Simple impedance converter... Guitar -> 10Kohm Line input

[T-Dogg]

I'd like to make a simple impedance converter to interface my guitar with the line input of a preamp (and subsequently feed into a virtual software amp sim program) using one of the following I have floating around at home:

--some old russian germanium transistors...
--a bunch of NE5532's...

My thinking was a simple buffer will do the trick, no need for a more complex active circuit or full-blown DI.  I know germaniums aren't terribly stable, and are probably noisy, etc...  But I got a bunch of them, so why not put em to use here...  do you think the simplicity of a single transistor in an emitter-follower configuration might be just the ticket?  The less is more approach, versus pushing the signal through all the internal voodoo of an opamp?  I feel like I can live with whatever self noise the tranny has if that's the main issue, as it's still quieter than any single tube stage?    

Haven't thought it through much yet, but if I carefully bias the tranny with a split supply I'm thinking I should theoretically be able to avoid a blocking capacitor or transformer as well, right?   or is that just bad design?

Tim

 

[Gus]

Tim

Balanced out?

Something like this?  I have not built it yet.  Bootstrapped input for OK input resistance and balanced out.

http://twin-x.com/groupdiy/displayimage.php?album=random&cat=10055&pos=-3274

Call if you want.

Gus

 

[T-Dogg]

Perfect -- thanks Gus!  Land-line-based auditory PM on the way

 

[Bo Hansen]

T-Dogg,

Take a look on my "simple FET impedance converter" here:
www.hansenaudio.se/techpage.htm

--Bo

 

[T-Dogg]

Thanks Bo -- just checked it out, nice simple design!  Don't have any FET's to play around with at the moment, but i'll see if i can scare one up.  I definately like the balanced nature of Gus' schematic, but with my (very) elementary understanding of electronics, I'm still trying to wrap my head around how the "bootstrapping" concept and differential signals are working in that one (specifically the feedback path form the emitter to R3/R4 node?)

btw -- PM box is full gus  

 

[fazer]

Gus ,  I have to try that 1 transistor circuit for a simple DI.  I suppose you could use a Fet in front for a higher impedance with some gain if needed(mosfet booster).  I have an API 2488 and have been thinking about direct in's to the mic preamps for Synth/Samplers  (Hardware based).  The input impedance should be fine just the way it is for that. 

 

[recnsci]

T-Dogg,

bootstrapping (references to Gus schematic): voltage at Q1 emiter is (aprox.) the same as base voltage. So current trough R3 is (Vb-Ve)/R3 in audio band, thanks to C1. That current is much smaller than without C1, so R3 "apears" larger than it is. C1 performs bootstrapping, and preformance of bootstrapping depends on how close gain at emiter is to unity gain. What is left unaccounted for is base input impedance which could be as low as 200k (a tad too small for guitar p'ups)  with emiter ressistance this small. Also, Ic is set way to high IMHO. Noise figure will go trough the roof for typical p'up impedances. Finaly, this circuit doesn't have "true" balanced output, since impedance at emiter is much smaller than at colector, so you will have impedance disbalnce (output range will also be reduced, but that is not of paramount importance i you plug only guitars in this). JFET follower is good idea, but you must be carefull that FET has small enough gate capacitance not to mess with p'up resonance (for instance 2SK170 has 30pF of gate C, which could be too high for some p'ups).

If you really want to go with BJTs, I would use complementary feedback pair, with something like 2SA1455 at input set to Ic~20uA, and with simple impedance balanced output (that is just with balanced impedances and no differential driver).

cheerz
urosh

 

[Gus]

recnsci

Good to read someone looked at the circuit and noted the "faults".  You make good points.  There are reasons for why the circuit is drawn that way.  I often will post but not give the why I did something.  This is for guitar

What can one do with one transistor?

If a passive guitar pickup is loaded it often tend to lose high end and this circuit is for a simple DI with no other frequency shaping, You might not want that going into a guitar amp but going into a board or PC EQ can make a difference.

470 ohms were picked for a 9VDC supply, like a regulated external guitar effect power supply and I do not know what the circuit will be connected to.  Some extra noise might not be a problem with single coil pickups.

It can be fun to drop different hfe transistors in a circuit like this, for "better" ones like a MPSA18,2n5088 etc

Yes it is not a perfect balance DI but I assumed it would be used for guitar and close to the input of the board or PC

If clean and more perfect I would use a nice DI that can be all kinds of circuits like what BO posted or what you posted or a nice opamp circuit.

 

[sahib]

What is left unaccounted for is base input impedance which could be as low as 200k (a tad too small for guitar p'ups)  with emiter ressistance this small. Also, Ic is set way to high IMHO.

The effective input impedance works out roughly;

[R5xR4xR2 / R5+R+R2] x  Beta.

I took the beta as 200 and it works out around 12MOhm.

 

[recnsci]

[R5xR4xR2 / R5+R+R2] x  Beta.

That's one strange equation, unit of result is squred ohms. Maybe (R2xR4xR5/(R2xR4+R2xR5+R4xR5))xBeta ?




Gus,
yeah, I know. What I wrote whas nitpicking. Further more, ckt you posted will 100% work for OP, if he dropped CFP on bredboard it would probably sing (and not in a way he wants it). Me personaly, I would ditch that collector output, put some resistor in series with emiter output and be done with it.

 

[sahib]

I checked it again if I was remembering wrong but it seems right. Source is Solid State Circuits by Mannie Horowitz. However, since you aroused my suspicion I fiddled with other equations and without cluttering the post the effective resistance that the source sees is R3/1-Av. Take Av as 0.995 and this time it is working out 4.4Mohm. The problem might be far too many equations in this business but whichever way we look at it the impedance seen is working out sufficiently high for a guitar pickup. Well, at least on the paper.

 

[recnsci]

the effective resistance that the source sees is R3/1-Av.

I think that is "effective" impedance source sees insted of R3, but there is still base impedance (which is even a bit reduced). At least that is my interpretation of what bootsttrapping does, and I could be wrong. OTOH, that equation seems off since it is ohmXohmXohm/ohm, so result unit is ohmXohm. I've came up with R3llRpi + (hfe+1)xR2llR4llR5

 

[sahib]

You have already mentioned that the idea of bootstrapping is to make R3 appear much larger than what it is and this is correct. Remove the biasing network and what you have is beta times R2 and this can be taken as 200K with a beta of just over 400 as you indicated. But the source sees the whole shebang.

In terms of the equation Vout = Av x Vin. So the voltage accross R3 becomes  Vin - Av x Vin. Equally the current over R3 is V3 / R3. 

So the effective resistance is Vin/I3

= Vin/(1-Av) x Vin / R3

simplifies to R3/1-Av

Sounds o.k. to me but my wife is calling for bed now. I'll look up couple of references tomorrow and see if I can come up with other ways.

 

[PRR]

> around 12MOhm

There's an error there.

I get 80K. That's with a transistor of hFe nominal 400 but more like 160 at this operating point.

There's two impedances.

The transistor hss rE say 8 ohms and 470 in the emitter.

The 22K base bias is bootstrapped. The 8:470 split will give gain of 0.983 and boot 50 times higher as long as the bootstrap scheme holds up (it fails below 40Hz). Say 1Meg.

The 470 times hFE=200 is about 100K into the base, and that won't bootstrap away.

 

[sahib]

Deleted.

 

[recnsci]

That's with a transistor of hFe nominal 400 but more like 160 at this operating point.

How's that? Fairchild datasheet claims 500 at worst around 10mA of Ic.

There's two impedances.
...
The 470 times hFE=200 is about 100K into the base, and that won't bootstrap away.

Yeah, that's how it looks to me. Zin of base in paralel to whatever bootstrapping did to bias network impedance. For midaudio range I got that last equation by simply shorting cap in small signal equivalent circuit (otoh, I included only Rpi and controlled current source)

 

[recnsci]

rL = R2llR4llR5 = 61.5K

R2 is 470 ohm. rL is around 450 ohm. Ic is about 7-8 mA

 

[sahib]

Yes, I shouldn't have done it with the kids running around me. Bad timing. I deleted the whole thing.

 

[Gus]

Now this was drawn to drive an unknown, that is why the 470ohms.

One can change things increase the C and E resistors, increase the node to base resistor , increase the voltage divider resistor values, use a High Hfe transistor.

It will load the pickup as drawn and maybe you want that.