Help me understand and summarise this simple mixer pre-amp

[Kingston]

Hi,

I'm studying a late nineties mixer. The design is seems to be ripped off from similar designs by other companies (Midas for example) as far as I understand it. I've got the general topology and signal chain of the mixer itself somewhat down, but I lack the theoretical knowledge needed to understand the preamp section specifically.

1. What exactly are the transistors doing here, and how were they biased for the task?
2. Why was this transistor selected and are there better choices?
3. Are the small signal diodes here just for clipping/protection?
4. Does the opamp here have other task than to buffer the stage to the next one in chain? (pretty standard parametric filter irrelevant to my questions here)
5. What is the purpose of that 470uF cap next to the gain pot? And why such a big value?

Thanks.
Mike

 

[kooma]

1. are those just for push&pull amp biased with those diodes?
.
.
4. imo. no, just buffer&filter.
5. blocking dc for that pot?

 

[JohnRoberts]

Hi,

I'm studying a late nineties mixer. The design is seems to be ripped off from similar designs by other companies (Midas for example) as far as I understand it. I've got the general topology and signal chain of the mixer itself somewhat down, but I lack the theoretical knowledge needed to understand the preamp section specifically.

1. What exactly are the transistors doing here, and how were they biased for the task?

Transistors current bias comes from resistors to + and - supply. Difference between devices feeds into opamp and feedback loop returns to emitter of TR2

2. Why was this transistor selected and are there better choices?

typically these are selected for low noise as they will add hiss to audio if not quiet.

3. Are the small signal diodes here just for clipping/protection?

Diodes are to prevent damage if inputs are momentarily shorted with phantom power on. This will drive bases almost 50V negative (or try to).  The diodes backwards across the base emitter junction are to prevent them from zenering if base rises 7V above emitter. This can make low noise transistors noisy.

4. Does the opamp here have other task than to buffer the stage to the next one in chain? (pretty standard parametric filter irrelevant to my questions here)

Opamp also provides gain and tries to hold current constant inside transistors for low distortion. Better designs use two opamps to wrap feedback around each input transistor.

5. What is the purpose of that 470uF cap next to the gain pot? And why such a big value?

To prevent noise when you change the gain.. The very small DC differences between the transistors with several tens of dB voltage gain would make any pot sound scratchy. 

Thanks.
Mike

This general topology is pretty widely used, and this particular version looks like it should be reliable enough for commercial production but this is an inexpensive approach I wouldn't copy if looking for high performance.

JR

 

[MikeClev]

Looks similar to this: http://sound.westhost.com/project66.htm

The opamp debalances the signal and performs common mode rejection duties.

With the big cap, I think the larger the value, the lower the bass response...?

JR knows far more about this than I do!

 

[abbey road d enfer]

Looks similar to this: http://sound.westhost.com/project66.htm

This one and the one in the OT are not the best designs in terms of performance, specifically regarding headroom. A nicer version, that has an opamp complementing each input transistor and providing feedback to the emitter has approximately 10dB more headroom. Another opamp is needed to recombine the outputs of both halves to produce an unbalanced output.

 

[JohnRoberts]

Looks similar to this: http://sound.westhost.com/project66.htm

The opamp debalances the signal and performs common mode rejection duties.

With the big cap, I think the larger the value, the lower the bass response...?

JR knows far more about this than I do!

Actually the preamp you link to uses local negative feedback at the input transistors but no overall NF from the opamp output.

Another cheaper approach that trades performance for cost.

JR

 

[JohnRoberts]

This one and the one in the OT are not the best designs in terms of performance, specifically regarding headroom. A nicer version, that has an opamp complementing each input transistor and providing feedback to the emitter has approximately 10dB more headroom. Another opamp is needed to recombine the outputs of both halves to produce an unbalanced output.

+1  I like at least one opamp per input transistor to linearize and keep them happy.

I used the three opamp approach before working at Peavey, and there they used a two opamp variant that covers most bases adequately.

Imagine adding a unity gain inverting opamp from the output of IC1A to the ground side of R9. The only thing you forfeit relative to the 3 opamp approach is 3 dB of S/N since you only use one of the two outputs. Note: you need to add a RxC across the two input device collectors to stabilize for the extra delay in the feedback loop.

If cost is no object three opamps, but two opamps can get you most of the way there. One opamp, not very close at all.

JR

PS: Nowadays there are some pretty competent mic preamp chips.

 

[Kingston]

Thank you people. I'm surprised about the function of those diodes here. I was almost sure they were clipping protection.

Yes indeed this is from a somewhat inexpensive live oriented mixer. It has survived remarkable abuse over the years and has a good amount of gain.

I have noticed the opamp plays quite a significant role in the "sound" of this thing, which is no longer a mystery to me given the fact it has such numerous roles to play in that position. All important distortion mechanisms at play quite likely. I have experimented with quite a few using SG's opamp book as a rough quide.

This mixer has had phantom disabled for years. I will earmark those diodes to be removed for at least one stereo pair to test just how much effect they may have on the noise around those transistors.

 

[abbey road d enfer]

I used the three opamp approach before working at Peavey, and there they used a two opamp variant that covers most bases adequately.

Imagine adding a unity gain inverting opamp from the output of IC1A to the ground side of R9. The only thing you forfeit relative to the 3 opamp approach is 3 dB of S/N since you only use one of the two outputs.

Yes, I'm familiar with this topology. It has nearly 6dB less admissibility and non-linearities come much sooner. The only justification for it is when real-estate is at premium.

  Nowadays there are some pretty competent mic preamp chips.

Yes; I think these half-discrete topologies are of purely historic and educational value.