• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
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I have not built Bruce Rosenblitt circuit. However, I recall reading some letters in Glass Audio about people who have built the amp…very positive response.

I used to build OTLs in the late seventies-mid eighties mainly because no one here in Australia would wind an output transformer for me. I believe that OTL reigns supreme in the musical enjoyment category. However, they should be presented with simple (purely resistive…ideally speaking) loads. I used to have Magnepan MG IIB then, which presented a simple resistive load to the OTLs. Later, when I changed over to electrostatics, I tried higher (70W) power OTLs with not so exciting results and a host of other problems including oscillation etc..

Recently (two years ago) one of my friends built a 30W OTL and we played it through Tannoy GRF, I believe. When we substituted the OTL with Audio Research D76A, Radford and Dynaco amps, we lost a fair bit of the dynamics, speed and transient response (in short what I would call “intimate” presentation). OTLs are not for the common man. They need regular tweaking.

Bruce’s topology is a no-nonsense simple circuit based on Julius Futterman’s topology. I would rework the power supply a little. All in all, highly recommended if you have speaker that represents a simple load or is it time to change your speakers?

I'd suggest building the OTL circuit in "Audio Reality" if you're dead set on building a monster OTL. It's a more stable, refined design than the glass audio circuit and EL509's seem to be more consistent quality-wise than 6AS7's (although not as cheap). I've had my T8 OTL monoblock copies for a couple years now without any troubles or failures (although I prefer to listen to music/movies mainly with my 1.5watt parafeed amp).

If you're not into DIY metalwork, you could also go with the transcendent kit that uses 6C19PI's.

Tom §.
diyAudio Senior Member
Joined 2002

This is just plain lovely.
For years I sought people out in order for them to understand OTL circuits...
I'm glad to see you coincide.
Personally I have a couple of favs amongst OTL's being Aitos (incredible Sota Built) Croft being shy of the real thang lately( he prefers to play it safe with a couplin cap on the output)
As far as high-end circuits go I've never seen enything like the Aitos gear.This is state of the ART.Perfect to disbelief really.I have no idea whether this company is still in business...
Ok,I agree they are speaker sensitive.Apogees are not their kind of thing but then what is ?
I'm pretty sure that if I could convince people of their merits..Think of a modern speaker...needs current right? OTL.would do..
Regarding Bruce Rosenblitt's design let me just say it blows your socks off.Cheap to build too,guess some folks still have this populist streak in their veines...
Futterman gave OTL a bad rep on reliabity,it's time to change that.

diyAudio Senior Member
Joined 2002

I have not heard of the Aitos amp. What is the topology like?

Hello Mohan,

There is not much I could find on the web regarding AITOS,this link
is a dealer.


I once had a customer who brought a set of those monoblocks to my lab asking if I could do a check up.From what I recall they used a EF184 pentode as input and EL519 for the output.What struck me was their marvelous handbuilt quality.Everything was hardwired with painstaking attention to detail.
Not that many were ever sold I think,they were frightningly expensive.Still when you looked at the built quality it is easy to see why.
From what I heard about them they were all handbuilt by a retired Philips engineer in the Netherlands.
If anyone would have more info I'd appreciate it.

diyAudio Senior Member
Joined 2002
Hello Mohan,Tom et all,

For starters I'd love to see every single bit of information on OTL amp design available.
There is something about (immediacy,intemacy ?) OTL amps no other circuit has been able to deliver.There are drawbacks for sure but none that cann't be overcome.They definitely benefit from active regulation (most circuits do).
I have this imaginary gas factory in mind where you would have as much (or more) tubes regulating the output stages as you would have output tubes.
There sure is a future for OTL's...

Here are some names I know have to do with OTL:

Croft/Eminent Audio

**I agree that PSU could be bettered but then most others could as well.

Anyway I look forward to yr feedback ( come to think of it : OTL embraces the speaker into the global feedback loop...)

Cheers and thanks to all,
Joined 2001
Paid Member
Tom S. said:
The schematic isn't available online but there is a simplification of the design here. The output tubes have a separate power supply from the driver stages. The driver stages share one power supply but there is adequate decoupling methods applied.

That TubeCAD issue is a review of Rosenblitz's Audio Reality book which is a good read and has full details of his OTL amp (25 & 80W versions). Recommended. Rosenblitz always likes to start out with a fresh sheet of paper and there are some other novel designs in the book as well. The grounded grid line-stage gets good reviews from users and is designed to be quite inexpensive to build.

The little dig that the editor has re Class A & AB is the contention he has with Atmosphere claiming their amp is Class A.

I bought AR about the time it was first published which was nearly 3 years ago (then built the OTL and GG). Every once in awhile I'd go and thumb through it a little bit. I can't help but think that the book is real odd in that half the book is aimed at people who've never really touched a soldering iron or would want to touch a soldering iron. Yet, the last half of the book contains plenty of info that only experienced DIY builders would find valuable. It's like going from 1st gear to 5th gear.

It seems like a logical thing to do would be to sandwich "Beginners Guide to Tube Audio" between the theory and project chapters of AR. Then again, I'm sure Bruce would just rather have people buy both books.

Tom §.
There is not much I could find on the web regarding AITOS
Aitos is a VERY (I don't know exactly, but something like three people) small Dutch company dedicated to OTL design and building. They do the design and building themselves and all by hand. One of the founders is Danny Cabell. He also (still) works at Multifoon (a dutch audio shop) So If you want to know more you might contact him through Multifoon.

Over ten years ago I graduated as an industrial designer from Delft University. My graduation project was to design a chassis for one of the new Aitos Otl amps. Alas the design was never produced..... But I did hear some nice mucic in those days.

diyAudio Senior Member
Joined 2002

Hello Peter,

Thks for sharing the info.


Does yr. OTL use the EL509 or the 6P19C triode ?Any comments on reliability of these ?
The GG preamp is a linelevelpreamp isn't it?
Comments ?
These kits really sound almost too good to be true and I mean pricewise.

Does yr. OTL use the EL509 or the 6P19C triode?


Any comments on reliability of these ?

Can't say anything about the 6C19Pi OTL as I don't own one. The T8 monoblock copy I built works just fine after 2 and a half years of on and off use. It's quite reliable.

The GG preamp is a linelevelpreamp isn't it?

Yes. In AR there is a GG phono pre circuit as well.

FWIW, I find the designs I built a little hot in the treble character (the GG moreso than the OTL). At first, this will sound very exciting to the listener and almost create a sense of "wow, this is so clear." However, after long term listening (at least in my system) it tends to make for a less enjoyable music experience as you become more focused on the "trees" instead of the "forest." That does sound a bit vague but it's the only way I can describe my reaction. Don't mistake my impression as the designs being bass shy as these designs are not. Bass character is very strong.

Bruce has stated a number of times that he strives for as much of an uncolored sound as possible. I think this does show through in his designs. To me, uncolored is uninteresting. It's like eating prime rib that was boiled in water.

Tom §.
diyAudio Senior Member
Joined 2002

Hello Tom.

Care to read this ?

Other than that I see what you mean.I can't really point my finger to the cause of it,slight phase shift,tonal balance not to your liking ? I don't know.
Still from his designs and previous work in Glass Audio I noticed and heard his preponderance for extreme neutrality.
I try to strive for the same thing , but it's like walking a very thin line.One may easily err on the wrong side of things.(Ying and yang.)
To me it proves just that different people have different tastes,nothing wrong with that really.
If it's any help to you I'd try replacing the coupling caps in the preamp with metallized polycarbonate ones.If memory serves me right the Roederstein (amongst others)MKC 1831 may put things into balance for you.
There are of course other ways you can travel... if you don't want to put a soldering iron in your gear.
What I often see wanting in the so called high end designs is a lack of the unpalpable yet most important factor called musicality.
Guess this is what you try to describe using the "rib" analogy?
To me a set of this gear would allow me to use it as one would use a scope i.e. a tool to find flaws in other componentry.
I think Bruce sets the same goals for himself.
It's all a matter of balancing things out.Can't have your cake and eat it too.

Anyway thanks for sharing,

Care to read this ?

It's just Giz blowing hot air at Bruce who had blown hot air about the futterman circuit. *shrugs shoulders*

If it's any help to you I'd try replacing the coupling caps in the preamp with metallized polycarbonate ones.

Used solens then RELcaps. I'm not going to mess with it as it no longer interests me anymore. I now use a dirt simple 6H30Pi single tube preamp now. I was using a 26 dht preamp but I'm in the middle of cannibalizing parts of that box for a 101D preamp (and I'm also waiting for additional parts). It's a bit abnormal to be using DHT tubes for line stage use but I'm abnormal as well.

There are of course other ways you can travel... if you don't want to put a soldering iron in your gear.

I don't think there's any other way for me other than using a soldering iron.

Tom §.
diyAudio Senior Member
Joined 2002

Hey Tom,

Couldn't help but grin when I read through this.Who wants normal?Is there a valid definition?
What keeps on surprising me though is that our Japanese friends haven't bought the single last DHT yet.Must have been that economical crisis back in the early nineties.
Ten years ago these were scarse as it was.Has anyone bothered to keep track of the prices of this stuff?Any tube for that matter?

Bruce who had blown hot air ...

Doesn't sound like him.*shrugs shoulders*
Julius Futterman sure was no fool.It is about time people gave him credit for what he's achieved.
Bruce and Harvey should see things in perspective,relative to the time in which it was conceived.
So much for a little philosophy...
Funny you mention the 6H30P,this is one I'd love to try out.
Kind of a super 6DJ8 as I understand it.I'll post a new thread for that so your of the hook.
Thks again Tom and chin up,your new preamp is coming up.
Just kidding.

Hello everyone,

I know that there are a lot of knowledgeable people in this forum who are content to remain on the side lines. If there is substantial interest in a DIY OTL, then I think that we should pool all of our knowledge and resources and design one right here in this forum. My suggestion is that we should try a SE OTL design first. Perhaps there may evolve more than one design. Members can then build different designs and report to this forum. Any takers? Anyone interested?

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