Question about the tc integrated pre / fortin grind design

[Rob Strand]

I would really like to know how this is called. Does anyone know?

I'm not sure it has a name.  It's really only a CE amp.   You also see two transistor differential amps preceding the opamp in a similar way.  It was a common technique to lower noise back in the days of noisy old opamps like LM741 and LM301A.

In the back of my mind I'm thinking compound amplifier but it's just making up a name for no good reason.

[nonoxxx]

Forin use TL071 , on my previous chuggapre builds I used opa134 and it sounded great I will try NJM2068 and NE5532  to see if it's better

[nonoxxx]

I would really like to know how this is called. Does anyone know?

I'm not sure it has a name.  It's really only a CE amp.   You also see two transistor differential amps preceding the opamp in a similar way.  It was a common technique to lower noise back in the days of noisy old opamps like LM741 and LM301A.

In the back of my mind I'm thinking compound amplifier but it's just making up a name for no good reason.
[/quote]

The chuggah pre without this transistor is really a quiet unit , I wander if it worth the effort to try it or not

[Fancy Lime]

I would really like to know how this is called. Does anyone know?

I'm not sure it has a name.  It's really only a CE amp.   You also see two transistor differential amps preceding the opamp in a similar way.  It was a common technique to lower noise back in the days of noisy old opamps like LM741 and LM301A.

In the back of my mind I'm thinking compound amplifier but it's just making up a name for no good reason.

The chuggah pre without this transistor is really a quiet unit , I wander if it worth the effort to try it or not
[/quote]

I think Rob is talking about the right transistor in the "Triangulum" schematic, called Q1, and nonoxxx is talking about the left one, called Q2, right?

Andy

[Fancy Lime]

I would really like to know how this is called. Does anyone know?

I'm not sure it has a name.  It's really only a CE amp.   You also see two transistor differential amps preceding the opamp in a similar way.  It was a common technique to lower noise back in the days of noisy old opamps like LM741 and LM301A.

In the back of my mind I'm thinking compound amplifier but it's just making up a name for no good reason.

It is certainly very interesting. Why and how does it lower noise, I don't understand though. Can you elaborate. Also: any idea how to properly bias the JFET version of this sort of thing. I feel a booster project crystallizing on the horizon... or maybe a hifi-ish 80's preamp?

Andy

[Rob Strand]

The chuggah pre without this transistor is really a quiet unit , I wander if it worth the effort to try it or not

Modern opamps probably won't gain much by adding the transistor.  Some low power opamps might be helped.   Using modern opamps is probably a better solution.   The added transistor is solving a problem which doesn't need to be solved these days (except in specific cases).

t is certainly very interesting. Why and how does it lower noise, I don't understand though. Can you elaborate.

There's two ways it reduces noise: the first is it provides gain before the opamp.  So if you have a gain of 6dB then the signal to noise would improve by upto 6dB.    The second is you can tune the bias current of the opamp to get the least noise for a particular source impedance.   I don't actually know if in that circuit the transistor helps at all.   It comes down to a lot of details.   The idea *can* work but it only works if it's done right.   

With guitar stuff JFET inputs are often a better choice since you don't have to tweak bias currents to match the guitar, and when you do that it's actually non-optimal when you drive it with a buffer or effect.

Also: any idea how to properly bias the JFET version of this sort of thing.

In principle you would have to connect the gate to yet another bias point, one lower than Vr.   I suspect it would take some fiddling around to get it to work and might need more fiddling or trimpots  to cope with JFET tolerances.  It's one of those thing you need to analyze to see where you stand.

[PRR]

> What is the purpose of the first Bc550 transtor on the left

Q2, I have NO idea.... one of many mysteries in that plan.

Q1 is a simple preamp. Historically the input transistors inside chips were just so-so. For a buck more you could put better transistor(s) in front to improve some aspect of performance.

The ancient '709 opamp had many good features, but was not low input current. The early datasheets showed using a pair of JFETs to fix this:


Here input current is no issue, but hiss may be. The later opamps, particularly '072, have little input devices which are low-hiss in high-Z circuits but hissy in low-Z circuits. Coming out of 150 Ohm microphones we now normally put a pair of BIG transistors in front, and then the hiss of the chip hardly matters. This "Green Preamp" plan is over-wrought with all features, but Q1 Q2 give a better hiss factor than U1 could in 150 Ohms.
http://mhumhirecords.org/DIYpages/Green/Schematic.gif

But here Q1 is run at very low current. While this suits a hi-Z guitar input, the fat BC550 is not an obvious choice for very low current. In fact in this zone it will hardly be better than a good TL072.

The DC biasing is quite odd, but a (simplified) sim shows it works out OK:

[Rob Strand]

Some funky examples,
http://www.johnhroberts.com/p-10_art.pdf
http://www.johnhroberts.com/P10.gif

[Vivek]

Ultimately you tweak the Vref for maximum signal swing.  However in this circuit there is a need to make Vref higher than vcc/2 so the transistor operates correctly.

Suppose maximum swing occurs when the opamp sits at vcc/2.   Since the transistor has an emitter resistor, and emitter current, the emitter voltage must sit higher than vcc/2.   Because the opamp's input is connected to Vref the collector is sits at Vref. So the voltage drop across the collector resistor is (Vcc-Vref).  Since the emitter resistor is twice the collector resistor the emitter must sit at Ve = Vcc/2 + 2*(Vcc - Vref).  In order for the transistor to operate the collector must be higher than the emitter,
       Vc  > Ve
       Vref  >  Vcc/2 + 2*(Vcc - Vref)
So,     3Vref >  2.5 Vcc
or          Vref > (2.5/3) Vcc
or          Vref  > 0.83 Vcc

The other thing we need to be certain about is that a base current can flow when we account for the Vbe drop and the drop across the base resistor.   I'm not going through all the calculations.

The main point is Vref needs to be pretty high for the circuit to work; more specifically the transistor.

Sorry for resurrecting an old post !!!

about 2 months ago, I entered the schematic of the TC Intergrated preamp into LT SPICE

It does not seem to work if the IC receives less than 15 V power supply.

But surely it is possible to design a circuit with 9 V power supply and expect at least 2Vpp signals max out of it.

It seems to be a great puzzle on how to bias the IC out to 4.5V with a 9 volt supply. I tried XL spreadsheet goal seeking and I tried tweaking / stepping resistors on SPICE, but found it too difficult.

When I tried to analyse the circuit, I came to same conclusions as Rob Strand, but he stopped at the point where things started to get really interesting:  "The other thing we need to be certain about is that a base current can flow when we account for the Vbe drop and the drop across the base resistor. I'm not going through all the calculations."


Can anyone redesign this circuit for IC output biased at 4.5V when driven by 9V power supply ?

Or conversely prove that it is not possible.

[m4268588]

From https://www.diystompboxes.com/smfforum/index.php?topic=128429.msg1237537#msg1237537

Biasing on the TC Electronics Integrated Preamp at different supply voltages

Inexplicable simulations.

Don't hide any elements.

Code Select Expand


Version 4
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WINDOW 3 21 64 Right 2
SYMATTR Value {VCC}
SYMATTR InstName V1
SYMBOL voltage -32 -160 R0
WINDOW 3 27 64 Right 2
SYMATTR Value {Vref}
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SYMATTR Value 2N3904
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WINDOW 0 18 48 Left 2
WINDOW 3 -30 64 Left 2
SYMATTR InstName U1
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SYMATTR InstName R1
SYMATTR Value 6.8Meg
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SYMATTR Value 220k
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TEXT 0 40 Left 2 !.OP\n \n*.STEP Param CASE List 0 3 1\n.Param CASE 2\n.Param VCC=tbl(CASE, 0,9V, 1,15V, 2,20V, 3,32V)\n.Param Vref=tbl{CASE, 0,6.5V, 1,10.3V, 2,15.4V, 3,26.6V}\n.Param R101=tbl(CASE, 0,296.89764K, 1,264.45579k, 2,434.0448k, 3,627.99043k)


[m4268588]

Following just a pedantry.

Vref = ((0.65 + Vcc/2) + 2.2*Vcc) / 3.2
Must be within the "Common mode input voltage range" of OP-Amp.

-----------------------------
Vc = Vb(=Vref)
Vbe = 0.65
(Approximately equal.)

Ie = (Ve - Vcc/2) / 220k (=Ic)
100k * Ic = Vcc - Vc

100k * (Ve - Vcc/2) / 220k = Vcc - Vc
100k * (Vc-0.65-Vcc/2) / 220k = Vcc - Vc
100k*(0.65+Vcc/2) + 220k*Vcc  = 100k*Vc + 220k*Vc

[ElectricDruid]

Sorry m4268588, what's the point you're making here?