[Review] Bent Audio "NOH" pre-amp

Bent Audio "NOH" pre-amp

You may remember that I decided to go the pre/power route for my amplification upgrade. I decided to build myself a power amp based on the ZAPPulse 2.2SE modules from LC Audio and then get a transformer based passive pre from Bent Audio.

Well, the power amp project is currently stalled until I receive the case I ordered nearly a month ago . Everything else is ready to go. All the bits and all the tools.

Still, it's not all bad because it means I have time to evaluate my new pre-amp properly without getting distracted by the power amp build. It arrived yesterday from Canada. This time the customs bureaucracy was relatively painless and not that expensive either - even allowing for the fact that I payed less than I should have done because John @ Bent Audio had marked the value as $1000 (max. insurance value) instead of the $1575 it actually cost me (shipping included) .

Description

The NOH preamp is a transformer based pre which uses two Stevens & Billington TX102 transformers to provide attenuation instead of the resistors used in most volume controls. This overcomes the 2 most common issues with passive pres using resitive attenuation, namely the high output impedance and the sensitivity to cable lengths and impedances (the two issues are related). The transformers also provide input to output isolation which lowers the noise floor of the system. Lastly, using transformers also makes the job of converting from RCA inputs to XLR outputs or vice versa trivial.

The actual volume control is a Seiden 45SG 23-position switch which switches the output to the 23 different secondary output windings of each transformer that provide the different levels of attenuation. It has two "decks", one for each channel. They're not cheap switches at $150 each! I got the optional remote control which is done with a step motor and rubber belt to drive the volume knob The remote control receiver/control PCB and motor are powered by a 9V "wall wart" supply which is entirely separate from any of the audio wiring. The remote option also adds a nice little blue LED in the volume knob as a position marker.

Input selection is done separately for each channel! Once you take a look at the internals of the thing you can see why. Each input selection switch is a Seiden 32SG 6 position 2-pole switch for switching the inputs to the TX102 transformer primary winding.

The whole thing is immaculately put together in an aluminium chassis with black perspex sides and the aluminium top panel is also attached to the black perspex front panel. The rear panel has stoppers to fill the unused input/output holes allowing you to add extra inputs in the future if you think you can handle the intricate wiring required! The pictures make it all a lot more obvious.

I personally think it looks absolutely stunning and it's clear from using it that you're dealing with a very well built high quality bit of kit. I got mine made for me by John Chapman (the man behind Bent Audio) but all configurations are available in kit form. IMO whilst it might look easy there's a hell of a lot of fiddly little wires in there and I'd only recommend DIYing it if you're very handy with a soldering iron and extremely patient!

Pictures

Front view showing the central volume knob and the L & R input selector switches. The position indicator on the volume knob is a blue LED. What appears as a white dot to the left of the volume knob is the remote IR receiver.


Rear panel. Mine has one XLR input and two RCA inputs. In the centre are the outputs, RCA and XLR. There are spare holes for extra inputs and outputs and also headphone outputs. Each one is built to order so you can choose your spec. In the centre of each input panel is a toggle switch which switches between "hard" ground (input and output ground are connected) or floating ground. I prefer to use the floating ground position for total source to output isolation. The two toggle switches in the middle of the output panel switch between RCA and XLR outputs. On the right side of the panel you can see the input for the 9V supply for the remote control board.


Inside. Pretty straight forward really. The two big transformers, the volume switch and the two input selectors. PCB is for remote control. The blue/black wiring around the volume control shaft are for the blue LED inside the knob. Lots and lots of intricate wiring! One thing I didn't understand at first is that a lot of the wiring appears to be bare copper and I couldn't see how there weren't loads of short circuits. Well, it's not bare wire, it's coated with a thin transparent enamel apparently.


Hooking it up

In order to use my Arcam FMJ A22 as a power amp I had to open the case and flick a little internal switch which puts it into power amp mode. In that mode it's functionally identical to a P25 power amp and you use the "Power In" inputs. Cables next. I was using some MF NuVista RCA cables with Bullets between my DAC64 and the Arcam but the NOH preamp has XLR inputs so I wanted to use those. In anticipation of it's arrival I'd already bought myself two pairs of Van den Hul D102III XLR cables at £81 a pair from hificables.co.uk (the other set will go from pre to power once I've finished the power amp). I used the MF NuVistas now to go from the NOH to the Arcam as the Arcam only has RCA inputs.

For use with my AV system I've got the front pre-outs from my AV receiver going into one of the RCA inputs and then for AV use I just crank the volume to full (unity gain). The remote receiver is switchable to respond to either Daewoo or Sony TV volume commands so it's extremely unlikely to interfere with other bits of hifi or AV kit - particularly useful when using it in bypass mode for AV. I just have to make sure I turn the volume down again before switching back to the DAC input :MILD:

How it Sounds
This is the bit of writing any review I'm absolutely useless at. I also think it's quite impossible to describe how hifi sounds using words in a way that anyone else would have even the remotest idea of what I meant. It's like trying to describe the taste of wine. I'm also a "big picture" kind of person and am completely unable to describe detailed differences. "Better", "Worse" or "the same" is about the extent of my descriptive abilities when it comes to hifi. Don't ask me "better, how?" because I won't be able to tell you but I know it's better . So, an improvement then? You could say that . I'm over the moon with it, especially as it was such a shot in the dark purchase. I'd say it was one of the biggest improvements I've made to my system. Certainly the biggest since I got my DAC64. I'll do my best to expand a little. The most obvious thing when listening at low volumes last night was that at low volumes (even the lowest volume step) the system sounded no different, just less loud. All the soundstage, dynamics and texture that you hear at "annoy the neighbours" volumes was still there which was quite incredible. I've never heard a system play so well, so quietly. With the Arcam as an integrated if I couldn't listen above a certain volume for whatever reason then I just didn't listen at all as the total collapse in sound quality just took away any possible enjoyment. Now I can listen happily at minimum volume - a huge bonus for late night musical pleasure. This low volume capability is of course one of the well known benefits to using transformer based volume controls allthough I don't quite understand why that is.

Apart from that I would say that everything was just so much clearer. There was much more detail, particularly noticeably in fact on transients. You can really hear the attack of a piano note or guitar string properly. Lastly, there was not a hint of harshness or agressiveness. I wouldn't say it was "smooth" but somehow "creamy" and "chocolatey" are words that come to mind. What did I tell you about trying to describe hifi?

In short, it's taken my system up a notch or two and now I can't wait to get the power amp built. People on AudioAsylum who have a NOH have compared it more than favourably against the likes of the SimAudio P5 and other pres costing upwards of $5000. I don't have any experience with pre-amps so I can't comment but I think that for US$1575 (including shipping to Europe) it's an unbelievably good bit of kit, especially at current exchange rates. Prices vary according to specification of course. See the website: http://www.bentaudio.com/index2.html

Incidentally John Chapman, the guy who builds the preamps is an extremely nice guy to deal with and always responds to e-mails quickly. We kept in touch regularly over the 4-5 weeks it took from order time to delivery. Payment is only required a few days before delivery.

Michael.

 

Moved this to the HiFi section temporarily as it would appear that no-one visits the Reviews forum...or no-one cares

Michael.

 

Been following this with interest, Mike, but silently. Looks good. If I ever take steps along the vinyl route I'd need some kind of source selection/volume control (as I'd probably have a separate phono stage), so would be interested to hear how it sounds wrt just the direct connection I have atm. Nice write-up (until the "how it sounds" section anyway ). Really interested in how the ZAPpulse turns out, in case my pwr amp ever dies.

 

Thanks Graham. I'm really interested to see how the ZAPPulse turns out too! Since you've gone the "Class A with balls" route with your Patriot, you might prefer a couple of LC Audio 118W Class A monoblocks (the ZAPsolute 4.1 .

Michael.

 

Congratulations... :MILD:

Wasn't it supposed to have a volume bypass for AV, it is pretty dangerous the way you are using it...

Why dont you calibrate the AV to, lets say, mid level of the volume control, less dangerous and still easy to set...

 

Very nice Michael and cheaper than expected (I actually benefitted in the same way as yourself when importing. Maybe the Yanks ain't so bad afterall ). No need to ask if you're a happy bunny .

We wait with baited breath for the first time you forget to turn the volume down :micro: :SWMBO: .

 

I'm going to have to leave VERY CLEAR instructions to SWMBO for the 2 weeks when I'm in London each month. Otherwise could end up being :cry: .

After discussing the build with John he said that a bypass was difficult to do properly with a mixture of RCA/XLR inputs/outputs because he's relying on the transformer to do the RCA to XLR conversion so it can't just be a straight bypass (input connected to output).

The way it is is fine really as it gets over the 2 main issues with amps that don't have a specific bypass:
- finding the same volume setting for AV each time. Since there are 23 distinct, fixed volume steps it's not hard to find the same setting again and max volume is actually a proper unity gain.
- having the remote volume on the amp respond to the same volume commands as the AV amp/processor. The NOH uses Daewoo TV commands so not much chance of a conflict there

It's not actually that dangerous. In order to be a problem there would have to be a CD playing when switching back to the CD input which is unlikely. You have to switch inputs manually so it can't happen by accident and I'll just make a simple rule of always turning the volume to 0 whenever changing inputs (a good rule to follow anyway).

Worst case, since you change inputs one channel at a time it will only blow one speaker, not both

I haven't calibrated the AV amp yet so I might do as you said and calibrate it to a lower volume setting on the pre.

I'm extremely :banana: :banana: about this pre.

Michael.

 

Must confess I too have been stalking the NOH saga following my audition of various bits and pieces, one of which was a Music First Audio Pre.

This is also Billington tranformer design and I was well impressed The only downside is no remote. I've been in touch with MF and they say remote is not a possiblility due to size considerations so the NOH is a possible alternative.

THe MF Audio is much funkier looking though , the NOH does seem a bit er... functional aesthetically ! Might be better in the flesh so to speak.

The NOH would probably work out slightly cheaper (the MF is £1500) so an interesting comparison ]

 

that's the beauty of the cyrus pre x / av8 combination (when connected via "mc-bus"), you switch the input to "VI", the AV8 switches itself on automatically, and the pre X winds up the volume to match. switch back to cd, and the pre x snaps the volume back to the previous setting. only the av8 doesn't switch itself off again :-(
michael, wasn't ignored by me in the reviews section, i've been watching your developments with interest too. brave move, to buy "blind" (should that be "deaf" ?) but that really paid off !
plus i can 2nd you on the difficult bit of reviewing - actually describing how it sounds. for example, had my pre x for a couple of weeks, and it sounds .... much better.

 

Here's a pic of it in my system:



I think it looks rather nice

My NOH cost me $1575 including shipping + €230 in VAT and customs on arrival which adds up to about £1050 alltogether at current exchange rates. Quite a bit cheaper than the MF. The MF has more inputs and only a single source selector (easier to use I suppose) but no remote.

Bent Audio apparently get their TX102s made specially by Stevens & Billington with their own spec OCC coper wire (or silver wire for the much more expensive option) but I doubt that would make fundamental differences. I imagine that the MF and the NOH (or any TX102 based passive) would sound pretty similar.

If you want to see the TX102 based passive for mugs take a look at this:

http://www.avtac.com/Products.htm



Uses the exact same TX102 transformers but they've stuck it in a fancy case and added some of those cute nixie tube displays. Yours for a trifling $8500 .

Michael.

 

Ah.... if money was no object Funky looking piece though and although its Canadian $ admittedly it's still £3.4K

The NOH does look good in your rack I must say, great silver/black theme !!!

I can't see noh reason why the MF and the NOH should sound much different either !!!

 

Hi,

All this sounds very interesting. I may have a dabble with one. I'm looking into it now.

SCIDB

 

Hi Michael,

Is your power ready...

 

No, not yet. The case only just arrived this week and is only now going through the usual customs crap (it came from the US). Hopefully I'll have it next week and then I can start building

Michael.

 

Hi,

With reference to transformer pre amps, I have seen this.
The Django pre looks very interesting. This uses the same Steven & Billington transformers as the Bent Audio & Music First pres. It's doesn't have a remote control but it's at a cheap price to have a dabble.


SCIDB

 

Our British 'Sowter transformers' make a tapped volume transformer as well, to keep the money in the UK..

 

The Stevens & Billington transformers are also made in the UK

Michael.

 

Hi,

Been looking more into this. Most of the transformer pre amps on the market (most are US based) use the Stevens & Billington transformers (UK made). The Django is about half the price of the Bent Audio kit. It uses the same transformers & volume switches (Seiden). All the comments on the S & B transformers have been more than very good but the Sowter transformers look interesting.

The Sowter transformers are £86.63 + VAT each. I don't know how much the S & B transformers are in the UK but Bent audio sell them for approx £111 each. The switching costs around £100 from Bent as well but you may be able to get the same make cheaper. Different & cheaper switching can be used of course. The Django is about £360 + postage (& maybe duty tax etc).

I would interesting to see how much the S & B transformers are to buy from the Uk compared to getting them from the far east & US.

SCIDB

 

The Django does look like a good intro but it does have limited inputs 1 balanced and 1 unbalanced

Jeff

 

Hi,

Here is another review of the transformers.

SCIDB