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Posts: 3604
Location: Pacific Northwest Inland Empire | Anybody have a spare FET 3 laying around? I'm restoring a 1712 and it needs a replacement preamp. email: tweeterclark@juno.com
Thanks!! |
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Posts: 15654
Location: SoCal | Why not go to an OP 24 or beyond? |
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Location: Pacific Northwest Inland Empire | Because I'm cheap, Paul! This is a low-budget project. My funds are currently committed to the 1115 project at The Mothership.
For all you right-brainers out there:
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Location: Pacific Northwest Inland Empire | Fuhgitboutit. I'm puttin' in a different unit.....It....almost......fits.......There, that oughta do it. I figure, if the thing was shipped to The Mothership, they would have replaced the bowl anyway- I think the bulletholes are a nice touch. |
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Location: Pacific Northwest Inland Empire | Changed my mind, after the prospective preamp failed to function. I'm running straight off the piezo, into the cord jack and using the amp controls. I made a cover for the preamp opening as a temporary measure. So, I'm back in the market.
Hey, is there some kinda list of preamps, and body opening size, for interchangeability?. Yes, I realize this is a tempting subject for all the latent types out there. The present orifice is 1 7/16" X 2 13/16".
Actually, this critter has a pretty strong voice already, but what the hell- I'm puttin' in the electronics anyway! |
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Location: NJ | Chris;
I believe you're still gonna need an outboard preamp for impedance-matching. |
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Location: Pacific Northwest Inland Empire | OK, Cliff, so what happens if I continue this configuration? And, how do I determine when the impedance has been matched? |
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Location: NJ | While I've never really toyed with it, it's my understanding that the signal coming out of the pickup'll be way too "hot", and that the purpose of the preamp is to "temper" the signal (kinda like a regulator on a scuba tank). I would assume (and again, I'm not speaking frome experience), if you had an acoustic effects pedal, it should have a preamp circuit in it (my ART does), and that may suffice.
g'luck with the project. |
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Posts: 3604
Location: Pacific Northwest Inland Empire | Thanks, Cliff! I think I understand your analogy. I found this in a website info column, and I think it says the same thing. In a practical sense, I wonder if I put some electronic attenuation device, e.g., a resistor, in the line if that would accomplish the task. I would need to know the relative values of the piezo output and the input on the amplifier, though, right?
This is the material I am referencing:
Q3.2 - What is meant by "impedance matching"? How is it done? Why is it
necessary?
We can talk about the characteristic impedance of an input, which is to
say the ratio of voltage to current that it likes to see, or how much
it loads down a source. (You can think of this as being an "AC resistance"
and you would be mostly right, although it's actually the absolute
magnitude of the vector drawn by the resistive and reactive load
components. Dealing with line level signals, reactive components
are going to be negligible, though).
In general, in this modern world, most equipment has a low impedance
output, going into relatively high impedance input. This wastes some
amount of power, but because electricity is cheap and it's possible to
build low-Z outputs easily today, this is not a big deal.
With microphones, it _is_ a big deal, because the signal levels are
very low, and the drive ability poor. As a result, we try and get the
best efficiency possible from microphones to get the lowest noise
floor. This is often done by using transformers to step up the voltage
or step it down, to go into a higher or lower Z load. Transformers
have some major disadvantages in that they can be significant sources
of nonlinearity, but back in the days of tubes they were the only
solution. Tubes have a very high-Z input, and building balanced inputs
with tubes requires three devices instead of one. As a result, all
mike preamps would have a 600 ohm balanced input, with a transformer,
driving a preamp tube. Today, transistor circuits can be used for
impedance matching, although they are often more costly and can be noisier
in cases.
As a result of the expense, consumer equipment was built with high-Z
microphone inputs, and high-Z microphones. This resulted in more noise
pickup problems, but was cheaper to make. Unfortunately this still
held on into the modern day of the transistor, and a lot of high-Z
consumer gear exists. Guitar pickups are generally high-Z devices,
and require a direct box to reduce the impedance so that they can go into
a standard 600 ohm mike preamp directly.
Many years ago, the techniques that were used in audio came originally
from telephone company practice. Phone systems operate with 150 or 600
ohm balanced lines, and adoption of this practice into the audio industry
caused those standards to be used. In the modern age where lines are
relatively short and transformers considered problematic, the tendency
has been to have low-Z outputs for all line level devices, driving
high-Z inputs. While this is not the most efficient system, it is relatively
foolproof, and appears on most consumer equipment. A substantial amount of
professional gear, however, still uses internal balancing transformers or
resistor networks to match to a perfect 600 ohm impedance. [Scott]
[Ed. note: Modern equipment works on principles of voltage transfer
rather than power transfer. Thus a standard audio circuit today is
essentially a glorified voltage divider. You have a very low output
impedance and a very high input impedance such that the most voltage
is dropped across the load. This is not an impedance-matched circuit
in the classic sense of the word. Rather, it is a "bridged" or
"constant voltage" impedance match, and is the paradigm on which
nearly all audio circuits operate nowadays. -Gabe] |
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Location: the BIG Metropolis of TR | Hey seesquare,
Could you repete that please.....I lost you right after you said: "Thanks,Cliff!" :D :confused: :rolleyes: :eek:
****
Mike :cool: |
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Posts: 3604
Location: Pacific Northwest Inland Empire | When you pull something out of the microwave; it's too hot to handle. So, the point here is to figure out how to cool down the signal before putting your mitts on it! |
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Posts: 3604
Location: Pacific Northwest Inland Empire | OK, Campers, reminiscent of "The Graduate"- ONE WORD: "Buffers". I have determined this is the requirement between a high-impedance piezo undersaddle transducer output and the relatively low-impedance amplifier input. I have figured out that my amp will probably not blow up with an unmatched impedance, but I will lose some signal from the pickup, for sure.
I'll keep you all posted on buffer circuit fabrication. This is a HIGHLY esoteric adventure, and it would certainly be cheaper just to purchase a preamp. Apparently, I'm a fan of self-flaggellation and mental masturbation. |
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FET 3 Preamp Wanted
