Class D amplifier with TPA 3250 board

I have built several Class D amplifiers.  I used a TPA3255 by 3e audio.  I switched out a few key caps on the board. I used a 12 amp switching supply and added 120,000 mfd for the supply.  I engaged the PFFB circuit which makes the frequency response stable at all impedance loads, I wired it for balanced operation.  

Hello, new here. Been looking at the 3E Audio 3255 PCB for my next build. Did you find it an improvement over the Aiyima A07 TPA-3255 based unit soundwise?

Thanks much.

@rolox The Amptastic vanished from existence. This amp was my first choice, the price was very reasonable at around 100€. And the design was perfect.

The Trends was 200€ and with the same on paper specs as the Sure. I read a review somewhere comparing the Trends with the Sure and the conclusion was that differences were negligible.

Granted, the Sure amp is a bit of a false bargain because the price hikes to 80€ with external power supply + shipping.

On a side note I’m shocked how much the REL T5x improves the sound. It makes me wonder how the far cheaper but still delightful Q Acoustics 2020 sounds like with the REL.

Do I really need such bespoke speakers (OGY) if I have a REL?

Answering that question would require spending more money and time. So I won't bother to find out. 

@kokakolia yeah but you chose to get the cheapest of the bunch, which is fine, but you might have gotten a better volume pot AND more bass (to be honest I'm surprised about the lack of bass, but of course in the end 10 watts is just 10 watts) with a Trends or an Amptastic ;) 

As long as you're happy it's all that matters. Enjoy :)

OP,

Great. Glad to hear it. Enjoy the music and relax. Sounds like you had a stressful journey.

@rolox Thanks. I have to stress that the class D cheapie doesn't impress without a sub! This amp is all about the mids and highs. It's hard for me to recommend the Sure amp in absolute terms. You get 0 versatility. No bass. Only 10W of power. And the volume knob sucks. The Bluesound Node makes this thing usable. 

The Sure amp just happens to sound great with the OGY. I think I lucked out. 

@kokakolia I was in Italy on vacation and somehow was unable to answer any post here on Audiogon from my phone (which is actually very annoying, I can only do it on my laptop)

I'm glad, and not really surprised, that you like your new class D cheapie. I have a feeling a lot of the "bad" rep comes from people who have never actually took the time to really try one in their system, or just took the cheapest crap they could find (like a Lepai) connected it to some garage speakers through lamp cord and are judging the whole bunch based on that.

My system is far from perfect but I never ever had someone telling me "you are using cheap class D amps, aren't you?" based on listening only. They usually say it sounds good then raise their eyebrows when they see the tiny amps on their shelf.

Enjoy your new toy!

As long as you don't push them too hard, it can be a bit surprising how good many of these little amps can actually sound. Of course then you start to wonder how good it can get... In our own work on class D amps (we have a design entirely our own) I've come to the conclusion that power amps based on tubes are on life support.

@atmasphere Looking at those specs the TA2024 looks like junk haha. Who on Earth wants just 10W at 10% distortion LOL?

@dekay It looks like I have been pissing off the neighbors this past week. I'm still rocking that puny little Sure amp. It's really cheap junk amp. The volume knob spins around freely and the volume adjustment doesn't seem to do much. But I couldn't care less because it sounds so good! And the Bluesound Node has basic volume adjustment from the phone. 

Oh yeah, the tube amplifier is still at the workshop. The poor artisan is struggling to tame down the noise floor. I kinda forgot about him. I'm having too much fun. 

I never thought that my audio journey would end with a 40€ amp the size of a matchbox... Everything about it screams "cheap junk". It's like the Toyota Echo of amplifiers. 

As far at the argument that an amp was tested and it was 10 percent distortion???

@timlub That came from the specs published by the manufacturer of the chips:

TA2024 10% at full power (10W)

TA2020 10% at full power (13W)

TA2022 10% at full power (125W)

Tk2050 10% at full power (117W)

TAS5613 10% at full power (150W)

TAS5630 10% at full power (160W 8Ohms)

...and so on; in fact I've yet to see any class D chip that did not spec 10% at full power.

Typically they are operated at considerably less than full power, but if you are looking at specs and power is one of those things in which you're interested, its a good idea to look closely to see what the claim actually is. In the case of the raw chips this isn't such a big deal since a designer has to implement the design and will spec the resulting amplifier when that's done.

But I've noticed a lot of the Chinese offerings tend to state the '10% full power spec' when the power supply accompanying the amp won't support it- if you don't watch it, you'll think you're buying a more powerful amp and one of a lot less distortion. IMO its a bit misleading.

Glad to read/hear it, and...

I told you so (regarding the sub;-).

DeKay

News update: The tube amplifier is still under repair. The artisan is struggling to fix the problem. 

I added the REL T5X subwoofer just now. And it FIXED everything! Now I have an excellent setup, even at low volumes. I am very happy right now! Yay! 

Now that I think about, the Sure T amplifier is the cheapest component in my setup. Even the cables cost more. To summarize:

Speakers: Closer Acoustics Ogy

Subwoofer: REL T5x

DAC/Streamer: Bluesound Node (2021)

Amplifier: Sure AA-AS32157

Cables: Adeqwat brand (from Boulanger). 

I'm done! Finally.

I'm not really sure what is trying to be accomplished here.  I can comment that I have built no less than a dozen class d amps from modules.  TA2024, TA2020, TA2022, Tk2050 TAS5613, TAS5630, TDA7498, TPA3255, IRS2092, a couple different Sure/Wondom tri path boards..... yada yada.  The Tri paths, when critical parts were switched out and a solid power supply was put under them, they all sounded at least decent.  A couple ended up very good. As far at the argument that an amp was tested and it was 10 percent distortion??? Take a world class Class A amplifier and drive it hard enough, it will produce high distortions too.  The question is "Can some of these little Class D amps sound good?"  You bet... Beware, some of the Class D amps that I built were not good.  The TPA3255 That I mentioned earlier turned out quite good. as did a Wondom 400 what per channel board, sorry, I don't remember the chipset on that board... it was called 400 watts per channel, when I was finished, it spec'd out at around 60 watts to 8 and 120 to 4, but either way, ended up being a nice sounding little amp.  I am only saying that when used with the right speakers a few of these little amps can perform admirably, no, not quite world class, but very satisfying, good performing amplifiers

@rolox So I came back from an 8h train ride and finally had a chance to try the Sure amplifier. I hooked it up to the Node in a jiffy and listened to a bunch of tracks with Deezer/BluOS (subwoofer still in box). Yo be fair, I was in a cranky mood and I was also in the mood for finding flaws. 

First impressions: ??? I was impressed at first then I ended on a meh.

So there is a noise floor but you have to stand like 30cm from the drivers to hear it. So it gets a pass. 

I can't figure the volume knob, it spins freely over 360° and doesn't seem to go very loud. But volume is sufficient for my appartment use. I'll just play music moderately loud. Fine. 

Bass just doesn't slap like on the tube amplifier. It's kinda there but not really, like the orange flavor in Fanta. 

Midrange and Treble are remarkably clear. And that's a bit terrifying in a good way. I heard stuff on the Sure that I didn't hear on the tube amplifier. 

Buuuut I have to get used to the Sure's kind of dry presentation. It's CLEAR, almost to a fault. But you just don't feel much of the instrument decay/reverberation in the recording. Fleet Foxes Sunblind sounded like (really good) elevator music, because the Sure lacked some punch. 

I think I can get used to the Sure, and then the tube amplifier will sound woolly in comparison. I'm hoping the subwoofer will help. I could use more bass. Maybe the dinky power supply I ordered really doesn't help. It's 3A 12V 36W "laptop brick". 

I'm probably the only idiot pairing a set of $2k speakers with a 40€ amp. And the results are just fine. 

Go on Amazon ad check out a Willsenton R8 or the Muzishare X7 these are only $1500 and you get Amazons return policy. They will run or EL34 or KT88 which I prefer.

@kokakolia in my experience, and in my system, and to my ears, those little class T / D amps -when they are well built- can be far better sounding than any mass-market class AB solid state amps, like entry level Japanese. Then when you go higher in prices the scales revert but let me tell you, you can still be very surprised: I have a friend who bought some massive vintage French speakers (AudioReference 126DC, 97dB) and he first tried to use them with his big Musical Fidelity M6i, and the results were extremely "meh". Then I came to his place with a tiny Nuforce Icon amp, and the speakers came alive. The sound was more fresh, more vivid, more detailed. There is a certain quality that somehow brings you closer to the music (specially with TA2024 and TA2020 chips)

I’ll be curious to read what you think of it.

Hey, if it sounds loud to me I ain't bothered! 

Speaker specs are intentionally confusing anyways. The OGY are so easy to drive because they're just 4" paper drivers with a 1/4 transmission line inside a 30x30x15 cm box. 

@kokakolia Seriously, getting an amp to be quiet on a speaker that's only 91 dB should be a walk in the park! 15W seems surprisingly low for a speaker rated with that low sensitivity FWIW!

@atmasphere 50W is a lot! My terrible tube amp doesn’t even go to 20W and I have the volume set to minimum.

My room is about 24m^2, maybe less. My speakers are rated at 15W max. I am also listening from a distance of 1~3m.

Yeah the artisan sucks. Tell me something new. I’m gonna be using my Sure amplifier for the time being (when it arrives home). You know the cheap T amp.

At this point my expectations have been lowered so far down that any amplifier which doesn’t exhibit a loud noise floor will sound like angels singing from the sky.

@kokakolia

Sure, but if you have a subwoofer already it might compensate right?

If you drive the sub from the preamp, yes.

My OGY speakers are very efficient at 91db SPL.

That's not high efficiency! That's a moderate efficiency; you enter high efficiency territory when the speaker is about 97dB or higher.

If you are listening nearfield I can understand using such low power. In a typical American room you'd want about 50-60 Watts to make a speaker of that efficiency play properly.

The only conversion from AC to DC I can think of for the amp (to which you referred above) would be creating a DC power supply for the filaments of the tubes in the amp. FWIW this is sending up a bit of a warning flag for me. We use AC filaments in almost all of our amps and they are plenty quiet enough to use on horn speakers. In addition, if you are getting a hum from the filaments of the tubes, the hum would be 50Hz and not 100Hz (because its induced at the line frequency)!

So while I am keeping all this in abeyance, I must admit to having lost some faith in your 'artisan'. Of course this is all hearsay...

I have built several Class D amplifiers.  I used a TPA3255 by 3e audio.  I switched out a few key caps on the board. I used a 12 amp switching supply and added 120,000 mfd for the supply.  I engaged the PFFB circuit which makes the frequency response stable at all impedance loads, I wired it for balanced operation.  

It was very good.  I had several amps selling on marketplace and sold a few. That amp was snatched up first.  

My OGY speakers are very efficient at 91db SPL. I only need a few watts. The manufacturer rates the OGY at 15W 8ohm max. If I get 4W out of the amp it should be enough to annoy the neighbors. 

My #1 priority is noise floor, since my speakers are quite sensitive. But even my Yamaha RN402 had a small tiny noise floor which was only noticeable when you stood very closely to the tweeter. So I'm not very fussy I suppose. My tube amplifier was just that bad. 

My other big concern is that people seem too kind with these chip amplifiers online. The rock bottom price plays a huge part in that. I say this because there's no shortage of branded A/B amplifiers on the used market between 80€ and 400€. If this T amplifier can't compete with 150€  used Yamaha A/B amplifier then it's not worth my time. 

So I'm crossing my fingers. I'll keep you updated on Sunday. 

@kokakolia  okay, I'm curious (although I'm wary of the "barely any difference" affirmations) it's probably a neat little amp, I wonder if the TA2024 is an original tripath or a brand new Chinese copy (as long as it works it doesn't really matter)

Looks like it's mainly surface mount components inside (?) so tweaking might be difficult (maybe not needed), and the RCA and speaker connectors seem very cheap / small, but that's ok.

I believe you will be pleasantly surprised, give it a couple days of break in before judging its sound quality, and don't forget it's just barely 8watts / channel before distortion starts to raise (but 8 watts can be surprisingly loud).

Let us know how it sounds!

Sure, but if you have a subwoofer already it might compensate right?

One thing to watch in these less expensive amps is the bass response. Not unlike what Radio Shack did with some of their lower powered amps back in the early 1970s, a lot of them I've looked at have the low frequencies curtailed- likely for the same reason Radio Shack did this so long ago, to prevent bass notes from gobbling up amplifier power.

@rolox My curiosity peaked and I bought the Sure amplifier with the Tripath TA2024 chip. Some reviewer online compared the Sure amplifier with the Amptastic and the Trends and stated that the differences were pretty negligible. Since the Sure is 60€ (total 80,8€ with power supply and shipping) on sound imports VS 280€ for the Trends, the choice was easy. 

I am more worried about the noise floor at this point. And 80€ is the price of drinking/dining/clubbing for night in Paris anyways. 

@atmasphere I'm not familiar with the Topping amps but, aside from the Trends and Breeze Audio used in my main system, I own a few SMSL cheapies and I don't find them boring sounding. YMMV of course, and they certainly aren't as good and refined as the Trends or Amptastic - they do provide more power tho. They are best used with sensitive speakers and well within their limitations, and the ones that allow for input potentiometer bypass benefit from a good full size preamp. They also respond very well to a better non SMPS power supply. But yeah obviously they won't provide the euphonic sound of a good tube amp.

@kokakolia most likely place would be eBay

Trends TA10.1, Amptastic Mini1, there are plenty of others like the Bantam Gold (a bit more expensive)

You could also experiment with the other Chinese stuff, like SMSL, Topping, Aiyma - they sound good and powerful and lively but there is slightly less attention to passive components making them a little bit less refined sounding. Still, great little amps for not much money. I live in Belgium, by the way. ;-)

I also order a LOT from AliExpress if you can't find what you want on Amazon or eBay, and I never had any issue so far (choose the right vendor with plenty of good reviews)

Alright, so I do not have the amplifier with me.

The amplifier always made that hum, even on the shorted inputs. I only use 1 out of 4 inputs so it's quick to find out. The hiss is gone. 

But the amplifier is currently in repair. The artisan told me that he was gonna add diodes to convert the AC current into DC and that should kill the hum. He told me the hum was 100 Hz. 

From memory, the hum was just annoying enough to ruin quieter songs and unnoticeable on busy songs. But I love Billie Eilish and her tracks are equal parts quiet and loud, so you notice the hum. So I really hope it's the AC current killing the vibe with the hum. 

Anyways I can't do anything until Sunday. 

Folks on another forum I shall not name seem to heavily imply that all amplifiers should sound the same (or very similar). They rave about these cheap tiny Topping/Aiyima amplifiers with class D TPA 3250 amplifier boards. These same boards are used in Genelec active monitors, so they must be good? I'm flustered because there no direct comparisons between these TPA 32xx amplifiers and more conventional/expensive branded amplifiers. The same folks on the forum I shall not name imply that I'm a dunce for spending so much money on a tube amplifier (quote: it's a distortion factory and it can't play grindcore metal music so it sucks).

@kokakolia Tube amps can play grindcore just fine!

Since this is a buzz (not a hum, hum in France would be 50Hz only; since you have 100Hz you probably also have artifacts you can hear in the tweeters) and not a hiss there may be things you can do.

The first thing to do is find out if, when you short the amplifier's inputs, does the amp still make the sound?

Or does it only make it when the amp has something connected to its inputs?

If you can answer those questions we might be able to make some progress without any serious investment.

With regards to the TPA3250 and other chip-based class D amps, I think if you look at their specs you'll (and those over at ASR) you'll be surprised to find their distortion at full output is about 10%, more than your tube amp! I've got the Topping amp (the little '30 Watt' unit) and its quite boring to listen to- almost to make you think that specs are more important than the music 😁  Which they are not.

You need good specs to get the music right and plain and simple, while those chip based class D amps do some things fairly well, their specs suck. This is not to say class D does not work! But chip based class D amps don't represent the class of operation...

So answer my questions above and let's see if progress can be made.

You're talking about cars? Ha! no. Unfortunately I drive a company car for work. It's a Renault Kangoo diesel 85hp. It gets me all around Paris. And I don't pay for fuel (which costs 2.20€/litre BTW). 

PS:

Damping factor will not be much of a concern at all if the low frequency duty is taken over by your subwoofer.

I do not used subs (just 8" single drivers running full range) which is why low damping makes my speakers sound fuller/better.

I tried a sub years ago (Gallo TR2) which I liked, but due to my using a high level connection (speaker wire in/out) the sub’s crossover was fixed @ around 100 Hz (needed to go lower and didn’t want to fuss with splitting the signals from my basic/budget preamplifier in order to use the low level inputs).

If I lived where you do I would be investing my money in a nice 2CV or DS.

DeKay

@rolox This is music to my ears! Where do you buy these amplifiers? I live in France. 

I own a very high sensitivity, fully horn loaded system (averaging 105dB sensitivity).

After many years swapping various "classic" amps, and being more or less happy, the death of the last pure class A amp I was using prompt me to try on a very low power class D amp, with a Tripath tA2024 chip. The amp is a heavily modded (all passive components) trends TA10.1

Let me tell you, it was a shock: the tiny cheap amp not only sounded very good, it was also more silent, and of course stays cold and doesn't draw much expensive electricity.

Since then, I've replaced most stuff in my system, and I'm now tri-amping, but the tiny class D amp stayed. I use another, even cheaper, class D amp on the 60 to 500Hz range, and only the subs still use class AB amps.

I'm thoroughly happy with how it sounds, in fact I think my system is at its best with those amps, they are completely devoid of noise and very lively and detailed, almost like a SET amp. The system stays on 24/7, the amps stay cold, they are completely fool proof, very transparent (you hear every upwards change immediately), for me there's no going back! No more headaches!!!

I encourage you to try one, but be aware that they are not all made equal, maybe start with something that has some "audiophile" pretentions like a Trends or a Charlize or an Amptastic. You will be VERY surprised (always with a good preamplifier in front! The Trends and Amptastic allow you to bypass their input level potentiometer, which gives even more transparency.

They are also very sensitive to the power supply and swapping the cheap SMPS for a higher current, very low noise linear PSU and good power cords and umbilical DC cables brings huge improvements and raise the sound quality of those amps from "suprisingly decent little toys" to truly audiophile. Really, I'm not lying.

@dekay You’ll laugh but the artisan’s workshop had a bunch of noisy neon lights. The artisan told me that he couldn’t hear them anymore.

Thanks for the advice. Musical Fidelity sells the V90 amp new for 350€. The reviews seem kinda mixed. Everyone treats it like a cheap amp. Someone even says that it has a noise floor. 

There's no mention of low damping. I guess I'll give it a pass. I have to wait until Sunday anyways.

PS:

Couldn't resist and this is not a serious question, but does your amp builders demo/listening room look something like this?

DeKay

You got low damping factor pretty much spot on.

Not modern (and no measurements available that I'm aware of), but I have a 20 year old Musical Fidelity X-A1 SS integrated amp that seems to have a very low damping factor as I've used it with many different types of speakers and it mimics the characteristics of a tube amp with "measured" very low damping.

I've had to open it, and the separate power supply, a few times to remedy what was basically shoddy assembly work and discovered that it had wire wound resisters on the speaker outputs (one per channel).

I also have an early 80's Carver SS receiver that also sounds as if the damping factor is quite low (Carver MXR130).

Anyway, hopefully switching to DC on your artisan amp will remedy the hum you are getting.

Also, definitely try the sub you own as doing so should give your single driver speakers a helping hand (making them richer/fuller sounding - more effortless) if the sub settings can attenuate the low frequency output of your single driver speakers.

DeKay

I understand what you're saying. You went to a reputable company, not some guy. Some guy convinced me that he could build a wonderful amplifier for me and that I could see him anytime if I have issues. In essence I paid for the same things you paid for. But the difference between you and I is the reputation and quality. 

In hindsight, I was lured by the prospect of finally experiencing tubes. My experience couldn't be worse and it's hilarious. I notice that my OGY speakers have more bass and fuller midrange with tubes compared to the Yamaha RN402 I regrettably sold. From memory, the differences are subtle but still there. It's like 20% better with tubes (listening to busy tracks where the hum disappears), and that's a huge improvement in audiophile terms. But the quality of the recording and my mood play a bigger part (IMHO).

Perhaps if I use the powered sub, the quality of the amplifier should matter less.

I researched low damping factor amplifiers briefly. It looks like today's transistor amplifiers are built for high damping factor by default. Low damping factor amplifiers are most commonly tubes. A high damping factor doesn't allow a woofer cone to vibrate much. So you get harder upper midrange and less bass with fullrange drivers. That's all theoretical and I'm probably out to lunch on the understanding. 

I don’t often make a big mistakes, but when I do… I do what you are doing. Do the best to mitigate the problem… then sell it. Get what you can. Or throw it in the trash and caulk it up to experience. It was great of you to support a local artisan. Live and learn.

Unfortunately, if you do detailed research you get what you pay for. My amp cost $22K… and well worth it. Incredible performance, from a reputable company that has been around for 50 years. If I had a problem my dealer would come over, give me an interim replacement until he could deliver a fully functional replacement… but then, I am paying for that.

In summary, yeah. I would gladly spend 500€ or 1000€ on a good class A/B amplifier afterwards. Something without surprises. But the situation is hairy. This guy doesn't want to back down. He's insistent on his abilities which I don't care for anymore. Looking closely at the innards of the amplifier, his soldering is sloppy. Some tin was floating around in the casing. And I was driving as gently as possible with the anxiety that vibrations and bumps would desoldure components.

The artisan has a well equipped workshop and a great listening room. He has everything to succeed. That's why I stuck around for so long. I doubt that I'll ever get my money back. I don't want to deal with him anymore. The anxiety and amount of hours spent on driving is a burden on my life. But he'll be back next Sunday. 

Presently, the amplifier displays an audible "100 Hz hum". There was hiss prior to that, it is now resolved. 

Yeah the situation sucks but I'm in good health and my financials have never been better. I can afford to lose some cash (just not every month during the year like that). Heck! If only we lived in a world where we could mass produce good amplifiers for a few hundred bucks LOL 

Appearance, evidence, and reports suggest this "artisan" is not an engineer, technician, or competent designer and probably lacks the proper tools to measure and evaluate the device to isolate, determine, and identify its inherent defects you should seek an immediate, full, and complete refund.

Confusing thread -- first hiss, now hum! Two distinctly different problems!

News update: I brought the amplifier back to the artisan.I told him about the annoying hum.

He told me that he thought I would never notice. He told me that the issue is due to the AC current feeding the circuit. He said that he could make the amp quiet by using diodes to turn the AC current into DC. That should eliminate the 100 Hz hum.

I’m like whatever dude! I also said that I’m not driving back to his house (60km south of Paris) again. It’s so far away and the traffic jams are killing me. He said that he will show up to my house next Sunday.

And I’m like "k fine".

The amplifier was tested at the artisan's shop with an old CD player and older speakers for testing. It was still hissing but less. I previously had a Yamaha RN402 with the OGY and no problems. The RN402 is run of the mill class A/B from Yamaha. 

How do you know if an amplifier has a low damping factor? 

Class A/B has nothing to do with noise -- it simply describes how the amp is biased -- class A mode at low power, switching to class B operation as the power increases.  That switch can take place at a fraction of a watt to many watts, but doesn't generate or eliminate noise by itself.

I've not used either specific amp, but do know that Yamaha is a good company if you like their sonic character. I'm not familar with Atoll.

Finally,I doubt that a power conditioner would affect the hiss at all -- hiss is not the typical way in which power line noise affects sonic playback.

Not familiar with your Bluesound Node +, but it still could be a gain issue.

Aside from that do you have any other musical source gear that you can try (with the amp) besides the Bluesound, as the hiss may be caused by it (the Bluesound) and a gain/other issue)?

If not, I would try the product I suggested (or something like it) before tossing a 2K US$ unit back to it’s builder without a refund.

I listened to your speakers (YouTube) and though this is not the best way to audition speakers they seemed fairly bright with little bass/mid-bass to me (this is coming from someone who has been using wide band single drivers since 2002) and I would suggest implementing your unused subwoofer at some point.

All my single driver speakers mate best with single ended tube amplifiers and/or PP tube/SS amplifiers that have a very low damping factor as I do not use subwoofers.

DeKay

Yeah I just think that the amp isn’t built very well and that it’s crazy noisy. It doesn’t seem to bother the artisan. I essentially flushed 1800€ down the toilet. The tubes are JJ and new. I don’t want to go down a rabbit hole of spending more money to fix a lemon. I’ll return the unit tomorrow. I don’t expect a refund. I just don’t want to create more landfill.

Two amps caught my attention:

- Yamaha A 670 (brand new) 330€. It’s a 20W class D amplifier with PWM tech. It should sound very clean/neutral.

- Atoll IN30 (used, like new) 250€. Your typical A/B amplifier. Plenty of power. Shouldn’t have much noise because A/B.

My dad told me to get a power conditioner etc... But that’s just throwing more money at the problem and I don’t have much confidence in the craftsmanship. I also forgot to mention that I have a REL T5X never unboxed. So bass shouldn't be an issue.

One other thought -- have you tried different tubes in your amp? I know nothing about the design of your amp, but if the hiss is equal in both channels, it may be a bad preamp tube. Ones like 12AX7, for example, are dual-channel tubes so one tube can affect both channels.  Bad driver or output tubes typically affect only one channel unless the whole complement of tubes is getting old.

You could experiment with something like this (inline attenuators)

This works in most cases, unless your custom designed amp is just crazy noisy or has way more gain than the average amp. I have 9 tube amps, all are dead quiet. 

I have Bluesound Node + Ogy speakers + hissy tube amplifier