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Re-biasing mic input for bass guitar
Re-biasing mic input for bass guitar
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Old 3rd June 2015, 02:08 AM   #1
Civitello is offline Civitello  United States
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Default Re-biasing mic input for bass guitar

Hey guys,

I have a neat 70's solid state PA amp with some great sounding germanium output transistors.

It currently has two 1/4 TS mic inputs that I would like to make inputs for bass guitar. Currently, when plugged in, the bass is a bit crunchy sounding.

So what am I looking at doing here? Do I need to re-bias the preamp transistor or simply change the input resistance?

I can pass along more info of the preamp circuit if needed.

Thanks
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Old 3rd June 2015, 09:39 AM   #2
DF96 is offline DF96  England
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Posting in the right forum area would help: Instruments and Amps.

You may need to reduce the input sensitivity i.e. reduce the gain. At present you are probably grossly overloading the input.

Bear in mind that germanium output transistors are more fragile and more prone to overheating problems than silicon, and probably difficult to obtain spares too.
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Old 3rd June 2015, 10:08 AM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Make and model of the amplifier would be helpful but I would reduce the input sensitivity with a potential divider. Re biasing will not help if the microphones sound OK. Beware, as DF96 states, they can easily be destroyed!
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Old 4th June 2015, 01:23 AM   #4
Civitello is offline Civitello  United States
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Sorry for posting in the wrong forum.

The amp is branded a Realistic MPA-20, one of Radio Shack/Tandys brands. I actually not only have two models of the amp, I have 4 back up germanium transistors ready to go if I muck anything up.

So basically I just need to pad the input down with a resistor?

Sorry for the basic question. I'm doing my best to teach myself this stuff.
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Old 4th June 2015, 01:46 AM   #5
kevinkr is offline kevinkr  United States
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Re-biasing mic input for bass guitar
This really does belong in Instruments & Amps and will get a lot of more helpful responses there. I'll move it. Please note the report button which can be used to request mistakes like this be fixed.
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Old 4th June 2015, 01:49 AM   #6
kevinkr is offline kevinkr  United States
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Re-biasing mic input for bass guitar
Find a photofax or the original schematic of the amp.. Padding IMO is not a brilliant idea, ideally bypassing the mic pre-amp altogether might be better..

Bass guitar active or passive?
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Old 4th June 2015, 02:24 AM   #7
Civitello is offline Civitello  United States
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I would die for a schematic. That's my issue; I've been teaching myself audio electronics for a few months and although I'm starting to get it, I'm in no way comfortable with just modding circuitry on my own and without a schematic.

I want it to use for a P-bass. There is also an aux input, but I'd like to preserve that. I have some pictures of the amp posted in another thread.

Realistic - Album on Imgur

I have tons of pretty cool stuff in my parts bin. I could certainly build a nice solid state bass preamp, (I love transistor sound on bass), but I just don't know where to start.

I have a few Yamaha Pm-700 channel strips (although they use IC op amps) and I have a few other discrete transistor boards from the 70's that might make a handsome pre. Maybe untilize the Pm-700 input transformer somehow?

Thanks for your patience guys.
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Old 4th June 2015, 03:59 AM   #8
kevinkr is offline kevinkr  United States
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Re-biasing mic input for bass guitar
Not sure if you know about this, but there is a lot of information on converting a MPA-20 here: Realistic MPA-20 Solid state 40W P.A amp used for guitar
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Old 6th June 2015, 10:23 AM   #9
Civitello is offline Civitello  United States
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Also, does anyone known if I'll have any issues making my unit a standard 3-prong ground? I understand the germanium ouput transistors are PNP and positive ground rather than the usual negative ground. Will I encounter any issues keeping the hot and neutral the same and grounding to the chassis ground lug?

Thanks
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Old 6th June 2015, 02:49 PM   #10
JMFahey is offline JMFahey  Argentina
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By all means, ground your amp the proper way.

It's absolutely unrelated that the transistors are "positive or negative ground" which is an internal problem, handled by its own power supply.

In fact ground is ground, always the reference point and so neutral by definition; calling it negative or positive is inaccurate and an unfortunate heritage from old car wiring names.

It simply meant that the car battery negative or positive terminal was connected to car chassis, but I repeat ground is always neutral.

And chassis ground must always be connected to outlet ground, for safety.
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