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Old 15th December 2009, 09:32 AM   #1
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Default Gz34 + otl 6n23p

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

Click the image to open in full size.

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:
Click the image to open in full size.
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:
Click the image to open in full size.

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:
Click the image to open in full size.
Click the image to open in full size.

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

First the CDP970 only:
Click the image to open in full size.
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:
Click the image to open in full size.
1st: -72dB and 2nd: -81dB


Then the whole amp, no load:
Click the image to open in full size.
Same as the previous one. It seams that CF is not making any distortion when not loaded.

Then amp with 510 Ohm load:
Click the image to open in full size.
1st: -73dB and 2nd: -97dB - Even better!

And finally the amp with 60 Ohm load:
Click the image to open in full size.
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!

Last edited by BlackBLOOD; 15th December 2009 at 09:39 AM. Reason: correct pics
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Old 15th December 2009, 08:01 PM   #2
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Ups, first picture is wrong, this was the previous version. Down is the new and measured version:
Click the image to open in full size.
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Old 15th December 2009, 08:53 PM   #3
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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.
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Old 16th December 2009, 10:38 PM   #4
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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!
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Old 17th December 2009, 07:58 AM   #5
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I think i will try this supply configuration:

Click the image to open in full size.

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.

Last edited by BlackBLOOD; 17th December 2009 at 08:00 AM.
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Old 17th December 2009, 12:12 PM   #6
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split the choke you thinks that is better ?

I need a psu for my phono I try to learning .....
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Old 5th May 2012, 10:11 PM   #7
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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:

Click the image to open in full size.

Don't mind the peaks not related to the 1kHz. They come from my measurement setup.
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Old 5th May 2012, 11:29 PM   #8
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So the next step for me is quiet radical:

Click the image to open in full size.

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!
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Old 11th May 2012, 10:55 AM   #9
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Great job!!!
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Old 11th September 2012, 04:40 PM   #10
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Quote:
Originally Posted by BlackBLOOD View Post
So the next step for me is quiet radical:

Click the image to open in full size.

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
Click the image to open in full size.
your design of upgrade Ciuffoli
its very interesting
claudiomas
https://picasaweb.google.com/1180125...eat=directlink
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