EMG SA (original series): 9V, 18V questions

[ssstonelover]

There is a lot on the internet about 'headroom' and 'sterility' with EMG active pickups, especially the older generation type, and even more so on their bass and full size humbuckers, but that differences exist even with SA single coil sized pickups, like the SA set used by David Gilmour for years so effectively.
As we know EMGs were designed to run on 9V batteries, but are fully capable of also running on 18V without a hitch, and this is supposed to help dynamics especially when played clean, so is a common modification --something our group explores a lot (of course).

The question then arises, what is the best way to wire EMGs to 18V?
-Should the user be able to toggle between 9V and 18V at will? (i.e. is there any benefit to this)
-Should wiring be rigged up with the 2 x 9V in parallel (i.e. double the current) and with the 18V in series (twice the volts, but normal amount of current) or some other way? (and what would that look like)
-Does having the 2 x 9V batteries in parallel have any detriment effect on their battery life and voltage? (example one is old and only showing 8V but the newer one at 9.5V, would the effective voltage be only 8V? Is this something to avoid, and hurt the newer battery in the parallel chain)

I set up the following schematic (very close to the old EMG schematic I found) with 3 alternatives to 18V.
-Type 2 and 3 are very similar (series/parallel) and only differ as to whether UP is more voltage or DOWN is more.
-Type 1 is a simple daisy chain series arrangement, so a dedicated 18V wiring.

What is the best wisdom here? I've only used newer electronics (Artec and GFS, and that was on EQ only) so never had to try 18V to 'wring out' extra performance from pickups. This is the first time I've used active pickups....

[sumgai]

Bethany,


The bottom line for you is this: batteries have an internal resistance, it goes along with the equation of Volts = Ohms X Amps (E = I x R, in Ohm's Law parlance). It is that resistance that works against you when connecting batteries in parallel. Battery A will "see a load" consisting of battery B, and thus will provide power to that load. As soon as A's power level drops below that of B, B begins supplying power to A. And the cycle repeats until both batteries are pretty much unusable. Left tied together long enough, one or both of them will die an irrecoverable death. Hence, absent an external means of recharging them,  series is the name of the game.

As to headroom, I'll let others chime in, my head is not in a good place this evening.  Sorry.

HTH





sumgai

[ssstonelover]

Oct 30, 2023 22:41:58 GMT -5 sumgai said:

Bethany, absent an external means of recharging them,  series is the name of the game.
sumgai

I like your description of a problem existing of parallel batteries weakening each other and your summary too. It makes sense.
Ergo it would seem pointless to have a switch that can shuttle between 9V and 18V, thus making it best to wire up 18V full time with a simpler daisy chain connection.

It seems EMG active systems can operate down to 4.5V (thus giving some measure of useful battery life) but if headroom and dynamics are best at around 18V (only small gains at even higher voltages) then I'd prefer a half dead 18V series system (9V) much over an almost completely dead 9V system limping along at an actual 4.5V.

[mikecg]

This post may be of interest, if you have overlooked it:


guitarnuts2.proboards.com/thread/8989/emg-response-curves

[kitwn]

The only good reason for connecting batteries in parallel is to reduce the volt-drop when feeding a high current demand. Active pickups take relatively little current so this does not apply. Extending the time between battery changes, or to protect against mid-gig embarrassment would be better done with a more active changeover circuit of some kind with a warning when battery-1 is flat. Series connection to get 18v will help hold up the voltage as the batteries fade and it's worth noting that rechargeable batteries tend to fade more quickly than ordinary alkaline ones so might be a mixed blessing. Good battery management is the key here.

The possible improvement in sound with 18v may be an issue with 'slew rate' which measures how well the amp can follow a fast change in input voltage such as the transients at the start of a note. A higher supply voltage will raise the slew rate of the same amp. We humans are surprisingly sensitive to starting transients, they affect how we perceive the timbre and loudness of the rest of a note. Can you tell I've been Googling 'psychoacoustics' quite a lot lately? 🤪

Kit

[ssstonelover]

Oct 31, 2023 17:34:35 GMT -5 mikecg said:

This post may be of interest, if you have overlooked it:
guitarnuts2.proboards.com/thread/8989/emg-response-curves

Thanks, I had seen it, but thanks for reminding me about it again. Antigua always has interesting tests and observations.

[ssstonelover]

Nov 1, 2023 18:24:37 GMT -5 kitwn said:

Series connection to get 18v will help hold up the voltage as the batteries fade ....

The possible improvement in sound with 18v may be an issue with 'slew rate' which measures how well the amp can follow a fast change in input voltage such as the transients at the start of a note. 
Can you tell I've been Googling 'psychoacoustics' quite a lot lately? 🤪

Kit

All good points. I also checked out a rival 24v option 24v which is very compact (fits in the space of a 9v battery box), plus I had some conversation with EMG tech support on alternatives like the 24v system.

At the end of the day 18v is pretty good (for this application) both in terms of Amp-Hour capacity (i.e. real world time between battery changes being needed) and performance (transients, clarity, or what not). The 24V using 2 of the tiny 23A 12v batteries has no where near the battery life, though it may be slightly better in its audio performance. I guess it's the old 80-20 rule. 80% of the perfect audio performance for 20% of the aggravation of battery life, and likely even 0% as I could ride the batteries down to 4.5v total (though of course I then wipe out any gains from spending time doing the mod!)

[Yogi B]

Oct 30, 2023 18:23:05 GMT -5 ssstonelover said:

The question then arises, what is the best way to wire EMGs to 18V?
-Should the user be able to toggle between 9V and 18V at will? (i.e. is there any benefit to this)

In the long term, I don't really think so — maybe temporarily as a good way to quickly (and thus effectively) compare 9V to 18V.

-Should wiring be rigged up with the 2 x 9V in parallel (i.e. double the current) and with the 18V in series (twice the volts, but normal amount of current) or some other way? (and what would that look like)

Paralleling the batteries won't magically double the current flow through the circuit. What it does do is double the the amount of current that can be supplied to the circuit for a given voltage drop occurring due to the batteries internal resistance (i.e. halve the voltage drop for a given amount of current draw), but this should be more or less negligible in either case (a few ohms multiplied by a few milliamps is in the tens of millivolts at most).

Series is the only way to significantly increase headroom. The idea is that the output of any (straightforward) active circuity is limited to be within the power supply rails minus some margin, with any larger signal being clipped. For example using a single 9V battery, the output voltage (Vo) will at best be bounded by 0 ≤ Vo ≤ 9. But, like I said there is some margin on that too, a typical op-amp might only provide output up/down to around 1V away from the power rails, so around 1 ≤ Vo ≤ 7, a maximum peak-to-peak voltage of 7V. Whereas, doubling the supply voltage to 18V gives roughly 1 ≤ Vo ≤ 17, or V_PP = 16V — a little over twice the maximum output as with the 9V supply. As such, voltage spikes that get clipped when running on 9V should either have significantly reduced clipping or avoid clipping at all when running on 18V.

Where this can make a significant difference is in some kinds of fuzz/distortion pedals, where vast swathes of the signal are intentionally clipped against the power rails (though, in discrete transistor designs, the voltage change likely also throws the biasing somewhat askew). With an EMG preamp, where the gain is much less, only the very loudest initial transients of a pluck/strum should come close to clipping, even at 9V. antigua's tests of an EMG 85 in the previously linked thread showed maximum peak-to-peak voltage of around 4V and no sign of clipping.

-Does having the 2 x 9V batteries in parallel have any detriment effect on their battery life and voltage? (example one is old and only showing 8V but the newer one at 9.5V, would the effective voltage be only 8V? Is this something to avoid, and hurt the newer battery in the parallel chain)

As sumgai has said the older battery will continue to discharge the newer until they reach some equilibrium point, however I can't see either battery being particularly happy undertaking that journey. The internal resistance is generally quite low (a few ohms), potentially allowing a lot of current (hundreds of mA) between the batteries and causing a lot of heat. But aside from that, you'd also effectively be attempting to recharge the older battery, which in the case of single-use / non-rechargeable batteries is a bad idea.

But, if the batteries start out in equilibrium, the two should theoretically discharge evenly avoiding the above problems — whilst giving twice the capacity, thus twice the runtime, of a single 9V.

Conversely, whilst a series connection sounds like it should also give extended runtime: i.e. an almost dead 9V battery giving only 5V will result much reduced headroom, two in series would give 10V, still more than a single fresh battery. In reality, at this point the voltage will already be quickly dropping of a cliff — that, in combination with the fact the circuit running at a higher voltage will draw more current, probably makes the 18V runtime equal (or even less than) that of a single 9V.

---

Nov 1, 2023 18:24:37 GMT -5 kitwn said:

The possible improvement in sound with 18v may be an issue with 'slew rate' which measures how well the amp can follow a fast change in input voltage such as the transients at the start of a note. A higher supply voltage will raise the slew rate of the same amp.

Hmmm, interesting.

The commonly held belief is that EMG used to use the now obsolete LM4250 (see, for example, Electrosmash's analysis of an EMG 81). With a brief glance at the middle-left graph on page 6 of the LM4250 datasheet, it appears that the supply voltage affects the slew rate only a little: a factor of less than one and a half, for an order of magnitude difference in V_CC, so we might as well treat that as constant for a mere doubling of the voltage. However, it's important to not miss that's a plot against I_SET which does have what is basically a linear relationship to the supply voltage (i.e. if we ignore the variability of the drop from Q9 in the op-amp, see the schematic on page 9 of the datasheet).

Using this SPICE model I get slew rates of about 170mV/μs at 9V and 360mV/μs at 18V which seem reasonable based upon the datasheet graphs. Adding a little context to that, we can use the relation f ≤ Slew Rate / (π V_PP) to determine the maximum frequency of a sinusoid at a given voltage level that will be immune to slew rate limiting. That is: for a sinusoid of 7V_PP (again, about the maximum we might reasonably assume we could get from a 9V supply), we get f ≤ 7.7kHz with a 9V supply, but f ≤ 16kHz with an 18V supply. However, the initial transients may of course have a steeper gradient than a sinusoid.

If slew rate really was a huge issue, a better solution would've been a different op-amp. For example a TL061 has a typical slew rate of 3500mV/μs, though that does come at a cost of an increased current consumption of around 175—200μA for 9—18V. By comparison, with the given 1Meg R_SET resistor, the LM4250 draws around 80μA at 9V and 180μA at 18V — though it's also worth mentioning the up-to-date specs for an EMG 81 (presumably now with a non-obsolete op-amp) list it as having 400μA of current draw at 9V, and so all of this may no longer be relevant.

[ssstonelover]

These particular pickups have an “HGK” sticker on the back, meaning they were manufactured in November 1987, so likely have an ancient obsolete op-amp.

[ssstonelover]

Switched out to 18V. Simply I cut out the bottom of the battery box, deepened the cavity with the 9V battery with my trusty router, and stacked one 9V on top of the other and hooked them up in series.

Sorry, no audio of the before-after test, and as an hour passed between the before-after, it becomes hard to say (or quantify) exactly how much changed, except that it 'seems' better, a totally subjective cop-out and maybe confirmation bias (if we were to get psychological about it).

BTW, I really like the SPC pot, seems to push the pickups a bit harder when engaged.
The overall set gives pretty nice Strat type sounds, so this all may be a keeper, the SPC adds some oomph

The use of a passive tone pot seems to also work well, so if I want to shave off a little high, that is on tap. If I had used the EXP instead of the passive tone I could have only added treble, and that is not needed (at least not in my test/playing so far). If any of you have the David Gilmour DG20 loaded pickguard this is a worthwhile mod, if you find you are not playing it much. In other words either keep the EXP at "0" if you find using it makes the guitar too shrill, or use this passive mod giving you the ability to subtract.
Note: In an ideal world EMG would have made the EXP capable of adding and subtracting, but no, that's not how it works.

Someone actually posted a video about the mod, so you can check that out here: EMG DG20 modification