active dummy coil, series or parallel

[Amplexus]

I'm thinking about doing a low impedance air core dummy coil with op amps for hum level matching to 3 strat single coils. What is likely to preserve more single coil tone, series or parallel? Op amps will probably be lm4562's.

[Mike Sulzer]

I'm thinking about doing a low impedance air core dummy coil with op amps for hum level matching to 3 strat single coils. What is likely to preserve more single coil tone, series or parallel? Op amps will probably be lm4562's.

In a passive system you want to use a low impedance coil in series. The question need not be relevant in an active system.

[jbforrer]

I'd be interested in a project like yours too -- so please keep us posted.

Thought you might be interested in some work I did a while ago on dummy coils attached to regular Strat, single-coil pickups. This experiment used inexpensive pickups marketed as a "Vintage Strat" set. I made a couple of dummy coils and epoxied them to the back of the pickups, wired out of phase with the main winding.

My dummy coils are about 2.5K/0.6H (with ferrite cores) vs. the pickup coils that were in the 5.8 - 6.3K range. Attached please see a picture of my piggy-back pickup and the measured frequency response plots recorded installed in a guitar (500K volume/250K tone pots). These plots were produced following the procedure described in Lemme's article using a computer sound card. The software is by Ivo Mateljan, called "STEPS".

The following nomenclature need to be added to the plots for clarification:

RED -- My reference pickup, a true vintage Strat pickup (1960's)
YELLOW -- the aftermarket "Vintage" pickup.
PURPLE -- Pickup with attached dummy coil wired in series, out of phase.
PINK --- Dummy coil.

A few observations FWIW:

1. One can safely ignore any signal response beyond 10KHz as that will be lost in the audio chain and certainly never make it to the speakers.

2. The aftermarket pickup (YELLOW) is a little brighter (more mids/highs) than a real vintage Strat pickup (confirmed/supported by listening tests).

3. With the dummy coil attached (PURPLE), there is very little difference in the response of the piggyback pickup. Actually, a slight increase in upper mids is evident in listening to the modified pickup.

4. Please note the dummy coil response (PINK); Since the dummy coil is merely glued to the bottom of the pickup with no magnetic screen between the coils, it does sense a little bit of the main pickup's magnetic field, albeit some 18 dB down and it covers most of the audio spectrum. This is will be to the detriment (or advantage maybe) of the character of main pickup sound, however, not much as shown in the PURPLE trace..

I did not make any serious efforts to measure/balance out the magnitude of the noise component and can imagine an active system offering that possibility. My conclusion was that 60Hz hum was well under control, though this arrangement did still leave a trace of higher-frequency noise (like from a computer CRT monitor) --- perhaps not a problem for live gigging. Noise reduction is good, but not as good as a well-designed split-coil arrangement.

Trust this is of interest.

JBF.

[Amplexus]

I'd be interested in a project like yours too -- so please keep us posted.

Thought you might be interested in some work I did a while ago on dummy coils attached to regular Strat, single-coil pickups. This experiment used inexpensive pickups marketed as a "Vintage Strat" set. I made a couple of dummy coils and epoxied them to the back of the pickups, wired out of phase with the main winding.

My dummy coils are about 2.5K/0.6H (with ferrite cores) vs. the pickup coils that were in the 5.8 - 6.3K range. Attached please see a picture of my piggy-back pickup and the measured frequency response plots recorded installed in a guitar (500K volume/250K tone pots). These plots were produced following the procedure described in Lemme's article using a computer sound card. The software is by Ivo Mateljan, called "STEPS".

The following nomenclature need to be added to the plots for clarification:

RED -- My reference pickup, a true vintage Strat pickup (1960's)
YELLOW -- the aftermarket "Vintage" pickup.
PURPLE -- Pickup with attached dummy coil wired in series, out of phase.
PINK --- Dummy coil.

A few observations FWIW:

1. One can safely ignore any signal response beyond 10KHz as that will be lost in the audio chain and certainly never make it to the speakers.

2. The aftermarket pickup (YELLOW) is a little brighter (more mids/highs) than a real vintage Strat pickup (confirmed/supported by listening tests).

3. With the dummy coil attached (PURPLE), there is very little difference in the response of the piggyback pickup. Actually, a slight increase in upper mids is evident in listening to the modified pickup.

4. Please note the dummy coil response (PINK); Since the dummy coil is merely glued to the bottom of the pickup with no magnetic screen between the coils, it does sense a little bit of the main pickup's magnetic field, albeit some 18 dB down and it covers most of the audio spectrum. This is will be to the detriment (or advantage maybe) of the character of main pickup sound, however, not much as shown in the PURPLE trace..

I did not make any serious efforts to measure/balance out the magnitude of the noise component and can imagine an active system offering that possibility. My conclusion was that 60Hz hum was well under control, though this arrangement did still leave a trace of higher-frequency noise (like from a computer CRT monitor) --- perhaps not a problem for live gigging. Noise reduction is good, but not as good as a well-designed split-coil arrangement.

Trust this is of interest.

JBF.

I'll have to check out the sound card software. Unless I hear something to the contrary I'll probably buffer the dummy coil and all three strat pickups, split the signal from the dummy and run it into a summing node of another op amp for each strat coil (one will need to run through an inverting op amp). Does anyone know why emg went with a differential configeration running the coil into both the inverting and non inverting inputs instead of just using a hi impedance noninverting setup? Is there any chance the op amp they use is really a dual? Also why would you not want to keep harmonics out to 20khz and beyond?

[Mike Sulzer]

(one will need to run through an inverting op amp).... Also why would you not want to keep harmonics out to 20khz and beyond?

If I understand what you are doing, you do not need an inverter since you can reverse the polarity of the dummy coil.

Guitar speakers produce essentially nothing above about 5 KHz. Keeping harmonics to 20 KHz before a non-linear amplifier can produce bad sounding intermodulation distortion.

[Amplexus]

thanks
I can easily filter out the highs. As to the inverting amp won't I need one between the dummy coil buffer and the middle strat pickup, assuming I use one dummy coil to cancel all 3 strat pickups?

[Mike Sulzer]

thanks
I can easily filter out the highs. As to the inverting amp won't I need one between the dummy coil buffer and the middle strat pickup, assuming I use one dummy coil to cancel all 3 strat pickups?

Yes, you are right, if you keep the middle pickup reverse-wound, reverse polarity. I had not thought of that.