low THD amplifier suggestion for speaker testing purposes

OscarJr

So I was recently testing out speakers with a Class D amp (Peavey IPR-7500) and realized that I definitely need to go back to class A/B.  Previously I had used a Peavey CS-800X which tops out at 0.030% THD, but with my AMT's measuring in the 0.2x % THD range, that 0.03% ends up making a difference.  I don't even want to think about how much THD the IPR-7500 might have been contributing, LOL.

So, what should I be looking at for an amplifier with < 0.01% THD @ 50W into 4Ω?  I don't need anything super power powerful, since it will just be for testing, so 50W would suffice.  

BrannigansLaw

Don't know what your budget is, but these fit the bill:

https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-neurochrome-modulus-286-amp.6443/

https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-hypex-nc400-diy-amp.5907/

jr@mac

Great big pro audio amp, they will be squeaky clean for the first 20 watts, older ones are still Class AB. 

BrannigansLaw

another contender:

https://kenrockwell.com/audio/hafler/dh-200.htm#thd

WRT big power amps, I suspect JR is right... however, for a 'known reference', if I were attempting definitive testing, I'd want something that had independent bench tests available somewhere. That's just my inherent skepticism of manufacturers' specs though.

dcibel

Don't blame the amp class for poor performance. (see Ncore data above). I use an old DIY LM3886 amp for testing usually. It's not the standard LM3886 circuit, see "My_Ref" project at Diyaudio for details. When I need more power, I usually grab a hefty old HK6900 at 200wpc. Can't comment on their 0.01% THD specs, but they're good enough for what I do. If I had money to burn, I'd go for ncore in a heartbeat, Hypex makes some relatively affordable DIY monoblock kits.

Keep in mind as well when looking at "THD" specs, that the error is usually in the high frequencies. Here's a clip from the datasheet for LM3886, this is an IC rated as 0.03% THD typically:

OscarJr

The hafler looks good, but I didn't see any specs into a 4Ω load, only 8Ω.  Nevermind, I see them now, lol.  Budget is perhaps around $500.  Not looking for a kit, I don't need yet another project, LOL.  The used Hafler 200's look to be a great deal for the money.

rjj45

re: Hafler - that's what I would recommend. Old vintage stereo amp/receiver.  I've got an old Kenwood KM-208  that I love. Also got an old Hafler 280 and an old Yamaha - all from Ebay or Criagslist for about $100.  I've got a box full of Class-D amps that I only use in an emergency. Just strongly prefer a clean Class AB amp.
Here ya go:
https://www.ebay.com/itm/Hafler-DH-200-Stereo-Amplifer-Audiophile-Tested-and-Working/173865004047?hash=item287b296c0f:g:ilQAAOSwJxRcqQoy

rjj45

There were a couple of intensive mods to Hafler amps that greatly increased fidelity. I heard somewhere that the XL-280 incorporated the best mods. 
https://www.ebay.com/itm/Hafler-XL280-Amplifier-in-Excellent-Condition-Great-Sound-w-copy-of-manual/283458344393?hash=item41ff6f15c9:g:16EAAOSwYnhcu4CU

Shawn_K

I use a Sony a SRP-P50 Professional amplifier; 50W x 2 (8ohm), 70W x 2 (4ohm). This amp is rare I bought it 5 years ago on Craigslist for $130. It's a Class AB design with Toroidal transformer; 2 pairs of output transistors (which is weird for a amplifier with this low of power output rating). It has Elna Silmic capacitors and separate volume attenuators for both channels. The volume pots have detents which is nice for setting the output level the same when measuring driver sensitivity. It's 1U rack size and weighs about 18 lbs. When I bought this amp I was going to use it on some active speakers but it makes a better testing amp.

JasonP

Huh, old high-end AVR tested well there too: https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-denon-avr-4306.5087/

kennyk

Any age class a/b should do. I don't know any that have issues with 4-8 Ohm loads and quite a few can handle 2 Ohms. 
Been using GAS amps for years.

OscarJr

Thank you guys for the recommendations.  kennyk,  my class A/B amp(s) that I have available (Peavey CS-800X's) work just fine into 2Ω as well, but I am specifically looking to have less "± fudge factor" with these low-distortion AMTs.

Example. 

If an AMT measures 0.2% THD as an average, and the amp may have up to 0.030% of it's own, then the actual THD as high as 0.23%.  I want to reduce that uncertainty, but I may not actually know how much unless that specific amplifier is bench-tested.    With a lower distortion amplifier, the uncertainty is reduced, which is what I want.

OscarJr

I found a guy who bench tested the Peavey IPR2-7500.  I'm trying to see if he has his 4Ω distortion test saved as a spreadsheet, so I can continue using mine.  Or I'm going to try to convince him to bench test mine, so I can have a true and meaningful baseline to factor into my spreadsheets.  Which means retesting everything, LOL.

BrannigansLaw

Exactly! In your context, it's best thought of as a piece of lab equipment rather than an audiophile piece or even an ultra-robust workhorse.

jholtz

I'll echo the older Haflers, specifically the 220's and 280XL's. B&K, Marantz, Adcom along with many others will work just fine to as long as you make sure to get an A/B design.

Another one to consider is an Emotiva A-100 ( https://emotiva.com/collections/amps/products/a-100 ). This wouldn't be my choise for a main system amp since its 50/80 watts but it sounds like it would be perfect for what you're looking for.

HTH

Jim

OscarJr

No dice with the actual spreadsheet for the 7500, but I think I might convince him to bench test me another amp, and that Emotiva A-100 looks like a great choice!  Shouldn't be too much to ship back and forth!  That way if can get the spreadsheet with the amp THD curves, I can factor it into my speaker THD measurements and filter them out right from the get-go!

jholtz

OscarJr said:

No dice with the actual spreadsheet for the 7500, but I think I might convince him to bench test me another amp, and that Emotiva A-100 looks like a great choice!  Shouldn't be too much to ship back and forth!  That way if can get the spreadsheet with the amp THD curves, I can factor it into my speaker THD measurements and filter them out right from the get-go!

Emotiva used to have free shipping when you bought something from them. This series comes with a 3 year warranty. If you have any questions about measurements etc. you used to be able to talk to the engineer that designed it or possibly Dan, the owner. They have excellent customer service.

Jim

OscarJr

Sounds good, but I doubt they would do custom THD sweeps for me, lol.  I have seen one THD sweep, bu it was into a 40W dummy load.  I'm trying to convince the guy who can do THD measurements, since he also has an A-100 in his kitchen, to do custom sweeps for me (1V RMS, 2V RMS, 2.83V RMS).   If he does that would be killer!  That way I can do the most precise THD sweeps I possibly can on my end.  I just need the data in a spreadsheet format that Excel can open!