BAJA discrete differential low current low noise guitar preamp  [documentation]

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Serge75
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Post by Serge75 »

What a great project! do you think this preamp would be feasible with a jazz bass? Thanks a lot Bajaman

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Post by Jarno »

You'll need two 😀
And, if you are using single coil pickups as original jazzbass pickups are, you don't have the noise reduction benefit of the inverting pickup.
You could do a dummy coil pickup without polepieces and magnet, but you'll need tuning to make sure noise is cancelled.
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Post by bajaman »

Serge75 wrote: 28 Aug 2021, 07:18 What a great project! do you think this preamp would be feasible with a jazz bass? Thanks a lot Bajaman
yes - what pickups are they please ? Do you know the LCR (inductance, capacitance, resistance values for them) ? Are they Fender, or something else - if you let me know I can do some simulations and load the schematic for you. by the way, a single preamp should be sufficient for both pickups ;-)

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Post by Serge75 »

I hace two jazz oriented bases, Germán warwick fortress 5with original mec dynamic correction picks, its passive/active, but played passive all the time, and an old jazz that wears original Marcus Miller pick ups that i bought from a Guy that was changing them to put Bartolini s. So far i have exclusively played them passive. I can take a look for the inductance on line. I dont have a inductance meter. I ll keep telling you. Thank you

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Post by dylan159 »

Thanks dylan159 for the simulations and observations
Thanks to you bajaman. I'm glad my observations were constructive and that we both benefited from the discussion. There's few people I can discuss guitar circuit design with, and I treasure each one.
I must admit that i was not paying enough attention to headroom issues - most pickups have only small output levels in the 100mV to 200mV peak region and in practice I have not heard any nasty distortion in my own working examples
You can get much bigger peaks if you try! But being peaks, this distortion isn't usually audible, at least as such. Many "clean boosts" around probably have worse headroom, just for having the same or more gain, same supply, and the restricted headroom of an op-amp. So I don't think it's that big of an issue as of now.

About the circuit, in true engineering way, you really can't have everything at once, so all things considered you did well with the low current limitation. I've checked output impedance, and feedback saves the day there keeping it low even with the 150k collector load. I also checked again noise, and while it went up, it's not by much with respect to other causes.
Still one point though:
  • for single ended operation, there's no need to distribute the gain between the feedback network and the input voltage divider. This is attenuation before amplification and will result in higher noise both by itself and because of the series resistor. The gain is 3 because it's 0.75x from the attenuator and 4x from the amplifier. As I've shown, using a ratio of two for the feedback divider and a bias resistor equal to the desired input impedance gets to the same result with lower noise (and parts count :D ). But admittedly the differential configuration is the more interesting one
  • differential input. Here my concern is completely different. For a floating source, impedances at both inputs don't necessarily need to be equal, but in this case I don't think we're dealing with a floating source at all, since the coils are grounded at one end. So while a floating source will have the same common mode current in both inputs, in the humbucker example the first coil "sees" an input resistance of about 159k while the second "sees" just a bit more than 300k. I think that for best common mode rejection those two should be equal, and it's easy to achieve the same gain and input impedance.
    Source on balanced inputs and CMRR: https://sound-au.com/articles/balanced-io.htm part 3.
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Post by bajaman »

Hi Serge75
You could always try this preamp with the marcus miller pickups ;-)
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bajaman
Fender vintage 74 jazz bass pickup with 43uA preamp example 280821.GIF
Fender vintage 74 jazz bass pickup with 43uA preamp example 280821.GIF (10.2 KiB) Viewed 4054 times
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Post by bajaman »

a slight improvement - I simulated the non inverting response with the inverting input grounded and adjusted the input impedance for critical damping - +6dB gain
then i simulated the inverting input, again adjusting the input impedance to damp the coil resonant peak - +6dB gain
then I simulated the preamp in full differential mode and - +12dB gain, BUT, the output response clearly showed that the inverting input coil was being overdamped causing a droop in response of around -7.5dB @ 8kHz relative to midband gain ! So, i readjusted the R3 and R4 resistor values upward (about 10%) until the droop was no longer visible and the output response was now-3dB down @8kHz with a smooth rolloff above this frequency - here is the revised example:
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bajaman
Baja high output humbucker pickup preamp example 43uA version rev 1 290821.GIF
Baja high output humbucker pickup preamp example 43uA version rev 1 290821.GIF (11.57 KiB) Viewed 3998 times
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Post by CheapPedalCollector »

dylan159 wrote: 28 Aug 2021, 09:16
Thanks to you bajaman. I'm glad my observations were constructive and that we both benefited from the discussion. There's few people I can discuss guitar circuit design with, and I treasure each one.
Maybe if you worked on your people skills you would have more people you could discuss it with.

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Post by Jarno »

Reading back this thread, not sure that comment was needed, at least not in the context of this particular discussion.
Please be mindful of the fact that there are a lot of different people on forums, from a lot of places and backgrounds, each with their own "manual" so to speak. It helps if we give eachother a little leeway.
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Post by DaveLTX »

Jarno wrote: 29 Aug 2021, 11:46 Reading back this thread, not sure that comment was needed, at least not in the context of this particular discussion.
Please be mindful of the fact that there are a lot of different people on forums, from a lot of places and backgrounds, each with their own "manual" so to speak. It helps if we give eachother a little leeway.
Just to give you a bit of background... on a certain discussion place we were a little... suspicious of what he's saying and decided to call us snobby know it alls. Thus hunting down dylan's thread.
So yes, that comment was done with ill intention

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Post by deltafred »

DaveLTX wrote: 29 Aug 2021, 11:49 Just to give you a bit of background... on a certain discussion place we were a little... suspicious of what he's saying and decided to call us snobby know it alls. Thus hunting down dylan's thread.
So yes, that comment was done with ill intention
What happens on other discussion places has little bearing here and, not that it matters, we only have your side of the story.

As far as I can remember, and I read most of the new posts, dylan has upset nobody here and I see no reason for people to follow him here to insult him.
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Post by bajaman »

Here is a very nice collection of pickup specification that fellow forum member Antigua has compiled (hope that it is okay with him, my posting it here)
Amongst other measurements it gives resistance, inductance and capacitance figures for a very large collection of pickups :-)
cheers
bajaman
http://www.echoesofmars.com/pickup_data/viewer/#~(eddyRp~1000000~eddyVal~1600~sim_ns_cover~true~sim_br_cover~true~graph_debug~true~graph_with_load~true~sort~(~'~'Mfg~'Model~'Position)~deletions~(~)~ascdesc~(~null~'asc~'asc~'asc)~graphs~(~)~filters~(~)~filters_not~(~)~filter~'~approx_dB_diff~false~selectedRow~65)
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Post by bajaman »

Here is a little vero board (strip board) layout i whipped up using bancifa's excellent DIY layout creator program
the first picture is the view from the component side and the second picture shows the track side as a mirror image of the component side.
enjoy
cheers
bajaman
discrete differential preamp 43uA 230821.png
discrete differential preamp 43uA track side 230821.png
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Post by dylan159 »

bajaman wrote: 30 Aug 2021, 02:34 Here is a very nice collection of pickup specification that fellow forum member Antigua has compiled (hope that it is okay with him, my posting it here)
Amongst other measurements it gives resistance, inductance and capacitance figures for a very large collection of pickups :-)
cheers
bajaman
http://www.echoesofmars.com/pickup_data/viewer/#~(eddyRp~1000000~eddyVal~1600~sim_ns_cover~true~sim_br_cover~true~graph_debug~true~graph_with_load~true~sort~(~'~'Mfg~'Model~'Position)~deletions~(~)~ascdesc~(~null~'asc~'asc~'asc)~graphs~(~)~filters~(~)~filters_not~(~)~filter~'~approx_dB_diff~false~selectedRow~65)
Since it's on a website, so already publicly accessible, I think you shouldn't worry about it, it's ok to share it. This will come in handy when I'm tired of always using the same values for estimating the response of a circuit with a pickup source :D
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Post by bajaman »

Here is an example of using the preamp in differential mode with a hum cancelling dummy coil mounted with double sided thick 3M mounting tape, under a strat style pickguard, between the neck and middle pickups.
I wound the dummy coil on a standard humbucker plastic former - from memory, i wound around 5000 turns of 42awg polynylon covered copper wire, but that was a few years ago and I did not document it at the time. The dummy coil was stuffed with 6 nickel plated steel rod slugs - as used in most PAF style humbucking pickups. The coil measured 2.18H, 4.92k ohms and 96pf.
The strat pickup coils were wound to approximate Fender Vintage 65 specs. The middle pickup measured 2.58H, 6.3k ohms and 125pF.
The veroboard preamp was mounted with doubled side 3M mounting tape to the underside of the pickguard between the pickup selector switch and volume control - I cut a hole in the back of the guitar and fitted a 9v battery holder feeding the wires through to the control cavity.
With regards to the preamp design, i was aiming at loading the pickup coils with exactly the same resistance as in a standard passively wired alnico 5 pickups stratocaster guitar - approximately 125k ohms (the neck and middle pickups see this resistance although the bridge pickup sees 250k because there is no tone control for this pickup ;-) ) I have seen many guitars modified to allow the middle and bridge pickups to share a tone control but it is worth bearing in mind that doing so does damp the bridge pickup's resonant peak far more than originally intended by Leo Fender - reduces some harshness ;-)
I simulated the response with the dummy coil connected to the inverting input and trimmed R3 and R4 resistor values to provide an essentially similar top end response frequency characteristic - i had to add some additional capacitance across the dummy coil to better approximate this.
When i wired everything up I was pleasantly surprised that i must have got the dummy coil connected with the correct polarity, because there was almost no buzz or hum from a guitar which previously had hummed and buzzed if I got within a metre of my amplifier - now i could hold the guitar very close to the amplifier with only a very small amount of expected hum - certainly closer than i would ever get to it while playing ! The sound of the guitar is very clean and clear and i honestly could not hear any hiss from the preamp whatsoever - very satisfied with the results
cheers
bajaman
Baja strat pickup preamp with hum cancelling dummy coil example 43uA version 310821.GIF
Baja strat pickup with hum cancelling dummy coil 43uA preamp 310821.png
Baja strat pickup with hum cancelling dummy coil 43uA preamp track side 310821.png
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Post by bajaman »

strictly speaking, the previous circuit works well with the strat neck and middle pickups, but the bridge pickup "bite" at the top end is missing - not necessarily a bad thing though as many feel the bridge pickup can be a bit ice picky . To truly model the bridge pickup requires a 250k loading not 125k because there is no tone control pot in parallel with the volume control when using this pickup.
Here is the same preamp - same voltage gain but with values chosen for just the bridge pickup. As you can see it involves changing the four resistor values (R1,R2,R3,R4). To switch between these values would require a 4pdt mini toggle switch and at least 4 additional resistors, in addition to the existing strat 5 way selector switch - interestingly, this would function in all selector switch positions to give unloaded tone control response (treble boost) for all pickups - similar in effect to the Fender TBX no load tone control ;-).
I will draw up the amended circuit with 4pdt switch etc in a future post, in the meantime, here is the preamp with 250k loading resistor values
cheers
bajaman
Baja strat pickup preamp with hum cancelling dummy coil example 43uA version 250k loading 310821.GIF
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Post by bajaman »

Baja strat pickup preamp with hum cancelling dummy coil example 43uA version 250k or 125k  loading 310821.GIF
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Post by bajaman »

Today I built the 125k or 250k switchable preamp as in the previous post - managed to find a 4 pole double throw mini toggle switch in my stash of parts :-) I wired 39k, 82k, 170k, and 341k resistors directly to the switch contacts and ran 6 flying leads to connect to the circuit board.
With the 125k ohm setting the pickup coil is critically damped and shows a very slight (less than 1dB) rise around resonance frequency - however, the 250k ohm loading clearly shows the pickup coil is now under-damped with a +4dB rise in response around 8kHz
How does it sound? Could I hear any difference ? Because the peak in response is around 8kHz I was only able to detect a small difference in sound - but notice it i could - if anything it adds a touch of "brittleness" to the sound but in no way is it detrimental - I feel that the sound change would be much more noticeable with respect to a PAF humbucker which generally has a lower resonant peak in it's response.
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Post by ppluis0 »

Hi bajaman,

It's possible to employ this design of your preamp with a piezo pickup placed into an acoustic guitar ?

BTW congratulations for your innovative designs !!!

Jose

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Post by bajaman »

ppluis0 wrote: 18 Oct 2021, 20:27 Hi bajaman,

It's possible to employ this design of your preamp with a piezo pickup placed into an acoustic guitar ? [...]
In general - no, a piezo needs to see a VERY high input impedance such as a mosfet - i have found it necessary to adjust the input coupling capacitor incrementally down in capacitance to lessen the "thump" that these devices inflict on speakers ;-)
[...] BTW congratulations for your innovative designs !!!

Jose
- thank you - we all try and help each other here as best we can - I may be old but always willing and happy to learn from the "hive" mind !
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