HiZ input for any preamp

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V

valerio

Member
Joined
Nov 7, 2005
Messages
18
Hello!
Is there any schematic about an active HiZ input ,trasformerless, to be connected in a mic preamp?
I found this two Jensen schematics:

http://www.jensen-transformers.com/as/as004.pdf

it could be perfect....but i guess that 1M input impedance it is not enough for any use...for example for an acoustig guitar.

The second:

http://www.jensen-transformers.com/as/as098.pdf

It is perfect in the input but it need a transformer to be connected in the mic input.

Is it possible to increase the input impedance of the first schematic or change the output stage of the second schematic?
Or if you have any other schematic to do that...

Thanks!!
Valerio
 
The first one could have a higher input impedance as well. The 2M (R1) can be removed (or replaced with a much higher value).
Then the input-Z is mainly determined by R2 // R3, which could be increased as well. More practical though is to keep these at their present values (or scale these both down by a factor of 10), connect a decoupling cap (47uF...) from the midpoint to ground, and insert say 10M between that midpoint (R2, R3 & the added cap on one side) and the rest (C2, Q1-gate on the other side).

Note though that both circuits you mentioned are quite alike, it's just the biasing for the JFET that's a little bit different. And the second one has some gain.
The first one connects to a gtr-amp (J4), so not a mic-pre either. It's convenient to use a transformer (or even required in case of eventual hum-issues), but maybe you can simple skip the TX and connect the micpre to a tap on R7 (orig) 10k.

The second circuit originally has an attenuation of roughly 25 dB in total:

+12 dB (the CFP / Sziklai-pair Q1, Q2) & 12:1 (the TX) & 6:1 (the pad), so you may want to 'mimic' that
for the tap on R7. The numbers fit nicely here for the second circuit: a 150 Ohms resistor does both give a nice source-impedance to the micpre and give a usable attenuation close to the original situation

In other words:

put a 150 Ohm resistor 'under' R7 (so the 150 is between R7 & circuit-ground now).
Skip R9, R10, R11 & the TX.
Wire your to-micpre-XLR like this:
* pin #2 to the tap of the inserted 150 and R7
* pin #3 to circuit-ground (the lower side of the 150 Ohm)
* pin #1 as shown in the Jensen-pdf


Regards,

Peter
 
I´m confused. Is indispensable to add a new circuit like this ones or is optional?
Well, those circuits will work as a Hiz input for any preamp..
But can´t be modified the actual resistors on the mic input stage to modify the Z input, or is needed something else that require an specific circuit fot the instrument input?
 
[quote author="lagoausente"]I´m confused. Is indispensable to add a new circuit like this ones or is optional?
Well, those circuits will work as a Hiz input for any preamp..
But can´t be modified the actual resistors on the mic input stage to modify the Z input, or is needed something else that require an specific circuit fot the instrument input?[/quote]
It depends on your mic-pre circuit. For mic-pre's like for instance the G9 it's very easy to add a Hi-Z input without adding additional circuitry. Hey, it even has already such an input. That method can be used for other alike pre's as well. But it won't work for say your few-discretes-and-an-opamp-type of mic-pre.

G9: http://www.gyraf.dk/gy_pd/g9/g9_sch.gif (J1-jack)

You skip the input-TX though, so eventual colour (...) from that one won't be there, while you will get it when adding the mentioned circuits above in from of your pre-amp.

Regards,

Peter
 
Would work on this one? http://www.akustische-kunst.org/audio/supergreen/supergreen_schematic.pdf

It has a switch below, R37 can be putted in paralell with R5, maybe could be changed the values to get a HiZ/Low Z input? or is any other reason why this type of pre can´t have the DI input without adding a new circuit?

Thanks a lot.
 
[quote author="lagoausente"]Would work on this one? http://www.akustische-kunst.org/audio/supergreen/supergreen_schematic.pdf

It has a switch below, R37 can be putted in paralell with R5, maybe could be changed the values to get a HiZ/Low Z input? or is any other reason why this type of pre can´t have the DI input without adding a new circuit?

Thanks a lot.[/quote]
Fiddling with those values could probably make it a 'Mid-Z' input but I'm not sure you could take it to 1M or higher (and still have not too much clutter hanging on it that increases capacitive-load).
Choice up to you: try it and you might be able to find a compromise between 'normal mic-pre' use and a Hi(gher)-Z mode. You'll learn the most from it, even if it doesn't work well. If you're just in need of a Hi-Z input that's more tried & tested then just put a FET & PNP in perfboard and be done with it.

Regards,

Peter
 
The phantom power resistors will limit the input impedance to less than 2*6k8. Even if you would switch these out as well there is still almost 6 nF across the input--you might calculate the low-pass cutoff-frequency you get with a 1M source impedance. And even if you remove this as well noise will be pretty bad for typical source impedances (the reason for this is somewhat complicated). So the answer is: no.

Samuel
 
[quote author="Samuel Groner"]The phantom power resistors will limit the input impedance to less than 2*6k8. Even if you would switch these out as well there is still almost 6 nF across the input--you might calculate the low-pass cutoff-frequency you get with a 1M source impedance. And even if you remove this as well noise will be pretty bad for typical source impedances (the reason for this is somewhat complicated). So the answer is: no.

Samuel[/quote]
Source impedance won't be 1M (assuming a passive bass or guitar), but 6 nF is indeed a lot more than the typ input capacitance of a 'Hi-Z' kind of input. Usually the capacitance of a 6 meter guitar-cable is the dominant capacitance 'at the input-node', but even for bad cable (200pF/meter) it's still far from that 6. So the PU-resonance will shift even lower, a task usually not desirable for a DI-kind of input.

Regards,

Peter
 
[quote author="lagoausente"]So the solution could be something like the Go Bewteen DI from JLM?
http://www.jlmaudio.com/JLMDI.htm[/quote]
Yes sure, just like it says already over there. It looks to be exactly what you're looking for.
But by the time the kit arrives from Austr you could have made quite a few of those discrete circuits though :wink:

Regards,

Peter
 
[quote author="Samuel Groner"]
Source impedance won't be 1M (assuming a passive bass or guitar).
Click to expand...
It's not resistive, [/quote]
Assuming a (perhaps too) simple equiv schematic of 'R, L & shunt-C', the magnitude of the impedance at the res-peak is equal to sqr(Q)*R, so resistive.

but the magnitude surely does come close to 1M: http://online.physics.uiuc.edu/courses/phys498pom/498emi_guitar_pickup_results.html

Samuel
Click to expand...
That's higher than I recalled but indeed. FWIW, the first Helmut Lemme book (Elektro Gitarren, Git.-Elektr., Teil 1, 1977) even mentions a PU that manages to come above that 1M at its PU-resonance frequency (big Gibson EB3 PU).

But it's all for a 'naked' PU.
Adding one or more pots & stuff to that (as most guitars/basses have), in practice the magnitude of the resulting source-Z (as seen by the preamp at the other end of the cable) will drop below that value.

Regards,

Peter
 
But by the time the kit arrives from Austr you could have made quite a few of those discrete circuits though Wink
Click to expand...

Maybe I could be starting to make an insolation box? My experiences with the pencil and acid, was not very good. But I don´t know if there is a pcb layout so... would must try.

I´m not sure uf understands both pdf. The circuit that starts on J3 is part of the DI circuit, or is just part of the mic pre itself?

On the second one, If I understand ok, the XMFR output goes to any of the other "to guitar circuit", and "to mic pre". Is on this one where I could skip the tx?
At this moment my skills are very limited, and in relation with the TX, I don´t know what on should I take, or how could I build it, really.
 
[quote author="lagoausente"]Maybe I could be starting to make an insolation box? My experiences with the pencil and acid, was not very good.[/quote]
No acid, just 'veroboard'/perfboard. If you have the parts already the JFET + PNP-circuit can be done fairly quickly, depending on skills & what's on TV.
 
[quote author="clintrubber"][quote author="lagoausente"]Maybe I could be starting to make an insolation box? My experiences with the pencil and acid, was not very good.[/quote]
No acid, just 'veroboard'/perfboard. If you have the parts already the JFET + PNP-circuit can be done fairly quickly, depending on skills & what's on TV.[/quote]

Sorry, I was editing while you was posting.
I really don´t watch TV. Maybe I could being playing guitar, but I´m spending much time now here. I´m Spanish, so the language not helps.
What´s veroboard/perfoboard? what do you mean? Edited: is a prototype boar, isn´t? I haven´t. Haven´t also the components.
Was you suggesting me to DIY instead of buying the kit?

I´ll quote what I was editing in my previus post, and what I think you couldn´t read.
I´m not sure uf understands both pdf. The circuit that starts on J3 is part of the DI circuit, or is just part of the mic pre itself?

On the second one, If I understand ok, the XMFR output goes to any of the other "to guitar circuit", and "to mic pre". Is on this one where I could skip the tx?
Click to expand...
 
[quote author="SonsOfThunder"]http://www.groupdiy.com/index.php?topic=14150

Click the link that Brad gives on the Great River DI circuit.

HTH!
Charlie[/quote]

Could you explain where are switched the J4 when pluging the jack, and how works Q7?

http://www.avensonaudio.com/tech/MP-2NV%2520DI.jpg
 
Adding one or more pots & stuff to that (as most guitars/basses have), in practice the magnitude of the resulting source-Z (as seen by the preamp at the other end of the cable) will drop below that value.
Click to expand...
That makes sense. Do you have any data what the effective output impedance is?

Samuel
 
[quote author="Samuel Groner"]
Adding one or more pots & stuff to that (as most guitars/basses have), in practice the magnitude of the resulting source-Z (as seen by the preamp at the other end of the cable) will drop below that value.
Click to expand...
That makes sense. Do you have any data what the effective output impedance is?

Samuel[/quote]
I guess it'll be up to the pot-value(s) & their settings. What have we... vol-pots of 500k, 250k etc ?
(I don't recall 1M-pots, but who knows)

Assuming most people have the vol-pot fully CW (and ignoring tone-pots for now) the resulting numbers will be 5...8...15 kOhm // the pot-value // cable-capacitance (below resonance) and the 500k...1M // the pot-value // cable-capacitance (at resonance). So below resonance the output-Z remains fairly unaffected but the pot lowers the res-peak and the cable-capacitance shifts the res-peak down in frequency.

The Lemme book presents various circuits to tweak this response in a controlled fashion; in essence an output-buffer inside the instrument preceeded by switchable caps & also a Q-control IIRIC. He then goes on to make the position of the pickups variable as well (on rails) and you'll understand you want to keep your own guitars well away from him :cool:

No 'circuit-values', but let's add it anyway:
http://www.muzique.com/lab/pickups.htm

BTW, see you soon - off for some hiking in CH :wink:

Regards,

Peter
 
[quote author="lagoausente"]What´s veroboard/perfoboard? what do you mean? Edited: is a prototype boar, isn´t? I haven´t. Haven´t also the components.[/quote]
Yep, protoboard, a search for those words will give some more info.

Was you suggesting me to DIY instead of buying the kit?
Click to expand...
In fact, yes, but it was just a suggestion. You can of course determine best what suites you the most.

Enjoy & good luck building the one you choose,

Peter
 

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