Review: Audio Research D-70 mkII Tube amp

Category: Amplifiers

This is a brief review of the ARC D-70 Mk2. These amps are now going into their 27th year and since I recently acquired one which I have rolled tubes into, I thought a review of how this amp compares against some of the more current tube amps on the market that I have heard, might be of interest.
First of all, my amp is the Mk 2 version ( although when I bought it I had understood it to be the Mk1 version) What is the difference between mk1 and mk 2?.. Not very much... In the driver stage of the mk2 we find 6FQ7's vs. 6DJ8's or as is now more common , just 6922's tubes in the Mk1. Also, according to Leonard at ARC, the Mk2 amp was updated with Fet's to work with the 6FQ7's and that's about it. This review is of the Mk2 version, i have NOT heard the original Mk1.
Upon viewing the amp, it is of the classic look from the 80's period of ARC production, meaning a 19" wide rack mountable faceplate with the typical logo and the power switch and line/screen fuse mounts in the lower center of the
faceplate. Along with that are two green LED's that light upon power up and when screen voltage is reached. On the back panel are the old-fashioned strip barrier posts for the speaker connections allowing for 4/8/16 ohm connection and two RCA inputs. The power cord is hardwired into the chassis and that is about it on the back panel.
At first, I had a problem with speaker cables, as the current day spades are too wide to fit the barrier strips. The solution was to acquire a bi-wire pair of Nordost Red Dawn mk11 cables with standard spades at the speaker end and small spades at the amp end.
Now at first fire-up I had a problem with one of the original GE 6550's going up in smoke. A pity, as these tubes are apparently very fine in this amp. Anyway after my tech looked at the amp and gave it a clean bill of health except for the tubes, I replaced the whole tube set with a matched quartet set of SED winged 'c' 6550's in the power section and a mix of NOS GE 6FQ7's and JJ 6922 tubes in the driver section. My tech did the biasing procedure( NOT for the faint of heart or layman such as myself and definitely not recommended unless you have some technical ability ).
The amp now resides in my system with the aforementioned speaker cables and is driven by my CAT SL1 Signature pre-amp and drives my SF GH's.
(See my Virtual system).
I have heard several tube amps in this system, (see similar products) and I currently also use a Jeff Rowland Model 8 with top plate mod and choke as my ss reference.
After inserting the D-70 into the system, I immediately noticed that the amp was able to drive the fairly inefficient SF's with little problem, but compared to the model 8, the D-70 was more in the sweet spot of the preamp and amazingly seemed to have as much bottom end slam as the 8! This was on the amps 4ohm tap, ( which after some experimentation i feel is best for this speaker).
What I also heard was a wide soundstage that was able to expand from just outside the edges of the speakers and with depth that seemed to expand much deeper than the Model 8 and even the prior champ in that area the BAT VK75SE. Micro detail was also there in spades, on the Andreas Vollenweider disc 'Book of Roses' I noticed more detail in the background than i had noticed with the other tube amps i have had in my system. To see how the amp would handle dynamics I put on the disc, "jimmie Lee Robinson" track: "The Boll Weevil' IMHO a good test of this aspect. WOW!! this amp has 'BALLS'!
The dynamics just scream... The model 8 is able to keep up here, as it should do with 250 watts per/ch vs. 65 watts for the ARC, BUT it certainly isn't better. The D-70 easily bested
a friends MR RM9 when i inserted that amp into the system and to both our surprise the RM 9 seemed to loose detail as well vs. the ARC.
Another friend supplied a ARC VT 100 Mk3 to AB against the D-70... to nobody's surprise the VT100Mk3 equalled and maybe slightly exceeded the D-70's bottom end reproduction and would probably be a better amp with speakers that can do more bass than my SF's.However, in the midrange, where the ARC D-70 is one of the best amps i have ever heard, the VT100Mk3 was outclassed! On the Fi/Analogue productions Sampler disc track 1.. Sonny Boy Williamson with Matt "guitar' Murphy; Sonny's guitar was simply in the room with the D-70, with the VT100 we all felt that the guitar wasn't quite as realistic. Sonny's playing seemed a little more strained and slower/less palpable with the VT100.
What are the strengths of the ARC D-70MK2... that's easy.. midrange reproduction that has to heard to be believed and is IMHO still SOTA, a bottom end that is clean and amazingly deep for a tube amp, superb pinpoint imaging with fabulous depth reproduction and black backgrounds. Highs are reproduced with superb micro-detail and with seemingly endless extension.
Now here's the icing on the cake...Currently, one can pick up one of these amps for less than $1000!!!! I am amazed at what a SCREAMING BARGAIN this amp is in today's market. For it to knock off far more expensive and recent amps really says something about Mr.W.Z.Johnsons design for this circuit.
Weaknesses: the biasing procedure is not for the layman and like all tube amps, it throws off more heat than a ss amp.
IF this amp doesn't become a classic, I will be VERY VERY surprised. I like it so much, I will probably pick up another one while the pricing is still such a steal.

Associated gear
Click to view my Virtual System

Similar products
BAT VK 60, BAT VK75SE, CJ Premier 1, CJ Premier 8, CJ70LPS,Tenor 75, VAC Renaissance 70, ARC VT100Mk2, ARC VT100 Mk3,VTL st 85,Music Reference RM9
Lol I see your listing of the amp on Ebay already.
What happened? Can't deal with the pentode sound or the instability of the screen grid regulator?
The amp has the biasing stats in the manual. If you cannot source one, ARC has the parameters, I'm sure they would supply to you. As I stated above, the biasing procedure isn't for the faint of heart or less technically myself. Have a good tech do it for you if you aren't up to the task.
BTW, I'm fairly sure that KT120's will NOT work with the transformers in the D70Mk2. I use the SED winged C 6550's and like them a lot. Agree with your friend's findings as to the sound...actually bettered the VS110 that I heard as well.
IMHO, this amp is one of the best in the ARC line...even to this day.

I’m late to the party.... At one point, I had two of these babies.. but in a fit of madness I sold one.

I still have one, as a back up... wonderful amp.

Biasing? Come on, it’s a piece of cake. Multimeter and plastic tool and you’re set. Just bias for 65 mV ( I think that what it is... ).

Of course, I’m an engineer by trade... but seriously this is easy stuff. It’s not like you need a scope and a function generator... seriously!

Sound? Fabulous with a 90 db/w speaker... just don’t try Maggies... for that the D115 is the bare minimum.

Oh, I've been running Sovteks  and Svetlanas since the late 90s...  best 6550 for these amps.  Down a bit in power, about 59 wpc... but you can't hear the difference.

@esporma if I remember correctly, the biasing didn’t just require one to adjust the power tubes to 65mv, but also to adjust other parameters as well, for which you did need a scope. Also, according to my tech, the opportunity for one to get zapped was pretty high with placing the clips on the test points. Compared to my self-biasing amp that I now own, it was almost criminal!

One thing I agree with @esporma … the sound of the ARCD70 mk2 is fabulous. I still miss this amp to this day. It really made a great impression on me, and IF one can live with its idiosyncrasies, one of the best low powered amps I have heard.