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Hi-end 845 with only two stage
Hi-end 845 with only two stage
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Old 21st April 2009, 12:27 PM   #181
Zoran is offline Zoran  Serbia
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Hi Andrea, really beautiful and simple...
cheers
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Old 21st April 2009, 12:51 PM   #182
panos29 is offline panos29  Greece
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AE Europe got an excellent 1:1 interstage which i have tested and is one of the best I ever tried. It is also very cheap compared to performance.
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Old 17th May 2009, 05:36 PM   #183
audiodesign is offline audiodesign  Italy
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Yesterday Mr. Bartolucci was in my house.

We hava compared the sound of the all Tango solution to the all Bartolucci set in the GM70 amplifier.
All the persons present have considered the Bartolucci result better than Tango.

We have also compared the sound of the my 2 stage GM70 to the amplifier used by Bartolucci in the last Hi-end meeting in Milano (Italy)
http://nuke.nonsoloaudiofili.com/Por...09%20B/175.jpg
This is a 3 stage amplifier using the 2A3 to drive the GM70 and separated vacuum tube power supply per each channel.
The my 2 stage has been considered better for many factors.

After these tests has been tested on this 845 amplifier a new prototype of interstage created with a different way (not two-wire and with a turn ratio 2:1).

The result is great, I have seen a flat frequency response in the range 20-20KHz.

Soon I will have the final version of these new interstage with a turn ratio 1.5:1.
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Old 18th May 2009, 05:37 AM   #184
otto88 is offline otto88  Australia
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Hi Andrea

Later this year Ill be doing an 845 SE.
Its to run into class A2: a small benefit of A2 is extra power, but Ive heard a an SE class A2 on an 808 (radio transmitter tube) based amplifier with just modest parts in a seven way amplifier comparison. To my ears and one other, it was the best, on music I really like. http://www.tubelab.com/845SE.htm has some test results on A2 vs A1.

For prts - I already have some very good Tamura output trannies, and a very good high voltage toroidal power transformer.

I was going to do a three stage but can imagine the benefit of only two stages.

With properly rated output transformers, etc, would your design (adapt to) run into class A2?

How far off recommendations or a partial kit are you?
In particular for a sweet spot on value interstages, the INT.XR-B, the INT8C NEW, a later INT8C or a newer one; or AE Europe?

Thank you
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Old 18th May 2009, 06:41 AM   #185
audiodesign is offline audiodesign  Italy
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About the transation in A2 class there is a good article on:
http://www.geocities.com/bobdanielak/se_sv572.html

A2 class amplifier give a bad sound and it is necessary a feedback to compensate this problem.

Any A1 class amplifier can go in A2 class if the driver current is enough but if we keep in A1 the sound is better.

Using 1100v with these output tubes like the 845, 211 and GM70 it is possible get a very high power near to 20w and never go in A2 class.

In A2 class forget 1-2% thd without feedback.

Many years ago I have created a prototype amp. for the Aria company, they must use power supply voltage under 600v to keep the insurance rates low.
I have used SV811-3 driver by EL34 and 600v, the power was high going in A2 but the sound was terrible if compared to my 2A3 Reference PSE.
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Old 18th May 2009, 07:04 AM   #186
audiodesign is offline audiodesign  Italy
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I prefer increase the power supply voltage and keep in A1 class.

In any case this 2 stage design have enough current on driver to support some peak in A2 class.

The final interstage transformer is still in develop state and it will be a 1.5:1 so the 25mA will be 37.5mA on secondary.

Using the 6Hv4A there is enough voltage gain to use a 1.5:1 instead of 1:1 and the amplifier will need about 1Vrms to get 18-20w.

I hope to get these Bartolucci interstage very soon, probably in the next two weeks.

Also the output transformer can support many more power until 30w without increase the distortion.
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Old 18th May 2009, 01:14 PM   #187
otto88 is offline otto88  Australia
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Hi Andrea

(Not really fair to compare an SV811-3 in A2 in a 2A3 Reference PSE . .)

Thank you for the link Bob Danielak’s article. I like the easily re-configurable circuits.

I interpreted Bob results summarised as:
“in Class A2 Grid Drive (Mode 1) sounds really powerful”. And “Mode 3 sounds very similar to Mode 1. . Warm and loud. Perfect for Rock-n-Roll. Does quieter stuff real well too”.
(That was also my distinct impression of an A2 amp).
And “If you like the "Single-Ended" sound usually associated with really nice 300B and 2A3” class A1 is the appropriate choice.

Bob implies the A1/ A2 choice is almost subjective and/ or depends on your favourite music.


Bob’s results differed to George’s (tubelab). Tubelab did 3 experiments to explore A2 operation, each with different current amplification. Have you read his reports?

Eg: “In normal operation the two amps sound similar until a sharp percussive transient occurs. The A1 amp will clip
The A2 amp does not. I find that the A2 amp does a much better job of recreating highly dynamic music which contains a lot of transients. Snare drums, rim shots, the sound of a cello being smacked sharply with the bow.”
Measured distortion is shown.

A better explanation of tubelab results with A2 starts half way down at http://www.tubelab.com/PDcookbook.htm
Eg: “Operation below 20 watts remains unchanged”
“A driver stage that can supply all the grid current the tube demands as the grid goes positive” for A2 is essential.
Though I personally believe a small amount of feedback is ok, tubelab apparently didn’t use it with any of the 3 options.

Most importantly “The distortion numbers improve at EVERY POWER LEVEL. So does the damping factor” referring to TubeCad simulations, verified with measurements and listening tests. Ie, class A2 can also be cleaner.

I’m looking forward to hearing the further refinement of your amp. How far into A2 would the current on your driver stage be likely to allow?

Thanks again
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Old 18th May 2009, 01:24 PM   #188
audiodesign is offline audiodesign  Italy
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I want tell you that also using a strong driver with many current in A2 class we will get a higher distortion and lower damping factor at the same power.

A2 class need a global feedback to compansate these effects.
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Old 18th May 2009, 02:24 PM   #189
audiodesign is offline audiodesign  Italy
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If we see the grid current plot for a similar tube like the SV572-3 we need 5mA at Vg=+20v, 20mA at +40v and 30mA at +80v so with 35mA we can support large peak in A2.
Attached Images
File Type: gif 572-3.gif (38.2 KB, 914 views)
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Old 18th May 2009, 09:09 PM   #190
hemgjord is offline hemgjord  Sweden
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First of all I think your work is very interesting but I wonder a little about your aversion against A2 operation which seems quite cathegorical.

Quote:
Originally posted by audiodesign
I want tell you that also using a strong driver with many current in A2 class we will get a higher distortion and lower damping factor at the same power.

Could you please elaborate this statement a little? How can a strong driver negatively affect the output stage damping factor? (or any driver for that matter) Also I wonder about the distortion statement since you cannot have the same output power in A1 and A2 for the same idle conditions and the same tube. If you stay in A1 territory I cannot see how a stronger driver could lead to higher distortion.

/Olof
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