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Inward Connections Vac Rac TSL-4
4.75 4.75 out of 5, based on 2 Reviews

The TSL-4 is two channel tube limiter by Inward Connections. It is based on the TLM-1 module of the original Vac Rac system, but presented in a 3U rack mount chassis with several features to modernize the unit.

30th September 2013

Inward Connections Vac Rac TSL-4 by LVP

  • Sound Quality 5 out of 5
  • Ease of use 5 out of 5
  • Features 4 out of 5
  • Bang for buck 4 out of 5
  • Overall: 4.5
Inward Connections Vac Rac TSL-4

To give you a little of my background, I have owned the TSL-4 for a little over a year. My studio work primarily consists of producing, engineering, and mixing rock and indie records. I have a good amount of experience with the common compressors that you will see in many studios. I do not have any experience with the previous iterations of the Vac Rac so I can not directly compare my experience with the TSL-4 to the earlier versions of the Vac Rac. The TSL-4 can also be ordered as the TSL-4V which is the same unit with NOS tubes. My unit being reviewed here is the basic TSL-4 which ships with Electro-Harmonix tubes.


The TSL-4 comes in a solid 3U chassis covered in the signature Vac Rac rust colored finish. The unit has two channels that can be operated separately or linked for stereo use. Each channel features a 40 detent pot for threshold and gain make up. Each channel also has three switches. One to activate the high pass filter in the detector circuit, one to switch between gain reduction and output metering, and one to bypass the compression on that channel. There are two gorgeous backlit VU meters, one for each channel. The VU meters are huge which makes them easy to read across a room. Lastly, each channel has a little thumb screw to allow you to zero out the VU meter in the case that they drift a bit away from zero over time. There is one link switch on the unit for stereo operation. Both the audio inputs and outputs of the unit are transformer balanced. The previous TSL-3 had a transformerless unbalanced output. The high pass filter and the 40 detent pots are also new features to this revision of the Vac Rac.

The the unit is very straight forward to use, but also gives engineers a feature or two that sets this unit apart for some other tube opto contenders. The high pass filter in the detector circuit gets a lot of use in my studio. It's a great way to keep vocal plosives from hitting the compressor too hard, or on drum buss duties from letting the kick compress the drums too much. Ultimately the high pass filter is going to give you two different compression feels at the flick of a switch. The 40 detent pots are a nice feature, but perhaps almost too small of increments to really allow for accurate recall from say a recall sheet. In recall situations I have had to just count how many clicks from the left I was to give me accurate recall information. The same goes for matching settings between channels as 20 clicks up on channel one might look ever so slightly different than 20 clicks up on channel two. That said, it is nice to be able to match gain and reduction exactly for stereo material.

In Use

When describing the tone of the TSL-4 there are two tonal characteristics that should be considered. The sound the TSL-4 imparts on audio passing through it and also the sonic signature of how the unit compresses audio. First I will address the former. I would not describe the tone of the TSL-4 as heavily colored. Compared to many other tube compressors that I have used I find the sonic finger print to be less pronounced. The TSL-4 does add a subtle smoothness to signals that pass through it, but it does not add the amount of color to a signal that a LA2A or Lisson Grove would add. That said this is a tube limiter and does have a color, but it's color may be more subtle than other comparable products. I don't often find myself reaching for this box because of it's color, but rather for it's compression characteristics which brings me to the second sonic category: the sound of it's compression.

The place I really find that the TSL-4 shines is in the amazing transparency of it's compression. Even with high amounts of compression, the source still retains all the feel of the source's original dynamics. When applying larger amounts of compression the TSL-4 does not thin out the sources sound like many other compressors will. The combination of those two compression characteristics are a huge win for this unit. In mixing, I can throw the TSL-4 on a vocal or bass track and use the amount of compression I apply to seat it more forward in the mix without sacrificing the tone or feel of the original source. Because the TSL-4 is so forgiving it's also great for tracking situations as I don't have to worry about something coming out sounding over compressed. Here are a few specific places I have used the TSL-4 and found it to really stand out.

Vocals - Can't go wrong with it here. You can really compress a vocal hard and have it still sound great. I would use it similar to how I use a LA2A on vocals in mixing or tracking. The LA2A's I have used tend to start to roll off highs with higher amounts of compression where the TSL-4 leaves the highs intact with similar amounts of compression. If I wanted a more aggressive rock vocal sound an 1176 or Distressor would probably be preferable. The TSL-4 will sound smoother than either of those units on vocals and will not be able to add the potential edge and aggression that compressors of that type can offer.

Bass - Similar characteristics to the LA2A in this application too. I have used it to just add small amounts of bass compression while mixing, and also higher amounts to really reign in performances that were very dynamic. I gravitate towards the TSL-4 to compress bass source that has a great feel and tone, but needs dynamic control. If I want to manipulate the feel of the attack of a bass track I often prefer my 1176. I have used the high pass filter on bass performances that had high notes that were much louder than the lower end content to even out the volume differences between the two.

Acoustic Guitar - I have used the TSL-4 on acoustic guitar a few times. It does a great job of adding a small amount of compression to acoustics, but at higher levels of compression I tended to prefer other units. With acoustic tracks with complex finger picking I found myself really wanting something with release controls to dial the feel of the compression in to the specific feel of the track.

Electric Guitar - I don't usually compress electric guitars unless I really want to add a distinct compression sound the the track. An 1176 or the Softube CL 1B will often do the trick for me in that case. Occasionally I will get a track to mix where the dynamics of the performer just need to be smoothed out without effecting the tone. The TSL-4 does a great job at doing this without making the guitar sound more compressed.

Mix Buss/Mastering - When I am mixing, the TSL-4 is usually getting used on individual tracks or sub groups. I don't fancy myself as a mastering engineer, but on a few occasions have been asked to master singles or EP's. One great application I have found for the TSL-4 in those mastering situations is to use it in conjunction with my SSL buss compressor. I will set up the TSL-4 in the signal chain prior to the SSL and engage the high pass filter on the TSL-4. This allows the TSL-4 to compress the mix based on the midrange and treble content. The SSL will then pick up the compression duties from the low end content. That combination has been super successful for me. I can get really great dynamic control on a mix without it feeling overly compressed.

Drum/Bass - This is a really specific application that I found while mixing once that I really loved. I had a mono drum buss and the bass track on a soul/R&B tune. Both the drums and bass had been compressed in tracking and were dynamically in a good place but they didn't quite groove together dynamically the way I wanted. By running the drum buss through one channel and the bass through the other in link mode, I was able to add compression to the bass guitar from the dynamics of the drums. This small amount of compression really glued the groove of the two tracks together in a pleasing way. Although this is a specific scenario, the independent channel controls of the unit allow for some experimentation when running two unique signals through the unit in link mode.


Over the year I have had the TSL-4 it has become one of the most used compressors in my studio. The feature set is a great marriage of simplicity and flexibility. Because the TSL-4 imparts less color on my tracks than my other opto type compressors, it can be used in more situations where the goal is dynamic control rather than color. Warmth and color are often considered desirable features of an outboard unit these days, but the relatively clean yet far from sterile imprint of this unit is to me one of it's biggest strengths. At the price point you get a great stereo unit that can also be used as two mono units. Whether you are on a budget and can only afford one opto limiter for your studio or you have a large amount of gear on hand, the TSL-4 is going to be a great purchase that will get lots of use. Inward Connections did a great job at delivering a device that does it's job very well and I see this device being a classic that is revered by engineers for many years to come.

1 week ago

Inward Connections Vac Rac TSL-4 by carolcappetta

  • Sound Quality 5 out of 5
  • Ease of use 5 out of 5
  • Features 5 out of 5
  • Bang for buck 5 out of 5
  • Overall: 5
Inward Connections Vac Rac TSL-4

This is a WOW compressor. The HPF is so very useful with a bass guitar track or a kick drum. Steve was a pleasure to speak with, so very helpful and very informative. The tubes add a warmth and even on the highest compression settings this unit is smooth like velvet.Im looking for another one :-)