Low current driver!?

I am working on an incarnation of the preamp in my old, beloved Chilton QM3 console, and Ive hit a little snag.

(schematic attached)


If I load TP4 with an output transformer, I have quite a bit of current on the transformer, and correspondingly, quite a bit of signal (voltage) loss.

but thats not all...

I also have an active stage I'd like to be able to switch into the after circuit after TP4 with a high impedance, opamp input. When I switch it in, current through TP4 is restricted, but even though I have no signal (voltage) loss, it introduces hum (I am guessing that the lower drive current and relatively higher voltage messes with the transistor biasing -  I'd like to know if that explanation makes any sense, or I have taken a wrong turn thinking).


So for these reasons, I'm thinking the solution would be to put "something" after TP4 of this circuit that does not put a load on source current and puts out a low impedance signal that retains the signal voltage.

A low impedance input, low impedance output driver/buffer!? I only have examples of low impedance output drivers, but none with the low impedance input I would like to try.


...Also...Vbe on Q3 is 0.3V no matter how the circuit is loaded...thats odd, right? So maybe I "just" have some problem that should be solved, which would result in a lower source current on TP4 and cancel the need for the interstage.

Gustav

Q3 and T2 are a fixed 20dB gain block. The quiescent current is only about 4mA so it has nowhere near enough capability to drive a transformer. Your schematic shows the Chiltern equivalent of the first two preamps in a typical Neve channel. What is missing is the big meaty output stage and its transformer. You need to find the Chiltern equivalent of that stage and add it to the end of your signal chain.

Cheers

Ian

ruffrecords said:

Q3 and T2 are a fixed 20dB gain block. The quiescent current is only about 4mA so it has nowhere near enough capability to drive a transformer. Your schematic shows the Chiltern equivalent of the first two preamps in a typical Neve channel. What is missing is the big meaty output stage and its transformer. You need to find the Chiltern equivalent of that stage and add it to the end of your signal chain.

Cheers

Ian

Click to expand...

Thanks so much for this, Ian.

Trying to make sense of it, I was under the impression that the drive current (corresponds to what I measured) was too high. I have never measured drive current or looked at it before, so the reasoning was that the voltage drop happened due to too high current on the transformer (V = I x R ), and the problem hitting the opamp input as described above.  (no blushing icon?). Ill re-visit it with your comment and try to make sense of it - Hope I can decipher the schematic to lift the missing stage.


Gustav

The direct outs in the QM3 look like they come much later in the circuit.  After EQ, filters, and fader.  There is another gain stage with a 5532.  Here's a page from the manual that might help others help you ;-)

mjrippe said:

The direct outs in the QM3 look like they come much later in the circuit.  After EQ, filters, and fader.  There is another gain stage with a 5532.  Here's a page from the manual that might help others help you ;-)

Click to expand...

There seems to be no meaty 600 ohm output drive circuit on that schematic. The 5532 just seems to drive the post fader buses.

Cheers

Ian

> with an output transformer

What kind of transformer??

Assume "600r". Since the output T2 is resistor loaded, check for a load resistor smaller than 600 Ohms. Hmmm, I see 2,700 Ohms. This is dandy for pulling a 10K load, sucks for 600r load.

Which matches your symptom.

The small value of C16 is another clue. For some loss at 20Hz, 22uFd can support 500 Ohms, true. But there are many-many such caps in mixer and studio. Each one is typically sized for "no" loss at 20Hz. We would expect to use 100++uFd to drive 600 Ohms.

The 24V won't support BIG output Voltage like we expect from +/-15V supplies or transformer-loaded SE stages (Neve), unless done push-pull (many German broadcast consoles used two 553x to drive the output).

Why do you need a transformer? If you have ground-loops, put a 10K:10K at the INput of the box you go to.

See attached.

> Vbe on Q3 is 0.3V no matter how

Are you measuring Vb-Ve?

When you poke Vb, your meter loads-down the 270K+ Base bias network; Ve is much more solid.

The difference implies about an 11Meg meter. This happens to be a VERY common meter impedance, VTVM or DMM.

Use a *battery-power* meter and measure directly from B to E. You will find around 0.6V.

PRR said:

> with an output transformer

What kind of transformer??

Assume "600r". Since the output T2 is resistor loaded, check for a load resistor smaller than 600 Ohms. Hmmm, I see 2,700 Ohms. This is dandy for pulling a 10K load, sucks for 600r load.

Which matches your symptom.

The small value of C16 is another clue. For some loss at 20Hz, 22uFd can support 500 Ohms, true. But there are many-many such caps in mixer and studio. Each one is typically sized for "no" loss at 20Hz. We would expect to use 100++uFd to drive 600 Ohms.

Click to expand...

Thanks!

PRR said:

Why do you need a transformer? If you have ground-loops, put a 10K:10K at the INput of the box you go to.

Click to expand...

Need is too strong a word, I guess. I was trying with a transformer for balancing the output...

Even with the signal loss, I have just enough total gain (not a ton of headroom, but I knew this from the console)

The other portion of circuit/go-between was inserted after TP4, before the output transformer, and I thought I had wrapped my head around some feasible explanation that accounted for both symptoms (hum on the opamp, loss on the trafo). I was mostly trying to see if I had learned something here....maybe next time!

PRR said:

measure directly from B to E. You will find around 0.6V.

Click to expand...

I was measuring both with reference to common ground and noted the difference of 0.3V. Measuring directly from B to E gives me 0.6V, as you say.  8)

Back to the bench - Ill try to implement the modded circuit.

Gustav

Hey Gustav ! did you ended up doing anything with that circuit ? I was looking for Chilton preamp projects... Let us know