer is obviously not going to go do work to make his products enough worse to make the specs come true, right?

Still, 150Hz is not all that good for guitar. So I used a trick. Transformer bandwidth is tremendously impedance sensitive. To get the best bandwidth in a passive situation, you simply have to drive it from the expected input impedance and load it to the expected output impedance. But I wasn't building a passive box, I was building a box to make the best use of cheap transformers.

So I took note of the fact that the low end of a transformer is limited by the core inductance eating up the input signal before it could be transformed. If you supply a primary from a quite low impedance, it can supply the extra current that the transformer core is trying to eat up, and still supply the signal being transformed too. In effect, the extra power extends the low end response on the secondary side.

In the circuit I set up, I ... measured... 60Hz to 22kHz, which I thought was good enough for rock 'n' roll.

[Auke Haarsma]

edit: removed because I replied on a msg RG was still editing.

[R.G.]

1-RG, are you confirming alfafalfa?

No, I'm confirming you. I just accidentally hit the send key before I was ready. Read it again now.

[Auke Haarsma]

Thanks for your explanation RG!

The PCB can be populated now 

[alfafalfa]

Thanks for the very interesting and clarifying explanation R.G.

That's good news Auke , let me know how it works . I have a couple of theose xformers as well.

Alf 

[alfafalfa]

Thanks for the very interesting and clarifying explanation R.G.

That's good news Auke , let me know how it works . I have a couple of those xformers as well.

Alf 

Sorry , I thought it was the modify button !!

Auke what about the distortion level of those xformers ?  Do you have any info on that ?

Alf

[Auke Haarsma]

I have no other info available about those statistics.

I've put it on my testbank right now. It seems to output has a bit more volume than the input. A couple of questions, it's not humfree yet:

-On the scope I see that the output of the opamp is higher than the input signal. Am I correct in thinking that the 11k resistor sets the opamp gain? Can I lower it to, say 10k, to make it unity gain?
-I didn't have a 2.2uF NP cap. I used two 1uF NP in parallel instead. This is OK, right? (what is the function of this 2.2uF cap btw?)
-Should the enclosure be connected to GND or to VR? I'd say for shielding purposes I can connect it to GND.

Alf, could you mail me your layout please? It's still not available at the DIY Layout creator page, and I'd love to have a look. My email: aukehaarsma at gmail dot com. Thnx!

[Auke Haarsma]

also:
-into the ltc1044 goes 7.97, but it puts out: +7.97 and -7.76. Shouldn't this be +7.97 and -7.97? Now there's a 0.2V gap. Can this cause problems/hum?
-also, the hum (with the vox ac30 at max volume btw) does respond to the volume knob on my guitars. The intensity of the hum changes (not the frequency). When I plug the guitars directly into the Vox there's no hum (or very very little). This id tested with 1 amp (the vox) on the direct out, and another (Epi Valve Jr) on the isolated out. Removing either amp from the splitter doesn't make a difference.

[R.G.]

Auke what about the distortion level of those xformers ?  Do you have any info on that ?

The distortion of a transformer depends heavily on how hard and at what frequency it's driven.
The only distortion mechanisms transformers have are B-H nonlinearity and nonlinear core dissipation.

B-H nonlinearity is a low frequency issue, as it takes the product of voltage and time to move the core across a large part of its BH curve. So at high frequencies distortion decreases and core distortion is only an issue at the lowest end of the frequency range for all practical purposes. Nonlinear core dissipation can happen at all frequencies, but it's most severe at the top end. The cure for that is to use thinner iron. However, it's rare that iron nonlinearity is an issue at audio.

Bottom line - for small signals like less than audio line, the transformers in question should be quite good.

-On the scope I see that the output of the opamp is higher than the input signal. Am I correct in thinking that the 11k resistor sets the opamp gain? Can I lower it to, say 10k, to make it unity gain?

The gain as drawn is 1+11K/10K or about two. However, the audible difference at such low gains is almost undetectable. It seems to be more "clarity" than more level to most listeners. If you want unity gain, leave the 2.2uF NP cap out.

-I didn't have a 2.2uF NP cap. I used two 1uF NP in parallel instead. This is OK, right? (what is the function of this 2.2uF cap btw?)

It's OK. It's also OK to leave it out, as above. It's purpose is to give unity DC gain but allow a gain of about two in the buffer.

-Should the enclosure be connected to GND or to VR? I'd say for shielding purposes I can connect it to GND.

Sorry, I didn't dig through your circuit in detail until now. Why are you constructing a "Vr" different from ground? Use ground everywhere you have Vr now, and leave out those Vr parts. You don't need them and things may be quieter. Connect the input jack sleeve to the enclosure, and the direct output. Isolate the two transformer coupled jacks from the enclosure to start with. Depending on the local AC hum field situation, you ... may... need 100 ohm resistors from the transformer secondaries to the splitter local ground. Hum is hard to defeat totally. Oh, yeah. Use a steel, not aluminum box or your isolating transformers can actually pick up hum.

-into the ltc1044 goes 7.97, but it puts out: +7.97 and -7.76. Shouldn't this be +7.97 and -7.97? Now there's a 0.2V gap. Can this cause problems/hum?

No, and you can simply ignore it. The LTC1044 and all other charge pumps actually put out slightly less than their input. The gap will be ignored by modern opamps, which will work fine with powre supplies that are not perfectly mirror imaged.

-also, the hum (with the vox ac30 at max volume btw) does respond to the volume knob on my guitars. The intensity of the hum changes (not the frequency). When I plug the guitars directly into the Vox there's no hum (or very very little). This id tested with 1 amp (the vox) on the direct out, and another (Epi Valve Jr) on the isolated out. Removing either amp from the splitter doesn't make a difference.

I believe that this is the hum from the guitar and cord being buffered into the splitter. If it responds to the guitar volume, it has to be on the input side of the splitter, right? I think that your guitar volume is changing the loading on the input cord wiring and this changes the input hum. The splitter can't fix what comes in on its input. How long are the cables and what kind of guitar do you use?

What the splitter ... should... do is make the hum into two amps no bigger than the hum into one amp.

[Auke Haarsma]

Thanks for your detailed response!

I'll check all the things you said, but I'll reply now on the guitar part:

-I'm using a '96 Gibson LP Standard with stock pups, and a PRC-clone with Golden 50's humbuckers.
-There's a pretty short (10ft) cable going to my Vox
-There's a bit longer (20ft) cable from my guitar to the splitter
-and another 20ft (old) cable from the splitter to the Epi Valve Jr Head.

[Auke Haarsma]

As RG suggested I made Vr the same as GND. I also removed R14 and R15 (the 10k's from the ps-section) and got a nice stable splitter now!
I also removed the 2.2 uF since I had too much gain. Right now it's (close to) unity gain.

I've compared the direct output with the isolated output and they sound very much alike. I think I'll record and loop a phrase and then record my vox onto my pc. While recording (and looping) I'll swith from direct to isolated output. Seems to me a good way to compare the output. Ears can be deceiving.

[Auke Haarsma]

Finalized this week. I used the 'improved quick n dirty oscillator' and my oscilloscope to finetune it. Added a 50k trimpot between the xformer and the first isolated-output. This allows for some treble roll off.

Pics are coming up anytime soon.

[Auke Haarsma]

And here are the pics. Outside:



And of more interest, the inside:



Hopefully it stays humfree! Thanks for all the help!

[familyortiz]

Yo All,
  This has to do with the performance the transformers specified for R.G. Keen's Hum free splitter.
As explained by RG in previous posts, the Xicon 42TM018-RC transformers, available from Mouser, gives a 3dB range of approx. 30-20kHz when driven with a low impedance output such as an opamp.
What has not been mentioned yet, is that there can be significant noise induced when the transformer shows some leakage between primary and secondary windings, indicated by a resistance of 1-3 Mohms between primary and secondary leads.
Of 3 pieces obtained from Mouser, 2 showed this small amount of continuity between the windings. The effect on the signal is such that it shows a thicker trace on an oscilloscope than a circuit with a transformer that has no measureable continuity.
In short, I would measure any transformer before installing into the design, and look for any measureable resistance between the primary and secondary.  

[alfafalfa]

Auke in the datasheet of the Mouser Xformer 42TM018  it says that they have a freqency range of 300 Hz tot  3.4 KHz within 3 db  only !

In my opinion it narrows down the performance too much especially in the low end.
That's a common mistake among people who don't understand transformers and datasheets, alf.

I only just read this today !

Thanks for the info R.G.  !  I wish I had known this before , could saved some money because I bought two special xformers from Sowter. They do a great job but are quite expensive.





Alf