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
Nice to see an excellent, well written review of the Audio Research D-70 mk2 amplifier. Having owned one of these amplifiers, I can agree with most of the comments of the review. For the current used market price, this unit along with the ARC D-115 mk2 are amazing amplifiers for the money.
If your speakers can be driven with 65 watts per "tube" channel the D-70 mk2 is an excellent amplifier.

I can also comment on some other Audio Research amplifiers that I was able to directly compare to the D-70 mk2. When comparing the D-70 mk2 to it's big brother the D-115 mk2 you'll get get a much better bass response on the D-115 mk2 due to the doubling of the power supply. Also, the 110 watts per channel on the D-115 mk2 will give you much better bass slam. The high end response on the two amplifiers was very similiar.
I did directly compare the ARC D-70mk2 and the ARC D-115 mk2 to an ARC VT-130 and found the ARC VT-130 beat them both. This should not be a huge surprise, as the VT-130 (regular edition) retailed for over $5,500 and is almost a decade newer design with high resolution transformers and newer internal parts. The differences I noticed was the D-70 mk2 and D-115 mk2 had more of that "classic tube sound" which was a little colored in nature. The VT-130 sounded more accurate or neutral, and had a lower noise floor.

Concerning the weakness of the D-70 or D-115 amplifiers...I agreee, the biasing of the units is probably easy for the technically minded owner, but for your casual audiophile it's a more difficult challange. It should be noted that the D-70, D-115 and D-250 ARC amplifiers all have a mild turn off noise which is perfectly normal for these units. The mild turn off noise according to Audio Research is caused by a temporary DC offset and does no harm to the amplifier or speakers in any way.
This article just kills me. I owned a d70 in college. I am counting the amps and especially the $$'s I just could've saved! thanks for a trip down memory lane. great review, very nice to compare to modern day amps...
It feels great when one finds a steal in the audio market, I have always paid respect to the ARC line but my experience owning any of their line is scarce, on the other hand I have many friends that have just changed models but keeping themselves within the ARC product line.

Nice review Dave, as I said in my other reply, I find hard to believe it sounds that good compared to your 8 (which I am very familiar) - have you ever considered trying the ARC matching preamp of that time to compare?

Also it would worth considering one per channel, I mean ' for that price!

Enjoyed your review, thanks for posting it.
Hi Fernando,
I too was a little surprised that this amp can sound as good as it does.
However,I had known from other 'phile friends that this amp was one of the true 'sweet spots' in the ARC line in the last 25+ years. I think it also says a lot that a superbly well designed tube amp from probably any era is still highly competitive with amps of both genres today.
IMHO, an example of this, one can see by the many owners and admirers of the Mac 275 that this is true.
OTOH,my JR 8 with choke is still my ss reference, and not likely to be replaced any time soon.

BTW, I actually think that the CAT mates very well with the ARC. I suspect that if I were to try an ARC preamp of similar vintage, that it wouldn't be able to compete with the CAT. ( except maybe the model SP10Mk2... BUT that's not really that affordable in the market right now)
I am considering one D-70 per channel, however, not sure about the ability to strap these for mono operation. ( I don't like the idea of simply attaching one speaker cable to one side of the amp and leaving the other some other listeners have done; IMHO that is NOT the way to go). Nonetheless, that's probably a ways out as I'm enjoying the amp so much right now as it is.
If you think D70 is difficult to work on try biasing a D-115 mkII like the one I just got! Even tigher space and I ended up having to mod md DVM probes with heatshrink.
LOL Johnsonwu, I suspect that they are equally as difficult to bias. My tech, whom has worked on everything from ARC to Mac to Bat etc., wasn't at all pleased with this amp's biasing. He told me that ARC had to have been out of their tiny minds if they thought a typical consumer had any chance at biasing these amps. I do agree with him, the biasing scheme is criminal.
Frankly, it does annoy me that now I have to replace a noisy driver tube, that I have to take the amp to my tech again for what should be a simple procedure.
OTOH, once I fire the amp up and take a listen, all is forgiven....that is until the next time..:0)
Drivers are capctatively coupled to output
You just need to tweak the 10k pot not the power tubes bias pots.
Much easier than biasing power tubes.
Thanks for a trip down memory lane , my introduction to the joys of tube components was an ARC SP8/D70 Mk2 combo driving Acoustat3a speakers , I wish I had stopped and gotten off the upgrade merry go round then .... I loved my D70 but curiously when I 'upgraded' to the D115 I wasn't as happy , everytime it had to be biased it came back sounding different ..... I think that period was the early exciting glory days of ARC , certainly later products are more refined and better sounding & the aesthetics / reliability are also better , but I look back on my D70 with fondness , I remember back in the mid 80's a D70 retailed in Australia for not much change out of $4000 , thanks to high tariffs .
Chris, interesting remembrances of your D70. Basically agree with your thoughts, except I do not think that ALL of the later ARC products are more refined and better sounding. A few are, but IMO only a few. For instance, as I stated in the OP, the VT100Mk3 was in fact inferior. I recently also had a chance to listen to the ARC VS110 and again IMO, the D70Mk2 was superior. The D70Mk2's midrange reproduction was far more realistic than the VS110's. The D70Mk2 and the M100's were truly from the glory days of ARC as you correctly pointed out. I think ARC has had a number of hits and misses since then.
Hi Davey ,
I was ' in the wilderness ' for a few years and missed the VT100's etc , when I came back it was with the ARC CA50 integrated amp , I then jumped to the VS115 and then the Ref 150 & an LS27 , I think that combo certainly bests many of ARC's earlier efforts . Speaking of the M100's , guess what the original Australian Audio Research importer ( now retired ) has as his personal system ? Sp10/M100's -:)))) , I would be interested to know what your opinion is of the other ' hits ' , Regards .
Chris, Naturally I haven't heard all of the ARC pieces over the years. However, of the one's that I am familiar with, only the Ref 150 and the bigger Ref 250 have been significantly superior to the D70Mk2. In my system, both of the Ref amps have considerably more low end ability and slightly more overall air. I cannot say about the VS115 as I have NOT heard that in my system. I can also say that the new Ref 75 is not an amp that I would trade for my D70Mk2. It has a sound that I found far more sterile than the D70. Sorry, but I am not a fan of the LS27, I think the Ref 3 and Ref 5 are far more appropriate to the Ref 150's and I do think that the Ref 150's would benefit from those preamps- or up the range....say Anniversary?
I've got my D70 MK2 on the way! Very excited. Bought it when a friend did some comparisons with current ARC amps and came to the same conclusion. Ref 150 and up better, Ref 75 not. Nothing else in their line matched it although I don't know if he's heard the VS series.

A couple of questions: How did you find the biasing instructions? Are they in the manual... mine doesn't come with one? And have you tried the KT120s with amp yet? I'm a big fan of that power tube and I've read that it's compatible with the MK2.
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.

@daveyf   The instructions to bias the tube specify only adjusting the bias 65mV, not adjusting anything else.  Of course, the instructions also state to get a quad set of matched tubes...

It's not hard to do, your tech just wants your money.  The safest way to do it is to use clips and long leads for each tube -when the amp is turned off- and then carefully, clip you meter on each pair while you twist the pot.   

One thing though, it gets hot.. don't drop sweat on the circuits.. I use a fan to cool me down.

Depending on your speakers... 90db/watt or more... the D70-II is all the amp you need.... However, I also have some Nelson Pass derived clones that will blow your mind away too.

@esporma   Thanks for your thoughts. I have moved on from the D70Mk2 to a auto biasing set of tube mono blocks, which actually sound even better than the ARC.

Nonetheless, I do remember my tech stating how poor the biasing procedure was on the amp, and telling me that just setting the bias voltage to 65mv. per tube was insufficient. The instructions also mention further adjustments are required IF the driver tubes are replaced. The attachment points for the meter clips are also right in the high current area...just a nasty design, IMHO.