Hi-end 845 with only two stage

[audiodesign]

My experince with OTL is very old, many many years ago

I don't consider the OTL a good choice if you are searching a reference amplifier.

This because OTL need feedback and to use low global feedback it is necessary use many output tubes so the cost it is near to high power single ended.
Another point is that the OTL use bad vacuum tubes, ofter voltage regulator like 6080 and 6c33c-b with high distortion so other feedback it is necessary to compensate this.
Another point is that in the OTL is always used a bad power supply with no inductance and the power supply transformer are never enough big.
So for these aspect I don't like these amps and I don't susggest.
But many DIY have build my OTL with a relative low cost and now are happy.
So I suggest it only if you can get sulplus material like capacitors and transformers

[Villaw]

Thanks for your comments, appreciated.
I was under the impression that getting rid of the coupling transformer is the way to go in high end audio, but have never had the opportunity to play around with any single ended designs mostly due to the fact that they have received some negative publicity from some "gurus" for low power limited frequency range and impedence matching limitations.

Nevertheless I will take your word for it and go for a single ended design from your web site. Do you think the the 845 design surpasses the dual 300b reference amp you have built in sonic quality?

[audiodesign]

Here the first test result using Bartolucci Interstage transformer

http://www.audiodesignguide.com/Clau...test_ver2.html

[rdf]

Hi Andrea. What is the full power input sensitivity? Wondering if a step-down interstage is an option.

[audiodesign]

Nick,
many commercial SE amplifiers give bad result because in the projects to get high power use low impedance transformers.
This give output impedance near 3-4ohm so the damping factor is terrible.
These amplifiers can give a good sound only if you use fullrange loudspeakers with impedance near to 16ohm.
All my vaccum tube amplifiers have an output impedance less than 2ohm so these can drive without problems any loudspeakers.
In my medium size room I am using this 845 amplifier to drive Martin Logan Aerius and this give a reference sound also with a good output level.
The 300b is not a good vaccum tube because the sound is not neutral and it have a characteristic. The medium frequency of the 300b is very natural but bass and high frequency have very large limitations.
I consider very well the 2A3 and 845, soon I will test GM70 (both versions).
The SV572-3 and SV811-3 are good like the 845 but these are not in the current production.
I hope my bad english can be understand.

Soon I will test and compare the Tango X-10S to a new Bartolucci output transformers designed for this project.

[audiodesign]

Richard,
why are you interested to an input step-down ? to reduce input capacitor ?

[rdf]

The 6HV5a has a lot of gain. With that B+ it's also good for around 30ma plate current comfortably. If I have it right, a step down would provide more current to the output grid and a higher plate load to the driver in A2, as well as a lower DCR from grid to ground on the secondary for quicker recovery. The step down reduces input stage noise and, as you say, the capacitive load on the driver. If the circuit has gain to burn it seems worth considering.

BTW, that's not my quote in the signature, but my name is Richard so it still works out.

[audiodesign]

Yes, an input step down is possible

I suggest the Lundahl LL7903 with turn ratio:

1+1+1+1 : 2+2+2+2

so it is possible use it in 4:2

This transformer has been tested in my DAC Final test.

http://www.audiodesignguide.com/DAC_final/DacFinal.html

I still have a pair of these on my house so if someone is interested I can make some measurements and ear possible sonic variations.

[Villaw]

Andrea

I am interested to know your final recommendations on the interstage and output transformers since I have to make some orders. Would you recommend Lundahl or Bartolucci or a combination?

[audiodesign]

I should receive in the next week a new interstage, more expensive, from Bartolucci so wait one week before order parts.

I am waiting also a pair of output transformers designed for this circuit.