What Is A Line Output Converter? The Definitive Guide

In this extensive guide, I’ll explain:

  • What a speaker level to line output converter is – and how they work (with diagrams).
  • What you need to know about their use and installation.
  • Sound quality facts with real-world measurements & testing I’ve performed.
  • Using line output converters for home stereo systems.

…and a lot more!

What is a line output converter?

car line output converter example use installation diagram

A line output converter is a device that converts speaker output signals to RCA line level preamp signals. This is done by scaling down the higher voltage input (amplified) signals used to power speakers that cannot be connected to RCA input jacks as they require a preamplified (“preamp”) input signal level.

Also called a LOC, hi-low converter, or speaker level converter, a line out converter makes it possible to connect factory audio systems or aftermarket head units with no RCA preamp jacks to an aftermarket amplifier. They’re extremely helpful for many installations as they make it possible to customize or upgrade your audio system by connecting to factory speaker wires nearly anywhere.

In other words, they’re an impedance matching device that connects an incompatible audio signal source to nearly any amplifier, EQ, or aftermarket digital audio processor you like!

They provide several great benefits despite being very affordable:

  • Allow you to keep an existing factory stereo or audio amplifier, avoiding the need for pricey replacements.
  • For systems lacking good bass response, an amp and subwoofer can be added relatively easily. This is one of the most common uses, as the factory bass roll-off is often severe and sounds poor.
  • Line out converters can prevent ground loop noise picked up by amplifiers. (Also called “alternator whine.”) similar to how a ground loop isolator works.
  • Factory audio packages that would otherwise require expensive car dealer upgrades or replacement parts can be bypassed.
  • Allow you to retain factory radio controls, especially stereo wheel controls while enjoying the sound you want.

A similar principle is used in speaker volume controls to lower the speaker signal supplied to loudspeakers directly.

What are the different types of line out converters?

line output converter examples

There are two types of speaker level to line level output converters:

  1. Passive – These use passive (non-powered) components to work and are the most common. However, some versions require a power connection to provide an amplifier remote output signal when sound is present.
  2. Active (electronic) – These perform the same function as a traditional passive line converter but also typically include additional features and even a stronger output signal. The cost more and are less common. They work using electronics such as op-amps and other solid-state components.

Passive line out adapters are the most common, as they’re affordable and easily found. They provide a good compromise between ease of installation and sound quality, as you’ll see later. 

Products with multiple RCA jacks are available for multi-channel amplifiers.

How does a line output converter work? Diagram and schematics

Line level output converter diagram

how a line output converter works diagram

While they may have small differences, nearly all common speaker level signal converters use the same general technology for each stereo channel:

  1. The LOC is connected to an amplified musical signal (factory radio or another speaker source).
  2. The higher voltage speaker signal input supplies a pair of input resistors which form a voltage divider. These resistors also serve as a resistive load to work with radios or amps that need a speaker load connected.
  3. The voltage divider reduces the incoming higher voltage, providing ½ the speaker voltage signal across each resistor. (This is the first stage of attenuation of factory audio levels.)
  4. This reduced input supplies a small step-down transformer that does the following:
    • Scales that voltage down further.
    • Blocks direct current (DC) voltage often present at head unit speaker outputs.
    • Isolates the input circuitry from the output jacks.

4. On the output side, each channel’s transformer supplies the low level signal to the RCA output jack and across a fixed resistor or variable resistor, if provided. These transformers are very similar to those used in a ground loop isolator which also isolates the input from the output.

line out converter internal parts labeled

Many line level speaker converters feature variable resistors, also called potentiometers, which allow you to adjust the output voltage level as needed. Typically, these are 1K Ohms (1,000Ω), one per channel output.

Some home or car stereo systems may behave poorly if no speaker load is present, and some may trigger an error or open-circuit/failed speaker condition if a resistive load isn’t present at the outputs. The line out converter’s input resistance (the input impedance) prevents this.

Speaker level vs line level voltage comparisons

speaker level to line level voltage comparison diagram oscilloscope

Image showing a 1V test signal on the speaker input side and the resulting reduced output signal. In this case, at the maximum voltage setting, the output is lowere by about 15:1 – safe for amp preamp inputs.

A typical speaker output from a head unit may be below 10 volts while amplified speaker signals from a stereo receiver or amplifier can be much higher: typically around 28V to 32V or so at maximum output.

While stock speakers may have fairly poor power, voltages at this level are way outside the acceptable input voltage range of most amplifiers with preamp (RCA) jacks and can even cause damage permanently. At the very least, you’ll get nothing but distortion and garbage.

The maximum input voltage of most preamp RCA inputs can vary in range but are typically from 0.5V to around 4V.

As you can see from the image above, the line level signal output of a converter is the same as the input signal but greatly reduced. This is often by a ratio somewhere around 10:1 or 15:1.

Unlike a standard home stereo or car amp, car stereo speaker outputs often use a DC output voltage signal called DC bias. This is done because they don’t have a sophisticated internal power supply that can supply the positive and negative voltages needed to create an amplified musical signal like a “real” amplifier can.

Car stereo speaker outputs and DC bias

dc biased speaker line level converter diagram

DC biasing makes it possible to “bias” or create positive or negative sound wave signal cycles relative to a midpoint voltage. This would be somewhere around +6V, typically. The problem is that amplifiers should not be fed a DC signal as it can result in a DC voltage output that can burn out speakers.

Line level converters prevent this problem as their internal transformers block DC. That’s because a transformer can pass only alternating current voltage signals like those found in musical content.

Amplifier remote wire output feature

line output converter remote out diagram

Some products also feature an amplifier remote turn-on lead. These types add basic electronics and require a +12V power source and ground. They’re often sold for installations where a switched +12V wire is unavailable or inaccessible.

Typically these switch on their remote wire output when the signal sense circuit detects a musical signal or DC bias from a car stereo speaker output. While they’re good in concept, in my personal installation experience, this feature isn’t always reliable (or may not even work at all!) so my advice is to run a remote wire of your own.

Line level converter schematic

line level converter schematic diagram snapshot

(Click image to enlarge)

In case you’re interested, here’s a schematic diagram I created based on a real-world line out converter, the Install Depot HL300 sold in stores and online, one of several I’ve used for test purposes.

You can download the full schematic in Adobe format here.

Should I disconnect speakers when installing a line level adapter?

line out converter installation diagram

In nearly all cases, there’s no need to disconnect factory speakers when installing a line level converter. Leaving one connected to stock rear speakers is one of the most common uses.

Nearly all sold today are designed to allow them to be used with speakers connected in parallel.

[TIP: It’s very important to make the proper connections when wiring an LOC. If one set of inputs is reversed, this can output one channel that’s reversed vs the other channel, which causes sound cancellation at the speakers.]

Is the total Ohms load of a line level converter safe for a radio or amp?

Most LOCs sold today have an internal resistance designed to provide a safe total Ohms load on your amp or stereo when connected in parallel with speakers. There are some instances where it’s a bit low, but that’s generally not an issue with most you buy.

In this table, I’ll provide some example LOC input resistance values and the total Ohms when used with common speaker Ohm values.

Line output converter + speaker total Ohms table

Brand/model Input Ω 4Ω Spkr 6Ω Spkr 8Ω Spkr
PAC SNI-35 16 3.2 4.4 5.3
Install Depot HL300 42 3.7 5.3 6.7
KY K-28 (generic) 92 3.8 5.6 7.4
Install Bay IBLOC01 1,200 4 6 7.9

A few models, like the PAC SNI-35 for example, aren’t the ideal choice for stereos or amps requiring a minimum speaker load of 6Ω or 8Ω. However, most on the market provide a safe total load resistance that nearly any stereo or amplifier can handle.

Unfortunately, many manufacturers don’t specify the input resistance, but it’s generally between 40Ω to 100Ω. Some major name brands like Kicker and Pacific Accessory Corporation (PAC) may provide this value in the specifications.

Do line output converters affect sound quality? The facts with real measurements

line out converter sound quality comparison diagram

Image: frequency performance measurements for 4 examples line out converters. Note that the signal voltage is shown in dBu (decibel voltage units, referenced to 0.775V) where a reduction of 2.5dBu is a reduction of about 0.6V.

Typical line output converters have good sound quality but this varies by brand and quality. Some models, like the PAC SNI-35, offer nearly perfect frequency response, while units with poor performance can have reduced bass and high-frequency sound due to losses.

As a general of thumb:

  • Don’t buy the absolute cheapest, but you don’t need the most expensive.
  • Expect to spend about $12-$20 for a good performer with adjustable outputs. Premium LOC options offer more features but definitely aren’t critical.
  • Most sold in this price range will have similar sound quality and installation features.

Very good, average, and poor LOC audio performance comparison

As you can see from the measurements I’ve made above, your average LOC adapter isn’t perfect. It’s common to lose some signal strength because of variances in the design (mainly how the transformers perform over the entire 20Hz to 20KHz audio range).

Some are great performers while most are ok for average, everyday speaker systems, and most people would be hard-pressed to hear much of a difference. However, it’s definitely best to avoid poor performers or “el-cheapo” units for this reason. Those can have poor bass, treble, and even both, hurting sound quality.

For sound quality fans, you’ll want a brand & model specifically and clearly advertised as having an almost flat output, with 0.5dB+/- (shown at the top) being a great example. Some models offer a higher preamp signal output level which can help with the noise ratio of audio signals.

Why use a line output converter instead of an amp’s speaker level inputs?

line output vs amplifier speaker level inputs

There are several reasons why it’s better to use a line output converter instead of an amp’s speaker level inputs:

  • While speaker level inputs are convenient, they don’t provide a guaranteed isolated signal connection like a line output converter to prevent noise or ground problems.
  • In some cases, an amplifier’s speaker level input connections aren’t a good match for the radio or amp being used as the signal source, which can cause problems and even damage!
  • Connecting speaker wire to a multi-channel amp is an installation hassle, can be messy, and can take longer than the other option.
  • Plan ahead: using a LOC and standard RCA cables means it’s easier and fast to change or upgrade your system including adding additional amps.

If you don’t have the extra money, then using speaker level inputs is usually fine. However, after years of installation work, I never do it personally or for professional work. The amount of money you save usually isn’t worth the time & effort you have to spend.

[**NOTE: This does not apply to home stereo subwoofers with speaker level inputs, as these are often a great way to add a subwoofer when no subwoofer outputs are available.]

How do speaker level inputs on an amplifier work?

Alpine amplifier input stages block diagram

Amplifiers with speaker level inputs work similarly to line out converters but do not use step-down transformers in the signal path. They accept high level input voltage signals and scale them down using a resistor voltage divider directly connected to the line-level input stages.

Since they don’t use isolation transformers like LOCs, they rely on the existing op-amp based isolation stages to prevent noise.

car amplifier speaker level input schematic snapshot

(Click image to enlarge)

In this image, you can see the actual design of a popular Alpine car amp. The speaker level inputs are connected to the same signal path as the preamp inputs before the isolation op-amp circuits.

Using a line level converter with a factory amplifier

connect factory amp to speaker level outputs diagram

Above: A diagram showing how to Integrate your aftermarket amp or other components when dealing with an amp or factory head unit with separate speaker outputs.

A factory source unit with full-range outputs is simple to work with – connect to the speaker outputs directly via the LOC and to the amp you like. However, it’s more complicated for factory amps with separate tweeter, midrange, and/or woofer outputs.

In cases where the factory audio signals provide separate high, midrange, woofer, and bass outputs, you cannot simply connect the speaker outputs together and then connect to an inexpensive LOC. You need a device with a signal summing feature to recombine them into a full range musical signal properly.

It’s relatively easy, however, to upgrade your bass by tapping off of the factory subwoofer speaker output a premium stock amp often provides.

An example of this is the AudioControl LC6i. Additionally, some car audio digital signal processors (DSPs) may offer this as well.

NOTE: If you’re wondering why you can’t just connect a stock head unit to an aftermarket amp and bypass the factory amplifier, it is possible with some adapters but not always an option. Factory head unit outputs that provide low-level signals are often incompatible with standard car amps, so you’ll need a workaround using an LOC.

Using line level output converters with home AV receivers and amplifiers

Home stereos, AV receivers, tube amplifiers, and more are a different case because they typically work with 6, 8, or even 16Ω speakers. Because of the relationship between voltage and current in relation to power, their amplified speaker output have a higher voltage than those with 4Ω rated outputs.

This higher voltage can exceed the maximum input voltage. This can potentially saturate or even burn up standard car stereo line out adapters. However, they’re fine for general use at low to moderate volumes.

For example, models rated 80W can be used with a home amp with up to 40W-50W of power supplied without issues in most cases.

Using a car stereo line out adapter with home stereos and amps

car line out converter with home stereo amp diagram

If you’ll be using an amp or stereo with a high power rating or at higher volumes, there’s a simple solution: add a series power resistor inline with one of each pair of speaker inputs. The internal resistors will receive a fraction of the input voltage much closer to car stereo use as intended.

Follow my diagram above and the table for recommended values. However, note that it’s a compromise for higher power amps and receivers. The speaker voltage the LOC receives internally will be lower.

That’s one reason it’s ideal to have a model with an adjustable output.


About the author

Marty is an experienced electrical, electronics, and embedded firmware design engineer passionate about audio and DIY. He worked professionally as an MECP-certified mobile installer for years before moving into the engineering field. Read more »

Your comments are welcome.
  1. Can i use a LOC in reverse? Meaning i have a new head unit that has RCA outputs and I want to use the factory Amplifier which has speaker level inputs. If i hook the RCA from the head unit into the LOC then tap into the audio signal lines that used to go from the factory radio to the factory amp, will this work?

    • Hi there Chad. That’s a creative idea! My thought at the moment is that technically, in principle, it could work. However, I believe the output signal on the speaker level side will be affected by the input resistance on most typical LOCs.

      To find out for 100% sure I’d need to test one, which I can do. I happen to still have some left over from other testing so if you need me to check let me know. Best regards!

  2. Hi Marty. This great info. I’m wondering if you can help with the following. I’m a guitarist and I have an amp on which the speaker output is muted when you plug in headphones. I’ve seen it stated that an auto LOC can be put between the speaker terminals and a 1/4″ jack to enable a headphone or lineout connection e.g. to a mixer, while still getting output on the speaker. Is this true and if so can you advise on the wiring? I assume the resulting output will be mono, since the main speaker output is mono. Is that correct?

    • Hi Adrian yes you can definitely do that with a LOC. Yes it will be mono as the source signal is mono, and you’ll only need one side of a stereo LOC to get a line level signal out. The other channel won’t need to be used in this case (since it’s a mono signal you could use a “Y” RCA adapter on the output if you like).

      You’ll just connect the speaker level inputs to the speaker wiring as seen in my diagrams above. It’s very simple. Best regards.

      • Thanks Marty. I’ll give that a go! One last question: if I use a 2 channel LOC, will the unconnected pair of wires generate any kind of interference or hum, or can they just be insulated and left alone?

        Best regards

        • Hi, they’re electrically separate to it won’t matter. So yep, you can just leave them insulated and unused until you need them for some other project later on.

          By the way, I forgot to mention earlier that an adjustable output LOC will be a better choice than one without the adjustment. The input signal (speaker signal) will affect the output level. Therefore being able to adjust the output can be helpful sometimes for non-typical projects like this.

          All the best!

          • Hi. I have a Fosi Audio BT10A amp with a 50+50w rating. I want to use a Line level converter to hook up an aftermarket audio system to the amp. Maybe I will use it at max volume so I’m planning to use resistors but I’m wondering if these would have an effect on the impedance (ohms).

          • Hello there. I’m a bit confused by your comment as you mentioned both using a line level converter *and* using resistors. You shouldn’t be using resistors – you need to use a line output converter to ensure you avoid any major issues and also keeps the signal grounds isolated too.

            As I mention in my article most line output converters (LOCs) have a high enough input resistance that it won’t cause a problem for the source stereo or amplifier. Best regards.

  3. Hi, nice content. Considering the scenario of having both speaker level and RCA inputs (as such as Alpine’s reference design mentioned), I see that RCA ground is not indeed grounded. Do you have any reference design of a circuit allowing both speaker level input and RCA (but RCA with common grounding)?

    • Hello Sirvo. Yes, it’s true that there’s not a ground connection. However, you can’t use both RCA inputs and speaker level inputs at the same time as you’ll end up with the higher-voltage (speaker level) inputs creating a current path that can potentially damage components, particularly op amps, especially due to backfeeding.

      If the speaker level inputs apply a higher voltage to the line level adapter, it could also backfeed with a stepped-up voltage to the RCA outputs at the source which is definitely not desireable.

      It might not destroy anything in every case but that will depend on the particular design.

      > Do you have any reference design of a circuit allowing both speaker level input and RCA (but RCA with common grounding)? I’ve never seen anything like that as it’s an either/or situation. Is there some particular goal you have in mind for using both?

      Best regards.

  4. Hi Marty, I might have missed writing that speark-level and RCA input would not be used simultaneously. Either speaker-level OR RCA. The idea is having the same ADC input to support both. I came into a similar circuit as suggested in your article. However, my client wants to have common ground between RCA and differential in spear-level. I could not figure out a way to allow both in the same circuit.

    • Hello Sirvo. Both RCA and differential inputs do not use a ground reference for the signal; the ground is “floating” normally on the input side to amplifiers, etc.

      But if you want to for some reason you could potentially use the input stage’s power supply ground as a reference. I can’t give a good answer to this without more information.

      If the end goal is just to mix speaker level inputs and RCA (line level) input signals, that isn’t so difficult. You can do this:

      1) Speaker input circuit: similar to amp speaker level input stages (scales down the input voltage), and then through an op-amp buffer.
      2) RCA input circuit, similar to #1, but no scaling down is required, and through an op-amp buffer.
      3) Non-inverted outputs of both #1 & #2 go to a op-amp summing circuit. The output will then be the combination of both.

      This basically would be just separating the speaker-level and line-level input sections, then adding them together safely. You can use example circuits for all 3 above to base this on.

      A common ground would only be used as a reference on one side of the signal inputs and would be “floating” as I mentioned. If you have more detailed things to ask, it’s better to reach out via my Contact page. Best regards.

  5. Marty I am planning on building a Line Output Converter with your example schematic. My transformer knowledge is limited my thought is a 115v input and 10v output would be 15:1. I am thinking the amperage is fairly low and not a huge concern.

    • Hi Johnny. You’re right on both those things: 1) line-level signal current is tiny (I’m sure it’s on the order of milliAmps), and 2) that ratio is about right.

      The thing that’s an obstacle is getting the right transformer. The issue is that unfortunately many transformers are terrible for audio and will have very poor frequency response when used for audio. For example, common AC-AC/DC transformers can’t usually support audio without affecting the output badly (it would drop off badly over a portion of the audio range).

      If you have the right equipment you can test transformer frequency response yourself. If you’re only using it for subwoofer purposes though they may work ok. Best of luck with your project!

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