Autoformer for Voltage gain with Low Impedance guitar pickup

discussion of magnetic volume control design for both line and speaker levels.

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malamutewolfdog
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Joined: Wed Mar 06, 2019 3:54 am

Autoformer for Voltage gain with Low Impedance guitar pickup

Post by malamutewolfdog »

Hi Dave,

I have a couple of your $200 AVC autoformers that work great for attenuation with my HiFi kit but I am currently using one in reverse to provide voltage gain for some superb Low Impedance guitar pickups borrowed from a 1971 Les Paul Recording.

The $200 autoformer certainly provides usable voltage gain, but I can't help feel that at the levels of gain that I am using (24dB, 30dB and 39dB are a typical use case) there is scope for improving 1st order effects by adjusting the inductance or air gap configuration and I'd love some help here to know where you think the ideal inductance ballpark is and how to get there (even if this means buying another autoformer or new shims or lams).


I think that my main aim is to maximise voltage output to the amp at the full gain setting without introducing too much distortion and perceptible nonlinearity in the bass through upper midrange frequencies. I think for my application distortion in the higher frequencies is much less of an issue than perceptible frequency response nonlinearities and a HF rolloff is undesirable, even up to 20k Hz (it is a very bright electric guitar!).
The sound with the unmodified $200 autoformer seems to alter by more than just loudness noticeably from 27dB of gain upwards, and after 33dB of gain a little more so, with 42dB definitely seeming significantly coloured.
Of course my tube amp has nonlinearities that will interact with the changes in gain, but I am sure the sound could be fuller too and this is also what I want (warm low midrange without bloomy bass).

Please let me know if expecting to get much closer to a linear frequency response compared with the default $200 autoformer configuration at these levels of gain is unrealistic within the bounds of the parameters below.


On a separate but related question, if I wanted to experiment with saturating the autoformer core when the guitar was played hard then what sort of inductance would I need and possibly how could I get there?


Most guitar amps have a 1meg Ohm input impedance (including mine).
These 2 pickups have 10.3 Ohms and 10.5 Ohms DC resistance compared to the approx 5k to 10k Ohm DC resistance of most guitar pickups!
Their inductances are both very close to 8.5mH (measured with a DER EE DE-5000 (4-wire LCR meter)).
I think it is also safe to assume that the magnets are AlNiCo (I am guessing type 5) so I believe someone with your experience could guesstimate the largest voltage / current they would be likely to produce?
Each pickup is wired directly with both wires connected to a DPDT selector switch (with no caps or resistors between the pickup and the amp).
Guitar cables have a fair bit of capacitance (50pF to 300pF are typical for me).

Your help is greatly appreciated!
Thanks,
-Chris
dave slagle
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Re: Autoformer for Voltage gain with Low Impedance guitar pi

Post by dave slagle »

hey.

you really had me saying whaaaaat until i saw that your low impedance pickups were in the 10Ω range... which puts the device I would wind in the category of a MC step up. Given the limited range of the frequencies involved i suspect the high frequency peaking if the autoformer as step up are well beyond the problem zone but would still feel "better" looking into something specifically designed.

distortion is directly related to the turns on the core and the turns required to get enough inductance for the low end will for the most part eliminate the possibility of distortion given the singnal levels involved. (what are the output levels of the low Z pickups?)

I would also think that hum would be an issue but since you are using a presumably unshielded autoformer with success.....

I would also think going to a really small core might give a device that will fit inside the body but that might be a step in the wrong direction...

the inductance just has an effect on the lowest frequencies so more inductance = more low frequencies.

the capacitance of the cables will eventually roll off your top end as your step up ratio is increased.

sorry to be all over the place on the reply... we can continue the conversation to get into more of the specifics.

dave
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malamutewolfdog
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Joined: Wed Mar 06, 2019 3:54 am

Post by malamutewolfdog »

Hi Dave,

Thanks for replying so swiftly and I totally understand why you would read most of the post with bemusement - though the way you put it made me chuckle.
I guess it bothers me more than many guitarists how much high impedance pickups can colour the sound, but most people wouldn't think to use low impedance pickups because they want a "hotter" output than these would normally give without a step-up, plus Lo-Z pickups are so esoteric.

An MC step up makes total sense now that I have looked at these, since the idea of using one for this application had not occured to me.
I am not sure how much these differ in terms of inductance and construction from the little "impedance matching" transformers in low impedance mics (or mic adaptors) that often provide about 24dB of gain.
I guess those are laminated steel EI transformers too?

How would I go about measuring the "output level" of the low Z pickups?
I have Rigol 100Mhz oscilloscope, but at risk of sounding like an idiot I am not sure of the best method to calibrate/verify the AC or DC voltage readings on the scope, or how you would recommend producing output - hook it up and hit the strings really hard? =P

Yes, hum is definitely picked up by the autoformer despite being in a solid aluminium box (I did use nylon screws and a plastic shim to mount the transformer, but connected the box at one point to the signal output ground).

I am testing a small core transformer inside the body as this would enable traditional style volume and tone controls, so this is a good shout, but I want to maximise the HiFi nature of the sound and possibly provide the ability to step the gain in the same way the $200 autoformer lets me do now (I can actually squeeze this in the body if other approaches don't give a better sound).
If a big external transformer sounds better then I'll use this as a reference for deciding whether an inside-the-body approach is too much of a compromise.
It would of course negate the ability to use long cables without treble roll-off that Lo-Z pickups inherently have, but I need a reference first.

It's helpful to know that inductance mostly affects lower frequencies and you're absolutely right about the cables causing treble roll-off.


I don't think your reply was any more all over the place than my question and I really appreciate your advice a whole lot.


Would you be able to provide a quote for an MC step-up of the type that you recommend?
if you need further information to pin this down then I'll do my best, including measuring the output if you can advise how to go about this.

Thanks!
-Chris
dave slagle
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Post by dave slagle »

malamutewolfdog wrote:Hi Dave,

An MC step up makes total sense now that I have looked at these, since the idea of using one for this application had not occured to me.
I am not sure how much these differ in terms of inductance and construction from the little "impedance matching" transformers in low impedance mics (or mic adaptors) that often provide about 24dB of gain.
I guess those are laminated steel EI transformers too?

mic transformers would be a great place to look... the UTC O-1 is a 500/2000:50K step up transformer which will net you 20dB of gain and I have a single wiht an O-17 25dB magnetic shield I can sent you to try. I figure why reinvent the wheel at the outset and anything I wind will be expensive and should be considered after the ground work is done. I would guess that all of the mic / low level transformers use nickel cores.


[qupte] How would I go about measuring the "output level" of the low Z pickups?
I would use a data logger or record it into audacity. chances are you will need a fair bit of gain to get above the noise floor but this will give you some real world numbers.
hook it up and hit the strings really hard? =P
essentially yes...
Yes, hum is definitely picked up by the autoformer despite being in a solid aluminium box (I did use nylon screws and a plastic shim to mount the transformer, but connected the box at one point to the signal output ground).
aluminum does very little for magnetic shielding you need steel and mumetal for this.

Would you be able to provide a quote for an MC step-up of the type that you recommend?
if you need further information to pin this down then I'll do my best, including measuring the output if you can advise how to go about this.

I'll send you the O-1 ouncer and we can go from there.

dave
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malamutewolfdog
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Joined: Wed Mar 06, 2019 3:54 am

Post by malamutewolfdog »

Hi Dave,

It is very kind of you to offer to lend me an O1-ouncer.

I did send you an email regarding this with my postal address (in London, UK) but I am not surprised this received no reply as you must get a lot of spam since you have your email address in plaintext on the website.

I can't send you a PM until I hit 5 posts so do you have a suggestion for how best to get this address to you?

Many thanks,
-Chris
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