Input Impendance Question

[Schau_ins_Regal]

Hi,

I have a question to the electronics guys here.

Let´s say I have a K&K PWM and a Schertler Jam 100.

The K&K wants to see 1mOhm input impendance but the Schertler high-z inputs have 47kOhm.

What I read here several times is that many people just solder a 1mOhm resistor into the guitar cable from tip to ground.

I think in this case it wouldn´t change anything because the two resistors are now parallel which means the resulting resistance will be less than 47kOhm.

I think that 1mOhm should just be soldered to the lead of the jack but not to ground so that a series connection of the resistors is established.

Am I right or am I not seeing something here?

[James May]

Originally Posted by Schau_ins_Regal

Hi,

I have a question to the electronics guys here.

Let´s say I have a K&K PWM and a Schertler Jam 100.

The K&K wants to see 1mOhm input impendance but the Schertler high-z inputs have 47kOhm.

What I read here several times is that many people just solder a 1mOhm resistor into the guitar cable from tip to ground.

I think in this case it wouldn´t change anything because the two resistors are now parallel which means the resulting resistance will be less than 47kOhm.

I think that 1mOhm should just be soldered to the lead of the jack but not to ground so that a series connection of the resistors is established.

Am I right or am I not seeing something here?

That would give you an impedance of 1.047Mohm, but you'd also be creating a 27dB attenuator, so not a good plan.

[Schau_ins_Regal]

Okay, so what would be the best solution in this situation?

[martingitdave]

K&K works well with anything from 400 to 1mOhm. This is from their own documentation. My advice... Do nothing. It probably won't sound any better (or possible any different at all) with a 500 kOhm change.

[MrErikJ]

An easier solution may be to add an active enpin preamp to your Pure Mini as I've never heard of a high-z input only being 47kohm. I've heard anything over 500k is sufficient for most passive pickups but 47k is pretty chill. To James' point, adding a resistor will attenuate your signal so your next best option (to me) besides changing the Schertler is preamping your signal before it reaches it. An endpin preamp or beltclip unit should buffer it enough to reduce any impedance issues.

[endpin]

Attenuation and frequency response are the only two things you can change by messing around with resistors - What is the objective you want to achieve?

Personally, I wouldn't do anything to it.

[Ed-in-Ohio]

Originally Posted by martingitdave

K&K works well with anything from 400 to 1mOhm. This is from their own documentation. My advice... Do nothing. It probably won't sound any better (or possible any different at all) with a 500 kOhm change.

Totally agree. All this impedance talk is mostly the Emperor's New Clothes. Plug it in and play...I bet it sounds killer.

[Schau_ins_Regal]

Originally Posted by martingitdave

K&K works well with anything from 400 to 1mOhm. This is from their own documentation. My advice... Do nothing. It probably won't sound any better (or possible any different at all) with a 500 kOhm change.

you mean from 400 Ohms or from 400 kOhms?

[martingitdave]

K ohms. Typing on phone. Sorry. Also, I thought you said your amp was 470 kOhm, not 47 kOhm. I was on the go. If you want to match impedance you’ll probably need an active/preamp or DI. However, I think Schertler is supposed to sound very good with K&K straight in.

[Doug Young]

47K ohms is unlikely to sound good with K&K. People who try the resistor trick are trying to lower a 10 M impedance down to K&K's claimed 1M. In reality, anything over 500K or so will work fine with K&K (including 10M or even 20M) in my experience. However, below 500K you will start hearing low end roll off. 47K is likely to be pretty thin sounding. This amp would work fine with active pickups that have low impedance outputs, it's unlikely to sound good with a passive piezo. Of course, depending on what you like, you never know. Some people's objection to K&Ks is too much bass, and 47K would definitely solve that!

BTW, here's an impedance test I did a while back with various impedances. I don't have a 47K example, but I have 330K and 10K. 47K would be in-between, closer to the 10K. (10K is the 3rd sample in the list)



impedance sequence: 1M, 10M, 10K, 1M, 20M, 330K

[lschwart]

Such a low input impedance on the "High-Z" inputs on an amp designed to be used with amplified acoustic guitars seems like an odd design choice, given that many guitars have passive piezo pickups that wouldn't sound good into an input like that. Are we sure that that's actually the right figure for the input? I can't get clear information poking around on line, but several sources indicate that the high-z inputs have a "bootstrap" design that adapts the impedance to the nature of the signal.

Bottom line: try it first. If it sounds good, it's likely because it's designed to work properly with a signal like the one that comes from a K&K mini. It would be odd if it weren't so designed--although it's possible, I suppose. If it does sound bad, and you don't want to get a different amp, get a preamp or EQ pedal with at least a 1mOhm impedance to buffer the signal. Or a DI box to go into one of the mic inputs.

Louis

[Doug Young]

Originally Posted by lschwart

Such a low input impedance on the "High-Z" inputs on an amp designed to be used with amplified acoustic guitars seems like an odd design choice, given that many guitars have passive piezo pickups that wouldn't sound good into an input like that. Are we sure that that's actually the right figure for the input?

I actually think that outside of AGF, passive piezo pickups are rare. There are zero (not 100% sure, but almost certain) guitars sold commercially (Taylor, Martin, etc) with passive pickups. 99.9% of the market (to make up a number) is active pickups. Passive pickups like K&K are almost entirely the domain of DIY enthusiasts who are customizing their guitars. The one type of passive pickup that is fairly common is magnetics, and 47K is a good match for those.

I do find it to be telling when a company doesn't provide good specs. I know Schertler is a reputable company, but pro-quality gear generally provides detailed specs. When I see specs that only say things like "high impedance", a voice whispers "run the other way". (I have heard good things about the Jam 100's tho - they appear to be nice amps)

[lschwart]

Originally Posted by Doug Young

I actually think that outside of AGF, passive piezo pickups are rare. There are zero (not 100% sure, but almost certain) guitars sold commercially (Taylor, Martin, etc) with passive pickups. 99.9% of the market (to make up a number) is active pickups. Passive pickups like K&K are almost entirely the domain of DIY enthusiasts who are customizing their guitars. The one type of passive pickup that is fairly common is magnetics, and 47K is a good match for those.

I do find it to be telling when a company doesn't provide good specs. I know Schertler is a reputable company, but pro-quality gear generally provides detailed specs. When I see specs that only say things like "high impedance", a voice whispers "run the other way". (I have heard good things about the Jam 100's tho - they appear to be nice amps)

You may be right. The lack of clear specs is certainly surprising.

Louis

[SalFromChatham]

I definitely am one who does not believe the k&k impedance issue is nothing. I have two k&k guitars, I gig in bars with them, and their sound is fantastic with the right setup.

Choices?

1. K&k preamps
2. Fire eye red eye preamp
3. Used older Boss Tuner pedal. $50 or a used TU-3Wazacraft ($80)

[Doug Young]

Originally Posted by themissal

I definitely am one who does not believe the k&k impedance issue is nothing. I have two k&k guitars, I gig in bars with them, and their sound is fantastic with the right setup.

The effects of impedance on passive piezo pickups is absolutely real. But people often get confused about it, and there seems to be a bit of marketing designed to confuse people. James May posted one of the clearest technical descriptions I've seen here recently, so it's worth looking that up. But for K&K's, it's pretty simple, and comes down to anything above around 500K (+/-) is fine, including the 1M, 5M, 10M, 20M, etc. Below 500K or so, you start losing low end. Above that, it makes no discernable difference (you might want to be careful with very long, low quality cables at very high impedances, tho). There are lots of anecdotal reports that contradict that, but it's always with different preamps, different brands, different gain structures, etc. It's not surprising that different products sound different, but there's more to preamps than impedance, and two different designs are, well, different. Get a preamp with variable impedance, like the Grace Felix, and you will be able to compare apples to apples and see when impedance matters and and when it doesn't without changing 100 other variables at the same time.

Of course in the end, it all comes down to "does product X sound good to me". I've yet to encounter a preamp or amp that sounds noticably "bad" with a K&K. Some seem to sound better (with any pickup), but I've never seen anything to indicate that the reason was impedance (unless the impedance was very low, like 47K).