DIY Amps for NX-Oticas

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Jaytor

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DIY Amps for NX-Oticas
« on: 9 Aug 2020, 11:40 pm »
I've spent the last few months SLOWLY buidling a pair of monoblock amps to use with my NX-Oticas. I've only been able to work on these on the weekends, and I took my time since this is my first audio electronics build since about 1980. Back then, I built a Pass A-40 and a couple of my own designs until I got heavily involved in digital electronics and never had a chance to get back to audio stuff.

As my first (recent) foray into DIY electronics, I decided to build another amp based on one of Nelson's designs (I have fond memories of the A40). This is what is referred to as an F5 Turbo. It's based on the First Watt F5, but uses higher voltage rails (32v in my case), doubled output transistors, and a cascoded input stage. I also used a power supply with a capacitance multiplier to reduce ripple on the rails. There is also a soft start board under the power supply board, and thermal cutoffs mounted to each heatsink.

Output power is about 60W RMS into 8 ohms. I haven't finalized the bias yet, but I'll probably set the bias to around 1 Amp (which will give me class A to about 25W) to keep the amp running a little cooler. The NX-Oticas are fairly efficient, so this will keep me in class A almost all the time.

The power supply board for each channel has 72,000 microfarads of capacitance in a CRC arrangement, but most of the ripple reduction comes from an active Cap Mx circuit.  I used an Antek 400VA toroidal transformer for each monoblock.

The input circuitry has a switchable Jensen transformer for balanced input. There is also a switchable MiFlex copper cap for lower frequency roll-off since I'm using the powered OB servo subs. For outputs, I've got tube connectors in parallel with Cardas binding posts.







I haven't hooked these up to my NX-Otica's yet. I am running them in on a cheap pair of speakers first, to make sure that they are operating reliably. Once they've burned in for a couple weeks, I'll get them hooked up and let you know how they sound.

I'm currently using a Parasound JC-5, which is a pretty nice sounding amp, so it will be interesting to see how these compare. But I had a blast building these, so I'm sure there will be more DIY audio electronics in my future.
« Last Edit: 10 Aug 2020, 01:00 am by Jaytor »

UtopiaNemo

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Re: DIY Amps for NX-Oticas
« Reply #1 on: 9 Aug 2020, 11:58 pm »
This is incredible. Before finding this site, the idea of a DIY amp sounded ludicrous to me. I’m so impressed! Please keep us informed as to how it sounds in comparison to your Parasound!

Early B.

Re: DIY Amps for NX-Oticas
« Reply #2 on: 10 Aug 2020, 12:04 am »
I'm currently using a Parasound JC-5, which is a pretty nice sounding amp, so it will be interesting to see how these compare.

It ain't a fair fight. You may as well put your Parasound up for sale. 

poseidonsvoice

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Re: DIY Amps for NX-Oticas
« Reply #3 on: 10 Aug 2020, 01:04 am »
The power of DIY! Enjoy the fruits of your labor. I rotate my DIY Sony VFET from time to time and have the Aleph J as well as MOFO monoblocks on the building queue...

Best,
Anand.

WC

Re: DIY Amps for NX-Oticas
« Reply #4 on: 10 Aug 2020, 01:43 am »
Enjoy.

I have a 4 channel Mod-86, Mod-686 monoblocks, M2X Monoblocks, Aleph J monoblocks, Folsom 7297, 3255 evm, and 4 channel 3e 3255 amps all in various stages of construction.

Currently using my ACA 1.1 monoblocks.

RonP

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Re: DIY Amps for NX-Oticas
« Reply #5 on: 10 Aug 2020, 02:18 pm »
Keep us posted Jaytor!


Jaytor

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Re: DIY Amps for NX-Oticas
« Reply #6 on: 10 Aug 2020, 07:46 pm »
Thanks everyone.

WC - that's quite a collection of DIY amps. I'm also quite interested in the Neurochrome amps. How did you decide between the Mod-86 and the Mod-186?

- Jay

Skilly

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Re: DIY Amps for NX-Oticas
« Reply #7 on: 10 Aug 2020, 08:10 pm »
Jay,

Can I ask what you are budgeting per amp and your parts source?

Scott

Jaytor

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Re: DIY Amps for NX-Oticas
« Reply #8 on: 10 Aug 2020, 09:48 pm »
Hi Scott - That's not so easy to figure out since I ended up changing the design a few times along the way and ordered parts I didn't end up using (at least for these amps). I'd guess somewhere between $800 and $1000 for each mono amp.

I bought the chassis on ebay from a vendor in China, the transformers with covers from Antek, the connectors from Parts Connexion, the F5 PCBs from DIYAudio Store, the soft start and power supply PCBs from XRKAudio Etsy store, the Jensen transformers (for balanced input) used from an ebay seller, the Copper input caps from SonicCraft, and all the rest of the parts from Mouser. I had the small PCB boards for the balanced input circuit made by a shop in Taiwan.

Building amps like this isn't a way to save money. In addition to all the parts, I also bought a nice soldering station, a Variable AC transformer, a heat gun for heat shrink, two nice Fluke multimeters (I had a cheap one), and a 4-channel oscilloscope. The scope isn't really necessarily, but everything else is. You can get away without the variable AC transformer, but it significantly reduces the risk of blowing parts and/or starting a small fire if you've got a wiring error, and they are not very expensive.

It's definitely possible to build an amp similar to this for a lot less. For example, the standard FirstWatt F5 uses a single chassis, a shared simple power supply, no soft start (other than a single simple thermistor), no balanced input, no copper input cap (which is used to roll-off the low end for use with OB subs), no tube connectors and no thermal shutoffs. I would expect you could build one of these for well under $1K for a two channel amp.

- Jay

AKLegal

Re: DIY Amps for NX-Oticas
« Reply #9 on: 10 Aug 2020, 10:07 pm »
Enjoy.

I have a 4 channel Mod-86, Mod-686 monoblocks, M2X Monoblocks, Aleph J monoblocks, Folsom 7297, 3255 evm, and 4 channel 3e 3255 amps all in various stages of construction.

Currently using my ACA 1.1 monoblocks.

What are your impressions of the Modulus amps?  I just finished a Folsom 7293 and I am looking at the Modulus 186 as my next build.

Skilly

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Re: DIY Amps for NX-Oticas
« Reply #10 on: 10 Aug 2020, 10:44 pm »
Jay,

Great response, thank you. This is a project I have been contemplating since I sold my Nakamichi PA-7. I really like that beast, but I needed to do it. I have been lurking at DIYaudio for some time. They never seem to have complete kits available. Its weird, how they have boards but no parts or parts but then no boards. No links for transformers.

I appreciate your answers.
« Last Edit: 10 Aug 2020, 11:53 pm by Skilly »

WC

Re: DIY Amps for NX-Oticas
« Reply #11 on: 11 Aug 2020, 03:00 am »
Thanks everyone.

WC - that's quite a collection of DIY amps. I'm also quite interested in the Neurochrome amps. How did you decide between the Mod-86 and the Mod-186?

- Jay

I bought the Mod86 boards before there were Mod-186 boards. The Mod-186 costs more, but comes as an assembled board and is supposed to be better, but even Tom says they sound very similar.

WC

Re: DIY Amps for NX-Oticas
« Reply #12 on: 11 Aug 2020, 03:19 am »
What are your impressions of the Modulus amps?  I just finished a Folsom 7293 and I am looking at the Modulus 186 as my next build.

Don’t have any impressions yet, since neither of them are yet complete. I am missing parts for both amps. For the Mod-686 I have the partial amp boards, power-86, and transformers. Just need the heat sinks and the rest of the parts for the chassis and the amp boards. For the Mod-86, I have everything except for parts for the Power-686. I move slowly with my builds. Just finished building 2 Rythmik 15” subs and three pair of speakers with full range drivers. Now I can turn my attention and finances to building amps and DACs to use with the speakers and subs.

Jaytor

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Re: DIY Amps for NX-Oticas
« Reply #13 on: 2 Oct 2020, 11:35 pm »
Well, after listening to my F5 Turbo monoblocks for a few weeks, I decided I wasn't really happy with them. They had good dynamics and no noise (hum or hiss), but just didn't have the clarity (detail and imaging) I was used to with my Parasound JC-5.

I suspect the quality of the components and/or wires I used was not good enough, and I didn't do an adequate job of component matching. The meters I had when I built these didn't provide any accuracy or precision when measuring small value resistors. I've since purchased a desktop multimeter which has much better precision.

At some point, I'll get around to rebuilding the front-end boards with better quality and matched components, and rewiring with higher quality signal wires to see if I can improve the SQ.

In the meantime, I built another pair of amplifiers based on the Neurochrome Mod-286 with a beefy power supply. These have plenty of power for the NX-Oticas and really sound fabulous. I'd say at least as good as the JC-5 and that's with lesser quality speaker cables. I've just ordered a speaker cable kit from Danny to make some short cables to use with these new amps.







Rock Ball

Re: DIY Amps for NX-Oticas
« Reply #14 on: 4 Oct 2020, 12:27 pm »
In the meantime, I built another pair of amplifiers based on the Neurochrome Mod-286 with a beefy power supply. These have plenty of power for the NX-Oticas and really sound fabulous.

Nice job!  That looks like a fun build.  Was it a kit or did you source all the individual components yourself?

Do you have a build thread for these on any forums?

Jaytor

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Re: DIY Amps for NX-Oticas
« Reply #15 on: 4 Oct 2020, 04:25 pm »
Thanks Rock Ball.

These weren't done from a complete kit, but the Modulus-286 amplifier module is sold assembled by Neurochrome.

https://neurochrome.com/products/modulus-286

The chassis is the 3U Mini-Dissipante from ModuShop available through the DIYAudio Store.

https://diyaudiostore.com/collections/chassis/products/mini-dissipante-3u?variant=12173317060

The soft start board is from XRKAudio, available through his Etsy shop. I used this in my FirstWatt F5Turbo as well.

https://www.etsy.com/listing/769363642/soft-as-a-feather-pillow-sfp-solid-state

I bought the power supply boards on Ebay - fairly cheap but fit in the case and provided what I needed. This is a CRC type design - two capacitor banks split by parallel resistors - which provides better ripple reduction than a single capacitor bank supply. I built mine with 2x15,000uF per rail in the first bank and a 22,000uF per rail in the second bank. The board doesn't include a snubber circuit to cancel transformer secondary resonances, so I added this to the back of the board.

https://www.ebay.com/itm/CRC-6-Capacitors-PASS-Amplifier-PSU-Power-Supply-Rectifier-Filter-Bare-PCB-Board/133283479788

All the parts for the soft-start board and power supply board are from Mouser.

The Transformer is the Antek AS-4225 (400VA, dual 25v secondaries). They also sell the steel cover for this transformer.

https://www.antekinc.com/as-4225-400va-25v-transformer/

The power supply is overkill for this amp. In fact, Tom Christiansen, the designer of the Modulus-286, recommends a supply a quarter this size. But I've generally found a larger supply provides better dynamics and fullness in the bass and lower-midrange. And since I don't have to worry about building to a budget, I went with the largest supply I could fit in the enclosure.

This was a fairly easy build because the amp modules are pre-built. The hardest part was getting the case together with the tight fit of all the components, but that was partly because I didn't realize that the mounting holes for the top and bottom covers aren't completely symmetrical and had already drilled the holes in the bottom plate before I figured it out. So the panels didn't go together quite as easily as they would have if I had not made this mistake.

I had Modushop cut the holes in the back panel - they do a great job for a very modest price. I drilled the holes in the bottom panel myself which was fairly easy. Overall, I'd estimate it took me about 30 hours to build the pair, but I like to take my time. This is a hobby, after all.

Next projects - finishing up a preamp for which I'm currently waiting for the case from Modushop which has a LOT more custom machining than these amps had, and then building another pair of monoblocks using the Pufifi 1ET400A class-D modules with a 1000VA linear power supply and a custom triode front-end. Oh, and a pair of Line Forces once Jay (captainhemo) gets the enclosures done for me.

AKLegal

Re: DIY Amps for NX-Oticas
« Reply #16 on: 4 Oct 2020, 04:34 pm »
Its great to see good reviews of the Neurochrome amps.  I'm building a pair of Modulus 186 monos for my Super 7s.  Great job on the build.