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Old 6th July 2006, 07:41 AM   #1
liberty is offline liberty  Italy
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Join Date: Jul 2004
Location: Ancona
Default Otl Se

7 years ago I decided to have an OTL SE for my headphone.
Since I had a Grado SR60(32 Ohm) I decided for this schematic:

http://utenti.lycos.it/pcclubfermo/OTLa.jpg


I buyed all components but I never finished the ampi.
Now I decided to use the components to try an OTL for 8 Ohm speakers, improving the project.
First of all I tryed this schematic:

http://utenti.lycos.it/pcclubfermo/OTLb.jpg

As you see I left the driver(e88cc) as it was and I added a 6080 in output stage. I know that this configuration give about 1W, but it is enough for me. My idea is to add one more 6080 in output stage, but first of all I wanted to know how the ampli sound.
The problem is that the sound(with 98db loudspeakers - 8 ohm) seems to be little distorted and with poor high freq.

Where I got wrong?
3 x 6080 need improved driver?
Or Cathode need lower res. value than 160 ohm(for example 120 ohm)?
I know 3 x 6080 in this configuration has an output of about 20 ohm, but many people say that it's enough to drive 8 ohm speakers.

Help needed! Thanks!
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Old 6th July 2006, 11:54 AM   #2
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Hi,

In order to produce significant output you need to ensure that the current through the tubes and the cathode resistor is much higher. 1A standing current will give you a theoretical max output power of 4W as the peak output current can never exceed 1A and output power is Ipk^2/2*Rl. 2A will give you 4 times as much output power or 16W, the real output power will be lower as it is impossible to use all current swing.

A problem will be power dissipation as the circuit will dissipate 170V * the standíng current so you will dissipate 170W to get 4W output power.

It seems that with 40V over the cathode resistor and 170V B+ a 6080 will draw 160mA which will be 1A for 6 sections in Parallell, 160mA is a bit higher than allowed but should probably be OK, anode dissipation will be ~21 per section which is too high but it will work at least for a while. The cathode resistor should be 40Ohm and will dissipate 40W continous.

I don't recommend to build an amplifier like this but the example above illustrate the problems with building a class A OTL and this is the reason why all OTLs on the market are working in class AB except for the low power SET OTL offered by Trancendant which gives 1.5W

Regards Hans
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Old 6th July 2006, 01:04 PM   #3
liberty is offline liberty  Italy
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Join Date: Jul 2004
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First of all thanks for the answer.
The reason I am trying with this kind of amp is simple:
I have the components and I am very curious about the type sound it can produce.
And also I have many things to learn about tubes, and I hope to starting here in this nice forum with your competent and kind help.

Now, if I understand well, you say that the problem is in the output section(not in the ecc88 SRPP driver, that was thought for only 4 sections of 6080).

So I have "only" to try to change the value of R in the cathodes of 6080s.
For example starting using an 80 ohm res, or lower(you said 40 Ohm, but I am afraid to go so "out" of plate dissipation of 6080s).
I prefer starting lowering a little, for first time.

It's correct, or I miss something?

P.s. How I can calculate actual real anode dissipation per section(with my 160 Ohm cathode resistor)?

Thanks again.
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Old 6th July 2006, 02:54 PM   #4
liberty is offline liberty  Italy
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Btw, I tryed to change 160 Ohm res with a 80 Ohm res.
It seems to sound louder.
I notice, once again, poor high freq(it seems to be like a cut on high frequencies, muffled sound) and little rough sound(like a very little but advertible distorsion).
I don't know why I have so poor freq. res. and this sound.

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Old 6th July 2006, 03:03 PM   #5
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Hi,

Using 80 ohms will give you lower output power as the current will be lower, to find the current with any resistance you need to look at tube curves like these http://www.triodeel.com/6080_p4.gif and find the point where the grid voltage is R/I where R is the cathode resistance and where I is the current you want to have for all tubes in parallell. (Example -40V grid voltage gives a current of approx 160mA for a anode voltage of 130V so the resistor should be 40/(6*.16) = 40ohm).

To drive cathode followers like this is easy as the input capacitance is very low but you need to provide enough voltage, I would imagine that a 4W version will need something like 80V Pk-Pk which should be possible from the ECC88 (which BTW is coupled like a mu follower and not as a SRPP)

Regards Hans
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Old 6th July 2006, 03:47 PM   #6
liberty is offline liberty  Italy
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Thanks again for answer.

So using 80 ohm cathode res give lower than 160 Ohm Res?
I am confused...
I made in this way:
I checked the voltage across the cathode res. With 160 Ohm res I found 48V. With 80 Ohm res I found again 48V.
My conclusions were:
With 160 Ohm res I have 0,3A across this res. Each section of 6080 draws 0,05A.(or not?)
With 80 Ohm res I have 0,6A across this res. Each section of 6080 draws 0,1A.(or not?).
So with 80 Ohm res I reached the max. value for 6080.

Why you say I have lower power with 80 res than 160 res?

And, also why my sound changes a little or nothing with this change?
Again, why I have poor high freq in both cases?

Excuse me for make you loose your time answering me, but at this point I am very curious to know more about the sound of this schematic!

Thanks again.
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Old 6th July 2006, 05:55 PM   #7
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Hi,

When I wrote lower power with 80ohm I compared with my original recommendation of 40ohm, 80 ohm will give lower current and therefore lower max output power than with 40 ohm.

The loss of high frequencies is probably due to the cap of 500pF to ground in the mu follower, I don't understand what it is doing there, it seems to a deliberate attempt to limit high frequencies, remove this cap. (I didn't notice this cap from the beginning)

If you have the possibility please measure with an oscilloscope on the grid of 6080 to see if the mu follower give enough voltage and expected wide frequency response.

BTW, I encourage the idea to test a simple OTL circuit, when you discover the superiour sound of this compared to practically anything else I believe you probably want to build something more practical like this http://www.tubetvr.com/otl.html

Regards Hans
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Old 7th July 2006, 04:08 PM   #8
liberty is offline liberty  Italy
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Hi,
I am glad to here you encourage OTL design!
Yes, my idea is to discover the sound of OTL amps, and I expect to find good news in them(I ever used classical triode- single ended amps in my system). I know single ended OTLs have some limitations. The big output cap in one of them.
Do you think I can improve the sound bypassing it with a 5uF - paper/oil?
Next step is to try OTL amps like your nice web-site show.

BTW now I need to adjust this single ended OTL, just to know how OTL can sound.
I haven't an oscilloscope, so I cant meausre.
I removed the 500pF cap, but I have no improvment at all.
My sound is "darky" and seems to be little distorted, with bad dynamics impression and no "crystal" at all.

I don't know if the drive stage(mu follower) is capable to deliver the correct Voltage, but I noticed that the sound is bad also at very very low volumes.
I have to find where I am wrong.
The V+ come from a power supply with a tot. of 5000uF(5 caps of 1000uF), I think it's enogh.
Pheraps the problem come from mu follower, but I don't know how to change it, since I made a search in the web and I found nothing serious about mu follower design(formulas).
SO I don't know what I have to change to increase gain in the circuit without destroying E88CC, and also I can't control if the circuit is well designed for the purpose.

Thanks again for answers and for the link to your nice web-site.
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Old 7th July 2006, 10:30 PM   #9
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Remember the 6080's max dissipation of 13watts, so dont go over 100mA per triode section with this psu voltage. Although these tubes are super tough, I've run them much harder, by accident, and they are still running fine.
I agree with Hans in regards to how awesome OTL amps are. Something must be wrong since I've never had high frequency problems with OTLs.
Since the sound is slightly distorted, perhaps the tubes are faulty? Try another E88CC. Or maybe a resistor is the wrong value? Is the problem in both channels?
I know this circuit works ok, it doesnt take much to alter the circuit you now have to this, try it...
You really need an oscilloscope of some sort, audiotester is a great tool using your soundcard, it cost a little but well worth it. Just remember the soundcard has low input impedance,(will load any circuit you measure), and doesnt handle high voltages. A tube buffer is a good project too...
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Old 7th July 2006, 10:37 PM   #10
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Another thing about these power triodes, 6080 and 6AS7, is they have poor matching. One triode will perhaps have 30mA thru it and the other 60mA with same bias and voltages. So you really should have cathode resistors on each leg. I use a whopping 10ohm resistance, and even without any loop feedback the amp has full control. (My speakers have two 8" bass drivers in series, so the amp sees 16ohms in the bass, which may explain this.)
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