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1/4 Guitar to Balanced Amplifier input
1/4 Guitar to Balanced Amplifier input
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Old 31st October 2019, 01:35 AM   #1
thebentern is online now thebentern
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Question 1/4 Guitar to Balanced Amplifier input

So a bit about my scenario... I'm considering making a guitar amp for fun. I have come into some IcePower 50asx2-btl modules salvaged from Galaxy audio monitors (one was converted back to an SE by adding an inductor for home use). Now the trouble with this amplifier is that it only accepts a balanced input. I'm familiar with basic differential to balanced converter boards using opamps and such, but I'm really familiar with amplifying an unbalanced instrument signal from an electric guitar into a balanced line level signal.
Are there standalone boards that can accomplish this? Or will I need to use an instrumentation amp into a diff. to balanced board?

I have a big knowledge gap in this area, so some insight into my issue would be greatly appreciated.

Thanks
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Old 31st October 2019, 02:33 AM   #2
Gnobuddy is online now Gnobuddy  Canada
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There are several ways you can approach this, both DIY and using an off-the-shelf solution.

The simplest way is to buy and use a DI box that will accept an unbalanced input, and put out a balanced signal. There are tons of DI boxes out there, here is one fairly inexpensive one from a trustworthy manufacturer (I've always found ART projects punch well above their weight, excellent quality at affordable prices): Amazon.com: Art dPDB Dual Passive Direct Box: Musical Instruments

For half the price, there is this Donner box: Amazon.com: Donner Guitar Bass Passive DI Box Instrument to Balanced & Unbalanced Micro Direct Box Pedal: Musical Instruments

Note that the Donner box is NOT passive, as we can tell from the fact that it needs +9V power to work, and also it can produce up to +20 dB of gain (which requires amplifying circuitry instead, making it an active device, not a passive one.) So the ad copy is WRONG.

The Donner includes a cabinet simulator; you didn't mention what sort of speaker you're going to be driving with your solid-state power amp, but if it's a flat-response, full range speaker (P.A. speaker or similar), you will need a cabinet simulator of some sort between guitar preamp and power amp. The cab sim in the Donner is of unknown quality, but if you need something for this task, it's almost surely better than nothing. (And it may be great, who knows.)

A third option is the well-respected Hughes & Kettner Red Box: Amazon.com: Hughes & Kettner Red Box 5 DI and Speaker Simulator: Musical Instruments

The Red Box includes a nice, and very tweakable, cab simulator.

Note that all the passive boxes tend to have very low output signal level - not line level, but only mic level. This means you may need a balanced preamp between your passive DI box and your power amplifier.

Which brings me to option number 4: Amazon.com: ART Tube MP/C Tube Pre-Amplifier/Opto-Compressor-Limiter Project Series: Musical Instruments

This is a preamp with a lot of nice features, including accepting both balanced and unbalanced inputs, and providing both balanced and unbalanced outputs. Outputs can be switched to instrument or line level. There's lots of gain available if you need it. There is even a nice optical compressor available if you need it. BUT - there's no cab sim.


-Gnobuddy
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Old 31st October 2019, 07:51 AM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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You could build one:
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Old 31st October 2019, 12:03 PM   #4
thebentern is online now thebentern
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Quote:
Originally Posted by Gnobuddy View Post
There are several ways you can approach this, both DIY and using an off-the-shelf solution.

The simplest way is to buy and use a DI box that will accept an unbalanced input, and put out a balanced signal. There are tons of DI boxes out there, here is one fairly inexpensive one from a trustworthy manufacturer (I've always found ART projects punch well above their weight, excellent quality at affordable prices): Amazon.com: Art dPDB Dual Passive Direct Box: Musical Instruments

For half the price, there is this Donner box: Amazon.com: Donner Guitar Bass Passive DI Box Instrument to Balanced & Unbalanced Micro Direct Box Pedal: Musical Instruments

Note that the Donner box is NOT passive, as we can tell from the fact that it needs +9V power to work, and also it can produce up to +20 dB of gain (which requires amplifying circuitry instead, making it an active device, not a passive one.) So the ad copy is WRONG.

The Donner includes a cabinet simulator; you didn't mention what sort of speaker you're going to be driving with your solid-state power amp, but if it's a flat-response, full range speaker (P.A. speaker or similar), you will need a cabinet simulator of some sort between guitar preamp and power amp. The cab sim in the Donner is of unknown quality, but if you need something for this task, it's almost surely better than nothing. (And it may be great, who knows.)

A third option is the well-respected Hughes & Kettner Red Box: Amazon.com: Hughes & Kettner Red Box 5 DI and Speaker Simulator: Musical Instruments

The Red Box includes a nice, and very tweakable, cab simulator.

Note that all the passive boxes tend to have very low output signal level - not line level, but only mic level. This means you may need a balanced preamp between your passive DI box and your power amplifier.

Which brings me to option number 4: Amazon.com: ART Tube MP/C Tube Pre-Amplifier/Opto-Compressor-Limiter Project Series: Musical Instruments

This is a preamp with a lot of nice features, including accepting both balanced and unbalanced inputs, and providing both balanced and unbalanced outputs. Outputs can be switched to instrument or line level. There's lots of gain available if you need it. There is even a nice optical compressor available if you need it. BUT - there's no cab sim.


-Gnobuddy
Thanks for the all of the ideas here. For some reason, the fact that DC powered direct boxes might exist didn't come to mind because all of my experience has come from phantom powered ones connected to the mixer at church. The cab simulator will definitely be something I need to get as well, if I want to use this for electric guitar.
I may need to go with something with full pre-amp functionality, because I wouldn't have a way control levels with only a directbox / cab simulator. I suppose I could try to find a discrete balanced pre-amp module to run between the direct box and the amplifier.

Thanks again
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Old 31st October 2019, 12:24 PM   #5
russc is offline russc  England
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Are you sure this has balanced inputs?
I'm seeing single ended inputs on the data sheet.
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Old 31st October 2019, 01:58 PM   #6
thebentern is online now thebentern
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Originally Posted by russc View Post
Are you sure this has balanced inputs?
I'm seeing single ended inputs on the data sheet.
There are two configurations of the 50ASX2 boards. One is SE (50w x 2), and the other is BTL (170W).
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Old 31st October 2019, 03:17 PM   #7
conanski is offline conanski  Canada
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Originally Posted by thebentern View Post
Now the trouble with this amplifier is that it only accepts a balanced input.

You are mistaken. Even though the module may only have a balanced XLR or TRS input you can still connect an unbalanced source. If the module has a TRS input just plug in a regular TS guitar cable, if it only has an XLR input you just need a TRS to XLR adapter or cable where pin 3 of the XLR isn't used.
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Old 31st October 2019, 03:59 PM   #8
thebentern is online now thebentern
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Originally Posted by conanski View Post
You are mistaken. Even though the module may only have a balanced XLR or TRS input you can still connect an unbalanced source. If the module has a TRS input just plug in a regular TS guitar cable, if it only has an XLR input you just need a TRS to XLR adapter or cable where pin 3 of the XLR isn't used.
For some reason I remember reading a thread here where someone mentioned that if you try to use an unbalanced input with the BTL version, there is an unbearable noise problem. I could be conflating this with another mono btl amp board though. I've read too many different amp board threads.
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Old 31st October 2019, 07:41 PM   #9
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by thebentern View Post
...if you try to use an unbalanced input with the BTL version, there is an unbearable noise problem...
I'm unfamiliar with this particular class-D module, but I've used several inexpensive class-D power amp boards which have only unbalanced inputs. If the grounding is properly done, I can't think of a good reason for huge noise problems to occur.

I almost mentioned the unbalanced option, and some DIY options, but I could see I was headed towards re-writing Tolstoy's "War and Peace", so I trimmed my post and kept it to off-the-shelf solutions.

The little $30 Donner pedal might be just what you need - it includes cabinet simulation (quality unknown), and up to +20 dB of voltage gain in case you need it. (Instrument level is roughly 20 dB below line level, so you might find you do need that gain.)


-Gnobuddy
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Old 31st October 2019, 07:56 PM   #10
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by thebentern View Post
I may need to go with something with full pre-amp functionality, because I wouldn't have a way control levels with only a directbox / cab simulator.
I missed this earlier. I'm a bit confused - you know that a raw guitar signal is far too small to feed directly into a power amplifier, right? Not to mention, an electric guitar plugged straight into a solid-state power amp is a recipe for horrendously bad guitar sound.

So you need a guitar preamp of some sort (even if it's only a chain of guitar FX pedals), to bring up the 20 mV - 100 mV from the guitar to the roughly 1 - 2 volt line level most power amps want. And that preamp will presumably include the usual controls - tone, gain, volume, etc.


-Gnobuddy
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