Friday, August 18th, 2017

Review of the Fredenstein ‘Bento 10’ 500 Series compatible Rackframe

By editorApril 18, 2016


As a long time user of the API 500 Series Lunchbox, I’ve been looking for a 19” rackmount version for quite a while.  The Fredenstein ‘Bento 10’ is relatively new on to the market and is one of the very few 10 slot racks with integral mains PSU.

So I decided to take the plunge and see how it performed both mechanically and electronically.


Mechanically the unit is very robust, doesn’t flex when lifted and the fit of the cards into the connectors is smooth.  The card side of the multiplane backplane pcb has a very nice rubber type insulating sheet protecting the rack’s backplane pcb which is great considering how often you end up missing the card slot when trying to insert 500 series modules!  This is not a criticism of ‘Bento 10’ : the lack of module guides is a ‘feature’ of the 500 series format.

When you insert a 500 series module into the Bento frame’s sockets, the module’s gold plated pcb fingers don’t go nearly as far into the socket as in my API lunchbox, about 5mm less in fact.  They seemed to mate securely enough but I was surprised that Fredenstein didn’t stick to the API standard dimensions in this regard.  In any case, electrical connection seems fine.


The rear panel has the usual IEC mains socket, mains fuse and rocker off/on switch.  There is no need for a mains voltage switch as the unit’s integral universal power supply accepts mains voltages from 90VAC to 240VAC, 50 to 60Hz.  There’s also a ground lift switch which isolates audio ground from chassis but I had no need to engage this during the test period.

In my now elderly API Lunchbox, I had noticed from new that hum from the PSU did find its way into the last couple of models to some degree, so I was always a bit wary about plugging mic amps into the last two slots.  I was interested to see how the ‘Bento 10’ fared in this regard.

First to say that the Fredenstein rackframe uses a switched mode PSU which are a lot more efficient than linear PSUs and one of the reasons it is capable of supplying 3.2A to each of the ±15V supply rails.  I’m happy to say that the PSU is very low noise.


Using the test set-up above, I compared the output noise of each of the three mic amps under test when they were set to 0dB gain (so basically a low noise line amp).  Each mic amp measured a close to identical output noise spectrum.

If you want the figures (and I know you do!) this meant -71dBu broadband and -93.5dBu within the 22Hz to 22kHz bandwidth. Popping a 400Hz filter into the test set path indicated that the rackframe PSU was worsening the noise figure by around 0.5dB of mains hum related components.  I also checked for high frequency switched mode noise components but these were so low as to be unmeasurable above the audio noise floor.  This is a very good performance for any audio rackframe PSU never mind one which is churning out over 3A per rail.

The PSU itself provides ±15V supply rail LED indicators separately for channels slots 1 to 5 and slots 6 to 10 plus a single indicator for +48V phantom power.  With all five lit (as they should be) then it’s a bit of a too bright Christmas tree effect, so perhaps Fredenstein might consider dimming them just a touch!

I also did a quick check of interchannel crosstalk at 1kHz and 10kHz by feeding the mic amp in slot 1 with high level input tone and then measuring at the adjacent slot 2 mic amp output.  No crosstalk was measureable.  Another great result.

So we have a very high performance rack frame which is great but Fredenstein have added some nice additional features which I’ll briefly describe.

bento10-3First up is support for the API standard pin 6 compressor buss link.  This allows two or more compatible compressors to have their sidechains linked for stereo or multi-channel operation.  Each slot has a miniature buss link toggle switch and you can have up to 5 separate link busses.  A really useful facility.

Next up is the ability to directly route the output of one module to the input of the adjacent slot module, achieved via rear unit toggle switches.  Again this daisy chaining is extendable, so e.g. you could have the output from a mic amp feeding into an adjacent EQ module which in turn feeds into a compressor.  Nothing you couldn’t do with linking cables but nevertheless a thoughtful facility.

You’ll also notice that each slot has four instead of the usual two XLR’s.  This allows certain Fredenstein modules to get two additional audio paths in or out of the rackframe and Fredenstein have already used this facility in four of their 600 series modules (yes 600 series!  Don’t ask me why?)

If you want even more features then the Bento 10D adds an LCD Display and a rotary encoder which allows you to set internal routing and other parameters with non-volatile storage. In addition, you can display the operating parameters of certain 600 series modules.  Brave new world indeed!

At a street price of around £450 in the UK (around €560, US$600) the Bento 10 rack frame is excellent value for money.  Ten slots plus integral PSU in a standard 3RU x 19” frame is just what I was looking for, and the Fredenstein Bento 10 met the bill really well.

Robert Campbell
April 2016

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