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
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
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.
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.
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.
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.
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.
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.
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.
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
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
Thanks
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.
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.
Not entirely true - only under certain specific circumstances, and for class I devices.
Adding an earth to an item that was designed specifically without it should only be done if you understand fully and exactly what you're doing, and why.
Seconding JM's advice. Given the vintage of this amplifier and its maker it MUST be grounded for safety. It is never a bad idea to add earth ground for safety reasons regardless of the insulation class of the device discussed, the only issue of concern is ground loops which can be effectively dealt with.
Again, I would disagree - earthing isn't automatically safe - it makes a device 'more safe' under certain specific conditions (and likewise can make it 'less safe' under other specific conditions).
My problem isn't with with earthing this particular unit, which 'probably' won't make it any less safe, and 'may' make it more so - but with the blanket statement that earthing anything automatically makes it safer
Earthing of a class II device will almost certainly make it 'unsafe' to use, as you are removing it's legal approval - and it would certainly fail any PAT (Portable Appliance Test) in the UK, both visually and under electrical testing.
Dear Nigel, a guitar amplifier where the player is connected to it by a copper cable (not a radio wireless ) IS a Class 1 device, by definition.
The player IS touching CHASSIS GROUND which so MUST be directly grounded.
This "item" was designed specifically without it just because it was designed 30 years ago, where it was not strictly mandatory yet.
In fact I remember US made equipment already using grounded cords, whike Japanese stuff such as Roland still shipped with 2 prong cords.
It may have been tolerated for a couple years until all old stuff was sold, but today it's mandatory.
This amp might very well have been built during that last "tolerance" period, it definitely does not make it any safer and must be properly grounded.
In fact, any serious Service Tech will add 3 wire grounding to equipment he services or he may be sued as co-responsible if an accident happens, because he can not claim ignorance like a Customer would.
FWIW this causes endless strife between Techs and "vintage equipment" owners who complain their "modded" stuff lost collector value. No kidding.
You don't state those "specific conditions" so .......
Anyway, if problems arise, you can defend in Court modding a device to comply with official Safety rules, but it will be very difficult to argue the opposite.
Yours is also a blanket statement.
Again, an electric guitar amplifier is NOT a Class II device.
By the way, what legal approval?
A guitar amp will never ever receive a Class II certification, unless guitar audio gets there wireless
Now if its only input were, say, a Bluetooth receiver (with matching Bluetooth transmitter in the guitar) then it might do.
And that if it does not have earphone or speaker outputs or CD/MP3 inputs, because in that case we are back in square one.
Please look inside any modern amplifier and see by yourself the green/yellow wire firmly attached to its own grounding terminal.
Even using a transformer screw to attach it is frowned upon.
From: Appliance classes - Wikipedia, the free encyclopedia]Appliance classes - Wikipedia, the free encyclopedia
Class I
green/yellow ground
Class I symbol
These appliances must have their chassis connected to electrical earth (US: ground) by a separate earth conductor (coloured green/yellow in most countries, green in the US, Canada and Japan). The earth connection is achieved with a 3-conductor mains cable, typically ending with 3-prong AC connector which plugs into a corresponding AC outlet. The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that if a fault occurs the supply will be removed automatically (this is sometimes referred to as ADS = Automatic Disconnection of Supply)
A fault in the appliance which causes a live conductor to contact the casing (metallic chassis, not the wooden or plastic external cabinet) will cause a current to flow in the earth conductor. If large enough, this current will trip an over-current device (fuse or circuit breaker (CB)) and disconnect the supply . The disconnection time has to be fast enough not to allow fibrillation to start if a person is in contact with the casing at the time. This time and the current rating in turn sets a maximum earth resistance permissible. To provide supplementary protection against high-impedance faults it is common to recommend a residual-current device (RCD) also known as a residual current circuit breaker (RCCB), ground fault circuit interrupter (GFCI), or residual current operated circuit-breaker with integral over-current protection (RCBO), which will cut off the supply of electricity to the appliance if the currents in the two poles of the supply are not equal and opposite.
To boot, on the back panel you'll often see the Class 1 device symbol ...
An externally hosted image should be here but it was not working when we last tested it.
and if they didn't print it rest assured that it still is Class 1.
A guitar amp is not and never will be a Class 2 device, which do not require ground and protect the user by a double layer or double thickness insulated cover and have NO electrically connected metal parts which can be touched by the user which in an amp is the chassis and the input jacks.
The symbol, which you will see in electric drills, coffee makers and such, even TVs, is:
An externally hosted image should be here but it was not working when we last tested it.
A typical guitar amp green ground wire straight to its own chassis terminal:
That's an EXTREMELY dodgy place to be going
- I fully agree that not earthing the equipment 'could' mean the engineer would be liable, but also by modifying it he's making it fail any safety tests, unless he has it officially tested and reclassified for use as a class II appliance.
I think JM has adequately demonstrated his knowledge which is quite consistent with what little I know.
The internet and indeed the press are rife with stories of people inadvertently getting electrocuted by ungrounded guitar amplifiers, and PA gear. Even the few people I know who work in officially regulated compliance test labs under the auspices of UL, CSA, BSI or VDE will generally recommend that these sorts of vintage devices be reworked to add a safety ground when encountered.
Something to note here in North America is that if there is no UL or CSA sticker on the device it is not likely to have been safety tested at all. In those days (50 yrs ago) there were not necessarily any safety requirements to be met for sale in most jurisdictions. No Dyna amp or Dynakit, Heathkit, Scott Kit, or Fisher Kit sold that I have ever seen had a UL sticker on it for example. In those days connecting what we now call a "death cap" between one of the mains connections and the chassis was common practice - since unpolarized plugs were the norm you had no idea whether or not the cap was on neutral to chassis or hot to chassis. Paper dielectric caps were common here. Fuses were not always fitted.. I have seen the same sorts of things in some UK, Swiss and German made gear as well so it must have been common. Many devices that would be considered unsafe by today's standards had UL or CSA approval, and would not meet today's requirements for safety.. I suspect not fitting a grounding cable set here in the U.S. would make you more liable than not, as it could be regarded as negligent since it is considered good practice here.
The internet and indeed the press are rife with stories of people inadvertently getting electrocuted by ungrounded guitar amplifiers, and PA gear. Even the few people I know who work in officially regulated compliance test labs under the auspices of UL, CSA, BSI or VDE will generally recommend that these sorts of vintage devices be reworked to add a safety ground when encountered.
Something to note here in North America is that if there is no UL or CSA sticker on the device it is not likely to have been safety tested at all. In those days (50 yrs ago) there were not necessarily any safety requirements to be met for sale in most jurisdictions. No Dyna amp or Dynakit, Heathkit, Scott Kit, or Fisher Kit sold that I have ever seen had a UL sticker on it for example. In those days connecting what we now call a "death cap" between one of the mains connections and the chassis was common practice - since unpolarized plugs were the norm you had no idea whether or not the cap was on neutral to chassis or hot to chassis. Paper dielectric caps were common here. Fuses were not always fitted.. I have seen the same sorts of things in some UK, Swiss and German made gear as well so it must have been common. Many devices that would be considered unsafe by today's standards had UL or CSA approval, and would not meet today's requirements for safety.. I suspect not fitting a grounding cable set here in the U.S. would make you more liable than not, as it could be regarded as negligent since it is considered good practice here.
Nobody's mentioned that you should also be wary of if your amp can handle bass at all. A lot of small PA amps are actually targeted at public address systems for small business, etc. I have one such amp myself; it has a 150Hz cutoff. You mention a crackly sound, but is bass OK through a mic?
So here's what bass sounds like:
If I keep the mic input gain to ~3 (out of 10) and I play the bass with my normal dynamic level it sounds pretty good. But when I dig in with my right and hand play the bass a little harder (as you would at certain points in a song) that's when it becomes obvious I'm over the saturation point of the (I assume input) transistor.
Electric guitar sounds great -- really great in fact -- through the mic input. But there are two (identical) mic inputs and I was just wondering if anything could be done to make the preamp section of one of those preamps better suited to bass. At this point, I'm thinking it might be best to just build a small little bass-voiced preamp from parts I have sitting around. But before I went this route, I was just curious if there was anyway to modify the existing mic preamp to better suit bass guitar... but maybe bass through germanium OP transformers just isn't something that will sound good. I should inject an external preamp into the amp's aux as a test.
Thanks for all your help guys!
Indeed, you can use input jacks with switch contacts to make one jack high-gain and the other lower. There are really two issues to consider: input sensitivity and input impedance. The impedance is probably high enough, and a series resistor would reduce the gain before significantly affecting the impedance anyway.
The real question is noise. What happens if you turn down the volume on your bass guitar? You want what's called an "input level pad" attenuator, but for unbalanced high-impedance...it boils down to whether you really want just a series resistor, or an L-pad or T-pad... A potentiometer with a knob or one with a screwdriver-slot might work fine. You could make a short cord with a plug and jack, with the resistor in between for experimentation. If it's noisy, try a large resistor to ground right at the jack. You can find nomographs to find the exact values for input pads without doing the math, but trying it works OK for me.
Of course, as was stated, bypassing a stage might be better (quieter, better fidelity) than making gain and wasting it. But, your description sounds like it's close to where you want it already, so just burn off a little. I'd experiment with 5K or 10K in series and see if that's too much or too little. Have fun.
The real question is noise. What happens if you turn down the volume on your bass guitar? You want what's called an "input level pad" attenuator, but for unbalanced high-impedance...it boils down to whether you really want just a series resistor, or an L-pad or T-pad... A potentiometer with a knob or one with a screwdriver-slot might work fine. You could make a short cord with a plug and jack, with the resistor in between for experimentation. If it's noisy, try a large resistor to ground right at the jack. You can find nomographs to find the exact values for input pads without doing the math, but trying it works OK for me.
Of course, as was stated, bypassing a stage might be better (quieter, better fidelity) than making gain and wasting it. But, your description sounds like it's close to where you want it already, so just burn off a little. I'd experiment with 5K or 10K in series and see if that's too much or too little. Have fun.
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