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Old 20th April 2014, 08:07 PM   #1
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Default How much gain does a bass preamp need?

I want to design my own simple op-amp based preamp for bass because I've been studying electronics for about a year and am still a novice. But I'm not sure how much gain I need to drive a power amp to full power.

I assume this will have different answers depending on the signal strength of the individual bass. So maybe someone could give me a general idea of how much gain a preamp needs to drive a power amp (let's say one with a pretty high input sensitivity) with a passive bass.
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Old 21st April 2014, 07:19 AM   #2
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I generally start at around 24-30dB for my active basses (don't own any passive ones), but I prefer dead clean for the most part. You can find specs and schematics for commercial ones like the Ampegs, SWRs, etc. online pretty easily.
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Last edited by Passinwind; 21st April 2014 at 07:28 AM.
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Old 21st April 2014, 11:03 AM   #3
Enzo is offline Enzo  United States
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How long is a piece of string?

Consider what power amps you plan to use, or if no power amp, what level line out you expect.

Your pickups will put out a wide range of signal levels. That all depends upon the pickups themselves, plus how and what you play. For good examples, look at some typical Fender Bass amp. Most of their schematics have test conditions at the input and typical readings along the signal path. But you are using op amps, so every stage can have its gain level set by the resistors you use, and there should be MORE than enopugh fain stage to stage to back it off to useful levels. In fact, you could buy a handful of small trimmer pots to set the gain of each stage. Little Gallien-Krueger heads used to have exactly thaqt feature - each stage gain trim.
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Old 22nd April 2014, 08:23 AM   #4
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With an 'all-round' preamp, you want to be able to amplify the weakest bass to accommodate the poweramp with the lowest sensitivity. Anything in between can be handled with a volume/gain control.

Basses tend to put out a pretty big signal. 1Vpp is not uncommon, even for passive basses. And active doesn't always mean huge signal swings. Some onboard pre's are only there as an EQ and buffer, but don't actually increase the signal level.

Some poweramps only need a (pro) line level signal to drive them to full power, which is 3.4Vpp, but I've seen amps needing 15Vpp and up. I like 32dB (40x) amplification. It can amplify the weakest Bass to some serious levels. It also allows for different headroom inputs, e.g. an attenuated -15dB input. This is nice for tube inputs, since it'll give a less compressed sound if for example your first stage is a 12AX7 and the bass driving it is hot! But it will still give a descent output signal this way.
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Old 23rd April 2014, 06:55 PM   #5
teemuk is offline teemuk  Finland
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Quote:
I assume this will have different answers depending on the signal strength of the individual bass.
Naturally. As well as depending on the input sensitivity of your power amp.

So I think the only proper answer is: it depends.

You probably need to figure out some sensible average values and then add a bit of extra gain to the design to compensate for basses and amps with "lower" specs. Gain is easy to decrease by end user: just turn down volume or gain knob or whatever. Increasing gain would require that the end user modifies the amp.
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Old 24th April 2014, 01:19 AM   #6
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Thanks for the responses guys. I really was just looking for a ballpark minimum, I guess I should have stated that explicitly. Passinwind, your answer was especially helpful and made me happy because it lines up closely with some calculations I made for getting a typical bass signal to line level.
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Old 24th April 2014, 03:42 AM   #7
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Quote:
Originally Posted by nonex View Post
Thanks for the responses guys. I really was just looking for a ballpark minimum, I guess I should have stated that explicitly. Passinwind, your answer was especially helpful and made me happy because it lines up closely with some calculations I made for getting a typical bass signal to line level.
Best of luck. You can find build threads on half a dozen or so of my DIY bass preamp builds on Talkbass, same username there.
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Old 25th April 2014, 06:46 PM   #8
JMFahey is offline JMFahey  Argentina
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From a practical point of view, around 10X (20dB) for most good modern instruments; say 20X (26 dB) to accomodate some cheaper/older/weaker one.
It also depends on Bass Player´s "touch", I´ve had quite a few heavy handed players.
But, as said before, that´s what the volume control is for
We are talking flat bass amplification here.
If you use active equalization (Baxandall tone controls or graphic EQ) use a 10/20X first stage > volume control > tone controls.
For passive EQ (Fender/Marshall/old Ampeg) use 10/20X > pasive EQ > volume > 10/20X recovery stage.
This will accomodate 95% of instruments and players.

Last edited by JMFahey; 25th April 2014 at 06:49 PM.
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Old 25th April 2014, 07:16 PM   #9
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by nonex View Post
So maybe someone could give me a general idea of how much gain a preamp needs to drive a power amp (let's say one with a pretty high input sensitivity) with a passive bass.
I only use passive bass
playing 6 or 7 different ones now
and some more I have in boxes

so, I have a pretty good idea about the differences they can have

it varies a lot between the different instruments depending on pickups, and playing style etc

sometimes it feels like I could do with a bufffer, and really don't need any gain pre at all
and other times when it feels like I could use a bit more, but my ears tell me I have plenty

and not to forget the speakers either
and ofcourse, power amps can be different too

but your question is how much it takes to drive a power amp to full output, and a bit different matter
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Old 25th April 2014, 07:36 PM   #10
sreten is offline sreten  United Kingdom
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Hi,

Look at some bass preamp schematics. Any decent preamp has
high and low inputs, and they switch gain and input impedance
automatically by which socket is used.

You can try to reinvent the wheel, but you'll learn a lot more looking
at, and understanding the implementations of good bass preamps.

rgds, sreten.
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