Impedance matching for DI to Mic Pre

[Keriwena]

Originally Posted by shanx

Ok so basically Nigel suggested originally that the passive tone and volume controls do have an effect, and that also because of them being in the guitar circuit it is a factor in making neccessary a high impedance input on an amp.

Nigel stated "the values of the controls in a passive guitar/bass mean the following amplifier has to have a very high impedance input", suggesting a cause and effect relationship. I might have let "imply", "suggest", or even "indicate" go, but "mean" is misleading. Remember, this followed my suggestion that the control values confused people about the output impedance of the instrument. He was, basically, proving my point with his contrary argument and I couldn't let that stand unchallenged.

Originally Posted by shanx

For that matter, why go on with all this and [not] just try different things. A small audio transformer is not a big expenditure, and nobody gets hurt for trying. Maybe on a bass it will give a nice response. If not, run an active buffer first through to the transformer. Pretty straightforward.

I'm a big fan of experimentation. In fact, I believe most advances are made by people that "didn't know better" than to try something novel. It's the crazy people - you know the ones that sail off the edge of the flat earth - that actually make leaps in the progress of the human condition.

However, the more you understand about what's really going on, then the fewer tries it takes to nail the solution. And the the more you know about what isn't really understood, the more clues you have as to where the answer might lie.

Fezzle apparently knows that input transformer saturation in mic preamps and output transformer saturation in tube amps is often credited with improving the tone, and he wants to explore that. I'm just trying to give him some guidance as to what the other known parameters of good tone are, so he can focus his experiments on as few variables as possible.

Originally Posted by shanx

That is too bad. Maybe your values are different than some other people.

Well, I'm not a pickup-winder. They might find those charts useful. But when presented as "Heres some actual pickup data.", followed by, "Dosnt take much to see why one needs a high impedance input.", then I feel obliged to point out that awe of technology is a poor substitute for true understanding. I'm not saying it's impossible for a college class to seriously study electric guitars and do some fundamental research that benefits us all, I'm just saying I haven't seen it. Until I do, I'll match my 50 years of experience in helping guitar players get good tone against any professor's passing interest in engaging his students with a popular topic.

[shanx]

I'm a big fan of experimentation. In fact, I believe most advances are made by people that "didn't know better" than to try something novel.

I absolutely agree with this. It is good to get a handle on some theory, get advice from others and read some books to get an idea of what is going on. Fezzle is interested in exploring the transformer idea with the hopes that it could impart a nice characteristic to the bass. Good ''accidents'' can happen, and a lot of knowledge gained by trying.

[Nigel Goodwin]

Originally Posted by Keriwena

Nigel stated "the values of the controls in a passive guitar/bass mean the following amplifier has to have a very high impedance input", suggesting a cause and effect relationship. I might have let "imply", "suggest", or even "indicate" go, but "mean" is misleading. Remember, this followed my suggestion that the control values confused people about the output impedance of the instrument. He was, basically, proving my point with his contrary argument and I couldn't let that stand unchallenged.

That comment of mine was in response to your suggestion that only a low impedance input was needed - which you keep continually ignoring any mention of.

I mentioned the value of the pots as a further indication of the requirement for a high input impedance, not as the sole reason for it. You can't feed 250K/500K pots in to a low impedance, it messes every thing up.

The entire rest of this thread is based on your assertion that only a low impedance is required - would you care to comment on your reasons for saying that, or are you going to ignore it yet again?.

[Keriwena]

Nigel, perhaps you could quote the passage you're referring to?

[cbdb]

Originally Posted by Keriwena

Yes, we were.


Well, that's sort of my point. We were discussing the effects of different impedances, and you cite a paper where only one impedance was used.


I don't think you're qualified to make that statement. I do think though, the fact they were students suggests they were in a state of learning, not knowing.


Sometimes I'm not sure. It might be Alzheimer's.

What I'd like to do is help Dave (Fezzle) build a DI box that suits his stated purposes.


True, but as I stated earlier, if you're going to discuss this knowledgeably, then you ought to be able to look at one and infer the other.


I don't think you meant what you said. The input of the amp does not lower the response by a semi-tone.

If what you meant was, "The input impedance of the amp flattens the response", that's not what we want. (Well, you may be able to sell such a design to Jazz players, but they have special needs.)


Again I say, the amp's input does not "need to be high"; it needs to be right. It's not a subtle difference to a musician.

It's not about frequency response, really, it's all about resonance. We want the circuit to "ring" in a musical manner. The whole point of a DI box is to preserve the musicality of the instrument/amp interface when sending the signal to an input that was designed with flat frequency response as a goal. "Flat" is not a complimentary term when used to describe the sound of a guitar.

You need to learn some electronics. The paper was about pickup impedance and thats exactly what it shows. The test load to measure those impedances dosnt matter.

How would you know how qualified I am? You seem to use argumentative statements like that to cover your lack of real knowledge.

They were grad students working on a publishable paper. There is nothing wrong with there method or results (if you knew your electronics you would know this). Your lack of respect for education is glaring. As is your lack of education.

You also have trouble understanding basic audio electronics dialogue. I never said anything about changing pitch, I said freq response which is completely different. And I will say it again on the small chance you will get it this time: When the output impedance of a source (the guitar) is not flat across the freq range, the input impedance of the load (the amp) must be much higher than the source to even it out. Learn about Thevinin and voltage dividers.

Yes the resonant peak is important and the input Z does effect it but there is a lot more to the picture.

You say the input Z has to be right, not neccesarily high. If this were the case amps would have variable input Z (easy enough to do ), so why dont they? All the ones Ive looked at have a fixed Z of about 1meg.

And resonance is about freq response, which will have a bump at the resonant freq. Again, this is basic electronics.

Im done flogging a dead horse.

For those who believe other peoples data:

http://www.syscompdesign.com/AppNote...ar-pickups.pdf

[shanx]

Hello Fezzle,
I hope I can help you in the following way. Certainly there are a lot of things to talk about regarding impedance, but if you are looking to create a tonal response with a UTC type transformer as DI, then based on the list of the models and their specs, the model A27 is the only appropriate choice that I could find. It has 100 K (Split) nominal primary meant for xtal/Hi-Z to line level conversion with low loss & shielding. Secondary is tapped at 50, 200/250 and 600 ohm for your mic level input. That is most likely the version you have heard about as the rest are designed for other functions. They don't have a higher primary Z than that. On your active basses, should be fine. Your passives could work, and it is only a matter of adding an active buffer in front if you want to be flexible and give the transformer a little ''push''. I hope that is helpful.

Here is a link for the list
http://www.google.ca/url?sa=t&rct=j&...H_4ERg&cad=rja

Here are the connections (ignore the rest of circuit) for a x-tal radio
http://www.crystalradio.net/soundpow...utca27rev1.gif

That would be my practical answer.

[shanx]

I don't know if you will find an A27 type that is NOS or ripped out of an old radio or mixer.
Here is something similar:
Sowter Type 4243 DI Box transformer

[Nigel Goodwin]

Originally Posted by Keriwena

Nigel, perhaps you could quote the passage you're referring to?

Again then:

As to active basses having a lower impedance than a passive ones, I'd double check that.

[Keriwena]

shanx, thanks for good advice and keeping things on topic.


Nigel, I'm not sure how you construe a suggestion that the internet meme about an active instrument's output might be wrong to a statement about amplifier inputs. That seems like a real stretch to me.

[Keriwena]

cbdb, you expect me to discuss respect for education with someone that can't be bothered to spell things properly? Seriously?

You think I ought to learn more electronics, while mentioning Thévenin's Theorem in regard to a voltage source that is mostly an inductor?

You wonder why amps don't have a variable impedance input? Ever noticed guitars and basses usually have tone controls?

No, this isn't "simple electronics". No amount of electronic theory will help you understand the complexities involved. This is about making music, and the needs of musicians. If you didn't get the "flat" joke....