Fodera, Sadowsky, aguilar, and other bass preamps

Stompboxes circuits published in magazines, books or on DIY electronics websites.
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george giblet
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Post by george giblet »

Hello, here I have traced Ibanez onboard preamp with variable mids
Thanks a heap for that!

I've traced that board recently, but only from photos, like yourself.
Apart from unknown values your trace looks spot on.
Some part values were very unclear from the pics, and some I couldn't read at all.

How confident are you of the values?:
- 4n7 treble cap
[Later varimids use 6n8.]
- 51k resistors from the mids
[51k only gives about 6dB where as Ibanez always quoted +/-15 for varimid]
- 5k1 at the output
[quite hefty treble cut]
- JFET vs BJT at the input

The back and white graph, from about 1994, the one in your dropbox, show the mids at +/-15dB.
The exact 15dB on this graph always made me suspicious it was an "artists impresssion" and used
to promote the new varimid which was released around 1994. Treble plot frequency is a little higher,
perhaps matching the 4n7 value you have.

There's this graph which agrees with the graphs in the 2007 and 2010 bass catalogs:
Image
This graph also has +/- 15dB mids. I think the later varimids reasonably match the treble and bass on this graphs.

To get +/-15dB on the mids the 51k needs to be 12k or 15k but as I remember something didn't look right about the result. Since the new circuit matched the graphs, I also notice there was a possible design oversight in the vm old version around the differential amp.

I struggled with these issues for some time and gave-up until better pics or info comes about.

Anyone got one of these?


Here's the circuit of the newer varimid circuit from and Ergodyne EDC700 (maybe 1999).
Now I will point out.
- The pre-emphasis (C3 & R4, R5, R6) do not match the de-emphasis (R21, RV6, R22, C11)
- Also, I'm very suspicious these value may vary on different models because I saw different resistors on a board from an Ibanez BTB bass.

Don't compare these values with the circuit you traced because that one has *no* pre-emphasis. So the de-emphasis will only be a shelving treble cut.
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ibanez vm3lc edc700bp V14.png

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george giblet
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Post by george giblet »

To get +/-15dB on the mids the 51k needs to be 12k or 15k but as I remember something didn't look right about the result. Since the new circuit matched the graphs, I also notice there was a possible design oversight in the vm old version around the differential amp.
Sorry. Ignore this I stuffed up. The old files I used to play around with had a bug.

OK I just re-checked and the 51k it looks fine. I get +/-13.9dB with the more or less the same Q as the newer circuit I posted.

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lietuvis
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Post by lietuvis »

Hi George, all my traced preamp component values were done not by me, check text file in dropbox link.
And thanks to your newer schematic that helped me to sort little puzzle in also recent trace of this preamp PCB from picture found on web. Yes there is some resistor values not matching your schematic.
Here are my files:
https://www.dropbox.com/sh/ru6h2lyscly3 ... Urzwa?dl=0
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Ibanez preamp.JPG
PCB.JPG

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george giblet
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Post by george giblet »

all my traced preamp component values were done not by me
OK thanks. I did the EDC 700 the same way!
And thanks to your newer schematic that helped me to sort little puzzle in also recent trace of this preamp PCB from picture found on web.
I have traced that one as well. Find circuit attached.
*** I did not measure the caps ***
There is a small chance the treble cap is 5n6 but I doubt it.

The three vari-mids are:
- The original VM3, the one you posted. Used in SRxxx basses with a stacked Bass and Treble
- The EDC one, which is a special case (~1999). It's either a VM3-HIFI or a VM3-LC, but this name might have a different meaning for basses around 2000-2004. The PCB is through-hole and looks sort of like the one you did.
- The VM3-B (BTB). A special variant for BTB basses. The SMD board you posted the pic of.
If you look at the European 2007 bass catalog you can see there is Varimid BTB. The response is different to the one I gave previously.
Unfortunately the names Ibanez gave got a a bit blurred at one point.
Yes there is some resistor values not matching your schematic.
The pre-emphasis/de-emphasis thing wouldn't be expected to match. Also note the parametric is a different circuit and uses one less opamp.
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ibanez vm3b_BTB 5 knob smd v10.png

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george giblet
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Post by george giblet »

Hi George, all my traced preamp component values were done not by me, check text file in dropbox link.
Hi lietuvis, I was checking over your VM3 schematic. I found a bug, the Treble cap C2 is in fact 6n8, like the other Varimids circuits; the 6n8 value is in your text file. The board I traced had R1 = 510k; the pic was very clear and so there's no doubt about the value.

I also found one of the 2k7 treble resistors and one of the 2k0 frequency pots don't have designators on the picture. FYI, the older VM3 boards had the part designators marked on the PCB.

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george giblet
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Post by george giblet »

What about Kubicki.

1) Ex Factor (Schematic in PDF on Page)

http://mnats.net/kubicki_ex_factor.html


2) Fender Jazz Bass Plus V (around 1992). Kubicki designed Preamp
(schematic on page)
https://www.talkbass.com/threads/jazz-b ... c.1307840/


3) Fender Cowpoke (Precision) (around, 1990 to 1994). Kubicki designed Preamp
Also used in Fender Precision Bass Plus Deluxe of the same era.


Thread:
https://elektrotanya.com/?q=hu/content/ ... es-utanmie

Schematic:
https://elektrotanya.com/files/forum/20 ... sugori.jpg

I have traced this one and the schematic looks OK. The only difference I found was R12 & R13 were 270k on the unit I traced. The given schematic has the pots and the caps. I couldn't see all the caps on the pics I had.
Notice the treble pot and bass pot use different values. Is it really like that?

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Post by alexdepaulo »

lietuvis wrote:this is how it looks ...
download/file.php?id=10848&mode=view

download/file.php?id=10847&mode=view

Can someone draw the connection order of the potentiometers, IN, OUT and etc? Thank you! :thumbsup

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Post by x-tn »

lietuvis wrote: 28 Dec 2015, 18:44 Hi here I traced Bartolini MK1 onboard preamp using pictures from web, maybe someone do have one and can measure or already know smd capacitor values?
Detailed info about this preamp: https://corta4reloaded.blogspot.com/2120/03/

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Post by marsbel »

coming back on the Haz Lab preamps:
The treble control has only one 12k resistor. Does this mean that treble is boost only?

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george giblet
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Post by george giblet »

The treble control has only one 12k resistor. Does this mean that treble is boost only?
If you look at boost-cut plots for common tone controls the amount of boost/cut levels off to some value like +/-12dB or 15dB. This is called a shelving EQ.

With no 12k resistor on treble cut side the response does not shelve off at high frequencies. As frequency increases you get more cut, so what you end-up with a low-pass filter. The Haz-lab treble has a shelving boost but a low-pass cut characteristic.

A side effect of the removing the 12k is the centre position isn't quite flat, like on the common boost/cut shelving EQ, you end-up with small amount of high frequency cut due to the asymmetry in the boost/cut behaviour.

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lietuvis
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Post by lietuvis »

Hi. After quite long lurking on web trying to find more information about Alembic bass onboard electronics I would say there is almost nothing regarding schematics or wiring diagrams. But what I could find is some pictures of onboard preamp filters. So I decide to try to trace the circuit from pictures that are good enough in resolution to see what is going on. After tracing the bottom layout it wasn't so difficult to guess the overall circuit and where the connections should be on top layer. Then I decided to build one and as it is really simple circuit, even find a suitable dual C100K pot here https://www.blore-ed.com/product-page/p ... se-log-pot, and yes I like it more than any other preamp I had. In the end I made filter for each pickup on my bass. With this circuit you can Alembicize yor bass.
In this video you can hear what this filter sounds like:


Here https://mega.nz/folder/xFozSA6K#DA_vyCYLz1h04-1NfAkzaw you will find some pictures I worked from, LTSpice file for simulation and experiments, traced PCB files in Sprint Layout.

Image

Image

Here is both filters on my bass

Image

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george giblet
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Post by george giblet »

Alembic bass onboard electronics I would say there is almost nothing regarding schematics or wiring diagrams.
Great work.

I always thought a stripped down version of state-variable schematic was still bit much for on-board electronics.

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Post by subbass »

george giblet wrote: 25 Oct 2017, 11:39 The 90's Ken Smith and Fodera basses also use the same Haz Labs circuit.
The 3-band Ken Smith adds a mid control board that plugs into the other board.
...

If anyone has a means of checking it please do!
Hello George! Splendid job Sir! There were many invisible tracks on this one if you did this from photos too. Thank you so much for that!
I found some top and bottom shots of the mid board, then corrected perspective and traced with the help of your schematic. Found only one thing to correct which also was looking suspicious on the schema. The red dot is indeed a connection on the schema. Also the top cap value is definitely 4.7n. Not visible on the attached pics but found a few shots where it was visible.

Please see the attached images.

I built the main pre and it works great. I did some value changes too. Looks like what different brands were doing had all merit and mixing some of them produced the best result for me. 10n bass cap for example carries around 40Hz to a little bit higher. Can't say exactly but looks like 55Hz on spectrometer when I boost and cut. Didn't do maths. That's a more useful freq. because most sound guys will cut a lot down there, due to stupid tiny earphones and stuff. :D

I've got two questions if you could answer please.
-Would you integrate this mid board as per the original schematic just before the 4u7 cap or put it on the output. I think you thought putting it on the output would kind of isolate or mitigate interaction with bass and treb. section?
-What can be done to increase the Q?

I'll build the mid board as time permits and update again hopefully with good news.

Edit: I wasn't sure if posting board photos was allowed - as a newbie here. Please let me know. I can re-post those.
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hazmidcor.jpg

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george giblet
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Post by george giblet »

Also the top cap value is definitely 4.7n. Not visible on the attached pics but found a few shots where it was visible.
Thanks for the confirmation. The comments in the posts a few days after I posted had some extra info. lietuvis posted a schematic but the link is broken, his schematic fixed a few of the uncertain parts on my schematic, the changes were,
- the 4n7 you mentioned
- the gain pot was wired for boost and cut.
Something like input to 20k then to pot, pot wiper to opamp, pot to other 20k.
Basically an adjustable gain opamp.
- the input load resistor was 499k

As for the increasing the Q. It's not so simple. The idea is you can increase the ratio of the 4n7 cap to the 2n2 cap but keep the product of the two the same ie. 4n7 * 2n2 . However you can't just do that because changing the caps affects the amount of boost/cut. So you need to tweak the 20k's to set the amount of boost/cut but by doing that you need to re-tweak the cap values. All the parts are linked.

Here's a few examples where I slowly increase C1 to increase the Q then adjust C2 and R1 to match up the frequency and maximum boost.
R1 is the 20k on the original, C1 is the 4n7 and C2 is the 2n2.

f0 dB_max Q R1 C1 C2 Comment
410Hz 12.5dB 0.80 20k 4n7 2n2 Original
432Hz 12.5dB 0.89 18k 5n6 1n8
390Hz 12.0dB 0.90 18k 6n8 1n8
410Hz 12.8dB 0.95 16k 6n8 1n8
419Hz 12.1dB 0.99 15k 8n2 1n5
415Hz 12.9dB 1.08 12k 10n 1n5
457Hz 12.5dB 1.12 10k 12n 1n2

A Q of 1.1 to 1.2 is as high as you can get for that circuit for boosts around 12dB/15dB.

Feel free to increase or decrease any part by one value up or down to see how it sounds.
I built the main pre and it works great. I did some value changes too. Looks like what different brands were doing had all merit and mixing some of them produced the best result for me. 10n bass cap for example carries around 40Hz to a little bit higher. Can't say exactly but looks like 55Hz on spectrometer when I boost and cut. Didn't do maths. That's a more useful freq. because most sound guys will cut a lot down there, due to stupid tiny earphones and stuff. :D
No problems tweaking the bass. Some preamps have too much lows for me. If you read some of the posts following the schematic you will see the Hazlabs preamps had some variants and one of those actually used a 10n bass cap.

The later preamps have dip switches. I don't know what caps they have.
I've got two questions if you could answer please.
-Would you integrate this mid board as per the original schematic just before the 4u7 cap or put it on the output. I think you thought putting it on the output would kind of isolate or mitigate interaction with bass and treb. section?
The mid circuit definitely needs to go straight after the bass/treble section and before that RC circuit at the output. Putting the mid circuit after the RC network will load down the network and completely screw-up the frequency response (you will get some sort of fixed treble boost.)

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Post by subbass »

Thank you for taking the time for your detailed reply George. Love your work.

That's a great sounding preamp. I'm running this after a simple jfet buffer. One per pickup actually, for a nice active blend.
pickup > passive tone pots per pickup > buffer & blend > preamp.

I'm attaching the photos of the mid board. Most detailed ones I could find. Perspective-corrected. All the traces seem to match your schematic except that line from IC pin 7. Still not sure about that. All the values seem to match too.

Blue lines: back of the board, red lines: top.

I built it but it doesn't work like the way I posted.(with the red dot junction) Not sure if that was the reason but I'll keep trying. Well, if I can't do it I'll probably barrow a mid control section from some other circuit. Electric Druid's Hard Bargain is a good candidate. :D
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hazmidbot.jpg
hazmidtop.jpg

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george giblet
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Post by george giblet »

Thank you for taking the time for your detailed reply George. Love your work.
No problem, thanks.
That's a great sounding preamp. I'm running this after a simple jfet buffer. One per pickup actually, for a nice active blend.
pickup > passive tone pots per pickup > buffer & blend > preamp.
Good idea. It works out the same as tweaking the EQ in way you can't do with the basic circuit.
I'm attaching the photos of the mid board. Most detailed ones I could find. Perspective-corrected. All the traces seem to match your schematic except that line from IC pin 7. Still not sure about that. All the values seem to match too.
As far as your "red dot junction" fix goes. Your correction is 100% correct. It's a standard and common mid circuit and most people familiar with the circuit would just reconnect it in their heads. It's not like I mistraced the board. The bug is because the schematic package I use has a bad quirk which doesn't always connect overlapping wires. I try to check for this but sometimes one or two bad connections get through - so more an annoyance than a tracing bug.

Given it's such a common circuit there's no reason for it not work work as shown in your fixed schematic.
Blue lines: back of the board, red lines: top.
I retraced the board and it matches the schematic.

The funny thing is, the way the gain trimpot is wired looks like my original schematic and not like lietuvis's schematic. Not sure what's going on there.
I built it but it doesn't work like the way I posted.(with the red dot junction) Not sure if that was the reason but I'll keep trying. Well, if I can't do it I'll probably barrow a mid control section from some other circuit. Electric Druid's Hard Bargain is a good candidate. :D
Something weird is going on. They way you have drawn it should work. If you check all the DC voltages around the opamp they should read about 4.5V. The voltages should be fairly solid when the mid pot is varied across all pot positions - if not check you wiring and part values. The voltage on pin 6 might read lower due to meter loading.

The Electric Druid schematic is actually the same circuit just with different part values.

What's actually going on with your build?

The gain trim pot only provides boost so that might affect the blend. You might be better off just removing the gain trim altogether in a blend set-up. Another possible issue with blend is you need to make sure you that the phasing of the active and passive signal are the same, otherwise blend will work in a weird way. I suspect this might be the case. Here's how the preamp works,
- first opamp: non-inverting
- second opamp (Bass & Treble): inverting
- third opamp (Mids): inverting
- fourth opamp (gain): inverting

Overall the preamp is inverting which will interfere with the blend. I suggest you take the output from pin 7 of the mid stage instead of pin 1 and basically dispose of the gain and trimpot stage altogether. With this scheme the phase inversion of the Mids undoes the phase inversion of the Bass/Treble stage and that will fix the blend.

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Post by subbass »

I tried the mid board after output but it was a total failure. I'm sure I messed up somewhere whilst trying to make this as tiny as possible! I'll connect it to it's proper place and rebuild as well.

My blend is before the preamp and it blends between two buffered signals. Pickups don't see/load each other so I can do weird stuff like cutting tone on one completely and not affect the other. When mixed, you get what a "regular person who doesn't look behind the knobs" would expect to hear.

So the preamp should be "unaware" of what's going on before and it actually behaves like that. No interaction with passive tone control before buffers or blend position at all, as expected.

Yes, final gain isn't a must. Thanks for the tip.
Actually keeping the gain low means keeping the noise low too. There are so many ways to boost it after the output and out of the 9V battery zone.
Currently boosting with a Zoom ms60b which is always on for me. Mainly using it for precise HPF and some extra LPF. Basically, on most extreme and useless frequencies, which is much easier to do digitally. Such as cutting below 20-25Hz almost like razor! Also, the compressors in it are very nice.
Thank you and regards.

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george giblet
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Post by george giblet »

I tried the mid board after output but it was a total failure. I'm sure I messed up somewhere whilst trying to make this as tiny as possible! I'll connect it to it's proper place and rebuild as well.
The mid board has to be located between the Bass/Treble circuit and the RC output network. ie. it must connect to points M1.W and M2.Y.
If you wanted to bypass the gain section the M2.Y connection would move from opamp pin 1 to opamp pin 7.
My blend is before the preamp and it blends between two buffered signals. Pickups don't see/load each other so I can do weird stuff like cutting tone on one completely and not affect the other. When mixed, you get what a "regular person who doesn't look behind the knobs" would expect to hear.
I've got clearer picture now. It should work with or without the gain stage.

I suspect you have a bug in the build somewhere. There's no reason to suspect the circuit itself has a problem since this is a very common set-up. Maybe check your part values - it only takes one wrong value to make the controls work strangely. Measuring the DC voltages in the circuit would be a big help too.

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