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Connecting directly to a power amplifier

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A new system configuration

The Transporter and Squeezebox provide a novel configuration option for some stereo systems.

The Squeezebox or Transporter is capable of being the sole source of music in the stereo system. The Squeezebox can deliver ripped CDs, digitized LPs, and Internet Radio. If you wanted you could digitize your tapes too.

Once your music is all digital and saved on your music server (or available off the Internet), it is now possible to remove the other (now legacy) sources of music, including CD Transport, tape deck, radio and turntable.

If you choose to remove these other components, the question immediately arises "Can I now remove the preamp"?

The answer is yes, you can, with some care.

The really short answer is:

*If you remove the preamp and fail to ensure line level is properly matched to amplifier input sensitivity, you may harm your speakers, or ears, or you may get poor performance from your system.*

A slightly longer answer is available for the Transporter only. If you have the Transporter and you are using unbalanced connections, set the internal attenuation jumper so that when the Transporter shows full scale (full volume), you are at the maximum amplifier volume you want to use. Start with the attenuation jumper set to -30 dB and reduce the setting towards 0 dB until the maximum desired listening level is achieved.

Now the long answer.

What does a preamp do?

The preamp provides:

  1. source selection
  2. volume control
  3. line level balancing between sources
  4. equalization (optional)

The line level is the voltage between the source and the power amplifier. Volume control is effected by increasing (amplifying) or decreasing (attenuating) the line level. The preamp also ensures that sources with significantly different line levels are brought to a common level.

Why remove the preamp?

Several reasons exist why you might want to remove the pre-amp:

  1. Accuracy. Some audiophiles believe less is better - less gear between the source and the speaker results in less opportunity to add distortion, noise, or otherwise "color" the sound.
  2. Simplicity. If you don't need those sources anymore, you don't need source selection. Thus, removing the preamp simplifies the stereo system. It can be as simple as source (Squeezebox or Transporter) to power amp to speakers.
  3. Cost. If you are building a new system, removing the CD transport, tape deck, AM/FM radio, and preamp results in a system of much lower cost. You can then use your savings to buy better speakers!

So what's the problem?

Don't you just connect the Squeezebox or Transporter to the power amp? The connectors look the same.

Well, yes, you do, but...

You need to consider the function you eliminated when you deleted the preamp, namely, line level balancing. To understand the problem, we need to understand a couple of issues about sources and power amplifiers (or just skip to the next section).

Source line level

A source will usually have a statement of its audio analog output level, specified in Volts (V), usually either Volts peak-to-peak (Vpp) or Volts rms (Vrms).

These are the analog output levels for the Squeezebox3 and Transporter:

Device Unbalanced Level Balanced Level

Squeezebox 3

6 Vpp N/A


2 Vrms (5.7 Vpp) 3 Vrms (8.5 Vpp)

Notice that the output line levels vary somewhat by type of connection and between devices. This is typical of source devices, and while the Slim Devices output line levels are close together, other devices are not necessarily the same. This difference is one of the things the preamp controls. When the preamp is removed, only the uncontrolled source line level remains.

Power amp gain and input sensitivity

A power amplifier has three properties related to this issue:

  1. Gain, specified in decibels (dB).
  2. Rated Power, specified in Watts (W)
  3. Input sensitivity, specified in Volts (V)

Consider, as an example, the Bryston C-Series amplifiers and an older Rotel RB-890 amplifier.

14B SST 3B SST RB890
Input Sensitivity 29dB; 2.3V 29dB; 1.3V  ?; 1.0V
Rated Power 600 W 150 W 160 W

(Note that we are skipping over the fact that the Bryston amplifiers have selectable sensitivity for the moment.)

This table means that the amplifier provides 29dB of gain, or, in other words, amplifies (increases) the line voltage by a factor of 28.2. It also means that if the line voltage offered to the 14B line input is 2.3V, the output voltage is about 65 Vrms. Most importantly, this table means that at 2.3 Vrms on the amplifier line input, the maximum power output of 600W is achieved.

Finally, the problem - line level to input sensitivity mismatch

Let's take an example of the Squeezebox plugged directly into the RCA input connectors of the 14B.

The Squeezebox offers 6Vpp which is 2.1 Vrms. If you turn the Squeezebox all the way up to 100, you will offer 2.1 Vrms on the analog output. The 14B needs 2.3 Vrms to reach 600W. What will full scale on the Squeezebox get you?

2.1 Vrms x 28.2 = 59.2 Vrms. P = 59.2^2 / 8 = 438 W

In other words, the Squeezebox will leave 162 W unused (assuming loudspeakers with an 8 ohm nominal impedance).

However, if the Squeezebox had been plugged into a 3B SST, the problem is different. The 3B SST is rated at 1.3 Vrms to achieve full power of 150W. 29 dB gain on 2.1 Vrms still means 438W (if the amp could generate it). The end result in this case is different: the amplifier clips if the Squeezebox volume is set to full scale.

How do you keep that from happening?

In this example, the ratio of maximum source line level to input sensitivity is 4.2 dB. Thus, the objective is to reduce the line level by 4.2dB. This can be done with an attenuator (also called a pad), discussed in the next section.

Note that this example was not very extreme. 4 dB is 2/3 of one doubling in loudness. But amplifier input sensitivity may vary by manufacturer and can be lower, sometimes much lower.

For example, consider a Squeezebox plugged directly into the Rotel RB890. In this case, the gain is not known, but input sensitivity is. Our first objective is to reduce the Squeezebox maximum line level to match the RB890 input sensitivity. Recall the Squeezebox is 2 Vrms output, and the RB890 is 1 Vrms input sensitivity.

How much attenuation is required?

dB = 20 * log ( 1 / 2 ) = -6 dB

We need -6 dB attenuation to match maximum output to input sensitivity. This is the minimum recommended attenuation.

But let's say, in this case, that your 160 wpc amp is in a small room with bookshelf speakers rated to 100W. You do not want to go to full power as it will certainly induce speaker distortion, might damage your speakers and perhaps may wake the neighbours. Also, it is recommended that you avoid running the Squeezebox low in the volume range. Let's say you are currently playing the Squeezebox comfortably at 50 of 100 on the volume and you decide this is the maximum volume you want.

How much attenuation do we need to make the volume level you hear at 50 into an RB890 with no attenuation (line levels unmatched) be the same as when the Squeezebox volume is full scale (100) and levels are matched? Our objective is to get the listening level of 50 to become the listening level at 100 (full scale) using inline attenuation.

First, we known we need 6 db of attenuation to match maximum levels. Second, each step of the Squeezebox volume control down from 100 is .5 db of digital attenuation.

How much attenuation is 50? (100 - 50) * .5 = 25 dB.

Total attenuation required to match listening levels to full scale (volume of 100) = Maximum line level attenuation + listening attenuation = 6 + 25 = 31 dB.

So to get your listening levels matched, you will need a 31 dB attenuator as discussed in the next section.

In summary, it is essential to ensure the source output level is matched to the amplifier input sensitivity (the input level that achieves maximum power). Since you have removed the preamp, you must assess the mismatch, and if the source line level exceeds the amplifier sensitivity significantly, attenuation is recommended. Failure to ensure the source line level is properly matched to amplifier input sensitivity may result in:

  • too much power to the speakers, overdriving them to their mechanical stops and potentially damaging them
  • too much voltage to the amplifier, causing clipping and potentially damaging the speakers
  • accidentally increasing the volume control beyond a safe level and damaging your hearing
  • reaching full scale on the digital volume control and still having unacceptably low volume

How to resolve the problem?

Line level below input sensitivity

If the source (Transporter or Squeezebox) line level is below the rated amplifier input sensitivity then you may want to boost the line level. To do so, you need a preamp.

You might want to consider a small preamp that only does line level control and not all the other functions. Or you might want to leave your existing preamp in place.

Line level above input sensitivity

If the source (Transporter or Squeezebox) line level is above the amplifier input sensitivity rating for maximum power you need to decrease the line level using an attenuator in the line level connection between the source and the power amp.

Attenuation can be provided by:

  • a preamp (but this is what we are trying to remove)
  • the Transporter unbalanced attenuator jumpers (accessible inside the case via allen key)
  • an in-line commercial attenuator (either fixed or variable, balanced or unbalanced)
  • a DIY resistor network assembled into connectors and enclosure
  • the amp itself via a built-in potentiometer or selectable input sensitivity switch

Commercial Attenuators

Commercial attenuators are placed in line between the source and the power amplifier, either on balanced or unbalanced left and right channel connections. The Squeezebox has RCA analog outputs that require RCA style commercial attenuators. The Transporter has XLR analog outputs that require XLR style commercial attenuators.

Examples of inline commercial attenuators mentioned by users include:

Company Attenuation Style


fixed RCA, XLR


fixed RCA, XLR


stepped RCA, XLR

Creek Audio

variable RCA


fixed RCA


stepped RCA, XLR


variable RCA

A benefit of inline attenuators is that the attenuator impacts both the line level (the signal) /and/ the noise floor (the noise), preserving SNR.

Transporter Built-in Attenuation

The Transporter provides built-in line level attenuation for the RCA unbalanced connectors. Balanced attenuation must be provided externally.

The internal attenuation is provided using an internal jumper setting inside the case. Four attenuation levels are provided: -30dB, -20 dB, -10 dB and 0 dB.

What line levels can you expect using the internal attenuators?

Attenuation (dB) 0.0 -10 -20 -30
Line Level (Vrms) 2.0 0.6 0.2 0.06

Why not just use the digital volume control?

One reason not to depend on the digital volume control is that Slim Devices audio players are driven by software. There is always a risk (however small) of a software defect that causes the analog output to go to full levels by accident. This would result in the potential speaker or hearing damage if the levels are not matched.

Reducing the volume on the Transporter or Squeezebox occurs in the digital domain before the DAC. If you have to keep the volume very low to "fix" the line level mismatch, you are losing dynamic range and SNR.

An owner has tested Squeezebox audio performance to show the impact of operation in the low end of the digital volume control.

The Squeezebox head designer recommends attenuation for matching line levels to input sensitivity, and reserving the digital volume control for regular day to day listening control. He also discusses the impact of digital volume control on SNR in this article.

Practically speaking, however, if a mismatch is small (e.g. .5-1 dB), you might elect to use the Preamp Volume Control in the SlimServer GUI. This setting provides a fixed upper limit on the volume control range. While not recommended because it is effectively the same as using the volume control, a small mismatch is not going to risk hearing or speakers and maybe more convenient than adding small fixed attenuators. An alternative is to simply live with the small mismatch especially if there is headroom in the amp beyond published specifications. Some may choose a small attenuator over the Preamp Volume Control to ensure SNR is maintained.

Other Power Amp direct connection problems

Powering off the Squeezebox generates a hard click or pop

If power is turned off on the Squeezebox or Transporter while the power amp is connected, you may get a click or pop on the speakers. This is due to power being removed from the analog outputs and the power amp amplifying the resulting transient.

To resolve this,

  • log into your slimserver
  • select the Squeezebox that has the problem
  • select Audio from the top dropdown menu
  • under the section titled "POWER OFF AUDIO", select "Power always on" in the drop-down menu and push "Change"

Squeezebox with Power Amp results in high pitched tone in right channel

Users connecting their Squeezebox directly to the power amp have reported in this and this forum posting a high pitched tone only in the right channel tweeter that varies in volume due to brightness and pitch due to amount of text displayed.

The tone is 8865 Hz at -121 dBr. This issue is actually reported in the Stereophile review (Figure 7):

The right channel, however, produced ... a cluster of spectral lines in the 9kHz region. Peculiar.

To resolve the tone, add inline attenuation as described above. The audibility of the tone is due to the line levels not being properly matched, and the noise floor is being inappropriately amplified. You may also find that the speakers are "too noisy" as well.

A DIY resolution to the problem is via a grounded shield between the display and the PCB.

Squeezebox with a Balanced Power Amp results in loud hum

If you get a loud hum when you connect the Squeezebox's analogue outputs to a power amp with balanced inputs, see the discussion on the BalancedPowerAmps page.


Contributors: EricCarroll