Adding another audio output to an existing one - Page 1
Forum section: electronics / beginners

author: analogix
author_rank: Frequent Contributor
timestamp: May 31, 2025, 02:11:18 pm
content: I have a vintage computer (Atari STe) which already has two existing audio outputs, but I want to add yet another one.I suspect it takes more effort than just wiring up a new output jack attached to one of the existing outputs, because won't that cause impendance/level drop issues when attached or disconnected from whatever external amp, mixer etc. it's attached to?Below is a schematic of the audio output section.The PSG (Yamaha YM-2149F programmable sound generator) is fed into the LMC-1992 (digitally controlled stereo tone/volume control with 4-channel input selector) as a mono signal going intoL/R inputs 1andL/R inputs 2.DMA (Direct Memory Access) digital sampled audio (coming from a couple of D/A converters) are likewise fed intoL select inandR select inas a stereo signal.I've coloured the inputs in blue.The LM-1992 audio outputs (L front outandR front out) end up as a mono signal (which goes to the Atari 13-pin monitor connector; Atari branded monitors had a built-in speaker), and as a stereo output in form of two RCA connectors (for connecting to an external audio amplifier).I've coloured all the outputs in orange.I would like to add a third audio output which should be stereo, aiming to power a modern monitor's stereo sound-bar.

author: themadhippy
author_rank: Super Contributor
timestamp: May 31, 2025, 03:19:01 pm
content: I wouldnt bother,just wire em in parallel.

author: analogix
author_rank: Frequent Contributor
timestamp: June 01, 2025, 04:38:41 pm
content: Fair enough. I understand there are different ways of doing this.But I read that an op-amp, such as the LF-347 (U504B and U504C in the schematic I posted) gives a high impedance to its output and low impedance to its output resulting in less distortion and allowing for different output levels. I understand that it also "isolates" its output from the input so it won't affect any other outputs.Would a good solution be to duplicate the circuitry (highlighted in pink), connect it to the same outputs of the LMC-1992, but remove the connection between the L and R outputs (the red "X") which should result in a stereo output with a suitable level for the monitor's stereo sound-bar?

author: TimFox
author_rank: Super Contributor
timestamp: June 01, 2025, 04:52:33 pm
content: An op amp audio amplifier normally presents a lowsource impedanceto a highload impedance.Typically, the closed-loop source impedance is negligible (< 1 ohm) and the usual load impedance is 2000 ohms or higher.

author: analogix
author_rank: Frequent Contributor
timestamp: June 02, 2025, 10:54:19 pm
content: Quote from: TimFox on June 01, 2025, 04:52:33 pmOops! I see that I wrote "output" twice, but I got it wrong regardless.Honestly I only understand bit and pieces of how the amplification circuitry works,..... but I assume the output circuitry in this schematic with all of its components have a purpose; so if it's just a matter of wiring up a new output in parallel with the existing one (asthemadhippysuggested), what do the capacitors, resistors and op-amps in the output do?I believe some of the resistors/capacitors perform basic filtering (to remove noise), but there's got to be more to it than that?And how does the mixed mono output (the area I coloured in pink) work together with the RCA-connector stereo outputs?

author: planet12
author_rank: Regular Contributor
timestamp: June 03, 2025, 08:04:00 am
content: Quote from: analogix onYesterdayat 10:54:19 pmIf you're not versed in opamps, this might be a stretch.The two opamps in the mono circuit are in an inverting "virtual ground" configuration, where the inverting inputs (pins 6, 9) are at the analogue ground level.This means, from the LMC1992's perspective, it just sees 47uF in series with 10k to ground. The capacitor is to block DC currents, and the current through the 10k resistor is sensed by the opamp and used to generate an output voltage over R524 and R527 (is that 3.8kohm? I can't quite read it). The outputs of each channel are then unceremoniously jammed together via R540 and R541 and called a "mono" channel.Getting back to your original problem - adding another audio output - simply using a Y-cable is almost certainly going to be okay. Each output channel has a 1k resistor in series between the LMC1992 and the RCA jack, which should protect it from even a short-circuit on the output. Most audio amplifiers expecting consumer line-level audio will have >= 10k input impedance.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 03, 2025, 05:47:24 pm
content: Here's a thing. According to the schematic, the two inputs are combined (with a classy short circuit) into mono.I'm not familair with this device application but there is no stereo at the output. Stereo FX of a the combined mono input then?A simple splitter lead might do the job. I'd use a buffer though.I think a cheap preamp pcb will make a simple buffer the outputs. Feed it direct from the LMC1992s outputs.Yank the opamp on this little module and put in a lower voltage type eg TL972IP. BTW, I'd bet that the NE5532 opamp is a fake. 100% confidence level.Run it from the 8.2V (why?) supply.This one comes with a fixed gain of 5x. You'll probably want to reduce that to 1x.This means snipping out a couple of resistors, one on each channel. My very best guess given the component count is to attack it thus, 95% confidence level!https://www.aliexpress.com/item/1005007539582051.html?spm=a2g0o.productlist.main.1.7e014d66hl8xAb&algo_pvid=c79b2643-b807-428c-8de0-9bb569c0bbb7&algo_exp_id=c79b2643-b807-428c-8de0-9bb569c0bbb7-0&pdp_ext_f=%7B%22order%22%3A%22290%22%2C%22eval%22%3A%221%22%7D&pdp_npi=4%40dis%21GBP%211.08%211.08%21%21%2110.18%2110.18%21%40210391a017489692006174717e05db%2112000041209025451%21sea%21UK%213100120370%21X&curPageLogUid=FTxWBTYbknJI&utparam-url=scene%3Asearch%7Cquery_from%3AWaste your money buying it from amazon if you like.Beer o'clock here. Tada.

author: analogix
author_rank: Frequent Contributor
timestamp: June 04, 2025, 10:44:16 pm
content: Quote from: Terry Bites on June 03, 2025, 05:47:24 pmWell, there are two input sources to the LMC-1992:1) 3-channels of audio from the YM-2149 (programmable sound generator).For reasons unknown, Atari decided to connect these together, resulting in a mono signal instead of mixing them for stereo.There's also an external audio signal ("AUDIOI", probably for whenever an optional Genlock device is attached to the computer, for overlaying videos with text and audio) which joins that mono signal.2) DMA audio (8-bit sampled audio). Shown in blue text on the upper part of the schematic.This is a stereo signal.In summary: mono for the sound-chip generated sounds and stereo for 8-bit sampled sounds.As for the output, there's a mono output going to the Atari proprietary monitor connector (DIN13) passing audio on to its built-in amplifier and speaker.Then there's a couple of phono (RCA) sockets on the rear of the machine giving a stereo output for typically connecting to a home stereo's amplifier. The odd thing here is that it's way above normal line levels.I have no idea where the problem occurs (somewhere along the input stage or the output stage?), and I should really check by tracing a test signal, but unfortunately my Atari's currently not up and running, so that'll have to wait for later.But if someone catches an obvious hardware bug in the schematic I'd be happy to hear about itI hear other Atari STe users solving the problem by putting a resistor (can't remember the value) between each RCA output and the amplifier (i.e. modifying a phono-phono cable), but I don't know if it just lowers an already distorted sound, or if lowering it that way fully solves the problem.Anyway.... I want to add a third output, and a stereo one at that. It should drive the two speakers in a computer monitor's stereo sound-bar.QuoteYes! I think a buffer is what I've been vaguely thinking about without quite knowing how all this works and what I was after. A buffer serves as a protection to the rest of the circuitry, doesn't it? So whatever (within reason) happens with the new output (i.e. it's short-circuited or there's a damage to the sound-bar's amplifier causing harmful currents to enter the audio cable coming from the computer) the buffer will protect the computer from damage (even though the buffer itself might need replacing)?QuoteI'm not sure I understandWhich (fake) NE5532 are you referring to?Are you talking about modifying the existing circuitry inside the computer (by the way, there's no NE-5532 there, just an LF-347)? I'd rather not if I can avoid it, and was hoping to build some additional circuitry instead.Bothplanet12andThemadhippysuggest I attach a Y-cable (i.e. attach a 3.5mm stereo jack socket to the LMC-1992 stereo outputs (pins 14 and 16) and I see you agree with that, but with a buffer circuit in addition.Here's what I believe you're all talking about with the Y-cable connection (click image for full size):Your mention of buffers however got me searching for more information and I came across a page entitledamplifier buffer circuit diagram.If I've understood it correct, the first schematic from that page is what I need to create a new stereo audio output (with one of the following circuits for each of the L and R outputs):Quote from: planet12 on June 03, 2025, 08:04:00 amI agree, but I'm open to learnQuoteIs the purpose of the DC-blocking to remove any DC-voltage from the computer's power supply to enter the audio output chain, thus keeping the signal clean?And the two op-amps (and their associated components) act as a buffer, similarly asTerry Bitesabove suggested I have with my new output as well?QuoteAh! So that's why you all suggested a Y-cable!Do the 1K resistor in series with the LMC-1992 and the RCA jack only protect the LMC-1992 or does the protection go the other way as well (such as protecting an attached amplifier in case the Atari goes bad and sends out some damaging currents)?This makes me think about the RCA outputs which will most likely be connected to an audio interface for recording the Atari's audio output, and the RCA outputs don't appear to be buffered (like the mono output from the LMC-1992 is). Would it be smart to add some buffering circuitry to those outputs as well?I'm not sure of the impedanceof my audio interface (UPDATE: its line inputs have an impedance of 24K Ohms), but being a professional piece of gear it's likely different than that of a consumer grade stereo's amplifier.

author: MathWizard
author_rank: Super Contributor
timestamp: June 06, 2025, 11:18:16 am
content: IDK the signal levels involved , so I'm not saying you have to use a buffer, it might be overkill, but if you do...I'd use smaller resistors at the input, like say 2 100k's or 47k's, the op-amp will like it better.For AC purposes, the 2 resistors will look like they are in parallel to GND. And the input circuit will look like a series input cap, and then Rparallel to GND. So they form a voltage divider. And that's a high-pass RC filter, any DC , 0 Hertz signal, is blocked by the cap. And as the audio signal increases in frequency, the impedance (AC resistance) of the cap, will get lower and lower. The point at which the signal would be 1/2 the voltage, is called the -3dB point, or corner frequency, and that frequency is set by fc=1/(2pi*Rp*C)So for 100k resistors and a 100nF cap, w/ Rp=50k, fc = ~32Hz, which is a low bass signal. IDK the typical corner frequency for this stuff, maybe it would be 100Hz.Something that would also block any unwanted high frequency signals, with only a -0.85dB effect on most of the audio range, would be like this bandpass RC filter. And if you wanted to , you can not use a buffer configuration, but instead use an inverting amp, and add some gain.

author: planet12
author_rank: Regular Contributor
timestamp: June 09, 2025, 12:58:29 am
content: Quote from: analogix on June 04, 2025, 10:44:16 pmI'm not suggesting modifying the original circuit at all - just using two very simple passive 1x RCA to 2x RCA splitter cables (or a male RCA to two RCA female adapter, then standard male-male RCA cables), one for the Atari's left RCA output, one for the right RCA output, and then feeding one from each Y-cable into your gear so both get a stereo feed.Between the LMC1992's driving capacity (which isn't clearly specified in the datasheet, but the THD seems pretty okay down to about 2kohm from the graphs), and the 1kohm inline with each output, I'm pretty certain you're not going to damage anything (as in: I would be comfortable doing this with my own gear).

author: analogix
author_rank: Frequent Contributor
timestamp: June 24, 2025, 08:27:48 pm
content: Thanks for all the suggestions.I'm trying to make an informed decision on which route to take, hence all my questions, and also trying to get an understanding of how the existing output circuitry works.So, it appears the general consensus is that the "Y cable" route is the way to go.Is this because it's generally "good enough" here, or because you all conclude that I don't have the necessary skills to determine the correct component values needed for a buffer with filters etc?I'm still scratching my head as to why the original circuitry clearly involves more than "Y-cable" outputs if this is fine.

author: BillyO
author_rank: Super Contributor
timestamp: June 25, 2025, 01:55:59 am
content: Quote from: analogix on June 24, 2025, 08:27:48 pmThe "Y" cable is plenty fine unless the amplifiers being driven are very low impedance which, while not completely unheard of, is highly unlikely.The original circuitry involves using op-amps to combine the stereo to mono for the simple reason of keeping the stereo channels fully separated.The stereo outputs simply go through a 1K resistor and a DC blocking capacitor. The 1K resistor is there to protect the LMC1992 from accidental shorts. The DC blocking capacitor is there to protect the driven amplifier from any DC offset of the LMC1992.

author: analogix
author_rank: Frequent Contributor
timestamp: June 25, 2025, 09:16:03 am
content: QuoteI understand.What is considered low impedance for whatever amp/audio interface etc. will be connected to the output?QuoteExcellent explanation!Now I see why duplicating the existing circuitry for the new output(s) isn't the way to go.But if it turns out that I might need to connect the new outputs to something having a very low impedance -what kind of circuitry would I need to look at? A buffer as suggested earlier, or an inverting amp as two possible solutions?

author: BillyO
author_rank: Super Contributor
timestamp: June 25, 2025, 11:48:33 pm
content: Quote from: analogix on June 25, 2025, 09:16:03 amWell, considering the output impedance of the Atari is 1KΩ, then a load impedance of about 2.4KΩ (combined) would be within 3dB. So, I would consider anything less than about 2.5KΩ (combined) to be "low" impedance in this case.Quote from: analogix on June 25, 2025, 09:16:03 amThe exact response would depend on a number of things. Such as, do you want to build this in, as opposed to a self powered external device? Generally however, something like a TL082 connected like a couple of non-inverting buffers having their outputs pass through a 1KΩ resistor and a 47uF capacitor would produce an output with very similar characteristics to that of the Atari. You could power such a circuit from a couple of 9V batteries and get hundreds of hours of use on a set.

author: analogix
author_rank: Frequent Contributor
timestamp: June 26, 2025, 08:34:30 pm
content: Quote from: BillyO on June 25, 2025, 11:48:33 pmThanksQuoteSince the computer's motherboard will be recased, and the new 19" rack enclosure will house it along with various add-on boards, a new power supply etc. anything goes!I envision a small PCB that attaches to the rack enclosure's rear panel with its new output jacks etc.Two wires will go from this PCB to the motherboard's LMC-1992 output pins.QuoteSounds good!I see a slight problem with the TL082: it appears to need a dual-rail power supply and 18V, while my computer's power supply outputs +12V, +5V and GND.An additional, separate power supply is doable, but a hassle if there are other op-amps I could use. I guess I could generate the negative voltage rail with a voltage regulator IC, but then again my maximum available voltage is below the needed -18V...I'll see if I can find another op-amp which will work with my existing power supply voltages. Will any "general purpose" op-amp do, or are there specific specs (no pun intended) to keep in mind?Another thought: if I decide to go for a buffered output, I'm thinking I might createtwonew stereo outputs:1) for the monitor's stereo sound-bar2) for an external amplifier/speakers or audio recorder etc.The second one beinga replacementfor the existing two RCA output jacks (see the schematic in this thread's first post) because:a) it doesn't appear to be buffered already, hence won't protect an expensive audio interface, audio recorder, amplifier attached if something goes wrong with the computerb) the RCA outputs are for some unknown reason distorted by design. It's a well known problem with this specific computer.I'm not entirely sure where it originates, but if it's at the output stage I'd rather prefer to create a new output than modify the motherboardSo if I want to create two new stereo outputs, is it a matter of just building two identical non-inverting buffer circuits (such as the one you suggested with a TL082), bothconnected in parallel to the LMC-1992 output pins?

author: TimFox
author_rank: Super Contributor
timestamp: June 26, 2025, 10:51:36 pm
content: The datasheet for the TL082 shows that it works down to 10 V total voltage between PS+ and PS-.https://www.ti.com/lit/ds/symlink/tl082-n.pdfThe datasheet was written back in the day when we used symmetrical power supplies, before the demand for single-supply devices.As a result, you have to read the graphs carefully to understand the allowable common-mode input range for other than +/- 15 V supplies.As I read it, if you run it at +12 V (single supply), the common-mode range would be between about +3.5 V and +7.5 V, roughly symmetric about the mid-point (+6 V).

author: analogix
author_rank: Frequent Contributor
timestamp: June 27, 2025, 12:13:41 pm
content: Quote from: TimFox on June 26, 2025, 10:51:36 pmAh! Good news!Obviously I'm not that good at reading the more technical stuff in datasheets, so thanks for clearing that up.So in summary I conclude this means I can attach the TL082V+pin to my power supply's+12V,and the TL082V-pin to the power supply'sGND.

author: TimFox
author_rank: Super Contributor
timestamp: June 27, 2025, 02:02:40 pm
content: Quote from: analogix on June 27, 2025, 12:13:41 pmYes: but be sure to keep the common-mode input voltage and the output voltage within the somewhat narrow limits at 12 V total supply voltage.That particular datasheet is hard to read: better ones quote the common-mode ranges with respect to the + and - supply rails.