I owned the GM-20 OTL along with Graaf's 13.5B preamp until I very recently. I sold them simply because I needed more power to drive my not so efficient Kharma CE 2.2 speakers (89-db, 8-ohms) in a fairly large loft setting. I replaced the GM 20 with an Air Tight ATM-2 (80wpc) stereo amp, and the replacement for the preamp is a Supratek "Chardonnay" from Australia which has yet to arrive. I really enjoyed the Graafs. If you are a "near field" listner and/or have a small listening room, you will love the sound of GM 20. In my view the Graaf sound is a bit more accurate than the Air Tight.
I just got Graaf gm20. It's fantastic - one of the best amps I've ever heard. It is very nuetral tonally, incredibaly fast, transparent, detailed and dynamic as hell!!! Very powerful 20 watts. I highly recoment it.
I own a GM-100, which I use with Kharma 2.2's. The sound is very rich, dynamic and well layered. My only reservation is that the amp is alittle noiser than some. The amp has alot of gain, and the 6dj8/6922 input tubes are run very hard, so getting a quiet pair of tubes is quite a challenge. However, the signature of the 6dj8/6922 input tube is immediately obvious and easily disinguished.
The following is a review for the GM-20 by Ken Kessler - Hi Fi News Sept '99:
"With a decade's experience in output-transformerless design, Graaf's latest OTL, the GM20, should be something really special . . . . . .
Graaf's Giovanni Mariani is one of but an handful of electronics wizards brave enough to carry on with output-transformerless circuitry. Mariani has entered a new phase of excellence managing to produce something totally out of the ordinary yet completely in character with the amps of his which preceded it.
Yes, the GM20 is an OTL amplifier, like the GM100 and GM200 it's built to standards surpassed by no other valve amp and contains Italian parts wherever possible. This particular model uses a rather special tube which I don't believe has been used in OTL's before.
The benefits of this type of circuitry may be summarised as unrivalled transparency, speed and dynamics. That's because OTL/OCL designs are as close as it gets to direct injection of signal into speaker. In the GM20 the output stages are directly coupled to their loads, and the output tube in this case is the Russian military workhorse; the 6C33C, known for the bunch of 'nipples' on it's top.
Like it's predecessors, GM20 uses a fully differential and balanced configuration (i.e. symmetrical) DC-coupled between stages. This explains the low levels of hum by OTL standards and general immunity to noise, including noise from AC mains.
GM20's power supply is made up of six separate sections, four for the output stages and two sections for the driver and gain stages. Two of the aforementioned 6C33C triodes are used per channel configured to exploit the ability of these tubes to deliver high current with a low voltage power supply. Completing the valve complement is an input stage using two 6922 double triodes responsible for the voltage gain and operating as impedance adapters. The driver stages employ two triode-coupled EF184 pentodes to take advantage of the differential circuit and to work as phase splitters. Loudspeaker protection is guaranteed by a novel and sophisticated circuit which avoids the use of series relays or current limiters.
The GM20 is so damn cool looking that even if you don't like tubes let alone OTL's - you'll still fall in love with it.
Contained within the dimensions of the 350x215x440mm is a look somehow more svelt - still brutal and purposeful, but more 'styled'. There's a bold green light built into the on/off button, the cage is plated in some exotic 'black chrome' and the gloss-black paintwork was applied in the Ferrari restoration facilities.
Quite clearly, OTL's offer an immediacy and an intimacy which can only be present when the signal path is uncluttered and direct. OTL's deliver speed and detail a level of 'snap' in the upper mid and treble transients and a neutrality that almost disavows what the coverted feel are valve virtues. Warmth remains - that much is indisputable.
Despite power limits which can easily be reached if you use hungry speakers, the GM20 sounds big and powerful regardless of the actual SPL's. You get the full works whether you're playing at 65dB or 95dB.
But what sold me on the GM20, beyond the gloriously like-like midband, the trademark OTL clarity and detail, and the sheer scale of the musical recreation is the sense of what I can only describe as 'presence'. The sound of the GM20's possesses something else, a palpability which gives the performers more substance and body than I've experienced from any amplifier this side of the Marantz Project T1, the Audio Research Reference 600 and the 80k Krells.
What makes the palpability so remarkable via the GRAAF is the GM20's price tag: £2750. Merely as an objet d'art, the GM20 is worth double that.
The GM20 is too damned good for the majority of the world's audiophiles. Period."
Funny the Kessler wouldn't know that both the Joule and Tenor use the 6C33C Russian output tube. Don't know if Tenor was around when the article was written but Joule sure was.
I have heard a pair of Graaf mono's on an IRS 5 system and it was glorious as are all OTL amps in my experience.
One thing DRV, be careful with a small powered OTL with the Merlins. It might work well in a small room but if the room is too large, you may need/want more power. I owned a Transcendent T8-LN and while I loved the sound it didn't have enough gas in my 16' X 18' room.
Well, many good things already said. Graaf OTL amps are excellent even the GM 20, built to very high standards (Ferrari black and/or red paint, no ordinary tube sockets to mention just some). The only "drawback" if any, is if you have speakers of moderate sensitivity. Get one and... perhaps change your speakers!
I have had my Graaf GM20 for about 3 years now. It was an upgrade (very substantial) from a Sonic Frontiers SFS80. I find the Graaf sound to be immediate, engaging, muiscal, fast, fluid - very typical of OTL and somewhat similar to midrange Atmas - only much better looking. Is the sound colored - probably, but what matters most to me in the amp is its ability to deliver music through my speakers. I focus more on purity issues upstream of the amp, which is why I use a passive Placette preamp also great and a good improvement over my previous passive ReferenceLine). The system drives extra-ordinarily inefficient Alon IVs with an input impedance around 3 ohms.
The challenge is that Tube amps in general and OTLs in particular need to drive speakers with relatively high input impedance, say 8 ohms or more. The Graaf manual specifically wants to see 8 or better. The dealer who sold me the Graaf (and also sold me the Alons years earlier) did not alert me to this issue. So, I listened in some ignorance for about a year and half, always with that nagging feeling of why my system did not sound as good as I thought/knew it should. Finally, an Audiogon TT seller alerted me to an autotransformer as a solution to this impedance problem. The autotransformer is a speaker fix and basically increases the input impedance of the speaker to match the needs (through a simple trial and listen type approach) of the amp. The autotransformer called a Zero, with a link from the Atma-sphere webpage, dramatically improved the sound coming out of the Alons and I suddenly had a system I am more than pleased with. The mids and bass became alive and bloomed. Wonderful.
Be aware though, that some early GM-20s had some build problems and I was one of the few unlucky ones to get a crappy build. After three trips to the repair shop - all warrantied - the unit runs flawlessly now (I was out the amp for something like 6 months over the first 2 years of onwership due to repairs for capaciters, tube socket, and one other item I cannot remember). Ahh, patience. When the opportunity presents itself, I will likely buy a second GM-20. Oh, the amp runs hot enough to fry an egg on top, but I use a stove for that.
I have had the GM20 for about 3 years now, and it has been back for repairs about 4 times. Each time the problem is the same, i.e. one chanel starts to hum and one of the capacitors needs to be changed.
My speakers are JM Lab Utopias.
Has anyone had this experience, and if so, is there a fix? Or do I just have a crappy unit?
None of the above letters seems to indicate any problems, so I'm wondering if there is something wrong with my set that needs to be fixed, apart from changing capacitors every year.
I'm very happy with the sound, but I also dont want to have to struggle with repairs every few months.
ANY help would be appreciated.
Just read your graaf post and I posted just before you about a year ago. My post also cited problems - take a look. My graaf also required replacement of the large capacitors (one or two as I recall), a power tube socket, a resistor, and one other thing I can not recall. However, after I purchased some Zeros - autotransformers - that stepped up the speaker input impedance - all problems went away and the sound improved dramatically. With the zeros in place, the amp had no problems for well over a year of steady use. Before I sold the amp, I had the tubes replaced and the unit fully checked out by an authorized Graaf service facility and was perfect.
My speakers are the Alon IVs with an impedance range from 8 to 3 ohms! I looked up your speakers and it looks like their impedance is 4 ohms. The amp specs say that it wants to push against no less than 8 ohms. So, I suspect that your amp may be working too hard - I recommend the autotransferms by Paul Speltz (you can search direct and there is a link on the atmasphere web page (they make OTLs also). As I understand it, OTLs are very sensitive to speaker impedance, and while tube amps (unlike ss amps) are also sensitive to speaker impedance, the lack of an output transformer makes the OTL design more vulnerable. My dealer (Stereo Unlimited in Walnut Creek, CA) knowingly and ignorantly sold me the $5K amp to drive my Alons (which they also sold me) - I do not do business with them any more.
I have had the listening experence in Montreal hifi show this year. The combination of GM200 plus Bosendorfer loudspeaker VC7, and GM20 plus Bosendorfer VC2 are marvelous. Very good playback on piano and vocal. They actually hired live jazz group with female vocalist and a real Bosendorfer grand piano to do A/B comparison live!
More info can be found from link below:
I have heard enough praises of the Graaf GM20 and think it may be the time for me to physically listen to one or two (in bridged mode). I use several different loudspeakers including the high efficiency models like vintage JBL4333A difficult loading Quald ESL-57 and the modern moderate efficiency French Jean-Marie Reynaud Reference 25th Anniversary and Bosendorfer VC-7. I received a pair from the new US and Canada distributorfew weeks ago. After a few days burn in I listened the GM20 with my different loudspeakers. Switching to different loudspeakers didn't change the tonal character of the GM20, it is highly analytical yet very smooth, with very liquid mid to high and pin point localization of the sound sources. Those characters are very favorable to muscial reporduction and I like them very much. According to my experience with other high end amps, the merits are not only limited to Graaf GM20s; you can find these characters on most of the high end power amplifiers. What wonder me is I find ALL these elements perform at the same time through a samll output amplifier. What even shock me off is something which unique to the GM20s, I haven't heard that character very often before especially with all the other tonalities mentioned above from a single amplifier. Not so even on very powerful high end amps like the already very bass powerful Mark Levinson 33H. It is the power, authority and resolution in mid to low range in bass ( from app. 180 to 45Hz ). Quality and realistic bass in reproduced music becomes very rare and expensive nowaday, I expereince many super systems, including Wilson Grand Slamm, IRS and Genesis V and not many of them produced qulaity bass with speed, accuracy, resolution and QUANTITY that can move your emotion not only by the music but also by the air and vibration which is in pace with the music. I was shock by the first time I used a pair of mono bridged GM20 into my Jean-Marie Reynaud Reference 25th Anniversary speakers without hooking to my pair of highly modified Triad Gold Sub-woofers; a pair of 2 X 10" push pull highly modified very "musical sub-woofer", now I can listen to only the Jean-Marie Reynaud Reference 25th Anniversary and enjoy the sweetness, open, airly yet powerful, full range down to 25Hz bass and coherent musical experinece. I have no second thoughs after I change to the GM20s from a pair of Atma-Sphere OTL and very powerful Parasound JC-1. I am now free from hardware induced compromise and enjoying music like nothing before.
I have a set of Graafs GM20 (monoblocks) to drive the difficult Sonus Faber Eelcta Amator (first version, so without the passive radiator). This speaker especially goes down in Ohm in the lower frequencies and a single Stereo GM20, didn't pull it. But the monoblocks do!!!
I purchased a pair of Sonus Faber Guarneri's and have tested maybe 10 amps. I have kept the Graaf GM20 otl as my final choice. It is quiet, accurate, detailed warm.
I am listening to the system at low volumes in a 10x11 office so the mono route is not neccessary (but at louder volumes I find the bass control lags). This is an exceptional rare product that continues to fascinate me with musical enjoyment. I have ordered a GM200 for another system and will be recieving it soon, GRAAF makes products of high quality and refinement. Such products are becoming harder and harder to find in todays cluttered marketplace. I am glad I stumbled on this beauty.
You might also want to check out Mastersound, which I have been peased with, just to stick with the Italian theme. They make excellent amps.
I ahve beeen an audiofile for almost 30 years. For the last 25 years I listen my music through various Tannoy dual concentric loudpeakers. I have used almost all type of amplification (Tube and solid state of various power from 7W to 200W and various configuration single ended, class A push pull ) except OTL amps. Two months ago I have accuired a GRAAF GM 20 to drive my 15'inch prestige Tannoy GRFs. Since then every day has been a revelation to me. I love my Tannoys but now they sound so good that i have never heard them before. For those that are familiar with good 300B single ended sound I state that at least with my spaekers the result with the GRAAF is 10 times better. Excellent and very serious Amplifier. If you have sensitive 8 ohms loudspeakers you sould considerit very seriousely.
I have had a GM2 for 6 years and i can testify to about everything that has been said about it in the earlier posts.
The sound is big, smooth, fast, detailed, controlled. Never heard better, but I have not heard the Joules and Tenors. What I know is that everything else I have heard in comparison sounds slow and syrupy.
But owning the GM20 is not without hassle. You HAVE to buy matched output tubes. That is a must. You have to adjust the BIAS an DC offset about every 2-3 month. For that the amp must be turned up side down, the lid under unscrewed, warmed up, and then adjusted. (So watch out, you may kill yourself if you are really unlucky.)
You also have to replace the large electrolytics resting on top of the amps every other year. They last for about 3-5000 hours. Better replace them in advance in the summertime, before the listning season begins.
And then there is the 8 ohm issue. Take that serious.
And to make it sound the best I can tell you right away: Run it balanced. Do not waste your time using singel ended preamps. It may sound sweet, but you loose the bottom end control.
The best combo I have heard is the GM 20 and the GM13.5B. That is magic.
I also sounds very good if u run it balanced from a Wadia CD player.
But as of January this year, Graaf does not make it any more, so if you want one, you better hunt down a used one. (And then look for the newer types with metal bars on the tube pins. Look inside.) On the older ones, the PCB tends to be destroyed around the tube sockets due to the heat.
I have learned to live with the "trouble". I do most of the service work myself, so I don´t have to ship it away for repairs. Then it works very reliable. I use to say that if you own and drive a Ferrari, you don´t take it to the garage for service with a two year interval. A race car needs service more often to work reliable. But then again, driving a Ferrari is certainly not the same as driving a SUV.
So until I hear something that equals my GM20, that is my dream amp and it rests in my living room.
(Forgive my bad inglish, it is not my native language :-)
I had a Graff GM 100 that replaced a Symphonic Line RG1.The Graff was replaced by Chapter Audio.Go to Chapter Two power amplifier review:
Eye Fi Magazine, Norway Summer 2002-it is on the Chapter Audio website
The reviewer also repalced his reference GM 100 with the Chapter Audio Chapter Two amp.
I just want to point out that the GM100 is a totally different amp than GM20. Both sonically and with regards to speaker compabilty. They both have the musical Graaf "signature", but besides from that they sound very different. The GM20 is also a very good amp though.
Thanks Freshpuma.I actually had all the Graff amps, one time ot another, in my listening room and they all sounded magical.My point is,as a former tube man, I found a SS amp(in the Chapter) that took my listening experience to another level.
Well if you liked the Graaf amps, then I have to check out the Chapter amps :-)
BTW: Have you any experience with the GM200. Just wonder how it sounds compared to the GM20?
Yes I absolutely love the hot blooded and hot running GM200.It is alot more dynamic with more "air is there" and also more body to put Heidi Klumm to shame.However,the Chapter amp put my musical experience on another level.Now to be honest and sincere,this is my subjective opinion.And you can read what a reviewer said(and the one who got me to investigate):
Hy, I have two Graaf GM20 and the 13.5B II preamp. I absolutely agree with all the good reviews posted here. GM20 is a kind of magic.
I own a Graaf200 and the 13,5b II preamp.
This comb is simply one of the best and I enjoy them more and more each day.
Does anybody have the instruction for how to mono-bridge the GM20? I have just recieved the kit, and can se how to bridge it innside from your page Theo, but how do you connect it to your preamplifier and speakers?
Monostrapping any Circlotron-based OTL is an easy thing to do. The speaker terminals are placed in parallel and so are the inputs. This will usually more than double the output on smaller OTLs like the GM20 or the Atma-Sphere S-30 (which goes from 30 watt/ch into 8 ohms to 90 watts into 8 ohms, and runs a little cooler).
So: a 'Y' adaptor on the inputs and a couple of jumpers on the output terminals are all that you need. Just pay attention to phase on the outputs- get it wrong and the amp will make no power and be very hard on the power tubes!
Got feedback from Graaf. There should be no jumpers between output terminals and no "Y"-adaptor on the input. Everything is taken care of by the internal mono-kit.
QUOTED from Theo
.....You also have to replace the large electrolytics resting on top of the amps every other year. They last for about 3-5000 hours. Better replace them in advance in the summertime, before the listning season begins.....
Are you sure? Because it is rather expensive and gard to live with if true. Many tube amp need not change of this during its whole life....
You are correct, most amps dont. The problem with the GM-20 is the heat from the tubes gets the caps very hot, and so the caps don't last that long.
Hi , I have 2 Graaf Gm 20 used in biamp ( two stero amp one for the bass and one for the tweeter ) with a Graaf 13,5 bII and a Graaf Gm 70 phone stage.
I owned first only one Gm 20 with the same pre and i was absolutely wondered for the fastastc sound. With two Gm 20 the sound became amazing: detailed, terrific. Try it if you can, buy one and keep it .
Can anyone tell me if Graaf GM 13.5b can tape to 220v from 120v internally ? Anyone have schematic or manual of dong this?. Please help. Many thanks
8 years this this has been going!!! Drv what did you do???
Which Graaf amplifier can drive an ATC SCM40 in a medium sized room ?
Anyone.But i strong suggest you to find and try the Modena monoblock.Or place two Gm 20 in biamp connection
Yes i have had the GM 200 OTL for about 2,5 years and ive heard the GM 20 as well , i sold it off after i listened 3 months to them on my new speakers with a diamond high frequency cone .
The smooth topend what i had with the graaf tube amp and i was missing with SS was merely a tweeter problem in my case , my highs are almost as sweet with ss as with tubes (zanden exepted ).
Further very good amps, spatial/dynamic as well , i sold them merely because they need new tubes once in while , and with the 32 in the GM 200 it will not be chaep, the bass is a bitt more rounded than with the krells but also beatifull , i rank them in the top of poweramps
Replacing the entire set of 32 tubes on the GM200 costs an extremely reasonable 500 Euro for a matched set from the factory (you really want to order them from the factory as close matching is absolutely necessary). This is hardly expensive, especially when considering that these tubes have a 10,000 hour lifetime. With idle power consumption of about 1000w this is not an amp that you leave on all the time so, assuming that you use the amp for 2 hours a day, every day, the tubes will last for 14 years. Spare sets can be ordered from the new distributor, Audio Analog in Italy.
The capacitors are relatively cheap to replace but if you are worried about heat affecting the capacitors, use the tube covers that Graaf supplies. The amp is not quite as pretty with them on as it is with rows of exposed glowing tubes (although it remains a truly stunning looking amp) but Graaf is effectively doing the same thing as Porsche does with its Turbo cars - Porsche puts a thin dimpled aluminum heat shield in the 1" space between the exhaust and the very thin rear plastic bumper to stop the extremely hot exhaust from melting holes in the bumper). The tube covers protect the capacitors and transformers from heat damage in a similar way. In fact, in addition to the covers acting as a heat shield, air convection currents will pull in cold air into the bottom holes on the covers and hot air will exit through the top, cooling things down even further on the rest of the amp. A bit counter intuitive - you would think that putting the tube covers on would make things hotter but they don't.
My GM20 has just blown up for the second time this year, taking the driver of one of my Duevel Bella Lunas with it.
We're having some trouble fixing it and wondered if anyone has a service sheet with voltages and/or a circuit diagram?
I look forward to hearing from you, Zeb
PS Since it's spending more time being fixed than working these days I'm also interested in an inexpensive used amp that I could get for whilst it is being repaired. Ideally balanced, probably valve, maybe KT88s.
Zebshaw, likely the problem you are having has more to do with the power tubes than the amp.
A simple way to get greater reliability out of the tubes is, if you have a new set, put them in the amp and run it in Standby for about 2 days, and the bias up the amp and run it with either no music or very quietly for another day. This will double the life of the tubes and you will be far less likely to get premature arc-over failure!
We make fully differential balanced OTLs FWIW.