Thoughts on adding preamp to bridged NAD 3140 for bass guitar - diyAudio
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Old 20th February 2014, 04:02 AM   #1
ny3 is offline ny3  Australia
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Default Thoughts on adding preamp to bridged NAD 3140 for bass guitar

I have a NAD 3140 solid state amp that has been bridged for mono, powering a 12" 200W bass speaker. I want to add a simple high-impedence preamp stage to boost the signal from a bass guitar up to the input level the amp was designed for.

I've looked at the amp circuit. The lowest regulated voltage on the board is +/-26V. I was thinking of using a simple voltage divider to get, say, +/-15V to power a FET op amp such as an ADA4627 before the NAD amp input. I estimate a ballpark gain of 10 is required.

Aside from the op amp idea I also like the idea of a simple FET-based gain stage (like the Tillman discrete FET guitar preamp) but think I might have more success with an op amp because of the power supply and gain considerations.

Is this the op amp idea using the NAD power supply reasonable? (I understand some people may struggle to get their heads around the idea of using a NAD 3140 for a bass guitar).
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Old 20th February 2014, 04:57 AM   #2
Johno is offline Johno  Australia
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The 3140 already has a preamp, why not use that?
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Old 20th February 2014, 07:26 AM   #3
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Quote:
Originally Posted by Johno View Post
The 3140 already has a preamp, why not use that?
The level of the signal from a bass guitar is too low. I want to boost it up to the level the 3140 was designed to for. This was in the first sentence of my post.

I intent to continue to use the preamp the 3140 already has. I just need an extra gain stage to boost the bass guitar signal before it gets there.

Another reason for an extra gain stage is to provide a high-impedence input for the bass guitar. The input impedence of the 3140 preamp is lower than ideal for most electric basses and guitars.

That said, plugging the bass directly into the 3140 will work. Sort of. It's just very quiet, and if the bass has conventional passive high-impedence pickups it might sound a bit dull.
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Old 20th February 2014, 08:18 AM   #4
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Originally Posted by Johno View Post
The 3140 already has a preamp, why not use that?
Because it's completely unsuitable, wrong gain, wrong impedance, and wrong frequency response.

However, I'm more concerned about using a home HiFi amp for bass guitar - I wouldn't have thought it's going to last terribly well, particularly when bridged.

But for a preamp, a simple FET buffer and opamp gain stage are all that's needed, or build something based on this:

Bass Preamp
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Old 20th February 2014, 08:38 AM   #5
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Changing the value of the feedback components in the phono pre amp should more than suffice. Take a look at the circuit.
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Old 20th February 2014, 10:06 AM   #6
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Originally Posted by JonSnell Electronic View Post
Changing the value of the feedback components in the phono pre amp should more than suffice. Take a look at the circuit.
Altering the feedback of the phono preamp would sort out the gain and frequency response problems, but would still leave you with too low an input impedance.

Although you could do that and add a simple FET buffer to the front.

However, it hardly seems worth the effort on an amp which probably won't last very long.
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Old 20th February 2014, 08:08 PM   #7
effebi is offline effebi  Italy
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Seen on this forum short time ago.
It should do exactly what you intend.
Guitar preamp x stereo boombox
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Old 21st February 2014, 02:28 AM   #8
ny3 is offline ny3  Australia
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Thanks for the advice and suggestions - much appreciated.
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Old 18th March 2014, 05:43 AM   #9
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Update: My NAD 3140 has been converted to a quite nice-sounding bass amp. I tried a few ideas and ended up using an OPA2134 dual op amp as a high-impedance unity-gain buffer followed by a gain stage (about x15). This feeds into the NAD pre-amp stage.

An unexpected outcome is it also makes a nice-sounding clean guitar amp. But with more low end than most guitar amps.

One problem I did find was the the NAD's treble control wasn't very usable for guitar/bass because the treble frequency (being designed for a for a hi fi amp) was too high. However, this was easily fixed by adding a couple of capacitors to lower to treble frequency to a more useful value. I did consider bypassing the NAD pre-amp tone circuit completely and building a standard Fender-type tone circuit, but it wasn't necessary.
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Old 18th March 2014, 08:19 AM   #10
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Good for you. Thinking outside the box is fun!
I've made several practice amps for guitar and bass out of old stereos. Usually wire in a limit to the volume and never had a speaker blow (yet).
Most adventurous was to remove the power chip from a 15w SS guitar amp and wire in a triode pentode. The owner couldn't hear the difference in power and chose to use it on his subsequent Church gigs.
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