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Old 22nd August 2002, 04:39 PM   #1
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Question Otl

Hi,

Anyone els ever built the Bruce Rosenblitt OTL as published in Glass Audio ?
Like to get some feedback.(No pun intended.)

Thks,
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Old 24th August 2002, 10:37 PM   #2
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Frank,

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?

Mohan
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Old 24th August 2002, 10:55 PM   #3
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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 §.
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Old 24th August 2002, 11:07 PM   #4
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Mohan,

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.

Rgds,
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Old 24th August 2002, 11:28 PM   #5
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To Tom S.

Hey mista.
You leave us in the dark here.OTL? Tell us about it pls ...

Rgds,
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Old 25th August 2002, 12:51 AM   #6
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You leave us in the dark here.OTL? Tell us about it pls ...

I could write a lot in response to that question since it's so open-ended. Is there any particular aspect you'd like for me to focus on?

Tom §.
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Old 25th August 2002, 02:18 AM   #7
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Frank,

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

Mohan
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Old 25th August 2002, 09:33 AM   #8
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Default OTL

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.

www.multifoon.nl/aitos.html

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.

Greetz,
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Old 25th August 2002, 01:05 PM   #9
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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:

Atmasphere
BAT
Taki
Croft/Eminent Audio
Aitos
Futterman/NYAudiolabs
Rosenblitt**

**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,
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Old 25th August 2002, 02:44 PM   #10
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The schematic isn't available online but there is a simplification of the design here. The output tubes have a seperate power supply from the driver stages. The driver stages share one power supply but there is adequate decoupling methods applied.

Tom §.
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