Fodera, Sadowsky, aguilar, and other bass preamps

Stompboxes circuits published in magazines, books or on DIY electronics websites.

Re: Fodera, Sadowsky, aguilar, and other bass preamps

Postby george giblet » 30 Oct 2017, 01:10

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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Re: Fodera, Sadowsky, aguilar, and other bass preamps

Postby george giblet » 30 Oct 2017, 07:37

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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Re: Fodera, Sadowsky, aguilar, and other bass preamps

Postby lietuvis » 30 Oct 2017, 15:25

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/ru6h2lyscly30gj/AABX2iBB23DiB4k2OU8PUrzwa?dl=0
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Re: Fodera, Sadowsky, aguilar, and other bass preamps

Postby george giblet » 30 Oct 2017, 22:41

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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Re: Fodera, Sadowsky, aguilar, and other bass preamps

Postby george giblet » 02 Nov 2017, 00:34

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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Re: Fodera, Sadowsky, aguilar, and other bass preamps

Postby george giblet » 02 Nov 2017, 13:53

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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