Solo-in-place schem?

[rickc]

that's the circuit used in older API boards as their relay driver. it's useful because the input is active low, and it is several volts above ground (because of the zener) so you can have several diode drops in the path to ground, which is useful when you have some complex logic function that depends on several conditions being true.

I say older, because my experience was with a 32x24 in the mid 1970s.  In these boards, there was a large push button in the center of the meter bridge that activated the solo-in-place system. Essentially what happened is that any channel that wasn't solo'd was muted, and pressing the solo button on a channel grounded the SIP bus while removing that channel from the global mute bus. It's been a long time since I looked at the schematic (which I still have), so next chance I get, I'll look and figure it out further. The advantage to this destructive solo system is the signal took the exact same path to the 2 mix that it would during normal operation. This can be useful, say if you're doing something at bus level and need to hear what it's doing to a particular signal.

Muting in these consoles was an analog switch board; they called it a "soft switch" that used a pair of FETs in a series/shunt arrangement. The soft switches were problematic as I recall, and eventually they were replaced with relays by the studio's maintenance dept (this studio actually had maintenance techs on staff). I had to reverse engineer the circuit board to get the schematic, which I then sent to API marked, "is this correct?" and they responded by sending the blueprint. (they had feigned deafness at earlier requests).

The majority of other switching functions in these consoles were relays.

 

[jsteiger]

Yes, this is the solo/mute logic from the monitor side of my board. The input side is non-destructive and mono. So, there is a single, mono ACA for it. The control room source becomes the output of this ACA circuit when any input channel is solo'ed.

Kinda strange but probably par for the course back in the early to mid '70's. (pre SSL)

I hope you find it as useful as I do.

Cheers, Jeff

 

[Mark Slocombe]

If you want to use a relay to switch the audio this part from Mouser has Gold bifurcated contacts, is DPDT form and is only $2.52 in singles. It's very small and consumes very low power. pt#655-D3012, page 1708.

It's available in several coil voltages depending on how you intend on handling that.

The reverse coil discharge from a relay is a very high voltage but virtually no current. Experience taught me to use 1Kv diodes for the snubber across the coil. I mount the snubber diode right next to the relay coil connections.

Best of luck with your project

 

[plumsolly]

Wow! Thanks for all the responses!

A lot of the electronics I build are used in sound contracting situations where 24vdc is common. I've been building heavily with relays since 1981, I love them for their reliability. In my mixing console designs I have a separate 24vdc supply and ground, I use this for LED displays and powering relays.

I put a diode across the relay coil to snub the coil discharge. By implementing the separate power supply and the diode I have zero pops and clicks.

If you are switching audio with relays look for sealed relays with precious metal contacts that are bifurcated, gold contacts are best. I have one large installation I built in 1984 with these parts, they are "ON" 24/7 and have never had a failure.

I personally like destructive solo where everything drops out except what is solo'd. I never solo anything when I'm doing a final mix so destructive is fine. I also like doing it this way because it keeps me focused on the mix and hearing what each track or effect is contributing.

Best of luck with your construction project.

This is pretty much where I'm at: sealed, precious metal contact relays - diode across the coil - separate 24v power supply for leds, lamps, relays - no pops and clicks - prefer destructive solo. It's great to here that this approach has worked for you.

For a one off project it may not be worth the learning curve to design a JFET mute from scratch.

Definitely the case for someone at my level of electronics knowledge!

Thank you for your time. Much appriciated.

Dido!!

Hey Ben,

I have been using the following control circuit that I discovered in my console. Kinda like John mentioned!    Anyhow, it is for mute and destructive solo. Some minor rework would easily provide a dedicated stereo solo bus for non-destructive SIP. The parts are cheap and plentiful. Works like a dream. I like the TQ2's. Q1 is an NPN.

Oh, and out of respect for JR, I always try to spell bus correctly.  8)

Cheers, Jeff

Thanks a lot Jeff. This looks pretty similar to mine and analogical's circuits with the addition of a drive section for the relay. I have to admit, I still don't understand the purpose of drive section (or the zener or the caps really (ps decoupling?)) but that's to be expected as I am only on page 25 or so of The Art of Electronics. Any hints would be appreciated, but no expected . My circuit seems to work fine without these additions; it's silent and does what it's supposed to. So until I figure out that I need those pieces for something, I guess I am going to proceed with my circuit the way it is.

that's the circuit used in older API boards as their relay driver. it's useful because the input is active low, and it is several volts above ground (because of the zener) so you can have several diode drops in the path to ground, which is useful when you have some complex logic function that depends on several conditions being true.

Thanks Rick. I keep rereading this and trying make sense of it. It will make sense to me eventually .

The advantage to this destructive solo system is the signal took the exact same path to the 2 mix that it would during normal operation.

My sentiments exactly.

If you want to use a relay to switch the audio this part from Mouser has Gold bifurcated contacts, is DPDT form and is only $2.52 in singles. It's very small and consumes very low power. pt#655-D3012, page 1708.

It's available in several coil voltages depending on how you intend on handling that.

The reverse coil discharge from a relay is a very high voltage but virtually no current. Experience taught me to use 1Kv diodes for the snubber across the coil. I mount the snubber diode right next to the relay coil connections.

Best of luck with your project

Thanks Mark. That looks like a good part. I was using these: FBR211NED024M. I am doing this point to point (!) so the diodes are mounted on the pins of relays. 

Best, Ben

 

[analogical]

Thanks Jeff for the post.

Hehe I'll do to you plum what JR did to me:

http://en.wikipedia.org/wiki/Zener_diode

That zener is what makes the circuit so cool, cause when V dips below its breakdown, Q1 shorts nice and clean.

 

[rickc]

The api boards at this studio had the destructive DIP on the channel side of the board. Maybe they called it "mixdown solo."

the circuit works like this:
the input (junction of CR8 and R22) is active low. Pulling this node to ground causes the relay to open.

When the input is ungrounded, R22 pulls current thru CR8, R19, and the BE junction of Q1.
R19 provides a path for the mostly insignificant CB leakage of Q1, and it (more importantly) raises the current flowing thru CR8, to ensure that it operates in its avalanche region (i.e. zener working).

When R22 is grounded, then Q1 is starved for base drive, and Q1 goes open, and the relay opens. C15 puts a bit of a time constant on the circuit so that the relay doesn't open at EXACTLY the time that the input was grounded. It also slows the rate of change of Q1's collector current, which may have been an anti-spike measure.

C1 provides some local energy storage, and it would be wired so that the loop made by it, K1, and Q1 would be as small as practical. CR1 decouples the relay circuit from the 24v relay supply.

CR8 forces the voltage at the input to be 4.3 volts above ground; the voltage at this point would be the Vbe of Q1 plus the zener voltage of CR8 (~5v).
Although CR8 could be omitted, this would require a dead short from R22 to ground to open the relay. Normally in consoles like this (read: no microprocessors), the logic used in various switching functions depends on mechanical switches and diodes to OR functions together. You may have several diode drops in the ultimate path to ground for some function that depends on several other conditions being true. Having CR8 present lets the input of the circuit tolerate multiple diode drops to ground while still letting the relay open when the input is grounded.

You could have several diodes connected like CR7 to the input node, and each of these extra inputs would cause K1 to open, without being coupled to each other.

API had circuit boards with this circuit on it that they used for various switching functions within the console. Remember that a good part of their business was custom consoles, so it was quite common for them to add features, or to make something work the way that the buyer wanted. In the consoles that I worked on, the master section of the board was built on vectorboard. The studio paid about 96k, in 1973, for two identical consoles.

 

[plumsolly]

Wow, thanks Rick! I think I might understand now! So, since we cannot count on the logic input to be exactly at ground because of the diodes between the input and ground required for more complicated logic, we implement the zener and Q1 so that anything below 4.3v above ground will open the Q1 and connect 24v low to the relay?!!?? Thanks so much for walking me through this. Best, Ben

 

[boji]

we implement the zener and Q1 so that anything below 4.3v above ground will open the Q1 and connect 24v low to the relay?!!??

Not quite plum, please correct me if i'm wrong, but when R22 path to ground is offered by either switch or Solo/Mute Bus, the zener drops below its breakdown threshold and becomes a simple diode, cutting off 24v in from the base to Q1, which in turn makes the NPN stop supply of K1 to 24v low, and the relay, now not energised, opens.

 

[rickc]

boji: yes, you're right. The relay is normally energized, and pulling the control pin to ground makes it open.

In the consoles that I worked on, they had a dedicated circuit board, in the same form factor as a 325 line amp, that had a relay driver circuit on it and a 4pdt relay. There was an additional inverter that made it so that the relay CLOSED when the control pin was grounded. I can post the schematic if wanted.

Also, I found the schematic for their soft analog switch that was used for channel muting in these console. I can also post that.

 

[boji]

There was an additional inverter that made it so that the relay CLOSED when the control pin was grounded. I can post the schematic if wanted.

Also, I found the schematic for their soft analog switch that was used for channel muting in these console. I can also post that.

Please, Thank you!

 

[rickc]

this is the schematic to the relay card used by API in two consoles that were semi-custom and sold around 1972.

With the exception of the extra transistor to reverse the sense of the control input to the relay, the operation is much the same as the circuit already discussed.
also, the relay in the actual unit was 4pdt, rather than 2pdt/dpdt shown here. I got lazy. This circuit also shows expander inputs, etc. This allowed them to use the same card in many different places in the console.

 

[rickc]

This schematic is the soft-switch used for the channel mute in the 515E input module. The concept here was a 2-fet switch that provided a ramped turn-on and turn-off. There are 2 FETs used in a series shunt arrangement. The on and off slopes are ramped to prevent an audible click.

The circuit works, but (judge for yourself) it is overly complicated. It was built on a small daughter board that was stuffed inside the input module.

Eventually, the studio's tech department replaced this circuit with a simple relay circuit.

 

[JohnRoberts]

Indeed that design is generous with the number of components used for a simple FET mute.

I can't say that I follow what that 47 pF (C11) cap is doing from the first opamp output to the rail isolation resistor. The second opamp is adding in a buffered version of the drain voltage reduced 6 dB to the control voltage to reduce audible distortion, "while" the shunt FET is in the process of turning off. It appears a lot of attention was spent on reducing artifacts while switching.

I notice this mute buffer is inverting, so if replaced with a simple relay, some consideration might be needed to preserve absolute polarity. 

JR

 

[tv]

Perhaps the C11 is there to prevent some sort of oscillation if the output is accidentally short-circuited? In such case, the Q2 / Q4 would act like common-emitter super-pairs which (from top of my head) supposedly likes to act un-stable...

Why isn't it symetric (i.e. mirrored in bottom leg), I wouldn't know. But I'm sure THEY do.

 

[JohnRoberts]

Perhaps the C11 is there to prevent some sort of oscillation if the output is accidentally short-circuited? In such case, the Q2 / Q4 would act like common-emitter super-pairs which (from top of my head) supposedly likes to act un-stable...

Why isn't it symetric (i.e. mirrored in bottom leg), I wouldn't know. But I'm sure THEY do.

To prevent oscillation, with output shorted or capacitively loaded,  the obvious connection is from output to minus input. While this is a stretch, they may be tweaking the slew rate for symmetry in both directions, while I thought the 353 wasn't anything like the 356/357 family for exhibiting slew rate asymmetry. 

Or perhaps it's just some spare parts.

JR

 

[tv]

It's obvious that C11 is "compensating" for something, but for what exactly? The fact that the C9 isn't directly in UA1s FB path (ie connected to pin1 instead of the output Q's emitters) seems odd as well - if output is shorted or driving some cable's capacitance this would seem odd regarding that the whole circuit otherwise seems of "baroquesque" proportions. Designers surely have accounted for thet scenario. But, "they" surely know why it is as it is.

 

[JohnRoberts]

Rule #1 about pre CAD schematics, and even some post CAD. Don't ASSume the schematic is accurate.

That design appears to be well engineered with a lot of attention to detail about how it sounds in transition.

I can not explain or imagine a useful function for that part, across those two circuit nodes.

I would advise caution about blindly copying that (or any old schematic) verbatim. I would be inclined to review how that circuit acts with and without that part connected as shown, were I to use it.

Of course I don't know everything, so draw your own conclusions.

JR

 

[rickc]

JohnR mentioned:
Rule #1 about pre CAD schematics, and even some post CAD. Don't ASSume the schematic is accurate.

The schematic I posted came from a drawing that I got from API that I then redrew. The original was several generations down, and came from a blueline drawing, so it was murky and musty at best.

API was pretty proud of that circuit. We had schematics for every part of that console except the soft switch. BUT the soft switches kept failing and since we tried to be self sufficient (i.e. repair the problem in the studio's tech room), we needed a schematic. The circuit was built on a small pcb, with standup resistors, etc. in the tradition of a transistor radio. API kept dragging their feet about sending the print, so I spent part of an afternoon reverse engineering the board so we'd have a print. To tweak API, I sent them a copy (this predated fax) and attached a short note: "IS THIS CORRECT?"  I got that schematic shortly afterwards.

I'll 2x check the schematic that I captured against the one that I drew 37 years ago, just to see if there is a difference.

 

[JohnRoberts]

Just because I don't see what that cap is doing, doesn't mean it wasn't there. 

I can tell that somebody spent time and effort to make that circuit work well at doing what it did,,, It looks like it should exhibit good signal kill when muted, and make no extra distortion in the transition region from on to off and on again.

That one cap is the only odd piece that looks out of place to me. I can see what it does, just not why someone would do that.

Who knows, I don't.

JR

 

[rickc]

I realize that often there is a difference between what appears on a shematic, and what happened in production, so I was just going to see if the capacitor was present on the board that I reverse engineered.

Since the capacitor  runs from the collector of the output device, and thinking of the collector resistor as a partial collector load (so there is *some* signal developed there), that would amount to negative feedback to the input of the output buffer. So maybe it is a bit of compensation to make the whole thing stable once it is wrapped in the overall feedback loop of the opamp?

=========================================================================

Just because I don't see what that cap is doing, doesn't mean it wasn't there. 

I can tell that somebody spent time and effort to make that circuit work well at doing what it did,,, It looks like it should exhibit good signal kill when muted, and make no extra distortion in the transition region from on to off and on again.

That one cap is the only odd piece that looks out of place to me. I can see what it does, just not why someone would do that.

Who knows, I don't.

JR