I never said that.
You claimed that the pot values in a guitar were the reason a high impedance was needed.
My claim is that the resonance of the pickup coil is why guitar amps have a high impedance.
Since the required impedance for the coil is much higher than what the control pots require, your suggestion is misleading in terms of what's required to get good tone out of an amp or DI box.
Mostly, though, you just have this annoying attitude that your technical understanding trumps a musician's understanding of what's musical. It doesn't. Now, of course, it's become clear you can't even hold your own in a technical discussion, so I'm done with you.
Hi Dave,
as I had mentioned earlier maybe you would consider an active DI? Because you have both possibilities of passive pickup and active on-board preamps, it should be easier to do up a simple circuit to deal with both types. Your actives are already buffered, but you may want to introduce some more control ie gain control, and your passives will need some gain as well as the Hi-Z (high impedance input). Sorry to say that I have never been happy with the passive transformer based DI boxes, unless you are going for something different!
as I had mentioned earlier maybe you would consider an active DI? Because you have both possibilities of passive pickup and active on-board preamps, it should be easier to do up a simple circuit to deal with both types. Your actives are already buffered, but you may want to introduce some more control ie gain control, and your passives will need some gain as well as the Hi-Z (high impedance input). Sorry to say that I have never been happy with the passive transformer based DI boxes, unless you are going for something different!
Active preamp stage for DI
As (I think)everyone is basically agreed that high input impedance is required for optimal tone, If you build a simple 1 or 2 stage preamp and maybe add some tone control then buffer it out you would have a pretty decent and flexible DI.
This design was developed for guitar
http://www.diyaudio.com/forums/instruments-amps/27524-new-jfet-guitar-preamp-project.html
and was originally adapted from a bass preamp design by
Bass Preamp
Not only could you use the thing as a DI box, but it will give you all the advantages of your active pickup basses have if you use it first in your passive pickup chain. Once you get the input impedance high and your signal optimized from the preamp, from there you could send it through your isolation transformer, or an active circuit to the mic level.
As (I think)everyone is basically agreed that high input impedance is required for optimal tone, If you build a simple 1 or 2 stage preamp and maybe add some tone control then buffer it out you would have a pretty decent and flexible DI.
This design was developed for guitar
http://www.diyaudio.com/forums/instruments-amps/27524-new-jfet-guitar-preamp-project.html
and was originally adapted from a bass preamp design by
Bass Preamp
Not only could you use the thing as a DI box, but it will give you all the advantages of your active pickup basses have if you use it first in your passive pickup chain. Once you get the input impedance high and your signal optimized from the preamp, from there you could send it through your isolation transformer, or an active circuit to the mic level.
No I didn't, and the line you quoted didn't even say that - it was in response to your post here:
"As to active basses having a lower impedance than a passive ones, I'd double check that. There seems to be a similar belief that EMG pickups are low impedance, but they're not. Every spec I've found lists their output at 10K, and that's medium high for a passive guitar. I think people are being fooled by pot values, which have almost no effect on the total."
Where you claim active basses don't have lower impedances than passive ones, and that the pot's don't affect the output impedance.
Not exactly, it's because you need a high impedance to prevent the output of the pickup being overloaded and dampened, this will greatly reduce the output and the tone from the PU. The control values are chosen not to dampen it, and the amplifier has to do the same.
The entire PU, controls, amplifier impedance is a chain of impedances that need to prevent dampening.
The obvious exception is when you WANT the dampening effect, and some amplifiers offer adjustable input impedance for that reason, so you can add the effect at will for specific sound effects you might like. It's easily added to an amplifier if anyone wants to experiment with such a system (I never have, or felt the need to).
The pickup coil is not purely inductance, there is substantial resistance from the windings and also a small capacitance in parallel. Then there is the volume and tone circuit hooked up to that. The main fact is that with a pickup you have the equivalent of a small voltage ''signal generator'', with L, R in series and C in parrallel to consider. Because this weak signal generator is only capable of generating very low level signals in a range under a volt (unless your pete townsend) and is already being attenuated by it's internal L, R, C: The higher the input amp impedance the more ''true'' a signal you will get from the pickup into the amp. There is a practical limit to this. going towards infinite impedance invites all kinds of parasitic noise and charges to accumulate at the input of the amp, plus you need a reasonable return path to the pickup. You probably want as ''true a signal as possible'' to go through the preamp or DI box. Once the signal is shifted, attenuated or filtered it is hard to get it back in later stages without adding something and compromising the sound. That is what tone controls are for, at least there is some controls to play with. My first suggestion stands, that you should consider an active buffer with a hi impedance (1 M or even more) to do a DI. A passive transfromer just about kills the whole response.
Keriwena/Nigel ,if you 2 properly read each others posts you are saying the same thing. If your here just to argue for arguments sake (which it looks like), save everyone some time and take it else where.
Heres some actual pickup data.
http://www.ece.rochester.edu/courses/ECE140/resources/Guitar-Project/Electric_Guitar_Pickup_Measurements.pdf
Dosnt take much to see why one needs a high impedance input.
Heres some actual pickup data.
http://www.ece.rochester.edu/courses/ECE140/resources/Guitar-Project/Electric_Guitar_Pickup_Measurements.pdf
Dosnt take much to see why one needs a high impedance input.
cbdb, the link you posted uses a constant 1.5Meg load for all measurements. It has no data* relative to what we were discussing. Perhaps you should re-read this thread a little more closely.
* Actually, the exercise was likely useful to teach students how to use test equipment, but the recorded data has no real world value. The Q of those resonant peaks are all too high to sound good, demonstrating what we knew all along - guitar pickups need their associated controls to provide proper damping. (Of course, if you don't have the experience to look at a graph and relate it to a sound, then it's a bit like referencing text written in a foreign language you can't read, isn't it?)
Instruments are instruments. Anyone can make noise on a guitar, but there are very few Claptons among us. Same goes for Tektronix instruments. While many teachers seem to think electric guitars are something their students might relate to, I've never seen anything of value come from a school. If you want to cite someone, try someone with cred in the industry, like Lemme. His procedure leaves the guitar intact, so there's real value to his measurements.
* Actually, the exercise was likely useful to teach students how to use test equipment, but the recorded data has no real world value. The Q of those resonant peaks are all too high to sound good, demonstrating what we knew all along - guitar pickups need their associated controls to provide proper damping. (Of course, if you don't have the experience to look at a graph and relate it to a sound, then it's a bit like referencing text written in a foreign language you can't read, isn't it?)
Instruments are instruments. Anyone can make noise on a guitar, but there are very few Claptons among us. Same goes for Tektronix instruments. While many teachers seem to think electric guitars are something their students might relate to, I've never seen anything of value come from a school. If you want to cite someone, try someone with cred in the industry, like Lemme. His procedure leaves the guitar intact, so there's real value to his measurements.
Are you not discussing guitar amplifier input impedance?
So what, those graphs are of pickup impedances they wont change no matter the test load. They knew what they were doing do you?
Those peaks are impedance peaks, not output level peaks the two are not the same. Its the input impedance of the amp that flats the response out, thats why it needs to be high. (infinite input Z means the output Z has no effect on freq response)
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. Then why was it suggested that he was SO wrong? And why is it suggested NOW that the volume and tone controls are needed to provide the damping effect?
Isn't that generally agreement with what Nigel had said at the beginning?
For that matter, why go on with all this and 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.
That is too bad. Maybe your values are different than some other people.
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.
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.
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.
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.
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.
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?.
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/AppNotes/guitar-pickups.pdf
UTC transformer options
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...sg=AFQjCNGLO8oaeTtASZAdpk0pV19aH_4ERg&cad=rja
Here are the connections (ignore the rest of circuit) for a x-tal radio
http://www.crystalradio.net/soundpowered/matching/utca27rev1.gif
That would be my practical answer.
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...sg=AFQjCNGLO8oaeTtASZAdpk0pV19aH_4ERg&cad=rja
Here are the connections (ignore the rest of circuit) for a x-tal radio
http://www.crystalradio.net/soundpowered/matching/utca27rev1.gif
That would be my practical answer.
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
Here is something similar:
Sowter Type 4243 DI Box transformer
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.
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.
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....
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....
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