TBG Schematics? - Reverse engineered

[Nora]

Does anyone have the schematics for the TBG E610F2A stereo eq module? Can't find them anywhere.
It's a dual 3 band eq with rotary switches and inductor based mid band.
I have one unit racked up and it sounds amazing, especially the low and high bands.

I'd like to see how it works. As far as I can see, all transistors and single voltage psu.
Possibly a good candidate for cloning. The highs and lows require no special parts (like inductors) and they are very sweet.
The midrange has only a limited range (0,7 to 4K), sounds nice but not always that useful. I'd like it to go higher, so with a few caps and other inductor it might be amazing!

I could reverse-engineer it. But I thought I'd better check here first though.
Any other tbg eq schematic might be a good start for reverse engineering.

thanks!

 

[Nora]

If anyone is interested, here's the schematic for these nice little eq's...

Untested in real life, might have some faults, but seems to work in Spice.
The rotary for C and L switching is just symbolic, no values displayed (have to unsolder to read the caps...)

The boost/cut are also rotary switches but shown as pots in schematic.
20K for high and low, 5k CT for mids



To be honest, I haven't met too many all-transistor eq's, so this is pretty new to me.
I'm slowly starting to understand how this works. 
Any comments, suggestions for 'improvement' ?

It seems to handle 600R load trough a 1:1 transformer, but since it's relatively low psu voltage (+21V) headroom might be a problem (?)
I don't think it can drive a 1:2 on the output. How about increasing the 21volts?`
Any consequences as long as the transistors can handle it?
How do you calculate the max voltage swing from a "dual emitter follower" output stage like this

I have no idea what I'm going to do with it, I'm considering to make a board with a preamp or maybe add another freq band and use as eq-only. This thing sounds really.


Pictures of the racked module:





I like the switches, feel great too...

 

[abbey road d enfer]

Any comments, suggestions for 'improvement' ?

It seems to handle 600R load trough a 1:1 transformer, but since it's relatively low psu voltage (+21V) headroom might be a problem (?)
I don't think it can drive a 1:2 on the output. How about increasing the 21volts?`
Any consequences as long as the transistors can handle it?
How do you calculate the max voltage swing from a "dual emitter follower" output stage like this

The output capability depends very much on the load. Particularly if the resistor R1 is 470R. If this resistor was smaller, this output stage is relatively capable of delivering some current (about +16dBu into 300R, +18 into 600R), but with the resistor, there will be as much voltage lost in it than available to the load. Reducing it (making it 47R, as R12 for the 1st stage) would help, but anyway it is a single gain-stage opamp, and trying to load it will reveal some THD.
And definitely, loading it with a 1:2 xfmr is not a good idea.
I think your idea of increasing the supply voltage is worth investigating; you may have to rebias the FET, but there's already a trimmer so it's easy.
I don't see any reason why you could not increase the voltage to 30V. That would give a max output level comparable to most equipment powered with +/- 15V rails.
Beware that the output stage is not protected, though.
OK, how to calculate?
The unloaded output swing is rails voltage minus 2Vbe
Rais voltage is 21V minus R1.quiescent current
Quiescent current is very hard to determine here because it's governed by the difference between Vbe of transistors and Vd of diodes.
An estimate of 10mA, would make the rail voltage 21V - 4.7V, and output swing 21 - 4.7 -1.4 = 14.9 this is 5.2 Vrms => +16.6dBu

If the resistor R1 was much smaller, say 47R, the unloaded output swing would be +18.8dBu

When the output is loaded, the limiting factor is the current the resistor R22 or R20 can inject in the base of the NPN transistor when the output voltage is high. This, combined with the Beta of the transistor, will determine the current delivery.

 

[Nora]

OK, how to calculate?

The calculations always look self-explanatory when you look at them, until you have to come up with the formulas yourself  ;D

An estimate of 10mA, would make the rail voltage 21V - 4.7V, and output swing 21 - 4.7 -1.4 = 14.9 this is 5.2 Vrms => +16.6dBu

That means that, by raising to 30V, lowering R1 -> 47R, I could theoretically get up to 22dBu? That's very decent
(30V-0,47-2*0,7) = 28,13PKPK =>10VRMS => 22.2dBu (gently loaded)

Beware that the output stage is not protected, though.

Unprotected, as in short circuit/heavy loads burning the transistors?

I could replace the output amp with an opamp? but that would defeat the +30V option as long as it's a single powersupply.
And we live in a high load impedance era anyway so why bother with loading

Is there anything special about this design? Or is it simply an of the shelf (!) eq design.
I like how it sounds, I've not come across too many eq's that sound like this.
Except for the inductor, everything is just plain and simple R, C, and Q. Nothing special, so it has to be in the circuit right?

Thanksalot!

 

[PRR]

> I haven't met too many all-transistor eq's, so this is pretty new to me.

It's a tube circuit. The low-hi is the Baxandall but with a (mis-drawn*) silicon trio instead of a high-slope pentode.

(*) The T6 leg connected to T2 is surely working as Drain, not Source. Since source and drain are interchangeable for audio purposes on most JFETs, this may be in the original; but the trio MUST give voltage gain an T1 T2 give none, so the JFET must be a common-source gain-stage with Drain output.

There ARE inductors. Or one big custom tapped inductor. This works similar to the Baxandall except the shape network is an L-C series-resonant tank.

The input impedance falls to <1K at maximum boost.

> resistor R1 is 470R. ... with the resistor, there will be as much voltage lost in it than available to the load.

Not that bad. C7 holds voltage over an audio cycle. The average current for a stage like this, at maximum sine output, is like a resistor 6 times the load. An 8 ohm loudspeaker amp loads its supply like a 50 ohm resistor. Assuming 600r load, T1 T2 act like 3K6. C7 voltage will be 0.9 times supply voltage. For 300r, say 0.8. I agree that R1 perhaps is 47 not 47; but 470 may may pass performance specs.

The extreme limit is R22 6K8 divided by T1 T2 gain. Say hFE=100. Then a 68 ohm load would not swing half-voltage up, and we would expect a much higher load. 300 may be a good bet for minimum useful load.

If it is really 21V supply, I bet it is made for use "inside a system", and there will be boosters where internal signals leave the box. Like the old TASCAM boards worked at -10dBv internally (and local minor patches) but did give +4dBm at the main outputs.

> the output stage is not protected

I've seen dumber. R22 times hFE (plus R2) limits up-current to ~~0.4A. T1 may survive speech/music. Down-current is firstly limited by T6 and hFE, but that is 2-loosely specced and probably far over 1 Amp. Wait: reverse breakdown in T1 B-E junction against R2 limit near 0.7A. However longer-term the up and down currents must be equal; C1 will charge-up and T2 will settle down to near 0.4A max.

This is yet another reason to think these modules were not meant to drive outside the box.

> we live in a high load impedance era

Well, fah. Stick 470 ohms in series with C1. Dead-short, the transistors still see 470+10 a safe load.

> opamp? but that would defeat the +30V option as long as it's a single powersupply.

A bazillion plans use chip opamps on one-rail supply. Separate your DC bias grounds and your hard audio grounds. DC bias comes from a half-voltage splitter.

> it has to be in the circuit right?

Not much special about that.

 

[Nora]

This is indeed supposed to live in a console, probably somewhere between a mic/line amp and a fader amp.
I know they had transformers mounted in the frame, maybe they had separate output drivers too.

Since the mid boost could present a heavy load on it's driving equipment, I guess it might be better with an input buffer in front.

I checked, R1 really is 470R, the fet was mis-drawn indeed. (I correctly wired it in the spice sim  : )

I know many opamps are used with a single supply and half voltage bias, but I was referring to the 30V increased voltage for higher max output.
When an opamp is max-specced at +/- 18V supply rails, does it also mean +18V max single-rail?
Or is it: max 36 between Vcc and Vee => +36V single rail

Would there be any problem to add another mid band?
Duplicate the circuit from R4 to C9 and add it in series?
Spice seems to think that it's fine.

Ideally I'd like to make a pcb with a mic/line amp and this eq with added low mid tapped inductor.
I found another schematic of a tbg mic amp which is very similar, without all the caps ... and inductors... of coarse
working on updating the schematic now...

As always, your suggestions/comments are highly valued!

thanks!

 

[abbey road d enfer]

Since the mid boost could present a heavy load on it's driving equipment, I guess it might be better with an input buffer in front.

Yes, debalancer-buffer, good decision.

I checked, R1 really is 470R, the fet was mis-drawn indeed. (I correctly wired it in the spice sim  : )

I suspect the reason is to clean the power rail, since this stage doesn't have good PSRR. But I would reduce the resistor and increase the cap to 1000uF.

I know many opamps are used with a single supply and half voltage bias, but I was referring to the 30V increased voltage for higher max output.
When an opamp is max-specced at +/- 18V supply rails, does it also mean +18V max single-rail?

No.

Or is it: max 36 between Vcc and Vee => +36V single rail

That's it!

Would there be any problem to add another mid band?

Not a major problem, but you would invert phase (each stage is inverting). This can easily be taken care of if you add a debalancer at the input.

Duplicate the circuit from R4 to C9 and add it in series?

Yes.

 

[jensenmann]

I have uploaded what I could find about Geiling/TBG stuff on my HD here: http://www.twin-x.com/groupdiy/thumbnails.php?album=413

The desks internal level was -8dBu. Modules were supplied with +21V from the PSU but had an additional regulator on board. This gave a loss of another 2V, leaving +19V for the circuit. Line-ins were supplied with 5:1 step-down transformers to get german broadcast standard of +6dBu down, IIRC the output tranny was step-up again (don´t remember the ratio).

I did rack a pair of TBG Eqs quite some time ago and indeed had to increase the coupling caps to 1000uF, btw

 

[Nora]

I have uploaded what I could find about Geiling/TBG stuff on my HD here: http://www.twin-x.com/groupdiy/thumbnails.php?album=413

That's some really good stuff, thanks!
Especially the output stage.
Thanks for the gain-staging/internal levels info!

I suspect the reason is to clean the power rail, since this stage doesn't have good PSRR. But I would reduce the resistor and increase the cap to 1000uF.

low ripple power supply is expensive in a console, but not in a single channel unit.  8)

thanksalot!

 

[salomonander]

hey guys

i have a desk full of these eqs. and id love to add a second mid band to all of them. i also have a outboard tbg eq (same series) that uses different frequency points for a low mid control. thats what id like to add to the normal ones. it looks like it uses the same cicuit as the mid band mentioned but obviously different caps and a different inductor. since i assume that i will have to add this inductor to the added mid band i need to identify it. but i dont know how. could anyone help? i can supply the given frequency with the cap vaue. i could even reverse draw a schematic i guess.... but could anyone identify the inductor values when i provide that info?

cheers
jakob

 

[salomonander]

the inductors are made by siemens and labeled N48 400. if that helps...

 

[rafafredd]

would be nice to have the mH values.