Hi

Well, we blew up another amp - this time my 1980s Technics courtesy of a birthday card placed on the vent so I'm now sadly considering replacing my 'lockdown speakers' with a pair of active PA style ones (with XLR balanced inputs), but here is my dilemma:

I have two inputs: first is RCA line in from a Pi running PiCore Player, the second is an AUX input for when my children get fed up with my playlist.

I require three outputs: First, a physical volume control to the XLR, second RCA line out to a second smaller amp (for garden speakers) and third, a mono output to a WLED audio reactive controller that I recently built. Sounds simple enough, but I know nothing about impedance etc and cannot risk harming phones or laptops plugged into the AUX input!

I have found the following: https://forum.allaboutcircuits.com/threads/audio-splitting-signal-loss.127427/ got slightly excited by this: https://forum.allaboutcircuits.com/threads/audio-split-and-adjust-only-one-output.122066/post-978415 - and tracked down the circuit to here: https://www.eeweb.com/three-channel-audio-splitter/


This circuit will need to be duplicated for stereo, so I started transcribing to KiCAD but in doing so realised the op-amp 'volume' was controlled on the input side and I need to only control the active speaker output. I also realised that although I understand a bit more about op-amps (thanks to you guys), the rest of the circuit is a bit beyond my current comprehension. Can I 'fix' the op-amp by replacing the P1/R1 combination with a fixed value and then putting a new potentiometer in combination with R8?
It is a mystery to me what Q1 and Q2 are doing as they're on the power side of the op-amp - also no idea what the voltage should be - I think the NE5532 (P?) has an operating voltage of 0.9 to 1.5v but I thought RCA etc were 2V+... Edit: not sure where I got that from, just checked the proper datasheet - supply is +5V, -5V, so how do I ensure I stay withing the line-in limits?
On the input side I plan to use another potentiometer to 'fade' between the two inputs. And final question, if I create a stereo to mono output by putting resistors in R6 (and it's second channel partner) will that make all channels mono?

Thank you in advance.

 

The two transistors Q1 and Q2, and their associated components are linear voltage regulators, not a part of the amplifier scheme. Of course it is better to have stable and noise-free power for your distribution amplifier circuit,
I would not consider using such a low voltage supply for a good sound system because the supply voltage places a hard limit on the signal amplitude. UNless the whole system is intended to run on AA size batteries for extended intervals.

 

Thank you for your reply MisterBill,

The two transistors Q1 and Q2, and their associated components are linear voltage regulators, not a part of the amplifier scheme.

Aha, so the BZX79C18's that would give +1.2V and -1.2V and therefore a maximum 2.4V differential to the RCA outputs?

Of course it is better to have stable and noise-free power for your distribution amplifier circuit,

Is this a stable and noise-free power supply?

I would not consider using such a low voltage supply for a good sound system because the supply voltage places a hard limit on the signal amplitude. UNless the whole system is intended to run on AA size batteries for extended intervals.

I've always understood RCA (and XLR balanced?) to be operating at around 2V, so would the hard limit of +1.2V and -1.2V be the appropriate range? I'm trying to split a line-level input into three (2 amplifiers and a mono WLED input) without loss rather than amplify to drive a speaker. I would like this to run from a power source other than batteries.

 

The challenge with the really low voltages are that a lot of amplifier circuits are more linear when they are not close to the limits of the power supply voltage. THAT is why a whole lot of designs were supplied with +15 and - 15 volts when they were only handling signals of 10 volts peak. So a distribution amplifier with a 1 volt peak to peak output powered by a +/- 10 volt supply is a lot easier to design than one with a 3.3 volt supply. Just look at the supply voltages for the input stages of your high-quality amplifier.
As for supply voltage regulation, clean is more important than constant. And adequate supply filtering is not that difficult at low power levels.

 

The challenge with the really low voltages are that a lot of amplifier circuits are more linear when they are not close to the limits of the power supply voltage. THAT is why a whole lot of designs were supplied with +15 and - 15 volts when they were only handling signals of 10 volts peak.

That makes sense as there's a whole lot more headroom.

So a distribution amplifier with a 1 volt peak to peak output powered by a +/- 10 volt supply is a lot easier to design than one with a 3.3 volt supply. Just look at the supply voltages for the input stages of your high-quality amplifier.

Is this a 'distribution amplifier' that I'm attempting to create? If I were to use +/-10V power supply on +Vcc and -Vcc, would the Q1 & Q2 circuits have to dissipate a lot of heat?

As for supply voltage regulation, clean is more important than constant. And adequate supply filtering is not that difficult at low power levels.

I'm sure there will be an audio PSU design on this forum somewhere! I am using an iFi power supply for my OiCore player but they seem to have become quite expensive! I'd also have to work out how to get the +/- volts from a 5 or 9V DC supply.

 

The BZX79C18 zener diodes are 18V, not 1.8V. You do not need the zener diodes or the transistors that are used when the supply is +/-25V and higher.
The NE5532 opamp has a minimum supply of +3V and -3V producing a maximum signal output of about 2Vp-p. Its output with a normal higher supply voltage is 4Vp-p less than the total supply voltage.

 

The BZX79C18 zener diodes are 18V, not 1.8V. You do not need the zener diodes or the transistors that are used when the supply is +/-25V and higher.

Ah. I'm still not sure what that part of the circuit is doing then. Is there 18V+ going into the op-apm?
I've now gone to a more reliable source (than the first link in a google search), to Mouser.

The NE5532 opamp has a minimum supply of +3V and -3V producing a maximum signal output of about 2Vp-p. Its output with a normal higher supply voltage is 4Vp-p less than the total supply voltage.

So I can 'cap' the output voltage to line level only by controlling the input voltage? This is what I originally understood from previous threads but was a little confused by the above mentioned +/- 10 volt supply.

 

The 18V zener diodes and transistors are used when the power supply voltage is +/-25V to +/-50V from a power amplifier.
Preamps usually use a supply voltage that is much higher than the maximum output signal level so that the preamp output never produces severe clipping distortion (what you say is "capped"?).
Usually an input volume control limits the loudest peaks of volume but not when a microphone is dropped on the floor or is screamed at.

 

That makes sense as there's a whole lot more headroom.

Is this a 'distribution amplifier' that I'm attempting to create? If I were to use +/-10V power supply on +Vcc and -Vcc, would the Q1 & Q2 circuits have to dissipate a lot of heat?

I'm sure there will be an audio PSU design on this forum somewhere! I am using an iFi power supply for my OiCore player but they seem to have become quite expensive! I'd also have to work out how to get the +/- volts from a 5 or 9V DC supply.

YES, this is a simple DISTRIBUTION AMPLIFIER! That is because it is boosting a signal by some amount to send to other devices that are not transducers. It could also be a "line driver" amplifier except that probably the cables to the three systems are not long.
The benefit of having names for applications is that then folks can know what to ask about. And search-engines can know what to search for.

 

YES, this is a simple DISTRIBUTION AMPLIFIER! That is because it is boosting a signal by some amount to send to other devices that are not transducers. It could also be a "line driver" amplifier except that probably the cables to the three systems are not long.
The benefit of having names for applications is that then folks can know what to ask about. And search-engines can know what to search for.

I wish I'd known that term - I've been searching for splitters, pre-amps, record outs, everything except distribution amplifier! It's almost what I'm looking for but would probably do the job just by adding a pot between one pair of the line outs and the XLR inputs of the active speakers.

 

The 18V zener diodes and transistors are used when the power supply voltage is +/-25V to +/-50V from a power amplifier.
Preamps usually use a supply voltage that is much higher than the maximum output signal level so that the preamp output never produces severe clipping distortion (what you say is "capped"?).
Usually an input volume control limits the loudest peaks of volume but not when a microphone is dropped on the floor or is screamed at.

Or unplugged! My input will be limited by the source device but I'd need to (somehow) ensure the output was always within line limits. Or, as I've recently discovered, search for a 'distribution amplifier!

 

Assembling a dual input distribution amplifier will allow you to include adjustments for the gain of each input to each output so that it will not surprise you with sudden increases in volume.

 

Assembling a dual input distribution amplifier will allow you to include adjustments for the gain of each input to each output so that it will not surprise you with sudden increases in volume.

I would love to do that but now suspect I would need an awful lot of hand-holding! I started this thread with "How hard can it be? One RCA input, one AUX input, one volume controlled output, one straight output and one mono output!" Well, now I know. :|

 

One RCA input, one AUX input, one volume controlled output, one straight output and one mono output

Let's clarify some things.

One RCA input - Stereo (actually 2 RCA jacks) or mono (1 jack)?

One Aux input - Stereo (such as a 3.5 mm TRS (tip-ring-sleeve) plug/jack), or mono?

One volume-controlled output - is this the XLR? If yes, is it to be a balanced output (so two XLRs for a stereo pair), or one XLR with both channels (single-ended)?

One straight output - Connector preference?

If we sort through the input connectors, output connectors, and Left, Right, and Mono signal paths, filling in the boxes on a block diagram will be much more straightforward.

0 dBm is 1 milliwatt into 600 ohms. This works out to a signal amplitude of 2.2V peak-to-peak. Pro audio runs around at +4 dBm, while consumer level is around -10 dBm. Those might be old numbers, subject to updating by others. Back in my TV days, 0dB on the meter was +8 dBm. Things change.

Start with a decent "audio opamp" and +/-15 V power supplies. This will handle a 10x overload without clipping. There are a ton of audio circuit modules on ebay and alibaba, and it might be cheaper to build your system out of pre-fab modules rather than wiring everything from scratch.

Where are you located?

ak

 

Let's clarify some things.

One RCA input - Stereo (actually 2 RCA jacks) or mono (1 jack)?

One Aux input - Stereo (such as a 3.5 mm TRS (tip-ring-sleeve) plug/jack), or mono?

One volume-controlled output - is this the XLR? If yes, is it to be a balanced output (so two XLRs for a stereo pair), or one XLR with both channels (single-ended)?

One straight output - Connector preference?

Inputs:
- one pair of RCA jacks from my PiCore player
- one stereo AUX input - this can be RCA or 3.5mm phono

Only one input is active at a time - my plan was to use a pot as a 'fader' but that might cause groud loop isses?


Outputs:
- One pair RCA line level stereo output to low power amp (garden speakers)
- one mono output - can be RCA or 3.5mm jack. WLED only listens to the left channel but the I2S card receives a stereo 3.5mm jack
- One volume controllable stereo line level output. This can initially be a pair of RCA jacks and the upgrade/add on balance later. The speakers are less than 4m from the amp. I think RCA to XLR balanced cables are available?

If we sort through the input connectors, output connectors, and Left, Right, and Mono signal paths, filling in the boxes on a block diagram will be much more straightforward.

I can create a KiCAD schematic this evening (on mobile today, so please forgive the typos)

0 dBm is 1 milliwatt into 600 ohms. This works out to a signal amplitude of 2.2V peak-to-peak. Pro audio runs around at +4 dBm, while consumer level is around -10 dBm. Those might be old numbers, subject to updating by others. Back in my TV days, 0dB on the meter was +8 dBm. Things change.

Start with a decent "audio opamp" and +/-15 V power supplies. This will handle a 10x overload without clipping. There are a ton of audio circuit modules on ebay and alibaba, and it might be cheaper to build your system out of pre-fab modules rather than wiring everything from scratch.

Happy to do this - modules are my standard MO. Until my most recent effort, al my projects were PWBs (Printed Wiring Boards) rather than PCBs...

Where are you located?

ak

I'm in UK.
As an aside, I'm not aiming for HiFi quality sound (although close would be good) - I'd just like a bullet proof party music setup so we don't keep blowing up speakers & amps!

 

I'd just like a bullet proof party music setup so we don't keep blowing up speakers & amps!

Simple, use class D amps and don’t connected speakers that are rated for less the their max output.

 

Many of us use pieces that we acquire at different times. My original Marantz speakers matched the receiver, but presently they are way to large for the room. So the smaller speaker boxes that hang on the wall are rated a bit less power. I am sure that is an issue for many. And the TAN-8550 amp a friend gave me could overdrive all of them at once. Systems seem to grow for some of us.

 

If a simple mechanical switch between the two inputs is OK, then it sounds like almost everything else is unity gain. The only bit of design is the stereo-to-mono summer, which is a one-opamp inverting summer and a one-opamp inverter to re-establish the correct signal phase.

Optional: 2 dual opamp voltage followers to buffer the player and aux inputs
1 - DPDT switch to select between the two inputs
1 - stereo volume control
1 - Dual opamp voltage follower output driver for the main stereo output
1 - Dual opamp summer for the mono output

To this you can add stereo volume controls to the two input pairs to balance their levels.

ak

 

Simple, use class D amps and don’t connected speakers that are rated for less the their max output.

Yup. Did that - the first one, I think the crossovers over heated (it was a blazing hot evening and a small room filled with dancing humans), causing a cascade failure, the second one, my son left his birthday cards on top of the 40+ year old amp vents!

 

If a simple mechanical switch between the two inputs is OK, then it sounds like almost everything else is unity gain.

A mechanical switch will be perfectly fine - the changover only happens when my children tire of my playlist!

The only bit of design is the stereo-to-mono summer, which is a one-opamp inverting summer and a one-opamp inverter to re-establish the correct signal phase.

I understood this could be done with resistors ala: https://www.instructables.com/Simple-Way-to-Convert-Stereo-to-Mono/ (not where I originally found the method)?

Optional: 2 dual opamp voltage followers to buffer the player and aux inputs
1 - DPDT switch to select between the two inputs
1 - stereo volume control
1 - Dual opamp voltage follower output driver for the main stereo output
1 - Dual opamp summer for the mono output

Searching ebay for pre amplifier modules on the train (a Faraday cage) produces a limited range in UK:
1. https://www.ebay.co.uk/itm/405032689715 (THT)
2. https://www.ebay.co.uk/itm/296372858978 (THT)
3. https://www.ebay.co.uk/itm/375194778584 (SMD)
4. https://www.ebay.co.uk/itm/266783777308 (SMD + a bit of THT)

Number 4 is interesting as it appears to have through holes to add a pot in line with R5 and R7 - perhaps to add volume control? If my understanding of op-amps is correct (unlikely, but play along), the the amplification can be controlled by one of the resistors so bridging the right resistor on any of the boards should do that?

All of those boards though, only receive/require a positive input voltage (ie not +/- 15V) - I have yet to find a 'proper' audio module.

From your initial reply, my thoughts were to have the Line-in -or- AUX feed directly to the three op/pre amp modules (which would each take the lowered/divided input back up the line levels) but have a volume control on one of them - would this work or is the 'buffer' a requirement on both inputs?

To this you can add stereo volume controls to the two input pairs to balance their levels.

ak

I'm not sure of the need for volume controls to balance the input pairs - the PiCore (stereo RCA) has software volume and the AUX will be either a phone, tablet or laptop.

Thank you very much for you help - I'll do a block diagram in KiCAD tonight.