simple active tone control

[Lino22]

Guys is there a tone control (apart from well known Baxandall/James) i can stick in a BJT feetback so it becomes active? Can you rework the TS-808 tone control to work with transistors? Or BMP tone stack?
Just courious

[Rob Strand]

There's many ways to make a BJT based Baxandall,





You can use any of the variations of the Bass and Treble control circuits, just be careful of DC issues.

The TS9 tone control equivalent is something like a transistor version of the Ampeg Bass amp mid control (model V4 etc).  It's not quite the same as the output impedance isn't low until you add an output buffer.

I should add you can of course create a discrete opamp and use it in place of most opamps.  The above examples are more BJT orientated designs.

[Lino22]

Rob, thanks, but i know Baxandall (in Overdriver for example). I was courious if there is another variant of a BJT feedback tone control.

I would experiment myself, but such a control has to have 3 poles - in, out and the one connected to the feedback input, and that confuses me.

[antonis]

A Tilt EQ perhaps..??

Look at circut from C8 and right-hand till C11..
(Q7 buffer can be omitted)

[Lino22]

Nice, thank you

Are you planning to rework TS-808 to discrete BJTs?

Would Colorsound Overdriver benefit from adding a buffer stage and leading the feedback from its output instead of what is it now? Or maybe just a different Baxandal? I never liked the bass control there, it seems to be just very mild.

[antonis]

Are you planning to rework TS-808 to discrete BJTs?

Cant' see any reason for doing it, other than pure fun..

Would Colorsound Overdriver benefit from adding a buffer stage and leading the feedback from its output instead of what is it now?

Brute answer is YES..!!
(especially for Bass control..)

Could you guess the reason for this..??
Hint: Look at Q2/Q3 output impedances in conjunction to Baxandall respective ones and Q3 open and closed loop gain efficiency..

[antonis]

Something like this, perhaps: 



P.S.
R15 & R16 values can be lowered down to whatever current can be carried out according to Q5 & Q6 power handling capability and power supply rating..
(50mA should be more than adequate for monster-drive buffers so 100Ω should be the lower limit..)

[Lino22]

Antonis thank you very much! I was gonna play with the Baxandal values but the impedances eplain it 

[Lino22]

One more question considering the bias and RC/RE resistor values in the Q3 stage. I know we use certain ratios like R11 should be at least 3-10x bigger than R12, RC/RE sets the gain, but how to we decide on their real values?
Do we want them smaller for more current through the transversal paths, for some reason, or bigger to save on the current draw ... ?

[antonis]

[antonis]

One more question considering the bias and RC/RE resistor values in the Q3 stage. I know we use certain ratios like R11 should be at least 3-10x bigger than R12, RC/RE sets the gain, but how to we decide on their real values?
Do we want them smaller for more current through the transversal paths, for some reason, or bigger to save on the current draw ... ?

You can consider R11 & R12 values as a compromise between step 6. in chart above and stage input impedance value ( R11 // R12 // h_FE X 0.025 / I_Collector)..

[POTL]

you need a circuit in which you plan to install an equalizer. is this a pure booster? is this a preamp? is this fuzz? is this overdrive? is this distortion? pre-eq or post? what result is required? There are many equalizers and they work very differently, it all depends on the purpose and place of use.

[Rob Strand]

One more question considering the bias and RC/RE resistor values in the Q3 stage. I know we use certain ratios like R11 should be at least 3-10x bigger than R12, RC/RE sets the gain, but how to we decide on their real values?
Do we want them smaller for more current through the transversal paths, for some reason, or bigger to save on the current draw ... ?

If you are using a stage with feedback tone controls you want the stage to have a high gain (open loop gain), otherwise the response will not be like an opamp.   Normally you have a low emitter resistor or a bypassed emitter resistor.  You can see that in the two examples I posted.  The twin 25 circuit goes further because it bootstraps the collector to get more open loop gain.   There's nothing stopping you using feedback around a lower gain stage but you would need to check the response was OK because it would deviate from the ideal response you would get from an opamp circuit.

[Lino22]

@Rob: Rob what is the purpose of the 330k(?) resistor marked by a star at Playmaster 125?

[antonis]

The twin 25 circuit goes further because it bootstraps the collector to get more open loop gain.

Rob, I think the way EQ is fed back (not directly from T5 Emitter) might confuse OP..

[Lino22]

What confuses me is the cap between T4c and T5e ...

[antonis]

What confuses me is the cap between T4c and T5e ...

If you're talking about 10μF red circled cap, Rob told you above about T4 Collector resistor(s) bootstrapping..



In brief:
T5 Collector resistor of 20k is spilt into two equal values resistors with the lower one bootstrapped from T5 Emitter via 10μF capacitor resulting into this resistor high apparent value..
(a detailed formula for precise bootstapped value can be quoted but it's rather complicated and practically of no use.. here should be about x200, strongly dependent on Balance pot setting..)
10k/10k junction "follows" T4 Collector voltage swing (via T5) so (ideally) there isn't any voltage difference across that resistor, hence no current flowing through it..
That calls for a resistor of infinite value..!!, hence T4 stage infinite gain..!! 

P.S.1
In practice, due to T5 Emitter follower gain less than unity, there is a voltage difference across that resistor so its apparent value is lower than infinite... 

P.S.2
T4 Collector resistor total value could be split unequally, with the lower resistor of value much higher than the upper one, hence even higher apparent value (bootstrapped) but in such a case the upper resistor should significantly load T5 Emitter (consider it in parallel with 1k8+220Ω)..

[Lino22]

That's another good trick. Thank you for explainingf it to me.

[Rob Strand]

@Rob: Rob what is the purpose of the 330k(?) resistor marked by a star at Playmaster 125?

I wouldn't worry too much about those.   They are there to tweak the input sensitivity of the amplifier.   Basically a bit of DIY tweaking of the gain of the first stage.   Not really much to do with the tone control part, even though it messes with the bias of the tone control stage a bit.

[Rob Strand]

n brief:
T5 Collector resistor of 20k is spilt into two equal values resistors with the lower one bootstrapped from T5 Emitter via 10μF capacitor resulting into this resistor high apparent value..

The 820k feedback resistor used or biasing probably screws up the behaviour of the Baxandall tone control a bit.   Not sure how much as it's probably 30 years ago since I analyzed that circuit in detail.