Can anyone verify this onboard preamp?

[Nigel Goodwin]

No, it has a Gain of 3db not 3x, which isn"t much at all but in Practice it usually doesn"t add any Noticeable gain I have built this circuit many times and sometimes it actually reduces the output signal depending on the Fet.....

The gain of the circuit is dependent on the negative feedback, which gives it a gain of about three times, not 3dB.

Like in your attachment, bypass the source resistor to increase gain further.

 

[vangelis]

I've built this one several times. It gives you the option to change the center freq of the "mid" eq which can be very useful.

Hi Brion and others

Can you explain existence of last op-amp as follower. I think is not necessary. It's just decrease dynamics and put some noise and distortion...

Just my opinion, of course...

 

[Nigel Goodwin]

Hi Brion and others

Can you explain existence of last op-amp as follower. I think is not necessary. It's just decrease dynamics and put some noise and distortion...

Just my opinion, of course...

It's just a unity gain buffer, to isolate the tone circuit from the load, always a good idea to use one, as you can't guarantee what load impedance it may be called to feed. It won't have any negative impact on the sound at all.

 

[Andronico]

Hi Brion55, do you think that your preamp (with 3 band eq) is suitable to be used with a piezo pickup ? Have I to change something ? Thanks. 🙂

 

[Nigel Goodwin]

Hi Brion55, do you think that your preamp (with 3 band eq) is suitable to be used with a piezo pickup ? Have I to change something ? Thanks. 🙂

A piezo needs a much higher input impedance, the single FET preamp earlier in the thread could be used in front of the opamp one. As you don't need any gain from it, you could take the output off the source pin, and replace the drain load with a piece of wire.

 

[vangelis]

It's just a unity gain buffer, to isolate the tone circuit from the load, always a good idea to use one, as you can't guarantee what load impedance it may be called to feed. It won't have any negative impact on the sound at all.

Ok. man... if you say....
😉

 

[Brion55]

Sorry I didn't see this until now.

Yes the preamp would work for a piezo, but like Nigel says, the input impedence should be higher (just change the resistor going to Vref)

The output buffer is needed to keep from loading the mid-range filter. It shouldn't add any noticeable noise if low noise op-amps are used.

 

[macsong053]

Piezo pickups

I've just taken apart my 1985 Takamine acoustic/electric preamp to try to fix the gain slider on it... (you can read about my frustration and see a pic here- http://www.takamineforum.com/forum/viewtopic.php?t=238

I have traced the circuit out somewhat and find the resister at the input to be 2.2M ohms. So for this era of Takamine pickup, that input impedance would be good.

I can see that this is an old thread, but due to the fact that I'm very interested in acoustic preamps and would like to build one, if I can't fix this one on my Tak, I just wanted to keep it going... I think there's a LOT of interest on this subject, especially when it comes to repairs!

If ANYONE has contacts to buy a 50K or 100K slide pot with a 20mm throw in the USA... Check out my post at Takamine Forum for the specs and PLEASE let me know?



Hey and THANKS for a wonderful Forum, & access to great folks with larger brain cells than me! I've been reading here for a long time, just joined today!

 

[Brion55]

Mouser Electronics http://mouser.com has a lot of hard-to-find parts like that. Might be worth looking.

 

[macsong053]

Yes & thanks! But no, the only part that comes close is an Alpha model, which is not carried by Mouser, (even though they carry Alpha slide pots) and a Song Huei model, (Taiwan), and Mouser has declined any invitation to include either to their catalog.

I've got a lot of time on my hands and been looking for this slide pot for,... ummm THREE WEEKS. There are really NONE with this footprint, in the US of A.

If you check the link in the Tak Forum...

Song Huei is considering sending me "samples", since they usually deal in lots of >1000, i.e., a distributor! I'm not looking forward to the shipping charges? LOL But! I've provided them with "leads" to distributors here and just may become one myself!

I do hope they'll keep contact with me so that I can find a solution to replace this slider! And I do hope that it works!

What are your thoughts as to replacing, in a preamp situation as this, a 50k ohm slide pot, with a 100k ohm one? The original problem with the original slide pot is that it's first of all, just worn out. There's NO bottom end to it, and then at the 40-50 % mark, it is just BLASTING. Way too high for my needs. And keep in mind, that I'm comparing it to a Takamine guitar w/ preamp in a previous model, another guitar that I have used for 20 years in the same guitar amp, that I use all of the time with a mic in the second input jack. (now I haven't taken apart this first guitar of mine, as I bought it brand new in '78, and it's still like new....)

I just can't stand this new guitar's brazen loudness at only 40% of the gain?

Thanks for your reply and help!

 

[Brion55]

you might be able to replace the 50K with a 100K by "padding" it with 2 50K resistors placed between the wiper and both ends of the pot. Should be close enough.

 

[macsong053]

you might be able to replace the 50K with a 100K by "padding" it with 2 50K resistors placed between the wiper and both ends of the pot. Should be close enough.

??? Hmmm, this would add...., sorry, I'm at a loss? You show in the diagram a 100k pot where the existing is only 50k, and use the word "replace"? Do you mean to "add" resistance to the existing 50k pot? Or replace the existing 50k pot with a 100k pot and THEN add more resistance between the top and bottom end of the pot,... 2 @ 50k ohm?

But the original problem is that the existing 50k pot is essentially "open" at the low end.... no contact between the wiper and the pot. It's dead there? So if I follow you, adding a 50k ohm resister (IF there's room on the cb,... man, it'd have to be tiny!), would start my pot out at 50k ohm at the beginning of the wiper contact and then at 40-50% or almost halfway on the lever,.... BOOM! as soon as the wiper made contact on the middle of the pot,... it would add another 50k to it? Under this theory, maybe a 10k ohm resistor at the bottom end and a 40k ohm at the top end? But yet it's OPEN at the bottom end?

Hmmm, maybe just add a 50k ohm to the top end?

(They do say,... that on a acoustic guitar preamp, the closer to top end of your gain, the better the transference of tone & quality, to the guitar amp?)

"Splain here Lucy"? And please just consider this a sanity check on my part?

 

[Brion55]

What are your thoughts as to replacing, in a preamp situation as this, a 50k ohm slide pot, with a 100k ohm one?

Guess I thought you meant you had access to 100K but not 50K. What I showed was a way you could sub a 100K for the 50K that would probably work, that's all.

Don't know much about Takamine other than I've played on them and thought they were pretty good instruments. It surprises me that they don't have parts available or a direct replacement for the preamp.

 

[macsong053]

Gotcha Brion55! No, right now I have no access ('cept for Song Huei, which hasn't happened yet) to any replacement slide pot. But I did want to try a 100k, to replace the 50k, just because when this existing 50k slide DOES make contact, at about 40-50%, it's just way to loud?

THAT's what I was asking advice about..., besides finding one to fit.

In other words, how does adding a higher resistance pot affect this preamp? Maybe I'll get lucky and find both values and try one, then the other?

Thanks a bunch man!