Thoughts on Audio Research D70

I recently saw a very well priced D70. If I remember back in 80's these were highly thought of. Having no chance to hear, what do they compare with today or do they?
I had one from about 1984-1988. It had very good sound. Very tube like, yet precise, smooth, plenty of headroom, etc. The problem I had was that it runs its 6550s _very_ hot. I had two or three rather spectacular failures during the time I owned it. Each failure resulted in sparks, smoke, charred circuit boards etc. I would get it back two weeks and $200 ('80s money) later working and sounding perfectly, but I held my breath every time I turned it on! IN my book, it gets an A for sonics and a C- for reliability.

I had a D70 MK11 for about five years in the mid-80s. Excellent sounding amplifier with very few sonic flaws. My unit was totally reliable. The unit was very critical of the 12AX7 used on the input. If the tube was not quiet (LN or SLN) the sonics suffered as well as the life of the tube. After I changed to RAM SLN tubes all my problems went away. The D70 in the mark II version this is still one of the better sounding amplifiers available.
I owned a used one for a year or so in the mid 90's. Ultimately it developed a problem no technician could readily fix. So I traded it on a D125. The D70 was very warm sounding, musical and involving, but with less detail and air than more recent ARC amps; also a full upper bass, and much less deep bass. I liked it, but while I regretted being forced to sell it, the D125, or for that matter the more recent VT100 series, do sound better; in contrast the VT60 and CA50 gained detail but lost warmth; in the past month I've auditioned the VS55 and VS110, albeit neither properly run-in, and these seem to continue the predictable shift towards detail and control over overt warmth. If you do buy a D70 I'd clean all the circuit boards before use too, and probably put in new tubes, not from Audio Research as they're very expensive, but from or another secondary retailer. You'll also need an equivalent pre-amp - mine was and remains an ARC SP9MkIII, which has been utterly reliable and sounds great - I also auditioned a MkII version which was good enough, and some solid state (cheaper) alternatives that were hopeless. If possible, I'd try to audition a combo at home, for a week, and make sure it's reliable - some of my problems only surfaced when the amp had been switched on for an hour or so; others on start up. The sound might strike you as being wrapped up in big warm hug - just be careful it doesn't steal your wallet after the initial embrace. Cheers, Rob.
I have had a D70 MK2 with an Sp9 MK1 since 1988 with no problems.I bought the SP9 new when it came out ($1700 in 1988)and the D70 ($1000 1990 floor model )from a dealer with used tubes. I ordered ARC tubes and used them for a long time. The ARC tubes are better. Back then it was about $200 for a set. I have had the set for 20 years with Vandersteen 2Cs. Tired of the audio bug and just listened to music.
Anyone want to part with their D70? No tubes OK (even better). Itching to do a total rebuild project with it.
I work on them and retube them. Use ARC tubes only. Excellent amp. After 2000 hrs it's time for an output retube. I also set the bias down to 62-63mV @ 120Vac. Doesn't hurt the sound and runs a bit cooler. Very good amp. The D-115 is basically the same circuit with 2 more 6550's per channel.
As Hifigeek notes, the circuit is almost identical to the D-115 but with 4 output tubes instead of 8. This is a really nice tube amp, along with the D-115 and for the price of purchase on the used market, if in the market for a tube amp, you can't do better at the price, at least I haven't heard it. With improved caps and careful selection of input/driver tubes these amps can really compete if lacking the ultimate detail, air and bass control of much more expensive tube amplifiers. The build quality is superb as ARC is noted for. You can't lose, the prices used on these amps are higher now than 15 years ago indicating their continued value.
I pulled the trigger on an ARC D70mk2 about a year are my thoughts on it:

Agree with Tubegroover, I do not think one can buy a better tube amp on the market for anything near the price. Don't agree that the amp lacks ultimate detail, air or bass control of more expensive tube amplifiers. In an 'AB' with a BAT VK75 amp of recent vintage, all preferred the ARC and by no small margin. I do think that everything else in the system has to be up to what the D70 can bring to the table.
No doubt it's a good amp. The D115mkll has a bit more drive, dynamic range, and slam in the bass.
Indeed the D-115 Mk 2, which I owned for several years and is currently used by a friend in his system, has great bass drive and slam and is very dynamic HOWEVER it has less control and definition in the low bass than my current amp or some others I have listened to in my system. Also the presentation is a bit less airy, missing some of the ambient detail of some more resolving amplifiers. The D70 and D115 fall into the general category of "classic tube sound", a little rounder on the leading edges and thus can sound a bit fuller AGAIN a preference issue as often is the case in these matters. When more resolution is gained poorer recordings can come off as harsher than with a somewhat more forgiving amp. To be honest I could easily live with his amp as he has owned it for 12 years and done quite a bit to enhance its performance, again a great choice but still not as ultimately resolving as some that I've had in my system (all OTL's btw) but not enough of an issue to fret over for sure. Preferences and taste always comes more into play than absolutes but on balance these are truly excellent products and have aged quite well.
Thanks for the opinions. Now to find an old beat up unit for an affordable price so I can do a complete rebuild.
Found a D115 mkii. Can't wait to hear how it sounds.
Will definitely verify each test points voltage and go through the usual hexfred/schottky diode mods and coupling cap upgrades... then triode of course.
The circuit begs for a better driver tube than 6FQ7.
Will see if filament supply can support 6H30 (driver and power tubes have AC filament).
Johnsonwu, I will be very interested in your outcome after you have modded your D115Mk2. WZ Johnson got the circuit very right, IMHO. Changing what he designed could be a risky business, OTOH I applaud your experimentation.
Just the throw my 2 cents in, I'm not a big fan of Hexfreds. They can be very problematic down the road. One the other hand, some good soft recovery diodes in the filament supplies would work nicely, (non-Hexfred types). Also because 6H30's pull quite a bit more heater current I would avoid them in that amp least you over heat and damage the heater winding of the power transformer. Although the transformer might still be available from ARC, it is quite expensive. Instead, replace the stock coupling caps as well as the filter caps if they're old, and bypass them with good film caps. Make sure all the tube sockets are clean and tight, the 6FQ7's triode halves are well matched and the D.C. voltage around the 6FQ7's are properly adjusted. Check that all the voltage regulators are functioning properly and quiet.
@DaveyF: Totally understand the risks. I think the circuit is in many ways superior to the Classic 120s and VTXXXs I used to own, and provides more opportunities for asjustments, thats why my quest for this amp.

@HifiGeek1: Thx for the cautions. Guess I have been lucky. The only Hexfreds that have exploded on me was the IXYS bridges. Been lucky with fairchild 1200V stealth and ST 100V Schottkys. Knock on wood. The filament supply is good for 4 x 625mA (6FQ7), AC filament in parallel, and I am only changing 2 of those guys to 900mA. The 6922s use DC filaments (15VAC winding) and thats where I plan to use the 100V Schottkys.
Hi Johnsonwu, I may be misunderstanding your intent. If you are not planning on changing the circuit that is inherent in the amp, then there are no real risks that I can see.
BTW, I agree with you that the circuit is superior to the Classic 120's and the VT series..... this has been borne out in the listening tests that I and my a'phile group have conducted vs. the VT series and the Classic 60 that we also AB'ed against. ( presumably the Classic 120 is the same basic circuit).
I have yet to hear a better ARC amp in the mids than the ARC D70mk2, even in the ARC VS series of today.
The Classic series had triode configured output tubes. This tended to make the output tubes last longer and gave it a sweeter presentation. I believe the front end was fet transistor up to the driver tubes. Very different topology from the D-115mkII
@Hifigeek1: Yes indeed triode vs NON-Ultra-linear fixed bias tetrode mode. But same B+ @ output stage.
420V B+ is a bit low for my liking but with 4 tubes per side power should be more than enough.
I have every intention to convert that to triode at a later stage, after I am done with the caps and diodes.

I had the CL120 and CL150s before. CL120 at least has a tube driver stage (different plate voltage from D115 though). CL150 is just raw muscle.
Oh well. So much for the exciting anticipation for the amp.
Seller allegedly got talked OUT of including the original outer box by the FedEx people and the unit was shipped in the inner box only with less than a few pages of newpaper in the box so go figure what happened.
The CL-150 was a totally different animal and frankly pretty average by today's standards. It started life as an M-300 and used Chinese coke bottle 6550's which at that time, were just awful. Their were no Russian 6550's at that time, and the Sylvania 6550 was history. So the only choice was to use the Chinese 6550. The Chinese tubes would fail after a few months, but the heaters would still work. So basically the light was on but no one was home...ARC decided to change the amp to the Classic series and changed the output configuration to triode making easier on the Chinese 6550 output tubes. So it became a CL-150. It's got great slam and sounds good and is miserable to work on. The cooling fans blowing on the fets and tubes while the amp is on is not an advantage. Cooling tubes yes, cooling fets no. Funny thing is a modified M-300 to triode have bigger transformers then a stock CL-150.
Just to chip in for a minute, i have a pair of CL120's since 1990. Those have been modded by Steve Huntley and the inputs were also balanced.
I agree with Hifigeek about those Chinese bottles. The day i got them home from the dealer and plug them in, 2 tubes failed within 5 minutes. I have retubed those amps with 6550 SED Winged C many times since then and have not had a single issue with any of them. I have had the opportunity to "test" a VT100MKIII and VS110 and a VS115 in my system and i'm still holding on to the 120's. Having separate power supplies in monoblocks configuration have advantages which the D series don't have.

I would like to listen to a REF150 next. Last time i talked to Steve before he closed GNSC he recommended i listen to a 150 but only in my system in my room as at a dealer sound room with different ancillaries would not be a fair comparison.
So the verdict for my unit is:
Faceplate is dinged up and dented. Supporting bar bent. I cant get a replacement from Audio Research as they are out of spares.
Seller graciously offered a partial refund which is very nice of them.
The 1A fuse popped several times and made me worried the screen regulator circuit might have hidden issues. 3 x 1A fuses and a trip to the local store with a tube tester I am fairly certain it was a bad 12AT7 that tests weak on the tester. Replaced it and I have powered it up 10 times since with no issues.
I am hoping that the fuse pops were just a problem with the tube.
Since the unit is of late vintage and confirmed to be a mkII I think the lytics are still OK.

I meticulously adjusted the diff amp balances and bias and now it sounds better than initially, and so far the voltages (esp screen regulator output I am cautious of) look stable.

Over sound is that it's really foggy and a bit slow. Crank it up and I notice glare and roughness.
Space is real tight for any cap mods. Auricaps will fit but how much better are they than the Rel + Wonder paralleled.

A friend has a pair of Reference 300 that's in the store room for at least 4 years now since the last power up. Maybe this little project, if successful, can get him to power them up and play again.

@HifiGeek1: What do you think of the recent frenzy regarding those whatever 50 years treasure funny black bottles and the whatever black sable 6550s? PM me if you want to avoid flaming :)
Johnsonwu, sorry to hear of your travails. I would say that there must be something still wrong with your D115M2 if it sounds foggy and a bit slow.
Did you replace all of the tubes and check the caps?
All tubes are known good tubes from my collection. All voltages were checked and verified AOK.

I have no way of checking how good the lytics are cos I dont have a big cap meter and I dont want to pull out the lytics.

Foggy is expected of the Rel-cap/Wondercap combo, as I have over the years owned quite a number of Audio Research amps. No surprises there. With this amp, everything is like 95% there but not quite excellent. Nothing annoying, but nothing that leads me to say, wow this track plays good.

and I might have missed a few...
Most of them I ended up redoing the coupling caps with Infinicap-S, Dynamicap, Auricap, or Jensen (wish I had tried V-caps and worked on the all-tube models more before I sold each of them one after the other).

Problem right now is that space is at a premium and its very difficult to put in premium caps into this amp due to their size.
'Foggy', certainly isn't a term that comes to mind when I listen to my ARC!
Not sure what you are hearing, but if 'Foggy' is a factor, then I would suggest you look elsewhere in the chain for the offender.
No flaming. My attitude about spending gobs of money on NOS tubes is kinda like buying expensive flowers. Sooner or later there going to die...LOL I'm not a big fan of NOS output tubes. The D115mkII was designed around the Philips/Sylvania 6550. Look for a set of those. BTW just because the voltages are ok on the amp doesn't mean the voltage regs are quiet. That's important too. Noisy voltage regs. have a negative effect on that amp. Also, there is a B+ reg and a buffer reg. Make sure the voltage differential is the same or close to the circuit diagram. If the power supply caps are more then 20 years old, replace them. All new caps will make the amp sound better especially in the bass provided there's nothing else wrong with it. There are a few axial caps scattered throughout the amp. Replace those as well. On top and underneath. There is also a 27K ohm resistor near the main filter caps. Make sure it measures 27K. Those do open with tube arcs. There should also be red cap plugs on the capacitors. The support bars always get bent because people try to lift the amp with them and their aluminum. You can remove them and straighten them out a bit to get some of the kinks out. Use a rubber mallet, however the rack handles will come off too. Do one at a time. I strongly suggest you make sure that amp is in good working order before you attempt to modify it.
@DaveyF: No other offender. I am comparing it with my 6 other amps in the room.
Vac Phi 70 monos
Counterpoint NP220 Premium gold
Jolida 502 thats "re" modified from an already modified unit
just to name a few.

@hifigeek1: Thanks a lot for the suggestions.
I myself hate using NOS power tubes, I pity those who generic Sovteks and Svetlanas remarked as whatever the hell reissue and give them names like Black voodoo and blue snake oil. Chinese tubes even to this date are a complete joke. Just sad to see the feeding frenzy for those 50 year treasures. I myself use Svetlanas or Sovteks. They dont sound as good as the NOS stuff but with balancing and tweaking of the gain and driver stages I can live with the imperfections of run of the mill and reasonably priced power tubes.

Thank you very very much for the pointers. I scope the regulated B+s carefully. Thanks for the tip on measuring the 27k resistor. Been bitten before by a few resistors on my VT130 which measured wierd...disintegrated when I tapped them with a chopstick.

I am giving up on Hexfreds on this one, no way I can safely mount them unless I can find ones in smaller than T0220 package.

Thanks again.
@Hifigeek1 you are talking about the 270k drop resistor R75 thats a protection bleeder across the main B+ filters right? I cant seem to find a 27k in the schematic. Is that a WW or a Carbon comp?
Hmm I think I found it, goes from ground to pin4 of TL071, big 7W WW resistor. 26.4k and holding stable no change with tapping.
That's the one. Also make sure you measure 10 ohms at ground to common tap at output transformer.
all measured OK. Thanks HifiGeek1 for the pointers.

Took me 2 hours to carefully pull all the wondercaps and Relcap coupling caps out, and another 2 hours almost to install k40y9s into the amp (size and metal casing does NOT help at all).
The foggy when low volume and glare at high volume problem is mostly gone.
I get a lot more natural and airy sound with strings.
More to do but at least this IMO is going absolutely the right direction.
Here's an FYI. I own a D115MkII and called ARC about putting KT120's in the amp. They said don't do it as the heater in the KT120 draws alot more current than does the 6550s. He said that this will kill the power transfer over time. I would think that the D70 would also be in this camp.
I wouldn't put 8 kt120s into it. Consider how much more current the filaments will draw. 8 kt88s in tetrode mode should be able to power the hungriest of hogs. The apparent lack of power is from the stock caps. If you are not comfortable with DIY or non factory mods, ask Calvin at audio research if they offer recapping service for the coupling caps like they did i(infinicap and dynamicap) in the 90s. The relcap plus wonder cap combo is not conducive to good sound at all.
Sbrown, Thanks for sharing that info on the KT120's. I was wondering about this and was going to call Arc about it. You just saved me the call, LOL.
Too bad, as the KT120 is a very good tube. With decent 6550's getting a little hard to find, I'm glad I just re-tubed with SED's.
Sbrown, I just replaced the 6550 in the power supply of my CD-8. ARC sold me a 6550 WE. No idea what it is, but if ARC sells it, I'm not worried.
@sbrown if you are just after the sound of the kt120 and not the marginal increase in power you can use just 4 tubes in your 115. That's how I sometimes run my old vt130 anyway. Make sure you are using one tube per phase per channel. Filament supply is parallel so you will never be stressing the unit with half the power tubes.
I was asking about the kt120 more from a increased tube life perspective as I knew that I wouldn't get a power increase without mod's to the amp. I don't have any issues with power with 115 - it seems to be able to drive about anything I hook up to it. ARC did recomend the Sovtek 6550WE's which seem fairly easy to find.

I have no issues with a non factory mod. I am considering recapping it but haven't made up my mind on which cap(s) to go with. Any suggetsions??
Sovtek and Svetlana tubes are definitely good value for the money. I have owned no less than 8 Audio Research amps and I have only 1 tube blow out on me since I never run the bias rich. 50-55mA instead of pushing 65.

There are other ways to get richer sound and snappy bass than just biasing up the power tubes (eg. choice of coupling caps and making sure the diff amp tubes are fresh, and adjusted for symmetry with the trimpots)

I still remember fondly the conversations with Leonard and how he got me started with Inifinicap for my old Audio Research amps.

If cost were no object for this project, (hopefully sometime later), I'd be using V-cap teflon 0.22 - 0.33uF for the coupling caps (yes they are *that* good). Anything larger simply wont fit.
My next choice would be Jenseon oils but after having a few oil leak incidents I'd rather stick to the cheaper (but metal casing so use extreme caution) k40y9. Auricaps and Dynamicaps will fit easily, a few notches above the Relcaps, but not enough of an improvement to justify all the work esp on this amp IMO. Too many caps to replace.

All the above have leads which will not fit into the solder holes, you will need to do a 90 degree bend of about 2-3mm of the leads and solder them parallel to the pad. Also be EXTRA gentle and careful not to damage the traces when you are extracting the old caps.

The large can capacitors are always Sprague or Mallory or Cornell Dublier. Easy to swap out if you can find them in stock at Allied or Digikey or Mouser. Buying from Audio Research would be fail-safe of course.

Are you saying you'd replace the 1.0uF + 0.15uF caps with the 0.22 to 0.33 values? Seems like a big change.

KT-120's are not recommended by the factory for use in a D-115mkII. I would however use caps furnished by ARC as well as the red cap plugs that go on top of the cap .
@Sbrown: Check VHAudio V-cap's web page on sizing coupling caps. The -3dB rolloff point is lower than audible. Plus there is hardly over 2 inches of traces from plate to grid for each coupling stage.

Also even in the 90s when I used to call Leonard at Audio Research to get suggestions on how to upgrade the Classic 120s he's recommend 0.47 or 0.68uF Infinicaps (Dynamicaps werent available yet) to replace the paralleled 1uF Rel 0.01 Wondercaps.

Larger caps were used in the 70s and 80s to add a thicker sonic signature to mask the mid high glare.
@Hifigeek1: Those huge lytics are Sprague and Mallory and hard to find in-stock @ Digikey or Allied. The cap plugs are impossible to buy in qty < 1000.
Thanks for the tip the V-cap's website. Do you think that the mid high glare you refer to is from the Rel/Wondercaps??
My experience with stock vintage Audio Research gear is that all of them distort/smears quite early with complex passages esp. chorale and loud piano chords.
I also have a problem with constantly trying to adjust the volume when listening to music. To low and I miss resolution, too loud and the glare is mighty annoying.
Not sure if the Relcap/Wondercaps are not aging well which contribute to that or not.

Everytime after I re-cap these amps with any new caps I often find they get more headroom before they start smearing and exhibit glare.

I have tried Infinicap, Dynamicap, Obligato, and Auricap on my previously owned Audio Research amps with varying levels of success. Each has their strengths and weaknesses. All of them, while better than the Rel/Wonder combo, still exhibit glare IMO.

My old Counterpoints (except for the NPs) all use Wondercap. All have benefitted from changing to oil or Teflon caps.

However, I have not tried the state of the art caps like Mundorf SGO or Vcap or even Jensens on my previous Audio Research Power amps ($$$, number of caps involved) like i have on other gear but based on my experience with the k40y9s on other gear and on this D115Mkii I am confident that this amp can even be further improved with the class AA+ caps. (Audio Research uses Teflon nowadays, wonder who makes them?)

Also regarding the size of the caps, look at what other manufacturers use these days for similar circuits. Very few of them use 1uF or larger anymore. Preamp output caps are a different story.

This is only the first step in modding this unit. I am still trying to find a way to install fast recovery diodes and am considering upgrading the high voltage bypass as well. Not now but later.

Thanks for your input. I think the current ARC cap is the TRT Stealth cap. No outside reviews that I've been able to find. I do know that Aesthetix also uses these in their Eclipse series. From what I've read, I'm not sure these are teflon - they sound simular to the Clarity MR's if you read the write up. If you look closely at ARCDB for the Ref 110, there's a side photo that shows the gold & red Stealth cap with a small silver cap which looks to bypass the Stealth. I would venture a guess that the small silver cap is teflon.

Another interesting cap is the duelund Alexander's. In the smaller size range, they are not nearly as expensive as the teflon caps.
JohnsonWu, you say that every time you have used vintage ARC amps, even after replacing the caps, that they still exhibit smear and glare but at higher headroom.
In my system and with the original caps in the ARC, I am not exhibiting this symptom. As I asked before, could it be something else in your system that is the cause of what you are hearing? How sensitive are your speakers and what tap are you using on the amps to connect them to? I have a small dedicated room, so I am not driving the amp that hard ( which could be why I am not hearing your symptom ;))), what size is your room?
@Daveyf: I meant even after replacing the Rel+Wonders with the class B caps like Auricaps there is still glare remaining. Not so this time, I am using oil caps, not polyprop.

My 2 main pairs of speakers are Avalon Eidolons (too big for my 14 x 18ish room) and a highly modified pair of Utopia Divas. The Eidolons with their Accuton ceramic tweeter tend to exacerbate the glare and hardness more so than the Utopia.

I use the 4 Ohm taps.

I never have a glare problem with my Phi70s (well after all its an expensive amp with all 300Bs and Infinicap-S, they MUST not have glare) or my Counterpoint NPs (Vcap TFTF).

With the stock caps, and if you are not driving the amp hard, you wont hear a lot of details in the recordings.
Thats my main complaint of the Rel+Wonder combo... low volume and its muddy, high volume and it pounces at you.

Anything oil or Teflon will convey the microdetails better and be a lot more linear when going from soft to loud. I never got a chance to discuss with Leonard about teflon caps but figure why he recommended Infinicap and later on Audio Research went from Rel+Wonder to Dynamicap and now Teflon (in some of their amps), not sure about the TRT Stealth which should be a modern version of Dynamicaps.
Johnsonwu, i am following this thread with interest. I have has several ARC amps and preamps modded by GNSC )Steve Huntley) over the last 10 years.

I still own a pair of Classic 120's which i hope to put head to head with a REF150 soon. I also have a REF3 LE and REF Phono 2 both modded by Steve.

The only thing that Steve did not replace in the critical signal path are the interstage coupling caps (4 in each unit). He felt that the stock caps were good enough and need not be replaced. What ARC use in their SE upgrades for the REF5 and REF2's are teflon 150 vdc 10.0uf =/- 10%.
What similar caps would you recommend as replacement (Vcaps?). I doubt that ARC would release those proprietary caps for installation in the field. Any recommendations much appreciated. Thanks again.
@Smoffat: The reason why I sold my CL120 (and CL150 of course) was the solid state input and gain stages.
I believe in the Counterpoint (Michael Elliot) philosophy of getting the small signal stage as tubey and transparent as possible and use whatever as the output stage as it has the least influence to the sound.

The 10uF caps had to be output caps (from pre to power) or from phono to pre. Upgrading those to any exotic brand like Duelund or Mundorf or VCap would be cost/size prohibitive.

The coupling caps within each device had to be 2uF or under I imagine. But until I get to see the schematic I cant comment.

Anyway, upgrading the small signal coupling caps would have a more noticeable benefit than the output caps.
JohnsonWu, I just don't agree with your assessment of the sound that the older ARC amps exhibit. My D70Mk2 has wowed all of the a'phile group that has heard it in my system. So much so, that one of the group traded a BAT VK75SE for a pair of M100's ( with the same wonder caps) and never looked back after he heard my ARC/CAT set up.
I'm sure you are aware that ceramic tweeters will exhibit glare and hardness with aplomb. The same can be said, to a slightly lesser extent, for the beryllium tweeters in your Utopia's. A hard dome can be bright and exhibit glare IF any of the upstream components are at all bright...speaker cables, preamp, front end, amp etc.
I believe and i think many others are beginning to believe also, that a silk dome or a ribbon is far less likely to exhibit this behavior than a hard ceramic or metal dome. Which, if I'm not mistaken is one of the reasons that Dave Wilson chose a silk dome for his top of the line speaker, the Alexandria XLF. I guess when it comes to this discussion, YMMV.