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Transistor pair question


New Member
Hi, is that a Sziklai pair configuration?

I works as a low shelving filter (exaclty what I need).
How can I adjust the gain? If possible, to go bellow gain 1 for the filter attenuated frequencies.
Example range - gain 1 for the attenuated frequencies to gain 1 for the boosted frequencies.

I am also trying to set it for minimum consuption (total of 0,2 mA if possible).
On the simulator it seems it needs to be at least 0.4mA, to drive 50k load.
(3.2 v p-p, 50 hz signal)
But, I am just wiggling with the simulator, not sure what I do. Here it is the simulated circuit:


Is there any way have a headroom for more than 3,2 v p-p input signal?
- apart from lowering the total gain of the circuit


Well-Known Member
Most Helpful Member
Hi, is that a Sziklai pair configuration?
It's similar, but not a true Szlaki pair as there are other components within it, changing how it works.

I think that reducing the value of R19 and increasing C15 in the same proportion should reduce the gain, but also affect the filter attenuation; the resistor sets the gain at low frequencies, and the capacitor acts to bypass that at high frequencies.

The simplest way to control the overall level would be attenuate the input to compensate for the gain.

Nigel Goodwin

Super Moderator
Most Helpful Member
It's basically a VERY old preamp circuit, probably based on an old Mullard application note? - the feedback was often switched to alter gain, and to provide RIAA, tape or flat feedback.


New Member
Dont know how to strat proper design. Setting operation mode for Q1, or Q2?
Or, given I also look for minimal consumption, setting the min output current to drive 50k load?
The gain for bass could be played with changing some values. I guess +3 to +9 db is needed.

Gain 1 for the mids and highs should be ok. Attenuating them might result in poor signal to noise.
There is an EQ for trebble (9v powered) and it allready starts to make noise, when turned all the way up.
But it does not clip, even with old batery (less than 7v).

The bass boost is indeed to 'flatten' the source signal, so that the next EQ operates at proper range.
Now the bass is just OK when turned all the way up.
Turning down makes completely useless setting. Mid position is bass attenuated.


Well-Known Member
Most Helpful Member
Is there any way have a headroom for more than 3,2 v p-p input signal?
most preamps follow the defacto standard of 1.0 to 1.5Vrms for line level signals.... this is usually the reference level for full output power from power amplifiers as well... it hasn't always been this way, but it has been fairly standardized since the 1970s or even the 1960s. i'm pretty sure it was an IHF (Institute of High Fidelity) standard (now CEA, Consumer Electronics Association).
Last edited:


Well-Known Member
Most Helpful Member
Why do you need bass boost? Is the speaker too small or is not in a proper enclosure?


New Member
This is going to be ad-hoc correction for a bass guitar onboard EQ.
Dont really know the input levels (and cannot measure them).
But in general, it could easily go above 2-3 v peak, for some guitar pickups.
I hope it could manage gain of 2 for the bass.

I found out how to calc the most resistors, except R17.

It works in a pretty wide range in sumulator - f.ex. 22k-470k, so I dont know what to put there :D

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