Gz34 + otl 6n23p

[BlackBLOOD]

Hello everybody,
i would like to present you my new OTL amplifier, realised with cathode follower:



As you see, it is a standard schematic based on the projects of Andrea Ciuffoli in Headwize. It has no electrolytic capacitors, output caps are metal film AC caps. Output impedance is calculated to be ~40 Ohm.

Supply is with the lovely GZ34 and two 15H chokes:

No electrolytic caps here ether. Rated for 120mA at 300V the PSU is absolutely clean. PSU Designer II calculates output noise of less than 200uV.

This is the working model:


After finishing the fine tuning it will go in two separate boxes, that will stack one on top of the other.That is the reason the tubes are horisontal.

Here are the simulated tube regimes:



Here are some measurements made with my EMU0404 with Sony CDP970 used as a generator.

First the CDP970 only:

It is quiet clean, except the noise under 1Khz, but it is something related to the test setup and not relevant for the THD calculations.


Then measuring the driver only, between C1 and R8:

1st: -72dB and 2nd: -81dB


Then the whole amp, no load:

Same as the previous one. It seams that CF is not making any distortion when not loaded.

Then amp with 510 Ohm load:

1st: -73dB and 2nd: -97dB - Even better!

And finally the amp with 60 Ohm load:

Here the picture is quiet bad, but it does not really concern me since i will use the amp with high impedance phones.

So , i would be really happy to hear your opinion and critics to my realisation and design. I would really like advices on how to reduce THD even further without changing the topology. Any comment is welcome!

[BlackBLOOD]

Ups, first picture is wrong, this was the previous version. Down is the new and measured version:

[SY]

So it appears that the predominant source of 2nd harmonic at light loading is the first stage. You can reduce that a bit with a few circuit tricks, but at -72dB, really why bother? That's already pretty darn good.

If you're trying to win a spec war, go higher on the plate load resistor and raise the B+ to the first stage accordingly.

[BlackBLOOD]

Well it is fine, but i prefer to get as most as possible out of it, since it is a headphone amplifier and every distortion will be more noticeable.

And actually today i did that. I just wanted to see what will hapend with 15K anode load on the driver. I got something like 3-4db lower distortion when measuring between driver and CF , but when i load the schematic with 510R and measure it on the output it is 6dB higher distortion than before. It is pretty much strange to me. Probably it has to do something with R7 wich is only 1K for this set-up. It is possible that current movements in the CF are influencing the driver more with the smaller R7.

I will try to design a Power Supply with 4 different outputs for each tube. Hope that works better.

PS. I`m thinking about a double 6S45P in the CF. It will bring output impedance to less than 10R!

[BlackBLOOD]

I think i will try this supply configuration:



It is making the driver more independant from the CF and gives me more current to try diferent configurations. I hope it will bring some results.

[nicoch58]

split the choke you thinks that is better ?

I need a psu for my phono I try to learning .....

[BlackBLOOD]

Hi guys,

The last version of this amp I had tested was with 4 triodes in the CFollower for each channel - 60mA. A serious PSU decoupling between the CF stage and the driver delivered even better results:



Don't mind the peaks not related to the 1kHz. They come from my measurement setup.

[BlackBLOOD]

So the next step for me is quiet radical:



I would like to use a negative PS for the CF in order to achieve the following things:

1. I can put the attenuator between the Driver and the CF. Since the CF input impedance is ~1M it is better to be there then in front of the driver that has ~150K input impedance.

2. I can use low voltage caps in the output like the BlackGate NX 2200uF 6,3V. They will be polarized with the bias voltage ~3,5V - high enough!

3. I will get higher P suppression & less ripple/noise

The only trade-off that I see is that the PSU price will double. It will rely on 4 Hammond 193J chokes(10H/200mA/82R) for the CF and the two chokes from my previous design.

Other than that, complete separation between the driver and the CF PSU are planed. Also I have choose to use 6 triodes in total for each CF. That way I will get a nice 15R output impedance, 75mA current and 3,5V Bias. For 300R headphones, this should be perfect.

Please comment, give ideas, criticize!

[radiostar]

Great job!!!

[claudiomas]

Originally Posted by BlackBLOOD

So the next step for me is quiet radical:



I would like to use a negative PS for the CF in order to achieve the following things:

1. I can put the attenuator between the Driver and the CF. Since the CF input impedance is ~1M it is better to be there then in front of the driver that has ~150K input impedance.

2. I can use low voltage caps in the output like the BlackGate NX 2200uF 6,3V. They will be polarized with the bias voltage ~3,5V - high enough!

3. I will get higher P suppression & less ripple/noise

The only trade-off that I see is that the PSU price will double. It will rely on 4 Hammond 193J chokes(10H/200mA/82R) for the CF and the two chokes from my previous design.

Other than that, complete separation between the driver and the CF PSU are planed. Also I have choose to use 6 triodes in total for each CF. That way I will get a nice 15R output impedance, 75mA current and 3,5V Bias. For 300R headphones, this should be perfect.

Please comment, give ideas, criticize!

Hi!
i c'ant open the link

your design of upgrade Ciuffoli
its very interesting
claudiomas
https://picasaweb.google.com/1180125...eat=directlink