JohnRoberts

Re: Not So Excellent Article on Impedance
« Reply #20 on: September 09, 2019, 09:38:28 AM »
And that illustration is confusing as it discusses 'signal power' that isn't really a 'thing' in line level interconnection where we are looking at the voltage and need only enough current to produce that at the load.
To be fair it does go on to discuss more standard Lo-Z outputs / Higher Z loads but I do see that the article could be misleading and the other technical flaws that have been highlighted.
Signal power is "old school" termination philosophy. The strategy back then was to maximize "power" transfer that occurs when source impedance and load impedance is equal. Back in the day the standard interface was 600 ohms outputs and inputs. We still see 600 ohm terminations on old legacy equipment.

The old power transfer model has been superseded by a maximum voltage transfer philosophy AKA "bridging" terminations.  Bridging generally stipulates at least a 1:10 ratio between source and following termination impedance.

It is not an accident the low Z mics are nominally 200 ohm source impedance and feed into mic preamp inputs with 2k ohm input terminations.  Line level inputs with nominal 10k terminations are driven from 1k or less output impedances.

So power transfer "was" a thing, but now voltage transfer is the focus.

JR 
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


Phrazemaster

Re: Questionable Article on Impedance
« Reply #21 on: September 09, 2019, 01:14:50 PM »
Many thanks everyone. Ian, Newmarket, JR, wonderful.

A question remains to me - in building/cloning old classics, is it still a good idea then to terminate outputs with a 600R resistor?

From these discussions it sounds like no.

Thx

Mike
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Re: Questionable Article on Impedance
« Reply #22 on: September 09, 2019, 02:10:44 PM »
Many thanks everyone. Ian, Newmarket, JR, wonderful.

A question remains to me - in building/cloning old classics, is it still a good idea then to terminate outputs with a 600R resistor?

From these discussions it sounds like no.

Thx

Mike

Not for signal transfer no but, if it's a termination on a transformer output, then usually some sort of termination will tame any peaking up high.  It's not always necessary to use a full 600r termination and a lighter load might work, or a zobel network.
Better transformers will usually have a recommendation for that stuff.


Edit:  one thing that I've found is, with some transistor output stages, there is better behavior regarding thermal stuff if there is a load.  These older output stages were biased for a specific load and without it, I sometimes see worse thermal induced distortions and drift.  it sometimes seems to be a balancing act for minimizing gm doubling around transition from class A to class B too.
None of this related to impedance as such but, there you have it  :)

I suppose I might say, with a clone, if in doubt, terminate it.  It won't do any harm if it's not lower than what was used originally.
« Last Edit: September 09, 2019, 02:27:11 PM by Winston O'Boogie »

Phrazemaster

Re: Questionable Article on Impedance
« Reply #23 on: September 09, 2019, 02:37:42 PM »
Helpful, thx Winston.
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JohnRoberts

Re: Questionable Article on Impedance
« Reply #24 on: September 09, 2019, 03:15:31 PM »
Many thanks everyone. Ian, Newmarket, JR, wonderful.

A question remains to me - in building/cloning old classics, is it still a good idea then to terminate outputs with a 600R resistor?

From these discussions it sounds like no.

Thx

Mike
It depends on what you plan to use it with... Back a few decades ago when there was far more 600 ohm gear around, I designed my products to competently drive 600 ohms, but ignored using actual 600 ohm source impedance (I typically used 50-100 Ohm).

Op amps like 553x can drive 600 ohms, TL07x not so much (more like 2k). As long as you use 553x or equivalent in output sockets you will be OK no matter what you encounter.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

boji

Re: Questionable Article on Impedance
« Reply #25 on: September 09, 2019, 05:58:54 PM »
Thanks Ian and others! You too, Phrase for the seed, CJ for the loam.

Phrazemaster

Re: Questionable Article on Impedance
« Reply #26 on: September 09, 2019, 07:28:15 PM »
It depends on what you plan to use it with... Back a few decades ago when there was far more 600 ohm gear around, I designed my products to competently drive 600 ohms, but ignored using actual 600 ohm source impedance (I typically used 50-100 Ohm).

Op amps like 553x can drive 600 ohms, TL07x not so much (more like 2k). As long as you use 553x or equivalent in output sockets you will be OK no matter what you encounter.

JR
Thanks JR.

So if I’m interfacing old gear clones with modern gear, generally there should be no need for 600R termination? But it doesn’t hurt?

This has been a fun journey. Thanks everyone!

Mike
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PRR

Re: Questionable Article on Impedance
« Reply #27 on: September 09, 2019, 07:49:04 PM »
> guitars which have ~15K output impedance

Can easily show cases that go to 150k in a high narrow band. 100k inputs are less brilliant than 1Meg inputs. (A trick known to early "shrill" transistor g-amp makers.) Fender practice was 1Meg for one axe, >136k each for the secondary cases of one over-hot axe or two axes in one channel.

But impedance fitting really is no different than running a too-big lamp on a too-long too-thin extension cord and wondering why it is dim. Or washing your elephant with a pencil-thin hose.

Re: Questionable Article on Impedance
« Reply #28 on: September 09, 2019, 07:55:59 PM »
Or washing your elephant with a pencil-thin hose.

Haha! 😄

I'ma gonna use this as a euphemism for something.

Phrazemaster

Re: Questionable Article on Impedance
« Reply #29 on: September 09, 2019, 08:19:12 PM »
Very funny.

I distinctly remember in my high school physics class, the instructor, who was usually incredible, sort of drawling off about how impedance was really complicated...that's about all I remember. To be fair to him he might have followed that up with a great lesson, but somehow that's all I remember!

Even though the spawning article had some egregious errors, I do feel I have a bit better handle on this after all thanks to y'all.

Best

Mike
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squarewave

Re: Questionable Article on Impedance
« Reply #30 on: September 09, 2019, 09:27:37 PM »
> guitars which have ~15K output impedance

Can easily show cases that go to 150k in a high narrow band.
I suppose that was a rather simplified statement. More specifically, with the volume control at max, typical guitar output impedance is lowest at ~15K measured at 1KHz for a Fender strat single coil. Values may vary depending on the pickup(s). Z goes up rapidly with any other volume control position and fiddling with the tone control causes Z to drop at higher frequencies.

Phrazemaster

Re: Questionable Article on Impedance
« Reply #31 on: September 09, 2019, 10:41:42 PM »
I had no idea what a flashpoint this topic would be...thank you square wave. That was fascinating.
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Newmarket

Re: Not So Excellent Article on Impedance
« Reply #32 on: September 10, 2019, 03:54:29 AM »
Signal power is "old school" termination philosophy. ...

So power transfer "was" a thing, but now voltage transfer is the focus.

+1

PRR

Re: Questionable Article on Impedance
« Reply #33 on: September 10, 2019, 02:55:24 PM »
> guitar output impedance is lowest at ~15K measured at 1KHz

And what at 2kHz-4kHz??

(If it was flat at all frequencies, we would be justified in saying "resistance"; when it varies we use "impedance" to keep the variation in mind.)

The naked pickup is a large inductance. There's much variation between different designs, but 2H is a ballpark value. 12k at 1kHz, 37k at 3kHz. Plus DC resistance like 5k. The coil and wiring add capacitance. And there is usually a Volume pot on the end.

Vol-pot at 50% does swamp-out most of this reactance. But it is very common to set the pot full-up for important passages, so that is an important condition.

The guitar impedance runs from a few Kohms in bass through a dozen k at 1KHz and bumps up to near 150k at the L-C resonance (generally 2kHz-4kHz).

Now let's try various load (amplifier input) impedances (mostly resistive). A 50k load gives a "nice" well behaved roll-off. However the pickup's musical response is falling above 1KHz due to pole-piece size and string factors. We "depend" on the top resonance to bring-out the "zing in the string". A load greater than ~~150k hardly raises the 80Hz-1KHz response (low-Z pickup range) but raises the response at top resonance significantly. We can find 9dB of boost by raising load from 50k to 500k, and 5Meg a wee bit extra.


squarewave

Re: Questionable Article on Impedance
« Reply #34 on: September 10, 2019, 04:16:29 PM »
That's very interesting. But is this based on just your spice sim or did you actually take measurements to verify? I made some empirical measurements once and I don't recall getting anywhere near 150K. But IIRC I was doing stepped so I suppose it would be easy to miss a narrow band peak. It would be an interesting exercise if someone injected white noise into the sleeve and measured at the tip using a rig devise to plot impedance spectra. All of my gear is tucked away and I'm supposed to be writing code right now so it ain't gonna be me. Also this really deserves it's own thread. A detailed analysis of the output Z of various guitars would be very informative.

Re: Questionable Article on Impedance
« Reply #35 on: September 10, 2019, 05:37:48 PM »
I can't say I ever did testing to that level but, empirical changing of load while watching live spectral information shows that 500k is about minimum for most guitar internal arrangements of pickup and pots so I double that and 1M is always my default load. Anything lower than 500k seems to really just be another top end tone control.
More detailed measurements would be useful for modeling etc though 👍

ruffrecords

Re: Questionable Article on Impedance
« Reply #36 on: September 11, 2019, 03:54:25 AM »
I am surprised that these days all new guitars are not Bluetooth enabled.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

gyraf

Re: Questionable Article on Impedance
« Reply #37 on: September 11, 2019, 04:37:23 AM »
I am surprised that these days all new guitars are not Bluetooth enabled.

I think the latency problem has not yet been resolved, even in later protocols..

Jakob E.
..note to self: don't let Harman run your company..

ruffrecords

Re: Questionable Article on Impedance
« Reply #38 on: September 11, 2019, 04:46:30 AM »
I think the latency problem has not yet been resolved, even in later protocols..

Jakob E.

Good point. but would not some innovative axe player turn it into an effect?

Edit: and just how long is latency? If you stand 10 feet in front of your speakers there is already a  10mS delay due to sound propogation.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

cyrano

Re: Questionable Article on Impedance
« Reply #39 on: September 11, 2019, 07:37:55 AM »
The absolute minimum is 15 msec. Most stuff is around 35 msec.

I've always ignored BT for audio. But I have to provide a smartphone audio connection soon. And since 3,5 mm jacks are disappearing, I have to use BT. For audio only, the latency isn't important, so...

The biggest problem is that BT equipment isn't really standard. Some receivers can do more than others. Some are stereo/music only. Others are voice/phone only. Some do both. Haven't found the ideal one yet. So I'm just playing around with whatever shows up.

I've bought a small JBL BT speaker. Nice toy, but it's around 30 msec latency. Just a bit too much to watch movies in bed. Lipsync is gone. It's a nice toy, though. Doesn't sound to bad. Startup and shutdown chimes are well designed. It's waterproof and the battery lasts longer than the 5 hours that's in the specs. Can't DIY for 20 €...
Why is it people love to believe and hate to know?


 

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